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Energy Systems Analysis, Thermodynamics and Sustainability

IMECE2011-62083 pp. 1-8; (8 pages)
doi:10.1115/IMECE2011-62083

Shrouds are important for damping vibrations in gas turbine blades. In modern industrial high-output, high-efficiency engines, long turbine blades can require the use of a mid-span or partial-span damping ring. However, the inclusion of a mid-span damping shroud, or “snubber,” can have negative effects on the aerodynamic performance of the gas turbine stage and engine. Therefore, a method of iterative study and optimization was applied to minimize the drag force caused by the snubber, while maximizing the structural life of the blade. The approach used integrated design environment software to perform parametric studies of the design space in preparation for optimization of the blade snubber geometry. The drivers employed in Isight 4.0/4.5 [9] optimization software carried out the parametric study and reported the results to the designer. Considering these results, the designer chose the initial seeding geometry of the optimization driver which greatly reduced analysis time and the time required to reach the design objectives. This approach provides an integrated design workflow and facilitates parametric studies of advanced gas turbine blade component geometry, and the optimization of the component to meet targets of minimized aerodynamic drag force and maximized low-cycle fatigue life, goals crucial to the development of an advanced and efficient power generation gas turbine.

IMECE2011-62194 pp. 9-20; (12 pages)
doi:10.1115/IMECE2011-62194

Liquid piston Stirling engines (sometimes termed “fluidynes”) have been studied extensively and applied in a variety of energy conversion applications. They are attractive for low capital costs and simplicity of construction. In addition, their operation as external combustion engines allows for flexibility in primary energy sources which is a distinct advantage when a low-cost or free source of heat can be paired with their minimal construction costs. Disadvantages of these devices include relatively low efficiency and low power density. A solar-powered fluidyne test bed was constructed and operated at the University of Colorado at Colorado Springs. This test bed was composed of a fluidyne engine which was constructed from copper pipe and plastic tubing along with temperature and pressure instrumentation. The system was designed to be powered by a Fresnel lens concentrating solar energy. The concentrated solar energy from the Fresnel lens provided ample power to operate the test bed, and tests were run in a wide variety of conditions. Indicated work of this unloaded engine was shown to agree well with a simple theoretical model of a Stirling cycle.

IMECE2011-62319 pp. 21-26; (6 pages)
doi:10.1115/IMECE2011-62319

Mass balance closure and exergetic efficiency is evaluated for a bench scale fast pyrolysis system. The USDA Agricultural Research Service (ARS) has developed this system for processing energy crops and agricultural residues for bio-oil (pyrolysis oil or pyrolysis liquids) production. Mass balance closure cannot be achieved due to the system size and complexity of inputs and outputs. A linear programming optimization model is developed to use the experimental data to achieve improved closure of elemental balances without losing the overall representation of the pyrolysis products. Having improved the mass balance, it is then possible to evaluate the exergy of the system. Exergy flows are computed using statistical relationships and other standard techniques. Computational details and results are discussed for switchgrass, a typical candidate biomass. Solutions for the minimum and maximum bio-oil outputs were generated. These particular results indicated that bio-oil accounted for approximately 10% of the loss mass. Considering all products as useful, the exergy destruction is approximately 20%. If the bio-oil alone is considered useful, the exergy destruction is about 40%. Further exercise of the model can be useful in evaluating mass losses and exergy for other feedstock and experimental runs.

IMECE2011-62470 pp. 27-32; (6 pages)
doi:10.1115/IMECE2011-62470

The utilization of vegetable oils, and in particular the palm oil, as fuel in the power generation has had a remarkable development in the last few years. Generally the vegetable oil can be used with a particular marine-derived diesel ICE, with low rpm and an electric conversion efficiency of about 40%. The efficiency is strictly connected to the size of the plant. Moreover, the considerable amount of the required vegetable oil to feed the system forces to import the fuel. This is one of the most critical elements as the palm oil is subject to continuous and wide variations in prices. Due to this variation it is difficult to obtain a stable and convenient fuel supply over a long period of time. The present work is aimed at evaluating and estimating the economic, technical and environmental feasibility of a 20 MW plant for the stationary power generation fed with palm oil, enlightening the system solution (technical constructive aspects) and the economic appraisal, on the basis of variations in oil prices. Finally, the economic sensibility analysis based on the fuel cost and the European mechanisms of biomass incentives.

IMECE2011-62538 pp. 33-38; (6 pages)
doi:10.1115/IMECE2011-62538

The use of air cooled condensers in power generation is increasing in many arid regions of the world. The classical A-frame condenser design is implemented in most new installations despite significant empirical evidence that such designs suffer from poor efficiencies and weather effects, and therefore provide significant scope for improvements. An inefficient condenser results in higher back pressure on the turbine, over-sized condensers and increased fan power. This paper addresses the flow distribution from an air cooled condenser for a ∼400MW gas and steam power plant. The results indicate that the flow patterns from the large scale fans results in a severe inhomogeneous distribution of cooling on the condenser fins. These region of high and low velocity are closely related to the outlet flow pattern from the fans, where in the hub region the air mass flow rate is reduced, while in the tip region it is increased. These measurements provide an excellent basis for both understanding the existing deficiencies of the A-frame designs and moreover provide direction for improved designs in the future.

IMECE2011-62648 pp. 39-46; (8 pages)
doi:10.1115/IMECE2011-62648

This paper presents the analytical and experimental analysis of a membrane based air-dehumidification system for handling the latent loads efficiently. This is important for tropical countries like Singapore where the humidity content of ambient air is high and therefore, air conditioning systems need to handle large latent load. A detailed COMSOL simulation model was set-up in order to simulate the water diffusion through the membrane. Experimental results from a real size membrane dehumidification unit are used to validate the mathematical model. Our investigations show that the moisture content of ambient air may be reduced by more than 5 g per kg of air if the dehumidification process is driven by the gradient between the water content of ambient air and the water content of exhaust air form air-conditioned spaces. With the exception of low electricity requirement for air transport, there is no electric energy consumption in the system. Therefore, the membrane system discussed in this paper is an efficient and alternative way of air dehumidification for air conditioning applications, potentially reducing the electricity consumption of air conditioning system in tropics.

IMECE2011-62688 pp. 47-56; (10 pages)
doi:10.1115/IMECE2011-62688

An exergoeconomic analysis identifies the location, magnitude and sources of thermodynamic inefficiencies and costs in an energy conversion system. This information is used for improving the thermodynamic and the economic performance and for comparing various systems. A conventional exergy-based analysis does not consider the interactions among the components of a system nor the real potential for improving the system. These shortcomings can be addressed and the quality of the conclusions obtained from an exergoeconomic evaluation is improved, when for each important system component the values of exergy destruction and costs are split into endogenous/exogenous and avoidable/unavoidable parts. We call the analyses resulting from such splittings advanced exergy-based analyses. The paper demonstrates how an advanced exergoeconomic analysis provides the user with information on the formation processes of thermodynamic inefficiencies and costs and with suggestions for their minimization. In the first part of the paper, the advanced exergy-based analyses are applied to an air refrigeration machine. In the second part of the paper, we demonstrate that the information obtained in the first part can be used to modify the values of the decision variables to reduce the cost of the final product (cold) of the overall system.

IMECE2011-62689 pp. 57-65; (9 pages)
doi:10.1115/IMECE2011-62689

In the first part of the paper, the advanced exergy-based analyses are applied to an air refrigeration machine. In this part, we demonstrate that the information obtained in the first part can be used to modify the values of the decision variables to reduce the cost of the final product (cold) of the overall system.

IMECE2011-62890 pp. 67-73; (7 pages)
doi:10.1115/IMECE2011-62890

A scalable and modular solar thermal dish-Brayton system is proposed in response to growing demand for renewable energy and distributed power generation. Existing dish systems require large areas to achieve sufficient conversion efficiency for the cost of the system. Also, the conversion efficiencies are limited by the materials and manufacturing processes. This paper proposes a low cost, high efficiency solar absorber as the core of a dish-Brayton system with the capability to achieve much higher operating temperatures than current absorbers. A simple cylindrical part, forming a black body cavity, is fabricated from silicon carbide for high absorptivity at a low fabrication cost. The manufacturing process consists of a simple casting and sintering procedure, which is a common way of creating ceramic parts. Another cylindrical shell is fabricated to cover the outer surface of the black body cavity, creating a channel for air to pass through. The high thermal conductivity of the silicon carbide ensures the efficient heat transfer between the solar absorber and the air. The entire solar energy absorber is designed to heat air up to 1500 K, which would improve energy conversion efficiency of concentrated solar power generation based on 1270 K by 20%. Analysis and test results on a scaled device are presented.

IMECE2011-63073 pp. 75-81; (7 pages)
doi:10.1115/IMECE2011-63073

Results of analyses performed using the UniSim process analyses software to evaluate the performance of both a direct and indirect supercritical CO2 Brayton power plant cycle with recompression at different reactor outlet temperatures are presented. The direct supercritical CO2 power plant cycle transferred heat directly from a 600 MWt reactor to the supercritical CO2 working fluid supplied to the turbine generator at approximately 20 MPa. The indirect supercritical CO2 cycle assumed a helium-cooled Very High Temperature Reactor (VHTR), operating at a primary system pressure of approximately 7.0 MPa, delivered heat through an intermediate heat exchanger to the secondary indirect supercritical CO2 recompression Brayton cycle, again operating at a pressure of about 20 MPa. For both the direct and indirect power plant cycles, sensitivity calculations were performed for reactor outlet temperature between 550°C and 850°C. The UniSim models used realistic component parameters and operating conditions to model the complete reactor and power conversion systems. CO2 properties were evaluated, and the operating ranges of the cycles were adjusted to take advantage of the rapidly changing properties of CO2 near the critical point. The results of the analyses showed that, for the direct supercritical CO2 power plant cycle, thermal efficiencies in the range of approximately 40 to 50% can be achieved over the reactor coolant outlet temperature range of 550°C to 850°C. For the indirect supercritical CO2 power plant cycle, thermal efficiencies were approximately 11–13% lower than those obtained for the direct cycle over the same reactor outlet temperature range.

IMECE2011-63084 pp. 83-90; (8 pages)
doi:10.1115/IMECE2011-63084

A Stirling cycle micro-refrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated thermodynamically. The cooling elements are each 5 mm-long, 2.25 mm-wide, have a thickness of 300 μm, and are fabricated in a stacked array on a silicon wafer. A 0.5 mm-long regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms. The diaphragms are 2.25 mm circles driven electrostatically. Helium is the working fluid, pressurized at 2 bar and sealed in the system. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally 90° out of phase such that heat is extracted to the expansion space and released from the compression space. The bulk silicon substrate on which the device is grown is etched with “zipping” shaped chambers under the diaphragms. The silicon enables efficient heat transfer between the gas and heat source/sink as well as reduces the dead volume of the system, thus enhancing the cooling capacity. In addition, the “zipping” shaped substrates reduce the voltage required to actuate the diaphragms. An array of vertical silicon pillars in the regenerator serves as a thermal capacitor transferring heat to and from the working gas during a cycle. In operation, the push-pull motion of the diaphragm makes a 300 μm stroke and actuates at a frequency of 2 kHz. Parametric study of the design shows the effects of phase lag, swept volume ratio between the hot space and cold space, and dead volume ratio on cooling capacity. At TH = 313.15 K and TC = 288.15 K and assuming a perfect regenerator, the thermodynamic optimization analysis gives a heat extraction rate of 0.22 W per element and cooling capacity of 30 W/cm2 for the stacked system. Evaluation of the stacked system shows that the COP will reach 6.3 if the expansion work from the cold side is recovered electrostatically and used to drive the hot side diaphragm.

