Thermodynamics for Energy Systems

2008;():1-8. doi:10.1115/ES2008-54036.

The evaporator is a key component for Organic Rankine Cycle (ORC) system. Second law analysis of the evaporator was carried out in this work. Three processes were included and studied: pre-heating, boiling and super-heating. Firstly, ε–NTU method was applied to study the heat transfer area of evaporator. Then, internal entropy generation for three processes of evaporator was studied by entropy generation number. Thirdly, entropy generation distribution (ds/dA) was predicted by analyzing the temperature difference between the two sides of single stage counter-flow evaporator, for both pure and mixed working fluids. The results show that increase of waste heat fluid temperature increases internal irreversibility, and higher evaporator pressure decreases this irreversibility. The results also show that temperature difference at the end part of boiling process is larger for pure working fluids; and for mixed fluids, because of its’ increasing boiling temperature, this irreversibility decreases remarkably. In conclusion, second law analysis shows that the evaporating pressure plays a key role in evaporator design for ORC system; and both evaporator and working fluid should be well designed to minimize the second law loss.

Topics: Rankine cycle
Commentary by Dr. Valentin Fuster
2008;():9-14. doi:10.1115/ES2008-54240.

A two-step thermo-chemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After proof-of-principle, successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, and elaboration of process strategies presented in previous contributions, the present work describes a thermodynamic study aiming at the fine tuning of the redox system, at the improvement of process conditions, and at the evaluation of the potential of the process. For the redox material the oxygen uptake capability is an essential characteristic, because it is directly connected to the amount of hydrogen which can be produced. In order to evaluate the maximum oxygen uptake potential of a coating material and to be able to find new redox materials theoretical considerations based on thermodynamic laws and properties are helpful and faster than actual tests. Through thermodynamic calculations it is possible to predict the theoretical maximum output of H2 from a specific redox-material under certain conditions. Calculations were focussed on the two mixed iron oxides nickel-iron oxide and zinc-iron oxide. In the simulation the amount of oxygen in the redox-material is calculated before and after the splitting step on the basis of laws of thermodynamics and available material properties for the mixed-iron oxides used. For the simulation the commercial Software FactSage and available databases for the necessary material properties were used. The analysis showed that a maximum hydrogen yield is achieved if the regeneration temperature is raised to the limits of the operation range, if the temperature for the water splitting is lowered below 800 °C and if the partial pressure of oxygen during regeneration is decreased to the lower limits of the operational range. The increased hydrogen yield at lower splitting temperature of about 800 °C could not be confirmed in experimental results, where a higher splitting temperature led to a higher hydrogen yield. As a consequence it can be stated that kinetics must play an important role especially in the splitting step.

Commentary by Dr. Valentin Fuster
2008;():15-21. doi:10.1115/ES2008-54287.

In this paper, a direct internal reforming SOFC (DIRSOFC) integrated with a downdraft biomass gasifier is modeled thermodynamically. As a case study, wood is selected as the biomass material and performance of the system at different operating temperature levels are studied. Change of the operating cell voltage, air utilization ratio, power output of the SOFC and electrical efficiency of the system with current density are investigated. Results show that operating the system at low temperature level yields higher electrical efficiency and air utilization ratio.

Commentary by Dr. Valentin Fuster
2008;():23-32. doi:10.1115/ES2008-54336.

This paper undertakes a thermodynamic analysis of a high-temperature solid oxide fuel cell, combined with a conventional recuperative gas turbine. In the analysis the balance equations for mass, energy and exergy for the system as a whole and its components are written, and both energy and exergy efficiencies are studied for comparison purposes. These results are also verified with data available in the literature for typical operating conditions, the predictive model of the system is validated. The energy efficiency of the integrated cycle is obtained to be as high as 60.55% at the optimum compression ratio. These model findings indicate the influence of different parameters on the performance of the cycle and irreversibilities therein, with respect to the exergy destruction rate and/or entropy generation rate. The results show that a higher ambient temperature would lead to lower energy and exergy efficiencies, and lower net specific power. Furthermore, the results indicate that increasing the turbine inlet temperature results in decreasing both the energy and exergy efficiencies of the cycle, whereas it improves the total specific power output. However, an increase in either the turbine inlet temperature or compression ratio leads to a higher rate of irreversibility within the plant. It is shown that the combustor and SOFC contribute predominantly to the total irreversibility of the system; about 60 percent of which takes place in these components at a typical operating condition, with 31.4% for the combustor and 27.9% for the SOFC.

Commentary by Dr. Valentin Fuster
2008;():33-38. doi:10.1115/ES2008-54340.

In this paper, an attempt is made to investigate the thermodynamic characteristics of a photovoltaic (PV) system based on exergy. A new efficiency is developed that is useful in studying the PV performance and possible improvements. Exergy analysis is applied to a PV system and its components, in order to evaluate the effect of various parameters e.g., voltage, current, area of the PV panel, fill factor and ambient temperature on exergy efficiency. Effect of solar radiation on power conversion efficiency is also evaluated.

Commentary by Dr. Valentin Fuster
2008;():39-49. doi:10.1115/ES2008-54342.

In this paper, we undertake a study to investigate the performance of a hybrid photovoltaic-hydrogen system through energy and exergy efficiencies, improvement potential. This will help identify the irreversibilities (exergy destructions) for performance improvement purposes. Energetic and exergetic renewability ratios are also introduced for grid dependent hybrid energy systems. A case study is presented to highlight the importance of the thermodynamic parameters and show them using some actual and theoretical data. Three different energy demand options from photovoltaic panels to the consumer are identified and considered for the analysis. The minimum and maximum overall energy and exergy efficiencies of the system are calculated based on these options. It is found that the overall energy efficiency values of the system vary between 0.88% and 9.7% while minimum and maximum overall exergy efficiency values of the system are 0.77% and 9.3%, respectively. The monthly improvement potential of the system is also studied to investigate the seasonal performance.

Commentary by Dr. Valentin Fuster
2008;():51-59. doi:10.1115/ES2008-54344.

In this paper we thermodynamically assess the performance of an ammonia-water Rankine cycle that uses no boiler, but rather the saturated liquid is flashed by a volumetric expander (e.g., reciprocating, centrifugal, screw or scroll type expander) for power generation. This cycle has no pinch point and thus the exergy of the heat source can be better used by matching the temperature profiles of the hot and the working fluids in the benefit of performance improvement. The second feature comes from the use of the ammonia-water mixture that offers further opportunity to better match the temperature profiles at the source and sink level. This fact brings ∼10% improvement of exergy efficiency with respect to the case when a single substance (e.g., steam) is used as working fluid. The influence of the expander efficiency, ammonia concentration and the coolant flow rate is investigated and reported for a case study. The applications of this cycle can be found in low power/low temperature heat recovery from geothermal sources, ocean thermal energy conversion, solar energy or process waste heat etc where the cycle competes with Kalina, supercritical or multi-pressure steam implementations of the Rankine cycle.

Topics: Rankine cycle , Water
Commentary by Dr. Valentin Fuster

Fluid-Thermal Sciences for Energy Systems

2008;():61-85. doi:10.1115/ES2008-54072.

A one dimensional method of analysis and experimentally determined entrainment and compression ratios are presented with considerations made for constant pressure and constant area mixing. A set of three nozzles, one converging and two converging diverging, were used to study the isentropic characteristics of the ejector performance. Ejector efficiencies are calculated using the turbomachinery analogue of compressors where adiabatic and complete mixing of primary and secondary fluids is assumed before discharge. Efficiencies are characterized against non-dimensional parameters chosen in context. A generalized algorithm and corresponding MATLAB™ based computer program was developed for performance analysis. While exploring the possibility of a refrigeration system for automotive applications where the size of the ejector could play an important role, a compact experimental ejector was designed and tested.

Topics: Ejectors
Commentary by Dr. Valentin Fuster
2008;():87-93. doi:10.1115/ES2008-54086.

In this paper, heating and heat storage in passive solar heating room with greenhouse has been studied. The unsteady numerical simulation is employed to analyze the performance of the flow and temperature field for the typical sunny day of Wuhan, China, in winter in the heating system. The heat storage layer of passive solar heating room has a great effect on temperature distribution and gas flow in heat storage layer of this system. Properties of the bed worked as solar absorber and storage layer have also been studied.

Topics: Solar energy , Heating
Commentary by Dr. Valentin Fuster
2008;():95-104. doi:10.1115/ES2008-54129.

A solid particle solar receiver (SPSR) is a direct absorption central receiver that uses solid particles enclosed in a cavity to absorb concentrated solar radiation. However, the existing open aperture lowers the overall efficiency by convection heat transfer. Aerowindows have the potential of increasing the efficiency of an SPSR by reducing convective losses from an open receiver aperture and eliminate reflection, convection and reradiation losses from a comparable glass window. Aerodynamic windows consist of a transparent gas stream, which is injected from an air jet, across the receiver aperture to isolate its interior from the surrounding atmosphere. Even though, the wind conditions may still have important effect on the performance of SPSRs. In the present paper, the wind effect on the performance of an SPSR is investigated numerically. The mass, momentum and energy exchange between the solid particle and air flow are simulated by the two-way coupling Euler-Lagrange method in the realizable k-ε turbulence 3D model. The independence of the calculating domain is studied in order to select a proper domain for the numerical simulation. Solar ray tracing method is employed in calculating the solar radiation energy. The numerical investigation of the performance of the SPSR is focusing on optimizing the prototype design and finding out the best working condition for the SPSR. In order to investigate the influences of the wind speed and wind blowing direction on the performance of the receiver, different wind conditions of and different air jet injection conditions are simulated numerically. The cavity thermal efficiencies are calculated and the optimal injection conditions are analyzed for different wind conditions.

Commentary by Dr. Valentin Fuster
2008;():105-115. doi:10.1115/ES2008-54146.

The temperature field of the beam pipe was calculated at the maximum heat loads and designed flow rates of the cooling liquids. The maximum difference between the predicted and measured temperature on the outer surface of the beam pipe was found to be 0.6 K. In addition, the temperature field between the beam pipe and the inner barrel of the main drift chamber (MDC) was calculated assuming natural convection heat transfer of the air in the enclosed cavity under three different conditions. The results of the calculation showed that only the use of insulation covers on the transition sections of the beam pipe keeps the temperature of the inner barrel in the range of 293.0±1.0 K. The thermal conductivity of the heat insulators under the heat insulation covers must be less than 0.5 W/(m·K).

Topics: Temperature
Commentary by Dr. Valentin Fuster
2008;():117-122. doi:10.1115/ES2008-54153.

Thermal comfort in an area is directly controlled by terminal boxes in variable air volume (VAV) air-handling unit (AHU) systems. The terminal box either modulates airflow or adjusts the discharge air temperature. Reduced air circulation will cause thermal discomfort in a conditioned space if the airflow and discharge air temperature are not suitable. The objective of this study is to identify an optimal value for airflow and discharge air temperature that will maintain room thermal comfort. Optimal room airflow and discharge air temperature is analyzed, and the impact of room airflow and discharge air temperature on thermal stratification is verified through CFD (Computational Fluid Dynamics) simulations.

Topics: Air flow , Ducts
Commentary by Dr. Valentin Fuster
2008;():123-133. doi:10.1115/ES2008-54267.

The present study explores the thermofluid characteristics of a seawater-methane heat exchanger that could be used in the liquefaction of natural gas on offshore platforms. The compression process generates large amounts of heat, usually dissipated via plate heat exchangers using seawater as a convenient cooling fluid. Such an application mandates the use of a corrosion resistant material. Metals such as titanium, expensive in terms of both energy and currency, are a common choice. The “total coefficient of performance,” or COPT , which incorporates the energy required to manufacture a heat exchanger along with the pumping power expended over the lifetime of the heat exchanger, is used to compare conventional metallic materials to thermally conductive polymers. The results reveal that heat exchangers fabricated of low energy, low thermal conductivity polymers can perform as well as, or better than, those fabricated of conventional materials, over the full lifecycle of the heat exchanger. Analysis of a prototypical seawater-methane heat exchanger, built from a thermally conductive polymer, suggests that a COPT nearly double that of aluminum, and more than ten times that of titanium, could be achieved.

Commentary by Dr. Valentin Fuster
2008;():135-143. doi:10.1115/ES2008-54351.

