Solar test of an integrated sodium reflux heat pipe receiver/reactor for thermochemical energy transport

A chemical reactor for carbon dioxide reforming of methane was integrated into a sodium reflux heat pipe receiver and tested in the solar furnace of the Weizmann Institute of Science, Rehovot, Israel. The receiver/reactor was a heat pipe with seven tubes inside an evacuated metal box containing sodium. The catalyst, 0.5 wt% Rh on alumina, filled two of the tubes with the front surface of the box serving as the solar absorber. In operation, concentrated sunlight heated the front plate and vaporized sodium from a wire mesh wick attached to the other side. Sodium vapor condensed on the reactor tubes, releasing latent heat and returning to the wick by gravity. The receiver system performed satisfactorily in many tests under varying flow conditions. The maximum power absorbed was 7.5 kW at temperatures above 800°C. The feasibility of operating a heat pipe receiver/reactor under solar conditions was proven, and the advantages of reflux devices confirmed.     

Numerical Investigation of Energy-Efficient Receiver for Solar Parabolic Trough Concentrator

In this paper, a thermal analysis of an energy-efficient receiver for solar parabolic trough concentrator is presented. Various porous receiver geometries are considered for the performance evaluation of a solar parabolic trough concentrator. Numerical models are proposed for a porous energy-efficient receiver for internal heat gain characteristics and heat loss due to natural convection. The internal flow and heat transfer analysis is carried out based on a RNG k-? turbulent model, whereas external heat losses are treated as a laminar natural convection model. The numerical models have been solved using the commercial engineering package, FLUENT. The thermal analysis of the receiver is carried out for various geometrical parameters, such as fin aspect ratio, thickness, and porosity, for different heat flux conditions. The inclusion of porous inserts in tubular receiver of solar trough concentrator enhanced the heat transfer about 17.5% with a pressure penalty of 2 kPa. The Nusselt number correlation is proposed based on the extensive numerical data for internal heat transfer inside the receiver. The proposed model is compared with more well-known natural convection models. A comparative study is carried out with different porous geometries to evolve an optimum configuration of energy-efficient receivers.     

Solar flux distribution study in heat pipe cavity receiver integrated with biomass gasifier

Technological advances have taken place in the field of solar receivers, gasifiers, and heat pipes, however, the integration of these technologies is not significantly available. In this paper, the conceptual design of a novel biomass gasifier is presented. The system facilitates the solar capture and gasification process separately. It is fitted with a heat pipe arrangement to transfer heat from the solar receiver zone to the gasifier zone. Collection of heat pipes comprised of few straight tubes and few innovative semi-‘S’ shaped tubes. Solar receiver geometry is modified to semi-cylindrical shape and the evaporator section of heat pipes is arranged circumferentially inside the solar receiver. The conventional gasifier is modified with an arrangement to distribute uniform solar heat throughout the fixed bed of biomass feedstock. This paper aims to present the optical analysis of the proposed heat pipe embedded solar receiver. Heat pipe disposition inside the cavity, receiver positioning on focal planes and slope error are varied to perform optical performance of the proposed solar reactor. Average solar flux is found to increase up to 1.1-fold to 1.7-fold placing cavity receiver below focal height by 16 and 32 mm respectively. Also, the magnitude and flux profiles incident on surfaces are affected with concentrator slope error. Average flux reduces up to 21.7% with 4 mrad as compared to 2 mrad error.     

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.     

塔式太阳能热管接收器表面热流密度分布及光学性能研究

该文设计一种用于小型塔式太阳能电站的热管接收器。基于蒙特卡洛光线追迹（MCRT）算法和混合编程方法,开发光学仿真程序。详细研究单根热管表面热流密度时空分布规律和热管接收器表面能量动态分布规律,并分析接收器瞬时光学性能。研究结果表明单根热管表面热流密度具有强烈非均匀性。夏至日正午,吸热面中心单根热管吸收能量约为6.9 kW。春分日和夏至日时,接收器最大光学效率约为75%;冬至日最大光学效率约为61%。研究结果有助于进一步研究热管接收器的光热耦合机理。     

Flux and temperature distribution in the receiver of parabolic solar furnaces

In this study, a mathematical analysis is presented on the complete interface problem between solar concentration systems and high temperature thermochemical processes. This includes the thermal process starting from the incoming solar radiation up to the heat transfer to a heat carrier fluid or reactants in a given reactor. The system considered comprises a heliostat, a parabolic concentrator and a receiver. The hourly incoming radiation, the hourly reflection and absorption losses on the heliostat and concentrator systems, the radiation flux density distribution in the receiver space, the solar and IR bands radiation exchange and the useful heat transfer are all considered in the analysis. The parameters such as temperature distribution in the receiver as well as thermal efficiency can be calculated for a given case. The model has been verified using the experimental results obtained in two different systems. In addition, a parametric study has been carried out on the global receiver efficiency with respect to temperature.     

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.     

