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.     

Financial feasibility analysis of box-type solar cookers in India

A framework for financial evaluation of a box-type solar cooker, using cost functions for its capital cost, has been developed. The annual benefits accrued to the user are quantified in terms of number of meals cooked per year and monetary worth of the fuel saved per meal. Expressions for some financial performance indicators have been derived and results of typical numerical calculations are briefly presented.     

Estimation of design parameters for thermal performance evaluation of box-type solar cooker

The paper presents a simple test procedure for determination of design parameters to predict the thermal performance of a box-type solar cooker. A series of out-door experiments were performed on the double-glazed solar cooker of aperture area 0.245 m² with a fibre body to obtain two figures of merit, F₁ and F₂. The necessary design parameters—optical efficiency, F′η_o and heat capacity, (MC)′ of the cooker are calculated using the linear regression analysis of experimental F₂ data for different load of water. Based on the experimental results, a correlation for F₂ as a function of quantity of water (load) is proposed. The close agreement between experimental and calculated F₂ indicates the validity of the correlation. The proposed procedure is then applied to predict the heating characteristic curves of the solar cooker for different load of water. The predicted heating characteristic curves are validated by comparing with the experimental data from a series of cooker testing experiments. The results of present study reveal that F′η_o and (MC)′ are the critical design parameters required for the prediction of thermal performance of the solar cooker.     

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.     

Efficient orientation impacts of box-type solar cooker on the cooker performance

The addition of a plane reflector to a box-type solar cooker increases the obtained cooker temperature and this depends upon the efficient orientation of the cooker. In order to find out the effect of the cooker orientation on its performance, the present analysis is carried out. A method is outlined to find out a reflector performance factor and an orientation factor that depend upon the elevation angle of the sun, the solar surface azimuth angle and the reflector tilt angle. The analysis is applied to a cooker placed at Aden (Yemen). The results indicate that with proper cooker orientation the improvement in the performance of the cooker due to the reflector reached during winter is more than 100% at lower elevation angles and is more than 60% at higher elevation angles. It is concluded that with the help of the analysis the optimum position for any place, for any day of the year and for any specific time of the day can be found.     

The performance of a box-type solar cooker with auxiliary heating

In Bangladesh it is very difficult to use solar cookers during the months when the days are generally cloudy and at times solar cooking becomes impossible. To overcome such problems for a box-type solar cooker, we have used an auxiliary source of energy inside it. This is done with the help of a built-in heating coil inside the cooker or a retrofit electric bulb in a black painted cylinder. It is found that the use of auxiliary sources allows cooking on most cloudy days.     

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.     

Multiple use communal solar cookers

Solar cooking in boarding schools and communal centres in isolated areas demands the heating of large quantities of food. It is generally done in large pots of 30–40 litres. In a previous paper (Saravia et al., Energías Renovables y Medio Ambiente, ASADES, 1999, 6, 7–16), the solar cooker presented consisted of two separate units: a concentrator on one side and an insulated box containing the pot on the other. In this paper we introduce some changes which improve its working capacity. At present we can count on three different kinds of absorbers, which have been optimised to fulfill different functions in a concentrator of an area of 2 m². These alternatives allow the possibility of satisfying the needs of a communal dining centre, cooking for up to 30 children, once each concentrator has been installed.     

Evaluating the international standard procedure for testing solar cookers and reporting performance

The international standard procedure for testing solar cookers and reporting performance was proposed at the Third World Conference on Solar Cooking (Avinashilingam University, Coimbatore, India, 6–10 January, 1997) and revised by the committee over the following months. The standard sets limits for environmental conditions, specifies test procedures and calls for performance to be reported in terms of cooking power (W). While this value, like the fuel economy rating of an automobile, is not a guarantee of performance, it does provide a useful tool for comparison. The entire standard is presented in this paper. It was evaluated both by using it to analyze data previously collected and by using it to test one of the solar cookers in the historical data set. The test standard cooking power curve clearly distinguishes between solar cookers of differing design. Estimates of solar cooker performance for different locations and dates are fairly consistent when the test standard is employed. The criteria of being easy to use and presenting data predictive of thermal performance are also satisfied.     

Mathematical modeling of a box-type solar cooker employing an asymmetric compound parabolic concentrator

A novel design of solar cooker is introduced. The cooker is of box-type equipped with an asymmetric compound parabolic concentrator (CPC) as booster-reflector. It consists of an insulated box equipped with a vertical double glazing cover on a side, and a vertical absorber plate laid out just behind the transparent cover. The booster-reflector is fixed on the glazed side of the box. The absorber plate and the glazing form a vertical channel, open at the top and bottom, and enclosed at the sides. The two openings allow the inside air circulation. A mathematical model of the heat transfer processes involved with this solar cooker, containing a cooking pot loaded with water and deposited on the box floor; was developed and the effects of various parameters, such as solar radiation, load of water and clouds on the dynamic behavior of the cooker are studied.     

