The oven receiver: An approach toward the revival of concentrating solar cookers

Concentrating type solar cookers are expected to demonstrate high performance because of the large collection area employed. However, the net amount of heat used is still low. This is greatly attributed to the large amount of heat losses from the bare food pots used. Introducing the oven type concept as an alternative approach for collecting the concentrated solar energy would drastically boost the overall cooker efficiency. In this work, the transient heat balance equations were developed for predicting the thermal behavior of an oven type concentrating solar cooker. This simulation was used to show theoretically the great advantage of using a glass-sided oven over the conventional bare receiver pot. The resulting mathematical model was solved using numerical integration. The transient nature of solar radiation and effects of wind speed variation were all taken into consideration. The analysis showed that the oven type receiving pot has both a higher fluid temperature and overall receiver efficiency compared to the bare receiver type, working under similar conditions.     

Parametric model of solar cooker performance

This paper presents a model for prediction of the cooking power of a solar cooker based on three controlled parameters (solar intercept area, overall heat loss coefficient, and absorber plate thermal conductivity) and three uncontrolled variables (insolation, temperature difference, and load distribution). The model basis is a fundamental energy balance equation. Coefficients for each term in the model were determined by regression analysis of experimental data. The model was validated for commercially available solar cookers of both the box and concentrating types. The valid range of model application includes most of the feasible design space for family-sized solar cookers. The model can be used to estimate the cooking capacity of existing box type and concentrating type solar cookers. It can also be used to find the combinations of intercept area and heat loss coefficient required to cook a given quantity of food in a given climate.     

Solar cookers for outdoors and indoors

The solar cooking process has been investigated to develop safe, simple, portable, and reliable solar cookers. The concept of insulated and vapor-tight pots has been introduced and applied to oven, point-focus and heat-pipe cookers. A new, flat-plate cooker with heat pipes has been developed. It requires no tracking and allows cooking to be done in the shade or indoors. Also, a novel-portable cooker, the Mina Oven, featuring a vapor-tight pot and an integral collector with reflector flaps, has been constructed and tested. A second portable cooker that has been developed is the Arafa Cooker, which comprises a parabolic dish focused at a glazed and insulated receiver. Experiments indicated that all cookers yielded satisfactory performance, with cooking times of 25–45 min per kg of food per m² of solar collection area while operating from 10 a.m. to 4 p.m.     

A new solar cooker design

This paper deals with a new box-type solar cooker design with a single reflector at the hood. In this design, the base of the oven acts as the lid, unlike the conventional box-type solar cooker. With the introduction of this concept of the lid as the base of the oven, we solve the problem of preheating, as faced in the conventional box-type solar cooker. Thus, the performance of conventional box-type solar cookers can be appreciably improved by having the lid at the bottom of the oven and not at the top. It has been observed that, having cooked once, the preheating time for further cooking is greatly reduced as compared to the conventional box type. Hence, cooking twice a day by this cooker becomes easier, unlike cooking by the conventional one.     

Fast thermal cycling of acetanilide and magnesium chloride hexahydrate for indoor solar cooking

Solar cookers are broadly divided into a direct or focusing type, indirect or box-type and advanced solar cookers. The focusing and box-type solar cookers are for outdoor applications. The advanced solar cookers have the advantage of being usable indoors and thus solve one of the problems, which impede the social acceptance of solar cookers. The advanced type solar cookers are employing additional solar units that increase the cost. Therefore, the solar cooker must contain a heat storage medium to store thermal energy for use during off-sunshine hours. The main aim of this study is to investigate the influence of the melting/solidification fast cycling of the commercial grade acetanilide C₈H₉NO (T_m = 116 °C) and magnesium chloride hexahydrate MgCl₂·6H₂O (T_m = 116.7 °C) on their thermo-physical properties; such as melting point and latent heat of fusion, to be used as storage media inside solar cookers. Five hundred cycles have been performed. The thermo-physical properties are measured using the differential scanning calorimetric technique. The compatibility of the selected phase change materials (PCMs) with the containing material is also studied via the surface investigation, using the SIM technique, of aluminum and stainless steel samples embedded in the PCM during cycling. It is inferred that acetanilide is a promising PCM for cooking indoors and during law intensity solar radiation periods with good compatibility with aluminum as a containing material. However, MgCl₂·6H₂O is not stable during its thermal cycling (even with the extra water principle) due to the phase segregation problem; therefore, it is not recommended as a storage material inside solar cookers for cooking indoors. It is also indicated that MgCl₂·6H₂O is not compatible with either aluminum or stainless steel.     

An exergy based test protocol for truncated pyramid type solar box cooker

Developing a test standard/protocol for solar box type cookers has drawn a considerable interest among the researchers throughout the world. Recent publications on solar cookers emphasize the need of introducing the thermal performance indicators determined through exergy analysis. In the present paper, the time variation of instantaneous exergy output and energy output as function of its temperature and also of the instantaneous ambient temperature have been reported for truncated pyramid type solar box cooker and compared with those of box type cooker. Further, variations in the exergy lost with temperature difference have been depicted for the selected water temperature range from 60 °C to 95 °C. Based on this study, quality factor, exergy temperature difference gap product, and heat loss coefficient are determined and are proposed as benchmark parameters of solar cooker’s thermal performance.     

