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A new type of district energy network providing simultaneously heating and cooling services is being investigated. It is based on the use of CO2 as a heat transfer fluid by taking advantage of the latent heat of vaporization, to store and transfer heat across the network. The goal of the present study is to determine the performance of a CO2 network when applied to a real urban area. It focuses first on determining the requirements for the various thermal energy services for a part of Geneva's City Centre. The final energy consumption is first computed for the energy conversion technologies now in place in this area e namely heating oil boilers and air cooled compression chillers. Then the new final energy consumption is computed considering that a CO2 network is used instead of boilers and air cooled compression chillers. For the area considered the CO2 network's variant leads to a final energy consumption of 10,968 MWh of electricity. It represents a reduction of 84.4% when compared to the boilers and chillers case. A comparative profitability analysis of the two cases is also presented. The analysis takes into account investment, heating oil and/or electricity, equipment replacement, operation, and maintenance costs, as well as the sales of energy services. For an interest rate of 6%, a price of the delivered heating/cooling services at 0.108 V kW h 1 and a lifetime of 40 years, the conventional technology does not reach profitability while the CO2 network achieves a profit in present value of 69.59 million V and the break-even point is reached after 5.5 years of operation.

The development of an efficient isothermal turbine and compressor is essential for the realization of a small-scale compressed air energy storage (CAES). This article presents the theoretical development of an oil-free co-rotating scroll air compressor and turbine working with water injection to make the operations of expansion and compression as isothermal as possible. First experimental results in expander mode are shown. The theoretical performance is predicted with the help of a mathematical model using the equations of energy and mass conservation and the equation of state. This model takes into account the effects of water injection and volumetric losses. The experimental prototype is an oil-free scroll air compressor with the distinctive feature of having two mobile involutes working in synchronized co-rotation one relative to another. The prime-mover is an electric motor driving the two scrolls with two synchronizing belts. Water injection in the housing intends to provide a quasi-isothermal compression. The same device is used as an isothermal expander by supplying high-pressure air with water when it rotates backwards in expander mode, the electric motor acting then as a generator. Expected improvements to a standard scroll compressor and expander are a better volumetric efficiency and a greater power density due to a higher rotational speed of the scrolls, thanks to their symmetrical masses. The isothermal processes increase also the overall performance. (C) 2014 Elsevier Ltd. All rights reserved.

An attractive way toward a more rational conversion of fuel and biofuel is the decentralized power generation and cogeneration. The fuel cell-gas turbine hybrid cycles are emerging as the most promising candidates to achieve in decentralized generation comparable or higher efficiency than in large scale power plants. The present contribution is devoted to the design and optimization of an innovative concept of small scale Solid Oxide Fuel Cell – Gas Turbine hybrid cycle for residential applications. A 5kW planar SOFC module, operating at atmospheric pressure, is integrated with a micro gas turbine unit, including two radial turbines and one radial compressor, based on an inverted Brayton cycle. A thermodynamic optimization approach, coupled with system energy integration, is applied to evaluate several design options. The optimization results indicate the existence of realistic optimal designs achieving exergy efficiency higher than 66%. Sensitivity analyses on the more sensible parameters are carried out. The heat exchanger network definition is performed for an optimal configuration and a system layout is proposed. The cogeneration performance of the system and its integration in common residential buildings are discussed.

In this paper the effect of systematic errors in gas temperature measurements on the thermal control of a Solid Oxide Fuel Cell system is demonstrated. Three control schemes were tested under two different conditions. First, the real gas temperature at the point of interest was used as input for the controller. Second, the reading of the thermocouple that takes the measurement at the same point was used as input. The second approach included the bias on the measurement due to radiation and heat conduction on the thermocouple. The results showed an overestimation of temperatures with important influence on the system control. Furthermore, in one case the system could not follow the required power demand and it failed. A successful solution to the problem was obtained by increasing the dimension of the system's heat exchanger. (C) 2013 Elsevier B.V. All rights reserved.

Recently, TEES (thermo-electric energy storage) systems have been proposed as a new method for large-scale energy storage wherein electric power is stored as thermal energy by using a heat pump and retrieved from the TES (thermal energy storage) by using a heat engine. The advantages of TEES systems are their higher energy density and independence from geological formations in comparison with pumped hydro storage and CAES (compressed air energy storage). In particular, a TEES system based on hot water, ice storage and transcritical CO2 cycles is considered to be a promising method for large-scale energy storage. This paper reviews current TEES systems and proposes a novel isothermal TEES system with transcritical CO2 cycles. For the given efficiencies of the compressor and expander, the maximum round-trip efficiency decreases rapidly with an increase in the back work ratio. It is shown that the round-trip efficiency of the isothermal TEES system can be increased because of a lower back work ratio than in Isentropic case. For the isothermal compression/expansion, the water from hot storage tank is used by pump/motor to compress/expand the supercritical CO2 as the liquid piston, and a portion of the water is sprayed to cool/heat the supercritical CO2 by a circulation pump. (C) 2012 Elsevier Ltd. All rights reserved.