IMECE2011-63280 pp. 91-101; (11 pages)
doi:10.1115/IMECE2011-63280

Onboard energy storage system (ESS) plays a major role for vehicle electrification. The performance of hybrid electric vehicle (HEV), plug-in HEV (PHEV), extended range electric vehicle (EREV), fuel cell vehicle (FCV), and electric vehicles (EV) heavily depends upon their ESS technology. The ESS must be able to store sufficient energy for adequate pure electric range, provide adequate peak power for needed vehicle performance under various driving cycles, absorb energy efficiently during regenerative breaking, and have long operation life and low costs. At present, pure battery based ESS often cannot effectively meet all these requirements due to many trade-offs. In order to improve the overall performance of ESS, integration of two (or more) energy sources have been studied to best utilize the unique characteristics of each, leading to a hybrid energy storage system (HESS). Hybridization of high-energy batteries and ultracapacitors with complementary characteristics present a common choice today. In this paper, the necessity and superiority of a HESS are illustrated considering system performance, efficiency, costs, functional life, and temperature requirements. Three major types of battery-ultracapacitor HESS, passive, semi-active and fully active, are presented. Various HESS control strategies proposed in the past are then reviewed, including rules or reference curves and tables based control, fuzzy logic control, and closed-loop control. Building upon these review and analyses, a novel control strategy based on signal separation using sparse coding is proposed at the end.

IMECE2011-63517 pp. 103-108; (6 pages)
doi:10.1115/IMECE2011-63517

In a free-piston expander-compressor (FPEC) the work of an expanding gas or vapor is used to compress another gas or vapor through a direct connection of one piston-cylinder assembly to another, without rotary motion. Each section of the FPEC operates like the piston-cylinder of a shaft-connected reciprocating machine. The expander section operates like that of a steam engine: high-pressure gas or vapor is freely admitted through part of the expansion process, but stopped at the “cutoff” part way through the expansion. The compressor section is like that of a shaft-driven reciprocating compressor: low-pressure gas is admitted during the intake stroke, compressed during the compression stroke, and then discharged when it has reached the downstream high pressure. The design of an FPEC is complicated by the fact that the expander and compressor have different relationships between force and position. The force on the expander piston decreases as the stroke progresses, while that on the compressor piston increases. The force difference must be made up by momentum change of the pistons and connecting rod, which accelerate in the early part of the stroke and decelerate in the latter part. The balance between the expanding fluid, the moving mass, and the compressed fluid can be described either dynamically (force and momentum) or thermodynamically (work and energy). It is shown that the mechanical design (piston areas, stroke, oscillating mass, and frequency) of an idealized FPEC is highly constrained by the thermodynamics of the high-pressure stream expansion and the low-pressure stream compression. For simple cases, dynamic models in differential equation form can be compared to thermodynamic analyses in algebraic form. The thermodynamic models provide a baseline design point for an ideal FPSE. The dynamic models can then be used to study non-ideal cases.

IMECE2011-63665 pp. 109-119; (11 pages)
doi:10.1115/IMECE2011-63665

This article examines how hybridization using solar thermal energy can increase the power output of a geothermal binary power plant that is operating on geothermal fluid conditions that fall short of design values in temperature and flow rate. The power cycle consists of a subcritical organic Rankine cycle using industrial grade isobutane as the working fluid. Each of the power plant units includes two expanders, a vaporizer, a preheater and air-cooled condensers. Aspen Plus was used to model the plant; the model was validated and adjusted by comparing its predictions to data collected during the first year of operation. The model was then run to determine the best strategy for distributing the available geothermal fluid between the two units to optimize the plant for the existing degraded geofluid conditions. Two solar-geothermal hybrid designs were evaluated to assess their ability to increase the power output and the annual energy production relative to the geothermal-only case.

IMECE2011-63703 pp. 121-130; (10 pages)
doi:10.1115/IMECE2011-63703

Improving performance of combined cycle power plants has been the target of numerous investigations. Most of the researchers have focused their attention on the heat recovery steam generator (HRSG), the connecting equipment between the gas turbine group and the steam section. On the other hand, almost all equipment in a combined cycle is a fairly standard design available from a manufacturer, while the HRSG is one of the few components that may be somewhat customized. In fact, the HRSG provides many different design options with respect to the layout of heat transfer sections and their operating parameters. The aim of this work is the development of a model for optimizing the main operating parameters of the heat recovery steam generator of a CCGT. The thermodynamic behaviour of the power plant has been simulated through the commercial software GateCycle, whereas the optimization has been carried out using a genetic algorithm. The objective function to be minimized is the cost of electricity, evaluated through a cash flow analysis in constant or in current dollars. Two CCGT power plant configurations, with one or three-pressure reheat HRSG, are simulated and optimized, evaluating the influence of fuel price variation on the optimal operating parameters of HRSG.

IMECE2011-63952 pp. 131-144; (14 pages)
doi:10.1115/IMECE2011-63952

A common approach for simulation of energy systems at design and off-design conditions is presented, which uses the same concepts and terminology independently of system dimension, complexity and detail. The paper shows that the higher the dimension of the system, the simpler is the model of each part of the system, but concepts and approach to built the model remain the same, being those commonly used in the literature. The approach consists in organizing energy systems models according to some criteria, which help enhance system models comprehension, and build them more easily. For any dimension and level of detail of the system these criteria consist in identifying the design specification from the environment surrounding the system, choosing the independent variables depending on the nature of the model, organizing them into categories, defining performance curves (characteristic maps) of each part of the system and organizing mass and energy balances into categories. Particular emphasis is given on modeling of system units behavior, which is generally described by the mathematical functions (characteristic maps) linking outflow to inflow variables. Examples of characteristic maps of the system units at each level of detail are shown, and models are then completed by mass, energy and momentum balances linking the behavior of all system units.

IMECE2011-63956 pp. 145-150; (6 pages)
doi:10.1115/IMECE2011-63956

One of the noted benefits of concentrating photovoltaics (PV) is the reduced cell area which results in reduction of the overall system cost. A variety of studies have looked at the cost for concentrating PV systems and made comparisons to concentrating solar thermal power plants, typically resulting in concentrating solar thermal power having lower system costs. Recently a widespread design space was assessed for the potential efficiency improvements possible with a coupled hybrid PV/thermal solar energy system for electricity generation. The analysis showed that modest efficiency improvements could be made but no assessment of the economic impact was made. Although modest efficiency gains can be made such a hybrid system requires more components than one of the conventional stand alone concentrating solar power plant on its own resulting in significantly different system costs. As a result we look to compare the overall system costs of three different solar power technologies: concentrating PV, concentrating solar thermal, and the concentrating hybrid approach. Additionally we will focus on documenting the necessary hybrid efficiencies to make a hybrid system competitive as well as the feasibility and means for achieving these efficiencies.

IMECE2011-64009 pp. 151-158; (8 pages)
doi:10.1115/IMECE2011-64009

Computational modeling was completed on a simplified downdraft gasifier being installed at the University of Iowa Oakdale Power Plant. The model was created in Gambit and simulated in ANSYS Fluent. The process modeled was non-premixed combustion on biomass fuel with a fixed-bed. The Fluent coal model was modified based on (off-site) proximate and ultimate analyses of the biomass. Varying packing densities, oxidizer inlet velocities and fuel types were simulated and the impact on the combustion zone was assessed. It was found that packing densities around 0.5 with oxidizer inlet velocities less than 5m/s were ideal for modeling wood gasification and produced a temperature distribution that was the most analogous to previous experimental measurements. The resulting reaction field was mainly a large rich fuel combustion (RFC) zone where gasification and pyrolysis could occur. The different fuels were found to have similar temperature and mean mixture fraction patterns, although the maximum temperatures attained were very different (1080K for seed corn versus 678K for wood), with the wood showing a greater area of RFC for gasification and pyrolysis. The temperature contour corresponded to the mixture fraction figure perfectly and well explained the stable asymmetric combustion in a downdraft gasifier.

IMECE2011-64106 pp. 159-167; (9 pages)
doi:10.1115/IMECE2011-64106

As part of the Global Threat Reduction Initiative (GTRI) Reactor Conversion program, the fuel assembly at the University of Missouri Research Reactor (MURR) is undergoing a significant redesign. The proposed fuel structure is based on low-enriched uranium foils. The proposed aluminum-clad LEU foil fuel plates for the MURR core are significantly thinner than the currently used fuel plates. Further, the monolithic structure of the proposed fuel is fundamentally different than the current design based on powder metallurgy. Consequently, coolant flow reduction due to flow induced deformation of the proposed fuel plates is of concern. The goal of the current analysis is to estimate the amount of flow induced deformation of the proposed LEU-based fuel plates when subjected to coolant flow imbalance due to fuel plate assembly tolerances. Previous methods for assessing fuel plate deflection have relied heavily on analytic and experimental techniques. With the continued advancement of computational codes, new options are now available to assess structural stability. The current approach is to explicitly couple a commercial CFD code with a commercial FEM code. This paper will describe the convergence and stability criteria that were developed to obtain an accurate deflection solution. Time step management and pressure ramping strategies were effectively used as relaxation parameters to improve the computational stability. Additionally, mesh quality criterion were developed and are enforced during a simulation. Benchmarking of the numeric results to analytic calculations is also presented.

IMECE2011-64133 pp. 169-178; (10 pages)
doi:10.1115/IMECE2011-64133

This experimental study reports the adhesion rate and adhesion density of Chlorella vulgaris on hydrophilic glass, and hydrophobic indium tin oxide (ITO) surfaces at constant shear rate. Cultivation of algae as biofilms offers an energy and water efficient method for algal biofuel production. In order to design algal biofilm cultivation systems, algal adhesion and biofilm formation on substrates with different surface properties must be known. To assess this, a parallel plate flow chamber was used to quantify the adhesion rate of the commonly used algae Chlorella vulgaris to the surfaces under controlled shear rates. The contact angle and zeta potential measurements were made both for the algal cells and the adhesion surfaces to model adhesion. The experimental results were compared with the predictions of the Derjaguin, Landau, Verwey, Overbeek (DLVO), extended DLVO (XDLVO) theories, and the thermodynamic model. The experiments showed that the rate of adhesion over the hydrophobic surface was 81 cells mm−2 min−1 which was 3 times larger than that of the hydrophilic surface for the first forty minutes of the adhesion experiments. Moreover, the final adhesion density over the hydrophobic surface was 6182 mm−2 after an experimental duration of 320 minutes which was 2.7 times that of the hydrophilic surface. Detachment studies done with increased shear rates showed that the adhesion strength of algae was also higher over the hydrophobic surface. The experimental results fit best with the results from the XDLVO theory. However, the model was inaccurate in predicting high detachment rate from the hydrophilic surface with increased shear rates. Results show the importance of surface material selection for the initial adhesion of cells. These results can be used for selection and design of surface materials for optimizing initial adhesion of algae cells in algal biofilm photobioreactors. Furthermore, the results can also be used for the design of planktonic photobioreactors to avoid biofouling.

IMECE2011-64470 pp. 179-186; (8 pages)
doi:10.1115/IMECE2011-64470

The use of combined heat and power (CHP) systems to produce both electric and thermal energies for medium-size buildings is on the increase, due to their high overall efficiency, high energy prices and political and social awareness. In this paper, an energy-economic study is presented. The main objective is to implement an analysis that will lead to the optimal design of a small cogeneration system, given the thermal power duration curve of a multi-family residential building. A methodology was developed to obtain this curve for a reference B-class building located in the North of Portugal. The CHP unit is based on a micro gas-turbine and includes an Internal Pre-Heater (IPH), typical of these types of small-scale units, and an external Water Heater (WH). A numerical optimization method was applied to solve the thermo-economic model. The mathematical model yields an objective function defined as the maximization of the annual worth of the cogeneration system. A purchase cost equation was used for each major plant component that takes into account size and performance variables. Seven decision variables were selected for the optimization algorithm, including performance of internal gas-turbine components and electrical and thermal powers. The results show that, the revenue from selling electricity to the grid and fuel costs have the greatest impact on the annual worth of the system. The optimal solution for the small CHP is sensitive to fuel price, electricity feed-in-tariff, capital cost and to the thermal load profile of the building. High European energy prices point towards future micro gas-turbines with better electrical efficiencies, achieved via a higher pressure-ratio compressor and turbine inlet temperature.

