Design considerations for solar collectors with cylindrical glass honeycombs

A properly designed cell structure placed between the solar absorber and outer cover glass can substantially reduce natural convection and infrared reradiation heat losses. Glass has merit for such a cellular structure or honeycomb because it is an inexpensive, abundant and stable material with low thermal conductivity and outstanding optical characteristics. To optimize the design of a honeycomb structure, i.e. to minimize the cost of the solar energy collected Z, requires the determination of the honeycomb solar transmission as a function of incidence angles of the sun, infrared effective emittance, cell Nusselt number, and cell wall conductance as well as an estimate of appropriate costs. For an array of circular tubes, the design parameters are wall thickness b, cell diameter d_i, and cell length L. It is difficult to make b less than about 0.2 mm. Typically, d_i must be no larger than 1.6 cm. Increasing decreases reradiation and conduction losses, but also decreases solar energy transmission. For d_i = 9.53 mm and b = 0.198 mm, optimum values ranged from 3 to 12 for collector temperatures (above ambient air temperature) between 22°C and 100°C. Since the Z vs curves have fairly broad minimums, values less than 9 can be used with less than a 3 per cent penalty in cost at the higher temperatures.A comparison of collector effiency characteristics indicated that cylindrical glass honeycomb collectors with nonselective-black absorbers were markedly superior to single-glazed selective-black and double-glazed nonselective-black collectors, especially at higher collector temperatures. Cost effectiveness studies also indicated honeycomb collector superiority at temperature differences between the working fluid and ambient air greater than about 35°C.     

Practical design considerations for CPC solar collectors

Several practical problems are addressed which arise in the design of solar collectors with compound parabolic concentrators (CPC's). They deal with the selection of a receiver type, the optimum method for introducing a gap between receiver and reflector to minimize optical and thermal losses, and the effect of a glass envelope around the receiver. This paper also deals with the effect of mirror errors and receiver misalignment, and the effect of the temperature difference between fluid and absorber plate. The merits of a CPC as a second stage concentrator are analyzed.     

Design considerations for residential solar heating and cooling systems utilizing evacuated tube solar collectors

As solar heating systems become a commercial reality, greater efforts are now being employed to incorporate solar cooling components in order to obtain a complete solar heating and cooling system and thus take advantage of the cost-effectiveness of year-round use of the solar equipment. Because of the exceptional performance and high efficiency of evacuated tube solar collectors, these advanced collectors are receiving considerable attention for use in solar heating and cooling systems. While improved performance is readily obtained with these sophisticated solar collectors, there are also numerous difficulties and problems associated with their use in a solar system. This paper addresses many of the design considerations which must be included in any realistic solar system design. Most of the considerations presented here are based on the experience gained in the design and performance of the solar heating and cooling systems for CSU Solar Houses I-IV.     

Cheap effective thermal solar-energy collectors

A light-weight flexible solar-collector, with a wavelength-selective absorption surface and an insolation-transparent thermal-insulation protector for its aperture, was built and tested. Its cheapness and high performance, relative to a conventional flat-plate solar-collector, provide a prima-facie case for the more widespread adoption of its design.     

Potential of carbon nanohorn-based suspensions for solar thermal collectors

The optical characterization is reported of a new fluid consisting of single-wall carbon nanohorns and ethylene glycol for solar energy applications. Carbon nanohorns play a significant role in enhancing sunlight absorption with respect to the pure base fluid. The obtained results are compared with those obtained for fluids suspending more conventional carbon forms, i.e. carbon-black particles. We found that nanohorn spectral features are far more favorable than those of amorphous carbon for the specific application. This result shows that carbon nanohorn-based nanofluids can be useful for increasing the efficiency and compactness of thermal solar devices, reducing both environmental impact and costs.     

