Optical evaluation and analysis of an internal low-concentrated evacuated tube heat pipe solar collector for powering solar air-conditioning systems

An optical evaluation and analysis of an internal low-concentrating evacuated tube heat pipe solar collector designed to enhance the collection of solar radiation for medium temperature applications is presented in this paper. The internal low-concentrating evacuated tube heat pipe solar collector was designed with an acceptance angle of 20° given a geometrical concentration ratio of 2.92. The truncation of the upper part of the reflector giving a geometrical concentration ratio of 1.95 was carried and enabled the internal low-concentrating evacuated tube heat pipe collector to be enclosed by a borosilicate glass tube with 100 mm and 93 mm outer and inner diameters, respectively. Ray trace analysis at different transverse angles determines optical efficiencies, related optical losses and flux distribution on the absorber of the internal low-concentrating evacuated tube heat pipe solar collector. A detailed two dimensional ray trace techniques considering only the direct insolation component predicated overall ray’s acceptance of 93.72% and optical efficiency of 79.13% from transverse angles of 0° to 20°.     

Cusp mirror—Heat pipe evacuated tubular solar thermal collector

A solar thermal collector was constructed based on an internal 1.15X cusp concentrator, thermal insulation involving a vacuum and selective absorber, and thermal transfer to a manifold via heat-pipe action. Performance of the collector was compared with that of an evacuated, selectively coated, flat-plate absorber equipped with flow-through heat transfer. It was shown that with single collector tubes, mirror losses lowered the optical efficiency of the cusp, heat-pipe collector below that of the flat plate, while the smaller absorber area of the heat pipe reduced thermal losses at absorber temperatures above ambient. Thus, a crossover in efficiency occurred such that the flat plate was more efficient at low while the cusp-heat pipe was more efficient at high . Testing of modules showed that manifold losses and gains could dominate these collector effects when the collector area approximately equaled the manifold area.     

Thermal analysis of CPC collectors

Mathematical formulations were developed to study thermal processes in a compound-parabolic-concentrator (CPC) collector. The system under investigation consists of a CPC cusp fitted with a concentric, evacuated double pipe to serve as a heat absorber. Heat is transmitted to the circulating fluid flowing inside a U-tube via the heat getter slipped inside the inner pipe. The collector has a cover for dust protection. Four nonlinear, simultaneous equations were derived to predict heat exchange among various components in the system. Collector efficiency equations were also developed following the Hottel-Whillier-Woertz-Bliss formalism. These equations were subsequently used in a computer program to test the collector performance under varied operating conditions. Test results indicate that, because of the high thermal resistance between the receiver jacket and the envelope, the collector performance is quite stable and is nearly independent of many parameters tested. The efficiency of the collector is high and shows only a very slight drop at high operating temperatures. The predicted results were also compared with experiments.     

Comparison of the optics of non-tracking and novel types of tracking solar thermal collectors for process heat applications up to 300 °C

Evacuated CPC (compound parabolic concentrator) collectors with non-tracking reflectors are compared with two novel tracking collectors: a parabolic trough and an evacuated tube collector with integrated tracking reflector. Non-tracking low concentrating CPC collectors are mostly mounted in east–west direction with a latitude dependent slope angle. They are suitable at most for working temperatures up to 200–250 °C. We present a tracking evacuated tube-collector with a trough-like concentrating mirror. Single-axis tracking of the mirror is realized with a magnetic mechanism. The mirror is mounted inside the evacuated tube and hence protected from environmental influences. One axis tracking in combination with a small acceptance angle allows for higher concentration as compared to non-tracking concentrating collectors. Ray-tracing analysis shows a half acceptance angle of about 5.7° at geometrical concentration ratio of 3.2. Losses of well constructed evacuated tube collectors (heat conductivity through the manifolds inside the thermally insulated terminating housing are low) are dominated by radiation losses of the absorber. Hence, reducing the absorber size can lead to higher efficiencies at high operating temperature levels. With the presented collector we aim for operating temperatures up to 350 °C. At temperatures of 300 °C we expect with anti-reflective coating of the glass tube and a selective absorber coating efficiencies of 0.65. This allows for application in industrial process heat generation, high efficient solar cooling and power generation. A first prototype, equipped with a standard glass tube and a black paint absorber coating, was tested at ZAE Bayern. The optical efficiency was measured to be 0.71. This tube-collector is compared by ray-tracing with non-tracking market available tube-collectors with geometrical concentration ratios up to 1.1 and with a low cost parabolic trough collector of Industrial Solar Technology (IST) with an acceptance half angle about 1.5°, a geometrical concentration ratio of 14.4 and a measured optical efficiency of 0.69.     

