Measurement and simulation of flow rate in a water-in-glass evacuated tube solar water heater

This paper evaluates the characteristics of water-in-glass evacuated tube solar water heaters including assessment of the circulation rate through single ended tubes. A numerical model of the heat transfer and fluid flow inside a single-ended evacuated tube has been developed assuming no interaction between adjacent tubes in the collector array. Flow measurement using Particle Image Velocimetry (PIV) has been undertaken to validate the numerical model. The experimental rig consists of a single full-scale tube coupled to a storage tank. A non-dimensional correlation has been developed of the circulation rate through a single evacuated tube mounted at 45° inclination over a diffuse reflector. Simulation results show that the natural convection flow rate in the tube is high enough to disturb the tank’s stratification and that the tank temperature strongly affects the circulation flow rate through the tubes. Circumferential heat distribution was found to be an important parameter influencing the flow structure and circulation rate through the tube, hence a separate correlation needs to be developed if a concentrating reflector is incorporated into the collector.     

全玻璃真空太阳集热管光—热性能

从能量平衡议程导出全玻璃真空太阳集热管的3个光－热性能参数,即热损系数UTL、闷晒太阳曝辐量H和空晒性能参数Y与真空热管理的罩管玻璃的太阳透射比τ、选择性吸收表面的太阳吸收比a、发射比ε、和集热管内的气体压强p,太阳辐照度、环境温度和真空太阳集热管的几何尺寸等函数关系。理论计算的真空太阳集热管的3个热性能参数与实验的结果比较一致。上述全玻璃真空太阳集热管的光－热性能参数已经被国家标准《全玻璃真空太阳集热管》（GB／T17049－1997）采用。     

The calculation and analysis of glass-to-metal sealing stress in solar absorber tube

The failure or degradation of solar absorber tubes is the single largest cost factor for current parabolic trough solar power plant. The main failure reason is that there are residual stresses in the glass-to-metal joint which are generated during the cooling process of sealing. According to the thin shell theory and thermal stress theory, this paper presents the analytic solution for the glass-to-metal sealing residual stress. It also analyses how the thickness of glass tube, thickness of metal ring, and thermal expansion coefficient affect the residual stress distribution. In order to verify the calculation results, the photoelastic technique is used to measure the residual stress and the tensile test is used to obtain the point of the most dangerous stress and the tensile strength for the sealed specimens. It can be concluded that the maximum tensile stress happens at some distance near the sealing interface on the outer surface of glass tube. The seal strength increases when the thickness of the glass tube is increased. The analytic solution is proved feasible to analyze the residual stress of glass-to-metal seals in solar absorber tubes.     

The performance simulation of all-glass vacuum tubes with coaxial fluid conduit

A numerical investigation has been carried out for a solar system, which consists of all-glass (double skin) solar vacuum tubes. Water is heated as it flows through the coaxial fluid conduit inserted in each tube. The space between the exterior of the fluid conduit and the glass tube is filled with antifreeze solution. This is to facilitate heat transfer from the solar heated absorber surface to water and to prevent the functional problems due to freezing in frigid weather conditions. Results show good agreement when compared with other experimental data demonstrating the reliability of the present model. The one-dimensional numerical model could be used efficiently in designing all-glass solar collector tubes with different geometrical parameters other than those considered in the present analysis.     

Numerically assisted analysis of flat and corrugated plate solar collectors thermal performances

The paper presents the results of experimentally supported numerical analysis performed in order to investigate the possibilities to improve the thermal efficiency of plate solar collectors. Different numerical models were developed in order to asses the influence of design and operating parameters such as bond conductance between absorber plate and tube, tube diameter, glass cover to absorber plate distance, optical properties of absorber and flow rate on thermal efficiency of collectors. Following the results, two designs of collector without tubes, with parallel flat and corrugated absorber plates of chevron type, is further considered and shown to be an effective way to increase the thermal efficiency of solar energy conversion beyond that of commercial glazed and unglazed solar water heaters. Based on the results, the guidelines for design of a new collector prototype consisting of chevron type corrugated plates normally used in plate heat exchangers are provided.     

