Geometrical optical performance studies of a composite parabolic trough with a fin receiver

The geometrical optical performance characteristics of a composite parabolic trough (CPT) with a fin receiver have been studied. The variation of geometrical concentration ratio with mirror element size and the rim angle of the parent parabola has been studied and the results are presented graphically. The distribution of local concentration ratio over the receiver plane has also been determined for some typical concentrator designs.     

槽式太阳能集热管热性能评估方法

真空集热管是槽式太阳能聚光集热系统的核心部件之一.集热管工作过程中通过辐射换热、对流换热和热传导的方式将热量传递给环境,这部分传递到环境中的热量称为集热管的热损失.真空集热管的热损失是聚光集热系统热损失或总能量损失的主要组成,在很大程度上决定着聚光集热器的光-热转换效率,因此对集热管热性能的正确评估对聚光集热系统的研究至关重要.本文对槽式太阳能集热管热性能的计算、评估分析方法等进行了分析.     

Numerical investigation of parabolic trough receiver performance with outer vacuum shell

The useful heat gain of a parabolic collector system is directly dependent on the heat loss from the absorber at its operating temperature. Selective coatings with evacuated/non evacuated glass tubes are employed to control radiative and convective heat losses. A concentric glass shell under vacuum is investigated for its thermal performance as this method circumvents the need for direct sealing between the glass envelope and the metal receiver to maintain vacuum and its related technical challenges. The performance is compared against a non evacuated receiver and its influence under different wind velocities; emissivities are calculated by a one dimensional theoretical model and solved by an iterative method.     

非对称槽式抛物面聚光器的仿真与性能研究

基于光学成像原理,设计了一种由两个不同抛物面组合而成的、适于在太阳能中低温集热领域应用的新型非对称槽式抛物面聚光器。文章介绍了该聚光器的工作原理,并在计算机上三维建模,利用光学分析软件计算了其中所安装的圆管型接收器和平板型接收器的光学效率,分析了接收器上的光线接收率随入射偏角的变化趋势。研究结果表明,新型非对称槽式抛物面聚光器在入射偏角为6°时,圆管型接收器表面的光线接收率达到71%;在最大尺寸条件下,圆管型接收器的接收效果优于平板型接收器;实际测试所得的装置聚光热效率为34%。     

Performance simulation of a parabolic trough solar collector

A new analytical model for optical performance and a modified integration algorithm are proposed and applied to simulate the performance of a parabolic trough solar collector with vacuum tube receiver. The analytical equation for optical efficiency of each point at reflector is derived first, then the optical efficiency of the system is simulated by numerical integration algorithm. The cosine factor, receiver efficiency, heat loss and efficiency of conversion of solar energy into net heat energy at any time can be calculated with the program. The annual average efficiency is also simulated considering discard loss. The effects of optical error, tracking error, position error from installation of receiver, optical properties of reflector, transmittance and absorptivity of vacuum tube receiver on efficiencies of the trough system are simulated and analyzed as well as optical parameter.     

Effect of receiver geometry on the optical and thermal performance of a parabolic trough collector

In this study, the optical and thermal performance of a Parabolic Trough Collector PTC system is investigated theoretically. A series of numerical simulations and theoretical analysis has been conducted to investigate the effect of the receiver geometry and location relative to the focal line on its optical performance. The examined receiver geometries are circular, square, triangular, elliptical and a new design of circular- square named as channel receiver. The thermal performance of PTC is studied for different flow rates from (0.27 to 0.6 lpm) theoretically. Results showed that the best optical design is the channel receiver with an optical efficiency of 84% while the worst is the elliptical receiver with an optical efficiency of 70%. Thermally the best design is the elliptical receiver with a thermal efficiency of 85% while the worst is the circular receiver with a thermal efficiency of 82%.     

Modelling, optimisation and performance evaluation of a parabolic trough solar collector steam generation system

A parabolic trough collector (PTC) system used for steam generation is presented in this paper. PTCs are the preferred type of collectors used for steam generation due to their ability to work at high temperatures with a good efficiency. The modelling program developed called PTCDES is used to predict the quantity of steam produced by the system. The flash vessel size, capacity and inventory determines how much energy is used at the beginning of the day for raising the temperature of the circulating water to saturation temperature before effective steam production begins. Optimisation of the flash vessel presented here uses a simplified version of the program PTCDES. System performance tests indicate that the modelling program is accurate to within 1.2% which is considered very adequate. Finally, the theoretical system energy analysis is presented in the form of a Sankey diagram. The analysis shows that only 48.9% of the available solar radiation is used for steam generation. The rest is lost either as collector or thermal losses.     