Optimisation of evacuated tubular solar collector arrays with diffuse reflectors

The optical efficiencies η_o of arrays of evacuated tubular collectors incorporating plane, triangular and semicircular shaped reflectors coated with flat-white and gloss white paint have been studied experimentally using a calorimetric technique and theoretically using a ray tracing computer program. The results showed that the plane reflector is the optimum design. Detailed studies have been made of the dependence of optical efficiency and incident angle modifier as a function of collector tube separation for collectors incorporating the plane reflector. Two collector panels complete with heat extraction manifold and incorporating the plane reflector, but with different tube spacings were subject to detailed outdoor testing. The results indicated that it is cost-effective to space the collector tubes two or more absorber tube diameters apart.     

Thermal performance of linear Fresnel reflecting solar concentrator with trapezoidal cavity absorbers

Thermal performance of the four identical trapezoidal cavity absorbers for linear Fresnel reflecting solar device were studied and compared. The absorbers were designed for operating in conjunction with a prototype Fresnel solar reflector. Rectangular and round pipe sections were used as absorber by placing in the trapezoidal cavity. The absorber pipes were coated with ordinary dull black board paint and black nickel selective surface. The bottom of the cavity was provided with plane glass to allow the solar radiation to be reflected from the Fresnel reflector. The other three sides of the cavity absorber were insulated to reduce heat loss. Thermal performance of the Fresnel reflecting concentrator with each trapezoidal cavity absorber was studied experimentally at different concentration ratio of the reflector. The study revealed that the thermal efficiency was influenced by the concentration ratio and selective surface coating on the absorber. The thermal efficiency decreased with the increase in the concentration ratio of the Fresnel reflecting collector. The selective surface coated absorber had a significant advantage in terms of superior thermal performance as compared to ordinary black painted absorber. The round pipe (multi-tube) receiver had higher surface area to absorb solar energy as compared to rectangular pipe receiver. Thermal efficiency of the solar device with round pipe absorber was found higher (up to 8%) as compared to rectangular pipe absorber.     

Analysis of a fixed spherical reflector exposed to oblique incident rays

A fixed spherical reflector tracking absorber solar collector, popularly known as Fixed Mirror Distributed Focus (FMDF) collector, is described. An analysis has been performed to study the variations in optical concentration along the absorber. Optical concentrations have been determined for cases when the sun rays strike the reflector obliquely and have been compared with the values obtained for a noon condition when the sun rays would be normal to the reflector. An elemental ring on the reflector surface is taken up for analysis to determine the useful area of the reflector that would reflect the incident rays towards the absorber. The area of the sector of the elemental ring which illuminates the absorber is determined. The effect of the reflectivity on the optical concentration is also studied. The predicted values are indirectly compared with experimentally measured surface temperatures along the absorber of a laboratory FMDF unit.     

Optimized reflectors for non-tracking solar collectors with tubular absorbers

We present an approach to find optimal reflector shapes for non-tracking solar collectors under practical constraints. We focus on cylindrical absorbers and reflectors with translational symmetry. Under idealized circumstances, edge ray reflectors are well known to be optimal. However, it is not clear how optimal reflectors should be shaped in order to obtain maximum utilizable energy for given operating temperatures under practical constraints like reflectivity less than unity, real radiation data, size limits, and gaps between the reflector and the absorber. For a prototype collector with a symmetric edge ray reflector and a tubular absorber, we derive from calorimetric measurements under outdoor conditions the optical efficiency as a function of the incidence angle. Using numerical optimization and raytracing, we compare truncated symmetric edge ray reflectors, truncated asymmetric edge ray reflectors and free forms parametrized by Bezier splines. We find that asymmetric edge ray reflectors are optimal. For reasonable operating conditions, truncated asymmetric edge ray reflectors allow much better land use and easily adapt to a large range of roof tilt angles with marginal changes in collector construction. Except near the equator, they should increase the yearly utilizable energy per absorber tube by several percent as compared to the prototype collector with symmetric reflectors.     

