Cooking is one of the most important energy consuming tasks in the domestic sector and the energy spend for cooking accounts for about 50% of the total primary energy consumption. Solar cooking provides an alternative energy source for cooking, which is simple, safe and convenient without fossil fuel consumption and environment pollution. It is suitable for millions of people around the world with scarce fuel and financial resource to pay for cooking fuel. Solar cooker provides many advantages, on the health, time and income of the users and on the environment.
Solar cooking presents an effective method of utilizing solar energy for meeting the demand for cooking energy. Many solar cookers used presently are relatively inexpensive devices as they don’t need any fuel and operating cost. Many service organizations are promoting the use of solar cooker in order to aid fuel costs reduction and air pollution control and to slow down the deforestation caused by cutting down trees for firewood for cooking.
The basic principle of a solar box cooker is that it can cook the food as the interior of the box is heated by the solar energy. Both direct as well as reflected sunrays enter the solar box through the glass or plastic sheet provided on the top of the cooker. The temperature thus created inside the cooker will be sufficient for cooking food, boiling water etc. There are three heating principles that need to be considered in the design of solar cooker are:
Heat Gain: Heat gain can be achieved through greenhouse effect, with the use of reflectors and by the orientation of glass. Enclosed spaces in the cooker into which the sun rays enter through a transparent material such as glass or plastic heats up due to green house effect. The light energy that is absorbed by dark pots and the dark absorber plate which is placed underneath the pots inside the cooker is converted into heat energy of longer wavelength and radiates from the interior materials. Most of this radiant energy, because of its longer wavelength, cannot come out through the glass and is thus trapped within the enclosed space. The reflected light is absorbed by other materials within the space.. With the use of single or multiple reflectors additional sunlight will bounce through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures.
In case of glass orientation, if more directly the glass faces the sun, the greater will be the solar heat gain.
Heat Loss: According to the second law of thermodynamics, the heat always travels from higher temperature to lower temperature, i.e. from hot region to cold region. Heat trapped within a solar box cooker is lost through conduction, convection and radiation. In case of conduction, heat within a solar box cooker is lost as it travels through the molecules of tin foil, glass materials, cardboard sheets, air and insulation to the air outside of the box. The hot absorber plate inside the cooker conducts heat to the bottoms of the pots. In order to prevent the loss of heat by conduction through the bottom of the cooker, the absorber plate is raised from the cooker bottom with the help of small insulating spacers. In case of convection, the molecules of air move in and out of the box through cracks. Heated air molecules within the solar box escape, primarily through the cracks around the top lid, a side oven door opening, or through the imperfections in construction. Cooler air from outside the box also enters through these openings. To prevent this problem, all opening section must be sealed properly. In case of radiation, hot things or food within a solar box cooker, radiate heat to their surroundings. Most of the radiated heat waves from the hot pots within a solar box are reflected from the foil and glass back to the pots and bottom tray.
Heat Storage: As the density and weight of the materials within the insulated shell of a solar box cooker increase, box capacity to hold heat increases. The interior of certain solar cooker box are provided with heavy materials to heat up to increase the heat storage capacity. The incoming solar energy is absorbed and stored as heat in these heavy materials, slowing down the heating of the air in the box. These dense materials, charged with heat, will radiate that heat within the box, keeping it warm for a longer period at the day's end.
Classification of Solar Cookers
Solar Cookers can be classified mainly into three types as below:
- Solar Panel cookers
- Solar Parabolic cookers
- Solar Box cookers
Panel Type Solar Cooker
Solar panel cookers may be considered as the simplest type available due to their ease of construction and low cost material. In solar panel cookers, sunlight before reaching inside the cooker is concentrated. As the name indicates, panel type cookers have a flat panel which reflects and focuses enough sunlight for cooking and heating. This method of solar cooking provides a limited cooking power and is therefore not very desirable. Solar panel cookers make use of reflective equipment such as mirror in order to direct sunlight to a cooking vessel which is enclosed in a clear plastic bag.. The panel cookers integrate elements of box and curved concentrator cookers. Panel type solar cookers are commonly referred as a CooKit. In days with bright sunlight, the food is ready within two to three hours.