A new class of heat and mass transfer model for a desiccant wheel has been presented and implemented in a design tool. Having studied the behavior of the system in different conditions and sensitivity studies, two physical parameters have been chosen to make simplified models or correlations. Using 1500 data of model solutions, two correlations have been made by an optimization routine in Matlab. These equations correlate outlet air conditions of a desiccant wheel to inlet air conditions of air streams and also the wheel and air speeds. The correlations are limited to be used only in the given range of air conditions and wheel speed. However, the range covers the practical situation that usually happens according to the weather data. The behavior of air conditions in Mollier diagram shows that the error for simulation of a typical cooling cycle to calculate supply air conditions is reduced with a factor of almost 3 times smaller. This shows that even in those ranges with low accuracy the correlations are useful. These simplified equations will be used in the design tools as has been presented in details in this paper.

Topics: Design , Wheels
Commentary by Dr. Valentin Fuster
2008;():145-151. doi:10.1115/ES2008-54352.

A validated heat and mass transfer model for an air cooler is developed and implemented in a design tool. The overall heat transfer coefficient is derived from the experiments, which can be written as a function of the heat transfer coefficients on the water and air side. For a good agreement between the measurements of a wet cooler and model solutions a correction in the heat transfer coefficients substantially is done. In addition, results obtained from the measurements indicate the heat transfer coefficient itself and the correlation between heat and mass transfer need to be adapted. Acceptable results are obtained when mass transfer coefficient is adjusted by a correction factor. The decrease in heat transfer is observed with increase in dehumidification and the same is concluded from the coupling of heat and mass transfer. The fins are not completely wet, which means an analogy between sensible and latent heat only exists partly. The corrected model is implemented in the studies of advanced evaporative air conditioning systems where it is used to construct a graphical model based on rules for transients in the Mollier diagram of humid air. Preliminary calculations show that it is very accurate and useful for the simulation of controlled air handling systems although it is very fast resulting of the short calculation time of the simplified models.

Topics: Design
Commentary by Dr. Valentin Fuster

Energy Systems Miniaturization

2008;():153-163. doi:10.1115/ES2008-54244.

Energy recovery is gaining importance in various transportation, industrial process, and military applications because of rising energy costs and geopolitical uncertainties impacting basic energy supplies. Various advanced energy recovery/conversion technologies will require high-performance heat transfer characteristics to achieve energy recovery performance targets and requirements. System analysis of thermoelectric (TE) systems quantify potential power output, conversion efficiency, specific power and power flux in a unique, useful format on maximum efficiency — power maps. Lines constant specific power and power flux and their relationship to lines of constant hot side temperature and points of maximum power are demonstrated. Regions of preferred TE design are associated with not only higher conversion efficiency, but higher specific power and power flux that drives TE conversion designs towards use of microtechnology solutions. Water and He gas microchannel designs are investigated as potential solutions to achieve miniature TE energy recovery systems. Developing high-heat-flux thermal designs using microtechnology are key to enabling miniature energy recovery systems and should occur in parallel with ongoing research in advanced energy conversion materials.

Commentary by Dr. Valentin Fuster
2008;():165-172. doi:10.1115/ES2008-54266.

A micro heat engine, based on a cavity filled with a stationary working fluid under liquid-vapor saturation conditions and encapsulated by two membranes, is described and analyzed. This engine design is easy to produce using MEMS technologies and is operated with external heating and cooling. The motion of the membranes is controlled such that the internal pressure and temperature are constant during the heat addition and removal processes, and thus the fluid executes a true internal Carnot cycle. A model of this Saturation Phase-change Internal Carnot Engine (SPICE) was developed including thermodynamic, mechanical and heat transfer aspects. The efficiency and maximum power of the engine are derived. The maximum power point is fixed in a three-parameter space, and operation at this point leads to maximum power density that scales with the inverse square of the engine dimension. Inclusion of the finite heat capacity of the engine wall leads to a strong dependence of performance on engine frequency, and the existence of an optimal frequency. Effects of transient reverse heat flow, and ‘parasitic heat’ that does not participate in the thermodynamic cycle are observed.

Topics: Heat engines , Cycles
Commentary by Dr. Valentin Fuster
2008;():173-179. doi:10.1115/ES2008-54277.

A numerical simulation is performed to study two-dimensional cross flow over a staggered array of square cylinders in a microchannel using the non-isothermal Information Preservation (IP) method. The IP method works concurrently with the Direct Simulation Monte Carlo (DSMC) eliminating statistical noise from the DSMC results at low flow speeds. Pressure boundary conditions at the inlet and outlet based on the characteristic theory implemented. This study will form a base for our future particle-atomistic hybrid computations.

Commentary by Dr. Valentin Fuster

Climate Control

2008;():181-191. doi:10.1115/ES2008-54081.

The capillary plane terminal system is a novel HVAC system which can be used in office or residential buildings with dedicated outdoor air system. The capillary mats buried in the surface grout of the ceiling or the wall or the floor handle the interior sensible space cooling load with the handled dedicated air taking on the rest latent cooling load to keep the indoor parameters to be desired state. In this paper, with an example of a typical residential room where the capillary mats are installed on the ceiling, by using the method of CFD the indoor air flows under different modes in summer and winter are simulated and the temperature, velocity, relative humidity, predicted mean vote (PMV) and predicted percent dissatisfied (PPD) fields are presented and analyzed. Based on the simulation results, the indoor thermal environments are evaluated. And the optimal air flow mode is recommended correspondingly. These will be useful to the design and application of the capillary plane HVAC terminal system.

Commentary by Dr. Valentin Fuster
2008;():193-201. doi:10.1115/ES2008-54082.

Nowadays as a novel terminal air conditioning system, the capillary plane HVAC terminal system is being researched in China. In this system the terminal capillary pipes are buried in the surface of the ceiling or the wall or the floor with the purpose-built grout, the chilled or heated water circulates in the pipes and exchanges the sensible heat with the indoor air by radiation and convection to make the indoor air parameters stable. The capillary mat terminals usually combine with dedicated outdoor air system and the latter takes on the indoor latent heat. Compared with the floor radiant heating system, the capillary plane HVAC terminal system has the advantages of saving more indoor space and any position installation. In this paper, with the method of numerical calculation the heat transfer performance of the capillary pipe buried in the grout are studied, the average temperature of the radiation surface and average heat flow density in summer and winter are figured out, and the influence factors such as: the pipe spacing, pipe embedded depth, pipe diameter, average temperature of the supply and return water, and design indoor air temperature are analyzed respectively. The optimal mode or trend under given conditions are proposed and the relations of the influence factors are summarized. All these above will be good theoretical references for the design and application of the capillary plane HVAC terminal system.

Commentary by Dr. Valentin Fuster
2008;():203-208. doi:10.1115/ES2008-54154.

Terminal boxes are one of the major building HVAC components and directly impact building room comfort and energy costs. Current terminal boxes may cause occupant discomfort and waste energy if they have inappropriate operation control functions. The objective of this study is to develop and implement applicable optimal terminal box control algorithms. The thermal conditions and energy consumption are compared between conventional and improved control algorithms using measured data. The results of this study show that optimal terminal box control algorithms can stably maintain the set room air temperature and reduce energy consumption compared to conventional control algorithms.

Commentary by Dr. Valentin Fuster
2008;():209-219. doi:10.1115/ES2008-54185.

This paper presents the development of an equation based model to simulate the combined heat and mass transfer in the desiccant wheels. The performance model is one dimensional in the axial direction. It applies a lumped formulation in the thickness direction of the desiccant and the substrate. The boundary conditions of this problem represent the inlet outside/process and building exhaust/regeneration air conditions as well as the adiabatic condition of the two ends of the desiccant composite. The solutions of this model are iterated until the wheel reaches periodic steady state operation. The modeling results are obtained as the changes of the outside/process and building exhaust/regeneration air conditions along the wheel depth and the wheel rotation. This performance model relates the wheel’s design parameters, such as the wheel dimension, the channel size and the desiccant properties, and the wheel’s operating variables, such as the rotary speed and the regeneration air flowrate, to its operating performance. The impact of some practical issues, such as wheel purge, residual water in the desiccant and the wheel supporting structure, on the wheel performance has also been investigated.

Commentary by Dr. Valentin Fuster
2008;():221-229. doi:10.1115/ES2008-54292.

The work described in this paper relates to advanced control systems, specifically designed for heating, ventilation and air conditioning in office buildings. This work specifically focuses on the use of state of the art fan coil units with advanced instrumentation and control. The premise of the work is that control systems can be significantly enhanced by using real-time data from a distributed sensor network deployed in the building. Specifically, the performance of control systems can be improved by augmenting predictive (feed-forward) control operations with techniques to improve the accuracy of models. A control algorithm for heating, ventilating and air-conditioning systems is described in this paper that integrates an advanced feedforward control algorithm with conventional feedback control. This paper further contains a description of a functional prototype used to demonstrate the proposed control algorithm for indoor thermal environmental control. The test-bed used in this work — the Robert L Preger Intelligent Workplace (IW), at Carnegie Mellon University, involves a large number of variables and hence a complex control task, i.e., the test bed contains multiple sources of thermal energy, and multiple constraints and disturbances — both measurable and immeasurable. The algorithms demonstrated in this test-bed are expected to perform satisfactorily on other environments with smaller number of variables. This paper contains a description of experiments that were performed to validate the comfort and energy benefits of increased sensing using fan coil units that are in installed in two spaces in the IW.

Commentary by Dr. Valentin Fuster

Low/Zero Emission Power Plants

2008;():231-237. doi:10.1115/ES2008-54050.

A novel transcritical Rankine cycle is presented in this paper. This cycle adopts CO2 as its working fluid, with exhaust from a gas turbine as its heat source and LNG as its cold sink. With CO2 working transcritically, large temperature difference for the Rankine cycle is realized. Moreover, the CO2 in the gas turbine exhaust is further cooled and liquefied by LNG after transferring heat to the Rankine cycle. In this way, not only the cold energy is utilized, but also a large part of the CO2 from burning of the vaporized LNG is recovered. In this paper, the system performance of this transcritical cycle is calculated. The influences of the highest cycle temperature and pressure to system specific work, exergy efficiency and liquefied CO2 mass flow rate are analyzed. The exergy loss in each of the heat exchangers is also discussed. It turns out that this kind of CO2 cycle is energy-conservative and environment-friendly.

Commentary by Dr. Valentin Fuster
2008;():239-246. doi:10.1115/ES2008-54073.

The present study attempts to take nitric acid as absorbent to clean up SO2 and NO gases simultaneously from the simulated flue gas in the lab-scale bubbling reactor, this study was divide into the individual DeNOx experiments and the combined DeSOx/DeNOx experiments: the individual DeNOx experiments were carried out to examine the effect of various operating parameters such as input NO concentration, nitric acid concentration, oxygen concentration input SO2 concentration, adding KMnO4 as additive and taking NaOH as the secondary absorption processes on the SO2 and NOx removal efficiencies at room temperature, the results of the individual DeNOx show that NO removal efficiency of 70%–95% were achieved under optimized conditions. NO removal efficiency increased with the increasing nitric acid concentration and increased by adding KMnO4 into the absorbent as additive as well. The removal efficiency of NO can reach 95% when using the two-step integrated processes of (HNO3 +KMnO4 )-NaOH, the absorption solution of 50% nitric acid, 400ppm of input NO concentration. 0.5% oxygen concentration and without SO2 in the simulated flue gas. No improvement on the NOx removal efficiency was observed with the increasing of KMnO4 and NaOH concentration in the scrubbing solution. The results of the combined DeSOx/DeNOx experiments show that the maximum DeNOx and DeSOx efficiencies ranged from 36.6% to 81% and from 99.4% to 100.0%, respectively. The prime parameters affecting the NOx removal efficiency are the oxygen concentration and the input SO2 concentration.

Commentary by Dr. Valentin Fuster
2008;():247-261. doi:10.1115/ES2008-54166.

