Optimum performance characteristics of an irreversible solar-driven Brayton heat engine at the maximum overall efficiency

An irreversible cycle model of a solar-driven Brayton heat engine is established, in which the heat losses of the solar collector and the external and internal irreversibilities of the heat engine are taken into account, and used to investigate the optimal performance of the cycle system. The maximum overall efficiency of the system is determined. The operating temperature of the solar collector and the temperature ratio in the isobaric process are optimized. The influence of the heat losses of the solar collector and the external and internal irreversibilities of the heat engine on the cyclic performance is discussed in detail. Some important curves which can reveal the optimum performance characteristics of the system are given. The results obtained here are general, and consequently, may be directly used to discuss the optimal performance of other solar-driven heat engines.     

Maximum power-conversion efficiency for the utilization of solar energy

The overall efficiency of solar thermal power plants is investigated for estimating the upper limit of their practical performances. This study consists of the theoretical optimization of the heat engine and the optimization of the overall system efficiency, which is the product of the efficiency of the solar collector and the efficiency of the heat engine. In order to obtain a more realistic performance of the solar thermal power plant, the solar collector concentration ratio, the diffused solar radiation and the convective and radiative heat losses of the solar collector are taken into account. Instead of the classical Carnot efficiency, the efficiency at maximum power is used as the optimal conversion efficiency of a heat engine. By means of simple calculations, the optimal overall system efficiency and the corresponding operating conditions of the solar collector are obtained. The results of the present work provide an accurate guide to the performance estimation and the design of solar thermal power plants.     

EFFECT OF SOLAR COLLECTOR DESIGN PARAMETERS ON THE OPERATION OF SOLAR STIRLING POWER SYSTEM

An analysis has been carried out to find out the optimum operating temperature for solar Stirling power systems. The analysis has also clearly brought out the effect of solar collector design parameters, such as, concentration ratio, overall heat loss coefficient, and heat engine parameter on the overall efficiency of solar Stirling power systems. © 1997 by John Wiley & Sons, Ltd.     

Parametric optimization of a solar-driven Braysson heat engine with variable heat capacity of the working fluid and radiation–convection heat losses

An irreversible solar-driven Braysson heat engine system is presented, in which the temperature-dependent heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the irreversibilities resulting from heat transfer and non-isentropic compression and expansion processes are taken into account. Based on the thermodynamic analysis method and the optimal control theory, the mathematical expression of the overall efficiency of the system is derived and the maximum overall efficiency is calculated, and the operating temperatures of the solar collector and the cyclic working fluid and the ratio of heat-transfer areas of the heat engine are optimized. By using numerical optimization technology, the influences of the variable heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the multi-irreversibilities on the performance characteristics of the solar-driven heat engine system are investigated and evaluated in detail. Moreover, it is expounded that the optimal performance and important parametric bounds of the irreversible solar-driven Braysson heat engine with the constant heat capacity of the working fluid and the irreversible solar-driven Carnot heat engine can be deduced from the conclusions in the present paper.     

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.     

A parabolic dish/AMTEC solar thermal power system and its performance evaluation

This paper proposes a parabolic dish/AMTEC solar thermal power system and evaluates its overall thermal–electric conversion performance. The system is a combined system in which a parabolic dish solar collector is cascaded with an alkali metal thermal to electric converter (AMTEC) through a coupling heat exchanger. A separate type heat-pipe receiver is selected to isothermally transfer the solar energy from the collector to the AMTEC. To assess the system’s overall thermal–electric conversion performance, a theoretical analysis has been undertaken in conjunction with a parametric investigation by varying relevant parameters, i.e., the average operating temperature and performance parameters associate with the dish collector and the AMTEC. Results show that the overall conversion efficiency of parabolic dish/AMTEC system could reach up to 20.6% with a power output of 18.54 kW corresponding to an operating temperature of 1280 K. Moreover, it is found that the optimal condenser temperature, corresponding to the maximum overall efficiency, is around 600 K. This study indicates that the parabolic dish/AMTEC solar power system exhibits a great potential and competitiveness over other solar dish/engine systems, and the proposed system is a viable solar thermal power system.     

Performance characteristics of an irreversible solar-driven Braysson heat engine at maximum efficiency

A novel model of the solar-driven thermodynamic cycle system which consists of a solar collector and a Braysson heat engine is established. The performance characteristics of the system are optimized on the basis of the linear heat-loss model of a solar collector and the irreversible cycle model of a Braysson heat engine. The maximum efficiency of the system and the optimally operating temperature of the solar collector are determined and other relevant performance characteristics of the system are discussed. The results obtained here may provide some theoretical guidance for the optimal design and operation of solar-driven Braysson and Carnot heat engines.     

