Performance analysis and parametric optimum criteria of a class of irreversible fuel cell/heat engine hybrid systems

Based on the current models of solid oxide fuel cells and two-heat-source heat engines consisting of two isothermal and two polytropic processes, a general model of a class of fuel cell/heat engine hybrid systems is established, in which multi-irreversibilities existing in real hybrid systems are taken into account. Expressions for the efficiency and power output of the hybrid systems are analytically derived from the model. The curves of the efficiency and power output of the hybrid systems varying with the current density and the efficiency versus power output curves are represented through numerical calculation. The general performance characteristics of the hybrid systems are revealed and the optimum criteria of the main performance parameters are determined. The effects of some key irreversibilities existing in the fuel cell, regenerator and two-heat-source heat engine on the performance of the hybrid systems are discussed in detail. The results obtained here are very general and may be directly used to derive the various interesting conclusions of the hybrid systems which are operated under different special cases.     

Thermo-economical analysis between new absorption–ejector hybrid refrigeration system and small double-effect absorption system

The comparisons of coefficient of performance and the cyclic characteristics between the three-pressure absorption–ejector hybrid refrigeration system (AEHRS) and small double-effect absorption refrigeration system are carried out. The thermo-economical analysis models of the two systems in two cases of high-temperature heat resources: waste heat resources and natural gas fuel are presented. The thermo-economical performances of the two systems in two modes of the running hours per year (600 and 1000 h) are calculated and discussed to show the commercial perspective of the AEHRS.     

Modeling and optimization of a typical fuel cell–heat engine hybrid system and its parametric design criteria

A theoretical modeling approach is presented, which describes the behavior of a typical fuel cell–heat engine hybrid system in steady-state operating condition based on an existing solid oxide fuel cell model, to provide useful fundamental design characteristics as well as potential critical problems. The different sources of irreversible losses, such as the electrochemical reaction, electric resistances, finite-rate heat transfer between the fuel cell and the heat engine, and heat-leak from the fuel cell to the environment are specified and investigated. Energy and entropy analyses are used to indicate the multi-irreversible losses and to assess the work potentials of the hybrid system. Expressions for the power output and efficiency of the hybrid system are derived and the performance characteristics of the system are presented and discussed in detail. The effects of the design parameters and operating conditions on the system performance are studied numerically. It is found that there exist certain optimum criteria for some important parameters. The results obtained here may provide a theoretical basis for both the optimal design and operation of real fuel cell–heat engine hybrid systems. This new approach can be easily extended to other fuel cell hybrid systems to develop irreversible models suitable for the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Performance analysis of proton-exchange membrane fuel cell stacks used in Beijing urban-route buses trial project

Fuel cell hybrid vehicles' sustained development and commercialization are contingent on the reliability and durability of the fuel cell engines. In August 2008, official trial of the three proton-exchange membrane (PEM) fuel cell hybrid city buses commenced in a commercial-operation urban-route in Beijing for one year. In this paper, data from the performance analysis of the automotive fuel cells used in those city buses are presented and analyzed. The durability and reliability of the fuel cell engines under realistic conditions were evaluated by analyzing the standard deviation of the single-cell fuel cell voltages and by estimating the voltage vs. current characteristics obtained using the recursive least squares fitting method. After studying the degradation status by analyzing fitted results from the measured data, it is concluded that the fuel cell engines' performance meets the standard imposed by the driving conditions of the Beijing urban-routes, but that their performance degradation necessitates maintenance in order to ensure normal operation.     

Design and evaluation of hybrid propulsion ship powered by fuel cell and bottoming cycle

Fuel cells are a promising power source in the electric propulsion systems for zero-emission vessels. The electric efficiency of fuel cells can be increased to 55% practically, but significant amounts of remaining energy from the electrochemical reaction are wasted as heat. This article proposes a hybrid propulsion system for ships that utilizes both the electric energy and thermal energy generated by fuel cells. The electric power capacity of fuel cells and the steam generation capacity of recovered heat from fuel cell systems are calculated, and then the propulsion power of the hybrid system is simulated by MATLAB Simulink. The overall energy efficiency of the proposed ship propulsion system is compared with that of conventional systems by comparing fuel consumption rate. Simulation results indicate that the proposed hybrid propulsion system can increase energy efficiency by 22.5% by additional utilization of the recovered heat from fuel cells.     

