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.     

Solar transmittance characteristics of evacuated tubular collectors with diffuse back reflectors

The partition matrix between light sources and light sinks is introduced to evaluate the transmittance properties of optically asymmetric collectors constructed with evacuated tubes. A ray tracing model is used to compute its elements. Curve fits to the ray tracing results are provided. The effects of polarization are discussed. Comparisons with previously published results are presented in order to check the accuracy of the proposed model. The beam and diffuse transmittance of a tubular cover are calculated and their dependency on the tube orientation investigated. The biaxial incidence angle modifier product is extended to evacuated tube collector arrays with a diffuse back reflector, providing a simple interpolation process for the beam transmittance. An effective beam direction for diffuse radiation is computed so that transmittances for ground reflected radiation and sky diffuse radiation can be calculated from biaxial incidence angle modifier measurements.     

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).     

中高温直通式真空太阳集热管的研制与应用

中高温直通式真空太阳集热管(简称直通式真空管)是由北京桑达太阳能技术有限公司最新研制的一种新型太阳能集热元件.它完全采用桑达公司自有的知识产权和技术,材料和零部件完全来自国内.该管子在太阳能聚光集热系统上使用时,管内导热工质可以达到350℃,管子可以承受3MPa的压力.该管型的研制目前处于国内领先水平,产品性能接近国外同类产品,采用直通式真空管的太阳能集热器可以用于太阳能空调、太阳能工业加热、太阳能热发电等中高温系统,大大拓宽了太阳能热利用的应用领域.     

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.     

The progress and prospect of middle/high temperature evacuated tubular solar collector

The all-glass evacuated solar collection tubes, incorporating the dc sputtered double layer metal-aluminium nitride cermet selective surface, have been mass-produced by TurboSun in large quantities under license to the University of Sydney since 1995. A solar absorptance of 0.94–0.95 and emittance of 0.04–0.05 at room temperature has been achieved for the SS-AIN cermet solar coatings. These solar tubes are stable at 330–400°C. These M-AIN cermet tubes have widespread application for solar hot water and steam heaters, as well as the demonstration test units for solar thermal electricity. In China, the production of solar water heaters using all-glass evacuated solar heat collection tubes has rapidly increased since 1995. The experimental results show that the solar selective coatings incorporating dc sputtered tungsten and dc reactively sputtered aluminium nitride components in a cermet should be stable at 500°C in vacuum. It would be possible to produce solar collector tubes for solar thermal electricity application with superior solar performance at a much lower cost.     

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。     

全玻璃真空管太阳空气集热器

简要介绍了全玻璃真空管太阳空气集热器结构、工作原理和对其进行测试的结果，在空气流量为60m^3／h和90m^3／h时，瞬时效率曲线分别为η=0．73-5．54(Tm-Ta)／I和η=0．78-5．45(Tm-Ta)／I。实验结果表明：内插管式全玻璃真空管太阳空气集热器具有较好的集热性能。     

An internal cusp reflector for an evacuated tubular heat pipe solar thermal collector

This study involves the optical analysis of a slightly concentrating, symmetric cusp reflector inside a tubular glass envelope with a cylindrical heat pipe as the solar absorber. The basic design features of this non-tracking, evacuated, modular collector and the principles of heat removal are shown in Figs. 1 and 2. Differential equations of the cusp reflector optics, given the geometrical restrictions in Figs. 1 and 2, are derived, and solutions for the largest possible aperture inside a given diameter envelope and acceptance angle are presented.As an extension of the same study, the optical efficiency of a single collector tube has been simulated by means of a Monte Carlo Ray-Tracing Program. For a concentration ratio of 1.15, the flux distribution around the heat pipe is computed as a function of incidence angle. In addition, the impact of mirror defects and absorber misalignment on the optical performance are analyzed.     

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.     

Selective absorbing surface for evacuated solar collector tubes

The absorptance of AlN-Al(absorbing coating)/Al solar spectrum(0.34 ≈ 2.5 μ m) selective absorbing surface, containing 8-layer AlN-Al composite materials and with distinctive or non-boundaries between the layers, deposited by sputtering technology and heat-treated at 400 °C (60min) is about 0.95 and its thermal emittance is close to 0.07 ≈ 0.08(80 °C). But on the condition that the absorptance is unchanged, the thermal emittance dropped to 0.04 for the AlN-Al coating composed of matel volum fractionic 0.42, 0.25 and 0.00 films.     

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.     

Heat extraction efficiency of a concentric glass tubular evacuated collector

The results of detailed measurements and calculations of the properties of Sydney University/Nitto Kohki evacuated collector tubes have been used to develop a formula for the instantaneous heat extraction efficiency η of a collector panel incorporating the evacuated tubes. The instantaneous efficiency depends on ambient temperature, mean fluid temperature in the collector, solar flux and the design of the manifold used to extract heat from the glass absorber tubes. Manifold design determines the mean temperature difference between absorber tube surface and mean fluid temperature for given operating conditions, and strongly affects the efficiency η of a collector panel. Neither changes in the number of evacuated tubes per unit area of collector, nor variations in solar flux, significantly alter the efficiency decrement Δ η₀ associated with a particular manifold design. Calculated efficiencies agree well with experimental results for collector panels incorporating manifolds of various designs. The formula for efficiency η allows detailed analysis of the relative importance of various energy loss mechanisms in a collector.     

Theoretical flow investigations of an all glass evacuated tubular collector

Heat transfer and flow structures inside all glass evacuated tubular collectors for different operating conditions are investigated by means of computational fluid dynamics. The investigations are based on a collector design with horizontal tubes connected to a vertical manifold channel.Three different tube lengths varying from 0.59 m to 1.47 m have been modelled with five different inlet mass flow rates varying from 0.05 kg/min to 10 kg/min with a constant inlet temperature of 333 K. Under these operating conditions the results showed that:

• the collector with the shortest tube length achieved the highest efficiency,

• the optimal inlet flow rate was around 0.4–1 kg/min,

• the flow structures in the glass tubes were relatively uninfluenced by the inlet flow rate,

Generally, the results showed only small variations in the efficiencies. This indicates that the collector design is well working for most operating conditions.     

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.     

Gas heat conduction in an evacuated tube solar collector

We investigated experimentally the pressure dependency of the gas heat conduction in an evacuated plate-in-tube solar collector. A stationary heat loss experiment was built up with an electrically heated real-size collector model. The gas pressure was varied from 10⁻³ to 10⁴ Pa, the temperatures of the absorber and the casing were held at 150°C (electrical heaters) and 30°C (water cooling), respectively. Losses by radiation and solid conduction were determined experimentally at pressures below 0.1 Pa. At higher pressures these background losses were subtracted from the total heat losses, to receive the heat losses by gas heat conduction. The experimental results were compared with approximative theoretical models. The onset of convection is in agreement with the usual theories for parallel plates, taking the largest distance between the absorber and the glass tube as the plate distance. As a first approximation the pressure dependency of the gas heat conduction is described by the usual theory for parallel plates, taking the smallest distance between the absorber and the glass tube as the plate distance.     

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.     

Thermal performance of a solar pressure cooker based on evacuated tube solar collector

This paper presents the thermal performance of a community type solar pressure cooker based on evacuated tube solar collector. The developed design of solar pressure cooker has separate parts for energy collection and cooking unit and both are coupled by heat exchanger. The paper has presented the performance results of experimental study conducted on solar pressure cooker and a simulation model has been developed for predicting the cooker performance under a variety of operating and climatic conditions. The theoretical model is validated against the experimental results. The obtained results have suggested a possibility of several batches of solar cooking on a clear sunny day under typical conditions of Delhi.