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.     

Energy performance of a low-emissivity electrically heated oven

A low-emissivity oven (LEO) provides an energy-efficient alternative to a conventional domestic electric oven. A prototype domestic LEO is described and its energy performance is assessed and compared with that of a current generation domestic electric oven and previous test-rig ovens. In standard tests, the prototype achieved energy-savings of 36–57% when compared with the commercially-available oven, providing thermal performances similar to those of ‘idealised’ test-rig ovens. It is, therefore, recommended that manufacturers implement low-emissivity oven designs for domestic users as a means of speeding thermal performance, reducing energy consumption, and thereby reducing the emissions and peak electrical demands associated with domestic cooking.     

Heating characteristics of convection reflow ovens

The inhomogeneous temperature distribution in the reflow oven can cause soldering failures. In this project, we investigate how the construction of a convection reflow oven has an affect on its heating characteristics. In the convection reflow oven itself, the heating capability is mainly dependent on the heat transfer coefficient, which in turn is mostly determined by the gas flow parameters. Therefore, we study how the construction of the reflow oven affects the flow of gas in the oven. Our conclusions are then verified with experimental results. During our experiments, temperature changes were measured at different points located around the centre of the processing area in the oven. From this data, the 3D directional characteristics of the heat transfer coefficient was calculated using the heat equation of the investigated reflow oven. We consider that our results are important for the effective thermal modelling of the reflow soldering process [B. Illés, G. Harsányi, 3D Thermal model to investigate component displacement phenomenon during reflow soldering, Microelectronics Reliability 48 (2008) 1062–1068] and are also useful when calibrating and designing reflow ovens. During our work, we examine the latest reflow ovens constructed with the nozzle-matrix blower system.     

Performance studies of a large size nontracking solar cooker

A large size nontracking solar cooker has been designed, fabricated and tested. The cooker is based on the hot box principle. The cooker has been tested extensively and its performance has been compared with a solar oven, a hot box solar cooker and a solar cooker (tilted absorber). The stagnation temperatures are in increasing order for the hot box solar cooker, the solar cooker (tilted absorber), the large size nontracking solar cooker and the solar oven. The performance of this solar cooker is comparable with that of a solar oven. The former is not tracked towards sun while the latter is tracked every 30 min. The efficiency of a large size nontracking solar cooker is 24.9%. The energy saved by this new solar cooker has also been calculated and its payback period has been computed by considering interest, maintenance and inflation in fuel prices and maintenance cost. The payback periods are 1.10–3.63 years depending on which fuel it replaces. Relatively short payback periods show that the use of the cooker is economical, and it is easy to operate since no tracking is required.     

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.     

Modeling of transient natural convection heat transfer in electric ovens

Prediction of transient natural convection heat transfer in vented enclosures has multiple applications such as understanding of cooking environment in ovens and heat sink performance in electronic packaging industry. The thermal field within an oven has significant impact on quality of cooked food and reliable predictions are important for robust design and performance evaluation of an oven. The CFD modeling of electric oven involves three-dimensional, unsteady, natural convective flow-thermal field coupled with radiative heat transfer. However, numerical solution of natural convection in enclosures with openings at top and bottom (ovens) can often lead to non-physical solutions such as reverse flow at the top vent, partly a function of initialization and sometimes dependent on boundary conditions. In this paper, development of a physics based robust CFD methodology is discussed. This model has been developed with rigorous experimental support and transient validation of this model with experiments show less than 3% discrepancy for a bake cycle. There is greater challenge in simulating a broil cycle, where the fluid inside the cavity is stably stratified and is also highlighted. A comparative analyses of bake and broil cycle thermal fields inside the oven are also presented.     

Distribution of the heat transfer coefficient in convection reflow oven

Nowadays reflow ovens apply forced convection heating with nozzle-matrix blower system which generates numerous heater gas streams. In these types of ovens the heat transfer coefficients of the gas streams determine mainly the efficiency of heating. In this paper a measurement method is discussed which can determine the heat transfer coefficient distribution under the nozzle-matrix of a convection reflow oven. During the experiments, temperature changes have been measured and the heat transfer coefficient has been calculated using the heat equation of the investigated reflow oven. In the first step, the heat transfer coefficient of the heater gas streams has been studied under the nozzle-matrix in function of height. In the second step, the heating efficiency of the nozzle-lines has been compared and the distribution of the heat transfer coefficient has been calculated. The results are confirmed by a theoretical model. It is also presented that how the contamination of the nozzles affects the heating efficiency of the reflow oven. This measuring method can be useful for monitoring the operation of the reflow oven, soldering failure prediction and measuring input parameters of thermal modeling.     