In this paper two methodologies, able to avoid CO2 dispersion in atmosphere, have been analyzed: • treating exhaust gases in order to remove, liquefy and store the produced carbon dioxide; • de-carbonizing fossil fuels before using them in the combustion in order to inhibit completely carbon dioxide production. These methodologies have been implemented in advanced power plants based on gas turbine: a combined cycle power plant (CC), fed by natural gas, and an integrated gasification combined cycle (IGCC), fed by coal. The exhaust gas treatment is based on a chemical process of absorption, while the fossil fuel decarbonization is based on partial oxidation of methane, steam methane reforming and coal gasification. These systems require material and energetic integrations with the power sections and so the best interconnections must be investigated in order to obtain good overall performance. With reference to thermodynamic and economic performance, significant comparisons have been made between the above mentioned reference plants. An efficiency decrease and an increase in the cost of electricity have been obtained when power plants are equipped with systems able to reduce CO2 emissions. However, in order to obtain low CO2 emissions when coal is used, the coal decarbonization must be implemented: in this case it is possible to attain a global efficiency of about 38%, a specific emission of 0.1117 kg/kWh and an increase of kWh cost of about 32%. Vice versa, in order to obtain low CO2 emissions when natural gas is used, the exhaust gas treatment must be implemented: in this case it is possible to attain a global efficiency of about 50.7%, a specific emission of 0.0391 kg/kWh and an increase of kWh cost of about 15%. The clean use of coal seems to have good potential because it allows low energy penalizations (about 7.5 percentage points) and economic increases of about 32%. Because of the great availability, the homogeneous distribution and the low cost of this fuel, these results seem to be very interesting especially in the viewpoint of a transition towards the “hydrogen economy”, based, at least in the medium term, on fossil fuels.

Commentary by Dr. Valentin Fuster
2008;():263-267. doi:10.1115/ES2008-54187.

Anthropogenic carbon dioxide emission from its sources must be reduced to decrease the threat of global warming. Calcium oxide is considered as an effective carbon dioxide absorbent in biomass or coal gasification process as well as conventional power plants. It reacts with carbon dioxide to form calcium carbonate which can be decomposed into the original oxide and carbon dioxide at high temperature by calcination. In order to make this method practical for the carbon dioxide capture and sequestration, the performance of the calcium oxide absorbent must be maintained over a large number of carbonation/calcination cycles. For this reason, loss in the surface area of the absorbent due to pore plugging and sintering of particles in cyclic operation must be avoided. To prevent or minimize this problem, a simple and effective procedure for immobilization of calcium oxide on a fibrous alumina mat was developed in this study. The prepared samples were observed by SEM and the cyclic performance of the calcium oxide absorbent was evaluated by TGA experiments and compared to the previous studies in literature. 75% and 62% maximum carbonation conversions of the prepared absorbents with 23 wt % and 55 wt % calcium oxide content were achieved respectively and remained stable even after ten cycles whereas conversion in the literature data dropped steeply with the number of cycles.

Commentary by Dr. Valentin Fuster
2008;():269-278. doi:10.1115/ES2008-54268.

This paper presents the process design and the energy analysis for a coal-fired power plant based on pressurised oxycoal combustion and including carbon capture technologies. A combustion technology performing a pressurised combustion of coal in an atmosphere of O2 /CO2 /H2 O and including flue gases recycling has been selected. Combustion and steam production occur in separated equipments and the combustor’s design allows achieving high ash removal efficiency. The Rankine cycle has been chosen as the most viable thermodynamic cycle in a short-term scenario. Oxygen required by the combustion process is supplied by a cryogenic Air Separation Unit (ASU) and a double-reheat ultrasupercritical cycle is employed with main steam conditions of 250bar/605°C and reheat steam temperatures of 605°C/620°C. All choices related to thermal cycle selection and process design have been conducted upon the principle of feasibility and reliability. In order to increase net plant efficiency both sensible and latent heat is recovered from the flue gas stream before entering the purification and compression section. By operating in pressure it becomes possible to recover a larger amount of heat than in the atmospheric case. As a result, all low pressure steam bleedings and the corresponding regenerative heat exchangers can be eliminated. Process simulation is carried out in the paper and the expected efficiency is evaluated, as well as other cycle performance parameters. Since a relevant benefit may arise from the combustion of cheap coals, the impact of burning high-ash content and low ash-fusion-temperature coals is assessed. The impact of energy penalties associated to oxygen production and the benefit arising from high heat-transfer coefficients due to the increased pressure of the flue gas are deeply investigated.

Commentary by Dr. Valentin Fuster

Zero Energy Homes

2008;():279-286. doi:10.1115/ES2008-54044.

Sustainability is an issue of great importance in the building and energy sectors. In the United States, about 40% of total energy use is in buildings, 30% of raw materials are used in buildings, 30% of waste outputs come from buildings, 30% of greenhouse gas emissions are attributed to buildings, and 12% of potable water consumption occurs in buildings. Thus, there is a great necessity for the rapid deployment of highly sustainable buildings that are aesthetic and reliable. Solar houses are highly sustainable and can be designed to be reliable by using streamlined technologies, providing as much power as needed, and by minimizing the energy usage within the building. The US DOE Solar Decathlon offered a great opportunity to test these criteria which were at the same time the fundamental elements taken into consideration when designing the Santa Clara University (SCU) solar house in 2007 [1]. In this research the SCU solar decathlon solar house energy and thermal performances were analyzed. The energy and thermal loads were modeled using EnergyPlus™ software which helps to perform detailed modeling of the energy and thermal performances of buildings. The conditioned space of the building consists of two rectangular shaped zones, the living room and the bedroom, which together are approximately 45ft along the east-west direction and 11ft wide. Wood framing with R-19 insulation, made from recycled jeans, was used for the walls. The roof and the floor are constructed of a bamboo wood frame with foam insulation. Daylighting was maximized through active windows (i.e. electro-chromic), energy efficient appliances were incorporated along with solar thermal air conditioning, heating and hot water. Performance parameters for the mechanical systems were developed from conventionally available technologies and the control set-points were based on DOE Solar Decathlon 2007 (SD07) guidelines [1]. The thermal energy design decisions for the house were based largely on a combination of the solar decathlon contest requirements and that technologies were sustainable and commercially available. The house was tested in Washington DC in October 2007 during the competition and performed excellently well ranking at the top in the following categories: energy balance, thermal comfort, and hot water. Data collected during the event provide the unique opportunity of validating the simulated energy and thermal performances of the house with weather file created from the real-time weather data. The created weather file is used to run new simulations of the SCU SD07 house, from these results we can assess the accuracy of the simulation program used. If accuracy is high enough, annual simulations are executed to demonstrate how the house would perform under extreme climatic conditions in different regions.

Commentary by Dr. Valentin Fuster
2008;():287-295. doi:10.1115/ES2008-54179.

Building a zero energy home requires several major considerations: site selection for the home; considerations to use less; conservation of what you produce; and evaluation the best choices of renewable resources. This paper discusses the use of climate data collection software, heat loss and heat gain considerations and software; how to achieve a zero energy building and qualifying as a LEED (Leadership in Energy and Environmental Design) Certified Platinum home. Site selection is the first step. This first step can be taken after you have identified your goals. What alternative energy systems do you want to use? Do you want to use more than one alternative source? What other site considerations are important? In my case, I wanted to use solar energy as my primary alternative energy source. Why? I want comfort, reliability and ease of use. Other alternatives may require more maintenance. Wind power will be a second source that will be incorporated to produce electricity when the PV system can not produce. The site selected is a south facing mountain that has a steady breeze most of the time. Another consideration for me is the ability to use earth-sheltering as a measure of high efficiency construction. The south-facing mountain also provides the opportunity to “nestle” into the mountainside. Calculations and basis of design are presented. Using less is a key mindset that we all need to move toward. Using less does not mean that you suffer. This house will be comfortable year round with little effort because the house uses passive solar design for lighting and space heating, active solar hot water for additional heating of the floor and domestic hot water, and PV/wind/biodiesel generator backup to generate electricity for lighting and other typical electrical loads. The construction materials provide high R-values and green products that contribute to excellent indoor air quality. SIPs (Structural Insulated Panels) will be used as the structural components for the walls and roof. All electrical appliances, refrigerator, lighting, and washer/dryer were selected to use less electricity and water. Data describing the energy requirements are provided. Reuse all that you can. I am incorporating a masonry heater, also known as a Russian Fireplace. The combustion efficiency of the masonry fireplace is typically 92–94 percent with very low emissions. The masonry fireplace will provide passive mass for passive release of the woodburning energy during the evening and heat hot water coils in the fireplace as well (as the hot water backup system). The use of hard woods from the land will provide heat overnight, heat for cooking and supply additional BTUs for domestic hot water and radiant heat. Many of the building materials that are selected for construction are from the land; stone and whole cut wood from the land will be used for esthetic appeal and thermal mass thereby reducing harmful manufacturer’s emissions. My site is an excellent site for a hybrid solar and wind power (with a biodiesel generator as backup) system. The orientation and wind profile of the land is optimal for solar and wind energy applications. Site specific data and optimization of active solar, passive solar and PV/wind systems are presented. Life cycle costs are presented to show the cost comparison using Years-to-Payback and Return on Investment approaches for the energy systems and LEED certification costs for new construction.

Commentary by Dr. Valentin Fuster
2008;():297-304. doi:10.1115/ES2008-54255.

A detailed model of the Net Zero Energy Town House in Toronto is developed in TRNSYS, incorporating a ground source heat pump integrated with an in floor radiant heating system. In order to minimize the heating and cooling loads, the building envelope is well insulated with the exterior walls having an R-60 insulation value. Much of the work done previously on the use of thermal mass in buildings has been experimental in nature and has focussed mainly on conventional brick construction in hot climates such as Asia and Africa. This research will analyze the impact of using thermal mass with a building envelope that is highly insulated, and of a light construction, such as that used in Low Energy or Net Zero housing. Furthermore, this analysis would also evaluate the impact of using thermal mass in a cold climate such as that found in Canada. The simulations showed that, for colder climates, thermal mass can replace some of the insulation and still provide superior results. Also the impact of thermal mass was found to be more significant during the winter season than summer for Toronto.

Commentary by Dr. Valentin Fuster
2008;():305-312. doi:10.1115/ES2008-54290.

Building energy performance regulations and standards around the world are evolving aiming to reduce the energy use in buildings. As we move towards zero energy buildings, the embodied energy of construction materials and energy systems becomes more important, as it represents a high percentage of the overall life cycle energy use of a building. However, this issue is still ignored by many regulations and certification methods, as happens with the European Energy Performance of Buildings Directive (EPBD), which focuses on the energy used in operation. This paper analyses a typical house designed to comply with Irish building regulations, calculating its energy use for heating and how water with the Irish national calculation tool, which uses a methodology in line with the EPBD. A range of measures to reduce the energy performance in use of this typical house are proposed, calculating the reduced energy demand and moving towards a zero energy demand building. A life-cycle approach is added to the analysis, taking into account the differential embodied energy of the implemented measures in relation to the typical house base-case, annualizing the differential embodied energy and re-calculating the overall energy use. The paper discusses how a simplified approach for accounting embodied energy of materials could be useful in a goal to achieve the lowest life-cycle energy use in buildings, and concludes with a note on how accounting for embodied energy is a key element when moving towards zero energy buildings.

Commentary by Dr. Valentin Fuster
2008;():313-320. doi:10.1115/ES2008-54327.

This paper describes the mechanical systems, the DC-coupled electrical system, the simulation approach and the preliminary results of the University of Illinois entry in the 2007 Department of Energy Solar Decathlon competition. The competition showcased twenty net-zero energy solar powered houses. The University of Illinois entry was the only one that featured an all-electric design. No solar thermal collectors were used; space and water heating was accomplished primarily through heat pumps. Each of three house modules is sensibly conditioned with autonomous, custom mini-split heat pumps using all radiant and natural convection heat exchange for the interior side. Simulation methods are described and assumptions of wall and window properties, mechanical system performance and electrical system performance are disclosed. Details are provided on the theoretical analysis of internal heat transfer and the basic design of the custom mechanical system. The electrical system topology and equipment choices are presented and initial performance results are shown. Additionally, preliminary analysis is carried out on the data taken during the Solar Decathlon competition and on the observations of post-competition winter performance. The success in being awarded comfort conditioning points during the competition is discussed along with drawbacks not represented in the competition results.

Commentary by Dr. Valentin Fuster

Advances in Solar Hydrogen, Solar Chemistry and Bioconversion

2008;():321-327. doi:10.1115/ES2008-54085.

Effect of multiple operation conditions on behaviors of H2 production from organic substrate by photo-fermentation in immobilized-cells packed bed using Rhodoseudomonas palustris CQK 01 were investigated in a continuous culture. It revealed that in limited light intensity or below 590 nm of light wavelength the H2 production rate of the immobilized-cells bioreactor increased with increase of value of the experimental parameters. And the optimal operation factor of inlet concentration of substrate and inlet temperature of liquid for H2 production was 50 mmol/L and 30 °C, respectively. However, when these parameters value in the experiment became suppressive conditions the H2 production rate would went down with parameter ascending.

