An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

Optimization of a fixed solar thermal collector

Criteria are presented for optimizing solar thermal energy collection. These criteria are then used in setting the design of a fixed solar thermal energy collector. This design is obtained by proceeding carefully through a series of optimization steps. While seeking near optimum performance, features have been retained which should lead to low cost. Initial optimization steps lead to an all glass vacuum collector tube whose side and lower walls are internally silvered to provide optimal Winston concentration on an interior glass tube coated with a selective absorber. Heat transfer calculations, performed for an array module of these collector tubes, produce values for the radiation, heat conduction and pumping losses and indicate operating conditions which minimize these losses. Near this minimum, heat conduction and pumping losses are small and can usually be neglected. Liquids provide much better heat transfer than gases. For liquid heat transfer fluids, the minimum loss collector tube window width (setting the transverse scale) is ～3 cm and tube length ～4 m, depending somewhat upon array area and the weighting used for the various losses. A window width of～5 cm and tube length～2 m should provide lower cost fabrication, while still allowing operation near minimum loss. Skills now used in the glass and lighting industry are expected to lead to low cost production of these tubes.     

一种复合抛物面槽式太阳能聚光集热器的性能研究

对一种新型复合抛物面槽式太阳能集热器进行了光学仿真和热性能研究。介绍了聚光器的设计与工作原理,在计算机上对其进行了三维建模,利用蒙特卡洛光线追迹模拟不同入射角、不同安装误差偏角的情况下,圆柱形聚光接收体表面上能流分布特征及光学效率的变化规律。光学仿真结果表明,当镜面反射率为0.92,在太阳光线入射角为0~7.5°时,光线接收率为99.36%~51.51%,聚光效率为92.14%~48.1%。室外测试结果显示,装置热效率为43.2%。     

Advanced 3‐D CPC solar collector for thermal electric system

In this paper, the authors propose an innovative non‐tracking three‐dimensional compound parabolic concentrator (3‐D CPC) solar collector, which has excellent thermal efficiency for a high‐temperature range (100–200°C). In the past studies, in order to improve the thermal efficiency of the solar collector in a high‐temperature range, very high concentration ratios and tracking systems have been adopted. However, conventional high concentration solar collectors are not cost‐effective and are inappropriate for small‐rating thermal electric generation systems for residential use. The proposed 3‐D CPC collector has a moderate concentration ratio and does not need tracking. Initially, the tentative 3‐D CPC collector was fabricated and its thermal performance was tested. Next, numerical simulations of the optical characteristics of the 3‐D CPC collector were carried out via the ray‐tracing method. Finally, the specification of the optimal 3‐D CPC collector was clarified. Applications of the thermal electric system will also be mentioned. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 323–335, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20121     

Life cycle assessment of a solar thermal collector

The renewable energy sources are often presented as ‘clean’ sources, not considering the environmental impacts related to their manufacture. The production of the renewable plants, like every production process, entails a consumption of energy and raw materials as well as the release of pollutants. Furthermore, the impacts related to some life cycle phases (as maintenance or installation) are sometimes neglected or not adequately investigated.The energy and the environmental performances of one of the most common renewable technologies have been studied: the solar thermal collector for sanitary warm water demand. A life cycle assessment (LCA) has been performed following the international standards of series ISO 14040. The aim is to trace the product's eco-profile that synthesises the main energy and environmental impacts related to the whole product's life cycle. The following phases have been investigated: production and deliver of energy and raw materials, production process, installation, maintenance, disposal and transports occurring during each step. The analysis is carried out on the basis of data directly collected in an Italian factory.     

A generalized method for testing all classes of solar collector—II evaluation of collector thermal constants

This second part of a three part paper sets out how to evaluate collector thermal constants and the accuracy of these constants from test data. The correlating expression allows up to eight constants to be evaluated, which has been found sufficient to characterize glazed and unglazed flat-plate, evacuated tubular and concentrating collectors. A method of comparing tests carried out with different heat transfer fluids is also presented.     

