平板型太阳集热器热性能评价方法的研究

本文分析了风速对平板型太阳集热器顶部热损失系数的影响,导出了风速影响修正系数及经验公式。讨论了集热器基准瞬时效率方程和集热器名义能量收益概念,这两个概念有利于合理地评价平板型太阳集热器的热性能。     

与建筑结合的几种真空管太阳集热器

由于多数太阳热水器属于“事后安装”且品种规格各异，因此造成杂乱无章的无序状态，极大地影响了原建筑的整体外观形象，甚至影响到整个城市的建筑风貌。这一问题如果得不到很好的解决，势必会影响到太阳热水器的普及和推广。本文着重介绍北京桑达公司生产的几种真空集热管产品在国内外与建筑结合的成功范例。     

新型平板型太阳集热器热性能浅析

平板型太阳集热器有相对低廉的价格、成熟的制造技术及固有的热性能特点，在我国气候温暖的南方有很大市场，在寒冷的北方只有部分应用。这主要是由于平板型集热器保温性能差，与外界有热交换，高温效率特性不好的原因造成的。所以提高平板型集热器热性能指标，生产出新一代平板集热器是满足市场需求的唯一出路。     

太阳集热器热性能测试方法研究进展

以集热器热性能的测试和评价方法为研究对象,综述了太阳集热器热性能测试方法的发展现状,并对已有的具有代表性的集热器热性能测试方法做了较详细的分析,指出了各测试方法在测试过程和参数估计过程中所具有的优缺点,并在此基础上提出当前集热器热性能测试存在的问题和今后的研究重点.     

平板型太阳集热器热性能的快速测试方法研究

对快速测量平板太阳集热器热性能的瞬态测试方法进行研究，并将快速测试方法的测试结果与国标GB／T4271-2000(平板型太阳集热器热性能试验方法)规定的测试方法所得的结果进行比较。通过对比可见，基于瞬态测试的快速测试方法可以得到与稳态测试同样的结果，考虑到现有太阳辐射表的精度，只要太阳辐照度大于300W／m2瞬态测试即可有效，且对测试期间太阳辐照度的变化无要求。在除雨雪天的情况下，整个测试可以在1d内完成。     

平板太阳集热器列阵的二维模型与热性能

本文分析了管板式太阳集热器列阵的流动与传热。考虑了支管流量分布不均匀、吸热板二维导热及浮升力等因素,建立了离散型的二维数学模型,求得了数值解。计算结果表明,集热器列阵的布置方式和支管流量分布的均匀性对列阵的热性能有很大的影响。逆流布置(U型)的列阵由于流动的短路现象而使热效率明显降低。计算所得的数值结果与实验结果符合良好。     

