A unified model of combined energy systems with different cycle modes and its optimum performance characteristics

A unified model is presented for a class of combined energy systems, in which the systems mainly consist of a heat engine, a combustor and a counter-flow heat exchanger and the heat engine in the systems may have different thermodynamic cycle modes such as the Brayton cycle, Carnot cycle, Stirling cycle, Ericsson cycle, and so on. Not only the irreversibilities of the heat leak and finite-rate heat transfer but also the different cycle modes of the heat engine are considered in the model. On the basis of Newton’s law, expressions for the overall efficiency and power output of the combined energy system with an irreversible Brayton cycle are derived. The maximum overall efficiency and power output and other relevant parameters are calculated. The general characteristic curves of the system are presented for some given parameters. Several interesting cases are discussed in detail. The results obtained here are very general and significant and can be used to discuss the optimal performance characteristics of a class of combined energy systems with different cycle modes. Moreover, it is significant to point out that not only the important conclusions obtained in Bejan’s first combustor model and Peterson’s general combustion driven model but also the optimal performance of a class of solar-driven heat engine systems can be directly derived from the present paper under some limit conditions.     

Proposed improvements to a model for characterizing the electrical and thermal energy performance of Stirling engine micro-cogeneration devices based upon experimental observations

Stirling engines (SE) are a market-ready technology suitable for residential cogeneration of heat and electricity to alleviate the increasing demand on central power grids. Advantages of this external combustion engine include high cogeneration efficiency, fuel flexibility, low noise and vibration, and low emissions. To explore and assess the feasibility of using SE based cogeneration systems in the residential sector, there is a need for an accurate and practical simulation model that can be used to conduct sensitivity and what-if analyses. A simulation model for SE based residential scale micro-cogeneration systems was recently developed; however the model is impractical due to its functional form and data requirements. Furthermore, the available experimental data lack adequate diversity to assess the model’s suitability. In this paper, first the existing model is briefly presented, followed by a review of the design and implementation of a series of experiments conducted to study the performance and behaviour of the SE system and to develop extensive, and hitherto unavailable, operational data. The empirical observations are contrasted with the functional form of the existing simulation model, and improvements to the structure of the model are proposed based upon these observations.     

Finite-time power limit for solar-radiant Ericsson engines in space applications

The power output and thermal efficiency of a finite-time optimized solar-radiant Ericsson heat engine is studied. The thermodynamic model adopted is a regenerative gas Ericsson cycle coupled to a heat source and heat sink by radiant heat transfer. Both the heat source and heat sink have infinite heat capacity rates. Mathematical expressions for optimum power and the efficiency at optimum power are obtained for the cycle based on higher and lower temperature bounds. The results of this theoretical work provide a base line criteria for use in the performance evaluation and design of such engines as well as for use in performance comparisons with existing extra-terrestrial solar power plants.     

Exergy analysis of micro-organic Rankine power cycles for a small scale solar driven reverse osmosis desalination system

Exergy analysis of micro-organic Rankine heat engines is performed to identify the most suitable engine for driving a small scale reverse osmosis desalination system. Three modified engines derived from simple Rankine engine using regeneration (incorporation of regenerator or feedliquid heaters) are analyzed through a novel approach, called exergy-topological method based on the combination of exergy flow graphs, exergy loss graphs, and thermoeconomic graphs. For the investigations, three working fluids are considered: R134a, R245fa and R600. The incorporated devices produce different results with different fluids. Exergy destruction throughout the systems operating with R134a was quantified and illustrated using exergy diagrams. The sites with greater exergy destruction include turbine, evaporator and feedliquid heaters. The most critical components include evaporator, turbine and mixing units. A regenerative heat exchanger has positive effects only when the engine operates with dry fluids; feedliquid heaters improve the degree of thermodynamic perfection of the system but lead to loss in exergetic efficiency. Although, different modifications produce better energy conversion and less exergy destroyed, the improvements are not significant enough and subsequent modifications of the simple Rankine engine cannot be considered as economically profitable for heat source temperature below 100 °C. As illustration, a regenerator increases the system’s energy efficiency by 7%, the degree of thermodynamic perfection by 3.5% while the exergetic efficiency is unchanged in comparison with the simple Rankine cycle, with R600 as working fluid. The impacts of heat source temperature and pinch point temperature difference on engine’s performance are also examined. Finally, results demonstrate that energy analysis combined with the mathematical graph theory is a powerful tool in performance assessments of Rankine based power systems and permits meaningful comparison of different regenerative effects based on their contribution to systems improvements.     

