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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.

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.

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.

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%.

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.

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.

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.

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.

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.

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 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.

The operation of a cogeneration internal combustion engine with unscavenged prechamber ignition was investigated. The objective was to evaluate the potential to reduce the exhaust gas emissions, particularly the CO emissions, without exhaust gas after treatment. The investigation was carried out on a small size gas engine (150kW) and required the development of cooled prechambers and the modification of the engine cylinder heads. The limit of the conventional lean burn operating mode with direct ignition is discussed and the prechamber geometrical configuration is presented. Through the generation of gas jets in the main chamber, the use of a prechamber strongly intensifies and accelerates the combustion process. The prechamber operation reduces significantly the emissions of CO and THC for same NOx emissions. The use of a piston generating significantly more turbulence leads to a somewhat higher fuel conversion efficiency without affecting significantly the CO and THC emissions at low NOx emissions. Further improvement associated with the adjustment of the engine operating parameters and the turbocharger characteristics, as well as a comparison between direct and prechamber ignition operation are presented in the second part (II) of this publication.

In a first paper (part I), prechamber ignition in cogeneration natural gas engines has been shown to significantly intensify and accelerate the combustion process, offering a further potential to reduce the exhaust gas emissions while keeping efficiency at a high level. This second part discusses the influence of the engine operating parameters (spark timing and load) and the turbocharger characteristics with the objective of evaluating the potential to reduce the exhaust gas emissions, particularity the CO emissions, below the Swiss limits (NOx and CO emissions: 250 and 650 mg/m3N, 5% O2, respectively), without exhaust gas after treatment. The advantage of using an unscavenged prechamber is conditioned by a significant delay of the spark timing in order to generate substantial gas jets. This results in a large decrease in peak cylinder pressure and in an important reduction of NOx, CO and THC emissions. Minimum emissions are achieved at a spark timing of about 8°CABTDC . In comparison with the direct ignition, the prechamber ignition yields approximately 40% and 55% less CO and THC emissions, respectively. However, this also leads to about 2%-point lower fuel conversion efficiency. The optimisation of the turbocharger results in a recovery of about 1%-point in fuel conversion efficiency, but a consequent change in the exhaust manifold gas dynamics attenuates the reduction in THC emissions. At the rated power output (150kW), the prechamber ignition operation fulfils the Swiss requirements for exhaust gas emissions and still achieves a fuel conversion efficiency higher than 36.5%.

Although heat pump based district heating is often an obvious solution from an energy standpoint, adapting the delivery temperature to the most exigent users is detrimental to overall system performance. This pitfall can be avoided with a centralized plant of heat pumps, cogeneration units and an auxiliary furnace, supplemented by decentralized heat pumps. However, the problem of mixed energy production and delivery which this poses is complex and presents for the engineer the daunting if not impossible task of adequately, much less optimally, determining the best system for the job. In this first of a series of two articles, an environomic methodology for aiding in this task is described and the details of the environomic model for a district heating network based on centralized and decentralized heat pumps presented. This methodology is used to model the thermodynamic, economic, and environmental characteristics of such a system in order that its final configuration and corresponding component designs can be optimized. In the accompanying article [1], a complete set of results for the optimal synthesis, design and operation of the network is given and discussed. The resulting solution space is highly nonlinear, non-contiguous and is effectively searched using a genetic algorithm. The system’s environmental characteristics are introduced into the model through pollution damage cost terms and pollution penalty functions which adapt to the system’s changing emissions and to local and global pollutant conditions. Results are shown for various district heating user distributions and fuel and electricity prices. The approach presented is an attempt to respond at the synthesis, design and operational level of an energy system to the concept of sustainablility.

Steam production units (HRSG and HSSG) of ISCCS include several heat exchangers (economizers, evaporator, superheater, reheaters, etc.). The knowledge of the extended composites as a function of the solar input, allows the determination of the most critical zones for heat transfer but does not allow, in itself, the full knowledge of the real streams needed to be able to design an optimum heat exchanger network. The procedure proposed in this paper permits, from so called interaction factors which characterize the interdependancy between streams, the determination of the massflows in each stream. The choice of the best heat exchanger network must respect, for each operational conditions, the optimum evaporation levels (including pressures et temperatures) determined in part I, as well as the particular practical operational factors (independance or not between the various heat recovery units, etc.). The network design is done using the standard guidelines of pinch technology (respect of the minimum pinch DTmin for each heat exchanger close to the pinch temperature, separate design of the zone above and below the pinch temperature, etc.). The respect of the DTmin in the critical zones of heat transfer requires the use of stream splitting and the network includes heat exchanger tubes which are interlaced at the same level of the stack. One example of the best performing power plant designed on the basis of this approach is given.

