Study of transmittance in an evacuated flat plate collector with double cylindrical glass cover

A generalized theory for the computation of transmittance of a flat plate collector with double cylindrical cover is given. Effective transmittance for the collector at different azimuths and different orientations has been computed. This study gives an idea about collector performance at different orientations on a whole day basis.     

Experimental comparison of solar drying based on evacuated tube collector with desiccant drying for dehydrating potato and ginger

In this article, a comparative study of different drying techniques, namely, open sun drying (OSD), desiccant drying, solar drying (SD), and solar drying with desiccant (SDWD) was done for drying potato and ginger. The thermal efficiency and air temperature differences of the solar dryer based on evacuated tube collectors were also examined. The average efficiency of the solar dryer was 29.01%. Average drying rates of OSD, desiccant drying, SD, and SDWD bed are 0.435, 0.435, 0.59, and 0.635 g/min. The moisture content of the potato was reduced from 79.6% to 32.9% by OSD, 32.6% by desiccant drying, 9.6% by SD, and 6.9% by SDWD. The moisture content of the ginger was reduced from its initial value of 82.3%–25.3% by OSD, 25% by desiccant drying, 10.1% by SD, and 3.2% by SDWD. SD assisted with a desiccant bed proved to be the best method for drying vegetables. Drying with desiccants is not feasible for drying in winter and spring seasons of the Indian climate.     

Performance optimization of evacuated tube collector for solar cooling of a house in hot climate

Evacuating the space connecting cover and absorber significantly improves evacuated tube collector (ETC) performance. So, ETCs are progressively utilised all over the world. The main goal of current study is to explore ETC thermal efficiency in hot and severe climate like Kuwait weather conditions. A collector test facility was installed to record ETC thermal performance for one-year period. An extensively developed model for ETCs is presented, employing complete optical and thermal assessment. This study analyses separately optics and heat transfer in the evacuated tubes, allowing the analysis to be extended to different configurations. The predictions obtained are in agreement with experimental. The optimum collector parameters (collector tube length and diameter, mass flow rate and collector tilt angle) are determined. The present results indicate that the optimum tube length is 1.5 m, as at this length a significant improvement is achieved in efficiency for different tube diameters studied. Finally, the heat generated from ETCs is used for solar cooling of a house. Results of the simulation of cooling system indicate that an ETC of area 54 m², tilt angle of 25° and storage tank volume of 2.1 m³ provides 80% of air-conditioning demand in a house located in Kuwait.     

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).     

Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube

The thermal performance of a glass evacuated tube solar collector is numerically and experimentally investigated. The solar collector considered in this paper consists of a two-layered glass tube and an absorber tube. Air is used as the working fluid. The length and diameter of this glass tube are 1200 and 37 mm, respectively. Four different shapes of absorber tubes are considered, and the performances of the solar collectors are studied to find the best shape of the absorber tube for the solar collector. Beam irradiation, diffuse irradiation, and shade due to adjacent tubes are taken into account for a collector model to obtain a realistic estimation. In addition, a single collector tube with only beam irradiation is studied as a simplified model, and the results of the simplified model are compared to those of the collector model to identify the difference between these two models. The performance of a solar collector is affected by the shape of the absorber, incidence angle of solar irradiation, and arrangement of collector tubes. The results obtained from the simplified model are very different from those from the collector model, which considered not only beam and diffuse irradiation but also shade due to adjacent tubes.     

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。     

Numerical and experimental investigation of parabolic trough collector with focal evacuated tube and different working fluids integrated with single slope solar still

This study deals with the design and fabrication of parabolic trough solar collectors (PTCs) used to increase the yield of a single slope solar still. The designed parabolic trough solar collector is investigated numerically using Ansys Fluent 18.2. The proposed solar still is coupled with a parabolic trough solar collector with an evacuated tube receiver in its focal axis using different working fluids. The working fluids are water (case 1), oil (case 2), and nano-oil (CuO/mineral oil 3% vol; case 3). In the case when the working fluid is not water, then a heat exchanger serpentine should be used in the solar still basin. The PTC has a rim angle of 82° and an aperture width of 0.9 m and length of 2.8 m. An assessment of the performance for the studied systems was accomplished under the weather conditions of Ismailia, Egypt, during summer months, June, July, and August 2019. The outcomes of closed-loop working fluids different flow rates are investigated. The experimental results of the accumulated freshwater productivities record 2.955, 3.475, 4.29, and 5.04 L m⁻² d⁻¹ for the traditional solar still and the modified cases 1 to 3 solar stills, respectively. The modified solar still in case 3 has the highest daily accumulated freshwater productivity with a percentage increase of 71.2% than the traditional solar still. The maximum daily efficiency is 46% and 26.9% for the traditional and modified (case 3) solar stills, respectively. The cost of 1 L of fresh water is 0.057 and 0.062 $/L for the traditional and the modified (case 3) solar stills, respectively.     