Commentary by Dr. Valentin Fuster
2008;():329-334. doi:10.1115/ES2008-54090.

A prototype direct absorption central receiver, called the solid particle receiver (SPR), was recently built and tested on-sun at Sandia National Laboratories. The SPR consists of a 6 m tall cavity through which a 1 m wide curtain of spherical ceramic particles is dropped and directly heated with concentrated solar energy. The focus of this current effort is to provide an experimental basis for the validation of computational models that have been created to support the development of the solid particle receiver as a solar interface for thermochemical hydrogen and solar power systems. In this paper we present detailed information on the design and construction of the receiver as well as test data including the temperature change of the particles and internal cavity walls. We conclude with a discussion of the steps needed to demonstrate the overall feasibility of the SPR concept.

Commentary by Dr. Valentin Fuster
2008;():335-343. doi:10.1115/ES2008-54093.

A two-step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. A reactor concept has been developed in which the metal oxide is fixed on multi-channelled honeycomb ceramic supports capable of adsorbing solar irradiation. In the solar furnace of DLR in Cologne coated honeycomb structures were tested in a solar receiver-reactor with respect to their water splitting capability and their long term stability. The concept of this new reactor design has proven feasible and constant hydrogen production during repeated cycles has been shown. For a further optimization of the process and in order to gain reliable performance predictions more information about the process especially concerning the kinetics of the oxidation and the reduction step are essential. To examine the kinetics of the water splitting and the regeneration step a test rig has been built up on a laboratory scale. In this test rig small coated honeycombs are heated by an electric furnace. The honeycomb is placed inside a tube reactor and can be flushed with water vapour or with an inert gas. A homogeneous temperature within the sample is reached and testing conditions are reproducible. Through analysis of the product gas the hydrogen production is monitored and a reaction rate describing the hydrogen production rate per gram ferrite can be formulated. Using this test set-up, SiC honeycombs coated with a zinc-ferrite have been tested. The influences of the water splitting temperature and the water concentration on the kinetics of the water splitting step have been investigated. A mathematical approach for the reaction rate was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ/mole was found.

Topics: Ceramics , Cycles , Iron , Water
Commentary by Dr. Valentin Fuster
2008;():345-353. doi:10.1115/ES2008-54098.

The synthesis and hydrolysis of zinc nanoparticles are carried out in a tubular reactor. A key component of the reactor is a coaxial jet quench device. Three co-axial and multi-inlet confined jets mix Zn(g), steam and argon to produce and hydrolyze zinc nanoparticles. The performance of the quench device is assessed with computational fluid dynamic modeling and measurements of hydrogen conversion and particle size and composition. Numerical data elucidate the impact of varying jet flow rates on temperature and velocity distributions within the reactor. Experiments produce hydrogen conversions of 61 to 79%. Particle deposition on sections of the reactor surface above 650 K favors hydrolysis. Residence time for in-flight particles is less than one second and these particles are partially hydrolyzed.

Commentary by Dr. Valentin Fuster
2008;():355-360. doi:10.1115/ES2008-54118.

Gasification of coal, biomass, and other carbonaceous materials for high-quality syngas production is considered using concentrated solar energy as the source of high-temperature process heat. The solar reactor consists of two cavities separated by a SiC-coated graphite plate, with the upper one serving as the radiative absorber and the lower one containing the reacting packed bed that shrinks as the reaction progresses. A 5-kW prototype reactor with an 8 cm-depth, 14.3 cm-diameter cylindrical bed was fabricated and tested in the High-Flux Solar Simulator at PSI, subjected to solar flux concentrations up to 2300 suns. Beech charcoal was used as a model feedstock and converted into high-quality syngas (predominantly H2 and CO) with packed-bed temperatures up to 1500 K, an upgrade factor of the calorific value of 1.33, and an energy conversion efficiency of 29%. Pyrolysis was evident through the evolution of higher gaseous hydrocarbons during heating of the packed bed. The engineering design, fabrication, and testing of the solar reactor are described.

Commentary by Dr. Valentin Fuster
2008;():361-369. doi:10.1115/ES2008-54151.

A thermochemical two-step water splitting cycle using a redox system of iron-based oxides or ferrites is one of the promising processes for converting solar energy into clean hydrogen in sunbelt regions. Fe3 O4 supported on YSZ (Yttrium-Stabilized Zirconia) or Fe3 O4 /YSZ is a promising working material for the two-step water splitting cycle. In the water splitting cycle, an iron-containing YSZ or Fe2+ -YSZ is formed by a high-temperature reaction between Fe3 O4 and YSZ support at 1400°C in an inert atmosphere. The Fe2+ -YSZ reacts with steam and generate hydrogen at 1000°C, to form Fe3+ -YSZ that is re-activated by a thermal reduction in a separate step at 1400°C under an inert atmosphere. In the present work, the thermal reduction was performed in a higher temperature range of 1400–1500°C while the hydrolysis reaction was carried out at 1000°C. It was confirmed by XRD analysis that the cyclic redox reactions occurred based on the same reaction mechanism when using a thermal reduction temperature between 1400 and 1500°C. The conversions of Fe3 O4 to Fe2+ -YSZ were 20, 26 and 47% when the thermal reduction temperature were 1400, 1450, and 1500°C respectively, indicating that the x values in the formed Fe2+ -YSZ or Fex 2+ Yy Zr1−y O2−y/2+x were 0.08, 0.11, and 0.19 respectively, where y = 0.15. The conversions of Fe2+ -YSZ to Fe3+ -YSZ in the hydrolysis reaction (at 1000°C), however, decreased from 90% to 60% when the thermal reduction temperature increased from 1400 to 1500°C. As the results, the hydrogen production reactivity of Fe3 O4 supported on YSZ increased from 5.6 × 10−4 to 7.5 × 10−4 g per gram of Fe3 O4 /YSZ for one cycle on the cycle average by elevated thermal reduction temperature from 1400 to 1500°C.

Topics: Temperature , Iron , Water
Commentary by Dr. Valentin Fuster
2008;():371-383. doi:10.1115/ES2008-54156.

Ni-Cr-Al alloy foam absorber with high porosity was catalytically activated using a Ru/γ-Al2 O3 catalyst, and was subsequently tested with respect to CO2 reforming of methane in a small-scale volumetric receiver-reactor by using a sun simulator. A chemical storage efficiency of about 40% was obtained for a mean light flux of 325 kWm−2 . Furthermore, the activity and the stability of the metallic foam absorber were compared with those of a SiC foam absorber activated with the same Ru/γ-Al2 O3 catalyst for 50 h of light irradiation, and it was found that the metallic foam absorber has superior catalytic stability in comparison to the SiC form absorber. In addition, unlike ceramic foams such as SiC, metallic foams feature superior plasticity, which prevents the emergence of cracks caused by mechanical or thermal shock. The kinetics of CO2 reforming of methane over metallic foam absorbers were also examined for temperatures of 600–750°C using a quartz tube reactor and an electric furnace. The experiments were performed by varying the methane/CO2 ratios of 0.5–2.3. Moreover, the kinetic data were fitted to four different types of kinetic models, namely the Langmuir-Hinshelwood, Basic, Eley-Rideal, and Stepwise mechanisms. The kinetic model which provided the best prediction of the experimental reforming rates was the Langmuir-Hinshelwood mechanism.

Commentary by Dr. Valentin Fuster
2008;():385-390. doi:10.1115/ES2008-54170.

Hydrogen production by water-splitting thermochemical cycle based on manganese ferrite /sodium carbonate reactive system is reported. Two different preparation procedures for manganese ferrite/sodium carbonate mixture were adopted and compared in terms of materials capability to cyclical hydrogen production. According to the first procedure conventionally synthesized manganese ferrite, i. e. high temperature (1250 °C) heating in Ar of carbonate/oxide precursors, was mixed with sodium carbonate. The blend was tested inside a TPD reactor using a cyclical hydrogen production/material regeneration scheme. After few cycles the mixture resulted rapidly passivated and unable to further produce hydrogen. An innovative method that avoids the high temperature synthesis of manganese ferrite is presented. This procedure consists in a set of consecutive thermal treatments of a manganese carbonate/sodium carbonate/iron oxide mixture in different environments (inert, oxidative, reducing) at temperatures not exceeding 750 °C. Such material, whose observed chemical composition consists in manganese ferrite and sodium carbonate in stoichiometric amount, is able to evolve hydrogen during 25 consecutive water-splitting cycles, with a small decrease in cyclical production efficiency.

Commentary by Dr. Valentin Fuster
2008;():391-396. doi:10.1115/ES2008-54202.

Proton exchange membrane fuel cells (PEMFC) are good candidates for portable energy sources with a fast response to load changes, while being compact as a result of their capability to provide a high power density. Hydrogen constitutes the fuel for the PEMFC and can be obtained in situ to avoid transportation and safety problems. An efficient method to produce hydrogen is by methanol steam reforming in a micro-reactor, an endothermic reaction for which the highest efficiency occurs between 250°C and 300°C. Different methods have been used to reach and maintain these temperatures, including electrical heaters and exothermic reactions. We propose to use solar energy to increase the efficiency of the micro-reactor while taking advantage of a free, renewable energy source. The micro-channels, where the water-methanol mixture flows, are insulated from the surroundings by a thin vacuum layer coated with a selective material. This coating has a high absorptance for short wavelength incoming radiation and low emmitance for infrared radiation, reducing the heat losses. By using these coated insulation layers, the fluid temperature in the microchannels is predicted to be higher than 250°C. Hence, it is expected that the solar powered micro-reactor will produce hydrogen with a higher overall efficiency than the present reactors by taking advantage of the solar radiation.

Commentary by Dr. Valentin Fuster
2008;():397-402. doi:10.1115/ES2008-54281.

Solar H2 production by the two-step water splitting process with thermochemical reaction has been proposed to convert solar energy into chemical energy. We succeeded in repeating the cyclic two-step water splitting process composed of the O2 -releasing and H2 -generation reactions with metal (Fe, Ni) doped ceria. The metal doped ceria with low content of metal ion (Fe3+ , Ni2+ ) formed a solid solution with fluorite-type structure between ceria (CeO2 ) and metal oxide (Fe2 O3 , NiO). The empirical formula of the solid solution was Ce1-α Mα O2−δ (M = Fe, Ni), and it was assumed that the high reactivity on the two-step water splitting process was due to an oxygen deficiency in the solid solution. The metal doped ceria with different Ce:M mole ratio (Ce:M = 0.97:0.03–0.7:0.3) was prepared through the combustion method. The two-step water-splitting process with metal doped ceria proceeded at 1673K for the O2 -releasing reaction and at 1273K for the H2 -generation reaction by irradiation of an infrared imaging lamp for a solar simulator. The amounts of H2 gas evolved in the H2 -generation reaction with Fe-doped ceria and Ni-doped ceria with different Ce:M (M = Fe, Ni) mole ratio were 0.97–1.8 and 1.7–2.5 cm3 /g, respectively, and the evolved H2 /O2 ratios were approximately equaled to 2 of the stoichiometric value. The amounts of H2 and O2 gases evolved in the two-step water splitting process varied with the Ce:M mole ratio in the metal doped ceria. It was suggested that the O2 -releasing and H2 -generation reactions with metal doped ceria was repeated with the reduction and oxidation of Ce4+ -Ce3+ enhanced by the presence of Fe or Ni ions. Furthermore, the O2 -releasing reaction with Ni-doped ceria proceeded under a high O2 partial pressure atmosphere (pO2 = 0.05 atm) and at the temperature of 1773K. The progress of the O2 -releasing reaction under a high pO2 indicates that metal doped ceria can be applicable for the rotary-type solar reactor developed by Tokyo Tech group for solar H2 production.

Topics: Metals , Solar energy , Water
Commentary by Dr. Valentin Fuster
2008;():403-408. doi:10.1115/ES2008-54282.