微型燃机与燃料电池复合装置的应用

对微型燃机发电装置及与燃料电池复合装置作了简介,并比较了采用顶层循环的固体氧化物燃料电池-微型燃机复合发电装置与单独微型燃机发电装置各自的循环特点,以燃机功率为50kW的微型燃机及其复合发电装置为例,进行了两者的性能分析比较：在复合发电装置中,分析了余热利用的优越性,并对余热供热进行了计算分析.     

The potential role of hydrogen as a sustainable transportation fuel to combat global warming

Hydrogen is recognized as a key source of the sustainable energy solutions. The transportation sector is known as one of the largest fuel consumers of the global energy market. Hydrogen can become a promising fuel for sustainable transportation by providing clean, reliable, safe, convenient, customer friendly, and affordable energy. In this study, the possibility of hydrogen as the major fuel for transportation systems is investigated comprehensively based on the recent data published in the literature. Due to its several characteristic advantages, such as energy density, abundance, ease of transportation, a wide variety of production methods from clean and renewable fuels with zero or minimal emissions; hydrogen appears to be a great chemical fuel which can potentially replace fossil fuel use in internal combustion engines. In order to take advantage of hydrogen as an internal combustion engine fuel, existing engines should be redesigned to avoid abnormal combustion. Hydrogen use in internal combustion engines could enhance system efficiencies, offer higher power outputs per vehicle, and emit lower amounts of greenhouse gases. Even though hydrogen-powered fuel cells have lower emissions than internal combustion engines, they require additional space and weight and they are generally more expensive. Therefore, the scope of this study is hydrogen-fueled internal combustion engines. It is also highlighted that in order to become a truly sustainable and clean fuel, hydrogen should be produced from renewable energy and material resources with zero or minimal emissions at high efficiencies. In addition, in this study, conventional, hybrid, electric, biofuel, fuel cell, and hydrogen fueled ICE vehicles are comparatively assessed based on their CO₂ and SO₂ emissions, social cost of carbon, energy and exergy efficiencies, fuel consumption, fuel price, and driving range. The results show that when all of these criteria are taken into account, fuel cell vehicles have the highest average performance ranking (4.97/10), followed by hydrogen fueled ICEs (4.81/10) and biofuel vehicles (4.71/10). On the other hand, conventional vehicles have the lowest average performance ranking (1.21/10), followed by electric vehicles (4.24/10) and hybrid vehicles (4.53/10).     

Hybrid systems to address seasonal mismatches between electricity production and demand in nuclear renewable electrical grids

A strategy to enable zero-carbon variable electricity production with full utilization of renewable and nuclear energy sources has been developed. Wind and solar systems send electricity to the grid. Nuclear plants operate at full capacity with variable steam to turbines to match electricity demand with production (renewables and nuclear). Excess steam at times of low electricity prices and electricity demand go to hybrid fuel production and storage systems. The characteristic of these hybrid technologies is that the economic penalties for variable nuclear steam inputs are small. Three hybrid systems were identified that could be deployed at the required scale. The first option is the gigawatt-year hourly-to-seasonal heat storage system where excess steam from the nuclear plant is used to heat rock a kilometer underground to create an artificial geothermal heat source. The heat source produces electricity on demand using geothermal technology. The second option uses steam from the nuclear plant and electricity from the grid with high-temperature electrolysis (HTR) cells to produce hydrogen and oxygen. Hydrogen is primarily for industrial applications; however, the HTE can be operated in reverse using hydrogen for peak electricity production. The third option uses variable steam and electricity for shale oil production.     