IMECE2011-64479 pp. 187-196; (10 pages)
doi:10.1115/IMECE2011-64479

Cerro Prieto Geothermal Power Plant has a capacity of 720 MW. The earliest 5 units are 23 years old, and unit 5 from Cerro Prieto Uno was restored in 2008. This paper presents a thermodynamic analysis on the effects that has the increase of non condensable gases content in geothermal steam. Results show that the cooling water temperature will rise due to the energy entering the system with the water flow of the new vacuum system that feeds the condenser. Normal operation would be limited and there exists a risk of not sustaining the condenser’s pressure. The new vacuum system, should extract from the condenser a flow 4 times larger, requiring 27% more steam at a higher pressure, as well as 4.5 times the quantity of cooling water. At this condition, the water returning to the condenser is 4.3 times larger than the original at a higher temperature, increasing in 218% the associated energy. A thermal behavior model was obtained for the cooling tower. In the most likely scenario the cooling tower exit temperature will be higher than the required, and to maintain the equilibrium it will be necessary to lower the condenser thermal load by reducing the steam flow to the turbine and accordingly, the power delivered.

IMECE2011-64484 pp. 197-202; (6 pages)
doi:10.1115/IMECE2011-64484

Algae have created a lot of interest in recent years due to their high potential for yielding oil that might be used for biodiesel production while capturing CO2 and helping clean the environment. However, to reach high productions, more efficient photobioreactors are required. Different kinds of photobioreactors configurations have been proposed but only a few of them show high productivity levels. This article presents a comparison among various promising photobioreactors by evaluating the light regime, mass transfer and scale up to achieve an efficient operation of mass algae cultures. Finally, a promising configuration for reaching high productivity is determined.

IMECE2011-64550 pp. 203-209; (7 pages)
doi:10.1115/IMECE2011-64550

This paper reports the cellular photosynthetic rates of the green algae Chlamydomonas reinhardtii wild strain and its truncated chlorophyll antenna transformant, tla1 , as a function of local irradiance. It is hypothesized that reduction in the pigmentation of algae cells can enhance light peneration in mass cultures and increase productivity. Thus, an experimental setup was designed to expose each cell within planktonic algae cultures to a nearly uniform irradiance. An oxygen microsensor was used to monitor the photosynthetic rate as the irradiance onto the sample was varied. The results showed that the cellular photosynthetic rate of the wild strain, CC125, was greater than that of tla1 at all irradiances, by a factor that ranged from 1.7 to 4. Photoinhibition was observed in both strains, although the effect was more pronounced in CC125. Although less pigmented cells enable deeper light penetration in photobioreactors, their reduced phosotynthetic rate can negate this benefit.

Topics: Microsensors , Oxygen
IMECE2011-64649 pp. 211-217; (7 pages)
doi:10.1115/IMECE2011-64649

Piezoelectric materials have many applications including sensors, actuators, and motors. However, the ability of piezoelectric films to generate electricity from wind power has only recently been advanced. Piezoelectric films harvest wind energy by means of a layer of polyvinylidene fluoride (PVDF) that upon deformation generates an internal electrical voltage across two silver-ink electrodes. These films are allowed to freely flap in an airflow to cause deformation and thus electricity generation. A single small film (4.14 cm (1.63 in) × 1.63 cm (0.64 in) × 0.15 cm (0.06 in)) is tested at various wind speeds. The output of the film may be enhanced with the addition of a bluff body (in this case, a cylinder) upstream from the film. The vortex shedding from the cylinder produces a wake that can enhance the vibrations of the piezoelectric device, which in turn optimize voltage and/or power output. Voltage and power output is recorded across varying load resistances. The method of storing useful energy from the piezoelectric films is also of particular interest. Preliminary experiments using a LTC3588-1 energy harvester to various configurations of supercapacitors and Li-ion batteries are conducted. The LTC3588-1 is comprised of an efficient rectifier with a buck converter to allow the chip to efficiently charge the super capacitor while only requiring a small input to begin charging.

IMECE2011-65000 pp. 219-225; (7 pages)
doi:10.1115/IMECE2011-65000

The CO2 transcritical Rankine power cycle has been widely investigated recently, because of its better temperature glide matching between sensible heat source and working fluid in vapor generator, and its desirable qualities, such as moderate critical point, little environment impact and low cost. A reheat CO2 transcritical power cycle with two stage expansion is presented to improve baseline cycle performance in this paper. Energy and exergy analysis are carried out to investigate parametric effects on cycle performance. The main results show that reheat cycle performance is sensitive to the medium pressures and the optimum pressures exist for maximizing net work output and thermal efficiency, respectively. Reheat cycle is compared to baseline cycle under the same conditions. More significant improvements by reheat are obtained at lower turbine inlet temperatures and/or larger high cycle pressure. Work output improvement is much higher than thermal efficiency improvement, because extra waste heat is required to reheat CO2 . Based on second law analysis, exergy efficiency of reheat cycle is also higher than that of baseline cycle, because more useful work is converted from waste heat. Reheat with two stage expansion has great potential to improve thermal efficiency and especially net work output of a CO2 transcritical power cycle using a low-grade heat source.

IMECE2011-65326 pp. 227-235; (9 pages)
doi:10.1115/IMECE2011-65326

In this paper, mixtures of CO2 and SF6 were evaluated as working fluids for geothermal plants based on property measurements, molecular dynamics modeling, thermodynamic cycle analysis, and materials compatibility assessment. The CO2 - SF6 was evaluated for a reservoir temperature of 160°C. Increasing the efficiency for these low reservoir sources will increase the options available for geothermal energy utilization in more sites across the country. The properties for the mixtures were obtained either from thermodynamic property measurements and molecular dynamics simulations. Optimum compositions of the CO2 - SF6 were identified for a well reservoir temperature and a given water-cooling condition. Concerning the global warming potential, it was estimated that the equivalent CO2 emissions per 1kWh for a Rankine cycle operating with 100% SF6 would be approximately of 7.6% than those for a coal-fired power plant.

IMECE2011-65393 pp. 237-245; (9 pages)
doi:10.1115/IMECE2011-65393

Phase-change materials (PCM) are particularly promising for thermal storage in various energy plants as solar plants, district heating, heat pumps, etc. mainly because of the possibility to reduce the volume of storage tanks, but also because the problems related with thermal stratification are considerably reduced. On the other hand, research is necessary in order to address technical problems, mainly related to the heat transfer in the medium, which needs to be enhanced in order to achieve reasonable charging and discharging processes. The present paper describes the application of computational fluid-dynamics (CFD) for the analysis of PCM thermal storage systems. The numerical analysis is directed at understanding the role of buoyancy-driven convection during constrained solidification and melting inside a shell-and-tube geometry. The 2D model is based on a finite-volume numerical procedure that adopts the enthalpy method to take in account the phase change phenomenon. The time-dependent simulations show the melting phase front and melting fraction of the PCM and incorporate the fluid flow in the liquid phase. The obtained temperature profiles are compared to a set of experimental data available in the literature. The results show that during the melting process natural convection within the PCM has non negligible effects on the behavior of the system. The numerical simulations of the solidification process show that the increasing solid fraction of the PCM inhibits the buoyancy in the remaining liquid portion of the phase-change-material. Furthermore, the paper discusses the effects on the phase-change processes of the main operating conditions, including inlet temperature and mass flow rate of the heat transfer fluid.

IMECE2011-65396 pp. 247-254; (8 pages)
doi:10.1115/IMECE2011-65396

In this paper, the combined application of two different approaches to the diagnosis of energy systems is proposed. The first approach is based on thermoeconomic analysis. It consists on the filtration of effects due the dependence of the efficiencies of components on their operating condition. This is obtained through productive models which relate resources and products. With respect to physical models, these are generally less accurate but more compact and thus more suitable to deal with the available measurements in real plants. The second approach is an artificial intelligence (AI) technique. This is based on the calculation of appropriate indicators that experience shows to be affected by possible anomalies. Malfunctions are detected and recognized through the analysis of deviations registered by the indicators during plant operation. The diagnosis methods are applied to a gas turbine plant with real anomalies. These are investigated in order to highlight possible advantages and disadvantages of the two methods and the benefits that can be reached through their combined application.

IMECE2011-65711 pp. 255-261; (7 pages)
doi:10.1115/IMECE2011-65711

Energy microgrids are a key building block of smart grids. Energy microgrids can not only provide voltage and VAR support to the power grid but also reduce the emission footprint of the overall power generation infrastructure. While it provides added advantages like grid decongestion and reduced operating cost for system operators, it creates significant challenges in stable operation and meeting economic goals of the microgrid owners. Currently, energy microgrids are heavily subsidized through government grants/rebates and require high maintenance in terms of skilled operating staff and advance control systems. In this paper, we propose a microgrid energy storage architecture that could reduce the cost of ownership and simplify control and management of energy microgrids while retaining the advantages of reduced emissions and resource consumption. The controls existing in normal energy storage also offers unique opportunities in simplifying the control system of such distributed generation infrastructure and improving the reliability of microgrid in meeting local demand constraints. From a utility operator’s perspective, energy storage provides a reliable and dispatchable source as opposed to intermittent distributed energy resources.

IMECE2011-62678 pp. 263-270; (8 pages)
doi:10.1115/IMECE2011-62678

Exergy-based analyses are useful means for the evaluation and improvement of energy conversion systems. A life cycle assessment (LCA) is coupled with an exergetic analysis in an exergo-environmental analysis. An advanced exergo-environmental analysis quantifies the environmental impacts estimated in the LCA into avoidable/unavoidable parts and into endogenous/exogenous parts, depending on their source. This analysis reveals the potential for improvement of plant components/processes and the component interactions within a system. In this paper, the environmental performance of an advanced zero emission plant (AZEP) with CO2 capture is evaluated based on an advanced exergoenvironmental analysis. The plant uses oxy-fuel technology and incorporates an oxygen-separating mixed conducting membrane (MCM). To evaluate the operation of the system, a similar plant (reference plant) without CO2 capture is used. It has been found that the improvement potential of the AZEP concept is restricted by the relatively low avoidable environmental impact of exergy destruction of several plant components. Moreover, the endogenous environmental impacts are for the majority of the components significant, while the exogenous values are, generally, kept at low levels. Nevertheless, a closer inspection reveals that there are strong interactions among the components of the MCM reactor and the components constituting the CO2 compression unit. Such results are valuable, when the improvement of the environmental performance of the plant is targeted and they can only be obtained through advanced exergy-based methods.