Performance characteristics of a Diesel engine power plant

Performance of an actual Diesel engine power plant with a rated output of 120 MW is analyzed based on the first and second laws of thermodynamics. The plant consists of seven identical Diesel engines and various subsystems including turbochargers, fuel heating units and heat exchangers performing various useful tasks. The engine runs on heavy fuel oil, and the pollutant emissions from the engine are greatly reduced by effective treatment systems. The characteristics and performance parameters of the internal combustion engines of the plant are evaluated. The mass, energy and exergy balances are verified for each flow stream in the power plant. The work and heat interactions, the exergy losses and the efficiencies of various components based on both energy and exergy concepts are evaluated. The thermal and the exergy efficiencies of the plant are determined to be 47% and 44%, respectively. The engine irreversibilities are due mostly to the irreversible combustion process and account for 32% of the total exergy input and 57% of the total irreversibilities in the plant. Most of the remaining irreversibilities in the plant occur in the desulphurization, intercooler, compressor and lubrication oil cooler units. The results should provide a realistic and meaningful ground for the performance evaluation of Diesel engine power units, and it may be used in the design and analysis of such systems.     

Efficiency bound of a solar-driven stirling heat engine system

A solar-driven Stirling engine is modelled as a combined system which consists of a solar collector and a Stirling engine. The performance of the system is investigated, based on the linearized heat loss model of the solar collector and the irreverisible cycle model of the Stirling engine affected by finite-rate heat transfer and regenerative losses. The maximum efficiency of the system and the optimal operating temperature of the solar collector are determined. Moreover, it is pointed out that the investigation method in the present paper is valid for other heat loss models of the solar collector as well, and the results obtained are also valid for a solar-driven Ericsson engine system using an ideal gas as its engine work substance. © 1998 John Wiley & Sons, Ltd.     

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.     

Theoretical limits on the performance of stirling engines

This paper describes a mathematical model of engines operating with an ideal Stirling cycle and subject to limited heat transfer, internal thermal losses, and mechanical friction losses. The goal of the paper is to describe the fundamental effects of these imperfections on the performance of an otherwise ideal Stirling engine. These are defects suffered by all real Stirling engines, but the intent here is not to predict the actual performance of specific engines. Rather, the aim is to obtain some mathematical insight into the nature of the theoretical dependence of engine performance upon these imperfections. © 1998 John Wiley & Sons, Ltd.     

埃里克森热机的(火用)经济和生态学优化性能

研究存在热阻和回热损失的埃里克森热机的(火用)经济优化性能和生态学优化性能,得到一些新的性能参数,并揭示了(火用)经济优化性能与生态学优化性能间的联系,所得结果具有普遍意义,可为埃里克森热机的研制和优化设计提供些新理论指导.     

Energy system feasibility study of an Otto cycle/Stirling cycle hybrid automotive engine

The aim of this study was to investigate the feasibility of utilising a Stirling cycle engine as an exhaust gas waste heat recovery device for an Otto cycle internal combustion engine (ICE) in the context of an automotive power plant. The hybrid arrangement would produce increased brake power output for a given fuel consumption rate when compared to an ICE alone. The study was dealt with from an energy system perspective with design practicalities such as power train integration, location of auxiliaries, manufacture costs and other general plant design considerations neglected. The study necessitated work in two distinct areas: experimental assessment of the performance characteristics of an existing automotive Otto cycle ICE and mathematical modelling of the Stirling cycle engine based on the output parameters of the ICE. It was subsequently found to be feasible in principle to generate approximately further 30% useful power in addition to that created by the ICE by using a Stirling cycle engine to capture waste heat expelled from the ICE exhaust gases over the complete range of engine operating speeds.     

Introducing an integrated SOFC,linear Fresnel solar field,Stirling engine and steam turbine combined cooling,heating and power process

A new integrated combined cooling, heating and power system which includes a solid oxide fuel cell, Stirling engine, steam turbine, linear Fresnel solar field and double effect absorption chiller is introduced and investigated from energy, exergy and thermodynamic viewpoints. In this process, produced electrical power by the fuel cell and steam turbines is 6971.8 kW. Stirling engine uses fuel cell waste heat and produces 656 kW power. In addition, absorption chiller is driven by waste heat of the Stirling engine and generates 2118.8 kW of cooling load. Linear Fresnel solar field produces 961.7 kW of thermal power as a heat exchanger. The results indicate that, electrical, energy and exergy efficiencies and total exergy destruction of the proposed system are 49.7%, 67.5%, 55.6% and 12560 kW, respectively. Finally, sensitivity analysis to investigate effect of the different parameters such as flow rate of inputs, outlet pressure of the components and temperature changes of the solar system on the hybrid system performance is also done.     

