Optimum solar collector operation for maximizing cycle work output

Analysis is presented to determine the optimum outlet temperature for a given solar collector that will maximize the work output for various idealized heat engine cycles. The effect of radiative and convective heat losses from the collector is demonstrated, and the relative importance of each in different operating ranges is shown.     

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.     

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.     

A novel auto-cascade low-temperature solar Rankine cycle system for power generation

A novel auto-cascade low-temperature solar Rankine cycle (ALSRC) system is proposed. Compared to the single stage low-temperature solar Rankine cycle (SSLSRC) system, the ALSRC system is different because it consists of two solar collectors, two expanders, a regenerator, and an internal heat exchanger (IHE). The working fluid for the ALSRC is the zeotropic mixture Isopentane/R245fa. The main advantages of the ALSRC system is that heat from the exhaust stream of the expanders are reclaimed twice, once using an IHE and another time using a regenerator. System parameters such as regeneration, mixture composition, the outlet temperature of the low temperature solar collector, and the inlet temperature of two expanders are investigated to determine their effects on thermal efficiency. Results showed that with a regenerator, the thermal efficiency of the ALSRC system using a mixture of 0.32 R245fa by mass was significantly higher than that of the SSLSRC system. It was determined that regeneration, the mixture composition, and the outlet temperature of the low temperature solar collector are all important factors that affect the system’s thermal efficiency.     

Effect of heat transfer on the performance of an ait-standard diesel cycle

This paper presents thermodynamic analysis of an air-standard Diesel cycle. It presents the effect of heat transfer on the net work output and the indicated thermal efficiency of the cycle. The heat losses through cylinder walls are considered to be proportional to the average temperature during heat addition process. The effects of other parameters, in conjunction with heat transfer, such as cutoff ratio and intake air temperature were also reported. The results obtained from this work can be helpful in the design and evaluation of Diesel engines.     

Design and optimisation of a combination solar collector–thermal engine operating on Mars

A “dynamic” solar power plant (which consists of a solar collector–thermal engine combination) is proposed as an alternative for the more usual photovoltaic cells. A model for heat losses in a selective flat-plate solar collector operating on Mars is developed. An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performances. The output power is maximized. Meteorological and actinometric data provided by Viking Landers are used as inputs. Two strategies of collecting solar energy were considered: (i) horizontal collector; (ii) collector tilt and orientation are continuously adjusted to keep the receiving surface perpendicular on the Sun’s rays. The influences of climate and of various design parameters on solar collector heat losses, on engine output power and on the optimum sun-to-user efficiency are discussed.     

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.     

工质变比热和传热损失对Otto循环性能的影响

用有限时间热力学的方法分析空气标准Otto循环，由数值计算给出了存在传热损失和工质变比热时循环功率与压缩比、效率与压缩比以及功率和效率的特性关系，并分析了传热损失和工质变比热对循环性能的影响特点。通过分析可知传热和变比热特性对Otto循环性能有较大影响，所以在实际循环分析中应该予以考虑。     

Thermal optimization of solar dish collector for indirect vapor generation

This work presented the performance analysis of a solar parabolic concentrator prototype. The purpose of this paper is to achieve most quantity of vapor production with different water flows. The principal component of the solar concentrator is a new absorber concept that absorbs reflected solar rays and transports it to a heat exchanger in order to generate vapor. Climatic conditions, inlet/outlet oil temperatures of the tubular solar heat exchanger, water tank temperature, and inlet/outlet water temperatures of the mixed heat exchanger were recorded experimentally during three days in November 2018. The absorbed energy, losses energy, concentrated energy, and vapor heat energy of the system were determined. Results of this work, the solar system provides thermal energy efficiency varied from 60% to 70% and a concentration factor around 350 for three water mass flow rates. In this experiment, the optimum value of vapor mass is 6 kg/h with 0.016 kg/s of water flow. Consequently, to achieve the most quantity of vapor, the water flow should be decreased.     

