Thermal test procedure for box-type solar cookers

Some guidelines are provided for thermal evaluation of box-type solar cookers. Suitable thermal tests have been proposed and appropriate parameters identified, which pertain to the solar cooker and are relatively independent of the climatic variables as well as the products cooked. These parameters have been combined into two figures of merit.     

Evaluating the optimum load range for box-type solar cookers

This paper introduces a new concept of Optimum Load Range (OLR) for solar cookers. OLR gives the load values for which cooker preferably shows good thermal as well as good cooking performance; it may be considered a crucial parameter for solar cookers. This OLR concept is based on the dependence of rate of rise of load temperature on different heat transfer processes between load and cooker interior. This concept illustrates solar cooking in two simple steps. The total time required to complete these steps puts an essential constraint for cooking of any load amount. The maximum value of load (upper limit of OLR) till which cooker shows satisfactory cooking may be determined from this constraint. This constraint requires determination of two OLR parameters which are t_{step I} and t_{step II}. The load for which cooker remain almost 30% efficient, may be referred as lower limit (minimum value) of OLR. For the verification of OLR, experimental studies have been conducted with a solar cooker named SFSC. The OLR parameters along with different thermal performance parameters (TPPs) (second figure of merit (F₂), utilization efficiency (η_u) etc.) suggested by different researches for solar cookers in water load condition have been computed from the measured thermal profiles of different loads (0.8–3.0 kg). From the curve analysis of different TPPs with load, the existence of upper limit of OLR is observed. The values of rate of rise of load temperature at water temperatures 80, 85 and 90 °C for different loads also confirm the same. The OLR of SFSC is found to be 1.2–1.6 kg.     

Mathematical model of the thermal performance of box-type solar cookers

A mathematical model of the heat transfer processes involved with a box-type solar cooker, containing food, was developed. Solar radiation, including that from a flat reflector, enters the box and heat transfer within and out of the box was considered. Using small increments of time, the computer model finds the temperature of the air, food, interior walls and top cover as the solar flux varies throughout the cooking period. Sample results are presented, giving the food temperature as affected by latitude, month, wind, clouds, mass of food, thermal resistance of the box walls and adjustment of the box while cooking.     

Experimental testing of a box-type solar cooker using the standard procedure of cooking power

A box-type solar cooker with one (Model I) or four (Model II) cooking pots was constructed and tested under Tanta prevailing weather conditions. Experiments were performed during July 2002 using the cooker with or without load. The obtained results were employed to calculate the two figures of merit, F₁ and F₂, as well as the utilization efficiency η_u and the specific t_s and characteristic t_c boiling times. The obtained values of F₁ indicate that the cooker can be used twice a day for consecutive cooking. F₂ was found to increase almost linearly with the mass of the cooking fluid M_f. The cooker is able to boil 1 kg of water in 15 min when its aperture area equals 1 m². Furthermore, experiments also considered the requirements for the international standard test procedure for solar cookers. The cooking power P was correlated with the temperature difference ΔT between the cooking fluid and the ambient air. Linear correlations between P and ΔT had correlation coefficients higher than 0.90 satisfying the standard. The obtained values of the initial cooking power, heat loss coefficient and the cooking power at a temperature difference of 50 °C agree well with those obtained for small solar cookers. The present cooker is able to cook most kinds of food with an overall utilization efficiency of 26.7%.     

Heat loss coefficients for box-type solar cookers

The top and overall heat loss coefficients for the entire feasible operating range of box-type solar cookers are evaluated experimentally and presented in a graphical form as a function of the difference between mean plate temperature and ambient temperature with wind velocity and number of glass covers as parameters.

The range of plate temperatures considered is from 50°C to 180°C. While the wind velocity is varied from 0 to 3.33 m/s, and the number of glass covers considered are from 1 to 4.

Based on these experimental results, a correlation for the determination of top loss coefficient in terms of optical properties of cooker, the spacing between glass cover and absorber plate, wind velocity and number of glass covers, is derived and presented in the paper.     

Characterisation and design methods of solar cookers

The use of solar cookers is much needed in many regions with good solar radiation intensity throughout the world. The reasons are economical, as the price of fuel for cooking is no longer affordable by many families; ecological, as in many regions deforestation is also associated with the use of wood as an energy source; and social, as the money used to buy fuel could be used to buy food, medications and other needs to improve the quality of life. Because of the variety of solar cookers that has been presented in the literature, a general procedure to compare these cookers with one another is very complex. This article presents the general types of solar cookers, theirs basic characteristics, and experimental procedures to test the different types of solar cookers. The variables measured in these procedures are necessary to calculate parameters, which are used to compare the thermal performance of the solar cookers. In addition to these experimental procedures, a simplified analytical model is presented to design simple cooking systems. For more complex systems, results are shown and references are indicated in the text.     