Integrated solar combined cycle systems (ISCCS) represent, both economically and energetically, a promising alternative for the conversion of solar energy while offering a guarantee of a minimum power supply independant of the level of solar radiation. Their performances are however strongly dependant on the intensity of the solar input. The approach proposed in this paper allows, from the characteristics of the turbines (gas turbines and steam turbines) and of the solar field, to rationalize the choice of the pressure levels and of the massflows of a steam cycle with multiple pressure levels. It is based on the coupling of a pinch technology approach with a thermodynamic modeling allowing an optimisation with deterministic algorithms. Results are applied to a dual pressure steam cycle and accounts for the respect of the "cone law" for the steam turbines. It is shown that an increase of the exergetic losses linked to heat transfer in the steam generators is inevitable at certain operational regimes and depends directly on the level of solar supply. The variations of the main steam cycle parameters as a function of the thermal supply (combustion gases + solar thermal oil) are shown for an 80 to 120 MWel power plant equipped with two gas turbine and one steam turbine train.

The integration of a solar collector field generating steam into a conventional combined cycle in order to partially replace the fossil fuel required by the latter results in a substantial reduction in greenhouse gases, in an increase in the return on investments associated with the solar field and in an almost complete elimination of the need for solar energy storage. This paper discusses the design of such an integrated hybrid solar-fossil combined cycle with maximum daily and nightly power outputs of 88 MWe and 58 MWe, respectively. This cycle is currently being evaluated from a technical and economic risk feasibility standpoint for possible implementation as a pilot plant in Tunisia. This paper outlines pertinent design considerations utilized in the thermoeconomic optimization approach employed for developing the hybrid combined cycle proposed here. The approach shows that there are several advantages to this type of design when compared with a purely solar steam cycle or any of the several other hybrid solar concepts which exist today. In addition to these advantages, the design presented revolves around the definition of a number of degrees of freedom which allow the solar energy part of the cycle to be highly integrated into the conventional part. A discussion of them is given. Finally, from an environmental standpoint, the obvious advantage of this type of cycle is that due to the substitution of fossil fuel, there is a marked mitigation in CO2 and NOx emissions when compared to a conventional cycle and to other hybrid concepts. Pertinent results for these reductions are presented.

Lifetime prediction and improvement of solid oxide fuel cell (SOFC) devices require a reliable electrochemical model that supports the implementation of degradation phenomena. This study comprises two parts. This Part I describes the calibration of an electrochemical model based on physical principles for simulation at the stack scale. Part II presents the further implementation of degradation models. A distinction is made between the two most common cathode materials, lanthanum strontium manganite and lanthanum strontium cobalt ferrite. The experimental data used for the parameter estimations was gathered by two segmented setups. The calibrations enabled to reproduce adequately the measurements over a wide range of operating conditions. The optimal values of the physical parameters were inside the ranges reported in literature. Unambiguous discrimination could not be achieved between variations (i) in the choice of electrode rate-determining steps, (ii) data on the properties of the materials found in literature and (iii) empirical relations for the steam-methane reforming reaction. However, these model variations do not affect significantly the predicted magnitudes and distributions of the field variables assumed to govern the degradation processes at the SRU scale, compared with the uncertainties on the degradation phenomena to be implemented in Part II. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596433] All rights reserved.