As more and more solar tower thermal power plants are being operated, built or planned, effort is put both on the development and research to bring costs down and increase the plant efficiency. In those plants, the central receiver is one of the key components, accounting for a large investment share. Receivers have to sustain strong thermal stresses caused by irradiation transients, mainly due to cloud passages. To avoid premature failures, increase the receiver cyclic life, and allow longer daily operation periods, an anticipation of the most likely or the worst situations is required. First the calculation of the receiver incident flux distribution is performed, second the cloud and cloud passage characteristics are identified for a given location, third the most likely case is simulated by covering and uncovering the heliostat field, then a worst case configuration is presented, and finally a strategy for the start-up/shut-down of the heliostats is proposed. The value of terms such as the heat flux peak, the maximal flux gradient, the fastest flux transient and total power transients are needed to choose the control strategies regarding heliostat orientation and the receiver operation, as well as the elimination of some bad plant layouts during the design phase.

The degradation of solid oxide fuel cells (SOFC) depends on stack and system design and operation. A methodology to evaluate synergistically these aspects to achieve the lowest production cost of electricity has not yet been developed. A repeating unit model, with as degradation processes the decrease in ionic conductivity of the electrolyte, metallic interconnect corrosion, anode nickel particles coarsening and cathode chromium contamination, is used to investigate the impact of the operating conditions on the lifetime of an SOFC system. It predicts acceleration of the degradation due to the sequential activation of multiple processes. The requirements for the highest system efficiency at start and at long-term differ. Among the selected degradation processes, those on the cathode side here dominate. Simulations suggest that operation at lower system specific power and higher stack temperature can extend the lifetime by a factor up to 10, because the beneficial decrease in cathode overpotential prevails over the higher release of volatile chromium species, faster metallic interconnect corrosion and higher thermodynamic risks of zirconate formation, for maximum SRU temperature below 1150 K. The counter-flow configuration, combined with the beneficial effect of internal reforming on lowering the parasitic air blower consumption, similarly yields longer lifetime than co-flow. (C) 2012 Elsevier B.V. All rights reserved.

Large scale energy storage may play an increasingly important role in the power generation and distribution sector, especially when large shares of renewable energies will have to be integrated into the electrical grid. Up to now pumped-hydro is the only technology that has been widely used for that purpose. However the spread of this technology is limited by geographic constraints. In the present work, a particular implementation of the storage concept based on thermodynamic cycles, as introduced by ABB Switzerland ltd. Corporate Research, has been analysed thermoeconomically. This concept can be scaled up to a GWh range of stored energy, and is site independent. The underlying idea is to use electricity to power a heat pump transferring heat from a cold source to a hot source (charging mode), while storing the heat and the cold at both sources. Subsequently the energy that has been stored/removed to/from the hot/cold sources can be used to run a heat engine, hence re-delivering the electricity when it is needed (discharging mode). In the implementation presented here, using the ambient temperature as the hot source for both the heat pump and the heat engine allows taking benefit from daily variations of the atmospheric temperature. Indeed, the heat-pump can operate with a higher Coefficient of Performance (CoP) during the night and the heat-engine at higher efficiency during the day. This results in an effective increase of the ratio between the redelivered electricity (discharge) and the stored electricity (charging), compared to a situation where the hot source of both cycles are at the same temperature. The use of solar collectors is a further way to enhance this ratio by increasing the hot source temperature above the ambient temperature during the day. In charging mode, the system consists in an ammonia based Rankine heat pump interfaced at the evaporator with a latent heat cold storage, and at the condenser to the ambient through aircoolers. In discharging mode an ammonia based Rankine heat-engine is interfaced at the condenser with the latent heat cold storage, and the evaporator(boiler) of the thermal engine is interfaced with hot water produced by a field of flat plate solar collectors. A steady state multiobjective optimization of a 50 MW power output plant was carried out, with the objectives of minimizing the investment costs, and maximizing the ratio between electricity output and input. Several types of cold storage substances have been implemented in the formulation and two different types of solar collector were investigated. A storage efficiency of 57% at a cost of 1200 USD/kW was calculated for an optimized plant using solar energy. Finally, a computation of the behaviour of the plant along the year was carried out, reaching a yearly availability of 84.4%.

An alternative way to process data from polarization measurements for fuel cell model validation is proposed. The method is based on re- and subsampling of IV data, with which repetitive estimations are obtained for the model parameters. This way statistics such as standard deviations and correlations between the parameters may be experimentally derived. Histograms may also be produced, approximating the probability distributions that they follow. Two experimental case studies are discussed. In the first case, observations are made on the behavior of the parameter values for two mathematical models. As the number of data points (measurement points) employed in the estimation of the parameters increases, parameters with high variances converge to specific values. On the contrary, parameters with small variances diverge linearly. The parameters' histograms do not usually follow normal distributions rather they show a connection between the number of peaks in the graphs and correlations of the parameters. The second case study is an application on a fast degraded SOFC button cell, where the values and the histograms of the parameters are compared before and after degradation.