Optimization of Output Power and Thermal Efficiency of Solar‐Dish Stirling Engine Using Finite Time Thermodynamic Analysis

This paper presents an investigation on finite time thermodynamic (FTT) evaluation of a solar‐dish Stirling heat engine. FTTs has been applied to determine the output power and the corresponding thermal efficiency, exergetic efficiency, and the rate of entropy generation of a solar Stirling system with a finite rate of heat transfer, regenerative heat loss, conductive thermal bridging loss, and finite regeneration process time. Further imperfect performance of the dish collector and convective/radiative heat transfer mechanisms in the hot end as well as the convective heat transfer in the heat sink of the engine are considered in the developed model. The output power of the engine is maximized while the highest temperature of the engine is considered as a design parameter. In addition, thermal efficiency, exergetic efficiency, and the rate of entropy generation corresponding to the optimum value of the output power is evaluated. Results imply that the optimized absorber temperature is some where between 850 K and 1000 K. Sensitivity of results against variations of the system parameters are studied in detail. The present analysis provides a good theoretical guidance for the designing of dish collectors and operating the Stirling heat engine system.     

A high performance solar powered thermoelectric generator

The efficiency of a single couple solar powered thermoelectric generator utilising fine-grained Si---Ge alloy thermoelements and a selective solar absorbing coating is investigated as a function of operating temperature and solar concentration factor. Optical losses and heat losses are taken into account. Under optimal operating conditions, the overall efficiency of the device is computed to be in excess of 12 per cent when operating between room temperature and 1000 K.     

On optimized solar-pond-driven irreversible heat engines

Optimum operating temperature and design parameters are defined for a salt gradient solar pond in combination with an irreversible Carnot heat engine. The influence of the design limits of the heat engine and the design parameter of the pond on the operating temperature are considered. It was found that when a solar-pond-driven heat engine is designed to operate close to a maximum power point, the operating temperature must be increased. Apart from this, the heat engine will operate close to the maximum efficiency limit.     

Optimization of solar-powered Stirling heat engine with finite-time thermodynamics

A mathematical model for the overall thermal efficiency of the solar-powered high temperature differential dish-Stirling engine with finite-rate heat transfer, regenerative heat losses, conductive thermal bridging losses and finite regeneration processes time is developed. The model takes into consideration the effect of the absorber temperature and the concentrating ratio on the thermal efficiency; radiation and convection heat transfer between the absorber and the working fluid as well as convection heat transfer between the heat sink and the working fluid. The results show that the optimized absorber temperature and concentrating ratio are at about 1100 K and 1300, respectively. The thermal efficiency at optimized condition is about 34%, which is not far away from the corresponding Carnot efficiency at about 50%. Hence, the present analysis provides a new theoretical guidance for designing dish collectors and operating the Stirling heat engine system.     

Exergetic analysis of a solar thermal power system

This communication presents a second law analysis based on an exergy concept for a solar thermal power system. Basic energy and exergy analysis for the system components (viz. parabolic trough collector/receiver and Rankine heat engine, etc.) are carried out for evaluating the respective losses as well as exergetic efficiency for typical solar thermal power systems under given operating conditions. It is found that the main energy loss takes place at the condenser of the heat engine part, whereas the exergy analysis shows that the collector–receiver assembly is the part where the losses are maximum. The analysis and results can be used for evaluating the component irreversibilities which can also explain the deviation between the actual efficiency and ideal efficiency of a solar thermal power system.     

SECOND LAW ANALYSIS OF A SOLAR THERMAL POWER SYSTEM

This communication presents second law analysis based on exergy concept for a solar thermal power system. Basic energy and exergy analysis for the system components (viz. parabolic trough collector/receiver and Rankine heat engine etc.) are carried out for evaluating the energy and exergy losses as well as exergetic efficiency for typical solar thermal power system under given operating conditions. Relevant energy flow and exergy flow diagrams are drawn to show the various thermodynamic and thermal losses. It is found that the main energy loss takes place at the condenser of the heat engine part whereas the exergy analysis shows that the collector-receiver assembly is the part where the losses are maximum. The analysis and results can be used for evaluating the component irreversibilities which can also explain the deviation between the actual efficiency and ideal efficiency of solar thermal power system.     

