SUSPENDED BOX SOLAR OVEN to Boost Rural Entrepreneurship

Conventional box type solar cookers cannot follow the sun properly except during solar noon. Moreover, use of single reflector is not so suitable to boost up the energy collection. Box type cookers with multiple reflectors generally known as solar ovens are equipped with adjustable leg at back for changing the inclination with respect to the ground. But these ovens also cannot follow the sun in morning and afternoon as adjustment of these ovens with the ground is limited. It is due to the fact that after certain degrees of inclination the oven will be turned over in front for the weight of reflectors and also for the rotation of line of action of oven box weight towards front. Another problem with existing solar ovens with the typical adjustable back stand is that, when oven inclination is to be changed then at first oven is to be lifted against its weight before readjustment in its stand. By the use of fiberglass body and aluminium reflectors this handling problem is however partly solved but material cost is thus increased.

The solar oven model (Picture 1) discussed below (follows the sun at right angle almost throughout the day which is especially beneficial in summer for solar cooking as during summer, intensity of solar radiation is still sufficient to cook utilizing solar energy when sun is low in the sky. The oven is provided with one large cooking pot.

Constructional Details

The presently proposed suspended box type solar over consists of two boxes, a black painted aluminium inner box kept inside an outer box. The outer box is made of GI sheet. The space between the two boxes is filled up with loose packed glass wool insulation. Length, width, and depth of the oven box are 44 cm, 44 cm, and 36 cm, respectively. The upside of the oven box has double transparent glass covers keeping a gap of 20 mm in between. Neoprene rubber is used as cover gasket to seal the closed cover and inner box. The supporting frame of the glass cover is hinged with oven box and cooking pot can be withdrawn or handled by opening the glass cover.

The oven box is suspended from inverted ‘T’ shape main frame and can be easily adjusted from 5° to even up to 80° with respect to the ground by simply swinging the oven box and, thereafter, positioning of rectangular support frame in desired slot. Slots are formed by series of long studs provided in the mainframe base. Location of pivot points from where the oven is suspended is just few centimetres above the centre of gravity point of the oven box so that changing of inclination of oven box required a light force. (approximate CG point is detected by trial keeping oven box sidewall parallel to the ground with open reflectors). The sketch of proposed oven is shown in Figure 1.

In this oven, only one cooking pot is used mainly for cooking rice. The tilting cooking tray is square and ends of the tray are folded upwards. The cooking pot is kept on SUSPENDED BOX SOLAR OVEN to Boost Rural Entrepreneurship Picture 1: Suspended box solar oven Suspended Box Solar Oven this cooking tray. In this new oven, cooking tray along with cooking pot is suspended through MS strips from pivots fixed in the sidewalls of the inner box and these pivots are aligned with the horizontal axis of the cooking pot. For the stability of the pot a counter-weight of about one and a half kg is attached at the bottom of cooking tray. During changing of oven inclination, the tray along with pot rotates at its horizontal axis and maintains constant horizontal position to avoid spilling over of food items. Cooking pot and interior of the oven are painted black by automobile muffler paint to increase absorption of solar radiation.

The oven has four foldable mirror reflectors. Size of the reflectors is equal to the glass cover and these are hinged with oven box at four sides of the glass cover. The reflectors when open are held fixed at an angle of 115° with the oven face by the help of reflector supports and holding clips provided at the backside of the reflectors. Reflectors can be folded for keeping on the top of the oven box cover when not in use.

During reorientation of the oven, apart from changing of inclination of the oven box, the whole oven can be rotated at the ground at desired position with the help of caster wheels attached at the bottom of the supporting frame, but the position of reflectors remain unchanged throughout the working period. To get the best performance, reorientation in every 30 minutes interval is suggested.

Merits

• The new oven can follow the sun at right angle almost throughout the day
• Easy reorientation system to follow the sun
• Indigenous design of pot holding arrangement to keep the pot always in central position inside cooker box (i.e., not typical suspended pot holding system) for better utilization of all the reflectors at all inclinations of the oven.

Test Results

At first after construction of the solar oven, some routine tests, such as inner box leakage test, leakage test of upper and lower sides of cover plate, cover gasket leakage test, etc., are carried out in line with IS code. Other performance tests were also carried out. Reorientation is done in every 30-minute interval.

Test date, time, and location: April 6, 2015 to April 15, 2015, from 9:30 a.m. to 12:30 p.m., Jalpaiguri (26.32° N latitude, 88.46° E longitude), West Bengal

• Average ambient temperature: 27°C
• Average intensity of solar radiation during test period: 0.53 kW per m2
• Peak temperature of empty cooking pot: 170°C

Test date, time, and location: April 16, 2015 to April 25, 2015, from 10:30 a.m. to 12:50 p.m., Jalpaiguri, WB

• Average ambient temperature: 28°C
• Initial temperature of water: 26°C
• Average intensity of solar radiation during test period: 0.55 kW per m2
• Time taken for boiling of 2 L of water: 110 minutes.

Cooking Performance

Average time taken for cooking 1.5 kg (after preparation) of rice is 100 minutes

Project Economics

• Minimum economic unit is 30 ovens per month
• Total Investment is Rs 250,000 working capital for two months
• At an estimated cost of Rs 3,000 per cooker one can earn a profit of Rs 10,000 per month.

Mr Sankha Subhra Datta is a Senior Section Engineer (Mechanical) in the Diesel Locomotive Shed, NF Railway, Siliguri Junction, West Bengal, India. E-mail: subhradatta611@gmail.com