Currently the herringbone grooved journal bearing (HGJB) has applications in the domain of small, low power energy related applications, where high rotational speeds are required for reaching reasonable efficiency and where standard oil lubricated bearings are limited or even unable to operate over the life time required. Furthermore the presence of oil requires auxiliary control systems reducing the overall system efficiency and reliability. High-speed rotors running in hermetic units processing refrigerant vapor require the bearings to operate with this same gas at thermodynamic conditions that are potentially very close to the saturation curve. In this region the usually applied perfect gas theory does not yield valid results when applied to the Reynolds-lubrication equation. Furthermore, depending on the processed gas and its thermodynamic condition, the Knudsen number may reach values where it is advisable to take gas rarefaction effects into account. In applications where the rotor is heavily loaded in terms of mass and inertia, the bearing design highly affects the dynamic stability of the rotor. In order to reach good stability margins, the bearing geometry tends towards low clearances leading to higher power losses. It has to be noted that bearing losses not only deteriorate the overall efficiency, but also as the unit size are getting smaller and smaller, cooling becomes an issue. The narrow groove theory (NGT) is modified in order to take into account the rarefied gas as well as the real gas effects depending on the thermodynamic and physical properties of the processed gas and on its thermodynamic state. The bearing module implementing the modified narrow groove theory allows calculating the stiffness and the damping matrices for a given bearing geometry. It is then linked to a rotor dynamic model that enables to calculate the critical speeds and the corresponding dynamic stability for a given rotor supported on herringbone grooved dynamic gas bearings. The latter module is linked to a multi-objective optimizer based on evolutionary algorithms. In this paper the evolutionary optimizer is used for two objectives: maximizing the stability margin and minimizing the bearing power loss. The optimizer yields in a Pareto curve representing a family of optimum solutions. One has the choice between a solution with a high stability margin but with high bearing losses to get dissipated or vice versa. A low power rotor for a refrigerant gas process illustrates the optimizing procedure discussed in this paper.

The implementation of integrated energy systems within urban areas is a promising CO?2 emissions abatement measure. In this paper on-site heating, cooling and power generation based on a solid oxide fuel cell and gas turbine (SOFC-GT) combined cycle unit associated with a compression chiller and additional boilers has been considered from the viewpoints of cost and CO?2 emissions. Physical and costing modelling of such a unit has been integrated withn a new multi-criteria evolutionary algorithm for an assessment of the economic and environmental performances associated with optimal design and operation, for typical requirements of large office buildings in Tokyo.

This paper aims to illustrate a new holistic modeling environment through a case study implementing a district heating system to supply four groups of users identified in the city of Tokyo. The holistic modeling environment, called DOME (Distributed Object-based Modeling Environment) is being used by researchers in the Alliance for Global Sustainability (AGS) to develop a platform for assessing technologies and policies in order to significantly reduce CO2 emissions in large cities. DOME is used as an information based collaboration platform for supporting the decentralized integration of models built independently under various application tools and operating systems. It provides system engineers with the ability to access experts’ models over the Internet through user-friendly interfaces. The interfaces provide clearly identified input and output variables to support a facilitated integration. The DOME environment also provides tools for analyzing the resultant integrated system. In this district heating simulation, a GIS (geographic information system) based land-use database manager, implemented as an object in DOME, is used to obtain the total ground area, the total building floor area, and the number and type of buildings in a specified region of Tokyo. Using the land use database manager, four groups of users are established. The heat demands of the users are derived using specific heat consumption data for the types of buildings considered. Users are either connected to the district heating system through direct heat exchangers on the supply or return line, or through local heat pumps which adapt the temperature to the needs locally, allowing a lower network temperature. Two different options are considered for the district heat supply: 1) a heat pump based superstructure comprising a gas turbine and a gas engine as cogeneration units, as well as an auxiliary boiler for peak load, and 2) a gas turbine / steam turbine combined cycle based superstructure with various options like steam injection and/or CO2 separation unit. Physical and costing models for the two alternative production unit superstructures and for the district heating system are connected with the GIS database manager using DOME. The environomic methodology, a simultaneous consideration of thermodynamic, economic and environmental criteria expressed as an aggregated cost, is used to assess the alternatives under different scenarios characterized by the cost of natural gas and the degree of pollution cost internalization.

The technology gap between liquid and gaseous fuels presents an obstacle in realising the environmental and performance potential of CNG vehicles. Therefore a gaseous fuel injector has been developed to cope with the considerable larger volume flow rated required for a gaseous fuel without additional penalties in response time. This injector has demonstrated a significant potential to reduce emissions from natural gas powered engines during transient operation.