Parametric study of a thermal trap solar energy collector

A parametric study of a thermal trap solar energy collector with the help of a modified Hottel-Whillier-Bliss equation, is presented. The developed analysis is used to optimize the typical parameters, namely the trap's thickness and the number of flowing channels. The variation of the rating parameters of a collector with typical quantities, such as the fin distance, mass flow rate and thickness of the absorber plate, is discussed in detail.     

Performance analysis of a double-pass photovoltaic/thermal (PV/T) solar collector with CPC and fins

The use of PV/T in combination with concentrating reflectors has a potential to significantly increase power production from a given solar cell area. A prototype double-pass photovoltaic-thermal solar air collector with CPC and fins has been designed and fabricated and its performance over a range of operating conditions was studied. The absorber of the hybrid photovoltaic/thermal (PV/T) collector under investigation consists of an array of solar cells for generating electricity, compound parabolic concentrator (CPC) to increase the radiation intensity falling on the solar cells and fins attached to the back side of the absorber plate to improve heat transfer to the flowing air. Energy balance equations have been developed for the various nodes of the system. Both thermal and electrical performance of the collector are presented and discussed.     

Performance of a thermal trap flat-plate solar energy collector

The performance of a thermal trap flat-plate solar energy collector has been investigated theoretically and validated experimentally, from the point of view of stagnation temperature. The theoretical model is based on the periodic solution of the heat conduction equation and takes into account the internal emission of the thermal trap material. The model explains the experimental measurements fairly well. It is found that the trap material should have an optimal thickness in order to obtain the maximum plate temperature.The thickness of the air gap between the trap material and the cover has only a very marginal effect on the stagnation temperature.     

Performance of a concentrating photovoltaic/thermal solar collector

The performance of a parabolic trough photovoltaic/thermal collector with a geometric concentration ratio of 37× is described. Measured results under typical operating conditions show thermal efficiency around 58% and electrical efficiency around 11%, therefore a combined efficiency of 69%. The impact of non-uniform illumination on the solar cells is investigated using purpose built equipment that moves a calibrated solar cell along the line of the receiver and measures short circuit current. The measured illumination flux profile along the length shows significant variation, despite the mirror shape error being less than 1 mm for most of the mirror area. The impact of the illumination non-uniformities due to the shape error, receiver support post shading and gaps between the mirrors is shown to have a significant effect on the overall electrical performance. The flux profile transverse to the receiver length is also investigated. Peak flux intensities are shown to be around 100 suns. The impact on efficiency due to open circuit voltage reduction is discussed.     

太阳能集热器热性能测试系统

为满足工业生产及科学研究中对太阳能集热器测试的需要,按照GB/T 4271-2007《太阳能集热器热性能试验方法》设计了太阳能集热器热性能测试系统。系统由恒温控制台、恒温水箱、旋转平台、循环水泵和连接管路等组成,可对采用液体作为传热工质的集热器进行稳态和动态测试。选取了温度、流量、压力、风速及太阳辐照度传感器,设计了其硬件通讯电路,利用Labwindows/CVI软件为基础开发了测试系统的软件部分,实现了数据的采集、分析和显示。测试结果表明,系统能准确完成集热器的瞬时效率、时间常数、入射角修正系数及两端压力降等的测量,可为准确掌握集热器热性能提供试验平台。     

