Effect of collector components on the collection efficiency of tubular evacuated collectors with diffuse reflectors

The efficiencies η_o of arrays of evacuated tubular collectors with diffuse reflectors have been determined experimentally using a calorimetric technique and theoretically using a Monte-Carlo ray tracing technique. Results have been obtained on collector arrays with various collector tube separations and collector-reflector distance, using two types of reflector, and efficiencies are compared for collector tubes with and without antireflection layers on the glass envelopes. The variation of collector efficiency with angle of incidence for sunlight has also been studied for two collector tube separations. The reflecting properties of the reflectors, glass envelope and selective absorber have been modelled in some detail for the ray tracing calculations. Experimental and theoretical efficiencies agree within the experimental and theoretical uncertainties, and all the trends observed experimentally are predicted by ray tracing. The efficiency of the collectors is not strongly dependent on the reflectance of the diffuse reflector, but depends strongly on the collector tube separation. Antireflection layers which increase the transmittance through the glass envelope by 5% result in an increase of 0.02 (about 3 per cent) in collector efficiency.     

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.     

Optical collection efficiencies of arrays of tubular collectors with diffuse reflectors

The fractions of solar radiation collected by arrays of cylindrical absorbers with diffuse reflectors of various shapes have been found using a Monte-Carlo ray tracing technique. The results for configurations with black absorbers with no outer glass envelopes and various shapes and positions of the diffuse reflector show that there is little to be gained in having curved diffuse reflectors when the finite absorptance of the reflector is included, and that there is an optimum position for a planar diffuse reflector.The fractions of solar radiation collected for arrays of modules with outer glass envelopes and selective absorbers, situated above a plane diffuse reflector, have been calculated for variable module spacings in the range 1.5-3 absorber tube diameters. The calculations include the effects of polarisation and angle of incidence on the reflectance of the outer envelope and the selective absorber, and the effect of absorption in the glass envelope and by the reflector. Multiple reflections or second chance effects play an important role in increasing the fraction absorbed over what is predicted with simple theories.The curves derived for the model with realistic properties of the components are useful for designing collectors. Tube spacings less than two diameters produce only small increases in collector performance. Collectors made with the modules aligned E-W rather than N-S will be less efficient, but the difference will be small ( 5 per cent).     

Angular dependence of optical efficiency of evacuated tubular collectors with antireflection coatings and stationary specular reflectors

The efficiencies of η₀ of arrays of evacuated tubular collectors with non-imaging specular reflectors have been determined experimentally using a calorimetric technique and theoretically using a Monte-Carlo ray tracing technique. Results have been obtained for collectors incorporating reflectors of two concentrations, and efficiencies are compared with and without antireflection coatings on the glass envelopes for sunlight incident at angles 0–70°. The reflective properties of all optical components have been modelled in detail for the ray tracing calculations. Experimental and theoretical efficiencies agree within 0.02 for a wide range of angles of incidence. Antireflection coatings which increase the normal transmittance through a glass envelope by 5% result in an increase of 0.025 (about 4%) in collector efficiency. A theoretical study of the dependence of collector efficiency on absorptance of the absorber tube and specular reflectance of the reflectors is also discussed. Experimental and theoretical results have also been obtained for a collector incorporating a specular reflector with an accumulated dust cover. In this system, the reflector exhibits both specular and diffuse components of reflectance.     

Heat transport in Dewar-type evacuated tubular collectors

This study focuses on the performance of a particular type of manifold (single-ended metal riser system) used to extract heat from Dewar-type evacuated tubular collectors. This heat extraction concept is similar to liquid-in-glass systems which have shown to perform very well. However, the additional thermal resistance between the selective surface and the metal tube carrying the collector fluid results in a performance reduction. The heat transfer processes within this system are analysed experimentally and analytically. Excellent agreement is obtained between measurements and theoretical models based on conduction and radiation heat transfer within the collector. Important parameters affecting overall collector performance are discussed.     

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.     

