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.     

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。     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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。     

Water-in-glass manifolds for heat extraction from evacuated solar collector tubes

Measurements are reported on three novel manifolds of the water-in-glass type for evacuated all-glasssingle-ended tubular collectors. The manifolds provide for series connection of tubes, but because there is virtually no partitioning of the inner volume of the collector tubes, the manifolds are extremely simple and exhibit low impedance to fluid flow. The efficiency of heat extraction from the tubes has been determined by measuring temperatures at various points on the surface of glass tubes in a panel of area 1.2 m² while heating the tubes electrically to simulate solar energy input. Measurements have been made for a range of tube inclinations (0–80°), water flow rates (0.5–5 lmin⁻¹, water inlet temperatures (13–70°C), and effective solar fluxes (100–1000 W/m²) for two absorber tube diameters. The results show that for a wide range of operating conditions buoyancy effects alone result in efficient heat transfer to the tops of the tubes. The manifold designs described offer a possible low cost solution to the problem of manifolding evacuated collectors for sub-100°C heat extraction for domestic and industrial applications.     

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.     

不同朝向全玻璃真空太阳集热管的热性能比较

为了确定不同朝向全玻璃真空太阳集热管的热性能,从太阳累计辐射与真空管储水箱温差关系入手,对水平、倾斜以及垂直朝向的真空管进行了系列的试验。研究结果表明,自然循环时,不同朝向真空管具有不同的集热性能,测量日上午6:00-10:00,垂直真空管具有最高的集热性能,水平管最低,其他时间则是倾斜真空管最高,垂直管最低;倾斜真空管系统日总得热量为1.17×10~6J,水平及垂直真空管系统总得热量分别为1.12×10~6J和0.88×10~6J,各占倾斜管总得热量的96.35%和75.56%。     

Transit theory of a tubular solar energy collector

A simple analysis has been developed to predict the transient response of a tubular solar energy collector. A good agreement between the earlier reported measurements under no fluid flow conditions with the corresponding numerical calculations of present theory validated the applicability of the developed analysis. A selective coating over the receiver plate and the evacuation of annular space between the cover and the receiver plater suppressing the radiative and convective losses respectively, are found to play almost equally important role in improving the thermal performance of such systems especially for low fluid rates. The selective coating plays ore important role at temperature 120°C. A suitable mirror at the base of the collectors reflecting the radiation on the bottom of the collectors is expected to increase the thermal efficiency of such collectors considerably.     

U型管式全玻璃真空管集热器热效率及性能研究

在能量平衡分析的基础上，建立了U型管式全玻璃真空管太阳能集热器热效率方程，推导了集热器热损系数、效率因子等性能参数的计算公式，理论计算热效率与实验数据吻合良好。计算分析表明，真空管热损系数与吸热管和环境温差并非线性关系，将其关联式按环境温度分段整理将使计算结果更接近实际；涂层发射比对集热器的热效率影响较大，降低涂层发射比是提高集热器效率的有效途径；采取适当的措施降低吸热管与肋片间的接触热阻后，采用U型管连接方式不会时热利用系统集热器效率造成太大影响。     

Experimental comparison of open sun drying and solar drying based on evacuated tube collector

In this article, an experimental comparison between open sun and solar drying is done. The thermal performance of evacuated tube based solar dryer is investigated with drying characteristics of Phyllanthus Emblica (Anvla), Aloe Vera, Aegle Marmelos (Bel) and leaves of Azadirachta Indica (Neem), Aegle Marmelos (Bel) and Psidium Guajava (Guava). In this setup, an evacuated tube collector, shell and tube heat exchanger and drying chamber are used. It was found that the maximum temperature difference between hot air and ambient air is 35.4°C and maximum efficiency of the setup is calculated as 55%. The average drying rate of Phyllanthus Emblica (Anvla), Aloe Vera and Aegle Marmelos (Bel) is measured as 0.46?g/min, 0.44?g/min, and 0.39?g/min respectively which are higher than that of Open Sun Drying. The leaves of Azadirachta Indica (Neem), Aegle Marmelos (Bel) and Psidium Guajava (Guava) also get dried with faster rates of 0.18?g/min, 0.17?g/min, and 0.14?g/min respectively.     

Productivity amelioration of stepped solar still using phase change material and evacuated tube collector

This study investigates the individual and cumulative effects of phase change material (PCM) and evacuated tube collector (ETC) on the performance of a stepped solar still (SSS). Experiments have been performed on SSS, SSS loaded with PCM (SSS-PCM), SSS coupled with ETC (SSS-ETC), and SSS loaded with PCM and coupled with ETC (SSS-PCM-ETC). An innovative way of loading paraffin wax as PCM is introduced to utilize solar energy efficiently for the distillation process during off-sunshine hours. ETC is used to provide pre-heated RO wastewater to the distillation unit. The distillate output was observed maximum for SSS-PCM-ETC (4.97 kg/m² day) which is 99% more than that of the SSS unit. The PCM and ETC individually with SSS unit increases the distillate output by 31% and 24%, respectively. The working time of the distillation unit was observed to be increased by 3 h by the use of PCM. The total heat transfer coefficient was evaluated at the maximum for SSS-PCM-ETC and was observed in the range of 16.94–167.04 W/m² °C. The thermal energy efficiencies of SSS, SSS-ETC, SSS-PCM, and SSS-PCM-ETC were evaluated as 28.65%, 35.59%, 43.88%, and 44.04%, respectively. SSS-PCM-ETC is found to be the most economic with the best environmental conservation having maximum values of daily productive cost ($0.69) and carbon credits earned ($184.8).