The degradation of the electrochemical performance of solid oxide fuel cell (SOFC) devices is a major hurdle to overcome before commercialisation. The interplay between the phenomena and the long testing times complicate the research, which highlights the relevance of modelling to propose mitigation approaches. This study comprises two parts. This Part II proposes approaches for the simulation of the degradation induced by: (i) interconnect corrosion, (ii) loss of ionic conductivity of the ion-conducting materials, (iii) nickel particle growth in the anode, (iv) chromium contamination and (v) formation of insulating phases in the cathode. The literature survey highlights the lack of data for a completely consistent calibration of the models, despite the simplifications. The support for the implementation is the electrochemical model validated in Part I and a two-dimensional model of the cell and interconnection system. The cathode largely contributes to the degradation. The local overpotential predominantly governs chromium contamination, which can promote the formation of insulating phases, as operation proceeds. The local electronic current density has comparatively a weak direct influence on the degradation. Qualitative agreement with experimental data from the literature could be achieved, without dedicated adjustments of the parameters. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596435] All rights reserved.

Solid oxide fuel cell (SOFC) stack design must yield the highest performance, reliability and durability to achieve the lowest cost of electricity delivered to end-users. Existing modelling tools can cope with the first aim, but cannot yet provide sufficient quantitative guidance in the two others. Repeating unit models, with as degradation processes the decrease in ionic conductivity of the electrolyte, metallic interconnect corrosion, anode nickel particles coarsening and cathode chromium contamination are used to investigate their distribution, evolution and interactions in a stack. The spatial distribution of the degradation is studied for the operating conditions optimised in Part I for the highest system electrical efficiency during long-term operation under constant system power output. Current-voltage characterisations performed at different times underestimate the degradation. In the present conditions, the degradation of the cathode dominates. The lower and more uniform cathode overpotential in counter-flow configuration, combined with the beneficial effect of internal reforming on reducing the air-fuel ratio yields the highest lifetime, because it alleviates chromium contamination and interactions between the degradation processes. Increasing the operating temperature alleviates cathode chromium contamination. The beneficial decreases of the cathode overpotential exceed the detrimental higher release rate of chromium species from the metallic interconnect. (C) 2012 Elsevier B.V. All rights reserved.

The energy integration of industrial processes is becoming increasingly more effective thanks to new methodological developments such as pinch technology. This paper aims at extending the number of factors considered in pinch analysis towards a life-cycle optimisation and proposes new synthesis representation schemes. The original pinch method centres primarily on maximizing the internal heat transfer with the choice of appropriate ΔTmins. The proposed extension takes into account the complete heat transfer exergy losses, the pressure drop exergy losses and the exergy associated with the fabrication of the heat exchangers. The extended composite curves graphically represent the above-mentioned losses on a Carnot factor versus heat rate diagram. In a similar way, other high exergy inputs and outputs linked, for example, to the introduction of heat pumps and cogeneration units, are represented on a topping electricity versus Carnot factor diagram. Such an extended exergy synthesis results in an improved and more coherent exergy balance for comparing energy recovery schemes. It offers a new insight and permits the identification of solutions which are more stable in time and fairly independent of changing economic conditions. The proposed approach is suitable for future extension to include pollution and resource scarcity factors.

In urban areas, the use of thermal energy services is intensive and relatively heterogeneous by their nature, spatial distribution and temperature level required. In this study three concepts of district energy networks that could provide these services very efficiently to a specific area representative of mid Europe (Geneva) are compared. The focus is on the energy and technological aspects related to each of the proposed networks. These networks are characterized by similar temperature levels; between 9.5°C and 18°C, rely on free cooling for most of the cooling services and use a combination of centralized and decentralized heat pumps to provide the heating services. Two of these concepts exploit the latent heat of evaporation/condensation of CO2 and of the HFO refrigerant R1234yf to store and transfer heat across the network. The third concept is more conventional as it uses the sensible heat of liquid water, however with a small temperature spread (2 - 8 °C). The three proposed networks allow the waste heat emitted by the users requiring cooling to be collected, transferred and valorised by the users requiring heating, thus reducing the load on the central plant. For the area considered, where the annual heating and cooling demand are 53.1 GWh and 49.4 GWh respectively, the annual final energy (electricity) consumption required to supply the thermal services amounts to 10.87 GWh for the CO2 variant, 10.52 GWh for the water variant and 9.60 GWh for the R1234yf variant.