The performance of a solar cooker in Egypt

A simple, efficient solar cooker was designed and its performance in the Egyptian climate was evaluated. The cooker is of the hot box type with a plane booster mirror reflector. The performance of the cooker was measured experimentally for over two years under different working conditions. The test conditions included experiments with and without adjusting the cooker's position for maximum solar radiation and the tilt angle of the booster reflector for maximum concentration. Formulas for calculating the orientation angle of the oven and tilt angle of the reflector were deduced. The shadow effect caused by the sides of the oven was also calculated. The solar cooker proved to be reliable, trouble free and efficient. All kinds of food were cooked including meat, chicken, rice, peas, beans, potatoes, soup, eggs and cakes. A good meal for a family of four was cooked in 3–4 h. It was established that better heat transfer occurred when the cooking pot was covered with an airtight plastic transparent cover rather than using an ordinary metallic cover. The energy balance of the cooker from heat transfer considerations enabled estimates of both over and food temperature to be made. Calculated values agreed well with the experimental measurements.     

A new procedure for power efficiency optimization in microwave ovens based on thermographic measurements and load location search

In this work, a novel procedure for increasing the power efficiency of microwave ovens is presented. The procedure consists of iteratively moving the sample within the microwave oven until a maximum average temperature increment is detected across the sample. A new experimental set-up consisting of a multimode microwave cavity, a dielectric tracking system, a microwave power source and a thermographic camera has been built and tested. Results show that it is possible to find optimal positions for the sample within the oven for which microwave power transfer to the sample is maximized.     

Evaluation of solar dried thyme (Thymus vulgaris Linné) herbs

Thyme (Thymus vulgaris Linné) herbs can be dried at about 50°C reached an equilibrium moisture content after 12 h and 9.5 h using the wire basket solar dryer and oven drying method respectively. The initial moisture content (wet wt. basis), (final moisture content, dry wt. basis (dwb)) determined by the Dean–Stark toluene method, oven and microwave were 75.15% (10.0%), 75.12% (11.85%) and 72.31% (12.50%) respectively. Paired t-test (α = 0.05, 10 degrees of freedom) showed no significant difference between the Dean–Stark toluene and the oven methods, but a significant difference between these two methods and the microwave method. The % essential oils extracted after drying by the oven and the wire basket solar methods were 0.5 and 0.6% (per 100 g dwb) respectively. The % oleoresin and ash content were 27% for both drying methods and 1.60, 2.03 and 2.25% for the fresh, oven dried and the wire basket solar dried herb respectively.     

A review of solar drying technologies

Agricultural products such as coffee, tobacco, tea, fruit, cocoa beans, rice, nuts, and timber generally require drying through a consistent application of relatively low heat. Traditionally, crop drying has been accomplished by burning wood and fossil fuels in ovens or open air drying under screened sunlight. These methods, however, have their shortcomings. The former is expensive and damages the environment and the latter is susceptible to the variety and unpredictability of the weather. Solar crop drying is a happy medium between these two methods and it dries crops with more efficiency, uniformity, and less expense. A solar crop drying system does not solely depend on solar energy to function; it combines fuel burning with the energy of the sun, thus reducing fossil fuel consumption. In this paper, the status of solar drying technologies in developing countries is presented. The various designs of solar dryers, its types and performance analysis are reviewed. Special attention is given to the solar drying technologies that facilitate drying of crops in off-sunshine hours. The solar dryers specifically designed or tested using specific crops like the vegetable dryer, fruit dryer, grain dryer, grape dryer, and so on are also reviewed with details about the specifications and the results. In short, the state of art technologies and development of solar dryers are presented in this paper.     

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.     

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.     