Integrated collector storage solar systems with asymmetric CPC reflectors

New types of ICS solar systems were designed and outdoor tests of experimental models were performed. The systems consist of single cylindrical horizontal water storage tanks placed inside stationary truncated asymmetric CPC reflector troughs of different design. We used high emittance absorber surface, low cost curved reflectors, iron oxide glazing and thermal insulation at the non illuminated tank surfaces, aiming towards cost effective ICS systems with satisfactory heat preservation during the night. Four experimental models of different designs were constructed and tested to determine their performance regarding their mean daily efficiency and thermal losses during the night. The new ICS systems were compared to an ICS system with symmetric CPC reflectors of similar construction and dimensions and also to a typical Flat Plate Thermosiphonic Unit (FPTU). Test results showed that the ICS systems with asymmetric CPC reflectors present almost the same mean daily efficiency and better preservation of hot water temperature during the night, compared to the ICS system with the symmetric CPC reflectors. The comparison with the FPTU system confirmed the satisfied daily operation of all ICS systems and their moderate storage heat preservation during the night. Theoretical results showed acceptable thermal performance of all ICS systems regarding annual operation.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in the ground; the ground is covered with a glazed thermal trap. The heat is extracted by means of a flow of liquid in such a way that the collection temperature remains constant. An expression has been derived for the periodic rate at which heat can be retrieved to keep the collection temperature constant. Numerical calculations for a typical cold day in New Delhi predict that, for a collection temperature of 30°C and for a trap thickness of 0·03 m, the maximum possible integrated collection efficiency is 25·7 per cent.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in a mass of sand; the sand is covered with a glazed thermal trap. The heat can be extracted by flow of fluid in the pipes at a constant flow rate. An expression has been derived for the periodic rate at which useful heat can be collected, keeping the flow rate constant. Numerical calculations for a typical cold day in Delhi predict that the collection efficiency of the system is about 50% for flow rate of 10 kg/h. The efficiency increases with thermal trap thickness and with flow rate.     

简化CPC式全真空玻璃集热管太阳能高温空气集热器的传热模型研究

对一种新型简化CPC(非追踪式复合抛物线聚光板)式全真空玻璃集热管太阳能高温空气集热系统的传热过程进行了理论分析和数值模拟计算,通过实验数据对该传热模型进行了验证分析。该系统由多个集热单元组成,每个集热单元包括一个简化CPC集热板,一根全真空玻璃集热管,在玻璃集热管内安装一个U形铜管。流动空气在各级U形铜管内被逐级加热。计算研究表明:系统空气最大出口温度可达到200℃,系统平均集热效率达到0.3以上,整个系统表现了良好的高温集热特性。同时,计算也表明当系统工质流量增加时,只要系统增加更多的集热管以增加系统总功率即可满足工质温度达到200℃的设计要求。研究提出的新型简化CPC式全真空玻璃集热管太阳能高温空气集热系统是一种有工业实用前途的太阳能集热器;研究提出的传热模型模拟效果也可以满足一般性工程计算需求。     

Dual functioning flat plate thermal solar collector

The purpose of this paper is to investigate the cost effectiveness of a self-constructed and installed flat plate solar energy collector. The report analyzes the theoretical energy output of the solar collector, the experimental measured energy output and the theoretical insulation properties of the collector as constructed and installed.The results obtained indicate the collector is cost effective because of its insulation property when installed inside a single glazed window. The collector produces a relatively small energy income as a solar energy collector.     

Optical and thermal optimization of stationary non-evacuated CPC solar concentrator with fully illuminated wedge receivers

Stationary low concentrator collectors (C < 2), of the CPC type, are of great interest for thermal energy supply of industrial processes, at temperatures below or equal to 100 °C. In particular, concentrators with fully illuminated V inverted absorbers have attractive properties for thermal energy conversion.Numerical analysis of the geometric and optical characteristics of different low concentration CPC’s (C between 1 and 2) with fully inverted wedge absorbers, shows that the cavities with the minimal relationship between the length and height of the reflector surface and the aperture, (L/A) and (H/A), and the lower average number of reflections 〈n〉 correspond to the lowest angular acceptance concentrator. If a concentration of 1.2 is desired, the smallest ratios of (L/A) and (H/A) and mean number of reflections 〈n〉 occur for C = 2 (θ_a = 30°). However, when the annual generated thermal energy is also considered (for example, for Recife, tilt equals latitude, fluid temperature equals 50 °C, East–West orientation), a very large maximum value in the concentration region between 1.4 and 1.6 (acceptance angles of 38.68° e 45.58°) occurs. The simulation results indicate, that while the operational temperature rises, the ratio between the annual generated thermal energy by the CPC and a good quality flat-plate collector becomes greater than 1: for CPC with 1.2 concentration these ratios become 1.0 at 50 °C and 1.35 at 80 °C. The improvement in the reflectivity of the reflector surface of the CPC rises significantly this relation, i.e., if the reflectivity exceeds from 0.86 to 0.96 the CPC of the concentration relation 1.2, operating at 80 °C may generate 55% more thermal energy than flat-plate collector.