Novel solar cookers: suitable for small families

This paper presents the fabrication details and on-field experimental studies of two novel solar cookers, suitable for cooking requirements of small families; these are named as small family solar cookers (SFSC-1 and SFSC-2). Small size, good thermal performance, light weight, low-cost and short payback periods are some important features of these cookers. The values of some essential thermal performance parameters, first figure of merit (F₁), second figure of merit (F₂) and standard cooking power suggested by Bureau of Indian Standards and International Standard for box-type solar cookers, have been evaluated by experimental studies and found to be 0.116°C m²/W, 0.466, 30 W and 0.118°C m²/W, 0.488, 50 W for SFSC-1 and SFSC-2, respectively. A comparative analysis of the thermal performances of SFSCs with the solar cookers, developed by many authors, has also been presented here. The payback periods with respect to different cooking fuels for SFSCs have been found to be reasonably short.     

A review of the thermal performance parameters of box type solar cookers and identification of their correlations

One of the many thermal performance parameters such as efficiency, cooking power, figures of merit etc. are used to evaluate a solar cooker (including box type) based on test procedures which are non-identical. In the absence of an interrelation between the different performance parameters, it is very difficult to compare the cookers’ performance reported by different researchers and establish the criteria required for selection of a cooker which can accomplish cooking successfully and satisfactorily. In this review paper, some of the performance parameters and the related test procedures have been reviewed for box type solar cooker. Further an attempt has been made to identify common links between the different performance parameters in terms of a few objective parameters. This provides an enabling tool to the researchers to compare and correlate the different performance parameters. Three such objective parameters have been identified for box type solar cookers.     

太阳灶性能测试国际标准简介

〔第三届国际太阳灶学术会议于1997年1月6日至10日在印度哥印拜陀举行,会议提出了太阳灶测试和性能评估的国际标准。之后,美国农业部农业研究中心的科学家P.A.范克(Funk)利用实验数据对该标准进行了评估,认为该标准不仅易于使用,而且所提供的数据可很好的预测太阳灶的热性能。详细报道可见SolarEnergy(Vo1.68,No.1,2000)。—编译者〕     

Testing of solar cookers and evaluation of instrumentation error

Solar cooking technologies have large potential in developing countries. Many of the solar cookers (particularly box type and parabolic concentrating type solar cookers) have been commercialized in different parts of the world. An effective quality control is essential for a large-scale dissemination of solar thermal technologies on the products being offered by the industry to the end users. For this, there is a need to establish test procedures and methodologies for developing performance characteristic parameters, which could provide an equitable basis for comparison of performances of the products. A comprehensive review of various test procedures of solar cookers has been undertaken in this study.This study presents results of using various test procedures for characterizing box type and a family size parabolic concentrator solar cooker, based on detailed experimental investigations. The study is supported by a number of experiments carried at the location of New Delhi (latitude = 28.56°N, longitude = 77°E) under various climatic and operating conditions round the year. The overall error associated in the determination of performance parameters due to instrumentation has been estimated by using the root-sum square method. It has been estimated that instrumentation cause 1–5.5 percent error on the thermal performance parameters of solar cookers. The effect of instrumentation error has been evaluated maximum on second figure of merit, F₂, optical efficiency factor, F′η_o, and standardized cooking power P_s.     

A comprehensive procedure for performance evaluation of solar food dryers

Solar food dryers are available in a range of size and design and are used for drying various food products. Testing a dryer is necessary to evaluate its absolute and comparative performance with other dryers and the test results provide relevant information for the designer as well as the user. Literature reviews on existing testing procedures reveal that a comprehensive procedure for evaluating the performance of solar food dryers is not available. Therefore, selection of dryers for a particular application is largely a decision based on what is available and the types of dryers currently used widely. This paper presents a detailed review of parameters generally used in testing and evaluation of different types of solar food dryers. The inadequacies of the parameters generally reported are highlighted and additional parameters have been suggested. Based on this review, a procedure has been proposed, giving the methodology, test conditions and a sample evaluation sheet. This would assist in an unambiguous evaluation of solar dryer performance and facilitate comparing different solar food dryers.     