Energy storage systems are increasingly gaining importance with regard to their role in achieving load levelling, especially for matching intermittent sources of renewable energy with customer demand, as well as for storing excess nuclear or thermal power during the daily cycle. Compressed air energy storage (CAES), with its high reliability, economic feasibility, and low environmental impact, is a promising method for large-scale energy storage. Although there are only two large-scale CAES plants in existence, recently, a number of CAES projects have been initiated around the world, and some innovative concepts of CAES have been proposed. Existing CAES plants have some disadvantages such as energy loss due to dissipation of heat of compression, use of fossil fuels, and dependence on geological formations. This paper reviews the main drawbacks of the existing CAES systems and presents some innovative concepts of CAES, such as adiabatic CAES, isothermal CAES, micro-CAES combined with air-cycle heating and cooling, and constant-pressure CAES combined with pumped hydro storage that can address such problems and widen the scope of CAES applications, by energy and exergy analyses. These analyses greatly help us to understand the characteristics of each CAES system and compare different CAES systems.

In CO2 cycles with high-temperature heat sources that are used in applications such as nuclear power, concentrated solar power, and combustion, partial condensation transcritical CO2 (T-CO2) cycles or recompression supercritical CO2 (S-CO2) cycles are considered to be promising cycles; this is because these cycles cause a reduction in the large internal irreversibility in the recuperator owing to the higher specific heat of the high-pressure side than that of the low-pressure side. However, if heat is available in the low-temperature range, the T-CO2 Rankine cycles (or fully-cooled S-CO2 cycles) will be more effective than the T-CO2 Brayton cycles (or less-cooled S-CO2 cycles) and even than the partial condensation T-CO2 cycles (or recompression S-CO2 cycles). This is because the compression work is reduced while achieving the same temperature rise by heat recovery through the recuperator before the high-temperature heater. The proposed T-CO2 Rankine cycles or fully-cooled S-CO2 cycles using both the low- and high-temperature heat sources can maximize the power output of the CO2 power cycle with the given high-temperature heat sources. Moreover, the proposed CO2 cycles combined with the low-temperature thermal energy storage offer the advantage of load leveling over other CO2 cycles, with the given high-temperature heat sources. (C) 2012 Elsevier Ltd. All rights reserved.

Electrochemical and mechanical aspects in solid oxide fuel cell stack must be understood to meet the reliability targets for market implementation. This study presents a stack modelling framework that combines thermo-electrochemical models, including degradation and a contact finite-element thermo-mechanical model. It considers rate-independent plasticity and creep of the component materials and proposes periodic boundary conditions to model the stacking of repeating units. This Part I focuses on the effects of the operating conditions and design alternatives. In the present conditions, the stresses in both the anode and the cathode contribute to the probability of failure ( P f ), which can be lowered by adjusting the operating conditions. The requirements for mechanical reliability are here opposite to those that alleviate electrochemical degradation. Gas-diffusion layers (GDL) and interconnect design alternatives and stacking have a lower impact on the P f , but affect the contact pressure on the GDLs, which can cause electrical contacting challenges.

A dynamic model of a pure-solar combined-cycle power plant has been developed in order to allow determination of the thermodynamic and economic performance of the plant for a variety of operating conditions and superstructure layouts. The model was then used for multi-objective thermoeconomic optimization of both the power plant performance and cost, using a population-based evolutionary algorithm. In order to examine the trade-offs that must be made, two conflicting objectives will be considered, namely minimal investment costs and minimal levelized electricity costs. It was shown that efficiencies in the region of 18–24% can be achieved, and this for levelized electricity costs in the region of 12–24 UScts/kWhe, depending on the magnitude of the initial investment, making the system competitive with current solar thermal technology.

The concept studied in this work is a low power ORC-ORC heat pump system (providing about 20 kW heat at the condenser) and that is composed of an ORC power cycle driving a reversed ORC heat pump cycle, both cycles using the same fluid. The centrifugal compressor and the radial in-flow turbine are directly coupled on the same shaft rotating on self-acting refrigerant vapor bearings. The system has the advantage of being oil-free, fully hermetic and with low maintenance costs. The paper presents the development of an ORC-ORC prototype, with HFC-134a as working fluid. The main critical parts of the system are the compressor-turbine unit, the supercritical evaporator and the pump. The selected type of heat exchanger for the supercritical evaporation is the double tube coil (DTC). A first experimental setup has been built to test the pump and the supercritical evaporator. A comparison between the results obtained with an in-house supercritical evaporator simulation program and measurements made on the DTC is presented. The design steps of the compressor-turbine are briefly presented. The compressor-turbine unit has been balanced and tested, with air, at speeds up to 140'000 rpm.