Maximum equivalent efficiency and power output of a PEM fuel cell/refrigeration cycle hybrid system

With the help of the current models of proton exchange membrane (PEM) fuel cells and three-heat-source refrigeration cycles, the general model of a PEM fuel cell/refrigeration cycle hybrid system is originally established, so that the waste heat produced in the PEM fuel cell may be availably utilized. Based on the theory of electrochemistry and non-equilibrium thermodynamics, expressions for the efficiency and power output of the PEM fuel cell, the coefficient of performance and cooling rate of the refrigeration cycle, and the equivalent efficiency and power output of the hybrid system are derived. The curves of the equivalent efficiency and power output of the hybrid system varying with the electric current density and the equivalent power output versus efficiency curves are represented through numerical calculation. The general performance characteristics of the hybrid system are discussed. The optimal operation regions of some parameters in the hybrid system are determined. The advantages of the hybrid system are revealed.     

Solid oxide fuel cell technology

The use of high-temperature solid oxide fuel cell (SOFC) systems is discussed. Such cells show great promise for economical production of electricity and heat in a variety of commercial, industrial cogeneration, and electric utility systems applications. Pioneered by Westinghouse in the 1960s, this technology is based on the ability of stabilized zirconia to operate as a solid electrolyte at elevated temperatures. It is illustrated that the cells readily conduct oxygen ions from an air electrode (cathode) where they are formed, through the zirconia-based electrolyte to a fuel electrode (anode), where they react with fuel-gas CO or H₂ or any mixture, e.g., steam-reformed natural gas, and deliver electrons to an external circuit to produce electricity. These fuel cells operate at temperatures near 1000°C and are the basic building blocks for highly efficient combined heat and electric power, or all electric-power generators     

电子管理系统对现代发动机结构性能的影响

就电子技术对现代发动机增压系统、燃油系统以及发动机的特性曲线、排放技术和运行监控等方面的影响进行了深入分析,从而对电子技术对现代发动机技术发展的影响给出了一个比较完整的概念,以期对发动机的智能化发展有进一步深刻认识。     

Combined heat and power generation via hybrid data center cooling-polymer electrolyte membrane fuel cell system

Although there has been a lot of waste heat utilization studies for the air-cooled data center (DC) systems, the waste heat utilization has not been studied for the liquid-cooled DC systems, which have been rapidly gaining importance for the high-performance Information and Communication Technology facilities such as cloud computing and big data storage. Compared to the air-cooled systems, higher heat removal capacity of the liquid-cooled DC systems provides better heat transfer performance; and therefore, the waste heat of the liquid-cooled DC systems can be more efficiently utilized in the low-temperature and low-carbon energy systems such as electricity generation via polymer electrolyte membrane (PEM) fuel cells. For this purpose, the current study proposes a novel hybrid system that consists of the PEM fuel cell and the two-phase liquid-immersion DC cooling system. The two-phase liquid immersion DC cooling system is one of the most recent and advanced DC cooling methods and has not been considered in the DC waste heat utilization studies before. The PEM fuel cell unit is operated with the hydrogen and compressed air flows that are pre-heated in the DC cooling unit. Due to its original design, the hybrid system brings its own original design criteria and limitations, which are taken into account in the energetic and exergetic assessments. The power density of the PEM fuel cell reaches up to 0.99 kW/m² with the water production rate of 0.0157 kg/s. In the electricity generation case, the highest energetic efficiency is found as 15.8% whereas the efficiency increases up to 96.16% when different multigeneration cases are considered. The hybrid design deduces that the highest exergetic efficiency and sustainability index are 43.3% and 1.76 and they are 9.4% and 6.6% higher than exergetic and sustainability performances of the stand-alone PEM fuel cell operation, respectively.     

高温燃料电池与燃气轮机相结合的混合发电系统

高温燃料电池与燃气轮机相结合的混合发电系统具有高效、环保和可靠的特性，这种新颖的混合发电系统在未来分布式发电领域具有广阔的应用前景。有不少专家和学者对其系统构成度匹配、系统性能等问题做了大量研究。目前已有高温燃料电池与燃气轮机混合发电系统成功运行，但仍有很多问题需要进行进一步的研究和探索，以使该混合发电系统早日实现商业化运行。本文综述了高温燃料电池与燃气轮机混合发电系统的研究现状，展望了该混合发电系统在未来的研究度发展前景。     