聚光式太阳灶的设计与制作(Ⅰ)--抛物面聚光太阳灶是怎样工作的

一关于抛物线与抛物面 1什么是抛物线 抛物线是我们在抛出一个物体时,物体在抛力与重力的作用下所运行的路线.如你掷出一个小石子,它飞行的这一段轨迹,就是一条抛物线.抛力的大小不同,抛出的角度不同就会得到不同的抛物线,可惜你通常看不到这条线,我们可画出一条抛物线来先熟悉一下.图1为一个人抛出一颗石子,石子运行的轨迹,就是一条抛物线.     

A new method for testing the performance of flat-plate solar collectors

In this study, a new method is proposed for determining the thermal performance parameters of a flat-plate water heating collector. A parametric identification principle is applied to a mathematical model of the collector operating in a real system. The advantages of the method are that it does not need a sophisticated testing system, it avoids the problem of regulating the inlet fluid temperature (which must be held constant during the tests in the standard procedures) and a cloudless sky is not essential. Solar radiation of 600 W m⁻² was chosen as a threshold to start experiments. Applied to a flat-plate collector with a single-glazed cover, the proposed method gave satisfactory results for the determination of its thermal performance coefficients. To verify the adequacy of the present method, it was used to predict the outlet fluid temperature. The results indicate a satisfactory agreement between predicted and measured values with a correlation coefficient of 0.99 and an error of 0.4%. We propose that performance results will be presented as a technical sheet showing collector features and accompanied with a curve giving the correlation between estimated and measured output temperature of the collector fluid.     

聚光式太阳灶的设计与制作(Ⅳ)--选一选太阳灶的支架

针对太阳灶的使用对象主要为农牧民的特点，在满足太阳灶所应具备的功能的前提下，尽量考虑到便于使用和操作，同时还要尽力降低成本。     

聚光式太阳灶的设计与制作(Ⅲ)一一算一算要多大面积画一画灶面轮廓

人们在设计工作中首先遇到的问题，是要做多大面积的太阳灶，让我们首先研究这个问题。     

聚光式太阳灶的设计与制作(Ⅷ)--太阳灶技术要求和测试方法

作为一种产品,我们需要采用科学的方法对它进行检测,以评价它的各项技术指标的优劣.聚光型太阳灶行业标准(NY219-2003),系统地归纳了我国十余年来在聚光型太阳灶方面的科研成果和生产推广经验,提出了太阳灶的设计、型号、规格和测试方法,规定了太阳灶的技术要求、结构检测和性能试验方法,是世界上首次提出的太阳灶标准.为方便读者掌握和应用,我们把标准的主要内容介绍如下.     

聚光式太阳灶的设计与制作(Ⅵ)--太阳灶模具的制作

（二）太阳灶灶壳的材料和制作 太阳灶材料可分为灶壳材料、反光材料和支架材料三大部分。由于灶壳材料是关键，我们将重点加以介绍。水泥太阳灶、玻璃钢太阳灶、铸铁太阳灶、菱镁太阳灶等称谓，大都是指灶壳的制作材料而言的，太阳灶灶壳的制作材料对所制作出的太阳灶的特点和优劣以及工艺可行性起到了决定性作用。     

聚光式太阳灶的设计与制作(Ⅶ)--太阳灶模具的制作

（三）反光材料 聚光太阳灶实用效果的优劣，除了设计和制作工艺对太阳灶的影响之外，在很大程度上取决于反光材料的性能。选用反射率高、寿命长的反光材料，能提高太阳灶的热效率。在功率相等的情况下可以使反光面做得小一些，从而有利于操作，也有利于降低成本。     

太阳能集热器热性能测试系统

为满足工业生产及科学研究中对太阳能集热器测试的需要,按照GB/T 4271-2007《太阳能集热器热性能试验方法》设计了太阳能集热器热性能测试系统。系统由恒温控制台、恒温水箱、旋转平台、循环水泵和连接管路等组成,可对采用液体作为传热工质的集热器进行稳态和动态测试。选取了温度、流量、压力、风速及太阳辐照度传感器,设计了其硬件通讯电路,利用Labwindows/CVI软件为基础开发了测试系统的软件部分,实现了数据的采集、分析和显示。测试结果表明,系统能准确完成集热器的瞬时效率、时间常数、入射角修正系数及两端压力降等的测量,可为准确掌握集热器热性能提供试验平台。