Thermodynamic and exergoeconomic analysis of energy recovery system of biogas from a wastewater treatment plant and use in a Stirling engine

The aim of this research it is to show how the biogas biomethanisation from primary and secondary treatment of activated sludge from a wastewater treatment plant (WWTP), can be an alternative renewable energy option from fossil fuels, which offers competitive advantages and points out new horizons for the use of this fuel. This will allow to achieve some important priorities of energy plans in EU countries: to reduce the organic matter deposited in landfills and CO₂ emissions and to find viable solutions to minimize the environmental impact of sewage sludge (SS).This study analyses the biogas combustion and energy recovery processes from a thermodynamic, thermoeconomic and exergetic point of view.The results show that the boiler of the process is the main source of irreversibility and exergy destruction. Moreover, the energy and exergy economic value of exhaust gases from the combustion chamber, are significant and worthwhile to be exploited. For this reason, the present study explores the applicability and suitability of integrating a Stirling engine in such process. The study reveals that it is possible to create a small micro-cogeneration system which leads to sustainable waste management and energy savings in the treatment plant itself.     

Mobile hydraulic power supply: Liquid piston Stirling engine pump

Conventional mobile hydraulic power supplies involve numerous kinematic connections and are limited by the efficiency, noise, and emissions of internal combustion engines. The Stirling cycle possesses numerous benefits such as the ability to operate from any heat source, quiet operation, and high theoretical efficiency. The Stirling engine has seen limited success due to poor heat transfer in the working chambers, difficulty sealing low-molecular weight gases at high pressure, and non-ideal piston displacement profiles. As a solution to these limitations, a liquid piston Stirling engine pump is proposed. The liquid pistons conform to irregular volumes, allowing increased heat transfer through geometry features on the interior of the working chambers. Creating near-isothermal operation eliminates the costly external heat exchangers and increases the engine efficiency through decreasing the engine dead space. The liquid pistons provide a positive gas seal and thermal transport to the working chambers. Controlling the flow of the liquid pistons with valves enables matching the ideal Stirling cycle and creates a direct hydraulic power supply. Using liquid hydrogen as a fuel source allows cooling the compression side of the engine before expanded the fuel into a gas and combusting it to heat the expansion side of the engine. Cooling the compression side not only increases the engine power, but also significantly increases the potential thermal efficiency of the engine. A high efficiency Stirling engine makes energy regeneration through reversing the Stirling cycle practical. When used for regeneration, the captured energy can be stored in thermal batteries, such as a molten salt. The liquid piston Stirling engine pump requires further research in numerous areas such as understanding the behavior of the liquid pistons, modeling and optimization of a full engine pump, and careful selection of materials for the extreme operating temperatures. Addressing these obtainable research quandaries will enable a transformative Stirling engine pump with the potential to excel in numerous applications.     

有限热源斯特林热机的优化性能研究

应用有限时间热力学方法,探索有限热源、热阻和回热损失的斯特林热机的优化性能,得到一些新的性能参数,所得结论可为斯特林热机的研制和优化设计提供些新理论指导。     

Performance analysis of a novel eco‐friendly internal combustion engine cycle

The Miller cycle applications have been performed to diminish NO_x released from internal combustion engines (ICEs), in recent years. The Miller cycle provides decreased compression ratio and enhanced expansion ratio; hereby, maximum in‐cylinder combustion temperatures diminish, and NO_x formations slow down remarkably. Another less‐known method is Takemura cycle application, which provides heat addition into engine cylinder at constant combustion temperatures. In this study, a novel cycle including the Miller cycle and the Takemura cycle has been developed by using novel numerical models and computing methods with seven processes and a novel way to decrease NO_x emissions at higher levels compared with the single applications of known cycles. A comprehensive performance examination of the proposed cycle engine in terms of performance characteristics such as effective power (EFP), effective power density (EFPD), exergy destruction (X), exergy efficiency (ε), and ecological coefficient of performance (ECOP) has been conducted. The impacts of engine operating and design parameters on the performance characteristics have been computationally examined. Furthermore, irreversibilities depending on incomplete combustion loss (INCL), exhaust output loss (EXOL), heat transfer loss (HTRL), and friction loss (FRL) have been considered in the performance simulations. The minimum exergy destruction and maximum performance specifications have been observed with 30 of the compression ratio. Maximum effective power values have been obtained at range between 1 and 1.2 of equivalence ratio. The optimum range for exergy efficiency is between 0.8 and 1 of equivalence ratio. Increasing engine speed has provided enhancing effective power. However, an optimum range has been found for the exergy efficiency that is interval of 3000 to 4000 rpm. The results obtained can be assessed by researchers studying on modeling of the engine systems and designs.     