All-glass vacuum tube collector heat transfer model used in forced-circulation solar water heating system

The aim of this paper is to establish the heat transfer model of all-glass vacuum tube collector used in forced-circulation solar water heating system. In this model, the simplified heat transfer of collector is composed of the natural convection in single glass tube and forced flow in manifold header. Thus the heat balance equation of water in single tube and the heat balance equation of water in manifold header have been established. The flow equation is also built by analyzing the friction and buoyancy in tube. Through solved these equations the relationship between the collector average temperature, the outlet temperature and natural convection flow rate have been obtained. From this relationship and energy balance equation of collector, the collector outlet temperature can be calculated. The validated experiments of this model were carried out in winter of Beijing.     

Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.     

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.     

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°.     

Theoretical and experimental investigation of the filled-type evacuated tube solar collector with U tube

The filled-type evacuated tube with U-tube, in which the filled layer is used to transfer energy absorbed by the working fluid flowing in the U-tube, is proposed to eliminate the influence of thermal resistance between the absorber tube and the copper fin of the conventional evacuated solar collector. In this paper, the thermal performance of the filled-type evacuated tube with U-tube was researched by means of theoretical analysis and experimental study. The temperature of the working fluid in the flow direction was obtained, and the efficiency of the evacuated tube was also calculated, based on the energy balance equations for the working fluid in the U-tube. The effects of the heat loss coefficient and the thermal conductivity of the filled layer on the thermal performance of the evacuated tube were studied. In addition, the test setup of the thermal performance of the filled-type evacuated tube with U-tube was established. The evacuated tube considered in this study was a two-layered glass evacuated tube, and the absorber film was deposited in the outer surface of the absorber tube. The results show that the filled-type evacuated tube with U-tube has a favourable thermal performance. When the thermal conductivity of the heat transmission component is λ_c = 100, the efficiency of the filled-type evacuated tube with U-tube is 12% higher than that of the U-tube evacuated tube with a copper fin. The modelling predictions were validated using experimental data which show that there is a good concurrence between the measured and predicted results.     

A comparison of optical performance between evacuated collector tubes with flat and semicylindric absorbers

In this article, concepts of solar irradiance ratio and absorbed energy factor on the surface of the evacuated collector tube absorbers were presented respectively. For evacuated collector tubes with flat and semicylindric absorbers, we used a solar simulator as a light source, measured separately distribution of the solar irradiance ratio that varies with incident angles on various points on the absorber surface in a glass-covered tube, and gave their three-dimensional regressive equations correspondingly. Experimental measurement of solar irradiance ratio and solar absorptance of coatings on absorber surfaces was carried out. On this basis, rules of absorbed energy factors on absorbers in two shapes that vary with incident angles were analyzed and studied. According to clear-day model, the daily absorbed energy and its annual changes of single evacuated collector tubes with absorbers in two shapes placed under 40° northern latitude, 40° inclined angle and south orientation were calculated and compared. The results show that the annual absorbed energy of evacuated collector tube with a semicylindric absorber is 15.9% higher than that with a flat absorber. In addition, optimized incident angles for the absorber in two shapes of evacuated collector tubes operated in a whole year were tentatively investigated.     

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.     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.     

Thermal conversion with fluorescent concentrators

Fluorescent planar collector-concentrators are a new possibility for the conversion of solar energy into thermal energy. The collection and concentration of direct and diffuse radiation is feasible, using a transparent sheet of material doped with a fluorescent dye. The collector offers the advantage of separating the global irradiation into different spectral regions. This geometrical and spectral concentrated light can be converted with adapted highly selective absorbers into high temperature heat. Intensity and spectral region of the sunlight and the selectivity of the absorber determine the thermodynamically possible maximum absorber temperature. A test collector with a fluorescent concentrator area of 0.8 m² with an absorber pipe of 3 mm diameter in an evacuated glass tube was built. At a total irradiation of 850 W/m² on the fluorescent collector surface, a maximum stagnation temperature of 555°C (828 K) was reached. Under diffuse light conditions (150 W/m²), stagnation temperatures above 250°C (523 K) were measured. Thermodynamic calculation, experimental setup and results are given.     