Status of helium-cooled nuclear power systems

Helium-cooled nuclear power systems offer a great potential for electricity generation when their long-term economic, environmental, conservation and energy self-sufficiency features are examined. The high-temperature gas-cooled reactor (HTGR) has the unique capability of providing high-temperature steam for electric power and process heat uses and/or high-temperature heat for endothermic chemical reactions. A variation of the standard steam cycle HTGR is one in which the helium coolant flows directly from the core to one or more closed cycle gas turbines. The effective use of nuclear fuel resources for electric power and nuclear process heat will be greatly enhanced by the gas-cooled fast breeder reactor (GCFR) currently being developed. A GCFR using thorium in the radial blanket could generate sufficient U-233 to supply the fuel for three HTGRs, or enough plutonium from a depleted uranium blanket to fuel a breeder economy expanding at about 10% per year. The feasibility of utilizing helium to cool a fusion reactor has been included in most research studies on thermonuclear fusion and is also discussed in this paper. This paper summarizes the status of helium-cooled nuclear energy systems as a basis for assessing their prospects.     

Comparison of the theoretical performance potential of fuel cells and heat engines

Fuel cells have decided advantages including compatibility with renewable fuels such as hydrogen, methanol and methane. It is often claimed that they have greater potential for efficient operation than heat engines because they are not restricted by the Carnot limitation. However, in this paper a generalized (exergy analysis) approach is utilized to clarify the comparison of the theoretical performance potential of heat engines and fuel cells, in particular, to show that fuel cell conversion is restricted by the second law of thermodynamics in the same way as heat engines. The Carnot efficiency is simply a manifestation of the second law for the heat engine excluding the combustion process. It is shown that the maximum work obtainable from the conversion device is related to the change in flow exergy between reactants and products, that is in general, not equivalent to the change in Gibbs free energy. For equivalent reactant and product temperatures, the difference between the change in Gibbs free energy and the change in flow exergy is equal to the exergy flux of heat transfer that must be rejected by the device due to absorption of entropy from the reactant-product flow. The importance of exergetic (second-law) efficiencies for evaluating performance is demonstrated. Also, exergy analysis is utilized to resolve a number of efficiency related issues for endothermic reactions.     

Integration of a molten carbonate fuel cell with a direct exhaust absorption chiller

A high market value exists for an integrated high-temperature fuel cell-absorption chiller product throughout the world. While high-temperature, molten carbonate fuel cells are being commercially deployed with combined heat and power (CHP) and absorption chillers are being commercially deployed with heat engines, the energy efficiency and environmental attributes of an integrated high-temperature fuel cell-absorption chiller product are singularly attractive for the emerging distributed generation (DG) combined cooling, heating, and power (CCHP) market. This study addresses the potential of cooling production by recovering and porting the thermal energy from the exhaust gas of a high-temperature fuel cell (HTFC) to a thermally activated absorption chiller. To assess the practical opportunity of serving an early DG-CCHP market, a commercially available direct fired double-effect absorption chiller is selected that closely matches the exhaust flow and temperature of a commercially available HTFC. Both components are individually modeled, and the models are then coupled to evaluate the potential of a DG-CCHP system. Simulation results show that a commercial molten carbonate fuel cell generating 300 kW of electricity can be effectively coupled with a commercial 40 refrigeration ton (RT) absorption chiller. While the match between the two “off the shelf” units is close and the simulation results are encouraging, the match is not ideal. In particular, the fuel cell exhaust gas temperature is higher than the inlet temperature specified for the chiller and the exhaust flow rate is not sufficient to achieve the potential heat recovery within the chiller heat exchanger. To address these challenges, the study evaluates two strategies: (1) blending the fuel cell exhaust gas with ambient air, and (2) mixing the fuel cell exhaust gases with a fraction of the chiller exhaust gas. Both cases are shown to be viable and result in a temperature drop and flow rate increase of the gases before the chiller inlet. The results show that no risk of cold end corrosion within the chiller heat exchanger exists. In addition, crystallization is not an issue during system operation. Accounting for the electricity and the cooling produced and disregarding the remaining thermal energy, the second strategy is preferred and yields an overall estimated efficiency of 71.7%.     

