Solar Cookers

Advantages of a solar cooker

• No requirement of cooking gas or kerosene, electricity, coal or wood.
• No need to spend on fuel, as solar energy is available free.
• Food cooked in solar cooker is nutritious. About 10-20% of protein retention is more as compared to that in conventional cooking .Vitamin thiamine retention is about 20 to 30% more whereas vitamin A is retained 5 to 10% more when food is cooked in solar cooker.
• Solar cooking is pollution free and safe.
• Solar cookers come in various sizes. Based on the number of family members, the size of the cooker can be chosen.
• All cooking activities (like boiling, roasting) can be done using a solar cooker.
• There are government schemes which offer subsidies to purchase solar cookers.

Disadvantages of a solar cooker

• Adequate sunshine is required for cooking.
• Takes longer time to cook food than the conventional cooking methods.

Types of solar cookers

1. For household cooking : Box type solar cooker
2. For community cooking : Concentrator type solar cookers also called as Scheffler's model

Box Type Solar Cooker

It is like a hot box, in which food can be cooked. It is the most commonly used model for household cooking.

Important parts of a box type solar cooker

• Outer Box: The outer box of a solar cooker is generally made of G.I. or aluminum sheet or fibre reinforced plastic.
• Inner Cooking Box (Tray) : This is made from aluminum sheet. The inner cooking box is slightly smaller than the outer box. It is coated with black paint so as to easily absorb solar radiation and transfer the heat to the cooking pots.
• Double Glass Lid: A double glass lid covers the inner box or tray. This cover is slightly larger than the inner box. The two glass sheets are fixed in an aluminum frame with a spacing of 2 centimeters between the two glasses. This space contains air which insulates and prevents heat escaping from inside. A rubber strip is affixed on the edges of the frame to prevent any heat leakage.
• Thermal Insulator: The space between the outer box and inner tray including bottom of the tray is packed with insulating material such as glass wool pads to reduce heat losses from the cooker. This insulating material should be free from volatile materials.
• Mirror: Mirror is used in a solar cooker to increase the radiation input on the absorbing space and is fixed on the inner side of the main cover of the box. Sunlight falling on the mirror gets reflected from it and enters into the tray through the double glass lid. This radiation is in addition to the radiation entering the box directly and helps to quicken the cooking process by raising the inside temperature of the cooker.
• Containers: The cooking containers (with cover) are generally made of aluminum or stainless steel. These vessels are also painted black on the outer surface so that they also absorb solar radiation directly.

Box type cookers with electrical back up are also available in the market.

Process of cooking food using a box type solar cooker

• Keep the solar cooker in the sun in open space free from any shadow. Keep the cooker in the sun at least for 45 minutes before loading it with cooking pots. This will keep the cooker ready for cooking and reduce the cooking time.
• Adjust the cooker in such a way that the reflecting mirror faces the sun and the reflected rays fall on the transparent glass lid. Tighten the position fixing hinges of the mirror in this position.
• Open the glass lid of the solar cooker, place the cooking pots inside it and close the lid properly. Once the cooking vessels have been placed inside the cooker the lid should not be opened.
• Open the lid fully once the food is cooked. While removing the cooking vessels after the food has been cooked, use cloth napkins as the vessels are very hot.

Cost of a box type solar cooker

Cost ranges between Rs 2500 to 4,000. It varies based on the size and model.

Concentrator type solar cookers for community cooking

It works on the principle of solar energy concentration using a Reflecting Parabolic Solar Concentrator. A parabolic solar concentrator is used for concentrating solar radiation on a focal area where the cooking vessel is placed.

Parts of the concentrator type solar cooker

• Solar Concentrating Disc (Primary Reflector) -  The disc which helps in concentrating solar energy to a focal point
• Automatic Tracking System  - With the help of a simple automatic mechanical tracking system the solar disc rotates in the direction of the movement of the Sun to give continuous and accurate solar energy concentration.
• Secondary Reflector  - This is provided opening in the north-facing wall of the kitchen or the cooking place just below the cooking vessel. This reflector receives the concentrated solar radiation and reflects it on to the bottom of the cooking vessel.
• Cooking vessels

Process of cooking food using a concentrator type solar cooker

1. Installation

• The Solar disc is installed in the open shadow-free area or on terrace tops facing the South.
• The cooking place and vessels faces the North (at the same ground level). The reflection of the disc falls on a secondary reflector housed in an opening in the North kitchen wall.

2. Working of a Community Solar Cooker

• Daily in the morning the disc is manually oriented so as to face the morning sun – in the east.
• The daily orientation action winds the clock mechanism to work throughout the day and the disc starts rotating in the direction of the Sun guided by the Sundial.
• The Cooker begins to work automatically as the concentrated solar energy is directed to the cooking vessel.
• The primary concentrated reflection falls on the secondary reflector, which is placed right below the cooking vessel. The secondary reflector diverts the solar energy on the cooking vessel and the heating begin.
• The seasonal adjustment of the disc is required to be done once in six months due to shift of the Sun’s position, with respect to the Earth axis.

Cost

It ranges from Rs 7000 - Rs 50, 0000 depending upon the size and the model.

Make your own solar cooker

Simple things which are readily available around the house can be used to create a solar cooker. In your ‘do it yourself’ solar cooker you can cook a delicious meal without lighting a match.

Material required

• Two reasonably large cardboard boxes - a size that can hold two large bowls with some room to spare. One box should fit into the other.
• Straw / Old newspapers – Collect and crush a few old newspapers, better still if you can get some straw. These will be used for insulation.
• A transparent glass cut to the dimensions of the top of the box – to be used as the lid of the inner box.
• Aluminium foil
• Black acrylic paint
• Packing tape
• Scissors
• Glass sheet
• 2 aluminium bowls with metal lids
• Half bowl rice; vegetable of choice and spices

Methodology

• The inner Box: Clean out the box, tape up all the sides except the top. Ensure that the box is sturdy. With a pair of sharp scissors cut off the top four top flaps of the inner box. Tape up the rough edges. Now, paint the inside of the box a very very dark black - preferably using acrylic paint. Leave the box to dry.
• The outer box: Clean out the box, tape up all the sides except the top. Ensure that the box is strong. With the scissors cut out large pieces of aluminium foil - the kind that is used to wrap food. Paste the pieces on the inner side of the four top flaps. Leave the box to dry.
• The insulation : Use materials that are poor conductors of heat - rolled up newspaper, straw, husk etc. Place a layer of this material at the bottom of the big box and then place the smaller box on top. Next, fill out the gaps between the four sides of the boxes tightly with insulation material. Once this is done the two boxes should become one unit.
• The glass top : Place the glass top on the inner box.
• The meal : Select two non reflective bowls with lids. In one bowl put half a cup of washed rice and pour two cups of water. In the other bowl mix your favourite vegetable with a spoonful of oil, salt to taste, a pinch of turmeric and green chillies. Place both bowls carefully in your solar cooker and set it out in the sun. Wait for two hours, and the delicious food is ready, cooked only with the pure goodness of nature.

Source : Akshay Urja, Volume 4, Issue 4