Subject: and Green Technology Class: Second Year, II Semester Topic: Familiarization with Gadgets: Solar Cooker, Solar Water Heater Prepared by: Dr. Ajay Singh Lodhi Assistant Professor (SWE) College of Agriculture, Balaghat

FAMILIARIZATION WITH SOLAR ENERGY GADGETS Solar Cooker In our country energy consumed for cooking shares a major portion of the total energy consumed in a year. In villages 95% of the consumption goes only to cooking. Verity of fuel like coal, kerosene, cooking gas, firewood, dung cakes and agricultural waste are used the energy crisis is affecting everyone. It is affecting the fuel bills for those who use it for heating the houses and cooking their food. The poor of the developing countries who have been using dry wood, picked up from the fields and forests as domestic fuel, have been affected in their own way, due to scarcity of domestic fuel in the rural areas. At present, fire wood and cow dung too precious to allowed to be used for burning and cooking. It is very useful to improve the fertility of the soil; it should be used in proper way. The supply of wood is also fast depleting because of the indiscriminate felling of trees in the rural areas and the denudation of forests. There is a rapid deterioration in the supply of these fossil fuels like coal, kerosene or cooking gas. The solution for the above problem is the harnessing of solar energy for cooking purpose.

The most important is that the solar cooker is a great fuel saver. The department of new conventional energy source has calculated that a family using a solar cooker 275 days a year would save 800kgs of fire wood or 65 litres of kerosene. Similarly, an industrial Canteen or a Hostel mess using the larger community solar cooker which can cook for 20 to 25 people could save 400kgs of fire wood or 335 litres of kerosene per year.

Principle of Solar Cooking

The basic principles of all solar cookers are:

Concentrating sunlight: Some device, usually a mirror or some type of reflective metal, is used to concentrate the light and heat from the sun into a small cooking area, making the energy more concentrated and therefore more potent.

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Converting light to heat: Any black surface on the inside of a solar cooker, as well as certain materials for pots, will improve the effectiveness of turning light into heat. A black pan will absorb almost all the sun's light and turn it into heat, substantially improving the effectiveness of the cooker. Also, the better a pan conducts heat, the faster the oven will work.

Trapping heat: Isolating the air inside the cooker from the air outside the cooker, makes an important difference. Using a clear solid, like a plastic bag or a glass cover, will allow light to enter, but once the light is absorbed and converted to heat, a plastic bag or glass cover will trap the heat inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.

Types of Solar Cookers

Box type solar cooker: A box type solar cooker is an alternative food cooking technology with sunlight as its only energy source, which essentially consists of a black painted metallic trapezoidal tray (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 the insulation like glass wool. The incoming solar radiation falls on to the double glass lid and passes through it to strike the blackened cooking pots and the cooking tray. The glass covers, while transmitting radiation of short wavelength which form major part of solar spectrum, is almost opaque to low temperature radiation emitted within the box.

Thus, the temperature of the box rises until a balance is reached between the heat received through glazing and heat lost by exposed surface (greenhouse effect). In addition, a plane reflecting mirror (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 facilitate cooking. The mirror reflector is set in such a way to reflect the solar radiation falling on it to the cooker aperture.

Usually four black painted cylindrical pots are placed inside the box as cooking vessels and the cooking load is kept usually 8 lit/m2. The cooker takes 1.5 to 2 hours to cook items such as rice, lentils and vegetables. The cooker may also be used to prepare simple cakes, roast cashew nuts, dry grapes, etc. In good solar radiation conditions, it is possible to achieve the temperature up to 150oC inside the hot box of this solar cooker, hence best suited for boiling kind of cooking. It is an ideal device for domestic cooking during most of the year, except for the monsoon season and cloudy days. A family size solar cooker is sufficient for 4 to 5 members

2 and saves about 3 to 4 cylinders of LPG every year, while the life of this cooker is 10 to 20 years.

Fig. 1(a) Box type solar cooker with Single Fig. 1(b) Multi reflector type solar cooker reflector