Thermodynamic analysis and assessment of an integrated hydrogen fuel cell system for ships

In this thermodynamic investigation, an integrated energy system based on hydrogen fuel is developed and studied energetically and exergetically. The liquefied hydrogen fueled solid oxide fuel cell (SOFC) based system is then integrated with a steam producing cycle to supply electricity and potable water to ships. The first heat recovery system, after the fuel cells provide thrust for the ship, is by means of a turbine while the second heat recovery system drives the ship's refrigeration cycle. This study includes energy and exergy performance evaluations of SOFC, refrigeration cycle and ship thrust engine systems. Furthermore, the effectiveness of SOFCs and a hydrogen fueled engine in reducing greenhouse gas emissions are assessed parametrically through a case study. The main propulsion, power generation from the solid oxide fuel cells, absorption chiller, and steam bottoming cycle systems together have the overall energy and exergy efficiencies of 41.53% and 37.13%, respectively.     

Free piston Stirling engines: A review

This paper presents a detailed review on free piston Stirling engines (FPSEs) technology. Generally, the Stirling engines can be categorized into two broad classes comprising kinematic and dynamic converters among which FPSEs are known as the dynamic type. Other well-known dynamic Stirling converters are Fluidyne and thermosacoustic engines among which the thermosacoustic ones are the most advanced Stirling converters recently presented. In this research, the dynamic Stirling engines are first introduced and reviewed. Then, the review work is directed toward the FPSEs, one of the most reliable dynamic Stirling converters utilized in different applications such as combined heat and power systems (CHPs). Subsequently, the working principles of different types of FPSEs and their performance are summarized. Next, several manufactured FPSEs, as well as their corresponding features and applications, are discussed. Finally, the article is conducted to analysis and modeling approaches of FPSEs. Accordingly, linear and nonlinear analytical techniques of FPSEs are introduced, and some comparative data are provided to verify the modeling schemes. Then, various design parameters affecting the engine performance are introduced and studied. The outcomes of this review work demonstrate the potential of FPSEs for different applications and reveal that the perturbation-based model is likely the most comprehensive nonlinear method for modeling and design of the FPSEs.     

Performance optimization of Stirling engines

The search for an engine cycle with high efficiency, multi-sources of energy and less pollution has led to reconsideration of the Stirling cycle. Several engine prototypes were designed but their performances remain relatively weak when compared with other types of combustion engines. In order to increase their performances and analyze their operations, a numerical simulation model taking into account thermal losses has been developed and used, in this paper, to optimize the engine performance. This model has been tested using the experimental data obtained from the General Motor GPU-3 Stirling engine prototype. A good correlation between experimental data and model prediction has been found. The model has also been used to investigate the influence of geometrical and physical parameters on the Stirling engine performance and to determine the optimal parameters for an acceptable operational gas pressure.     

1D energy,exergy, and performance assessment of turbocharged diesel/hydrogen RCCI engine at different levels of diesel,hydrogen, compressor pressure ratio,and combustion duration

There is a growing concern about the feasibility of a new generation of internal combustion engines in a low-temperature and efficient way that can meet the emission regulations while maintaining the desirable power performance. This requires the controllability and flexibility over the ignition and reactivity within the cylinder by handling two fuels with different chemical reaction intensity. In this research, the reactivity controlled compression ignition (RCCI) turbocharger-assisted engine is proposed that operates with diesel-hydrogen fuels. After calibration and model validation, the effect of pressure ratio of compressor in turbocharger, hydrogen energy share, diesel mass per cycle, and combustion duration on temperature rise, entropy, pressure, heat release, and engine performance is evaluated. The design variables effects on the exergy share of work, heat, exhaust loss, irreversibility, and exergetic performance coefficient (EPC) are assessed and analyzed. Increasing the compressor pressure ratio can significantly increase the engine power and reduce fuel consumption. It is proved that turbocharging can reduce the entropy generation and thereby reduce irreversibility. The lower diesel injection is favored in terms of the work exergy and the EPC.     

Exergy analysis of the woody biomass Stirling engine and PEM-FC combined system with exhaust heat reforming

The woody biomass Stirling engine (WB-SEG) is an external combustion engine that outputs high-temperature exhaust gases. It is necessary to improve the exergy efficiency of WB-SEG from the viewpoint of energy cascade utilization. So, a combined system that uses the exhaust heat of WB-SEG for the steam reforming of city gas and that supplies the produced reformed gas to a proton exchange membrane fuel cell (PEM-FC) is proposed. The energy flow and the exergy flow were analyzed for each WB-SEG, PEM-FC, and WB-SEG/PEM-FC combined system. Exhaust heat recovery to preheat fuel and combustion air was investigated in each system. As a result, (a) improvement of the heat exchange performance of the woody biomass combustion gas and engine is observed, (b) reduction in difference in the reaction temperature of each unit, and (c) removal of rapid temperature change of reformed gas are required in order to reduce exergy loss of the system. The exergy efficiency of the WB-SEG/PEM-FC combined system is superior to EM-FC.