Optimum performance of thermal trap collectors

The “thermal trap effect” in semitransparent material and the trapping system in the conventional flat-plate collectors with two, three or four glass or plastic covers with air-gaps in between are analysed under a common heading of “thermal trap collectors”. In general, a thermal trap collector consists of one or many slabs of semitransparent material of finite thickness with air-gaps in between, and an ideal withdrawal mechanism at the base of the trapping system to withdraw all available energy. This approach makes a comparative study of the two types of collectors possible, and provides data to design the appropriate withdrawal mechanism and operating conditions. A steady state analysis which neglects internal reflections and body radiation shows the existance of an optimum performance in single-layer thermal trap collectors and its dependence on thickness. A model which includes internal reflections is then analysed and the existance of the optimum performance and its dependence on thickness is demonstrated by taking the example of a single slab of methylmethacrylate plastic. The model is extended to multilayer thermal trap collectors and two examples are considered; a multilayer methylmethacrylate thermal trap collector and a multilayer “glass” thermal trap collector. The results show that the two-layer methyl methacrylate thermal trap collector has, in general, a better performance than the corresponding single or three and four-layer systems. But at high withdrawal efficiencies of about 60 per cent, the single layer methyl methacrylate shows its uniqueness and becomes competitive with the two-layer system. But the three and four-layer “glass” thermal trap collectors perform better than the corresponding single and two-layer ones, with the three-layer system having an overall better performance. These results show that the number of slabs in addition to thickness are important parameters in the study of the performance of thermal trap collectors.     

Advances in liquid based photovoltaic/thermal (PV/T) collectors

In order to get more power and heat from PV/T system, it is necessary to cool the PV cell and decrease its temperature. This is not an easy task especially in hot and humid climate areas. There is a lack of an effective cooling strategy of PV/T panels. The liquid based photovoltaic thermal collector systems are practically more desirable and effective than air based systems. Temperature fluctuation in liquid based PV/T is much less than the air based PV/T collectors which subjected to variation in solar radiation levels. In this study a review of the available literature on PV/T collector systems which utilize water and refrigerant (working fluid) as heat removal medium for different applications has been conducted. Future direction of water-cooled and refrigerant hybrid photovoltaic thermal systems was presented. This study revealed that the direct expansion solar-assisted heat pump system achieved better cooling effect of the PV/T collector.     

Field study of various air based photovoltaic/thermal hybrid solar collectors

In the present work a comparative study for thermal and electrical performance of different hybrid photovoltaic/thermal collectors designs for Iraq climate conditions have been carried out. Four different types of air based hybrid PV/T collectors have been manufactured and tested. Three collectors consist of four main parts namely, channel duct, glass cover, axial fan to circulate air and two PV panels in parallel connection. The measured parameters are, the temperature of the upper and the lower surfaces of the PV panels, air temperature along the collector, air flow rate, pressure drop, power produced by solar cell, and climate conditions such as wind speed, solar radiation and ambient temperature. The thermal and hydraulic performances of PV/T collector model IV have been analyzed theoretically based on energy balance. A Matlab computer program has been developed to solve the proposed mathematical model.The obtained results show that the combined efficiency of collector model III (double duct, single pass) is higher than that of model II (single duct double pass) and model IV (single duct single pass). Model IV has the better electrical efficiency. The pressure drop of model III is lower than that of models II and IV. The root mean square of percentage deviations for PV outlet temperature, and thermal efficiency of model IV are found to be 3.22%, and 18.04% respectively. The calculated linear coefficients of correlation (r) are 0.977, 0.965 respectively.     

太阳能光电/光热一体化技术及其应用进展研究

太阳能光电/光热一体化系统主要由光伏电池组件和太阳能集热器组成,可同时实现光伏发电和光热利用,从而有效地提高了太阳能的综合利用效率。文章首先从光伏组件和光热部件着手,分析了PV/T系统的结构和各项性能;然后,概述了目前常用的PV/T热水系统性能评估方法;最后,提出了在推广PV/T系统时还须解决的问题。     

Potential of quarterwave interference stacks for colored thermal solar collectors

The architectural integration of thermal solar collectors into buildings is often limited by their black color and the visibility of tubes and corrugations of the absorber sheets. A certain freedom in color choice would be desirable, but the colored appearance should not cause excessive energy losses. Multilayered interference filters on the collector glazing can produce a colored reflection, hiding the corrugated metal sheet, while transmitting the non-reflected radiation entirely to the absorber. We investigate the potential of quarterwave stacks by simulation of their optical behavior, yielding the visible reflectance R_VIS, the solar transmittance T_sol, a figure of merit M = R_VIS/R_sol, and the CIE color coordinates. The necessary number of individual layers in the multilayer stack as well as the choice of refractive indices and thus of thin film materials are discussed. Finally, examples for realistic multilayer designs are proposed.     