Concentrating Type Solar Cooker

In the concentrating solar cookers, the cooking pot is placed at the focus of a concentrating mirror. The concentrating type solar cooker works on one or two axes tracking with a concentration ratio of up to 50 and temperatures up to 300oC, which are suitable for cooking. Concentrating cookers utilize multifaceted mirrors, Fresnel lenses or parabolic concentrators to attain higher temperatures. The various types of concentrating type cookers based on the mode of cooking are shown in Figure 2. Cookers that concentrate the light from above, and cookers that concentrate the light from below, are the two major types of concentrating solar cookers. In the first type, the light is concentrated from above (Figures 2a and b). Though this mode of concentration of energy from the top is not very desirable for cooking, there are several old designs, which have been developed. One of the most popular designs in this category is the solar panel cooker developed by Roger Bernard of France in 1994. Figure 4a shows the schematic of the panel cooker. It is elegant and simple looking, affordable, effective, and convenient. It requires a dark covered pot and a high temperature plastic bag per month. Within a few hours of sunshine, the cooker makes tasty meals for 4–5 persons at gentle temperatures, cooking food and preserving nutrients without burning and drying out. Figure 4b shows the schematic of a funnel cooker. It is simple to construct and store, and there is hardly any wastage of sheet. The funnel support may pose problems in the beginning; however, they can be solved by making a small hole in the ground, to hold the base of the funnel. Two small sticks or stones could be propped up at the back to hold the funnel in the right position.

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Fig.2: Concentrating type cookers: (a) panel cooker, (b) funnel cooker, (c) spherical reflector, (d) , (e) Fresnel concentrator and (f) cylindro-parabolic concentrator In the second type, the light is concentrated from below (Figure 2c-f), and this mode is the most convenient for routine cooking. There are several designs developed in this type of cooker. Figure 4c shows the schematic of a spherical reflector. Stam first suggested this design in 1961. It is the simplest type of concentrator and is easy to build and use. The cooking vessel could be hung from a tripod or a suitable stand, and positioned to meet the focus. Figure 1.9d shows the schematic of a parabolic reflector. It is popular among concentrating cookers, because the focus is much better and sharper than that of the other types of reflectors, but at the same time it is very sensitive to even a slight change in the position of the sun, and hence, the use of such reflectors requires constant tracking. Though the parabolic reflector is a perfect design, even good technicians find it difficult to construct parabolic reflectors even with the help of templates. Hence, the Fresnel reflectors as shown in Figure 4e, have gained importance. Three or four rings of Masonite (hardboard like material) are cut from a 40 x 40 sheet. Aluminium polyester is stuck to the rings, after which they are nailed to specially notched wooden reapers to form a Fresnel concentrator. The cooking pot is supported on a rod projecting from the centre. Figure 4f shows the schematic of the cylindro-parabolic concentrator. It focuses the rays into an insulated cylindrical box, in which two or more cooking vessels can be accommodated. This design was common in water heating; however, later it was

4 tried out for cooking as well, so as to cook food in more vessels. The advantages of concentrating cookers include high cooking temperatures, cooking any types of food and short heat-up times. The disadvantages are their size, cost, the risk of fires and burns, and the inconvenience to adjust the cooker, as it requires frequent directional adjustment to track the sun.

Solar Water Heater: Principle In a typical solar water heater, water is heated by the absorbed by the collectors. The hot water with lower density moves upwards and cold water with higher density moves down from the tank due to gravity head. A bank of collectors can be arranged in a series – parallel combination to get higher quantity of hot water. A typical 100 litres insulated tank with a 2 m2 collector area, will supply water at a temperature of 60- 80°C.

The term solar water heater includes conventional flat plate collector with either thermo-siphon or forced circulation flow system. A solar water heater normally consists of the following components:

• A flat plate collector to absorb solar radiation and convert it into thermal energy. • Storage tank to hold water for use and cold water feeding the flat plate collector. • Connecting pipes inlet and out let, for feeding cold water from the storage tank and taking hot water to the storage tank or point of use.

Solar Thermal Devices The solar water heater consists of a solar collector, insulated stainless steel tank and piping connections. The total system can be mounted on the roof top, on one side the piping connects it to the overhead tank and on the other side to the usage points. The smallest capacity available is 100 litres per day at 60o C temperature, suitable for a family of 4-6 members.

Based on the collector system, solar water heaters can be of two types-

1. Flat Plate Collectors (FPC) based Solar Water Heaters

The solar radiation is absorbed by Flat Plate Collectors which consist of an insulated outer metallic box covered on the top with glass sheet. Inside there are blackened metallic absorber (selectively coated) sheets with built in channels or riser tubes to carry water. The absorber absorbs the solar radiation and transfers the heat to the flowing water.

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Fig.3 Flat plate type solar water heater

Evacuated Tube Collectors (ETC) based Solar Water Heaters

Evacuated Tube Collector is made of double layer borosilicate glass tubes evacuated for providing insulation. The outer wall of the inner tube is coated with selective absorbing material. This helps absorption of solar radiation and transfers the heat to the water which flows through the inner tube.

Fig.4 Evacuated Tube Collectors (ETC) based Solar Water Heaters

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