Effect of non-uniform θ-temperature distribution on a receiver of an evacuated tubular collector on its performance parameters

A performance of an evacuated tubular collector (G.E. design) fixed at the focus of a compound parabolic concentrator is investigated. In the G.E. design, heat is transmitted to the circulating fluid inside a U-tube. The U-tube is in contact with the receiver only on a line along the length of the receiver. This results in a non-uniform temperature distribution on the receiver in the θ-direction. The effect of the non-uniform temperature distribution on the performance parameters of the collector, viz. overall heat loss coefficient, plate efficiency factor and heat removal factor, has been studied. The results are presented in the form of a graph.     

全玻璃真空太阳集热管（器）寿命测试方法的分析

综述了太阳集热管（器）寿命研究及选择性吸收材料老化性能检测方法的研究进展,分析认为单盖板平板型太阳集热器寿命研究同太阳选择性吸收材料的老化性能的检测方法一样要考虑温度（循环）、湿度及环境气氛的影响,而全玻璃真空太阳集热管（器）在长期运行过程中则会发生惰性气体及水分通过罩管玻璃向夹层渗透、吸气剂的变化而降低夹层真空度,气体导热损失增大,涂层（镀膜）和基底材料的相互作用、罩管积灰（尘）及内管结垢等改变涂层（镀膜）的光学性能,进而导致热性能的降低。最后,提出全玻璃真空太阳集热管（器）的寿命研究并形成检测方法的基本思路。     

Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit

The thermal performance of a prototype solar cooker based on an evacuated tube solar collector with phase change material (PCM) storage unit is investigated. The design has separate parts for energy collection and cooking coupled by a PCM storage unit. Solar energy is stored in the PCM storage unit during sunshine hours and is utilized for cooking in late evening/night time. Commercial grade erythritol was used as a latent heat storage material. Noon and evening cooking experiments were conducted with different loads and loading times. Cooking experiments and PCM storage processes were carried out simultaneously. It was observed that noon cooking did not affect the evening cooking, and evening cooking using PCM heat storage was found to be faster than noon cooking. The cooker performance under a variety of operating and climatic conditions was studied at Mie, Japan.     

Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles

To performance comparative studies, two sets of water-in-glass evacuated tube solar water heater (SWH, in short) were constructed and tested. Both SWHs were identical in all aspects but had different collector tilt-angle from the horizon with the one inclined at 22° (SWH-22) and the other at 46° (SWH-46). Experimental results revealed that the collector tilt-angle of SWHs had no significant influence on the heat removal from solar tubes to the water storage tank, both systems had almost the same daily solar thermal conversion efficiency but different daily solar and heat gains, and climatic conditions had a negligible effect on the daily thermal efficiency of systems due to less heat loss of the collector to the ambient air. These findings indicated that, to maximize the annual heat gain of such solar water heaters, the collector should be inclined at a tilt-angle for maximizing its annual collection of solar radiation. Experiments also showed that, for the SWH-22, the cold water from the storage tank circulated down to the sealed end of tubes along the lower wall of tubes and then returned to the storage tank along the upper wall of solar tubes with a clear water circulation loop; whereas for the SWH-46, the situation in the morning was the same as the SWH-22, but in the afternoon, the cold water from the storage tank on the way to the sealed end was partially or fully mixed with the hot water returning to the storage tank without a clear water circulation loop, furthermore, such mixing became more intense with the increase in the inlet water temperature of solar tubes. This indicated that increasing the collector tilt-angle of SWHs had no positive effect on the thermosiphon circulation of the water inside tubes. No noticeable inactive region near the sealed end of solar tubes for both systems was observed in experiments.     