IMECE2011-62785 pp. 271-285; (15 pages)
doi:10.1115/IMECE2011-62785

Cases of death during heat waves are most commonly due to respiratory and cardiovascular diseases, with the main contribution from the negative effect of heat on the cardiovascular system. In an attempt to control the body temperature, the body’s natural instinct is to circulate large quantities of blood to the skin. However while trying to protect itself from overheating, the body actually harms itself by inducing extra strain on the heart. This excess strain has the potential to trigger a cardiac event in those with chronic health problems, such as the elderly. Those in the U.S.A. between the ages of 65 and 74 are at a higher risk of mortality during heat waves when they are single, have a history of chronic pulmonary disease, or suffer from a psychiatric disorder. In the older group, 75+, single people are again more vulnerable as well as women. The relationship of mortality and temperature creates a J-shaped function, showing a steeper slope at higher temperatures. Records show that more casualties have resulted from heat waves than hurricanes, floods, and tornadoes together. The significance of this is that the U.S. suffers the highest damage total from natural catastrophes annually. Studies held from 1989–2000 in 50 U.S. cities recorded 1.6% more deaths during cold temperature events, as opposed to a staggering 5.7% increase during heat waves. People are at risk when living in large metropolitan areas, especially those mentioned above, due to the heat island effect. Urban areas suffer heat increases from the combination of global warming effects as well as localized heat island properties. It is flawed to claim that the contribution of anthropogenic heat generation to the heat island effect is small. Analyzing the trend of extreme heat events (EHEs) between 1956 and 2005 showed an increase on average of 0.20 days/year, on a 95% confidence interval with uncertainty of ±0.6. This trend follows the recorded data for 2005 with 10 more heat events per city than in 1956. Compact cities experience an average of 5.6 days of extreme heat conditions annually, compared to that of 14.8 for sprawling cities. The regional climate, city populace, or pace of population growth however does not affect this effect. Statistics from the U.S. Census state that the U.S. population without air conditioning saw a drop of 32% from 1978 to 2005, resting at 15%. Despite the increase in air conditioning use, the positive affects of it may have run their course as a critical point may have been reached. A study done by Kalkstein through 2007 proved that the shielding effects of air conditioning reached their terminal effect in the mid-1990s. Heat-related illnesses and mortality rates have slightly decreased since 1980, regardless of the increase in temperatures. This may be in part to the increase in availability of air conditioning, and other protective measures, to the public. Protective factors have mitigated the danger of heat on those vulnerable to it, however projecting forward the heat increment related to sprawl may exceed physiologic adaptation thresholds.

Topics: Heat
IMECE2011-62950 pp. 287-292; (6 pages)
doi:10.1115/IMECE2011-62950

The University of Iowa Power Plant operates utility generation and distribution for campus facilities, including electricity, steam, and chilled water. It is desirable to evaluate the optimal load combination of boilers, engines and chillers to meet the demand at minimal cost, particularly for future demand scenarios. An algorithm is under development which will take into account the performance of individual units as part of the mix which ultimately supplies the campus and determine the degree that each should be operating to most efficiently meet demand. The algorithm will be part of an integrated simulation tool which is specifically designed to apply traditional optimization techniques for a given (both current and possible) circumstance. The second component is to couple the algorithm with accurate estimates and historical data through which expected demand could be predicted. The process will be able to account for theoretical circumstances which will be highly beneficial for strategic planning. It is also necessary to determine the unique operating characteristics of the system components. The algorithm will rely upon performance curves of individual system components (boiler, chiller, etc) and those must be developed and refined when possible using the most accurate information from experimental testing and commissioning or manufacturer supplied data. The heuristics that will be examined include combinatorial approach, a gradient-based incremental method, and simulated annealing.

IMECE2011-63408 pp. 293-303; (11 pages)
doi:10.1115/IMECE2011-63408

The paper presents the results of a two years research on the field of energy management systems. These systems range from the largest ones found in industrial plants down to the smallest utilized by the residential consumers. The goal is to define an energy management system specific to supermarkets sector including hypermarkets operating in large scale retail. The need to ensure continuity and quality of energy services, the high energy consumptions, the complexity of operation and maintenance facilities and, in general, the lack of strategies in energy end uses are just some of the reasons that have oriented the research in this sector. On the other hand, the groups operating in the large-scale retail, in order to contain costs, have always walked the path of maximum standardization of plants characteristics or, in most cases, the implementation of economy of scale in purchasing and maintenance; strategies not sufficient to ensure real savings considering the rising costs of energy. The starting points of the research have been some energy efficiency actions applicable to this type of consumers: buildings and facilities design, optimization of energy purchasing, management of maintenance, monitoring and collecting energy data, promotion of best practices in end uses, energy benchmarking, etc. For the purpose an energy audit was developed in the biggest supermarket and hypermarket chain in Italy during the last two years; the results led to the implementation of an operative protocol that makes possible to achieve energy savings in excess of 20%. The large number of supermarkets analyzed, the rigorous approach to the measurement and the monitoring of energy data, the possibility to verify the results in a ongoing way and the use of simulation models and software, permit a feasible extension to similar contexts.

IMECE2011-63434 pp. 305-311; (7 pages)
doi:10.1115/IMECE2011-63434

Hospitals are complex building systems and they uses a large amount of energy in many different ways. Generally energy efficiency is not one of the main priorities in such structures, where the management mainly focuses on the continuity and the reliability of the energy supply. As a consequence this sector shows a high potential for energy savings. Furthermore the energy efficiency measures enforce the energy systems reliability. The first step in implementation of energy efficiency objectives and measures is the audit phase where the initial energy performance is assessed. Generally a complete audit needs the collection of many parameters and data (indicators) and that makes this procedure very long and expensive. Sometimes it is impossible to completely estimate the energy performance because of a lack of information. This paper proposes an algorithm able to operate a substantial reduction of all the indicators to be collected for the evaluation of the performance of energy end users. The model makes possible to estimate all the energy performance indicators (dependent) as a function of a small number of indicators (independent) whose estimation is simpler. It has been applied to the hospitals sector, but can be applied also to domestic energy sector and to a large number of the tertiary and commercial end users. The model operates the implementation of the mutual relationships among all the indicators involved in the energy performance evaluation. The indicator values are defined by applying mainly a statistical analysis of an adequate number of energy efficiency case studies. Furthermore design standards are applied in order to improve the indicators definitions. The model has been tested and validated in some case studies discussed.

IMECE2011-63830 pp. 313-321; (9 pages)
doi:10.1115/IMECE2011-63830

In this research an optimization model was used to determine the sensitivity of the revenue, net cash flow (defined as revenue less amortized capital costs, fixed and variable operating costs, and return on investment), and operational characteristics of a compressed air energy storage (CAES) facility to certain technical factors in the Electric Reliability Council of Texas (ERCOT) zonal market. The technical factors considered were compressor capacity and storage capacity relative to turbine capacity, non-spinning reserve market participation, minimum allowable runtime of the compressor and turbine systems, and costs associated with startup of the compressor and turbine systems. Additionally, the work showed that the nine-year optimization problem could be decomposed into nine single-year optimization problems with decreased computation time and minimal divergence from the nine-year solution. Previous work had determined the optimal compressor and storage capacities for a given expander capacity; the current work expanded on the previous work to show that the economics of CAES are reasonably insensitive (defined as within 5% of the maximum net cash flow) to compressor capacity within a range of 0.45 to 0.8 MW per MW of turbine capacity in the West zone of ERCOT and 0.25 to 0.5 MW per MW of turbine capacity in the non-West zones in ERCOT. Similarly, the economics of CAES are reasonably insensitive to storage capacity within a range of 20 to 60 MWh per MW of turbine capacity in the West zone of ERCOT and 12 to 35 MWh per MW of turbine capacity in the non-West zones. Previous work had determined that participation of the turbine-generator system in the non-spinning reserve market increased the revenues and net cash flow and reduced the amount of electricity bought and sold in the balancing energy market. This work confirmed the previous finding and also determined that the participation of the motor-compressor as dispatchable load in the non-spinning reserve market increased the revenues and net cash flow and increased the amount of electricity bought and sold. The increase in electricity sales due to the motor-compressor participation in the non-spinning reserve market only partially offset the decrease in the amount of electricity sold due to the turbine-generator participation. The net effect of both systems participating in the non-spinning reserve market was an increase in revenue of 29% to 37% and net cash flow of 130% to 250% and a decrease in the amount of electricity bought and sold by about 10%. This work also found that a CAES facility is sensitive to minimum runtime constraints and startup costs. Minimum runtime constraints reduce the net cash flow by 11% to 13% and increase the amount of electricity bought and sold by 1% to 3%, for a minimum runtime of 4 hours. The effect of startup costs is to reduce both the net cash flow by 5% to 6% and the amount of electricity bought and sold by 4% to 5% for startup costs of $2/MW-start.

Topics: Energy storage
IMECE2011-64131 pp. 323-334; (12 pages)
doi:10.1115/IMECE2011-64131

Recycling plastics is widely accepted as the most beneficial end use of plastic products. Consequently, many cities are turning towards single-stream recycling to make it easier for consumers to recycle and to increase the total amount of municipal solid waste (in particular, energy-dense plastic waste) that is diverted to recycling facilities. However, single-stream recycling Materials Recovery Facilities (MRFs) are now faced with sorting more diverse material flows with increased contamination from the mixing of recyclable and non-recyclable materials, leading to roughly 5–10% of the incoming material being sent to landfills. Converting the energy dense MRF waste material into solid recovery fuel (SRF) pellets creates an additional use for the products, diverts the material from the landfill, and displaces some fossil fuel use. However, there are some non-obvious energetic and environmental tradeoffs that require analysis to quantify. That is the intent of the research presented here. To analyze the potential of SRFs as viable alternative fuel sources, a first-order thermodynamic materials and energy balance was constructed using cement kilns as a test-bed. The proposed methodology allows for a range of traditional fuels to be compared with and without supplemental SRF. The SRF case can be benchmarked against the reference case, or conventional plastic end-of-life pathway, landfilling of the non-recycled plastic. The comparison includes transportation and processing steps required for each pathway, including any additional sorting needed for creating the SRF as well as the pelletization process itself. A robust methodology was created that allows for the MRF residue to be adjusted on a compositional basis because residue composition varies by season and location, which affects the analysis. Additionally, proximity to SRF conversion facilities and cement kilns will vary for each MRF and can impact the analysis so the methodology allows these factors to be adjusted. A test case was studied to compare the landfilling or combustion of MRF residue in a cement kiln at a rate of 0.9 metric tons per hour (7884 metric tons for a one year period). The analysis details the total energy consumed, landfill avoidance, amount of fuel displaced, and the total equivalent CO2 emissions of each scenario. The methodology successfully models the reference and SRF case and is robust enough to be used with a wide variety of potential SRF scenarios. A few parametric studies were performed on the transportation and landfill variables to determine their relative effect on results. It was found that additional transportation would have minimal effect of total energy consumption. When using SRF as a supplementary cement kiln fuel, the equivalent CO2 reductions are higher in scenarios with low methane capture efficiency at the landfill. Overall, it was found that using SRF as a supplementary fuel at cement kilns reduces the total fossil energy consumption and total equivalent CO2 reductions by 6% and 76%, respectively.

IMECE2011-64199 pp. 335-342; (8 pages)
doi:10.1115/IMECE2011-64199

NASA has been evaluating closed-loop atmosphere revitalization architectures that include carbon dioxide (CO2 ) reduction technologies. The CO2 and steam (H2 O) co-electrolysis process is one of the reduction options that NASA has investigated. Utilizing recent advances in the fuel cell technology sector, the Idaho National Laboratory, INL, has developed a CO2 and H2 O co-electrolysis process to produce oxygen and syngas (carbon monoxide (CO) and hydrogen (H2 ) mixture) for terrestrial (energy production) application. The technology is a combined process that involves steam electrolysis, CO2 electrolysis, and the reverse water gas shift (RWGS) reaction. Two process models were developed to evaluate novel approaches for energy storage and resource recovery in a life support system. In the first model, products from the INL co-electrolysis process are combined to produce methanol fuel. In the second co-electrolysis, products are separated with a pressure swing adsorption (PSA) process. In both models the fuels are burned with added oxygen to produce H2 O and CO2 , the original reactants. For both processes, the overall power increases as the syngas ratio, H2 /CO, increases because more water is needed to produce more hydrogen at a set CO2 incoming flow rate. The power for the methanol cases is less than pressure swing adsorption, PSA, because heat is available from the methanol reactor to preheat the water and carbon dioxide entering the co-electrolysis process.