Due to its suitability for using wet biomass, hydrothermal gasification is a promising process for the valorization of otherwise unused waste biomass to synthesis gas and biofuels. Solid oxide fuel cell (SOFC) based hybrid cycles are considered as the best candidate for a more efficient and clean conversion of (bio) fuels. A significant potential for the integration of the two technologies is expected since hydrothermal gasification requires heat at 673–773 K, whereas SOFC is characterized by heat excess at high temperature due to the limited electrochemical fuel conversion. This work presents a systematic process integration and optimization of a SOFC-gas turbine (GT) hybrid cycle fueled with hydrothermally gasified waste biomass. Several design options are systematically developed and compared through a thermodynamic optimization approach based on First Law and exergy analysis. The work demonstrates the considerable potential of the system that allows for converting wet waste biomass into electricity at a First Law efficiency of up to 63%, while simultaneously enabling the separation of biogenic carbon dioxide for further use or sequestration.

Energy storage systems are becoming more important for load leveling, especially because of the widespread use of intermittent renewable energy. Compressed air energy storage (CAES) is a very promising method for energy storage because CAES relies on existing technologies, is less expensive, and easier to site and permit, as compared to pumped hydro storage. But, in the case of CAES employing hard rock caverns or man-made air vessels, although the smallest possible cavern volume is desirable in order to minimize the construction cost and optimize utilization of the given space, the operating pressure range in the cavern must be limited in order to reduce the deterioration in efficiency of the CAES system at off-design conditions. In this paper, a new constant-pressure CAES system combined with pumped hydro storage was studied to address the current problem associated with the conventional CAES systems. An energy and exergy analysis of the novel CAES system was performed in order to understand the operation characteristics of the system according to several different compression and expansion processes; we then examined the effects of the height of the storage cavern and heat transfer between two media (air, water) and the cavern on the performance of the novel CAES system. (C) 2011 Elsevier Ltd. All rights reserved.

Stirling and Ericsson engines have great potential for many applications, including micro-cogeneration, solar power, and biomass. However, ideal cycles of both types of engines are difficult to achieve in practice because neither isothermal compression nor isothermal expansion is practical with reciprocating piston engines or with turbomachinery. On the other hand, scroll compressor and expander can be very suitable for effective cooling and heating because of the high area-to-volume ratio of scroll geometry or the application of two-phase flow. To achieve quasi-isothermal compression, either a large amount of liquid is injected into the inlet of the compressor or the compressor is externally cooled by liquid. Similarly, for quasi-isothermal expansion, either hot liquid, such as thermal oil, is injected into the inlet of the expander or the expander is externally heated by a heat source. In this current study, we have undertaken a theoretical investigation of thermodynamic analyses of several kinds of scroll-type engines, in particular with regard to associated compression and expansion processes, adiabatic or quasi-isothermal processes, and the highest cycle temperature. We selected power density, or thermal efficiency, as an objective function, and then deduced optimal design parameters for the scroll-type engine.

Following the pioneering work of the AGS Tokyo half project, a geographical information system has been developed to model the energy requirements of an urban area. The purpose of the platform is to model with sufficient detail the energy services requirements of a given geographical area in order to allow the evaluation of the integration of advanced integrated energy conversion systems. This tool is used to study the emergence of more efficient cities that realize the integration of energy efficiency measures, of energy efficient conversion technologies, and that valorize the use of endogenous renewable energy. The model is illustrated with case studies for the energetic planning of the Geneva district (Switzerland).

An optimal planning method is proposed for the renewal planning problem of an energy supply system installed into building from economic viewpoint. In this method, the objective function to be minimized is the average annual total cost during the evaluation period of the objective system based on the annualized costs method, and the renewal year is considered as a decision variable together with the renewal system’s structure, equipment capacities, and system’s operational strategy. This problem is formulated as one of the mixed integer linear programming problem, and a numerical study is carried out for an office building with the total floor area of 15 000m2 by using the GAMS/CPLEX solver. Impact analysis of the carbon tax introduction is mainly studied on the renewal planning mentioned above, and the following results are obtained: a) The optimal renewal strategy is not affected by the future carbon tax introduction at the standard rate such as 655 Japanese yen/t-CO2 argued presently in Japan. b) By increasing the above tax rate more than 13 times higher, structure of the optimal renewal system changes suddenly, and equipment with high energy conversion efficiency becomes to be installed in spite of its expensive initial cost.

Energy storage systems are becoming more important for load leveling, especially for widespread use of intermittent renewable energy. Compressed air energy storage (CAES) is a promising method for energy storage, but large scale CAES is dependent on suitable underground geology. Micro-CAES with man-made air vessels is a more adaptable solution for distributed future power networks. In this paper, energy and exergy analyses of a micro-CAES system are performed, and, to improve the efficiency of the system, some innovative ideas are introduced. The results show that a micro-CAES system could be a very effective system for distributed power networks as a combination that provides energy storage, generation with various heat sources, and an air-cycle heating and cooling system, with a energy density feasible for distributed energy storage and a good efficiency due to the multipurpose system. Especially, quasi-isothermal compression and expansion concepts result in the best exergy efficiencies.

