Power losses in an asymmetric compound parabolic photovoltaic concentrator

An asymmetric compound parabolic photovoltaic concentrator (ACPPVC) of geometrical concentration ratio 2 was designed, developed and evaluated for building façade integration. Despite the two times theoretical concentration, the maximum output power achieved was only 1.62 times that of a similar non-concentrating system for a wide range of solar radiation intensities due to a combination of optical and electrical resistance losses. In this paper, the power loss of the ACPPVC system is explained by a comparative power loss analysis of the non-concentrating photovoltaic system for long- and short-tabbed solar cell strings that showed an average of 3.4% electrical power loss due to resistance in the interconnections between each individual solar cell. The optical losses of the ACPPVC were 15% caused by the combined effect of the number of reflections at the reflectors and the misalignment of the imperfection in the reflector geometry solar cells in addition to the power loss due to increased temperature of 0.6%. Good agreement was found between the measured and calculated optical efficiency of the system.     

A novel advanced box-type solar cooker

An advanced version of the box-type solar cooker is presented: a fixed cooking vessel in good thermal contact with a conductive absorber plate is set into the glazing; the results are improved thermal performance, easier access to the cooking vessel and less frequent maintenance due to protection of all absorbing and reflecting surfaces. Outdoor tests show that 5 L of water per sq m of opening surface can be brought to full boiling in less than one hour. A finite element simulation model of the advanced box cooker is presented. It is shown that the most decisive parameters are absorber-to-pot heat transfer and absorber conductivity. Field tests in Ethiopia and India are under way, local production in India has started.     

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%。     

Micro-structured reflector surfaces for a stationary asymmetric parabolic solar concentrator

One of the main problems in using parabolic concentrators with standard photovoltaics (PV) cells is the highly non-uniform illumination of the cells. The non-uniform irradiation causes high resistive losses in the standard cells due to their relatively high series resistance. This results in a considerably lowered efficiency. To solve the problem, we introduce three different structured reflectors that will create a more uniform illumination, and also increase the concentration ratio in certain cases. The structures were evaluated in an existing trough system by Monte Carlo ray tracing, and it was found that structures improve the system performance mainly by homogenizing the light on the cells. The yearly irradiation collected in the evaluation system is slightly lower than for a reference with smooth reflectors, but the more uniform illumination of the cells will generate a net increase of the total system performance compared to a system that was optimized with smooth reflectors. The benefit of the increased concentration ratio is increased flexibility in designing new systems with concentration ratios surpassing the limit of existing trough concentrators.     

Losses in a three-dimensional compound parabolic concentrator as a second stage of a solar concentrator

In this article, the losses due to reflection properties in a three-dimensional compound parabolic concentrator (3-D C.P.C.) are calculated and the effect of these losses on the concentration is analyzed. The 3-D C.P.C. is used as a second stage in two configurations of a two-stage concentrator: (a) a parabolic dish as a first stage, and (b) a Cassegrainian as a first stage. The nonplanar rays play an important role in 3-D C.P.C. losses. Thus, a ray-tracing procedure is needed to evaluate the losses. In this study, a rigorous three-dimensional ray tracing program was specially developed. The reflection losses and their effect on the concentration were determined from the reflection distribution derived by the developed ray-tracing program. As a result, the reflection losses were approximated by a simple empirical linear model useful in practical ranges of the 3-D C.P.C. acceptance angles and reflectivities. This model facilitates design and system optimization by analytical methods without resorting to a ray-tracing procedure. The approach presented in this article may be used to compare the performance of the two-system configurations, taking into account the 3-D C.P.C. losses.     

Productivity enhancements of compound parabolic concentrator tubular solar stills

The performance of compound parabolic concentrator assisted tubular solar still (CPC-TSS) and compound parabolic concentrator-concentric tubular solar still (CPC-CTSS) (to allow cooling water) with different augmentation systems were studied. A rectangular saline water trough of dimension 2 m × 0.03 m × 0.025 m was designed and fabricated. The effective collector area of the still is 2 m × 1 m with five sets of tubular still – CPC collectors placed horizontally with north-south orientation. Hot water taken from the CPC-CTSS was integrated to a pyramid type and single slope solar still. Diurnal variations of water temperature, air temperature, cover temperature and distillate yield were recorded. The results showed that, the productivity of the un-augmented CPC-TSS and CPC-CTSS were 3710 ml/day and 4960 ml/day, respectively. With the heat extraction technique, the productivity of CPC-CTSS with a single slope solar still and CPC-CTSS with a pyramid solar still were found as 6460 ml/day and 7770 ml/day, respectively. The process integration with different systems cost was found slightly higher but the overall efficiency and the produced distilled water yield was found augmented.     