District energy systems can potentially decrease the CO2 emissions linked to energy services, thanks to the implementation of large polygeneration energy conversion technologies connected to buildings over a network. To transfer the energy from these large technologies to the users, conventional district energy systems use water with often two independent supply and return piping systems for heat and cold. However, sharing energy or interacting with decentralised heat pump units often results in relatively large heat transfer exergy losses due to the large temperature differences that are economically required from the water network. Besides, the implementation of two independent supply and return piping systems for heat and cold, results in large space requirements in underground technical galleries. Using refrigerants as a district heating or cooling fluid at an intermediate temperature could alleviate some of these drawbacks. A new system has been developed, that requires only two pipes, filled with refrigerant, to meet heating, hot water and cooling requirements. Because of the environmental concerns about conventional refrigerants, CO2, a natural refrigerant, used under its critical point, is considered an interesting candidate. A comparative analysis shows that both in terms of exergy efficiency and costs the proposed CO2 network is favourable.

This paper illustrates a methodology developed in order to facilitate the analysis of complex systems characterized by a large number of technical, economical and environmental parameters. Thermo-economic modeling of a natural gas combined cycle including monoethanolamine absorption CO2 separation option has been integrated within a multi-objective optimizer based on a genetic algorithm in order to characterize the economic and environmental potential of such complex systems within various contexts. A natural gas combined cycle project in a district of Germany is given as a case study. The results show the influences of the configuration and technical parameter changes on the evolution of electrical efficiency of the combined cycle plant as well as on those of its sub-systems, such as gas turbine cycle and steam cycle. The optimum integrations of such a complex system under different situations are revealed by the Pareto Optimal Frontier obtained through the multi-objective optimization process, which provides information on the relationship between power generation cost and CO2 emission performances. Such information is of direct relevance for policy makers to define coherent emission tax levels, or for utility owners or project investors to choose the appropriate emission levels to be reached by the new plant, or for power generation technology suppliers to identify the market potential of their products as well as the most appropriate design for a given power unit, under given policies and economic contexts.

Different types of energy storage systems are described, in particular hydro-pneumatic storage systems. In one, energy is stored by compressing gas in a chamber (44,45,54,55) with a liquid piston and released by gas expansion. A spray head or grid at the top of the chamber (44,45,54,55) supplies liquid as a shower through the gas being compressed or expanding in the cylinder (11,12) to maintain an isothermal condition. In another, energy is stored from an array of solar cells connected to an array of supercapacitors forming an auxiliary storage, and a main energy storage device such as a hydro-pneumatic storage system, for supply to an AC or DC network. The efficiency is improved by connecting the solar cells via the array of supercapacitors to the AC or DC network. An immersed hydro-pneumatic storage device for off-shore/on-shore power generation systems comprises a cylinder that is immersed in a liquid mass, wherein energy is stored by compressing gas with a liquid piston and energy is released by gas expansion. The mass of liquid maintains an isothermal condition in the cylinder during compression and expansion.

Decentralized power generation and cogeneration of heat and power is an attractive way toward a more rational conversion of fossil or bio fuel. In small scale power production fuel cell – gas turbine hybrid cycles are an emerging candidate to reach higher or comparable efficiency than large scale power plants. In spite of the advantages of this hybrid technology, many technical barriers have to be overcome to develop a highly efficient system. The present contribution introduces an innovative concept of hybrid cycle that allows to reach high efficiency maintaining the fuel cell operating under atmospheric condition and thus avoiding fuel cell pressurization technical problems. Carbon dioxide separation represents an additional advantage. A thermodynamic optimization approach, based on the system energy integration, is used to analyse several design options. The methodology proceeds in two steps: modelling the system for a set of decision variables and optimizing their values. The innovative system is analyzed and compared with state of the art fuel cell - gas turbine hybrid cycle. Exergy analysis has been performed. Optimization results prove the existence of designs that, neglecting the pressure drops, achieve exergy efficiency higher than 75%.

Concern for the environment has been steadily growing in recent years, and it is becoming more common to include environmental impact and pollution costs in the design problem along with construction, investment and operating costs. To economically respond to the global environmental problems ahead, progress must be made both on more sustainable technologies and on the design methodology, which needs to adopt a more holistic approach. Heat pumps and, in particular systems integrating heat pumps and cogeneration units, offer a significant potential for greenhouse gas reduction. This paper illustrates the application of a multi-objective and multi-modal evolutionary algorithm to facilitate the design and planning of a district heating network based on a combination of centralized and decentralized heat pumps combined with onsite cogeneration. Comparisons are made with an earlier study based on a single objective environomic optimisation of the same overall model. The advantage of the two objective approach is illustrated with the concurrent minimisation of CO2 emissions and heating cost for the given yearly requirement of part of a city. For the given 62.7 kW thermal rated plant meeting the needs of a set of new buildings with predominantly low temperature heating systems, Pareto curves illustrate the tradeoffs as well as the optimum supply temperature trends. The evolution of the optimum configuration of the central cogeneration and heat pump plant as a function of the CO2 emissions is also shown.