A detailed nonuniform thermal model of a parabolic trough solar receiver with two halves and two inactive ends

In this paper a detailed one dimensional nonuniform thermal model of a parabolic trough solar collector/receiver is presented. The entire receiver is divided into two linear halves and two inactive ends for the nonuniform solar radiation, heat transfers and fluid dynamics. Different solar radiation and heat transfer modes can be taken into consideration for these four different regions respectively. This enables the study of different design parameters, material properties, operating conditions, fluid flow and heat transfer performance for the corresponding regions or the whole receiver. Then the nonuniform model and the corresponding uniform thermal model are validated with known performance of an existing parabolic trough solar collector/receiver. For applications, the uniform thermal model can be used to quickly compute the integral heat transfer performance of the whole PTC system while the nonuniform thermal model can be used to analyze the local nonuniform solar radiation and heat transfer performance characteristics and nonuniform heat transfer enhancements or optimizations. Later, it could also be effectively used with an intelligent optimization, such as the genetic algorithm or the particle swarm optimization, to quickly evaluate and optimize the characteristics and performance of PTCs under series of nonuniform conditions in detail.     

Study of a new solar adsorption refrigerator powered by a parabolic trough collector

This paper presents the study of solar adsorption cooling machine, where the reactor is heated by a parabolic trough collector (PTC) and is coupled with a heat pipe (HP). This reactor contains a porous medium constituted of activated carbon, reacting by adsorption with ammonia.We have developed a model, based on the equilibrium equations of the refrigerant, adsorption isotherms, heat and mass transfer within the adsorbent bed and energy balance in the hybrid system components. From real climatic data, the model computes the performances of the machine. In comparison with other systems powered by flat plate or evacuated tube collectors, the predicted results, have illustrated the ability of the proposed system to achieve a high performance due to high efficiency of PTC, and high flux density of heat pipe.     

槽式太阳能电站集热管热性能测试

采用硅碳棒加热技术和热平衡法测试了桑普生产的具有自主知识产权的槽式太阳能电站集热管的热性能。在40～300℃温度范围内,共测试8个工况下集热管热性能。实验结果显示,集热管中低温性能与肖特公司的PTR70相差不大,完全满足中低温槽式太阳能电站和其他太阳能中低温利用领域的应用。红外图像结果表明,玻璃-金属封接温度明显高于玻璃外管温度,对集热管进行理论分析时不能忽略此部分漏热量。实验数据的获得为国内太阳能槽式电站的设计、建设提供了实验参数,为集热管漏热测试相关标准的制定提供了基础。     

Experimental performance investigation of modified cavity receiver with fuzzy focal solar dish concentrator

In this paper, thermal performance analysis of 20 m² prototype fuzzy focal solar dish collector is presented. The focal image characteristics of the solar dish are determined to propose the suitable design of absorber/receiver. First, theoretical thermal performance analysis of the fuzzy focal solar parabolic dish concentrator with modified cavity receiver is carried out for different operating conditions. Based on the theoretical performance analysis, the total heat loss (conduction, convection and radiation heat losses) from the modified cavity receiver is estimated. It is observed that the maximum theoretical efficiencies of solar dish collector are found to be as 79.2% for no wind conditions and 78.2% and 77.8% for side-on and head-on winds speed of 5 m/s respectively. Latter, real time analysis of parabolic dish collector with modified cavity receiver is carried out in terms of stagnation test, time constant test and daily performance test. From stagnation test, the overall heat loss coefficient is found to be 356 W/m² K. The time constant test is carried out to determine the influence of sudden change in solar radiation at steady state conditions. The daily performance tests are conducted for different flow rates. It is found that the efficiency of the collector increases with the increase of volume flow rates. The average thermal efficiencies of the parabolic dish collector for the volume flow rate of 100 L/h and 250 L/h are found to be 69% and 74% for the average beam radiation (I_bn) of 532 W/m² and 641 W/m² respectively.     

Numerical study of heat transfer enhancement by unilateral longitudinal vortex generators inside parabolic trough solar receivers

This study presents numerical computation results on turbulent flow and coupled heat transfer enhancement in a novel parabolic trough solar absorber tube, the unilateral milt-longitudinal vortexes enhanced parabolic trough solar receiver (UMLVE-PTR), where longitudinal vortex generators (LVGs) are only located on the side of the absorber tube with concentrated solar radiation (CSR). The novel absorber tube and the corresponding parabolic trough receiver with smooth absorber tube (SAT-PTR) are numerical studied by combining the finite volume method (FVM) and the Monte Carlo ray-trace (MCRT) method for comparison and verification from the viewpoint of field synergy principle (FSP). Then the effects of Reynolds number, heat transfer fluid (HTF) inlet temperature, incident solar radiation and LVG geometric parameters were further examined. It was found that the mechanism of heat transfer enhancement of this novel absorber tube can be explained very well by the field synergy principle, and that the proposed novel UMLVE-PTR has good comprehensive heat transfer performance than that of the SAT-PTR within a wide range of major influence factors of diverse working conditions and geometric parameters.     