Modeling study of soot formation and oxidation in DI diesel engine using an improved soot model

Particulate emission is one of the most deleterious pollutants generated by Diesel fuel combustion. The ability to predict soot formation is one of the key elements needed to optimize the engine performance and minimize soot emissions. This paper reports work on developing, a phenomenological soot model to better model the physical and chemical processes of soot formation in Diesel fuel combustion. This hybrid model features that the effect of turbulence on the chemical reaction rate was considered in soot oxidation. Soot formation and oxidation processes were modeled with the application of a hybrid method involving particle turbulent transport controlled rate and soot oxidation rate. Compared with the original soot model, the in-cylinder pressures, heat release rate and soot emissions predicted by this hybrid model agreed better with the experimental results. The verified hybrid model was used to investigate the effect of injection timing on engine performance. The results show that the new soot model predicted reasonable soot spatial profiles within the combustion chamber. The high temperature gas zone in cylinder for hybrid model case is distributed broadly soot and NOx emission dependence on the start-of-injection (SOI) timing. Retarded SOI timing increased the portion of diffusion combustion and the soot concentration increased significantly with retarding of the fuel injection timing. The predicted distributions of soot concentration and particle mass provide some new insights on the soot formation and oxidation processes in direct injection (DI) engines. The hybrid phenomenological soot model shows greater potential for enhancing understanding of combustion and soot formation processes in DI diesel engines.     

Comparative assessments of two integrated systems with/without fuel cells utilizing liquefied ammonia as a fuel for vehicular applications

In the current study, two different integrated systems for vehicular applications are presented and thermodynamically analyzed. The first system consists of liquefied ammonia tank, dissociation and separation unit (DSC) for decomposition of ammonia and an internal combustion engine (ICE) to power the vehicle. The second system is a hybrid system consisting of liquefied ammonia tank, DSC unit, a small ICE and a fuel cell system. In the second system, the main power unit is fuel cell and a supplementary internal combustion engines is also utilized. The exhaust gasses emitted from the ICE are used to provide the required heat for the thermal decomposition process of ammonia. The ICE is fueled with a mix of ammonia and hydrogen generated from the DSC unit that is installed in the two systems. Hydrogen generated from DSC unit will be utilized to operate fuel cell installed in system 2. The proposed systems are analyzed and assessed both energetically and exergetically. A comprehensive parametric study is carried out for comparative assessments to determine the influence of altering design and operating parameters such as the amount of ammonia fuel supplied to the two systems on the performance of the two systems. The overall energy and exergy efficiencies for system 1 and system 2 are found to be 61.89%, 63.34%, 34.73% and 38.44% respectively. The maximum exergy destruction rate in the two systems occurred in the ICE.     

MCFC-并联TTEG混合系统的性能优化

为有效回收熔融碳酸盐燃料电池产生的余热,提出一种由熔融碳酸盐燃料电池(MCFC)、两级并联温差发电器(TTEG)和回热器组合而成的混合系统模型。考虑MCFC电化学反应中的过电势损失和混合系统中的不可逆损失,通过数值分析得出混合系统的输出功率和效率的数学表达式,获得混合系统的一般性能特征,讨论MCFC电流密度与温差发电器的无量纲电流之间的关系,确定混合系统的优化工作电流区间。结果表明,该混合系统的功率密度和效率分别比单一的MCFC系统提高46.9%和35.0%。此外,考虑了热电材料性能指标等对热电能量转换的影响,为进一步开展燃料电池与两级温差发电器的混合系统的研究提供参考。     

A new analytical approach to evaluate and optimize the performance of an irreversible solid oxide fuel cell-gas turbine hybrid system

Based on the current models of the solid oxide fuel cell and gas turbine, a generic model of a solid oxide fuel cell-gas turbine hybrid system is established, in which the multiple irreversibilities existing in real hybrid systems are taken into account. Expressions for the efficiencies and power outputs of both the subsystems and the hybrid system are analytically derived. The general performance characteristics of the hybrid system are revealed and the optimum criteria of the main performance parameters are determined. The effects of some key irreversibilities existing in the fuel cell and gas turbine on the performance of the hybrid systems are discussed in detail. The results obtained may be directly used to discuss some special cases of the hybrid system.