Studies on a new combined concentrating oven type solar cooker

This paper describes a combined concentrating/oven type solar cooker. The device can be used for cooking, boiling and roasting of foods on clear days. The cooker makes use of both concentrator and flat-plate collector principles, wherein the sunlight entering the cooker is reflected onto a hood which is provided with a selective solar absorber coating which houses the cooking vessels. Hence, the cooker makes use of both direct and diffuse solar radiation. The important part of this new device is a stationary mode and maximum capture of energy through improved design. The new cooker has been found to be more practical in comparison with either the simple hot box type solar cooker or concentrator type cookers where one needs to direct it to follow the sun. The cooking trial shows that the new device can be used twice a day, even on winter days.     

THE SOLAR OVEN: DEVELOPMENT AND FIELD-TESTING OF USER-MADE DESIGNS IN INDONESIA

This paper considers the design, use, and social acceptance of solar cookers that are constructed by their users. Several generations of oven design are described and their field testing in Indonesia, are reported, the first generation design having been described in a previous paper. The second generation design reached 175°C in oven temperature, and it used only local materials in its manufacture. Results of field testing of sixty four units in East Nusa Tenggara Provence, Timor Island, Sulamu village, and at Maumere City of the Sikka Regency in Flores Island, all in Indonesia, are reported. Social observations via questionnaires and direct conversation were conducted as part of the field study. Preliminary results showed promising tendencies of acceptance (up to 28%), and the solar ovens proved their ability to cook effectively. The most recent generation of design has reached 202°C in oven temperature, while costing 10% less than the previous one. The dissemination of thirty units of this design in two villages in West Nusa Tenggara Province is reported on.     

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.     

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.     

Evaluation of usage and fuel savings of solar ovens in Nicaragua

Solar cooking technology has been promoted as a solution to both global poverty and environmental degradation, but relatively little research exists on the impact of solar oven usage on biomass fuel consumption. This study evaluates solar oven usage and wood consumption in northern Nicaragua during both the rainy and dry seasons, using surveys, temperature dataloggers, and direct measurements of fuelwood use. Solar oven owners reported usage on 79% of days during the dry season, and 41% of days during the rainy season. Comparison with oven temperature records confirmed usage on 50% of days during the dry season, and 16% of days during the rainy season. However, wood consumption measurements showed no statistically significant difference between days with solar oven usage and days without, suggesting that frequency of usage alone is not an appropriate proxy for fuel savings. Survey results suggest that a large part of solar oven usage came in addition to biomass cooking, as opposed to replacing it. These results suggest a need for further study of wood consumption in situ and more focus on the specific kinds of foods prepared in solar cookers, as well as local cultural and climatic conditions.     

Users and disusers of box solar cookers in urban India—: Implications for solar cooking projects

The purpose of this study is to understand the reasons behind the continued use or disuse of solar cookers, and to outline the implications from the results of this study for future solar cooker projects. Twenty-eight families in three urban sites in Gujarat, India who have a solar cooker have been interviewed. Their experience with solar cookers and solar cooking is studied. Direct discussions with families who have practical experience with solar cooking, brings forth significant practical issues. The study shows that many disusers of solar cookers do not have a suitable place for their solar cookers. Other disusers could not adjust their daily routines with what solar cooking requires, and some disused their solar cookers because they were not interested in using them. Both objective factors and aspects of practical interest have been shown to be important issues for understanding the conditions of prospective users and the shaping of the projects and the relevant technologies. It is concluded that project developers should consider the potential users as an important partner in project development processes. For example project developers can by close dialogue with them, uncover and define practical parameters which have important bearing on higher usability of solar cookers.     

Fast-response storage-type solar cooker

All the solar powered cookers presently available in the world market are of the so-called “Reflector Box” type. Despite the fact that they have been presented, for a good few years, to the market and are very simple and low cost they have not managed to establish any meaningful and noticeable presence in a viable commercial sense. This fact is even more in evidence in those markets they are primarily designed for. The reasons to the lack of commercial success for the reflector box type cookers are analysed in some details. The assessment of general features and performance of the reflector box type cooker led to the design of the Fast Response Storage Type Cooker presented in this paper.The new cooker is the result of combination of non-tracking compound parabolic reflector, fast response heat pipes, high-quality and low-cost thermal insulator, low-loss thermal storage battery, glass to metal seals, vacuum technology and bimetal automatic switches. It is capable of storing the solar energy during the day to facilitate cooking during the evenings and nights. A prototype is made and the performance of the elements of the system are presented. It shows how the “waste-energy” of the cooker can be used for other applications.     