太阳灶在印度的推广和应用

20多年来太阳灶在印度得到了成功的发展，已经被人们所接受。印度的太阳灶除了应用于炊事方面外，还在其它许多新的领域得到了应用，例如：处理污水，食品加工业，海水淡化，生产蒸汽，农产品干燥；空调和制冷。[编者按]     

Performance evaluation of three solar concentrating cookers

Three solar concentrating type cookers meant for domestic use were tested for their thermal performance and cooking abilities. Stagnation temperature, water heating and cooking tests were conducted. During the tests, the operational ease and problems in each cooker were evaluated.     

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.     

浅析太阳能灶在宁夏地区的发展

一引言 宁夏太阳能资源丰富,大部分属于太阳辐射一类区.据资料统计,宁夏每年太阳能辐射量为5781MJ/m2,其空间分布特征是北部多于南部,南北相差约1000 MJ/m2,灵武、同心最大,达6100 MJ/m2以上.宁夏的太阳能辐射量以直接辐射为主,散射辐射少,对宁夏太阳灶的开发利用十分有利.     

Thermal performance of solar air heaters: Mathematical model and solution procedure

A mathematical model and solution procedure for predicting the thermal performance of single-pass solar air collectors is presented. By omitting or providing a top glass cover over a plane absorber plate, or by providing a bottom plate under the absorber plate and circulating air over one or both of the air channels so formed, four common types of flat-plate solar air collector designs were considered. The surface temperatures of the walls surrounding the air streams were assumed uniform whereas the air temperatures were assumed to vary linearly along the collector. In the mathematical model, the solar collector was assumed sufficiently short for which the assumptions were valid. By considering a steady state heat transfer using the thermal network analysis procedure, a set of simultaneous equations for the mean temperature of the walls and the air streams were obtained. Instead of solving the simultaneous equations for mean temperatures explicitly, a matrix inversion method was employed using a standard sub-routine programme. Because heat transfer coefficients were temperature dependent, a set of mean temperatures was approximated which allowed the heat transfer coefficients to be evaluated as a first guess. An iterative process was then created that enabled the mean temperatures for the collector to be calculated. The newly-calculated mean temperatures were then compared with the initially-guessed temperatures. The iterative procedure was repeated until consecutive mean temperature values differed by less than 0.01°C. After this, another section of collector with a length equal to the previous one was added to the end of the first collector. The temperature conditions at the inlet of the second section were assumed equal to the outlet temperature conditions of the previous section. The iterative procedure to determine the mean temperatures was repeated for the next section. Additional sections were added until the required overall length of collector was considered. By this procedure, predictions of mean wall and air streams temperatures for a collector of any length could be obtained. Although only four, common single-pass types of flat-plate solar collectors are considered here, the solution procedure could be extended to encompass most other collector designs.     

A novel advanced box-type solar cooker

An advanced version of the box-type solar cooker is presented: a fixed cooking vessel in good thermal contact with a conductive absorber plate is set into the glazing; the results are improved thermal performance, easier access to the cooking vessel and less frequent maintenance due to protection of all absorbing and reflecting surfaces. Outdoor tests show that 5 L of water per sq m of opening surface can be brought to full boiling in less than one hour. A finite element simulation model of the advanced box cooker is presented. It is shown that the most decisive parameters are absorber-to-pot heat transfer and absorber conductivity. Field tests in Ethiopia and India are under way, local production in India has started.     

Mathematical analysis of the performance of cylindrical-parabolic solar concentrators

Attention is devoted to the application of cylindrical-parabolic solar concentrators in the intermediate temperature range for which wide receivers are used. Suitable indices that describe the performance of the concentrator are defined and evaluated. The effect of each individual parameter on total concentrator performance is investigated. The results of the analysis are presented as a set of graphs which can be used easily when designing parabolic concentrators.     

An analytic model for the long-term performance of solar thermal systems with well-mixed storage

A simple analytic method is presented for predicting the long-term performance of solar thermal systems with well-mixed storage and loads that do not vary significantly from day to day. Recognizing that most conventionally designed systems have effective relaxation times that are long relative to the time scale of variations in insolation, but short compared to the time scale for day-to-day variations in insolation, we invoke a constant radiation model and solve the problem within that framework. The results of the analytic method are simple closed-form expressions which enable the user to understand clearly the dependence of system performance on the physically important variables. Good agreement is found between the predictions of the analytic method and the corresponding results of the -f-chart method and computer simulations. The analytic method is furthermore applicable to all collector types and storage fluids and is not restricted to flat plates and hot water storage. The limitations of the analytic method are discussed and found not to introduce a serious restriction for commonly designed conventional systems.     

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.     

Mathematical modelling of salt gradient solar pond performance

This paper presents a mathematical model of the performance of the salt gradient solar pond. A lumped parameter model of the upper convective zone, non-convective zone and lower convective zone is used. This model enables the temperatures of the upper-convective zone and the lower convective zone of the solar pond to be predicted. The experimental results agree well with theoretically predicted values. The major error in the theoretical results is due to the difference between the theoretical value of the solar radiation inside the water and that observed experimentally. It is found that the experimental value of the solar radiation at a depth of 90 cm is approximately 26 per cent of the total solar radiation falling on the solar pond surface, whereas the corresponding theoretical value is found to be 33 per cent. The results conclude that the lumped parameter model can be used as a simple model to predict the performance of the solar pond.