IMECE2011-64488 pp. 343-351; (9 pages)
doi:10.1115/IMECE2011-64488

The use of Prismatic Skylights and its effects as a passive Energy Conservation Strategy in “Residential” and “Big Box Commercial Buildings” in hot and humid climate has been evaluated throughout this project. The potential benefits of using skylights reside in the fact that it reduces electrical lighting necessities but at the same time it contributes to an upsurge of the Cooling Loads of the conditioned space. Acknowledging the impact of skylights is fundamental to elaborate an optimized design of a building’s energy efficient mechanical system. To reach a sound conclusion, the evaluated buildings were modeled and their performance was simulated using the Department of Energy Simulation Program “Energy Plus”. To be able to compare the Energy Conservation Measure case (Using Skylights) with the Base Line (No Skylights), a photometric sensor was modeled to ensure that both cases sourced the same amount of light visible in the electromagnetic spectrum. Considering the Heating, Cooling and lighting energy consumption as variables, the variance between the ECM and the Base line for the residential case was 5% more energy consumption with skylights. For the Big Box Commercial Building there was a 5% deduction in energy consumption in the ECM case using 5% roof area covered with skylights. The results obtained from this investigation reveal a very promising effect in the implementation of skylights in “Big Box Commercial Buildings”, but not so optimistic in the case of “Residential Buildings” for hot and humid climate as shown by the simulation and monitoring data from the experimental case.

IMECE2011-64704 pp. 353-360; (8 pages)
doi:10.1115/IMECE2011-64704

New servers and data center metrics are introduced to facilitate proper evaluation of data centers power and cooling efficiency. These metrics will be used to help reduce the cost of operation and to provision data centers cooling resources. The most relevant variables for these metrics are identified and they are: the total facility power, the servers’ idle power, the average servers’ utilization, the cooling resources power and the total IT equipment power. These metrics can be used to characterize and classify servers and data centers performance and energy efficiency regardless of their size and location.

IMECE2011-64995 pp. 361-370; (10 pages)
doi:10.1115/IMECE2011-64995

This research is conducted to assess the present situation of energy demand and emission of air pollutants from road transportation sector in the Philippines along with the future forecasting of the environmental impacts from transportation sector. According to the published reports of the Department of Energy of the Philippines, transportation have the most energy-intensive sector amongst the sectors, which will account for the largest share in the country’s final energy demand registering an average of 34.5 percent. Hence, the past trend of energy consumption and emissions are applied in order to predict the future pattern. In addition, a model of transportation system using computer based software called “Long Range Energy Alternatives Planning (LEAP)” has been developed together with the associated Environmental Database (EDB) model. The framework of calculation utilized official transportation database, fuel consumption of certain vehicle type and corresponding emission of each vehicle type. The base scenario called as Business -As- Usual (BAU) is surveyed and other different alternative scenarios are presented and discussed. The model is run under the database of 2001 as the base year and extrapolated until 2030 to predict the impact of transportation. The main objective of this study is to achieve an optimal transportation policy which contributes in decline of energy demand as well as air pollution in the Philippines.

IMECE2011-65407 pp. 371-372; (2 pages)
doi:10.1115/IMECE2011-65407

The analysis of exergy losses of a system is a well-known way to determine the influence of the second law on existing systems. Thermo-economics combines this methodology with economic calculations. Using this methodology engineering becomes an evolutionary process. Since system structures are virtual, reversible system structures are possible and inevitable irreversibility is only caused by its real components. They can be described by their exergetic efficiency. Thus reversible system structures can be used as general valid benchmarks for system engineering. It allows easy comparison or a trade-off between possible solutions. The use of a few basic reversible processes allows the building of larger reversible structures including an effective management of released and demanded entropy within the system, as can be shown in different applications and missions. The effective use of renewable sources can be considered as well however bioprocesses are not investigated yet.

IMECE2011-65749 pp. 373-385; (13 pages)
doi:10.1115/IMECE2011-65749

In recent years, Integrated Gasification Combined Cycle Technology (IGCC) has been gaining steady popularity for use in clean coal power operations with carbon capture and sequestration. Great efforts have been continuously spent on investigating various ways to improve the efficiency and further reduce the greenhouse gas (GHG) emissions of such plants. This study focuses on investigating two approaches to achieve these goals. First, replace the traditional subcritical Rankine steam cycle portion of the overall plant with a supercritical steam cycle. Second, add different amounts of biomass as co-feedstock to reduce carbon footprint as well as SOx and NOx emissions. Employing biomass as a feedstock to generate fuels or power has the advantage of being carbon neutral or even becoming carbon negative if carbon is captured and sequestered. Due to a limited supply of feedstock, biomass plants are usually small, which results in higher capital and production costs. In addition, biomass can only be obtained at specific times in the year, meaning the plant cannot feasibly operate year-round, resulting in fairly low capacity factors. Considering these challenges, it is more economically attractive and less technically challenging to co-combust or co-gasify biomass wastes with coal. The results show that supercritical IGCC the net plant efficiency increases with increased biomass blending in the all cases. For both subcritical and supercritical cases, the efficiency increases initially from 0% to 10% (wt.) biomass, and decreases thereafter. However, the efficiency of the blended cases always remains higher than that of the pure coal baseline cases. The emissions (NOx , SOx , and effective CO2 ) and the capital cost all decrease as biomass ratio increases, but the cost of electricity increases with biomass ratio due to the high cost of the biomass used. Finally, implementing a supercritical steam cycle is shown to increase the net plant output power by 13% and the thermal efficiency by about 1.6 percentage points (or 4.56%) with a 6.7% reduction in capital cost, and a 3.5% decrease in cost of electricity.

IMECE2011-65836 pp. 387-400; (14 pages)
doi:10.1115/IMECE2011-65836

Oil resources are finite and production decline is a fact for this century. The question is, why there has been so little policy action? This paper proposes that dealing with the complex changes involved in the transition to oil supply contraction requires new kinds of engineering modeling and analysis. There are no miracle technologies that will mitigate the need for major policy, economic, infrastructure and land use changes. Researchers have the responsibility to develop new methods and tools necessary for policy makers and planners to manage this change in direction. Without the right tools, the policy choice is between denying the problem and hoping for miracles. With the right Transition Engineering tools, the policy choices involve changes in land use, incentives, taxes and investments that efficiently reduce vulnerability and risk, increase adaptive capacity and build resilience. For more than a decade, the research and development program at the Advanced Energy and Material Systems Lab (AEMSLab) has focused on Transition Engineering. The first Transition Engineering project assesses vulnerability and risk to essential activities from oil supply contraction in the near and long term. The risk assessment method employs a probabilistic model of future fuel availability and an impact model of travel behavior adaptation to meet the probable fuel constraint. The second project is to assess travel adaptive capacity of current travel behavior and of the current urban forms using a new kind of travel survey, and to develop adaptation models for different urban development scenarios. Another important analysis is the active mode accessibility of the current urban form. The model uses GIS data and an activity model based on the demographic profile. Future urban form development, technology and infrastructure investments and behavior change are modeled using the strategic analysis method.

IMECE2011-65891 pp. 401-411; (11 pages)
doi:10.1115/IMECE2011-65891

This paper puts forward a simple idea describing the time, space and relationship scales of survival. The proposed survival spectrum concept represents a new way to think about sustainability that has clear implications for influencing engineering projects in all fields. The argument for the survival spectrum is developed sequentially, building on theory, definition, examples and history. The key idea is that sustainability can be effectively addressed by emergence of a new field, Transition Engineering. This is a parallel of safety engineering but with longer time scale, broader space scale, and more complex relationship scale. The past 100-year development of safety engineering is examined as a model for development of sustainability risk management and mitigation. The conclusion is that the new field, Transition Engineering, will emerge as the way our society will realize reduction in fossil fuel use and reduction in the detrimental social and environmental impacts of industrialization.

IMECE2011-62232 pp. 413-419; (7 pages)
doi:10.1115/IMECE2011-62232

This paper presents an investigation into integrated wind + combustion engine high penetration electrical generation systems. Renewable generation systems are now a reality of electrical transmission. Unfortunately, many of these renewable energy supplies are stochastic and highly dynamic. Conversely, the existing national grid has been designed for steady state operation. The research team has developed an algorithm to investigate the feasibility and relative capability of a reciprocating internal combustion engine to directly integrate with wind generation in a tightly coupled Hybrid Energy System. Utilizing the Idaho National Laboratory developed Phoenix Model Integration Platform, the research team has coupled demand data with wind turbine generation data and the Aspen Custom Modeler reciprocating engine electrical generator model to investigate the capability of reciprocating engine electrical generation to balance stochastic renewable energy.

IMECE2011-63662 pp. 421-428; (8 pages)
doi:10.1115/IMECE2011-63662

The analysis and optimization of thermal performance of Li-ion battery packs are topics of great interest today. Most Li-ion batteries for motive, vehicular, backup power and utility energy storage applications are fitted with a microprocessor-controlled thermal management system including an array of temperature and voltage sensors and an active cooling system. However, as the complexity of the thermal management system increases, so does its weight, volume and parasitic power consumption, all factors that adversely affect the vehicle’s performance. In this sense, an improved thermal management system based on including passive solutions such as phase change materials or heat spreading technologies could decrease the load on active components and ultimately the weight and costs of the system. This paper describes an experimental and simulation study aimed at evaluating the effectiveness of flexible graphite materials for heat spreaders in battery thermal management systems. A commercial Li-ion battery pack for power tools applications was adopted as a case study. The electro-thermal behavior of the battery pack was characterized through combined experimental investigation and 3D FEM modeling to determine the heat generation rate of the battery cells during utilization and to evaluate the thermal behavior of the battery pack. A thermal management solution based on flexible graphite heat spreading material was then designed and implemented. The paper presents a comparative study conducted in simulation to evaluate the improvements in the pack thermal behavior.

IMECE2011-63762 pp. 429-437; (9 pages)
doi:10.1115/IMECE2011-63762

A dynamic process model of a steam turbine, including partial arc admission operation, is presented. Models were made for the first stage and last stage, with the middle stages presently assumed to have a constant pressure ratio and efficiency. A condenser model is also presented. The paper discusses the function and importance of the steam turbines entrance design and the first stage. The results for steam turbines with a partial arc entrance are shown, and compare well with experimental data available in the literature, in particular, the “valve loop” behavior as the steam flow rate is reduced. This is important to model correctly since it significantly influences the downstream state variables of the steam, and thus the characteristic of the entire steam turbine, e.g., state conditions at extractions, overall turbine flow, and condenser behavior. The importance of the last stage (the stage just upstream of the condenser) in determining the overall flowrate and exhaust conditions to the condenser is described and shown via results.

IMECE2011-63826 pp. 439-444; (6 pages)
doi:10.1115/IMECE2011-63826

In thermoacoustic device, the nonlinear phenomena are the source of secondary flows superimposed on the oscillating flow. To date, the acoustic streaming have been only characterized for progressive and standing waves in current systems. A method to characterize theoretically the secondary flows in thermoacoustic systems with more complex geometries is developed in this paper. This method is applied in the case of an annular thermoacoustic prime mover. The streaming fields are then provided. As the determination of the streaming fields requires the knowledge of the first order acoustic parameters, a linear model is used. The theoretical results of the acoustic pressure are compared to the experimental data. A relative error less than 10% is obtained, allowing the validation of the linear model.