The development impact of solar cookers: A review of solar cooking impact research in South Africa

Solar cooking is often considered “a solution looking for a problem”. Solar cookers have long been presented as an interesting solution to the world's problem of dwindling fuel wood sources and other environmental problems associated with wood fuel demand for cooking. However, recent GTZ field work in South Africa showed different benefits instead: the use of solar cookers resulted in appreciable fuel and time savings as well as increased energy security for households using commercial fuels. These observations are based on field tests in South Africa that started in 1996 to investigate the social acceptability of solar cookers and to facilitate local production and commercialisation of the technology. Impact studies and use rate studies have been carried out by a number of different organisations since the inception of the project and although commercialisation of the technology has not been achieved to its fullest potential, impact studies indicate that solar cookers have a positive development impact on households through fuel-, energy- and time savings. The article aims to summarise the findings of the various studies and present an overview of use rates and impact data. A variety of factors influence solar cooker use rates, which in turn determine impacts. Some factors are related to the user, some to the environment in which the cooker is used and some to the cooker itself. Ultimately, the data shows that on average, only 17% of solar cooker owners do not use their stoves after purchase and that active solar cooker users utilise their stoves on average for 31% of their cooking incidences. Since the majority of solar stove buyers actually use their stoves and obtain real benefits, this suggests that that solar cookers are indeed not a solution looking for a problem but a solution worth promoting.     

Fabrication of and performance studies on a low cost solar cooker having an inclined aperture plane

A low cost box-type solar cooker made of two paper carton boxes with crumpled newspaper balls as insulation has been fabricated with a tilted aperture plane. Comparative tests of this cooker have been conducted against a normal type costlier solar cooker with 1000 ml load of water in each of the cookers. It has been observed that on a sunny day water temperature initially increases more rapidly in the new cooker compared to the normal type cooker. But at temperatures higher than 90°C both the cookers perform similarly. Two figures of merit F₁ and F₂ have also been found to be satisfactory. An arrangement of low cost auxiliary heating using a 100w electric bulb inside a blackened metal casing allows the carton box cooker to reach cooking temperatures under cloudy conditions with ease when the normal type solar cooker fails. The cost of materials for the new cooker is within US $10 and has been observed to be as effective as the normal type solar cooker which requires between US $40 and $65 to fabricate.     

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.     

SOLAR COOKER ACCEPTANCE IN SOUTH AFRICA: RESULTS OF A COMPARATIVE FIELD-TEST

A one-year comparative field-test of 7 different types of solar cookers, involving 66 families in 3 study areas in South Africa, has been conducted by the South African Department of Minerals and Energy (DME) and the Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ). Overall, families use solar cookers on 38% of all days and for 35% of all cooked meals; they express clear preferences for certain cooker types. Solar cookers, together with wood (stoves and open fires, used on 42% of all days), are the cooking appliances most used. Fuel consumption measurements show overall fuel savings of 38%, resulting in estimated pay-back periods (through monetary fuel savings) from 8 month onwards, depending on the type and region. Economic analysis, acquisition of test cookers by users after the placement period as well as an independent market study, have led to a second programme phase of commercial pilot dissemination of locally produced cookers, adapted according to field-test experiences.     

Design, construction and study of a hybrid solar food processor in the climate of Costa Rica

During the last 25 years, the author has designed, constructed, studied and promoted solar oven, hybrid solar/electric oven, solar oven cum drier, solar cooker cum water heater and solar still. In different parts of the world, solar cookers have been made, studied, patented, however, their real uses are very limited due to many reasons—unstable climate, economic, cultural, social and single use, etc. In order to overcome part of the problems mainly the last one, author has recently designed one hybrid food processor (multi-purposes device) and studied various technical and practical aspects. It has been used for cooking, heating/pasteurizing water (to inactivate microbes) and distillation of small quantity of water (to remove different minerals) and drying domestic products (fruits, vegetables and condiments/herbs, etc.). For more than three years of use, author has found this to be a useful device, mainly from convenience, fuel saving, economic and also from ecological point of view. This device can be used at any time and for different uses but with the reduced consumption of conventional fuel.     

Experimental investigation of tracking paraboloid and box solar cookers under Egyptian environment

Two full tracking solar cookers, namely a paraboloid dish solar cooker (PDSC) and a booster mirror solar box cooker (BMSBC) have been designed, constructed and tested during a winter season in Cairo (30° N and 30° E). The cookers are made simple, durable and of low cost. Tests are carried out to compare the performance of these two cookers under the same operating conditions. The results showed that the rate of cooking of the PDSC is higher than that of the BMSBC. Also, the PDSC can handle various types of cooking even under the intermittent sunny and cloudy conditions of the winter.The effect of wind speed on the heat loss from the receiver is of utmost importance for the evaluation of PDSC performance. Therefore, the effect of wind speed on the heat loss coefficients from new or black painted receivers is experimentally determined, analysed and presented in graphical form. The overall heat loss coefficients are correlated in terms of wind speed and surface conditions (painted or unpainted). During the indoor test, the wind speed was varied from 0 to 8 m/s. Based on these experimental results, a wind shield has been designed to reduce the wind effect. Tests confirmed that the wind shield improves the PDSC performance and allows boiling to take place even for high wind speeds. This is confirmed by the thermal analysis which shows reductions of 24 to 35% in the heat loss from the receiver with the presence of the wind shield.