Optimum outlet temperature of solar collector for maximum work output for an Otto air-standard cycle with ideal regeneration

The optimum solar collector outlet temperature for maximizing the work output for an Otto air-standard cycle with ideal regeneration is investigated. A mathematical model for the energy balance on the solar collector along with the useful work output and the thermal efficiency of the Otto air-standard cycle with ideal regeneration is developed. The optimum solar collector outlet temperature for maximum work output is determined. The effect of radiative and convective heat losses from the solar collector, on the optimum outlet temperature is presented. The results reveal that the highest solar collector outlet temperature and, therefore, greatest Otto cycle efficiency and work output can be attained with the lowest values of radiative and convective heat losses. Moreover, high cycle work output (as a fraction of absorbed solar energy) and high efficiency of an Otto heat engine with ideal regeneration, driven by a solar collector system, can be attained with low compression ratio.     

盐梯度太阳池Kalina循环发电系统性能研究

Kalina循环发电系统是一种典型的低温热源发电系统,具有广阔的应用前景。盐梯度太阳池能够实现连续聚热和跨季节蓄热,可广泛应用于光热发电系统和光热供热系统。文章提出了一种以太阳池储热量为热源的盐梯度太阳池Kalina循环发电系统,并利用Aspen Hysys软件对该系统进行建模。而后根据模拟结果,研究了提热温度、运行压力和氨水浓度对该系统各项性能的影响。此外,还分析了典型工况下,该系统的热力性能。分析结果表明:随着提热温度逐渐升高,盐梯度太阳池Kalina循环发电系统的发电功率、热效率和效率均逐渐增加;随着运行压力逐渐升高,该系统的热效率和效率逐渐升高,并且存在最佳的运行压力1.75 MPa,使得该系统获得最大发电功率;随着氨水浓度逐渐增大,该系统的发电功率也会逐渐增大,但热效率和效率却逐渐降低;当氨水浓度为85%、运行压力为1.75 MPa、提热温度为90℃时,该系统的热效率和效率分别为7.93%,57.59%。     

Features of working process in stirling engines with direct and reverse cycles

The features of processes in thermodynamic transformers operating with direct and reverse Stirling cycles are considered. It is shown that nonstationarity of the heat exchange and gas dynamic processes influences the cycle efficiency. The influence of nonuniformity of solar radiant flux input to the heat absorber of the Stirling engine on solar energy transformation efficiency is evaluated.     

有限热源的热机供热率与效率关系的有限时间热力学分析

讨论了有限热源热机的供热率与效率关系。引人一种完善内可逆模型的方法，对高温热源侧与低温热源侧的传热优化问题进行了分析，得到一些新结论，它对太阳能热机、核动力装置和地热发电装置等热机设计均有一定指导意义。     

On optimized solar-driven heat engines

Heat engines will usually be designed somewhere between the two limits of (1) maximum efficiency, which corresponds to “Carnot” or reversible operation, albeit at zero power, and (2) maximum power point. Each of these limits implies a specific dependence of heat engine efficiency on the temperatures of the hot and cold reservoirs between which the heat engine operates. We illustrate that the energetically optimal operating temperature for solar-driven heat engines is relatively insensitive to the engine design point. This also pertains to solar collectors whose heat loss can range from predominantly linear (conductive/convective) to primarily radiative. Potential misconceptions are also discussed regarding the maximum power point and the Curzon-Ahlborn efficiency of “finite-time thermodynamics.”     

Maximum efficiency of solar energy conversion

Owing to the energy scattered or absorbed by the constituents of earth's atmosphere and self‐absorption in the outer layers of the sun, the spectrum of solar flux at earth's surface is different from that of a blackbody. Consequently, the second law of thermodynamics for heat engine cycles operating between thermal reservoirs needs to be revised to determine the maximum conversion efficiency. A thermodynamic model similar to those for multi‐temperature plasmas and non‐isothermal particle‐exchange heat engines is proposed to estimate the maximum conversion efficiency of a mechanical or solid‐state heat engine subject to a radiation flux not having a blackbody spectrum. An example is given to illustrate the calculation of the maximum power that can be converted from a solar flux with considerable gas absorption. Copyright © 2011 John Wiley & Sons, Ltd.     

Solar thermal engine systems using two stage concentrators

The overall efficiency of a system including a solar paraboloidal concentrator augmented by a new type of nonimaging second stage concentrator (shaped like the bell of a trumpet) and coupled to a heat engine is investigated. Typically, a relative increase of 10 per cent in system efficiency was found due to the second stage.