The rotary-type solar reactor has been developed for solar hydrogen production with the two-step water splitting process using the reactive ceramic (Ni, Mn-ferrite). The rotary-type reactor has the rotating tubular cylinder covered on a reactive ceramic and dual reaction cells for O2 -releasing and H2 -generation reactions. The successive evolutions of O2 and H2 gases were observed in the O2 releasing and H2 generation reaction cells, respectively, with the prototype (small) reactor (diameter of cylinder ; 4cm). There is an upper limit for the rate of H2 gas evolution in the case of the prototype reactor because of the slow rotation rate in a small irradiation area. To confirm the practical operation of the rotary-type solar reactor with the two-step water splitting process for the simultaneous production of H2 and O2 gases, a scaled-up rotary-type solar reactor with 400cm2 of the irradiation area was fabricated (diameter; 50cm). The scaled-up reactor made of inner and outer short tubular cylinders (stainless steel) has a quartz glass window for the irradiation of reactive ceramic coated on the inner tubular cylinder (cylindrical rotor) and reaction cells were aligned in the sharing spaces between the inner and outer short tubular cylinders with gas sealing mechanisms. In the reactor, reactive ceramic coated on the inner tubular cylinder was heated up to 1800K by using the infrared imaging lamps (solar simulator) with thermal flux = 600kW/m2 . The solid solution between YSZ and Ni-ferrite was used as reactive ceramic for the scaled-up reactor in order to prevent from sintering at a high temperature in the O2 -releasing reaction, since the sintering of reactive ceramic resulted in lowering the yield of H2 gas evolution in the H2 -generation reaction. The amounts of H2 and O2 gases evolved at the rotation rate of 0.3rpm were evaluated to 64cm3 and 30cm3 for 10min with the scaled-up rotary-type solar reactor, respectively, which were much larger than those with the prototype reactor. The simultaneous evolutions of H2 and O2 gases in the two-step water splitting process were repeated by employing the scaled-up reactor with the solid solution between YSZ and Ni-ferrite.

Topics: Fuels , Solar energy
Commentary by Dr. Valentin Fuster

Solar Thermal and Photovoltaic Power

2008;():409-415. doi:10.1115/ES2008-54016.

This research explores a new efficient approach for producing electricity from the abundant energy of the sun. A nantenna electromagnetic collector (NEC) has been designed, prototyped, and tested. Proof of concept has been validated. The NEC devices target mid-infrared wavelengths, where conventional photovoltaic (PV) solar cells are inefficient and where there is an abundance of solar energy. The initial concept of designing NEC was based on scaling of radio frequency antenna theory. This approach has proven unsuccessful by many due to not fully understanding and accounting for the optical behavior of materials in the THz region. Also, until recent years the nanofabrication methods were not available to fabricate the optical antenna elements. We have addressed and overcome both technology barriers. Several factors were critical in successful implementation of NEC including: 1) frequency-dependent modeling of antenna elements; 2) selection of materials with proper THz properties; and 3) novel manufacturing methods that enable economical large-scale manufacturing. The work represents an important step toward the ultimate realization of a low-cost device that will collect, as well as convert this radiation into electricity, which will lead to a wide spectrum, high conversion efficiency, and low-cost solution to complement conventional PVs.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
2008;():417-422. doi:10.1115/ES2008-54067.

Spacing between the adjacent collectors in a solar field is an important parameter which effects the shading and hence the energy conversion from collectors. Land value has an important bearing on the spacing between the collectors. A computer code has been developed to predict the change in incident energy on the collectors for various spacing distances between them. The code couples general shading models with the local weather data (TMY2). This can be used to calculate shadow area on the collectors from their adjacent collectors for different times of the day for various spacing distances. Variation of shadow area of the collectors for various spacing distances is presented for various time periods. It has been observed that near sunrise and sunset the percent shading area of a system is generally higher, but its influence on the overall energy collection is relatively less due to the decreased solar insolation available during those times of the day. The variation of annual energy received for various spacing distances is presented. Results are given for locations in the Southwestern region of the US including Las Vegas, Phoenix and Albuquerque. Economic implications of these results are discussed.

Commentary by Dr. Valentin Fuster
2008;():423-432. doi:10.1115/ES2008-54122.

Generating electricity from the sun using a combination of a compound parabolic concentrator (CPC) and a thermoelectric module (TEM) has been studied. The system was modeled, analyzed and tested. The model equations and the methodology used for the demonstration are presented and experimentally validated. The experimental setup comprised a manually fabricated CPC placed on a commercially available TEM. The results showed that the combination can generate and sustain enough power for a small appliance. It was also shown that there is enough dissipated heat from the system which could be harnessed for additional uses. The cost is still high, about $35/Wp, but if credit is given for the thermal energy the initial cost goes down.

Commentary by Dr. Valentin Fuster
2008;():433-441. doi:10.1115/ES2008-54124.

The performance of a network of five recently installed, grid interactive residential solar photovoltaic (PV) systems in Palm Beach County, FL is analyzed, and a probabilistic model for estimating the performance the network is developed. To first order, integrated network performance — whatever the combination of individual PV arrays — can be estimated using generalized tilt factors. These take into account basic geometrical information such as array size/orientation and solar position; as well as atmospheric effects, and module efficiencies. They are computed using a model termed the solar simulator that integrates the instantaneous solar irradiation striking a given PV system over a day. The resulting estimates for mean network performance are within ∼ 6% of the observed values. At present, work on estimators of higher moments of the energy production distribution is incomplete, but local meteorological factors that may influence their values as well as data (Pearson correlations and distribution skewness) useful for future developments are discussed.

Commentary by Dr. Valentin Fuster
2008;():443-448. doi:10.1115/ES2008-54134.

Jordan is considered one of the sun-belt countries, which possesses high solar radiation on its horizontal surface. The present study will be concerned on the uses of fuzzy sets methodology to perform evaluation between the most suitable solar technologies for power generation in Jordan, namely, solar ponds and photovoltaic (PV) technologies. The criterion of the evaluation were based on different parameters, i.e., power capacity, efficiency, availability, capacity factor, storage capability, cost, maturity, land usage and safety, they are planned as the technologies for the near foreseen term. Based on benefit to cost ratios, the results showed that photovoltaic technology found to be the better choice in terms of generating electricity, research and development and more effective programs of support and installation.

Commentary by Dr. Valentin Fuster
2008;():449-458. doi:10.1115/ES2008-54176.

The annulus of a parabolic trough receiver is normally evacuated to prevent heat conduction between the internal absorber pipe and the external glass envelope. In the past, this vacuum has sometimes been compromised by hydrogen permeation from the heat transfer fluid through the absorber pipe. Heat conduction, and consequently receiver thermal loss, can be significantly increased by the presence of hydrogen in the annulus. Supplying receivers with inert gases in the annulus, or injecting receivers with inert gases after the vacuum has been compromised, could mitigate these heat losses. This study measures parabolic trough receiver heat conduction in the transition, temperature jump, and continuum regimes for argon-hydrogen and xenon-hydrogen mixtures at an absorber temperature of 350°C. Test results show that small heat loss increases over evacuated values are associated with the 95% inert gas/5% hydrogen mixtures and that from a performance perspective gas-filled HCEs would likely induce a 1–3% plant revenue decrease relative to evacuated receivers, but would protect against hydrogen-induced heat loss as long as there was sufficient quantity of inert gas in the annulus. Sherman’s interpolation formula predicted the inert gas and 95% inert gas/5% hydrogen mixture test results within experimental and model uncertainty, but did not accurately capture the larger hydrogen molar fraction test results. The source of this discrepancy will be further investigated.

Commentary by Dr. Valentin Fuster
2008;():459-474. doi:10.1115/ES2008-54216.

A preliminary design and feasibility study has been conducted for a 200 kWe solar thermal power plant for operation in Ontario. The objective of this study is to assess the feasibility of small-scale commercial solar thermal power production in areas of relatively low insolation. The design has been developed for a convention centre site in Toronto, Ontario. The plant utilizes a portion of the large flat roof area of the convention centre to accommodate the collector array. Each power plant module provides a constant electrical output of 200 kWe throughout the year. The system is capable of maintaining the constant output during periods of low insolation, including night-time hours and cloudy periods, through a combination of thermal storage and a supplemental natural gas heat source. The powerplant utilized the organic Ranking cycle (ORC) to allow for relatively low source temperatures from the solar collector array. A computer simulation model was developed to determine the performance of the system year-round using the utilizability-solar fraction method. The ORC powerplant uses R245fa as the working fluid and operates at an overall efficiency of 11.1%. The collector is a non-concentrating evacuated tube type and operates at a temperature of 90°C with an average annual efficiency of 23.9%. The system is capable of achieving annual solar fractions of 0.686 to 0.874 with collector array areas ranging from 30 000 to 40 000 m2 and storage tank sizes ranging from 3.8 to 10 × 106 L respectively. The lowest possible cost of producing electricity from the system is $0.393 CAD/kWh. The results of the study suggest that small-scale solar thermal plants are physically viable for year round operation in Ontario. The proposed system may be economically feasible given Ontario’s fixed purchase price of $0.42 CAD/kWh, but the cost of producing electricity from the system is highly dependent on the price of the solar collector.

Commentary by Dr. Valentin Fuster
2008;():475-482. doi:10.1115/ES2008-54261.

Numerous earlier studies have compared the merits of different working fluids for use in Rankine power systems. Most often, however, these have considered a limited number of specific fluids for which the needed thermodynamic properties are known. In the investigation summarized here, the Redlich-Kwong fluid model was used to develop a thermodynamic similarity framework that can be used as a comparative model for evaluating the performance of Rankine cycle working fluids. This can be viewed as a reduced order model that, based on thermodynamic similarity, quantifies the characteristics of the working fluids in terms of a single dimensional coordinate space defined by the choice of critical temperature. The advantage of this framework is that it allows exploration of the performance advantages of working fluids for which full thermodynamic properties are not yet available. Predictions of the model for common fluids were examined and conclusions regarding optimal fluids for solar Rankine systems are discussed.

Commentary by Dr. Valentin Fuster
2008;():483-488. doi:10.1115/ES2008-54307.

A model is presented to simulate the energy production from a solar photovoltaic (PV) array in Southern Nevada and its energy produced for hydrogen production at a hydrogen filling station. A solar PV array composed of four single axis tracking units provides power to a Proton Exchange Membrane (PEM) electrolyzer, which produces hydrogen that is stored on site for use in hydrogen converted vehicles. The model provides the ability to predict possible hydrogen production at the site, as well as the amount of hydrogen required to sustain a prescribed level of vehicle usage. Together, these results made it possible to determine the energy required to produce sufficient hydrogen to sustain the vehicles, and the percentage of that energy generated by the solar PV array. For an average year in Las Vegas and a travel requirement of 57 miles/day, this percentage was found to be 33 percent. This simulation has the potential to be modified for different locations, array size, amount of storage, or usage requirement.

Commentary by Dr. Valentin Fuster
2008;():489-498. doi:10.1115/ES2008-54339.

A cogeneration system consisting of a solar concentrator, a cavity-type receiver, a gas burner, a thermal storage reservoir, a hot water heat exchanger, and an absorption refrigerator is devised to simultaneously produce heat (hot water) and cooling (cold chamber). A simplified mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics, mass and heat transfer, is developed. The proposed model is then utilized to simulate numerically the system transient and steady state response under different operating and design conditions. A system global optimization for maximum performance in the search for minimum pull-down and pull-up times is performed with low computational time. Appropriate dimensionless groups are identified and the results presented in normalized charts for general application. The minimum pull-down and pull-up times, found with respect to the optimized operating parameters are sharp and, therefore important to be considered in actual design. As a result, the model is expected to be a useful tool for simulation, design, and optimization of solar energy systems of the type presented in this work.

Commentary by Dr. Valentin Fuster
2008;():499-504. doi:10.1115/ES2008-54354.

The concept of combining both solar thermal and electric systems is not new yet the limited use and further development needed has been noted by both the Department of Energy in the U.S. [1] and the EU Coordination Action PV-Catapult in Europe [2]. These reports and the university’s solar house entry in the Department of Energy’s 2005 Solar Decathlon provided the opportunity for research and development of a hybrid roof system that combined both photovoltaics with a wet solar thermal system. The main goal of this research was to design and develop a hybrid roof system based on previous research. Once designed then build a prototype model for the purpose of performance analysis with the final stage being the implementation in the university’s solar house entry into the 2005 solar decathlon. This paper discusses the hybrid roof design and performance analysis. The design and development was initialized by a group of students and advisors from both the University of Missouri-Rolla and Crowder College with the intent to use the hybrid system as part of the solar houses in the upcoming solar decathlons. Previous research studies on hybrid roof systems have shown increased performance however the differences in the systems studied vary in their setups and use of materials. In the case of this study a series of copper tubes were integrated into a metal seam roof with an amorphous silicon panel encased in low iron glass. This experiment encompassed almost 160 square feet of hybrid Solar Thermal Electric Panel (STEP) system panels and performance data acquired was used for input to computer simulation software to optimize the system for application. Based on experimental tests the STEP system yielded overall efficiency of 50%. This is compared to a separate thermal and electric system with an estimated 26% for the same roof area. The glazed versus unglazed analysis yielded a glazed panel reducing the PV collection by 23% yet increasing the thermal collection by approximately 200%. In conclusion this paper will discuss experimental performance analysis on the STEP system thermal and overall outcomes.