A modular phase change heat exchanger for a solar oven

A modular energy storing heat exchanger designed to use pentaerythritol for thermal storage (solid-solid phase change at 182°C) is tested in an oven by circulating heat transfer oil which is electrically heated in a manner to simulate a concentrating solar collector. Three efficiencies for heating the system under controlled and measured power input are determined—the heat exchanger efficiency, the efficiency of the heater with distribution lines, and the total system efficiency. Thermal energy retention times and cooking extraction times are determined, and along with the efficiencies, are compared with the results previously reported for a nonmodular heat exchanger. The modular configuration provides a highly improved extraction rate for cooking due to its wrap-around character and its increased surface-to-volume ratio. A full scale glass model of the copper tubing of the system is described and flow observations reported demonstrating how uniformly the parallel pumping branches perform and how trapped air pockets affect pumping power. A technique for measuring pumping power is described and its application to our system is quantified to show that less than 1 watt is required to circulate the heat transfer oil even when the system includes the solar collector and its longer connecting tubes.     

High flux photochemical changes in Black Chrome solar absorbing coatings

Black Chrome samples have been exposed to concentrated solar radiation (350kW m⁻²–2.5 MW m⁻²) at elevated temperatures (250–500°C) over limited periods of time with the resultant observation that the coatings degrade less optically than equivalent coatings heated by IR radiation. The decreased degradation is a result of decreased oxidation of the Black Chrome coating under solar heating as compared to oven heating. This is proposed to be due to photo-desorption of gaseous oxygen bearing species from the surface of the film.     

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.     

Drying of Thin Films of Polymer Solutions Coated over Impermeable Substrates

The manufacturing of adhesive tapes, photographic films, magnetic media, and many other products involves the coating and drying of thin films of polymeric solutions or emulsions over an impermeable substrate. Wet thickness may vary from 1 micron to 500 microns and solid content from 3% to 50% or more. The drying process generally occurs in convective ovens, where air temperature, air flow rate, and solvent partial pressure may be adjusted. In many cases, it is the manufacturing step that controls the productivity; therefore, it is very important to optimize the oven conditions to minimize the oven residence time without causing defects on the coated film and keeping the residual solvent inside the final product within specification limits. A drying model and a simulation software based on it were developed in order to solve the transient mass and heat transfer equations that describe the drying of a polymer solution containing two different solvents. The resulting system of equations shows strong non-linearity due to the free surface and the dependence of diffusion coefficients on solvent concentration and film temperature. The integration in time was performed with a fully implicit method, and at each time step, Newton's method was used to solve the resulting non-linear system of algebraic equations. An example of using the drying simulator to optimize an industrial adhesive tape line is discussed. The gain in productivity achieved by the analysis was greater than 40%.     

Long Horizontal Vapordynamic Thermosyphons for Renewable Energy Sources

Thermosyphons of the large length are of great interest for being used as heat exchangers for recuperation of alternative energy sources and upgrading their potential with the aid of heat pumps. Some examples of vapordynamic themosyphons (VDT) application as the system of thermal control for solar collectors, in the air conditioning systems, devices combating snow drifts and icing on the active parts of the railway transport track structure, foodstuff baking ovens and roasters, driers, etc. are considered. It is concluded that VDT used for air conditioning, ground heat exchangers, and seasonal thermal storage systems connecting with solar thermal collectors have good opportunities.     

Drying saturated zeolite pellets to assess fluidised bed performance

This work studies the drying rates of zeolite pellets (Type 1 [D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley & Sons, New York, 1984]) in spouted beds and in conventional fan assisted ovens. Three temperatures were studied: 48, 85 and 133 °C for drying times up to 1 h. Heat transfer is deduced from measured mass transfer rates and literature correlations. Maximum heat transfer coefficients were found to be 115.6 W/m² K for the fluidised bed and 31.8 W/m² K for the oven using correlations from literature. Higher figures were found by experimentation and calculation, typically about 200 W/m² K. A finite element model was used to estimate the temperature profile within pellets, the predictions agree well with experimental temperature measurements.     

Silk cocoon drying in forced convection type solar dryer

The thin layer silk cocoon drying was studied in a forced convection type solar dyer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 °C. The cocoon was dried from the initial moisture content of about 60–12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.