Heat loss characteristics of an evacuated plate-in-tube collector

An experimental and theoretical investigation is described into the heat loss from a flat plate placed inside a partially evacuated glass tube. The conditions required to prevent convection losses are discussed and the effect of introducing a low thermal conductivity gas evaluated.     

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.     

Studies on anodic oxide coating with low absorptance and high emittance on aluminum alloy 2024

Anodization of AA 2024 in sulfuric acid bath containing glycerol, lactic acid and ammonium metavenadate has been studied to develop white anodic oxide coating. Investigation on the influence of various operating parameters — coating thickness, current density and ammonium metavenadate concentration on the optical properties was carried out to optimize the process. Infrared, atomic absorption spectroscopic techniques and scanning electron micrograph were used to characterize the coating. The obtained oxide coating provides a ratio of solar absorptance (α) to infrared emittance (), as low as 0.2. The optical properties and hardness values measured under optimum experimental conditions support its use as a thermal control coating.     

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.     

Thermal and optical analysis of an evacuated circular cylindrical concentrating collector

An evacuated concentrating circular cylindrical collector has been numerically investigated by ray tracing analyses. The optical efficiency of the collector is found by following incident rays onto the collector cover, calculating the amount of energy absorbed by the receiver for each ray, and then integrating the energy for all rays. Absorption and reflection losses in the collector materials are considered, as well as the formation of ray cascades. A thermal radiation exchange factor between the collector receiver and the cover, needed to find the thermal radiation losses, is also determined using ray tracing techniques. The collector overall efficiency was found for the case of a selective surface coating on the inner receiver cylinder and for the case of an absorbing fluid contained within a semi-transparent inner cylinder. The addition of a highly reflective thermal radiation coating of the inner surface of the cover, in order to suppress thermal radiation losses, was also evaluated.     

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.     

Aerogel and krypton insulated evacuated flat-plate collector for process heat production

We investigated the reduction of gas heat conduction in evacuated flat-plate collectors with operating pressures between 10³ and 10⁴ Pa. In stationary heat loss experiments with a real-size model the pressure dependency of the thermal losses was examined for air, krypton and for a nanostructured SiO₂-aerogel powder insulation between absorber and rear side of the casing. The theory for pressure dependent gaseous conduction was validated. Due to its high molecular mass, krypton as filling gas reduces gaseous conduction by 65% in the continuum range with respect to air. In a low emissive EFPC where gas conduction losses are comparable to radiative losses, the reduction in the total losses is about 30%. In principle, with unpacked aerogel the heat conduction could be reduced further: the nanostructures affect the gaseous heat conduction to be nearly totally suppressed below 10² Pa, because the mean free path of the gas is large compared to the pore dimensions of the aerogel. However, the applied packaging of PTFE-fabric compensates the excellent properties of the insulant due to solid conduction in the fabric.     

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.     

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。     

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.     

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%。     

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).     

An analytical expression for the instantaneous efficiency of a flat plate solar water heater and the influence of absorber plate absorptance and emittance

Standard test results to quantify the instantaneous efficiency, η, of a glazed flat plate solar water heater are normally expressed in terms of a reduced temperature parameter, x, and global insolation, G, as η = η₀ ? a₁x ? a₂Gx². We show that the Hottel–Whillier–Bliss relation for the efficiency can be expressed in the same form with each of the coefficients η₀, a₁, and a₂ in terms of algebraic expressions of standard mechanical, fluid and thermal parameters of a single glazed, finned heater, including the absorber plate absorptance, α, and thermal emittance, ε. The advantage of the derived expression is that the effect on the efficiency of changes in various heater parameters can be readily evaluated. Furthermore, it is shown that for selectivity α/ε > 2, each coefficient η₀, a₁, and a₂ can be expressed as η₀ = η_0C ? εη_0R, etc., in order to separate out the role of absorber radiation from other losses. This allows one to easily compare selective solar absorbers with different α and ε and, for example, to suggest an optimum coating thickness for thickness sensitive selective solar absorbers. In particular it can be seen that care should be taken in reducing ε at the expense of also reducing α in order to increase the selectivity, α/ε, since this will often be detrimental to the efficiency. The analytical expressions for η₀, a₁, and a₂ can be easily programmed on a spreadsheet and, for convenience, are summarised in an appendix.     

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.     

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.