Efficient orientation impacts of box-type solar cooker on the cooker performance

The addition of a plane reflector to a box-type solar cooker increases the obtained cooker temperature and this depends upon the efficient orientation of the cooker. In order to find out the effect of the cooker orientation on its performance, the present analysis is carried out. A method is outlined to find out a reflector performance factor and an orientation factor that depend upon the elevation angle of the sun, the solar surface azimuth angle and the reflector tilt angle. The analysis is applied to a cooker placed at Aden (Yemen). The results indicate that with proper cooker orientation the improvement in the performance of the cooker due to the reflector reached during winter is more than 100% at lower elevation angles and is more than 60% at higher elevation angles. It is concluded that with the help of the analysis the optimum position for any place, for any day of the year and for any specific time of the day can be found.     

Transient test of a solar air heater with a compound parabolic concentrator

A method for measuring the performance parameters of a solar thermal collector under non-steady conditions has been applied to an air heater with a truncated compound parabolic concentrator having an aperture area of 1.44 m² and a flat absorber with concentration ratio three. This type of collector was chosen in order to obtain high air temperatures in a tropical climate where the proportion of diffuse solar radiation is high. The parameters found were the optical efficiency, the linear and nonlinear heat loss coefficients, and the effective heat capacity. These parameters were determined individually in separate experiments, some with and some without solar radiation falling on the collector. Agreement within 2% was found between the optical efficiency measured in the experiments and the optical efficiency determined from direct measurements of the optical properties of the materials in the collector. The method can be performed all the year round in a tropical climate when steady conditions do not occur.     

The performance of a box-type solar cooker with auxiliary heating

In Bangladesh it is very difficult to use solar cookers during the months when the days are generally cloudy and at times solar cooking becomes impossible. To overcome such problems for a box-type solar cooker, we have used an auxiliary source of energy inside it. This is done with the help of a built-in heating coil inside the cooker or a retrofit electric bulb in a black painted cylinder. It is found that the use of auxiliary sources allows cooking on most cloudy days.     

A compound parabolic concentrator

A compound parabolic concentrator (CPC) for solar energy applications is presented in this work. A prototype was built and its thermal performance was determined. Operating temperatures of the order of 150 °C with a reasonable efficiency can be attained by means of a fixed CPC.     

太阳能建筑采暖系统槽式复合多曲面聚光器性能研究

为提高北方寒冷地区太阳能建筑采暖系统的适用性,研制了一套聚光比为3.9的非跟踪式复合多曲面聚光器(CPC)。该聚光器具有运行稳定、无运动部件及易于制造等特点。基于Light Tools光学软件,分析不同入射偏角和接收体偏差角度条件下的聚光特性,并对安装于呼和浩特市(北纬40°50')的太阳能建筑采暖系统在晴天和多云气象条件下不同安装倾角聚光器的光热性能进行对比研究。结果表明:当入射偏角为10°时,聚光器的光线汇聚率为55.2%,当板式接收体偏差角度为12°时,聚光器的汇聚率仍为93.4%;晴天运行时,安装倾角为45°的聚光器出口导热油温度比正入射时低3℃左右,多云条件下,接收体内导热油温度变化曲线平缓,说明聚光器对跟踪精度要求低,适合于应用到建筑采暖系统中。     

Experimental investigation of a box-type solar cooker with a finned absorber plate

This article provides the results and finding of an experimental work undertaken in the desert of Algeria. That aimed to compare experimental performance of a box-type solar cooker equipped with a finned absorber plate to a similar box-type cooker which absorber plate without fins. Tests have been carried out on the experimental platform of the Renewable Energies Research Unit in Saharan Environment of Algeria at Adrar. Adrar is located at a latitude 27° 53′ North and a longitude 0° 17′ West. Fins that have been used in solar air collectors enhanced heat transfer from absorber plate to air. Experimental tests have been undertaken as part of this project where was applied this phenomenon to a box-type solar cooker. The results of the experimental investigation have been rigorously analysed and showed that the stagnation temperature for box-type solar cooker equipped with a finned absorber plate was about 7% more than box-type solar cooker equipped with an ordinary absorber plate. The time required for heating water up to boiling temperature in both box-type solar cookers was reduced with about 12% when a finned absorber plate was used.     