我国太阳能集热器的现状及未来发展

对平板集热器、真空管集热器和热管集热器的研究进行了论述,通过对研究状况的分析,指出我国在太阳能集热管方面的研究应该做出努力的方向。     

全玻璃真空太阳集热器反射器材料及性能

全玻璃真空管太阳集热器所使用的反射器材料分为两种：漫反射和镜反射材料。 在光学中,反射分为三种,即镜反射、漫反射和一般反射 (包括镜反射和漫反射 )。如图 1所示。 前两种是在理想状态下才有的反射,一般材料的反射很难有全部是镜反射或全部是漫反射的,只能是以漫反射为主或以镜反射为主的反射材料,其表面简称漫反射表面或镜反射表面。 不同类型的反射材料,应用于不同的使用条件。例如太阳灶,它的反射板要求有很高的聚光能力,因此它的反射器就需要使用以镜反射为主的高反射材料制成。对于全玻璃真空管太阳集热器,用漫反射材料作反射器比较经济,也有用镜反射材料作反射器的。 一全玻璃真空管太阳集热器用的漫反射器 真空集热管的吸收表面呈圆柱形,即背向太阳的表面也有吸收涂层 (这与平板集热器有本质区别 ),怎样才能充分利用背向太阳部分的吸收涂层呢 ?基于这种考虑,就有了真空集热管背部的反射板。首先看一下真空集热管及反射板与光线之间的关系 (以正午时太阳垂直入射为例 ),如图 2所示。 如图 2(a)所示,在光线垂直入射时,如果选择的反射板是以镜反射为主的反射材料,那么,大多数光线会沿入射方向返回,不能被集热管上背向太阳的吸收涂层吸收,浪费了这部分能量;而如图 2(b)中所示,如果是以漫反射为主的反射材料作为真空集热管背面的反射板,那么通过漫射作用,就会有大部分光线到达集热管背向太阳的一面,充分利用了真空集热管为圆柱形吸收表面的优势,从而达到减少真空集热管用量,降低成本的目的。通过计算,集热管的前部与背部接收辐射能的比值为 2 3∶ 1,即集热管前部接收 70％总接收到的辐射,而集热管背部接收 30％总接收到的辐射。 由以上的分析可以看出,真空管太阳热水器是应该有反射背板的。但到底选什么样的材料作为反射板,还要考虑多种因素,如反射材料的光学性能、化学稳定性、强度及价格等。光学性能主要指反射材料的反射比,作为真空集热管太阳热水器反射材料,首先要求有高的太阳反射比,同时又有较高的漫反射比。化学稳定性主要是耐酸、碱腐蚀能力。强度是指抗大风能力。下面就介绍几种常用反射材料的光学性能及化学稳定性。     

Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

Optimum performance of thermal trap collectors

The “thermal trap effect” in semitransparent material and the trapping system in the conventional flat-plate collectors with two, three or four glass or plastic covers with air-gaps in between are analysed under a common heading of “thermal trap collectors”. In general, a thermal trap collector consists of one or many slabs of semitransparent material of finite thickness with air-gaps in between, and an ideal withdrawal mechanism at the base of the trapping system to withdraw all available energy. This approach makes a comparative study of the two types of collectors possible, and provides data to design the appropriate withdrawal mechanism and operating conditions. A steady state analysis which neglects internal reflections and body radiation shows the existance of an optimum performance in single-layer thermal trap collectors and its dependence on thickness. A model which includes internal reflections is then analysed and the existance of the optimum performance and its dependence on thickness is demonstrated by taking the example of a single slab of methylmethacrylate plastic. The model is extended to multilayer thermal trap collectors and two examples are considered; a multilayer methylmethacrylate thermal trap collector and a multilayer “glass” thermal trap collector. The results show that the two-layer methyl methacrylate thermal trap collector has, in general, a better performance than the corresponding single or three and four-layer systems. But at high withdrawal efficiencies of about 60 per cent, the single layer methyl methacrylate shows its uniqueness and becomes competitive with the two-layer system. But the three and four-layer “glass” thermal trap collectors perform better than the corresponding single and two-layer ones, with the three-layer system having an overall better performance. These results show that the number of slabs in addition to thickness are important parameters in the study of the performance of thermal trap collectors.     

无盖板渗透型集热器热性能的对比试验

利用太阳能空气加热系统实验台,对黑、深绿和深蓝3种颜色无盖板渗透型集热器的热性能进行了户外瞬态对比试验。试验结果表明:太阳辐射照度和风量是影响系统热性能的重要因素。在高档和低档两种风量下,黑色集热器的瞬时平均热效率分别为76.04%和67.50%,高于普通平板太阳能空气集热器;集热器表面颜色对其热性能有一定影响,在高档和低档两种风量下,深绿色和深蓝色集热器的瞬时平均热效率比黑色集热器低15%～22%,空气温升低3～4℃,但仍然优于普通平板空气集热器。从保持建筑立面美观考虑,无盖板渗透型集热器的集热板可以采用颜色较深的彩色,不会对系统热性能造成较大影响。     

The impact of optical and thermal properties on the performance of flat plate solar collectors

The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 °C operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 °C. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1%. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 °C. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 °C. Including external booster reflectors increases the expected annual output at 50 °C to 19.9–29.4% depending on reflector material.     