Topics: Solar energy , Testing
Commentary by Dr. Valentin Fuster

Concentrating Solar Components and Systems

2008;():505-513. doi:10.1115/ES2008-54048.

Concentrating Solar Power (CSP) dish systems use a parabolic dish to concentrate sunlight, providing heat for a thermodynamic cycle to generate shaft power and ultimately, electricity. Currently, leading contenders use a Stirling cycle engine with a heat absorber surface at about 800°C. The concentrated light passes through an aperture, which controls the thermal losses of the receiver system. Similar systems may use the concentrated light to heat a thermochemical process. The concentrator system, typically steel and glass, provides a source of fuel over the service life of the system, but this source of fuel manifests as a capital cost up front. Therefore, it is imperative that the cost of the reflector assembly is minimized. However, dish systems typically concentrate light to a peak of as much as 13,000 suns, with an average geometric concentration ratio of over 3000 suns. Several recent dish-Stirling systems have incorporated reflector facets with a normally-distributed surface slope error (local distributed waviness) of 0.8 mrad RMS (1-sigma error). As systems move toward commercialization, the cost of these highly accurate facets must be assessed. However, when considering lower-cost options, any decrease in the performance of the facets must be considered in the evaluation of such facets. In this paper, I investigate the impact of randomly-distributed slope errors on the performance, and therefore the value, of a typical dish-Stirling system. There are many potential sources of error in a concentrating system. When considering facet options, the surface waviness, characterized as a normally-distributed slope error, has the greatest impact on the aperture size and therefore the thermal losses. I develop an optical model and a thermal model for the performance of a baseline system. I then analyze the impact on system performance for a range of mirror quality, and evaluate the impact of such performance changes on the economic value of the system. This approach can be used to guide the evaluation of low-cost facets that differ in performance and cost. The methodology and results are applicable to other point- and line-focus thermal systems including dish-Brayton, dish-Thermochemical, tower systems, and troughs.

Commentary by Dr. Valentin Fuster
2008;():515-524. doi:10.1115/ES2008-54101.

CFD analysis has been conducted to obtain information on heat losses, velocity and temperature distribution of large molten salt Thermal Energy Storage (TES) systems. A two-tank 880 MWh storage system was modeled according to the molten salt TES containment design proposed for the 50 MWel commercial parabolic trough solar thermal power plants in Spain. Heat losses were established using the Finite Element Method (FEM), and used to determine the boundary conditions for the subsequent two- and three-dimensional Computational Fluid Mechanics (CFD) calculations. The investigations reveal that a high heat loss flux occurs at the lower edges of the salt storage tanks (between side wall and bottom plate). Thus the maximum temperature difference can be found at this location, resulting in the onset of local solidification within 3.25 days in the case of the empty cool tank. As a consequence, the detailed design of the lower edge has a large impact on both the overall heat losses and the period until the onset of local solidification.

Commentary by Dr. Valentin Fuster
2008;():525-534. doi:10.1115/ES2008-54258.

A method for correcting pyrometric temperature measurements of high temperature objects with unknown emissivity is presented. The method also estimates surface emissivity a the pyrometer wavelengths. It is particularly useful for correcting errors due to reflected light in solar heating applications. The method requires two or more narrow band pyrometers and can be applied to low-cost commercial instruments. The method analyzes the temperature measurements of multiple pyrometers operating at different wavelengths across a range of sample temperatures to solve for the surface emissivities that minimize the differences in temperature measurements. The temperature measurements are corrected using the new emissivities values. Simulated temperature data with both random noise and systematic errors is used to assess the robustness of the analysis method. When applied to temperature data from a solar furnace, it is shown to significantly decrease the difference in the temperature measurements of two single-color pyrometers. This method provides a potential low cost solution for pyrometric temperature measurement of solar-heated objects.

Commentary by Dr. Valentin Fuster
2008;():535-539. doi:10.1115/ES2008-54347.

The cost of solar tower power plants is dominated by the heliostat field making up roughly 50% of investment costs. Classical heliostat design is dominated by mirrors brought into position by steel structures and drives that guarantee high accuracies under wind loads and thermal stress situations. A large fraction of costs is caused by the stiffness requirements of the steel structure, typically resulting in ∼20 kg/m2 steel per mirror area. The typical cost figure of heliostats is currently in the area of 150 €/m2 caused by the increasing price of the necessary raw materials. An interesting option to reduce costs lies in a heliostat design where all moving parts are protected from wind loads. In this way, drives and mechanical layout may be kept less robust thereby reducing material input and costs. In order to keep the heliostat at an appropriate size, small mirrors (around 10 cm × 10 cm) have to be used which are placed in a box with transparent cover. Innovative drive systems are developed in order to obtain a cost-effective design. A 0.5 m × 0.5 m demonstration unit will be constructed. Tests of the unit are carried out with a high-precision artificial sun unit that imitates the sun’s path with an accuracy of less than 0.5 mrad and creates a beam of parallel light with divergence less than 4 mrad.

Topics: Solar energy , Mirrors
Commentary by Dr. Valentin Fuster

Advances in Solar Heating and Cooling Systems

2008;():541-550. doi:10.1115/ES2008-54053.

This paper deals with the development of a solar stove system that synthesizes the concepts of a reversible chemical reaction using CaO and water for heat generation, with that of the concentrated solar radiation for regeneration. A solar stove to take care of the needs of a standard family and which can be used at any time of the day has been designed, fabricated and used for experimental testing. A paraboloid solar concentrator has been conceived, designed and built to be used as a community facility in the neighborhood of the family for the regeneration of CaO from Ca(OH)2 . Different aspects encompassing heat transfer, reaction kinetics, water injection along with structural integrity and safety have been given due consideration as also the implementation technicalities with regard to capacity, cost, user friendliness, efficiency, and adaptation of locally available materials etc. These and the experimental test results of the heat generation part of the stove system are presented and discussed. This stove system has the innate potential to endear itself to the end user and upon completion of the testing of the regeneration part can turn out to be that long term solution, one is looking for.

Commentary by Dr. Valentin Fuster
2008;():551-559. doi:10.1115/ES2008-54097.

Prior studies of indirect water storage tanks that employ an immersed heat exchanger to discharge the stored energy have identified two potential methods of improving the rate of energy extraction: 1) an internal baffle to increase the velocity across the heat exchanger, and 2) a divided storage compartment to achieve thermal stratification. Thermal performance of these two options is compared to that of a conventional cylindrical tank during transient discharge. Each tank has a storage volume of 350 liters and a 10 m long, 0.3 m2 coiled tubular heat exchanger. For the specific configurations evaluated, the baffled heat exchanger provides the highest energy delivery rates and heat exchanger outlet temperatures. An analytic model shows the advantage of the divided storage depends on the NTU of the immersed heat exchanger. The heat exchanger employed in the present study is too small to realize the potential benefit of a divided storage. Both options, if used in the appropriate system, can improve thermal performance as measured by the rate and quality of delivered energy. The baffle is most appropriate when storage-side natural convection is the largest thermal resistance of the heat exchanger. The divided tank is useful when the NTU of the heat exchanger exceeds three.

Commentary by Dr. Valentin Fuster
2008;():561-568. doi:10.1115/ES2008-54102.

The efficient operation of a solar cooling system strongly depends on the chiller behaviour under part-load conditions since driving energy and cooling load are never constant. For this reason the performance of a single stage, hot water driven 30 kW H2 O/LiBr-absorption chiller employed in a solar cooling system with a field of 350 m2 evacuated tube collectors has been analysed under part-load conditions with both simulations and experiments. A simulation model has been developed for the whole absorption chiller (Type Yazaki WFC-10), where all internal mass and energy balances are solved. The connection to the external heat reservoirs of hot, chilled and cooling water is done by lumped and distributed UA-values for the main heat exchangers. In addition to an analytical evaporator model — which is described in detail — experimental correlations for UA-values have been used for condenser, generator and solution heat exchanger. For the absorber a basic model based on Nusselt theory has been employed. The evaporator model was developed taking into account the distribution of refrigerant on the tube bundle as well as the change in operation from a partially dry to an overflowing evaporator. A linear model is derived to calculate the wetted area. The influence of these effects on cooling capacity and COP is calculated for three different combinations of hot and cooling water temperature. The comparison to experimental data shows a good agreement in the various operational modes of the evaporator. The model is able to predict the transition from partially dry to an overflowing evaporator quite well. The present deviations in the domain with high refrigerant overflow can be attributed to the simple absorber model and the linear wetted area model. Nevertheless the results of this investigation can be used to improve control strategies for new and existing solar cooling systems.

Commentary by Dr. Valentin Fuster
2008;():569-576. doi:10.1115/ES2008-54127.

The deployment of solar driven air conditioning is a feasible target in all countries where high solar irradiation matches high cooling loads in buildings: the goal is to gradually replace compression chillers and reduce peak electricity demand during summer. Moreover, as solar thermal collectors are installed, solar cooling systems can be profitably employed during winter. In the present work a code has been implemented for the simulation and the design optimization of combined solar heating and cooling systems. The following system layout has been considered: in warm months the cooling demand is satisfied by means of an absorption chiller — driven by a solar collector field — and a reversible heat pump operating in series. A hot storage matches the variability of solar radiation, while a cold storage smoothes the non-stationarity of cooling demand. During winter, the reversible compression heat pump operates for space heating. Solar collectors are used as thermal source at the evaporator of the heat pump, increasing its coefficient of performance. The code, based on TRNSYS platform, is able to simulate the system throughout a year. Besides TRNSYS standard components a detailed model of the absorption chiller has been included, in order to accurately simulate its off-design operation. Using an optimization tool the size of each component is identified for a given space heating and cooling demand. The minimization of life cycle costs of the system has been chosen as the objective of the optimization. Results of a case study are presented and discussed for a solar heating and cooling plant in an office building. The optimization procedure has been carried out with simulations for a typical Northern Italy town (Alpine climate) and a typical Southern Italy town (Mediterranean climate).

Commentary by Dr. Valentin Fuster
2008;():577-582. doi:10.1115/ES2008-54141.

Presently, the performance of the air-conditioner or the air-conditioning system is usually valued based on some performance indices. However, it is deficient to evaluate the performance of the air-conditioning system which uses the new energy or renewable energy only based on the indices mentioned above. According to the defects and based on the works of the solar application, a new index defined PVI is put forward to evaluate the performance of the solar air-conditioning system rationally, which is significant to the new energy or renewable energy application.

Commentary by Dr. Valentin Fuster
2008;():583-590. doi:10.1115/ES2008-54200.

The center for building performance and diagnostic (CBPD) at Carnegie Mellon University has successfully designed, installed and tested a solar cooling and heating system to assess the feasibility of solar cooling for small scale commercial buildings or residential buildings with aspects of technology and energy efficiency. This solar cooling and heating system is primarily comprised of parabolic trough solar collectors, PTSC’s and a 16 kW dual energy source double effect (2E) absorption chiller. The 2E absorption chiller driven by PTSCs was tested to produce chilled water or hot water throughout a number of clear days in summer and winter. The analyses of the experimental data defined the system performance: the efficiency of the solar collector, the capacity and COP of the chiller, and the heat transfer coefficient of the heat recovery exchanger, by using a statistical approach, based on the energy balance equation. In the solar cooling tests during July 2007 in Pittsburgh, PA, the average efficiency of PTSCs was 35% when they were operated at about 155°C for driving the 2E absorption chiller and the chiller was able to provide 8 to 14 kW cooling with COP in the range 1.0 to 1.2; the overall system efficiency is in the range 0.35 to 0.41. In the near future, this solar absorption cooling and heating test system and its operation will be integrated with the cooling, heating and ventilation units for long term utilization.

Commentary by Dr. Valentin Fuster
2008;():591-597. doi:10.1115/ES2008-54246.