Topics: Flow (Dynamics)
IMECE2011-64354 pp. 445-452; (8 pages)
doi:10.1115/IMECE2011-64354

This paper presents a mathematical modeling of a modified voice coil generator, which consists of a moving coil within a fixed magnetic circuit. The simulation has been done with Comsol Multiphysics software, which is a powerful tool to demonstrate the pattern of magnetic field and calculate the induced current in the coil. In our simulations, the magnetic circuit consists of the magnetic conductor and the air gap. In this analysis, the magnetic flux density and the magnetic field intensity are calculated. Moreover, through calculation of the total reluctance of the magnetic circuit and employing the ohm’s law for magnetic circuits, the effect of the length and cross section of the total circuit on the magnetic flux are investigated. Finally, a pattern for the magnetic flux density are demonstrated and the simulation result indicates that the magnetic field is well concentrated on the coil area, therefore this prototype can capture and convert most of the kinetic energy to electricity. A prototype has been fabricated and tested on the shaker. The experimental results indicate that this setup is able to produce the maximum voltage of 0.326 V and the peak power equal to 2.605 mW in 35 Hz frequency and 1 mm peak to peak amplitude.

IMECE2011-65013 pp. 453-460; (8 pages)
doi:10.1115/IMECE2011-65013

Over the past decade numerous studies both conducted by and authorized by the US Department of Energy Office of Industrial Technology have identified significant energy savings potential by adjusting flow rates to meet process demands. As much as 40% energy savings have been achieved when variable flow pumping systems were implemented in some DOE demonstration projects. To date, only a small fraction of the identified companies in various industries which can benefit in energy savings resulting from adjustable pumping flow rates have installed the requisite capabilities. One reason for the slow rate of adoption of variable pumping is that there are few commercially available methods for adjusting pump rates. Electronic Variable Frequency Drives (VFDs) are the most commonly implemented method of varying pump speeds, usually resulting in reduced operating life of the electric drive motors and sometimes in significant costs of plant modifications. Veritran Inc. with the support of Team Technologies, Inc. is developing low-cost mechanical devices for varying electric motor speeds without the large initial investment associated with VFDs nor the other detracting features of the need to install larger electric motors and reduced motor life expectancy. Veritran’s Infinitely Variable Transmissions (IVTs), such as SM-15IVT (www.veritraninc.com ) are installed between the motor and the load, which allows for soft starts, and precise output set speeds, all under programmable microprocessor control. The amount of power demanded from the motor varies as the output speed of the transmission is changed or the load torque is changed. This paper will describe the engineering development that Veritran has been pursuing over the past decade of their novel IVTs, and will present some of the test data collected to date. Results will also be presented of systems analyses where IVTs are inserted into various industrial operations and significant energy savings result.

Topics: Flow (Dynamics)
IMECE2011-65818 pp. 461-470; (10 pages)
doi:10.1115/IMECE2011-65818

This paper reports the development of a third-order discontinuous Galerkin (DG) method for supersonic inviscid flow on a moving grid, as well as simulations of a simple model of crypto-steady supersonic pressure exchange (CSSPE) by solving 2D inviscid Euler equations. A total variation bounded (TVB) limiter is implemented for shock capturing. The third-order DG method is firstly validated using a case of supersonic vortex flow. Subsequently, the method is successfully employed to predict the crypto-steady supersonic flat-plate flow under various pressure ratios. In particular, at high pressure ratios between primary gas and secondary gas, a detached shock away from the trailing edge of the flat plate is accurately predicted. This study is our first step in approaching to developing a 3D numerical tool and modeling a novel pressure-exchange ejector.

IMECE2011-62043 pp. 471-476; (6 pages)
doi:10.1115/IMECE2011-62043

Micro-solid oxide fuel cell (SOFC) power plants are emerging as a promising alternative for power generation for portable applications due to their low emission of pollutants, high power density and fuel flexibility. Some of the challenges for developing such micro-SOFC power plants are geometrical compactness, fast start-up and self-sustainability at operating conditions. In this work, we present a hybrid start-up process for a micro-SOFC power plant using catalytic oxidation of n-butane over Rh-doped Ce0.5 Zr0.5 O2 nanoparticles in a small-scale reactor to provide the necessary intermediate operating temperature (500–550 °C) and syngas (CO + H2 ) as fuel for a micro-SOFC membrane. A short heating wire is used to generate the heat required to trigger the oxidative reaction. The hybrid start-up is investigated for partial oxidation (POX) and total oxidation (TOX) ratios at one specified flow rate. Additionally, the variation of electrical heating time and its influence on the hybrid start-up is evaluated.

IMECE2011-62304 pp. 477-487; (11 pages)
doi:10.1115/IMECE2011-62304

When the organic liquid carrier of hydrogen is used as the fuel on a vehicle, a key component is the hydrogen releasing device, which requires large surface area of catalyst, small size and light weight. Micro-reactors with internal structure of micro pin-fin array is considered due to its high area-to-volume ratio and intimate impaction-contact with the fluids. To support the design of a first experiment, a mathematical model based on laws of conservation and chemical reaction is established. A surface-reaction efficiency has also been included to consider the possible effect of surface nonwetting in the hydrogen two-phase flow. This model has been demonstrated using the N-Ethyl Carbazole as the liquid carrier and Palladium as the catalyst. Due to the large gas generation rate, the numerical modeling indicates that the system operates at high void fraction with large slip ratio. This leads to an alternative system design of using segmented reactors with a hydrogen separator located in between. This leads to reduced size of the system and less catalyst material used.

IMECE2011-62307 pp. 489-493; (5 pages)
doi:10.1115/IMECE2011-62307

Investigation of water production rate is one of the most important factors to evaluate the performance of a PEM Fuel Cell. In the present case water production rate was studied from the hydrogen and the air inputs with an additional consideration of the effect of the relative humidity of ambient air. A corrected model was used to evaluate the water production, and it was compared against experimental data. It was found that this new model is strongly enhanced and it becomes in a new focus of study. The fuel cell system was tested at steady state conditions during 2700 s at 245 W and 36 V. The mass flow rate of water produced by the experimental result was 1.444 grams/min, while the analytical result was 1.311grams/min; the error of the analytical data compared with the experimental data was 10.14 percent.

IMECE2011-62581 pp. 495-503; (9 pages)
doi:10.1115/IMECE2011-62581

Performance characterization and durability testing have been completed on two five-cell high-temperature electrolysis stacks constructed with advanced cell and stack technologies. The solid oxide cells incorporate a negative-electrode-supported multi-layer design with nickel-zirconia cermet negative electrodes, thin-film yttria-stabilized zirconia electrolytes, and multi-layer lanthanum ferrite-based positive electrodes. The per-cell active area is 100 cm2 . The stack is internally manifolded with compliant seals. Treated metallic interconnects with integral flow channels separate the cells and electrode gases. Stack compression is accomplished by means of a custom spring-loaded test fixture. Initial stack performance characterization was determined through a series of DC potential sweeps in both fuel cell and electrolysis modes of operation. Results of these sweeps indicated very good initial performance, with area-specific resistance values less than 0.5 Ω.cm2 . Long-term durability testing was performed with a test duration of 1000 hours. Overall performance degradation was less than 10% over the 1000-hour period. Final stack performance characterization was again determined by a series of DC potential sweeps at the same flow conditions as the initial sweeps in both electrolysis and fuel cell modes of operation. A final sweep in the fuel cell mode indicated a power density of 0.356 W/cm2 , with average per-cell voltage of 0.71 V at a current of 50 A.

IMECE2011-62582 pp. 505-512; (8 pages)
doi:10.1115/IMECE2011-62582

A three-dimensional computational fluid dynamics (CFD) and electrochemical model has been created to model high-temperature electrolysis cell performance and steam electrolysis in an internally manifolded planar solid oxide electrolysis cell (SOEC) stack. This design is being evaluated experimentally at the Idaho National Laboratory (INL) for hydrogen production from nuclear power and process heat. Mass, momentum, energy, and species conservation are numerically solved by means of the commercial CFD code FLUENT. A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, operating potential, steam-electrode gas composition, oxygen-electrode gas composition, current density and hydrogen production over a range of stack operating conditions. Results will be presented for a five-cell stack configuration that simulates the geometry of five-cell stack tests performed at the INL and at Materials and System Research, Inc. (MSRI). Results will also be presented for a single cell that simulates conditions in the middle of a large stack. Flow enters the stack from the bottom, distributes through the inlet plenum, flows across the cells, gathers in the outlet plenum and flows downward making an upside-down “U” shaped flow pattern. Flow and concentration variations exist downstream of the inlet holes. Predicted mean outlet hydrogen and steam concentrations vary linearly with current density, as expected. Contour plots of local electrolyte temperature, current density, and Nernst potential indicate the effects of heat transfer, reaction cooling/heating, and change in local gas composition. Results are discussed for using this design in the electrolysis mode. Discussion of thermal neutral voltage, enthalpy of reaction, hydrogen production, cell thermal efficiency, cell electrical efficiency, and Gibbs free energy are discussed and reported herein.

IMECE2011-62585 pp. 513-521; (9 pages)
doi:10.1115/IMECE2011-62585

A series of 5 cm by 5 cm bi-supported Solid Oxide Electrolysis Cells (SOEC) were produced by NASA for the Idaho National Laboratory (INL) and tested under the INL High Temperature Steam Electrolysis program. The results from the experimental demonstration of cell operation for both hydrogen production and operation as fuel cells is presented. An overview of the cell technology, test apparatus and performance analysis is also provided.

IMECE2011-62808 pp. 523-529; (7 pages)
doi:10.1115/IMECE2011-62808

High purity hydrogen is produced through a thermochemical water splitting process that utilizes iron reduction-oxidation (redox) reactions. An iron powder bed is fluidized to improve heat and mass transfer thus improving the reaction kinetics. Inert additives which act as sintering inhibitors, such as silica (SiO2 ) and zirconia (ZrO2 ), are added to the iron powder, and their effectiveness in inhibiting sintering in the oxidation step is evaluated. The influence of particle size, composition, mass fraction and bed temperature on reaction kinetics is investigated. Incorporation of zirconia in the powder bed is done by mixing it with iron powder or by coating the iron particles with a mixture of 1–3 μm and 44 μm zirconia particles. Two different batches of silica are used for blending with iron powder. The silica powder batches include particle diameters ranging from 0–45 μm and 200–300 μm. The mixing ratios of silica to iron are 0.33, 0.5, 0.67 and 0.75 by apparent volume. Experimental studies are conducted in a bench scale experimental fluidized bed reactor at bed temperatures of 450, 550, 650, 750 and 850 °C. It is verified that increasing the bed temperature and the steam residence time increases the hydrogen yield. Increasing the iron particle size reduces the specific surface area and reduces the hydrogen yield. It has been found that sintering can be completely inhibited by mixing iron with 0–45 μm silica powder and maintaining the reaction temperature below 650 °C.

IMECE2011-62967 pp. 531-538; (8 pages)
doi:10.1115/IMECE2011-62967

Ammonia borane (AB), a solid chemical under room temperatures, is a promising candidate material for on-board hydrogen storage. One of the major drawbacks that limit the use of this kind of materials is the difficulty to remove (transport) the spent fuels (reaction byproducts) out of the system. In the present study, a piston cylinder device was designed and used to study the spent fuel transport process and to find effective ways to improve the transportability of spent fuels. The piston, which has an area of 5.07 cm2 , is designed to push out the spent fuel through a clear acrylic die, which has a diameter of 3.175 millimeters. The pushing force on the top of the piston is provided by compressed nitrogen gas. The piston/cylinder assembly is able to operate under a pressure of 30 bars, which gives a theoretical force of 1,520 N. The assembly can be mounted either vertically or horizontally. Experiments were carried out with sand surrogates and spent fuel of BmimCl (20 wt%) aided AB (80 wt%) thermolysis, which was conducted within the temperature range of 80°C to 120°C. A series of experiments with sands were conducted with different sand to oil weight ratios from infinite, 10:1, 7:1, 5:1, to 4:1. Results of the experiments with sand showed that the sand/oil mixture was extruded out from the piston/cylinder assembly when the mixture ratios were 5:1 and 4:1. The transportability of the 4:1 mixture is better than that of 5:1 mixture, indicated by the piston travel distance. Experiment also showed that the powder-like spent fuel could be moved out from the piston/cylinder assembly smoothly without any oil added. This study demonstrated that effective removal of AB spent fuel in hydrogen storage systems is possible. Considering that only a small amount of spent fuel sample was used, tests with more spent fuel need to be carried out to further support the present observation.