可用于建筑采暖的槽式复合抛物面聚光器光热特性研究

文章针对太阳能建筑采暖系统集热面积大、换热介质抗冻能力差的问题,设计了一种新型的槽式复合抛物面聚光建筑采暖系统,并分析了该系统中槽式复合抛物面聚光器的聚光原理。文章还建立了槽式复合抛物面聚光器的三维模型,而后利用光学仿真软件分析该聚光器的聚光性能,并搭建试验台研究空气流速对该聚光器光热转化效率的影响。分析结果表明:在光线入射偏角为10°的条件下,当接收体中心与聚光器底部的间距为90 mm时,槽式复合抛物面聚光器的光线接收率和聚光效率最优,分别为65.54%和60.25%;在实际天气条件下,槽式复合抛物面聚光器光热转化效率随空气流速增加而升高,当空气流速为4 m/s时,该聚光器的光热转化效率达到最大值,为76.73%。     

The effect of dust accumulation on line-focus parabolic trough solar collector performance

Based on the major Department of Energy Solar Industrial Process Heat Program, it has been determined that the existing techniques for predicting the performance of parabolic trough solar collectors greatly overpredict the thermal output of these systems. The objective of the research reported herein is to improve the predictive capability of existing models by incorporating a factor that accounts for dust and dirt accumulation on the optical surfaces. This has been accomplished by modifying the optical efficiency with a dust factor to account for the reduced reflectivity of the mirror and reduced transmissivity of the cover glass. This technique has been developed independent of the test data used for verification. The dust factors have been developed from exposure tests conducted at six different sites, so that it is also independent of location and collector type. Wash frequency and optical degradation rate are input to the model to compute the time varying dust factors. Recommendations for these parameters are provided based on long-term observations. The complete model is then used to provide realistic predictions of real-world performance of solar IPH systems.     

Thermal performance analysis of small-sized solar parabolic trough collector using secondary reflectors

ABSTRACT

In this paper, theoretical analysis of receiver tube misalignment, the design of secondary reflector and experimental analysis of a small-sized solar parabolic trough collector (PTC) with and without secondary reflectors are represented. Experimental analysis of PTC has been done using a parabolic secondary reflector (PSR) and triangular secondary reflector (TSR) and compared with PTC without secondary reflector (WSR). The maximum outlet temperature of heat transfer fluid is observed as 49.2°C, 47.3°C and 44.2°C in the case of PSR, TSR and WSR conditions, respectively. The maximum thermal efficiency of 24.3%, 22.5% and 17.8% is observed in the case of PSR, TSR and WSR conditions, respectively. The circumferential temperature difference on the outer surface of the receiver tube is obtained more uniform in the case of PSR and TSR than WSR condition. This indicates that the use of a secondary reflector can improve the performance of a solar PTC system.     

槽式太阳聚光器的研究

提出了一种低倍聚光的抛物面槽式聚光光伏发电方式.从聚光器的聚光比入手,推导抛物面槽式聚光的能流聚光比的公式,分析了能流聚光比和各个参数的关系.依据这些关系式制成的低倍聚光装置适宜于普及,可节约光伏装置成本,增加光伏发电量.     

Influences of installation and tracking errors on the optical performance of a solar parabolic trough collector

The parabolic trough collector is an important component of parabolic trough solar thermal power generation systems. Coordinate transformations and the Monte Carlo Ray Trace (MCRT) method were combined to simulate the circumferential flux distribution on absorber tubes. The simulation model includes the optics cone with non-parallel rays, geometric concentration ratios (GCs), the glass tube transmissivity, the absorber tube absorptance and the collector surface reflectivity. The mode is used to analyze the effects of absorber tube installation errors and reflector tracking errors. The results are compared with reference data to verify the model accuracy. Influences of installation and tracking errors on the flux distribution are analyzed for different errors, incident angles and GCs. For a GC of 20 and 90° rim angle, X direction installation errors are −0.2%∼0.2%, Y direction installation errors are −1.0%–0.5%, and the tracking error should be less than 4 mrad. As the incident angle increases, the errors become larger, but the errors become smaller as concentration ratios are increased. The results provide foundations for heat transfer analysis of the absorber tube, for parabolic trough plant to ensure the safe intensity, and for economic analysis of the installation process and control system.     