Thermal mixing and stratification are explored experimentally in a horizontal cylindrical tank, which simulates a storage of water heated by a solar collector. The tank is 70 cm long and 24 cm in diameter. The study is conducted in a transient mode, namely, the tank is filled with hot water, which in the course of operation is replaced by the tap water in a stratified way or by mixing. The flow rates of 2, 3, 5 and 7 liters per minute are explored. Temperature of hot water is usually about 55 °C, while the tap water is about 20 °C. In the experiments, both flow visualization and temperature measurements are used. The effects of port location and deflector installation are examined. The experimental results are presented in a dimensionless form, as the normalized outlet temperature vs. dimensionless time. Three-dimensional transient numerical simulations, done using the Fluent 6 software, provide an additional insight in the process of mixing inside the tank.

Commentary by Dr. Valentin Fuster
2008;():599-609. doi:10.1115/ES2008-54260.

Use of solar powered absorption refrigeration to augment data center afternoon cooling has three advantages: (1) it replaces non-renewable electrically powered cooling with cooling provided by renewable power, (2) it reduces operating costs by reducing consumption of costly peak load electrical power, and (3) use of a carbon free energy source reduces the carbon footprint of the data center. In the investigation summarized here, a computational model of a lithium bromide (LiBr) and water absorption system performance was used to explore the advantages of using nighttime cooling and cold storage to enhance the performance of solar powered absorption refrigeration for peak cooling in data centers. In this study, the model accounts for thermodynamic property effects on the absorption cycle performance and finite heat exchanger effectiveness. The model is used to explore the impact of parametric changes on system coefficient of performance (COP) and system payback. The results indicate that COP enhancements above 15% can be achieved with simple cold storage strategies. The results indicate that that when optimally designed, this type of system achieves energy efficiency, offering environmental and economic advantages that make it an attractive initial step in incorporating solar powered absorption cooling into green data center designs. Strategies for maximizing the positive contributions of cold storage suggested by the results are discussed.

Commentary by Dr. Valentin Fuster
2008;():611-620. doi:10.1115/ES2008-54285.

Both solar and heat pump heating systems are innovative technologies for sustaining ecological heat generation. They are gaining more and more importance due to the accelerating pace of climate change and the rising cost of limited fossil resources. Against this background, a heating system combining solar thermal collectors, heat pump, stratified thermal storage and water/ice latent heat storage has been investigated. The major advantages of the proposed solar/heat pump heating system are considered to be its flexible application (suitable for new and existing buildings because of acceptable space demand) as well as the improvement of solar fraction (extended solar collector utilisation time, enhanced collector efficiency), i.e. the reduction of electric energy demand for the heat pump. In order to investigate and optimise the heating system, a dynamic system simulation model was developed. On this basis, a fundamental control strategy was derived for the overall coordination of the heating system with particular regard to the performance of the two storage tanks. In a simulation study, a fundamental investigation of the heating system configuration was carried out and optimisation derived for the system control as well as the selection of components and their dimensioning. The influence of different parameters on the system performance was identified, where the collector area and the latent heat storage volume were found to be the predominant parameters for system dimensioning. For a modern one-family house, a solar collector area of 30m2 and a latent heat store volume of 12.5m3 are proposed. In this configuration, the heating system reaches a seasonal performance factor of 4.6, meaning that 78% of the building’s and users’ heat demand are delivered by solar energy. The results show that the solar/heat pump heating system can give an acceptable performance using up-to-date components in a state-of-the-art building.

Commentary by Dr. Valentin Fuster

Advances in Energy Storage

2008;():621-629. doi:10.1115/ES2008-54043.

The addition of latent heat storage systems in solar thermal applications has several benefits including volume reduction of storage tanks and maintaining the temperature range of the thermal storage. A Phase change material (PCM) provides high energy storage density at a constant temperature corresponding to its phase transition temperature. In this paper, a high temperature PCM (melting temperature 80°C) made of a composite of paraffin and graphite was tested to determine its thermal properties. Tests were conducted with a differential scanning calorimeter (DSC) and allowed the determination of the melting and solidification characteristics, latent heat, specific heat at melting and solidification, and thermal conductivity of the composite. The results of the study showed an increase in thermal conductivity by a factor of 4 when the mass fraction of the graphite in the composite was increased to 16.5%. The specific heat of the composite PCM (i.e., CPCM) decreased as the thermal conductivity increased, while the latent heat remained the same as the PCM component. In addition, the phase transition temperature was not influenced by the addition of expanded graphite. To explore the feasibility of the CPCM for practical applications, a numerical solution of the phase change transition of a small cylinder was derived. Finally, based on the properties obtained in DSC, a numerical simulation for a known volume of CPCM in a water tank was produced and indicated a reduction in solidification time by a factor of six.

Commentary by Dr. Valentin Fuster
2008;():631-637. doi:10.1115/ES2008-54174.

Thermal energy storage can enhance the utility of parabolic trough solar power plants by providing the ability to match electrical output to peak demand periods. An important component of thermal energy storage system optimization is selecting the working fluid used as the storage media and/or heat transfer fluid. Large quantities of the working fluid are required for power plants at the scale of 100-MW, so maximizing heat transfer fluid performance while minimizing material cost is important. This paper reports recent developments of multi-component molten salt formulations consisting of common alkali nitrate and alkaline earth nitrate salts that have advantageous properties for applications as heat transfer fluids in parabolic trough systems. A primary disadvantage of molten salt heat transfer fluids is relatively high freeze-onset temperature compared to organic heat transfer oil. Experimental results are reported for formulations of inorganic molten salt mixtures that display freeze-onset temperatures below 100°C. In addition to phase-change behavior, several properties of these molten salts that significantly affect their suitability as thermal energy storage fluids were evaluated, including chemical stability and viscosity. These alternative molten salts have demonstrated chemical stability in the presence of air up to approximately 500°C in laboratory testing and display chemical equilibrium behavior similar to Solar Salt. The capability to operate at temperatures up to 500°C may allow an increase in maximum temperature operating capability vs. organic fluids in existing trough systems and will enable increased power cycle efficiency. Experimental measurements of viscosity were performed from near the freeze-onset temperature to about 200°C. Viscosities can exceed 100 cP at the lowest temperature but are less than 10 cP in the primary temperature range at which the mixtures would be used in a thermal energy storage system. Quantitative cost figures of constituent salts and blends are not currently available, although, these molten salt mixtures are expected to be inexpensive compared to synthetic organic heat transfer fluids. Experiments are in progress to confirm that the corrosion behavior of readily available alloys is satisfactory for long-term use.

Commentary by Dr. Valentin Fuster

Water Desalination Systems

2008;():639-647. doi:10.1115/ES2008-54075.

Creating vacuum conditions above liquids increases their evaporation rates. This phenomenon can be integrated into a practical continuous desalination process by repeatedly flashing seawater in vacuumed chambers to produce water vapor that condenses afterwards producing fresh water. Gravity can be used to balance the hydrostatic pressure inside the flash chambers with the outdoor atmospheric pressure to maintain that vacuum, while low grade solar radiation can be used to add heat to seawater before flashing. The proposed desalination system consists of a saline water tank, a concentrated brine tank, and a fresh water tank placed on ground level plus an evaporator and a condenser located several meters above ground. The evaporator-condenser assembly, or flash chamber, is initially filled with saline water that later drops by gravity creating a vacuum above the water surface in the unit without a vacuum pump. The vacuum is maintained by the internal hydrostatic pressure balanced by the atmospheric pressure. The ground tanks are open to the atmosphere, while the flash chamber is insulated and sealed to retain both heat and vacuum.

Topics: Vacuum , Solar energy
Commentary by Dr. Valentin Fuster
2008;():649-654. doi:10.1115/ES2008-54162.

Seawater desalination is one of the most suitable areas for the utilization of solar thermal energy due to the coincidence, in many places of the world, of water scarcity, seawater availability and good levels of solar irradiation. The solar assisted heat pump provides a new horizon in the seawater desalination. Experiments were conducted on solar assisted heat pump desalination system under meteorological conditions of Singapore. This system uses two types of flat-plate solar collectors. One is called evaporator-collector which is entirely unglazed. The other type is single-glazed collector used for feed water heating. A single stage MED (multi-effect distillation) evaporator is used in this system and the refrigerant R134a is used in the heat pump. The system has a Performance Ratio (PR) of around 1.3 and water production capability of 0.6 to 0.9 kg/hr. The Coefficient of Performance (COP) of the heat pump reaches a maximum value of about 9 for the meteorological conditions of Singapore.

Commentary by Dr. Valentin Fuster
2008;():655-664. doi:10.1115/ES2008-54253.

At present scarcity of potable and drinking water is a pressing issue in certain parts of the Middle East region. Important advances have been made in desalination technologies but relatively high capital and running costs restrict their wide application even in cases when solar energy is used. Flat-plate solar collectors mainly have been employed in the past to distill water in compact desalination systems. Currently, it is possible to replace the above collectors by more advanced evacuated tube ones, which are available on the market at a similar price. This paper describes results of experimental and theoretical investigations of the operation of a solar still desalination system coupled with a heat pipe evacuated tube collector with the aperture area of about 1.7 m2 . A multi-stage solar still water desalination system was designed to recover latent heat from evaporation and condensation processes in four stages. The variation in the solar radiation (insolation) during a typical mid-summer day in the Middle East region was simulated using an array of 110 halogen flood lights covering the area of the solar collector. The synthetic brackish lab water solution was used for experiments and its total dissolved solids (TDS), electrical conductivity and pH were measured prior to and after the distillation process. The system’s operation was numerically simulated using a mathematical model based on the system of ordinary differential equations describing mass and energy conservation in each stage of the system. The experimental and theoretical values for the total daily distillate output were found to be in good agreement. The results of tests demonstrate that the system produces about 6.5 kg of clean water per day and have the distillation efficiency equal to 76%. However, the overall efficiency of the laboratory test rig at this stage of investigations was found to be low at the level of 26% and this is due to excessive heat losses in the system. The analysis of the distilled water shows that its quality is within the World Health Organization guidelines. Further research is being performed to improve the performance of the installation.

Commentary by Dr. Valentin Fuster

Solar and Wind Resources Assessment

2008;():665-677. doi:10.1115/ES2008-54128.

The Chilean government’s energy policy and the power generation sector plans include wind, geothermal, hydro and biomass powerplants in order to introduce renewable energy systems to the country, but they do not mention solar energy to be a part of the plan. This apparent lack of interest in solar energy is partly due to the absence of a valid solar energy database, adequate for energy system planning activities. The only available solar radiation database is relatively old, with measurements taken in 89 stations from the 60’s onwards, obtained with high-uncertainty sensors such as Campbell-Stokes devices and pyranographs. Moreover, not all stations have measured incoming solar radiation for an adequate time span. Here, we compare the existing database of solar radiation in Chile with estimations made with satellite measurements, obtained from the GOES program through collaboration with the Brazilian space institution, INPE. Monthly mean solar energy maps are created from both data sources and compared, using Krigging methods for spatial interpolation. It is found that a maximum 30 percent deviation exist, with snow covers in the Andes Mountains adding additional uncertainty levels. The solar energy levels throughout the country can be considered as high, and it is thought that they are adequate for energy planning given proper diffusion and support by editing a Chilean Solar Atlas.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
2008;():679-687. doi:10.1115/ES2008-54194.

The engineering of wind turbines is not fully mature. There are still phenomena, particularly dynamic stall that cannot be accurately modeled. Dynamic stall contributes to fatigue stress and premature failure in many turbine components. The three dimensionality of dynamic stall make these structures unique for wind turbines. Currently flow visualization of dynamic stall on a wind turbine rotor has not been achieved, but these visualizations can reveal a great deal about the structures that contribute to dynamic stall. Particle Image Velocimetry (PIV) is a powerful experimental technique that can take non-intrusive flow measurements of planar flow simultaneously. High-speed cameras enable time resolved PIV can reveal the transient development. This technique is suited to gain a better understanding of dynamic stall. A custom 3.27 m diameter wind turbine has been built to allow such measurements on the blade. The camera is mounted on the hub and will take measurements within the rotating domain. Mirrors are used so that laser illumination rotates with the blade. The wind turbine will operate in controlled conditions provided by a large wind tunnel. High-speed pressure data acquisition will be used in conjunction with PIV to get an understanding of the forces associated with the flow structures. Many experiments will be made possible by this apparatus. First the flow structures responsible for the forces can be identified. Quantitative measurements of the flow field will identify the development of the stall vortex. The quantified flow structures can be used to verify and improve models. The spatial resolution of PIV can map the three dimensional structure in great detail. The experimental apparatus is independent of the blade geometry; as such multiple blades can be used to identify the effect of blade geometry. Finally flow control research in the field of aviation can be applied to control dynamic stall. These experiments will be subject of much of the future work at the University of Waterloo. Potentially this work will unlock the secrets of dynamic stall and improve the integrity of wind turbines.