The design of direct driven turbomachinery is an interdisciplinary task. Standard design procedures propose to split such systems into subcomponents and to design each one individually. This common procedure, however, tends to neglect the interactions between the different components leading to suboptimal solutions. The authors propose an approach based on the integrated philosophy for designing and optimizing gas bearing supported rotors. Based on the choice for herringbone grooved journal and spiral groove thrust bearings, the modeling procedure for predicting their properties and the linking to the rotordynamic behavior of a generic rotor supported on gas lubricated bearings is provided. The global model for gas bearing supported rotors is linked to a multiobjective optimizer for maximizing the dynamic stability and for minimizing the windage losses of the rotor and of the bearings. Two typical rotor layouts have been included in the optimization. The geometry of a proof of concept system, that has been designed previously using the fragmented component view, is represented as a comparison to the proposed integrated approach. It is shown that the integrated solution allows to reduce the windage losses by 25% or to increase the stability margin by 35%, emphasizing the advantage of the proposed integrated design tool. [DOI: 10.1115/1.4001381]

This paper introduces a novel of mini-hybrid solar power plant integrating a field of solar concentrators, two superposed Organic Rankine Cycles (ORC) and a (bio)Diesel engine. Turbines for the organic Rankine Cycles are hermetic scroll expander-generators. Sun tracking solar collectors are composed of rows of flat mirror bands (CEP) arranged in a plane, which focus the solar energy onto a collector tube similar to those used in SEGS plant in California. The waste heat from both the exhaust gases and block cooling of the thermal engine are also heat sources for the ORCs. Such units are considered to meet electricity, cooling and pumping needs of remote settlements in the sun belt areas or on special sites requiring low temperature heating such as swimming pools or greenhouses. The thermal engine guaranties a minimum level of both power and heat availability at night or during cloudy periods. Laboratory tests, made with the superposed ORCs only, confirmed adequate operational characteristics with good performances over a broad range of conditions. A few preliminary tests on the site of the solar power plant when coupled with the engine confirmed a reasonable behavior and the interest of the concept even at part load or during sharp variations of the thermal supply. The performances are satisfactory for a hybrid solar plant of this power range (10 to 25 kWe) and working so far with rather low temperature solar supply (165°C). First Law efficiencies of conversion in hybride mode is of the order of 41% when considerating only the fossil fuel input (total electrical power / LHV of the fuel). This already represents an efficiency increas of close to 50% compared to the Diesel engine alone. These results are encouraging particularly when one considers that excellent prospects exist for further increasing the supply temperature. However due to an over sizing of the turbines and a solar collector efficiency lower than expected, the tests reported here were made at very partial load and the efficiency in mode "solar only" was limited to 7.74%.

This paper presents the experimental investigation of a concept of Thermally Driven Heat Pump (TDHP), based on the coupling of a vapor compression heat pump cycle and an Organic Rankine Cycle (ORC). The studied concept uses a single stage centrifugal compressor directly coupled to a single stage radial inflow turbine. The shaft is rotating on gas bearings, which allows the system to be oil-free. Like most of the other TDHP's, this system has the advantage to work with a variety of fuels or heat sources like wood pellets, natural gas, solar heat, geothermal heat or waste heat. A prototype using R134a as working fluid and with a heating capacity of about 40 kW has been developed. The results of the first tests are presented. (C) 2014 Elsevier Ltd and IIR. All rights reserved.