Optimization of parabolic trough solar collectors

The results of a detailed optical analysis of parabolic trough solar collectors are summarized by a few universal graphs and curve fits. These graphs enable the designer of parabolic trough collectors to calculate the performance and optimize the design with a simple hand calculator. The method is illustrated by specific examples that are typical of practical applications. The sensitivity of the optimization to changes in collector parameters and operating conditions is evaluated.     

槽式太阳能供热光伏系统集热器集热性能的实验研究

文章基于沧州地区的气候特点,通过实验分析了传热工质进口温度、直射辐射量对槽式太阳能供热光伏系统中集热器集热性能的影响情况。实验结果表明:沧州地区3月份晴朗天气条件下,槽式太阳能供热光伏系统的集热效率为0.35～0.65;当运行温度为20～100℃时,随着传热工质入口温度逐渐升高,槽式太阳能供热光伏系统中集热器的集热效率和火用效率均得到明显提高;直射辐射量的增大,会导致槽式太阳能供热光伏系统中集热器的集热效率和火用效率随之升高。     

Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Alternative designs of parabolic trough solar collectors

Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. The conventional parabolic trough collectors are used in various applications of medium and high-temperature levels. However, there are numerous studies which investigate alternative designs. The reasons for examining different PTC configurations regard the thermal efficiency increase, the reduction of the manufacturing cost and the development of more compact designs. The objective of this review paper is to summarize the existing alternative designs of PTC and to suggest the future trends in this area. Optical and thermal modifications are examined, as well as the use of concentrating thermal photovoltaic collectors. The optical modifications include designs with secondary concentrators, stationary concentrators and strategies for achieving uniform heat flux. The thermal modifications regard the use of nanofluids, turbulators and the use of thermally modified receivers with insulation, double-coating and radiation shields. The concentrating thermal photovoltaics are systems with flat or triangular receivers which can operate in low or in medium temperature levels with the proper alternative designs. It has been found that there are many promising choices for designing PTC with higher thermal performance and lower cost. The conclusions of this work can be used as guidelines for future trends in linear parabolic concentrating technologies.     

Optimization of parabolic trough solar collector system

Process heat produced by solar collectors can contribute significantly in the conservation of conventional energy resources, reducing CO₂ emission, and delaying global warming. One of the major problems associated with solar process heat application is fluctuation in system temperature during unsteady state radiation conditions which may cause significant thermal and operation problems. In this paper a transient simulation model is developed for analysing the performance of industrial water heating systems using parabolic trough solar collectors. The results showed that to prevent dramatic change and instability in process heat during transient radiation periods thermal storage tank size should not be lower than 14.5 l m^?2 of collector area. Small periods of radiation instability lower than 30 min do not have significant effect on system operation. During these periods when water flow rate of collector loop is doubled the time required to restore system normal operating condition increased by a ratio of 1.5. Copyright © 2005 John Wiley & Sons, Ltd.     

Analytical determination of the optical performance of practical parabolic trough collectors from design data

A semifinite analytical formulation is presented to facilitate the efficient numerical computation of the concentrated radiant flux on receiver surfaces in the parabolic trough concentrator. The concentrated flux density is integrated to yield the interceptance ratio for the collector. By including models for the incident-angle dependent envelope transmittance and receiver absorptance, the concentration ratio for absorbed radiation is also calculated and integrated to yield the optical efficiency. Flux distribution results from this detailed analysis are compared with a point source model, and the optical efficiency results are compared with a first-order model. Since this model includes both a realistic nonuniform solar modeling as well as models for imperfect reflection, transmission, and absorption, it is capable of estimating the optical efficiency of practical collectors.     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.     

Potential of a parabolic trough solar concentrator for electric energy production

At present, parabolic trough technology is considered as the most low‐cost and powerful large‐scale technology to utilize solar energy for electricity generation and produce steam for different industrial usages. This article recommends the generation of electricity by using a parabolic trough solar concentrator in the central area of the Kingdom of Saudi Arabia (KSA) at Dawadmi city. Pressurized water is used as the heat‐transfer working fluid. A computer algorithm was built using the Matlab program to simulate the performance parameters of the Euro Trough collector (ETC). The input data included the properties of the working fluid (pressurized water) and the designing parameters of ETC. The output data were the outlet water temperature, the coefficient of heat transfer, the heat loss, and the thermal, solar, and global efficiencies. The obtained results indicated the ability of this type of parabolic trough in KSA to generate electric power due to the high‐performance parameters achieved. Also, the validity of using the simulation technique was measured and it showed good conformity.