The main driver for small scale turbomachinery in domestic heat pumps is the potential for reaching higher efficiencies than volumetric compressors currently used and the potential for making the compressor oil-free, bearing a considerable advantage in the design of advanced multi-stage heat pump cycles. An appropriate turbocompressor for driving domestic heat pumps with a high temperature lift requires the ability to operate on a wide range of pressure ratios and mass flows, confronting the designer with the necessity of a compromise between range and efficiency. The present publication shows a possible way to deal with that difficulty, by coupling an appropriate modeling tool to a multi-objective optimizer. The optimizer manages to fit the compressor design into the possible specifications field while keeping the high efficiency on a wide operational range. The 1D-tool used for the compressor stage modeling has been validated by experimentally testing an initial impeller design. The excellent experimental results, the agreement with the model and the linking of the model to a multi-objective optimizer will allow to design radial compressor stages managing to fit the wide operational range of domestic heat pumps while keeping the high efficiency level.

The presence of oil in domestic heat pumps is an obstacle toward higher efficiency, particularly for enhanced surface evaporators and for advanced concepts based on two-stage cycles. Very compact direct driven radial compressors supported on oil-free bearings represent a promising alternative. This paper presents the derivation of the specifications, the choice for an appropriate refrigerant fluid and the design of a proof of concept prototype with the various tradeoffs between the impeller characteristics to follow the seasonal heat demand, the bearing and rotordynamics for a stable operation. Heat pump simulation results, the design of the impeller as well as the layout of the experimental facility and first experimental results are presented. An impeller with a tip diameter of 20 mm has been tested at rotational speeds of up to 210 krpm reaching pressure ratios in excess of 3.3 and efficiencies above 78%. The refrigerant chosen for this first experimental approach is HFC 134a.

The locally-resolved degradation behavior was studied during 1900 hours in a SOFC repeat-element. In-situ measurements of local electrochemical performance were made on 18 locations over a segmented anode-supported cell. The evolution of local current densities, overpotentials and area-specific resistances was studied, showing a reorganization of the electrochemical reaction with time. The extent and the spatial distribution of degradation were established for different electrochemical reactions steps using impedance spectroscopy. The low-frequency cathode contribution was the mostly altered process, followed by the charge transfer reaction on anode side. Post-experiment analyses allowed to identify three major pollutants on the cathode side (chromium, silicon and sulfur), whose spatial distributions corresponded to the observed local degradation. Chromium was present only near the air inlet, while silica was found to affect a larger area, with decreasing amounts along the flow. Sources of pollutants were identified in system components as well as within the stack repeat-element.

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation. The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process-modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit. The different layers of the cell, i.e. anode, electrolyte, cathode and compensating layer were considered in the analysis by using the sub-modelling capabilities of the finite-element tool. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cells, operation under both thermal partial oxidation and internal steam-methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets. The results generated by the models are presented in two papers: Part I, focuses on the assessment of the risks of failure of the cell, which was performed by Weibull analysis, while the issues related to the other components are discussed in Part II. Only the anode support contributed to the probability of failure, since the other layers underwent compressive stresses independently of the operating conditions. The cell at room temperature after the reduction procedure was revealed as a critical case. Thermal gradients and the shape of the temperature profile generated during transient operation induced high probabilities of failure. The computed reliability is incompatible with commercialisation, but the scatter induced by the experimental data covers several orders of magnitude. Alternatively, the computed required strength of the anode material to fulfil a probability of failure of 10−2 in a 50-cells stack during steady-state operation appears achievable. Finally, extreme care is required when using the maximum thermal gradient or temperature difference over the SRU as an indicator for cell cracking.

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation. The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit. The different layers of the cell in exception of the cathode, i.e. anode, electrolyte and compensating layer were considered in the analysis to account for the cell curvature. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cell, operation under both thermal partial oxidation and internal steam–methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets. The results generated by the models are presented in two papers: Part I focuses on cell cracking. In the present paper, Part II, the occurrences of loss of gas-tightness in the compressive gaskets and/or electrical contact in the gas diffusion layer were identified. In addition, the dependence on temperature of both coefficients of thermal expansion and Young's modulus of the metallic interconnect (MIC) were implemented in the finite-element model to compute the plastic deformation, while the possibilities of thermal buckling were analysed in a dedicated and separate model. The value of the minimum stable thickness of the MIC is large, even though significantly affected by the operating conditions. This phenomenon prevents any unconsidered decrease of the thickness to reduce the thermal inertia of the stack. Thermal gradients and the shape of the temperature profile during operation induce significant decreases of the contact pressure on the gaskets near the fuel manifold, at the inlet or outlet, depending on the flow configuration. On the contrary, the electrical contact was ensured independently of the operating point and history, even though plastic strain developed in the gas diffusion layer.