Optical performance of an asymmetric inverted absorber compound parabolic concentrating solar collector

A computer-based numerical model has been developed utilising ray trace techniques, to simulate the optical characteristics of asymmetric inverted absorber line axis compound parabolic concentrating solar collectors (IACPCs). Cognisance is taken of the separate effects of the beam and diffuse properties of the total insolation on the optical performance. Increasing absorber gap height was expected to increase convection heat loss suppression and overall collector performance at the expense of a reduction in optical efficiency. The results are presented as graphical analyses of optical efficiency versus height of absorber gap, reflectivity and, acceptance and ray input angles.     

Numerical heat transfer studies of PCMs used in a box-type solar cooker

Theoretical investigations on the phase change materials (PCMs) used as the heat storage media for box-type solar cookers have been conducted in this study. The selected PCMs are magnesium nitrate hexahydrate, stearic acid, acetamide, acetanilide and erythritol. For a two-dimensional simulation model based on the enthalpy approach, calculations have been made for the melt fraction with conduction only. Different materials such as glass, stainless steel, tin, aluminum mixed, aluminum and copper are used as the heat exchanger container materials in the numerical calculations. The large value of thermal conductivity of heat exchanger container material did not make a significant contribution on the melt fraction except for at very low thermal conductivities. Based on the theoretical results, stearic acid and acetamide are found to be good compatibility with latent heat storage system. It is also found that the initial temperature of PCM does not have very important effects on the melting time, while the boundary wall temperature plays an important role during the melting and has a strong effect on the melt fraction. The results also show that the effect of thickness of container material on the melt fraction is insignificant. The results obtained in this paper show that in a box-type solar cooker, acetamide and stearic acid should be used as a latent heat storage materials.     

Raytrace for compound parabolic concentrator

This communication deals with the raytrace of tubular absorber of the compound parabolic concentrator. In order to investigate the location of the hot spots on the absorber, a raytrace was needed. There were different techniques to achieve an accurate raytrace. One of the technique was adopted is AutoCAD®. The analysis was carried out and applied to a CPC with an acceptance angle of 10° and a concentration ratio of x4.0. The objective was threefold. Firstly, it was necessary to calculate the profile of the CPC and the tubular absorber. Secondly, it was necessary to analyse the distribution of intensity around the circumference of the tubular absorber , Finally, to analyse the phenomena of the rays inside the CPC for different incident angle.     

The performance of a cylindrical parabolic solar concentrator

An experimental study has been conducted to determine the performance of a cylindrical parabolic concentrating collector with a novel design of the absorber. The experiments have been performed during winter and summer at Basrah, Iraq. It has been found that the concentrator performance depends mainly on water mass flow rate, and there is no significant change when the water mass flow rate becomes more than 10 kg/h.     

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.     

Top heat-loss factor of double-glazed box-type solar cooker from indoor experiments

The top heat-loss factor (U_t) of a box-type solar cooker varies with plate temperature, wind heat-transfer coefficient and ambient temperature. A method for correlating U_t with these variables is presented for a cooker with double glazing. A set of equations is developed for correlating data obtained in indoor experiments at different plate temperatures and wind speeds.     

Natural convective heat transfer in trapezoidal enclosure of box-type solar cooker

This paper presents simple thermal analysis to evaluate the natural convective heat transfer coefficient, h_c12 for a trapezoidal absorber plate-inner glass cover enclosure of a double-glazed box-type solar cooker. Several indoor simulation experiments in steady state conditions have been performed to measure the temperatures of absorber plate, inner and outer glass covers, ambient air, electrical input supply and wind speed. The experimental data has been correlated by an equation of the form, Nu = CRaⁿ. The values of the constants C and n, obtained by linear regression analysis are used to calculate the convective heat transfer coefficient. The heat transfer analysis predicts that h_c12 varies from 4.84 to 6.23 W m^{−2 o}C⁻¹ for the absorber plate temperature from 54 to 141 ^oC. The results of h_c12 are compared with those of rectangular enclosure for the same absorber-inner glass cover temperatures and gap spacing. The study reveals that the values of convective heat transfer coefficient and top heat loss coefficient for rectangular enclosure are lower by 31–35% and 7% respectively.     

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%。