真空管内吸热体接收太阳辐照量的研究

该文对真空管内吸热体分别为平板，圆柱体南向斜置，圆柱体东西横置3种形式在晴天时的日接受太阳辐照量进行了分析研究，推导出了它们与纬度、放置倾角和时间的函数关系，并对北京地区采用的3种安装角度对上述3种形式的集热管的日接收太阳辐照量进行了计算和比较。结果表明，不同纬度地区东西横置圆柱吸热体的日接收太阳辐照量是相同的，有一定倾角的朝南放置圆柱吸热体年平均日接收太阳辐照量高于其东西横置，并且还高于同一倾角的平板吸热体。     

Experimental Study on Performance Enhancement of Evacuated Tube by Constant Heat Flux Mode

The evacuated tube collector with U shape copper absorber tube is considered for the analysis. The experimental investigation is conducted on parabolic trough collector with U shape tube as absorber tube. The effect of the sudden fluctuations in the solar radiation on the performance of the collector is reduced by means of evacuated tube collector filled with thermic fluids. The analysis is performed with different thermic fluids such as dowtherm, therminol66, glycol water and ethylene glycol, are filled in the annular space between inner glass tube and U shape copper absorber tube. The experimentation is carried out at various mass flow rates from 20 to 100 LPH with the step-up flow rate of 20 LPH. A comparative study is carried out on various parameters such as effect of mass flow rate over instantaneous efficiency, useful heat gain and work input, etc. The characteristic curve of cylindrical parabolic trough collector (PTC) is also discussed. Experimental results show that, ethylene glycol gives better efficiency over mass flow rate and therminol66 gives best power heat ratio. Heat transfer mediums and its properties [specific heat capacity, thermal conductivity and dynamic viscosity] for all specified heat transfer fluids are also discussed. The results obtained with various specified heat transfer fluids filled in the annulus space of evacuated tube are compared with plain evacuated tube. It is observed that there is significant enhancement of overall instantaneous collection efficiency of the parabolic trough collector.     

Optimization of a fixed solar thermal collector

Criteria are presented for optimizing solar thermal energy collection. These criteria are then used in setting the design of a fixed solar thermal energy collector. This design is obtained by proceeding carefully through a series of optimization steps. While seeking near optimum performance, features have been retained which should lead to low cost. Initial optimization steps lead to an all glass vacuum collector tube whose side and lower walls are internally silvered to provide optimal Winston concentration on an interior glass tube coated with a selective absorber. Heat transfer calculations, performed for an array module of these collector tubes, produce values for the radiation, heat conduction and pumping losses and indicate operating conditions which minimize these losses. Near this minimum, heat conduction and pumping losses are small and can usually be neglected. Liquids provide much better heat transfer than gases. For liquid heat transfer fluids, the minimum loss collector tube window width (setting the transverse scale) is ～3 cm and tube length ～4 m, depending somewhat upon array area and the weighting used for the various losses. A window width of～5 cm and tube length～2 m should provide lower cost fabrication, while still allowing operation near minimum loss. Skills now used in the glass and lighting industry are expected to lead to low cost production of these tubes.     

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.     

Radiation collection by an optimized fixed collector

Criteria are presented for optimizing solar thermal energy collection. These criteria are then used in setting the design of a fixed solar thermal energy collector. This design is obtained by proceeding carefully through a series of optimization steps. While seeking near optimum performance, features have been retained which should lead to low cost. Initial optimization steps lead to an all glass vacuum collector tube whose side and lower walls are internally silvered to provide optimal Winston concentration on an interior glass tube coated with a selective absorber. Heat transfer calculations, performed for an array module of these collector tubes, produce values for the radiation, heat conduction and pumping losses and indicate operating conditions which minimize these losses. Near this minimum, heat conduction and pumping losses are small and can usually be neglected. Liquids provide much better heat transfer than gases. For liquid heat transfer fluids, the minimum loss collector tube window width (setting the transverse scale) is 3 cm and tube length 4 m, depending somewhat upon array area and the weighting used for the various losses. A window width of5 cm and tube length2 m should provide lower cost fabrication, while still allowing operation near minimum loss. Skills now used in the glass and lighting industry are expected to lead to low cost production of these tubes.     

Experimental studies on solar flat plate collector with internally grooved tubes using aqueous ethylene glycol

In order to cope up with the increase in energy demand and decline in fossil fuels, it has become imperative to use renewable resources efficiently. Among these renewable resources, solar thermal energy is abundant in nature. Solar water heating systems are one of the most important applications of solar thermal energy. Providing internal fins to absorber tube is the technique to improve heat transfer augmentation. Hence in the present study, experiments were performed on solar flat plate collector with different cross section of absorber tubes (plain tube and internally grooved tubes with different helix angles) and by varying the mass flow rates of the working fluids. This study reports the experimental results of flat plate collector, where the working fluid is water and aqueous ethylene glycol (50 : 50). Temperature profile of grooved absorber tube will be compared with plain tube. Since conversion efficiency of solar devices is low, the present study mainly focuses on improving the efficiency of solar flat plate collector.