Lignocellulosic biomass is considered as one of the most promising feedstocks for producing fuel ethanol because of its global availability and environmental benefits of its use. In this paper, the process of lignocellulosic ethanol production was investigated at its present state of development. The experimental data from the East China University of Science and Technology were used to develop a process model and evaluate the performance of the whole process design. For the process simulation, all relevant information about the process streams, physical properties, and mass and energy balances were also considered. Energy integration is investigated to identify the best ways to supply heat to the process, realizing also combined heat and power production from wastewater and residue treatment. The sensitivity on ethanol yield and the overall system performance are also investigated.

Extending the exergy concept to practitioners and policy makers is still a major challenge. Recently the “Canton of Geneva” in Switzerland introduced a new law governing the procedures of attribution of building permits for new or retrofitted city areas. Authorities were asked to define a procedure including the calculation of an exergy indicator to be quantified in each file concerning large projects submitted for acceptance. This paper summarizes the problem definition, a clarification of the limits expected from the exergy indicator as well as the spreadsheet tool and the tables used to facilitate this quantification both for heating and air conditioning. For simplification the overall system was divided into a superstructure formed by four subsystems including the room convector, the plant of the building, a possible district heating and cooling plant and an external power plant. Three temperature ranges were considered for the building distribution networks both in heating and cooling. Ten different technology combinations were considered ranking from the lowest heating exergy efficiency with nuclear electricity and joule heating to the best efficiency with hydroelectricity and District heating electric heat pumps using lake water.

A planar solid oxide fuel cell repeating unit, 50 cm2 in total active electrode size, consisting of an anode supported electrolyte cell bearing two 7 mm holes for fuel and air injection, and contacted to two dense metal current collector plates via gas distribution layers, was constructed with the aim of measuring local current densities rather than the integral current over the full area. The cathode side was entirely segmented (i.e. cathode layer, gas distribution layer, metal current collector plate) into 8 galvanically separated parts of ca. 6.5 cm2 each, with own current and potential leads.

The consumption of primary energy is often used to compare energy systems from a sustainability viewpoint at regional, national or even world level. For such comparisons to be “fair” and meaningful, this implies however to define primary energies in a coherent way for the various possible sources (coal, oil, gas, nuclear, renewables). This paper stems from the achnowledgement that the definitions presently used in the framework of energy policy studies do not really fulfill this requirement, and suggests approaches to correct such a methodological weakness; the importance of considering exergy as a key concept in this context is particularly emphasized. The authors’ reflections in this matter originated from the recent adoption by the Board of the Swiss Federal Institutes of Technology, soon followed by the Swiss Advisory Committee for Energy Research, of the objective to achieve a “2 kW-society” (2 kW-year/year) in Switzerland by 2050. Such a target, which is about 2.5 times smaller than today’s consumption, invites to better define what such a number concretely means. The concept of useful energy is also briefly discussed and linked to the only really pertinent consideration for the end-user in this context, i.e. the services (comfort, transport) he is looking for.

Reliability of SOFC stacks is a complex and key issue. This paper presents a simulation study including some degradation processes, namely interconnect degradation and the anode reoxidation potential. Quantification for these phenomena has been included in a repeat element model to simulate stack degradation and study the influence of design and operating parameters on the degradation. Interconnect degradation is based on Wagner's law for oxide scale growth, parameters applying to metallic interconnects used in planar SOFCs are used. Anode reoxidation is modeled by thermodynamic equilibrium which allows identification of the operating conditions where the anode is likely to be reoxidized. Simulations have been carried out for a large number of cases at different current density, fuel utilization and furnace temperature, for two different stack designs (base case and modified design). Using an appropriate criterion to express degradation, all these cases point to a clear trade-off between interconnect degradation and local temperature. The base case design is likely to be exposed to anode reoxidation.

Decentralized energy systems are thought to have great potential for supplying electricity, cooling and heating demands of buildings. A decentralized system combining a solid oxide fuel cell (SOFC) with two absorption-chillers and a heat-exchanger is proposed. In order to minimize the CO 2 -emissions and to compute the associated costs, a two-level optimization is performed. On the first level the design parameters of the system are optimized using an evolutionary algorithm, and on the second level the optimal daily operation of the system is optimized using a linear programming algorithm. The results are compared with the CO 2 -emissions and costs of a typical conventional system that obtains the base electricity requirements as well as electricity for an electric chiller/heater system from the central power grid. The fully decentralized SOFC-based energy system could result in a potential CO 2 reduction of 45% at an estimated cost increase of about 290% compared to the conventional system.