Parabolic Type Solar Cooker
Solar parabolic cookers can reach extremely high temperatures in a very short time. The main difference of parabolic cooker from the panel cookers or box cookers is that they do not need a special cooking vessel. Since the temperature attained in a parabolic cooker is high because of the concentrated power, there is a risk of burning the food if left unattended for long time. A solar parabolic cooker mainly consists of a parabolic reflector with a cooking pot and the pot is located on the focal point of the cooker and a stand to support the cooking system. On comparing with solar box type and panel cookers, parabolic cookers can attain much higher temperatures and can cook more quickly, but require frequent adjustment and supervision for safe operation. It needs more accuracy to focus the sunlight on the food in the pan. If the sunlight is not correctly focused on the food in the pan, the food will get cooked well. Many designs are available for parabolic solar cooking appliances. Temperature of between 200 and 300 degree celsius can be reached because of a combination of the circular design, the size and the polished aluminium.
Box Type Solar Cooker
Solar box cookers or solar ovens are the most common and inexpensive type of solar cookers. They are very simple in construction and are made up of low cost materials. Box type cooker consist of a tray which is called as cooking tray and is usually covered with a double glass window. It is kept in a metal or fiber glass outer casing and the space between the cooking tray and outer casing is filled with glass wool insulation. The incoming solar radiation passes through the double glass lid and it strikes the blackened cooking pots and the cooking tray. The glass cover transmits short wavelength radiations which form major part of solar spectrum and is almost opaque to low temperature radiation emitted within the box. Thus, the temperature of the box rises until a balance between the heat received through glazing and heat lost by exposed surface is reached. In addition, a plane reflecting mirror or booster mirror of about equal size as that the aperture area is used for augmentation of solar radiation on the aperture. The cooking tray is insulated on the sides and bottom.
The heat is absorbed by the blackened surface and gets transferred to the food inside the pots to aid cooking. The mirror reflector is set in such a way to reflect the solar radiation falling on it to the cooker aperture. Solar box cookers usually cook at moderate temperatures and often can accommodate multiple pots in it. It can reach a temperature of about 160 degree celsius. The solar box cooker, like other solar cookers, needs direct sunshine to operate and produces zero emission. However, the temperature is low and it cannot store and save solar heat for later use. History of solar cooking technology started with the invention of box-type solar cookers. The first solar box cooker was invented by Horace de Saussure in 1767, and it basically consists of an insulated box with a transparent glass cover and reflective surfaces to direct sunlight into the box. The inner part of the box is painted black in order to maximize the sunlight absorption. Maximum 4 cooking vessels are placed inside the box.
Merits and Demerits of Solar Cooker
Solar cooking can reduce the need of firewood, and thereby reducing the emissions. The merits of solar cooking are,
- Safe cooking and attention is not needed during cooking as in other devices.
- Eco-friendly and no smoke inhalation.
- Less water requirement.
- Require less maintenance and negligible maintenance cost.
- No risk of burning food.
- No soot accumulation on pots.
- Available both with and without electrical back up.
The demerits of solar cooker are,
- Cooking not possible during night, rainy and cloudy days.
- Cooking takes comparatively more time.
- Not efficient in retaining heat when compared with conventional cooking devices
- Foods which require high temperature baking cannot be cooked.
Solar cooker generally depends on the sun radiation, which is not consistent, and cannot cook the food under low radiation condition. This drawback can be solved by the storage unit associated with in a solar cooker. So that food can be cooked at low radiation condition. To overcome the reduced efficiency of solar cooker, many efficiency improvement strategies like providing the cooking pot with fins, use of internal reflectors, use of transparent insulation material, use of stone pebbles for thermal storage etc. can be adopted.