Numerical evaluation of a PEM fuel cell with conventional flow fields adapted to tubular plates

In this research a 3D numerical study on a PEM fuel cell model with tubular plates is presented. The study is focused on the performance evaluation of three flow fields with cylindrical geometry (serpentine, interdigitated and straight channels) in a fuel cell. These designs are proposed not only with the aim to reduce the pressure losses that conventional designs exhibit with rectangular flow fields but also to improve the mass transport processes that take place in the fuel cell cathode. A commercial computational fluid dynamics (CFD) code was used to solve the numerical model. From the numerical solution of the fluid mechanics equations and the electrochemical model of Butler-Volmer different analysis of pressure losses, species concentration, current density, temperature and ionic conductivity were carried out. The results were obtained at the flow channels and the catalyst layers as well as in the gas diffusion layers and the membrane interfaces. Numerical results showed that cylindrical channel configurations reduced the pressure losses in the cell due to the gradual reduction of the angle at the flow path and the twist of the channel, thus facilitating the expulsion of liquid water from the gas diffusion layers and in turn promoting a high oxygen concentration at the triple phase boundary of the catalyst layers. Moreover, numerical results were compared to polarization curves and the literature data reported for similar designs. These results demonstrated that conventional flow field designs applied to conventional tubular plates have some advantages over the rectangular designs, such as uniform pressure and current density distributions among others, therefore they could be considered for fuel cell designs in portable applications.     

Theoretical approach of a flat-plate solar collector taking into account the absorption and emission within glass cover layer

A rigorous theoretical approach of a flat-plate solar collector with a black absorber considering the glass cover as an absorbing–emitting media is presented. The glass material is analyzed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in one-dimensional case using the Radiation Element Method by Ray Emission Model (REM²). The optical constants of a clear glass window proposed by Rubin have been used. These optical constants, 160 values of real part n and imaginary part k of the complex refractive index of a clear glass, cover the range of interest for calculating the solar and thermal radiative transfer through the glass cover. The computational time for predicting the thermal behavior of solar collector was found to be prohibitively long for the non-gray calculation using 160 values of n and k. Therefore a suitable semi-gray model is proposed for rapid calculation. The profile of the efficiency curve obtained in the present study was found to be not linear in shape. Indeed, the heat loss from the collector is a combination of convection and radiation and highly non linear. The effect of the outside convective heat transfer on the efficiency curve is also studied. In fact, when the convection is the dominant heat transfer mode compared with the radiation one, the profile of the efficiency curve is more or less straight line. Consequently, the heat loss coefficient could be calculated using Klein model. It has been also shown that the effect of the wind speed on the glass cover mean temperature is very important. This effect increases with the increase of the mean absorber temperature.     

Theoretical approach of a flat plate solar collector with clear and low-iron glass covers taking into account the spectral absorption and emission within glass covers layer

Accurate modeling of solar collector system using a rigorous radiative model is applied for the glass cover which represents the most important component of the system and greatly affects the thermal performance. The glass material is analyzed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in one dimensional case using the radiation element method by ray emission model (REM²). The optical constants of a clear and low-iron glass materials proposed by Rubin have been used. These optical constants, 160 values of real part n and imaginary part k of the complex refractive index of such materials, cover the range of interest for calculating the solar and thermal radiative transfer through the glass cover. The computational times for predicting the thermal behavior of solar collector were found to be prohibitively long for the non-gray calculation using 160 values of n and k for both glasses. Therefore, suitable semi-gray models have been proposed for rapid calculation. The temperature distribution within the glass cover shows a good agreement with that obtained with iterative method in case of clear glass. It has been shown that the effect of the non-linearity of the radiative heat exchange between the black plate absorber and the surroundings on the shape of the efficiency curve is important. Indeed, the thermal loss coefficient is not constant but is a function of temperature, due primarily to the radiative transfer effects. Therefore, when the heat exchange by radiation is dominant compared with the convective mode, the profile of the efficiency curve is not linear. It has been also shown that the instantaneous efficiency of the solar collector is higher in case of low-iron glass cover.     

Experimental investigation of a domestic solar water heater with solar collector coupled phase-change energy storage

Phase change materials (PCMs) have good properties such as high thermal capacity and constant phase change temperature. Their potential use in solar energy storage is promising. Tests of exposure and constant flow rate are performed to investigate the thermal performance of a domestic solar water heater with solar collector coupled phase-change energy storage (DSWHSCPHES). Due to the low thermal conductivity and high viscosity of PCM, heat transfer in the PCM module is repressed. The thermal performance of the DSWHSCPHES under exposure is inferior to that of traditional water-in-glass evacuated tube solar water heaters (TWGETSWH) with an identical collector area. DSWHSCPHES also performs more efficiently with a constant flow rate than under the condition of exposure. Radiation and initial water temperature have impacts on system performance; with the increase of proportion of diffuse to global radiation and/or initial water temperature, system performance deteriorates and vice versa.