IMECE2011-63815 pp. 539-546; (8 pages)
doi:10.1115/IMECE2011-63815

The introduction of hydrogen as an energy carrier for light-duty vehicles involves concomitant technological development of an array of infrastructure elements, such as production, delivery, and dispensing, all associated with energy consumption and emission levels. To analyze these at a system level, the suite of corresponding models developed by the United States Department of Energy and involving several national laboratories is combined in one macro-system model (MSM). The MSM uses a federated simulation framework for consistent data transfer between the component models. The framework is built to suit cross-model as well as cross-platform data exchange and involves features of “over-the-net” computation. While the MSM can address numerous hydrogen systems analysis aspects, of particular interest is the optimal deployment scenario. Depending on user-defined geographic location and hydrogen demand curve parameters, the cost-optimal succession of production/delivery/dispensing pathways undergo significant changes (the most important of these being the transition between distributed and central H2 production with delivery). Some ‘tipping’ (break-even) points are identified.

IMECE2011-63833 pp. 547-552; (6 pages)
doi:10.1115/IMECE2011-63833

This numerical study presents the role of diffuse region of the electric double layer in both acidic and alkaline fuel cells. The numerical model is based on the Poisson-Nernst-Planck (PNP) and generalized-Frumkin-Butler-Volmer (gFBV) equations. The Laminar Flow Fuel Cell (LFFC) is used as the model fuel cell architecture to allow for the appropriate and equivalent comparison of acidic and alkaline cells. In particular, we focus on how each device behaves to changing reactant supply at the electrodes, including the overall cell performance and individual electrode polarizations. It is found that the working ion concentration at the reaction plane contributes to differing performance behaviors in acidic and alkaline fuel cells, including activation losses and reactant transport overpotentials. This is due to the working ion, and the electrode where it’s consumed, being opposite for acidic and alkaline fuel cells.

IMECE2011-64016 pp. 553-560; (8 pages)
doi:10.1115/IMECE2011-64016

In a PEM fuel cell, it has been shown that the compression under the land area is the main reason for the observed higher performance than that under channel areas. If the area under the channel can also benefit from such a compression the overall performance of the cell will increase. Since the areas under the channel are not directly compressed in an assembled fuel cell, it is the objective of this study to determine if a cold pre-compression treatment of the gas diffusion electrode (GDE) may have a significant positive effect on the overall performance of the cell. First, the GDE is cold pre-compressed to a level similar to the compression that would be experienced by the land areas in an assembled fuel cell. Then the pre-compressed GDE is assembled in a regular test fuel cell and the performances under various operating conditions are studied. Finally, the cell performance results are compared with the results obtained from a fuel cell with a regular GDE. The experimental results show that cold pre-compress of the GDE has significantly improved the overall performance of the fuel cell. Further experiments have also been conducted with five different levels of cold pre-compression to determine if there exists an optimal compression and its value if it exists. The experimental results show that the performance of the fuel cell first increases with the level of cold pre-compression, reaching a maximum and then decreases with the level of compression. These results clearly indicate that there indeed exists an optimal level of compression. Further studies using both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) have further corroborated the cell performance findings as well as the underlying mechanism. The results of EIS indicate that the ohmic resistance is hardly affected by the cold pre-compression, while the charge transfer resistance is significantly affected, especially in high current density region. The CV results show that the electro-chemical area (ECA) is higher with the cold pre-compressed GDE and there is an optimal compression that results in the maximum ECA. Therefore, the experimental results have shown that (a) the cold pre-compression treatment of the GDE is an effective and simple technique to increase PEM fuel cell performances; (b) there exists an optimal compression level at which the cell reaches its maximum performance; and (c) the increased performance is due to the increase of ECA resulting from the cold pre-compression treatment.

IMECE2011-64031 pp. 561-569; (9 pages)
doi:10.1115/IMECE2011-64031

Serpentine flow-fields are widely used in proton exchange membrane (PEM) fuel cells due to their various advantages, including providing a proper compromise between pressure drop and water removal capability. One of the advantages of serpentine flow fields is the cross-flow under the land through the gas diffusion layer (GDL) due to the pressure difference between adjacent channels. In this study, a three-dimensional PEM fuel cell model is developed to study the cross-flow effect under the land for both across and along the land directions. Simulation results of the flow distribution along and across the channel, and the relationship between the cross-flow and the pressure difference are presented. A parametric study is conducted to investigate the effect of the GDL permeability on the cross-flow rate. The cross-flow rate increases as the permeability becomes larger because the cross-flow velocity. However, cross-flow rate reaches an asymptotic value when the permeability is greater than 10−9 (m2 ) since the pressure difference between adjacent channels becomes smaller. The effect of the cross-flow on the local oxygen mass fraction is also investigated. The results show that oxygen concentrations in some locations are significantly higher due to the cross-flow under the land and secondary flows in the channel. Finally, by comparing average current densities between under the channels and the land areas, it is shown that the performance of the cell gradually decreases across the channel/land direction.

IMECE2011-64232 pp. 571-580; (10 pages)
doi:10.1115/IMECE2011-64232

A cell-level distributed electrochemistry (DEC) modeling tool has been developed to enable predicting trends in solid oxide fuel cell performance by considering the coupled and spatially varying multi-physics that occur within the tri-layer. The approach calculates the distributed electrochemistry within the electrodes, which includes the charge transfer and electric potential fields, ion transport throughout the tri-layer, and gas distributions within the composite and porous electrodes. The thickness of the electrochemically active regions within the electrodes is calculated along with the distributions of charge transfer. The DEC modeling tool can examine the overall SOFC performance based on electrode microstructural parameters, such as particle size, pore size, porosity, electrolyte- and electrode-phase volume fractions, and triple-phase-boundary length. Recent developments in electrode fabrication methods have lead to increased interest in using graded and nano-structured electrodes to improve the electrochemical performance of SOFCs. This paper demonstrates how the DEC modeling tool can be used to help design novel electrode microstructures by optimizing a graded anode for high electrochemical performance.

IMECE2011-64237 pp. 581-584; (4 pages)
doi:10.1115/IMECE2011-64237

Solid oxide fuel cell cathodes have been examined using non-destructive x-ray nanotomography. The cathodes examined were a composite of strontium-doped lanthanum manganite (LSM) and yttria-stabilized zirconia (YSZ), with three different starting powder sizes of 0.3 μm, 0.5 μm, and 1 μm. Differential absorption contrast imaging was performed over the manganese K-edge (6539 eV) for the identification of the LSM, YSZ, and pore phases. The three phases were each segmented from reconstruction of the tomography data. Three dimensional volumes of the segmented phases were used to calculate structural characterization parameters of the sample including porosity, pore size distributions, and mean phase sizes. These parameters are reported and some correlations are drawn to the starting powder size.

IMECE2011-64466 pp. 585-591; (7 pages)
doi:10.1115/IMECE2011-64466

By assuming the H diffusion coefficient and H adsorption rate to be exponentially and linearly dependent on H concentration, a physical model is developed to predict the hydrogenation process of Mg nanoblades. The predicted H uptake curves agree well with the experimental data from V-coated Mg nanoblades. The obtained H diffusion coefficients in MgHx between Mg and MgH2 have nearly three orders of magnitude variation. The characteristic time of H surface adsorption is longer than that of H diffusion in Mg but shorter than that in MgH2 for 100 nm thick nanoblades. Thus, as it proceeds, the hydrogenation process gradually changes from surface reaction-limited to diffusion-limited. In both one- and two-dimensional simulations, it is shown that a hydride shell is not formed during hydrogenation. In contrast, a hydride core is formed during dehydrogenation. The strong (exponential) concentration dependence of H diffusion coefficient throws profound influence on the stability and instability of a diffusion front, i.e., a H diffusion front in hydrogenation, and a H-vacancy diffusion front in dehydrogenation. In the latter case, the front tends to corrugate forming islands when the H2 release rate is high.

IMECE2011-64472 pp. 593-597; (5 pages)
doi:10.1115/IMECE2011-64472

Planar solid oxide fuel cells (SOFCs) are made up of repeating sequences of thin layers of cermet electrodes, ceramic electrolytes, seals, and current-collectors. For electro-chemical reasons it is best to keep the electrolyte layers as thin as possible. However, for electrolyte-supported cells, the thin electrolytes are more susceptible to damage during production, assembly, and operation. The latest-generation electrolyte-supported SOFCs feature metallic foam current-collectors which relay current between the energy-producing materials and the rest of the circuit. These foams are stamped into a corrugated shape which is intended to reduce the compressive loads which are transferred through the stack onto the brittle electrolyte, but the mechanical behavior of the foams remain to be fully understood. Characterization of the corrugated metal foams consists of comparison of load-vs.-displacement behavior between experimentally measured compression data and a single-component finite element model which isolates the foam from the rest of the stack. Mechanical properties of the foam are found using an iterative approach, in which the material properties used as inputs to the model are changed until the load-displacement data best agrees with experiments. The model explores the influence of elastic and plastic properties in combination with and without friction. Thus obtained, the properties can then be used in a stack model to determine which parameters can best reduce the demands on the electrolyte without sacrificing electrochemical performance.

IMECE2011-64795 pp. 599-604; (6 pages)
doi:10.1115/IMECE2011-64795

An experimental study has been conducted to assess the performance of electrode-supported solid-oxide cells operating in the steam electrolysis mode for hydrogen production. Results presented in this paper were obtained from single cells, with an active area of 16 cm2 per cell. The electrolysis cells are electrode-supported, with yttria-stabilized zirconia (YSZ) electrolytes (∼10 μm thick), nickel-YSZ steam/hydrogen electrodes (∼1400 μm thick), and modified LSM or LSCF air-side electrodes (∼90 μm thick). The purpose of the present study is to document and compare the performance and degradation rates of these cells in the fuel cell mode and in the electrolysis mode under various operating conditions. Initial performance was documented through a series of voltage-current (VI) sweeps and AC impedance spectroscopy measurements. Degradation was determined through long-term testing, first in the fuel cell mode, then in the electrolysis mode. Results generally indicate accelerated degradation rates in the electrolysis mode compared to the fuel cell mode, possibly due to electrode delamination. The paper also includes details of an improved single-cell test apparatus developed specifically for these experiments.

IMECE2011-63372 pp. 605-610; (6 pages)
doi:10.1115/IMECE2011-63372

We have developed a thermodynamic framework to calculate adsorption cooling cum desalination cycle performances as a function of pore widths and pore volumes of highly porous adsorbents, which are formulated from the rigor of thermodynamic property surfaces of adsorbent-adsorbate system and the adsorption interaction potential between them. Employing the proposed formulations, the coefficient of performance (COP) and overall performance ratio (OPR) of adsorption cycle are computed for various pore widths of solid adsorbents. These results are compared with experimental data for verifying the proposed thermodynamic formulations. It is found from the present analysis that the COP and OPR of adsorption cooling cum desalination cycle is influenced by (i) the physical characteristics of adsorbents, (ii) characteristics energy and (iii) the surface-structural heterogeneity factor of adsorbent-water system. The present study confirms that there exists a special type of adsorbents having optimal physical characteristics that allows us to obtain the best performance.

Topics: Cooling , Cycles
IMECE2011-63415 pp. 611-617; (7 pages)
doi:10.1115/IMECE2011-63415

The tip leakage flow rate can be directly linked to the loss and stall margin. In this paper, key factors affecting the tip leakage flow rate are explained based on a simple leakage flow model including viscous effect. Based on the numerical results, the flow model is verified in a low speed compressor rotor, and finally a simplified one-dimensional tip blockage model is established based on the Khalid’s model, which may be helpful in the design of compressor.