用于发电的太阳能聚光热管集热器

复合槽形抛物面和渐开线反射镜面的聚光吸收太阳能的热管集热器 ,平均集热温度高达16 4℃ ,并省去了复杂的跟踪机构。用热管作太阳吸收体 ,传热效率高 ,向大气反向传热损失少。圆筒形换热器的热水温度 16 0℃ ,饱和水蒸气压力 4 0 .6kg .f cm2 ,用于汽轮机发电和蓄能     

反光板对提高热管真空管热性能的研究

提出了对带反光板的吸热体接收太阳辐射量的分析方法，并对三种有渐开线反光板且吸热体形状不同的热管真空管和一种无反光板的热管真空管进行了对比实验。结果表明：渐开线反光板可大幅度提高热管真空管的热性能，而且其吸热体的形状对热性能提高的幅度有影响。     

Study of a solar water heater using stationary V-trough collector

There are various types of solar water heater system available in the commercial market to fulfill different customers’ demand, such as flat plate collector, concentrating collector, evacuated tube collector and integrated collector storage. A cost effective cum easy fabricated V-trough solar water heater system using forced circulation system is proposed. Integrating the solar absorber with the easily fabricated V-trough reflector can improve the performance of solar water heater system. In this paper, optical analysis, experimental study and cost analysis of the stationary V-trough solar water heater system are presented in details. The experimental result has shown very promising results in both optical efficiency of V-trough reflector and the overall thermal performance of the solar water heater.     

Effect of multiple reflections on the performance of plane booster mirrors

This paper presents an analytical study of a stationary V-trough concentrator. It consists of an array of east-west oriented trapezoidal channels with two side reflecting walls and a tubular absorber as a receiver at the base. The formulae for concentration factor and reflector surfaces have been derived. It is explicitly shown that the concentration ratio and reflector surface area depend upon the number of reflections the solar rays undergo before reaching the absorber, the cone apex angle, the coefficient of reflection and the acceptance angle. Results are presented graphically in such a way that one can choose the optimum configuration and the minimum material required to achieve a given concentration factor. The concentration ratio ranges from 1·2 to 3·6. The variations of the collector's efficiency with temperature difference for different numbers of reflections, acceptance angles, convective heat transfer coefficients and coefficients of reflectivity have been predicted.     

Measurement of radiation distribution on the absorber in an asymmetric CPC collector

A method to estimate the annual collected energy and the annual average optical efficiency factor is suggested. The radiation distribution on the absorber of an asymmetric CPC collector with a flat bi-facial absorber is measured for three different absorber mounting angles using a photo diode. The annual optical efficiency factors and a relative measure of the annual collected energy are determined for collectors with the absorber fin thickness 0.5 and 1 mm, and for a collector with a teflon convection suppression film mounted around the absorber. With the local optical efficiency factors and the annual incident solar energy distribution considered, the analysis indicates that the energy gain for a mounting angle of 20° is higher than for a collector with 65° absorber mounting angle. The annual collected energy is increased with 6–8% if the absorber fin thickness is increased from 0.5 to 1 mm. The annual average optical efficiency factor is relatively independent of the absorber mounting angle. It was found to be 0.87–0.88 for a collector with a 0.5 mm thick absorber fin and 0.92 for a collector with a 1 mm thick absorber fin or for a collector with 0.5 mm thick absorber fin with a teflon convection suppression film added. The low annual average optical efficiency factor is not caused by the uneven irradiance distribution but by the relatively high U_L-values.     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.     