Salt gradient stabilized solar pond collector

This paper presents a theoretical analysis of a salt gradient solar pond as a steady state flat plate solar energy collector. We explicitly take into account the convective heat and mass flux through the pond surface and evaluate the temperature and heat fluxes at various levels in the pond by solving the Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation as it passes through the pond water. These evaluations, in combination with energy balance considerations, enable the derivation of the expressions for solar pond efficiency of heat collection as well as the efficiency of heat removal. The efficiency expressions are Hottel-Whillier-Bliss type, prevalent for flat plate collectors. Numerical computations are made to investigate the optimization of geometrical and operational parameters of the solar pond. For given atmospheric air temperature, solar insolation and heat collection temperature, there is an optimum thickness of nonconvective zone for which the heat collection efficiency is maximum. The heat removal factor is also similar to that of a flat plate collector and the maximum efficiency of heat removal depends on both the flow rate and the temperature in the nonconvective zone.     

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.     

Model of optimized solar heat engine operating on Mars

An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performance. The output power is maximized. Meteorological and actinometric data provided by the Viking Lander 1 are used as inputs. Four strategies of collecting solar energy are considered. Results concerning the following three parameters are briefly reported: (1) optimum solar collector surface area, (2) optimum solar collector temperature and (3) maximum output power.     

A new calorimetric facility to investigate radiative‐convective heat exchangers for concentrated solar power applications

A new calorimetric facility for the aerothermal assessment of radiative‐convective heat exchangers in concentrating solar power applications has been developed and is described in this paper. The configuration of volumetric solar receivers enables concentrated sunlight to be absorbed and conducted within their solid volume, from where it is gradually transferred by forced convection to a heat transfer fluid flowing through their structure. Current design trends towards higher thermal conversion efficiencies have led to the use of complex intricate geometries to maximise temperatures deep inside the structure. The work presented aims to aid these objectives by commissioning a new experimental facility for the fully integrated evaluation of such components. The facility is composed of a high‐flux solar simulator that provides 1.2 kW of radiative power, a radiation homogeniser, inlet and outlet collector modules, and a working section that can accommodate volumetric receivers up to 80 mm × 80 mm in aperture. Irradiance levels and flow field nondimensional governing parameters are highly representative of on‐sun experiments at larger scales. Results from experiments with a siliconised silicon carbide monolithic honeycomb are presented, conducted at realistic conditions of incident radiative power per unit mass flow rate to validate its design point operation. Measurements conducted include absorber solid temperature distributions, air inlet and outlet temperatures, pressure drop, incident heat flux, and overall thermal efficiency. The relative influence of different sources of thermal loss is analysed and discussed.     

Multi-objective thermodynamic optimization of solar parabolic dish stirling heat engine with regenerative losses using NSGA-II and decision making

The proposed work investigates optimal values of various decision variables that simultaneously optimize power output, net-work output and second law efficiency of solar driven Stirling heat engine with regenerative heat losses, conducting thermal bridging losses using evolutionary algorithm based on NSGA-II in Matlab simulink environment. Effects of design parameters as absorber temperature, concentrating ratio, radiative and convective heat transfers are included in the analysis. Pareto frontier is obtained for triple and dual objectives and the best optimal value is selected through four different decision making techniques viz. Fuzzy, Shannon entropy, LINMAP and TOPSIS. Triple objective evolutionary approach applied to the proposed model gives power output, net-work output and second law efficiency as (38.87 kW, 1.24 kJ, 0.3156) which are 18.19, 16.78 and 31.51% lower in comparison with reversible system. With the objective of error investigation, the average and maximum error of the obtained results are figured at last.     

An analysis of solar heating systems that use vapor-compression cycles

This paper analyzes the technical and economic performance of solar heating systems that use vapor-compression cycles, circulating a compressible fluid as the working fluid. With conventional solar heating systems that use water or as their working fluid, the collector inlet temperature is equal to that of the storage outlet temperature. Operating the system on a cold day can result in large thermal losses to the surroundings and, thus, low useful heat gains. A vapor-compression cycle may be attractive because it allows the collector inlet temperature to be lowered so that the heat gain of the collector can be increased. Such a system is simulated and a preliminary economic analysis performed. The results indicate that the vapor-compression system can collect almost 50% more solar energy than a conventional system if the collector area of the two systems are the same.     