Optics of a two-trough solar concentrator

A linear focusing solar concentrator is described that uses two reflecting troughs, one is a large, stationary hemi-cylinder and the other is small and tracks the sun. The small reflector weighs less than a parabolic trough with the same concentrator aperture therefore diurnal steering is more feasible. Energy is carried to the job by solar heated fluid in a pipe at focus in the secondary reflector assembly. Theory is presented and comparisons are given for east-west and north-south oriented parabolic and two-trough collectors. In the ideal world, the parabolic outperforms the two-trough collector however in the real world, the simplifications due to the immobile, hemi-cylinder, large reflector may outweigh its disadvantages.     

Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.     

Numerical study of conduction and radiation heat losses from vacuum annulus in parabolic trough receivers

Parabolic trough receiver is a key component to convert solar energy into thermal energy in the parabolic trough solar system. The heat loss of the receiver has an important influence on the thermal efficiency and the operating cost of the power station. In this paper, conduction and radiation heat losses are analyzed respectively to identify the heat loss mechanism of the receiver. A 2-D heat transfer model is established by using the direct simulation Monte Carlo method for rarefied gas flow and heat transfer within the annulus of the receiver to predict the conduction heat loss caused by residual gases. The numerical results conform to the experimental results, and show the temperature of the glass envelope and heat loss for various conditions in detail. The effects of annulus pressure, gas species, temperature of heat transfer fluid, and annulus size on the conduction and radiation heat losses are systematically analyzed. Besides, the main factors that cause heat loss are analyzed, providing a theoretical basis for guiding the improvement of receiver, as well as the operation and maintenance strategy to reduce heat loss.     

热管式吸热／蓄热器在太阳能发电系统中的研究与应用

介绍了热管式太阳能吸热／蓄热器的工作原理,国内外对其所做的研究,以及在太空发电站中的应用,针对热管式吸热／蓄热器研究方向提出了几点建议。     

An experimental investigation of the heat losses of a U-type solar heat pipe receiver of a parabolic trough collector-based natural circulation steam generation system

The performance of a parabolic trough collector (PTC)-based steam generation system depends significantly on the heat losses of the solar receiver. This paper presents an experimental study of the heat losses of a double glazing vacuum U-type solar receiver mounted in a PTC natural circulation system for generating medium-temperature steam. Field experiments were performed to determine the overall heat losses of the receiver. Effects of wind, vacuum glass tube, radiation, and structural characteristics on the heat losses were analyzed. The thermal efficiency of the receiver was found to be 0.791 and 0.472 in calm and windy days, respectively, at a test temperature of about 100 °C, whereas the thermal efficiencies became 0.792 and 0.663, respectively, while taking the receiver element into consideration. The heat losses were increased from 0.183 to 0.255 kW per receiver for the two cases tested. It was shown that neither convection nor radiation heat losses may be negligible in the analysis of such U-type solar receivers.     

Effect of using nanofluids on efficiency of parabolic trough collectors in solar thermal electric power plants

In this paper, forced convection heat transfer nanofluid flow inside the receiver tube of solar parabolic trough collector is numerically simulated. Computational Fluid Dynamics (CFD) simulations are carried out to study the influence of using nanofluid as heat transfer fluid on thermal efficiency of the solar system. The three-dimensional steady, turbulent flow and heat transfer governing equations are solved using Finite Volume Method (FVM) with the SIMPLEC algorithm. The results show that the numerical simulation are in good agreement with the experimental data. Also, the effect of various nanoparticle volume fraction on thermal and hydrodynamic characteristics of the solar parabolic collector is discussed in details. The results indicate that, using of nanofluid instead of base fluid as a working fluid leads to enhanced heat transfer performance. Furthermore, the results reveal that by increasing of the nanoparticle volume fraction, the average Nusselt number increases.     

Thermodynamic optimisation of the integrated design of a small‐scale solar thermal Brayton cycle

The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate. Copyright © 2011 John Wiley & Sons, Ltd.     

Artificial neural networks approach on solar parabolic dish cooker

This paper presents heat transfer analysis of solar parabolic dish cooker using Artificial Neural Network (ANN). The objective of this study to envisage thermal performance parameters such as receiver plate and pot water temperatures of the solar parabolic dish cooker by using the ANN for experimental data. An experiment is conducted under two cases (1) cooker with plain receiver and (2) cooker with porous receiver. The Back Propagation (BP) algorithm is used to train and test networks and ANN predictions are compared with experimental results. Different network configurations are studied by the aid of searching a relatively better network for prediction. The results showed a good regression analysis with the correlation coefficients in the range of 0.9968–0.9992 and mean relative errors (MREs) in the range of 1.2586–4.0346% for the test data set. Thus ANN model can successfully be used for the prediction of the thermal performance parameters of parabolic dish cooker with reasonable degree of accuracy.