Optical collection efficiencies of tubular solar collectors with specular reflectors

The optical collection efficiencies of arrays of evacuated tubular collectors with specular involute reflectors have been studied using computer ray tracing techniques incorporating all the major physical features. The optical collection efficiencies under various representative assumptions concerning the angular distribution of incident radiation are presented as functions of the tube to tube spacing. The hemispherical collection efficiency obtained with collector aperture equal to absorber surface is in accordance with the expected loss resulting from the truncation of the reflector, and the losses associated with the mirror, with the envelope and with the absorber. At smaller apertures, the collection efficiency for hemispherically incident radiation agrees with a recent theory of O'Gallagher et al.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Optimization of stationary nonimaging reflectors for tubular evacuated receivers aligned north-south

Several manufacturers are developing solar collectors with tubular evacuated receivers aligned north-south. Adding low-concentration, wide-acceptance-angle reflectors to such tubes allows greater tube spacings, reducing the number of tubes per area of collector. It also improves collector efficiency, particularly for conditions of high , such as high temperatures or low light levels. This detailed study optimizes the reflector design for maximum daily energy collection and includes the effects of reflection losses, reflector-receiver alignment errors, variation of selective surface absorptance with angle of incidence on the receiver, and losses through the gap between the receiver and the reflector.Three general conclusions have been reached: The use of optimized nonimaging reflectors with tubular evacuated receivers will increase the energy collection efficiency—particularly for high-temperature and harsh environment conditions.Wide-acceptance-angle reflectors are forgiving to receiver-reflector alignment errors. It is neither necessary nor desirable to design reflectors for undersize receivers in order to compensate for misalignments that result from manufacturing tolerances.The daily energy collection of collectors using these reflectors having acceptance half-angles in a range. near 60° is not a sensitive function of the acceptance angle. Manufacturers' final reflector design decisions will probably be based on technical considerations related to fabrication and assembly techniques and possibly on market-related considerations such as collector appearance.     

Suitable selective absorbers for all-glass, evacuated, tubular, solar-energy collectors

Solar technologists working on all-glass evacuated tubular collectors are handicapped because the use of selective obsorbers is inherent in the design but most of the selective absorber coatings have been investigated and tested on metallic substrates. This study reviews methods to produce solar selective absorber surfaces which have been developed and tested on glass substrates to be used in evacuated collectors for the photo-chemical conversion of solar energy. It is also aimed to examine selective surfaces which are prospective candidates to be applied on glass tubes. This may provide an impetus to the scientists working in the area of solar energy materials to develop and test selective absorbers not only on metallic substrates but on glass tubes as well.     

Comparative study of fluid-in-metal manifolds for heat extraction from single ended evacuated glass tubular collectors

An investigation is reported of heat transfer between the glass absorber tubes of all-glass evacuated collectors and fluid-in-metal manifolds designed for heat extraction from the glass asborber tubes. The heat transfer is studied using a novel solar simulator which heats a panel of glass tubes electrically to simulate solar input to a collector panel. Measurement of the temperatures at various points on the glass tubes and on the manifolds gives a measure of the efficiency of heat transfer for each manifold under various operating conditions and allows calculation of the efficiencies η₀ of collectors incorporating the manifolds. The effect of fluid temperature, collector inclination and fluid flow rate has been investigated for four manifold designs of increasing simplicity. Experimental results for the manifolds are compared with calculations of heat transfer. Potential lifetime problems for the manifolds are also discussed. The simplest manifold design is shown to have good prospects for high-temperature (>100°C) heat extraction.     

Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold

A study is reported of thermosiphon circulation in solar water heaters incorporating glass tubular evacuated collectors and a water-in-glass manifold of extremely simple design. The manifold is characterised by the absence of partitioning of the inner volumes of the absorber tubes into inlet/outlet channels and buoyancy effects are utilised to convey heat to a header pipe at the open end of the tubes. Solar energy input to the tubes has been simulated by electric heating. The thermosiphon system design is unusual in that there are no risers within the collector tubes, thus, the pressure head responsible for thermosiphon flow originates entirely from relatively short pipe runs between manifold and storage tank. Thermosiphon flow has been measured for a number of system designs and a wide range of operating conditions. The relative impedances of the system components has been evaluated allowing optimization of the system design. An investigation of the effect of withdrawal of hot water from the storage tank, with associated injection of cold water to the bottom of the tank, has illustrated that the self-regulating nature of the thermosiphoning system results in a large proportion of heat stored in the wate filled collector tubes being effieciently transferred to the storage tank, providing some water is drawn off intermittently.     

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.     

Solar motors with flat-plate collectors

Research at the Laboratory of Meteorological Physics of the University of Dakar has been devoted for several years to the study of solarenergy utilization. An experimental solar motor has been operating since August 1962. This motor drives a pump lifting 8 to 10 liters water per minute from a depth of 13 meters. (Output 130 kg meters per MINUTE = 21 watt).