Commentary by Dr. Valentin Fuster
2008;():689-697. doi:10.1115/ES2008-54257.

In this paper the authors present a new set of solar radiation data taken at the latitude of 45.4°, in Padova, Italy. These measurements are performed within a new laboratory for the study of solar energy conversion systems. The global and the diffuse irradiance is measured on the horizontal plane and the global irradiance is also measured on sloped planes. The experimental uncertainty of the measurement of solar radiation is fully analyzed. In the design of a solar system, it is crucial to know the solar radiation on the inclined surface, but generally only data on the horizontal is available and solar radiation on the tilted plane is predicted using the information collected on the horizontal. There is need for assessment of prediction methods for estimating the solar radiation on inclined surfaces. In this paper, new data of global and diffuse radiation is compared to some most used correlations. Besides, the values calculated for the tilted plane are compared against those directly measured by a pyranometer installed on the sloped plane.

Commentary by Dr. Valentin Fuster
2008;():699-705. doi:10.1115/ES2008-54301.

Appropriate wind shear estimates are extremely important when assessing any regions’ wind power resource. Wind shear is used not only to estimate wind velocity at wind turbine hub heights other than the data collection height, but also as a siting tool to compare the wind resources in different locations when wind data are not available at a consistent height. Models for wind shear over land, as well as simple models for wind shear over open water have been found to correlate poorly with offshore wind data. This is thought to be partially due to the effect of changing wave conditions on wind shear as well as differences in thermal effects over bodies of water. In this study, offshore wind data from the South Atlantic Bight region is used to estimate the offshore wind shear conditions in this area. Data sets include collocated 10 m and 50 m meteorological data as well as wave data, all taken over a three and a half year time period. Offshore wind shear assessments from other studies are analyzed and compared to the current study as well.

Commentary by Dr. Valentin Fuster


2008;():707-716. doi:10.1115/ES2008-54010.

In the era of fossil fuel shortage and soaring oil prices under the condition of severe environmental problems we are facing now, an increasing need for sustainable development of new energy technology as a substitute of fossil fuel has become an issue of great concern throughout the world. Most of energy consumed in Korea, over 96%, is imported from foreign countries, especially Middle East. Korea is now ranked the 10th energy consumed country in the world. That is why we are interesting in hydrogen economy. As a result, hydrogen and fuel cell technology was selected as one of economic growth engines for next generation, and strongly supported by Korea government. Also, the government set Hydrogen Economy Policy in 2005. There are four R&D programs on hydrogen and fuel cell in Korea. Two of them are supported by MEST (Ministry of Education, Science and Technology) and others are funded by MKE (Ministry of Knowledge Economy). The hydrogen production technologies examined in Korea cover 3 main bases, fossil fuel, renewable energy including bio-hydrogen technology, and nuclear power. In October 2003, Korean government launched Hydrogen Energy R&D Center (HERC) as a member of the 21st Century Frontier R&D programs supported by the Ministry of Education, Science and Technology (MEST). The HERC has conducted research on the key technologies for the production, storage, and utilization of hydrogen energy for expediting realization of hydrogen economy based on renewable energy sources. The main purposes of this paper are to overview the current status of research programs conducted by Hydrogen Energy R&D Center based on the patent applications as well as research topics and to introduce specific achievements in each research program.

Commentary by Dr. Valentin Fuster
2008;():717-721. doi:10.1115/ES2008-54018.

In this study, the safflower seed (Carthamus tinctorius L.) was used as biomass sample for catalytic pyrolysis using commercial catalyst (Criterion-454) in the nitrogen atmosphere. Experimental studies were conducted in a well-swept resistively heated fixed bed reactor with a heating rate of 300°Cmin−1 , a final pyrolysis temperature of 550°C and particle size of 0.6–0.85 mm. In order to establish the effect of catalyst ratio on the pyrolysis yields, experiments were conducted at a range of catalyst ratios between 1, 3, 5, 7, 10, 20% (w/w). The bio-oils were characterized by elemental analysis and some spectroscopic and chromatographic techniques.

Topics: Catalysts , Pyrolysis
Commentary by Dr. Valentin Fuster
2008;():723-727. doi:10.1115/ES2008-54099.

Terpene hydrocarbons are high energy capacity hydrocarbons. The most known terpenoid biomass is Euphorbiaceae family. Euphorbia rigida, a member of Euphorbiaceae, was used as the biomass feedstock and natural zeolite was used as the catalyst in this study. In the experimental studies, firstly the raw material was analysed for its moisture, ash, volatile matter and fixed carbon. Then experiments were carried out in steam atmosphere in a fixed-bed reactor with a heating rate of 7 K/min, pyrolysis temperature of 823 K and mean particular size of 0.55 mm by mixing the catalyst to feedstock in different percentages. Experiments were performed with the catalyst ratios of 5, 10, 20 and 25 (weight-%) under steam atmosphere with the velocities of 12, 25 and 52 cm3 /min to determine the effect of catalyst and steam on the product yields and bio-oil composition. Steam velocities were considered as the average steam velocities in the inlet tube of the reactor. The maximum bio-oil was reached to a value of 39.7% when using catalyst ratio of 20% and steam flow rate of 25 cm3 /min. Pyrolysis oils were examined by using elemental analysis, IR and 1 H-NMR spectroscopy. The liquid products were also fractionated by column chromatography and the gas chromatographic analysis of n-pentane eluate was performed.

Topics: Biomass , Pyrolysis
Commentary by Dr. Valentin Fuster
2008;():729-737. doi:10.1115/ES2008-54230.

The National Park Service, the U.S. Fish and Wildlife Service, and the USDA Forest Service governmental agencies in southern Nevada have collaborated with the Center for Energy Research at the University of Nevada, Las Vegas to explore the feasibility of becoming energy neutral by 2010. The three federal agencies have set a goal to offset their combined annual energy demand (currently supplied by local utility companies) with an equal amount of power produced by renewable energy sources. The study results indicate that the three federal agencies above consume just over 3,000 megawatt-hours of electrical energy per year in and around the Las Vegas Valley. Upon researching various types of renewable energy, it was determined that wind, geothermal, and biomass technologies either failed to have sufficient resources available in southern Nevada or conflicted with the resource management philosophies of the federal agencies. Solar energy is the most abundant feasible source of renewable energy within the study area, and it was determined that a 1.5 megawatt fixed photovoltaic (PV) system is best suited for this project.

Commentary by Dr. Valentin Fuster
2008;():739-747. doi:10.1115/ES2008-54271.

For the past two years, Embry-Riddle has participated in the SAE Formula Hybrid competition. As part of the competition, a team of students analyze, design, and build a fully functional hybrid-electric race car. As an academic competition, the event is designed to allow a wide variety of system configurations and fuel choices. In order to optimize the vehicle characteristics, simulate vehicle performance, and build control laws, the design team created a Simulink model of the race car. As a recently created design competition, the SAE Formula Hybrid event offers an opportunity for both design innovation and system engineering. To develop a concept for the competition, the ERAU team developed detailed simulations of the vehicle in Simulink. Since the competition allows a variety of energy storage devices, engines, fuels, driveline configurations, and control systems, the development of a system dynamics model was not straight-forward. Further, system components for this project are constrained by some rules and practical constraints. The vehicle configuration was selected to be a parallel hybrid using a 250cc gasoline engine and 7.2kW DC motor with 1500F ultra-capacitor energy storage, with an unusual control strategy. The results of the Simulink model were used to predict how this vehicle configuration compares to other design choices including alternative fuels, energy storage devices and control strategies. The performance of the actual vehicle at the 2008 SAE Formula Hybrid competition, which occurs May 2008, will be presented at the conference.

Commentary by Dr. Valentin Fuster
2008;():749-757. doi:10.1115/ES2008-54279.

This work presents the comprehensive theoretical and numerical modeling efforts on the water transport phenomena in polymer electrolyte fuel cells and intends to elucidate an optimal water management strategy preventing condensed water formation in the anode and cathode channels. For a particular design of polymer electrolyte fuel cells, theoretical and numerical analyses on the inlet feed streams were conducted to set an optimal water transport strategy. The results showed that the favorable water transport scenario (half of produced water transported back to anode) requires less humidification at the gas channel inlets compared to the worst scenario (10% of the produced water diffused back to anode). It was also shown that for 80°C operation of fuel cells, the reactants should be super-saturated and the coolant temperature difference less than 5°C between the inlet and outlet has a significant effect on the humidification of reactant gases. For an optimal system, the inlet feed stream temperature and the temperature difference of the coolant should be carefully determined, considering the humidification capacity and the size of the radiator.

Commentary by Dr. Valentin Fuster
2008;():759-761. doi:10.1115/ES2008-54295.

In cold climates heating a home or building can consume a large amount of energy. The heating load on a building grows even larger when the cold is accompanied by wind. This paper proposes an environmentally friendly concept that offsets this load by directly and instantaneously using wind power. Current wind turbines that produce electricity have a high cost per energy unit. A large part of this cost comes from the generator, inverter, wiring, and their inefficiencies. This concept bypasses these components and creates low cost heat directly off of the turbine shaft. The heat is produced by rotating strong magnets inside a copper housing. Fields of the magnets induce a current in the copper housing which acts like a short circuit causing the energy to become heat. The heat that the housing produces can be transported by water lines or air ventilation to the building. With a promising low payback period this concept is very appealing to consumers interested in alternative energy systems. It can be added to any building design or any existing structure. The system is also maintenance free and in relation to other alternative energy systems its initial investment is very low. The turbine of choice for this concept is a Vertical Axis Wind Turbine. It performs well in industrial or residential areas even though these areas create turbulence in the wind. An experimental setup has been constructed for this concept and tested to find its performance characteristics.

Commentary by Dr. Valentin Fuster
2008;():763-773. doi:10.1115/ES2008-54325.

Integrated Gasification Combined Cycle (IGCC) is believed to be one of the most promising technologies to offer electricity and other de-carbon fuels with carbon capture requirement as well as to meet other emission regulations at a relatively low cost. As one of the most important parts, different gasification technologies can greatly influence the performance of the system. This paper develops a model to examine the feasibilities and advantages of using Ultra Superheated Steam (USS) gasification technology in IGCC power plant with carbon dioxide capture and storage (CCS). USS gasification technology converts coal into syngas by the endothermic steam reforming reaction, and the heat required for this reaction is provided by the sensible heat in the ultra superheated steam. A burner utilizes synthetic air (21% O2 and 79% H2 O) to burn fuel gas to produce the USS flame for the gasification process. The syngas generated from USS gasification has a higher hydrogen fraction (more than 50%) then other gasification processes. This high ratio of hydrogen is considered to be desired for a “capture-ready” IGCC plant. After gas cleanup and water gas shift reaction, the syngas goes to the Selexol process for carbon dioxide removal. Detailed calculations and analysis are performed to test the performance of USS gasification technology used in IGCC generation systems. Final results such as net output, efficiency penalty for CO2 capture part, and net thermal efficiency are calculated and compared when three different coal types are used. This paper uses published data of USS gasification from previous research at the University of North Dakota. The model also tries to treat the IGCC with carbon dioxide capture system as a whole thermal system, the superheated steam used in USS gasification can be provided by extracting steam from the lower pressure turbine in the Rankine Cycle. The model will make reasonable use of various waste energies and steams for both mechanical and chemical processes to improve the performance of the plant, and incorporate CO2 capture system into the design concept of the power plant.

Commentary by Dr. Valentin Fuster
2008;():775-778. doi:10.1115/ES2008-54362.

Where renewable energy sources, solar, hydro, wind are available the remote communities and businesses can be provided with the most reliable and affordable source of electrical energy. This paper presents a model of safari rest contains all the necessary services for the interested tourists who visit the safari Sinai desert. The PV energy system provides the rural energy needs of remote communities. A photovoltaic renewable energy system is designed to feed the global Ac and Dc electrical required load of this safari rest. The benefits of photovoltaic renewable energy at rural applications are its versatility and convenience. This model of safari rest must be taken in consideration by Egyptian Government as it will provide the tourism plane by new interested tourism field which put a big spot on Red sea area: El Ghordaka.

Topics: Seas
Commentary by Dr. Valentin Fuster

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