固-固相变蓄热材料在太阳能高温空气集热系统中的应用研究

为利用太阳能获得稳定持续的高温空气工质,除了有效集热外,还需要解决因太阳辐射强度变化导致输出工质温度波动的问题。在性能优良的太阳能集热系统中采用蓄热技术是解决此问题的有效途径。根据给定的设计目标,研究将固-固相变蓄热材料季戊四醇应用到太阳能集热蓄热一体化的实验装置中。实验结果表明:按集热蓄热一体化思路设计的实验装置,集热单元能够输出最高温度超过220℃的高温空气,蓄热单元能够将高温空气的温度稳定在蓄热材料的相变温度附近。并且随着蓄热管级数的增加,空气出口温度稳定的时间就越长,为利用太阳能获得稳定持续的高温热媒工质奠定了基础。     

Three-dimensional CFD analysis for simulating the greenhouse effect in solar chimney power plants using a two-band radiation model

The greenhouse effect in the solar collector has a fundamental role to produce the upward buoyancy force in solar chimney power plant systems. This study underlines the importance of the greenhouse effect on the buoyancy-driven flow and heat transfer characteristics through the system. For this purpose, a three-dimensional unsteady model with the RNG k–ε turbulence closure was developed, using computational fluid dynamics techniques. In this model, to solve the radiative transfer equation the discrete ordinates (DO) radiation model was implemented, using a two-band radiation model. To simulate radiation effects from the sun's rays, the solar ray tracing algorithm was coupled to the calculation via a source term in the energy equation. Simulations were carried out for a system with the geometry parameters of the Manzanares power plant. The effects of the solar insolation and pressure drop across the turbine on the flow and heat transfer of the system were considered. Based on the numerical results, temperature profile of the ground surface, thermal collector efficiency and power output were calculated and the results were validated by comparing with experimental data of this prototype power plant. Furthermore, enthalpy rise through the collector and energy loss from the chimney outlet between 1-band and two-band radiation model were compared. The analysis showed that simulating the greenhouse effect has an important role to accurately predict the characteristics of the flow and heat transfer in solar chimney power plant systems.     

Validation of CFD simulation for flat plate solar energy collector

The problem of flat plate solar energy collector with water flow is simulated and analyzed using computational fluid dynamics (CFD) software. The considered case includes the CFD modeling of solar irradiation and the modes of mixed convection and radiation heat transfer between tube surface, glass cover, side walls, and insulating base of the collector as well as the mixed convective heat transfer in the circulating water inside the tube and conduction between the base and tube material. The collector performance, after obtaining 3-D temperature distribution over the volume of the body of the collector, was studied with and without circulating water flow. An experimental model was built and experiments were performed to validate the CFD model. The outlet temperature of water is compared with experimental results and there is a good agreement.     

Optimum mass flow rate of water in a flat plate solar collector coupled with a storage tank and an organic Rankine cycle power loop

The optimum flat plate solar collector operation for maximizing solar organic Rankine cycle work output is investigated. A mathematical model for the flat plate solar collector water system integrated with a solar storage tank and an organic Rankine cycle loop is developed. The mass flow rate of water is searched for to obtain the maximum net work output from the integrated system during 1 yr of operation. Curve fittings of thermodynamic properties of some chosen organic fluids (R-11, R-21 and R-113) are derived for computer use. The Hooke and Jeeves optimization technique is used to determine the optimum mass flow rate of water in the collector for different values of collector inner tube diameters and boiler heat transfer coefficient-area product.     

The solar gas turbine for pump and electric generator drive

A turbine plant, using solar energy as a heat source, has been studied. The facility is used as a pump drive or electric-power generator. It is suitable for use in waterless areas, is easy to operate and maintain, and has high thermal efficiency. A computer-aided optimization was carried out for a regenerative solar gas turbine, including a parametric study of compressor, regenerator, concentrator, and turbine efficiencies. The effects of maximum cycle operating temperature and engine-pressure ratio on thermal efficiency and power output, as welll as corresponding optimum pressure ratios, were determined. The turbine and compressor efficiencies and the maximum cycle temperature exert the strongest influence on cycle thermal efficiency, power output, optimum pressure ratio for maximum work and efficiency: the regenerator has a greater effect than the receiver.