A generalized thermal analysis of tubular solar collectors

A generalized thermal analysis of tubular solar collectors such as the CPC, CPC with bare absorber and flat-plate (all having U-tube fluid carriers) is presented. The analysis is applicable for smaller as well as larger size troughs. It has been found that earlier published results of thermal analyses of the CPC collectors are not applicable to all CPC collectors encountered in the literature. The present analysis can, however, be successfully applied to all such collectors.     

Experimental comparison of two configurations of hybrid photovoltaic thermal collectors

The combination of a thermal collector and a photovoltaic module in a single system allows for increased efficiency of the total conversion of solar energy. A synergistic effect can be obtained in a structure combining these two devices in a judicious manner to those of thermal and photovoltaic system installed separately. Production of total energy from hybrid collector depends on the input (that is to say, the. energy of solar radiation, air temperature and wind speed) and output which is the electric production and the temperature of the system. Thin production also depends on the mode of heal extraction. In this paper, an experimental Study of two configurations of hybrid collectors is described. The configuration that the absorber is made by galvanized steel and in the second, the absorber is a copper serpentine. The advantages of the first configuration are mainly due to low cost and simplicity but the second configuration has the advantage of promoting the heat transfer between cells and fluid.     

A method of testing for rating solar collectors based on thermal performance

This paper describes a proposed test method for determining the efficiency of solar collectors under specified outdoor “steady-state conditions. The prescribed series of tests should provide useful data for the rating of solar collectors based on thermal performance. A study was made of existing theory, measurement practices and a number of collector test procedures in use prior to the publication of the proposed method.The test apparatuses and major components have been prescribed so a liquid or air can be used as the transfer fluid. The energy of the fluid entering and leaving the collector is determined by making appropriate measurements and these quantities are than compared to the energy incident upon the collector (also determined by measurement) in order to calculate the collector efficiency.The series of tests to be conducted consists of determining the average efficiency for 15 min periods (integrating the energy quantities) over a range of temperature differences between the average fluid temperature (average of inlet and outlet) and the ambient air. The test apparatuses have been designed so that the temperature of the fluid entering the collector can be controlled to a selected value. This feature is used to obtain the data over the temperature range desired. At least sixteen “data points” are required for a complete test series and they must be taken symmetrical with respect to solar noon (to prevent biased results due to possible transient effects).     

Solar thermal electric power systems: Comparison of line-focus collectors

Three types of line-focusing collectors: parabolic though, fixed slats with movable absorber and movable slats with fixed absorber, are evaluated to find those systems that are capable of producing the lowest costs of electrical energy. Minimum costs per kW-hr are found using sequential optimization techniques that consider variations in rim angle, reflectance, aperture width, length, orientation, tracking, contour error, slat width, slat curvature, tangent slat angle, slope, installation methods, materials, fabrication methods, absorptance, emittance, cover transmittance, field shape, layout, pipe sizes, insulation thicknesses and turbine-generator-cooling lower efficiencies and designs. This approach provides a uniform treatment of both cost and performance for a solar thermal electric power system. This uniform treatment of solar thermal electric power systems for all collector types insures that valid comparisons are made.     

Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions

New comparative tests on two different types of solar collectors are presented in this paper. A standard glazed flat plate collector and an evacuated tube collector are installed in parallel and tested at the same working conditions; the evacuated collector is a direct flow through type with external compound parabolic concentrator (CPC) reflectors.Efficiency in steady-state and quasi-dynamic conditions is measured following the standard EN 12975-2 and it is compared with the input/output curves measured for the whole day.The first purpose of the present work is the comparison of results in steady-state and quasi-dynamic test methods both for flat plate and evacuated tube collectors. Beside this, the objective is to characterize and to compare the daily energy performance of these two types of collectors. An effective mean for describing and analyzing the daily performance is the so called input/output diagram, in which the collected solar energy is plotted against the daily incident solar radiation. Test runs have been performed in several conditions to reproduce different conventional uses (hot water, space heating, solar cooling).Results are also presented in terms of daily efficiency versus daily average reduced temperature difference: this allows to represent the comparative characteristics of the two collectors when operating under variable conditions, especially with wide range of incidence angles.