Topics: Leakage flows
IMECE2011-64444 pp. 619-630; (12 pages)
doi:10.1115/IMECE2011-64444

Experimental evidence is presented and compared with theoretical predictions from Quantum Thermodynamics, (QT) to examine whether or not the claims of QT are consistent with the existence and generation of entropy at atomistic scales. QT makes the assertion that entropy is an intrinsic property of matter in the same way that inertial mass, energy, and momentum are and must, thus, exist even for single particles. Entropy as defined by QT is a measure of the distribution of a system’s internal energy at any given instant of time amongst the available internal degrees of freedom, i.e., the energy eigenlevels of the system. In this paper, it is shown that QT predicts the internal relaxation of a 5-level rubidium system that is consistent with the experimental data and not explained by current theory, i.e. by Quantum Mechanics. In addition, it is demonstrated that the decay of so-called “cat states” for single ions that are contained in Paul traps and that interact with a heat reservoir is also consistent with the idea of the existence of entropy and entropy generation at atomistic scales. This is accomplished by comparing experimental data with the predictions made using an extension of the equation of motion of QT that allows for heat interactions.

IMECE2011-62299 pp. 631-639; (9 pages)
doi:10.1115/IMECE2011-62299

This paper considers combined heat and power (CHP) systems based on topping cycles only, in which electricity is generated by a prime mover and heat is then recovered from the exhaust and utilized to offset all or a portion of the facility’s process and/or space heating requirements.. The objective of this paper is to develop a methodology to perform a topping cycle CHP assessment and feasibility study for industrial manufacturing facilities. In order to determine the best and most viable option for the facility in question, the proposed methodology can be used to size different systems which utilize diverse technologies and fuel sources, perform an economic analysis of each proposed option, and then compare the benefits and setbacks of each type of CHP system considered. The calculations performed in the economic analysis will then provide a broad insight as to which proposed system will show the best payback if installed. Examples are presented in this paper that describe in detail the application of this methodology, from equipment selection and sizing through economic analyses and proposed system comparisons, which is recommended for use in order to determine the most economically feasible CHP system for an industrial manufacturing facility.

IMECE2011-62542 pp. 641-650; (10 pages)
doi:10.1115/IMECE2011-62542

A combined heating and power system (CHP) can take the place of a conventional system with separate heating and power (SHP) where electricity is purchased from the grid. The CHP system provides electrical energy through a prime mover located near the building it serves, and waste heat from this generation is captured and delivered to the building to provide thermal energy. For a CHP system to show an economic advantage over a conventional system, its operating costs must be lower when providing the same amount of thermal energy and electricity that would have come from the SHP system. The spark spread (SS), or price difference between purchased electricity and fuel, is used as a simple indicator as to whether the CHP system is economically viable. Rather than using a single value of SS as a cutoff for viability of the CHP system, a more detailed spark spread expressed in terms of the efficiencies of the CHP system and SHP system components can be used to determine if a CHP system is economically viable. In an initial feasibility study, the calculation of the SS is based on estimates of a number of variables. It is important to assess the likely impact of changes in certain of some of these variables, as such changes can affect the SS calculations. This paper presents a sensitivity analysis to determine the effects of different parameters on the cost ratio which is used to calculate SS, including: reference heating system efficiency, power generation unit (PGU) efficiency and CHP overall system efficiency. Because CHP system efficiency itself is a function of the PGU efficiency as well as the thermal efficiency, these two parts of the total system efficiency are also investigated separately. Since the cost of purchased electricity and fuel varies by geographic region, the required spark spread for a given system may indicate favorable economics for a CHP system in one location while the CHP system shows no potential for savings in another location. Therefore, the sensitivity analysis is considered for three different U.S. locations.

IMECE2011-63083 pp. 651-669; (19 pages)
doi:10.1115/IMECE2011-63083

In the last decades one of the most difficult problems in the electricity market has been how to dispatch and manage the electricity in power generation plants. Despite of all the benefits of distributed poly-generation and combined heat and power systems, their penetration in the power market worldwide is quite modest and one of the barriers against their increasing participation is the high fees for back-up supplies, which is one of the problems addressed in this investigation. This paper introduces a pool of distributed generation units (named nerve-centre) able to economically optimise the generation schedule of gas turbine power plants and end-users interconnected through a mini-grid. A hybrid genetic algorithm adapted priority list was developed to solve the multi unit generation schedule optimisation problem. The algorithm developed in this study leads the optimisation mechanism to a faster convergence and a very low risk of non-convergence to the optimal result. Despite the power generation optimisation studies reported in the technical literature, none of them has been modelled for such a pool of distributed generators trading electricity in the competitive market. This investigation shows that the proposed nerve-centre concept can result in significant savings to generators/end-users.

IMECE2011-63311 pp. 671-676; (6 pages)
doi:10.1115/IMECE2011-63311

The underground temperature distribution and variation are important for the development and utilization of shallow geothermal energy and underground space. In this paper, a remote ground temperature monitoring system was established in Chongqing, located in southwest China and the ground temperature distribution and its variation with seasons of the 100m underground depth was measured from June 2010 to June 2011. The results show that, in the typical geo-structure of Chongqing, the variational ground temperature zone is from 0m to 10m, where the ground temperature is strongly affected by the change of ambient temperature. Below 11m depth, the ground temperature does not change with seasons, i.e., it is the constant ground temperature zone. In the constant temperature zone below 40 meter depth, the geothermal gradient is about 0.02°C/m. Baggs’s empirical formula was applied to predict the ground temperature distribution history at different ground depths. The results show a good agreement with the measured data.

IMECE2011-63318 pp. 677-683; (7 pages)
doi:10.1115/IMECE2011-63318

The design and performance optimization of ground source heat pump (GSHP) systems need the exact thermal properties of the soil, such as ground thermal conductivity and capacity, and the borehole thermal resistance of borehole heat exchanger (BHE). In-situ thermal response test (TRT) is the most widely used method to determine the overall thermal physical properties of the geological structure around the borehole. A TRT experimental apparatus has been developed and thermal response test was performed in Chongqing, southwest China. Both single-U and double-U borehole heat exchangers are studied in this work. The test duration is about 70 hours. Data direct fitting and parameter estimate method are both used to determine the soil thermal conductivity and the borehole thermal resistance. The results showed that the average ground thermal conductivity of the test region for single U and double U BHE conditions are 2.55 and 2.51 Wm−1 K−1 , and borehole thermal resistance are 0.116 and 0.066 mKW−1 , respectively.

Topics: Heat pumps
IMECE2011-63359 pp. 685-692; (8 pages)
doi:10.1115/IMECE2011-63359

In the next years Distributed Poly-generation Systems are expected to play an increasingly important role in the electricity infrastructure and market. The successful spread of small-scale generation either connected to the distribution network or on the customer side of the meter depends on diverse issues, such as the possibilities of technical implementation, resource availability, environmental aspects, and regulation and market conditions. The aim of this study is to develop an economical and parametric analysis of a Distributed Generation System based on gas turbines able to satisfy the energy demand of a typical Hotel complex. Here we show the economic performance of six cases combining different designs and regimes of operation. The software Turbomatch, the gas turbine performance code of Cranfield University, was used to simulate the off-design performance of the engines in different ambient and load conditions. A clear distinction between cases running at full load and following the load could be observed in the results. Full load regime can give a shorter return on the investment than following the load. Despite of CHP systems currently are not economically attractive, in the future this scenario may change due to the environmental regulations and unavailability of low price fuel for large centralised power stations. CHP has a significant potential but requires favourable legislative and fair energy market conditions to successfully increase its share in the power generation market.

IMECE2011-63660 pp. 693-699; (7 pages)
doi:10.1115/IMECE2011-63660

In this paper, a hotel with hybrid geothermal heat pump system (HyGSHP) in the Pensacola is selected and simulated by the transient simulation software package TRNSYS [1]. To verify the simulation results, the validations are conducted by using the monthly average entering water temperature, monthly facility consumption data, and etc. Three types of HVAC systems are compared based on the same building model and HVAC system capacity. The results are presented to show the advantages and disadvantages of HyGSHP compared with the other two systems in terms of energy consumptions, life cycle cost analysis.

IMECE2011-63681 pp. 701-705; (5 pages)
doi:10.1115/IMECE2011-63681

The U-tube sizes and varied thermal conductivity with different grout materials are studied based on the benchmark residential building in Hot-humid Pensacola, Florida. In this study, the benchmark building is metered and the data is used to validate the simulation model. And a list of comparative simulation cases with varied parameter value are simulated to study the importance of pipe size and grout to the ground source heat pump energy consumption. The simulation software TRNSYS [1] is employed to fulfill this task. The results show the preliminary energy saving based on varied parameters. Future work needs to be conducted for the cost analysis, include the installation cost from contractor and materials cost.

Topics: Climate , Heat pumps
IMECE2011-65489 pp. 707-716; (10 pages)
doi:10.1115/IMECE2011-65489

A 5-kW Organic Rankine Cycle (ORC) was designed for mobile 60-kW diesel engine waste heat recovery applications to provide additional electricity for powering air conditioners. The ORC uses a non-flammable, near-zero-global-warming-potential fluid (Novec649) in a supercritical cycle. The system conceptual design and some observations on the component specification are described. The system will utilize an advanced oil-free high speed direct drive turbine. The proposed power generation module has a volume of ∼3 ft3 and contains the turbine, generator, pump, recuperator, and electrical components. The heat rejection heat exchanger is located on the power generation module in a configuration similar to mini-split air conditioners. The heat recovery heat exchanger (supercritical heater) is attached to the diesel generator and placed in series before the OEM muffler. The supercritical heater must be carefully designed to prevent the refrigerant from overheating, while still maintaining a high effectiveness.

IMECE2011-65617 pp. 717-728; (12 pages)
doi:10.1115/IMECE2011-65617

This paper shares the results of an analysis of the energetic, emissions and economic balance of a simulated large-scale (multi-home) ground-source heat pump (GSHP) system in Austin, TX for heating and cooling. In contrast with traditional AC units and furnaces that use air outside of the house as a heat sink or source, GSHP systems use a ground loop heat exchanger to transfer heat to and from the soil where temperatures stay relatively constant throughout the year. GSHP systems operate more efficiently due to their use of the ground as a heat sink or source, whereas traditional AC units use the outside air. Consequently, GSHP systems typically use less energy than traditional HVAC units, but high capital costs have inhibited their widespread use. The monthly and annual energy costs for a home with a traditional AC unit-furnace are compared to a home with a GSHP system using hourly energy consumption data. The GSHP system modeled for the energy efficient home uses less energy (8.2 MMBtu) throughout the year compared to an efficient AC unit installed on the same home. Energy costs are lower using both the Austin Energy current rate schedule and for a proposed alternative schedule. Additionally, the GSHP system produces a net reduction in harmful emissions. Furthermore, GSHP systems reduce peak demand during the energy-intensive summer months, when air emissions are of greater concern. The installation costs for a large-scale residential GSHP system used in this research were based on interviews, current industry standards, and available pricing data. The analysis includes a sample case study of the cost of the GSHP system included with a home mortgage using a net monthly payment method and includes electricity price increases over time and federal tax incentives. If installed on a large scale, assuming best-case pricing with current federal tax incentives, the GSHP system case is $17 less per month than the traditional HVAC case. Federal tax incentives and high natural gas prices were found to make savings more significant for the GSHP system over time. Additionally, capital cost variability had a greater effect on savings than changes in electricity costs. Based on these analyses, large-scale GSHP systems are considered an economically feasible alternative to traditional HVAC units that will reduce energy consumption and air emissions.