The methods for designing, planning and managing integrated energy systems, while holistically considering the major economic and environmental factors, are still embryonic. However, the first phase of the design is often crucial if we want to manage resources better resource and reduce energy consumption and pollution. Considering integrated energy systems implies dealing with complex systems in which the synergy between the various components is best exploited (for example the thermal energy of a diesel engine produced during the night is complimented by the Ranking Organic Cycle of a solar thermal plant). The context of isolated communities further increases the difficulties when considering the long distance of transport required to supply fossil fuels. These sites are often located in very precarious environments, with limited or nonexistent resources except for solar energy, and with frequent additional needs for desalination (in arid zones). This paper illustrates a holistic method to rationalize the design of energy integrated systems. It is based on a superstructure (collection of models of all envisaged technologies) and a multi-objective optimisation (resources, demand, energy, emission, cost) using an evolutionary algorithm. The approach proposed allows the identification of more complete and more coherent integrated configurations characterizing the most promising designs (also taking into account the time dependency aspects). It also allows to better structure the information in view of a participative decision approach. The study shows that the economic implementation of renewable energy (solar) is even more difficult, compared to Diesel based solutions, in cases of isolated communities with high load variations. New infrastructure or retrofit cases are considered.

As energy plays a central role in the world development, it represents as well a major challenge for sustainable development. Today, more than 80% of the primary energy consumption is based on fossil fuels and the share is likely to remain high in the future. Even if technology developments will reduce the specific consumption, the world energy demand is likely to increase in line with its population. Energy and material efficiency and the integration of the renewable resources will therefore have to play a major role for sustainable development. The challenge concerns not only the technologies at the conversion and useful energy level, but also the energy management and infrastructures. The 2000 W per capita society initiative launched by the Board of the Swiss Institutes of Technology targets the identification of the major technological breakthroughs to reduce the per capita primary energy use of Switzerland by two-thirds within five decades. This study examined the energy saving potentials in the complete conversion chain “from primary energy to energy services” in the main sectors of economy: buildings, transport and industry. The report highlights the possible benefits which could be obtained from new materials, new technologies and an intensification of product and capital use. The systems integration appeared to be of prime importance in order to valorise the exergy potentials of the energy resources. The report emphasizes the political responsibility of developing the enabling society infrastructure: not only for the energy distribution (electrical, gas or heating networks) but also for the information technology, the appropriate regulations, the education and for the necessary integration of the lifetimes of the manufactured artifacts in the pathway to the 2000 W per capita society.

Results on solid oxide fuel cell stacks tested at 800° C with H2 fuel and using planar Ni-zirconia anode supported cells, 80x80x0.2 mm in size, are presented. Modeling and numerical simulation is used to interpret observed results and develop improved designs. Where neccessary, the models are calibrated with additional experimental data. Emphasis is placed on the critical issue of nickel anode reoxidation, related to the fuel flow field. Consideration is also given to gradients of temperature and current density developing over the cells and predicted by the models; local current density could be validated by measurement. Flow distribution within stacks is also illustrated by both experimental and modeling results.

A model for planar solid oxide fuel cell repeat elements and stacks has been developed. Distribution of concentrations, reaction rates and temperatures (both gases and solids) are computed as well as overall performance results. Specific experiments provide inputs to the model by a parameter estimation method. The modeling approach developed allows to compare several configurations. As the number of design parameters is large (from cell size, component thicknesses to gas flow configuration), the model is designed to change easily these parameters so as to explore as many cases as possible. This is particularly true for the flow configuration (inlet position, outlets) for which several options are considered. This model assists in choosing a configuration and allows to perform sensitivity studies in an efficient way (without having to produce a new mesh such as for CFD tools) or to be combined with an optimization tool. A first validation with experimental results, performed on a particular stack design, is presented. Issues of model accuracy and sensitivity to uncertain inputs are discussed.

A model for a 1000 kW class solid oxide fuel cell (SOFC) system running on biogas from a sewage sludge digestion plant was implemented in a process flow scheme using external steam reforming. The model stack consisted of planar anode supported cells operated at 800 degreesC displaying state-of- the-art electrochemical performance (0.15 W/cm(2) at 80% fuel utilisation). Real annual data from an existing sewage plant were used as input to the model. From the input of 43 m(3)/h biogas (63% CH4), equivalent to 269 kW (higher heating value, HHV), the SOFC stack was calculated to deliver 131 kW,l electricity (48.7%) using a steam-to-carbon ratio of 0.5. This would allow the sewage site to more than cover its own electrical needs, hence to depollute the waste stream at negative energy cost. In its current exploitation using a low efficient gas engine (130 M), the site is only approximate to50% self- sufficient. Special attention was given to the thermal balance of the stack. The stack developed heat (143 kW) could be balanced by endothermal reforming (78 kW) and by cathode excess air lambda (=3), allowing a temperature difference between stack inlet and outlet of 200 K. The case was compared to other fuel scenarios. Steam-added biogas behaves basically identically to steam-reformed methane. For partial oxidation of biogas or pure hydrogen feeding, electrical efficiency drops to under 43% while needs to be raised to 4.5 to maintain the 200 K thermal gradient over the stack.