Cost of paraboloidal collectors for solar to thermal electric conversion

Much is underway concerning technical and design aspects of solar collectors and associated costs of point designs. But little has been published on the tradeoffs between alternative designs and large-scale manufacturing possibilities. To fill this gap, close interchange is needed among those versed in technical and systems design, manufacturing and costing. The solar to thermal electric power study funded by the National Science Foundation and carried out by Colorado State University and Westinghouse Electric Corporation has led to the assembly of such a team. This team is obtaining new data on large-scale production of concentrating collectors of widely varying parameters.

Investment and other costs of paraboloidal collectors with various aperture widths, reflectivities, rim angles, and accuracies of contour and tracking are reported herein. In studying alternative design-cost-performance relationships, emphasis is placed on breadth of coverage rather than on detailed accuracy. Alternative aperture widths led to preferences for different production processes that are not particularly sensitive to changes in rim angle or reflectivity. Significant cost relationships in the manufacture, installation and maintenance of collectors were found; other relationships are less certain. Besides the above, study results indicate directions for further study.     

Heat withdrawal from multi-layer thermal trap collectors

The single-pass as well as the two-pass mode of heat withdrawal from a two-layer Thermal Trap Collector with slabs of variable thermal and optical properties are modelled mathematically. The working fluid is assumed to be transparent. Results are computed for slabs of methylmethacrylate plastic and “ideal” glass with air as the working fluid and these are compared with the air-heater experimental results due to Satcunanathan and Deonarine[1] and the novel collector theoretical results due to Caouris et al.[2]. From such a comparison the important features which increase the efficiency of a two-pass two-layer Thermal Trap Collector are identified.     

Design of chemical composition and optimum working conditions for trivalent black chromium electroplating bath used for solar thermal collectors

Black chromium is one of the selective surfaces which are commonly used in solar thermal systems. The aim of this research was to design an electroplating bath, using chromium trivalent ions instead of its hexavalent ions because of its toxicity, for black chromium coating to produce a good metallurgical and light properties at low cost and reproducibility. The effect of the additives was investigated and a coating with an absorption coefficient of 0.96 was prepared. The bath contained inexpensive constituents such as chromium sulfate, cobalt chloride, sodium fluoride, sodium hypophosphite and sodium dihydrogen phosphate and did not need any pH control. Thermal resistance, corrosion resistance and adhesion of the coatings as well as throwing power of the bath were investigated. The effect of bright Ni as an undercoat before black chromium electroplating was also investigated. SEM and XRD techniques were employed to characterize the surface microstructure and chemical composition. Spectroscopy was also used to measure absorptance of the coatings.     

A generalized thermal analysis of tubular solar collectors

A generalized thermal analysis of tubular solar collectors such as the CPC, CPC with bare absorber and flat-plate (all having U-tube fluid carriers) is presented. The analysis is applicable for smaller as well as larger size troughs. It has been found that earlier published results of thermal analyses of the CPC collectors are not applicable to all CPC collectors encountered in the literature. The present analysis can, however, be successfully applied to all such collectors.     

Energy delivery of solar thermal collectors in Zimbabwe

The long-term annual thermal energy delivery per unit of collector area of commonly used collector types and configurations, for a range of operating temperatures, are calculated for representative locations in Zimbabwe. A well-known model found in the literature is the basic tool of analysis, the only modifications being, the use of a locally-derived correlation of the monthly average diffuse fraction of hemispherical radiation to the monthly average clearness index, and the use of temperature-dependent collector heat loss coefficients. The results are presented as plots of annual specific thermal energy output against collector receiver temperature gain (T_r−T_a).The results, though founded on a number of simplifying assumptions on some collector parameters, provide a sound basis for the economic evaluation of solar thermal applications in Zimbabwe.     

Indoor simulation and testing of photovoltaic thermal (PV/T) air collectors

An indoor standard test procedure has been developed for thermal and electrical testing of PV/T collectors connected in series. For this, a PV/T solar air heater has been designed, fabricated and its performance over different operating parameters were studied. Based on the energy balance equations, in a steady state condition, a thermal model has been developed. Comparison between experimental and theoretical results were also been carried out. The thermal and electrical efficiency of the solar heater is 42% and 8.4%, respectively. This test procedure can be used by manufacturers for testing of different types of PV modules in order to optimize its products.