Test of the unscavenged prechamber ignition system on a 150kW cogeneration engine fed with biogas are presented. The objective was to reach at least 36% in fuel conversion efficiency with exhaust gas emissions lower than the Swiss regulations limits (NOx, CO: 400 and 650mg/Nm3, 5% O2 and 0% relative humidity) without exhaust gas after-treatment. With a compression ratio of 12.0, it is shown that 40% of CO2 induces a CO reduction of 15% and an HC reduction of 8% for the same NOx emissions but with a slight reduction of 1% in fuel conversion efficiency compared to pure natural gas. The results for the compression ratio of 13.3, a spark timing of 8 caBTDC, an air factor ?l of 1.57 and 39,5% CO2 are a fuel conversion efficiency of 37.7% while keeping the NOx and CO emissions below the Swiss limit. This study shows that using unscavenged prechambers in biogas engines is particularly beneficial to reach interesting conversion efficiencies with low emissions. It allows a significant boost in energy conversion from biogas compared to existing systems.

The conceptual design of a thermo-electric energy storage (TEES) system for large scale electricity storage is discussed in this work by showing the results of the thermoeconomic optimization of three different system configurations that were identified in previous works. The system is based on transcritical CO2 cycles, water storage and salt-water ice storage and is designed for a capacity of 2 h discharge and equal charge and discharge power of 50 MW. A two-step optimization procedure is used. The system intensive design parameters are optimized at the master level through a genetic algorithm. The optimal cycle mass flow rates are calculated in a nested linear programming step where the heat integration between the cycles is optimized subject to the heat transfer feasibility imposed through Pinch Analysis cascade calculations. The synthesis of the heat exchanger network and of the storage tank systems was solved through a set of heuristic rules. Equipment purchasing costs were estimated by means of cost functions that were built upon vendors quotations. The results are discussed by showing the Pareto fronts of the three optimization cases and the trends of the decision variables along each optimal front. Nine solutions are discussed in more detail by showing the values of the design parameters and the process flow diagrams including storage and heat exchanger layouts. Design guidelines are then formulated which can be used in future works for detail plant design. The topological features that are found to maximize the system performance at the minimum costs are: superheating before the CO2 heat pump, two independent systems of hot water storage tanks above and below the ambient temperature, and air cooling at the heat pump side. The design parameters that affect significantly the costs and performances are the cycle pressures. These are in fact directly associated with temperature differences both at the cold and hot storage sides which should be carefully optimized to obtain the best trade-off between exergy losses and costs for heat exchangers. Due to the change in specific heat of the supercritical CO2 along the temperature range of hot water storage, a system of multiple storage tanks was used. Intermediate storage tanks help reduce significantly the temperature differences at the hot storage side and therefore their number represent another critical design parameter that must be optimized to achieve the best trade-off between costs and performances.

Combined cycle power plants are currently one of the most important options for the construction of new generating capacity as well as for the replacement and repowering of existing units. Due to the complexity and the large number of options and parameters available to such plants, finding optimized solutions for system synthesis, design, and operation is very difficult if not impossible with these traditional methods such as case and parametric tradeoff studies. This is especially true when advanced options as well as thermodynamic, economic, and environmental criteria are considered. A thermoeconomic environomic methodology to deal with these difficulties is presented here. Results for the application of this methodology to a 50 MW cogeneration combined cycle power plant are presented and discussed.

Driven by applications using herringbone grooved bearings lubricated with a gas operating under conditions where the perfect gas assumption is not valid, the real gas effect has been introduced into the existing narrow groove theory. The enhanced model has been linked to a rotordynamic code and to a multi-objective optimizer resulting in optimum bearing geometries that differ from the ones under the perfect gas assumption. It is suggested that rotors on bearings that have been optimized under the perfect gas assumption may get unstable if they are submitted to a lubricant that operates close to the saturation line. (C) 2010 Elsevier Ltd. All rights reserved.

A novel ignition concept based on autoignition in an unscavanged prechamber is currently being developed at the Laboratory for Industrial Energy Systems (LENI). On a single cylinder test engine a series of experimental runs (CR=8.5-14, =1-1.6, RPM=1150/1500 min−1) have been realized with natural gas as fuel, comparing the new ignition concept to standard spark ignition. The comparison is based on fuel efficiency and exhaust emissions (CO, THC, NOx). The feasibility of operating the engine in auto-ignition mode has been demonstrated, and the potential of prechamber autoignition, in particular in the lean combustion regime, is indicated by the trends in fuel efficiency and emission concentration. The resistive heating of the prechamber walls has been shown to be an effective mean to trigger ignition. The prechamber could clearly be identified as primary ignition location. A reduction of the cycle-by-cycle variations - due to mixture fluctuations - is necessary to exploit the full potential of this engine concept.