A 3D simulation tool for solid oxide fuel cells is presented. The aim of this work is to predict current density, flow, temperature and concentration fields in order to compare and optimize repeat element geometry for a whole stack. A commercial CFD tool was used, solving mass, momentum and energy equations; whereas chemical kinetic equations are computed from external sub-routines. A steady-state case is presented, fed with hydrogen. The flow is laminar for both air and fuel. Radiative heat transfer is taken into account between inner surfaces. On boundaries, convective and radiative heat transfers are assumed at external surfaces between repeat element and oven. Due to the large range of dimensions (cells: 300 mum thick, gas channels: 1 mm height, whole cell: 80 mm x 80 mm) a fine mesh was needed. Data for conductivities and kinetics were estimated from experiments performed in- house. Simulation results are presented and compared to real repeat element test measurements for the current-potential characteristics.

Three perovskite-type compounds of composition La0.4Ca0.6Fe1-xCoxO3-d (x=o, o.25 and 0.5) were investigated for use as oxygen separation membranes for the partial oxidation (POX) of methane to syngas. Special attention was given to the question of their stability in real operating conditions. A permeation set-up was specially designed to measure oxygen fluxes through these materials when placed in a strong pO2 gradient. It also facilitated testing the long-term stability of the specimen. Permeation measurements performed in an air/argon gradient between 800 and 1000 °C showed that the highest fluxes were obtained with the highest content of cobalt (La0.4Ca0.6Fe0.5Co0.5 O3-d =La0.4Ca0.6Fe0.75Co0.25 O3-d >La0.4Ca0.6FeO3-d ). In addition, comparison between the fluxes of samples of different thickness gave clear evidence of surface limitations in the oxygen transport. The long-term stability test showed opposite trends: only the two lowest Co containing compounds (x = 0 and 0.25) sustained and air/(Ar + H2) gradient over more than 600 h. The other (x = 0.5) broke shortly after the introduction of H2. In the presence of H2, the oxygen flux was increased by a factor 10 compared to Ar and reached 0.83 mmol/cm2s for La0.4Ca0.6Fe0.75Co0.25O3-d at 900°C. Post-operation SEM examination of the cross-section and both surfaces revealed that the surface exposed to H2 had started to decompose resulting in the formation of a thin porous layer but the bulk of the material remained unchanged.

A small cogeneration system based on a Solid Oxide Fuel Cell (SOFC) fed on the renewable energy source biogas is presented. An existing farm biogas production site (35 m3 per day), currently equipped with a SOFC demonstration stack, is taken for reference. A process flow diagram was defined in a software package allowing to vary system operating parameters like the fuel inlet composition, reforming technology, stack temperature and stack current (or fuel conversion). For system reforming simplicity, a base case parameter set was defined as the fuel inlet of 60% CH4:40% CO2 mixed with air in a 1:1 ratio, together with 800 8C operating temperature and 80% fuel conversion. A model stack, consisting of 100 series elements of anode supported electrolyte cells of 100 cm2 each, was calculated to deliver 3.1 kWel and 5.16 kWth from an input of 1.5 N m3/h of biogas (8.95 kW LHV), corresponding to 33.8 and 57.6% electrical and thermal efficiencies (Lower Heating Values (LHVs)), respectively. The incidence on the efficiencies of the model system was examined by the variation of a number of parameters such as the CO2 content in the biogas, the amount of air addition to the biogas stream, the addition of steam to the fuel inlet, the air excess ratio l and the stack operating temperature, and the results discussed.

The effects and consequences of transferring the ignition point of the conventional combustion chamber into a small unscavenged prechamber were evaluated through a numerical simulation based on the CFD-code KIVA-3V. The prechamber flow characteristics at the location of the gap between the spark plug electrodes and in the crank angle period where the ignition is expected to occur were compared to the corresponding conditions in the conventional combustion chamber. The influence of the prechamber geometrical configuration was evaluated through variations of the nozzle orifice diameter, number and orientation, as well as prechamber volume and internal shape. The results show that the velocity magnitude is mainly dependent on the prechamber shape, and that it is generally of the same order as with direct ignition. Further, the turbulence intensity varies strongly with the geometrical configuration and reaches, in most cases, a much higher value in the prechamber. The partial dilution of the unburnt mixture with unscavenged prechamber residual gas generally leads to a slightly lower fuel to air equivalence ratio. However, nozzle orifices imparting a swirl motion or a prechamber shape with an almost uniform cross section can result in fuel concentration very close to or beyond the flammability limit. The prechamber charge and thereby the amount of energy available for the main chamber ignition depends on the prechamber volume and on the pressure drop across the nozzle orifices.

LENI activities in SOFC are focused on development of a new planar stack concept based on anode supported cells in collaboration with the company HTceramix SA. Modeling of repeat element, stack and systems is starting in parallel. This work presents a model of a SOFC stack in cross flow configuration. Concentrations, temperature and current density fields are computed, with a focus on sensitivity study for thermal parameters, while operating parameters are kept constant for the different cases. The main parameter considered was the thickness of the metallic interconnect and the related thermal conductivity. Stack height and cell size effects are presented as well. Importance of the temperature field on the design point and degradation behavior is briefly discussed.

Repeat element and short stack development is presented using ASE (anode supported electrolyte) cells for 0.5 kW/L.