Simulation and optimisation studies on a solar parabolic collector: an experimental investigation

The present paper has focused on parametric optimisation of a solar parabolic collector. This work has been performed in two phases. In the first phase, simulation studies have been done to identify the influential parameters. Based on simulation results, an experimental set-up of the parabolic collector has been fabricated using the materials selected by priority-score method. In the second phase, experiments are conducted according to Taguchi's L₉ orthogonal array. Parabolic collector parameters, namely reflector materials, absorber materials, positions of the absorber tube and angle of the absorber tube, are optimised with the consideration of multi-responses such as temperature, enthalpy, optical efficiency and thermal efficiency. The obtained experimental data are analysed using the desirability functional analysis approach and optimal levels of input parameters have been identified. Analysis of variance also has been performed to know the contribution of influential parameters on the responses.     

Heat transfer aspects of an elevated linear absorber

This paper describes aspects of the design methodology and heat transfer calculations for an elevated north–south oriented linear absorber. The absorber is part of a direct steam generation solar thermal concentrating system based on the Australian compact linear Fresnel reflector (CLFR) concept. The basic absorber design is an inverted air cavity with a glass cover enclosing a selective surface. This arrangement has been shown previously to offer good optical and thermal performance from measurements on a 4 kW_thermal outdoor test apparatus. Two main design aims are discussed here: Firstly to maximise the heat transfer between the absorbing surface and the steam pipes, and secondly, to ensure that the absorber surface temperature is sufficiently uniform so as not to cause thermal degradation of the selective surface. Results are given of the absorber temperature distribution obtained from finite element analysis. Sufficiently low temperature differences between the fluid surface and the absorbing surface (<20 K) can be achieved with satisfactory pipe separations and sizes, and with practical absorber plate thicknesses.     

Development and performance analysis of compound parabolic solar concentrators with reduced gap losses—‘V’ groove reflector

A system has been developed to use compound parabolic concentrators to collect solar energy and to generate steam. A CPC reflector profile with a V groove at the bottom of the reflector to reduce the gap losses was designed with a half acceptance angle of 23.5° for a tubular absorber of OD 30 mm. Five troughs fabricated with fiberglass substrate pasted over with UV stabilized self-adhesive aluminized polyester foil having high specular reflectivity joined together side by side comprise the CPC module with an aperture area of 2.04 m². Copper tubes coated with NALSUN selective coatings and enclosed by borosilicate glass envelope act as absorbers. The reflector absorber assembly housed in a single glass wool insulated wooden box forms the CPC collector. Using water as the heat transfer fluid efficiency tests were carried out with different inlet temperatures. In situ steam generation testing and possible application to steam cooking were also carried out. A theoretical modeling was developed by setting up different heat balancing equations and a reasonable agreement between theoretical computed values and the experimental values was observed.     

Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C_R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.     

Effect of off-south orientation on optimum conditions for maximum solar energy absorbed by flat plate collector augmented by plane reflector

A complete analysis of the off-south oriented, simple flat plate collector, augmented by flat sheet specular reflector, is developed. The enhancement of heat flux absorbed by solar collector due to the use of reflector is calculated as a funciton of solar altitude and azimuth angles, off-south orientation angle of collector and relative sizes and tilt angles of both collector. The shading effect due to the presence of the reflector is considered in the analysis. The collector and reflector variables are optimized for maximum solar energy flux absorbed by the collector during a pre-specified period of time. The Hooke and Jeeves optimization technique has been used in the analysis.     

Optimization of heat losses in normal and reverse flat-plate collector configurations: Analysis and performance

This communication presents a comparative analysis of flat-plate collector operated in normal and reverse configurations. Radiative and convective heat losses from the absorber and back collector have been minimised by optimizing the absorber to cover glass separation (gap width), providing low emissivity surfaces on the collector's back face and spacing an additional reflector supported on glasswool insulation behind the back face. Expressions for the transient and stagnation temperatures of the absorber plate have been derived. Experimental results are found to be in close agreement with the theory.