This experiment being successful, the construction of a more efficient pump was begun. It has a collector of 300 m² (3228 ft²) capable of supplying (during 5 hours/day) about 40 m³/hour of water lifted from a depth of 8 to 10 m (10,000 gallons water per hr raised 27 to 33 ft). This quantity of water represents the approximate daily requirement of a community of 500 persons. This engine, which is very simple and sturdily built, necessitating no servicing, opens a new area in the utilization of solar energy in tropical countries.     

Optimal tilt-angles of all-glass evacuated tube solar collectors

In this paper, a detailed mathematical procedure is developed to estimate daily collectible radiation on single tube of all-glass evacuated tube solar collectors based on solar geometry, knowledge of two-dimensional radiation transfer. Results shows that the annual collectible radiation on a tube is affected by many factors such as collector type, central distance between tubes, size of solar tubes, tilt and azimuth angles, use of diffuse flat reflector (DFR, in short); For collectors with identical parameters, T-type collectors (collectors with solar tubes tilt-arranged) annually collect slightly more radiation than H-type collectors (those with solar tubes horizontally arranged) do. The use of DFR can significantly improve the energy collection of collectors. Unlike the flat-plate collectors, all-glass evacuated tube solar collectors should be generally mounted with a tilt-angle less than the site latitude in order to maximize the annual energy collection. For most areas with the site latitude larger than 30° in China, T-type collectors should be installed with a tilt-angle about 10° less than the site latitude, whereas for H-type collectors without DFR, the reasonable tilt-angle should be about 20° less than the site latitude. Effects of some parameters on the annual collectible radiation on the collectors are also presented.     

利用同心套管的简化CPC式中高温真空管太阳能集热系统设计与实验研究

通过实验研究了一种利用简化复合抛物面聚光器(compound parabolic concentrato,CPC),全玻璃真空集热管和同心套管组成的太阳能中高温空气集热设备,可以满足工业过程对150℃至200℃的中高温度空气需求。该太阳能空气集热系统由8级集热单元串联而成。各单元都包括一个简化式CPC、一个双层玻璃真空管和一根铜套管。套管被安装在玻璃管内,空气在套管内逐级加热。对各种天气条件和流动参数对集热系统出口空气温度、系统功率和集热效率的影响进行了分析和试验研究。结果表明,整个系统具有良好的中高温集热性能。即使出口空气温度达到210℃,系统平均集热效率仍然达到20%;秋天晴天系统出口空气温度可达210℃,秋季阴雨天也可达168℃。试验结果确认这种简化CPC式全真空玻璃集热管和套管的组合装置是一种有工业实用前途的太阳能高温空气集热器。     

Bonding solar-selective absorber foils to glass receiver tubes for use in evacuated tubular collectors: Preliminary studies

The glass receiver tubes used in evacuated tubular collectors are currently vacuum coated with a selective surface in a batch process. Solar-selective absorber foils can be produced successfully by a number of methods and offer economic advantages over batch collector coatings in many instances. Bonding of these foils to a variety of surfaces is done at present with adhesives whose upper temperature limit is about 250°C. However, many absorber coatings are stable to much higher temperatures. Thus, suitable techniques for bonding solar-selective absorber foils to high-temperature (up to 400°C) glass receivers would be desirable. This paper describes possible foil-glass bonding techniques and the results of experiments on selected techniques, principally electrical field-assisted bonding and the use of glass sealing frits. Foil-bonded samples were thermally cycled in a vacuum to temperatures typical of stagnation in evacuated collectors, and changes in bond strength and foil optical properties were observed. Finally, the economic viability of foil bonding processes is discussed.     

Progress in the materials science of all-glass evacuated collectors

Materials problems in evacuated collectors are reviewed with emphasis on all-glass tubular systems. Collector design, including selective surface, vacuum insulation and layers for transmission enhancement of the glass envelope are examined in detail. In addition, design and materials aspects of reflectors and collector manifolding systems are discussed.     

Asymmetric CPC solar collectors with tubular receiver: Geometric characteristics and optimal configurations

We derive analytic expressions for the geometric characteristics of extremely asymmetric nonimaging solar concentrators (“asymmetric CPCs”) with tubular receivers, of fixed maximum concentration ratio C_max. We show that, at fixed C_max, there are nonideal concentrator configurations that minimize reflector area and average number of reflections, and determine configurations that are approximately optimal (maximizing yearly energy delivery at minimal material requirements), independent of climate and economics.