Solar Cookers

Importance:

• Cooking consumes a major portion of energy—about 64% in rural areas of India.
• Traditional energy sources for cooking include firewood, crop residues, and animal dung cakes.
• Collecting firewood is labor-intensive, mostly done by women who walk long distances carrying heavy loads.
• Forest depletion is increasing due to firewood demand.

Health & Environmental Concerns:

• Burning firewood and dung causes smoke-related health hazards, especially for women.
• Solar cookers reduce the use of firewood and fossil fuels, leading to cleaner cooking environments.
• They help save LPG, kerosene, and electricity in urban areas.

Limitations:

• Solar cookers cannot fulfill all cooking needs for all sections of society.
• Acceptance depends on adapting to traditional cooking patterns (e.g., large meals cooked outside sunshine hours).
• There is scope for improving thermal efficiency and developing hybrid solar cookers that can be used during cloudy or off-sunshine hours.

Environmental and Social Benefits

• Annually in India:
  • 150 million tons of firewood,
  • 52 million tons of dried dung, and
  • 36 million tons of agricultural waste are used for cooking.
• These non-commercial fuels contribute about 45% of the total energy supply.
• Solar cookers can reduce deforestation and reliance on dung cakes for fuel.

Advantages of Solar Cookers

• Can cook multiple items simultaneously (e.g., 4-pot cookers).
• Preserves nutritional value of food.
• Requires no constant attention during cooking.
• Eliminates risk of fire accidents and overcooking.
• Pollution-free, keeps kitchen and utensils clean.
• Useful for baking and roasting (except chapattis).
• Saves firewood and commercial fuels.
• Saves money and time.
• No recurring fuel or maintenance cost.
• Easy to manufacture, use, and maintain.
• Use can extend the life of gas cylinders.

Disadvantages of Solar Cookers

• Takes longer time to cook compared to conventional methods.
• Cannot substitute for all types of cooking (e.g., frying, chapattis).
• Requires additional mechanisms for use on cloudy days or at night.

Types of Solar Cookers

• Box Type Solar Cooker (Hot Box Type)
  • Uses solar heat without concentrating sunlight.
  • Suitable for cooking rice, dal, vegetables, meat, snacks, desserts, etc.
  • Cannot fry or bake chapattis (needs higher temperature).
  • Typical size: around 60 cm × 60 cm with 4 pots.
  • Cost ranges from Rs. 1800 to Rs. 3000.
  • Components include outer/inner boxes, thermal insulation, double glass cover, mirror, cooking containers with absorber paint.

• Concentrating Type Solar Cooker
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  • Concentrates sunlight from a large surface to a smaller area to achieve higher temperatures.
  • Useful for applications needing high heat.

• Hybrid Solar Cookers
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  • Combine solar cooking with electrical or thermal storage backup for use during off-sunshine hours or cloudy days.
  • Can be called solar ovens when electrical backup is included.

• New Designs for Flats and Urban Use
  • Cookers that bring sunlight into the kitchen through windows or south-facing walls.
  • Stationary systems integrated into buildings.
  • Allows cooking inside without going outside in the sun.
  • Suitable for pressure cookers and faster cooking.

• Window Type Solar Cookers
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  • Designed for kitchens with south-facing windows.
  • Often include electrical backup.

• Solar Steam Cooking Systems
  • Use solar concentrators to generate high-pressure steam for large-scale cooking.
  • Example: Tirumala Tirupathi Devasthanam (TTD) solar steam system cooks for 15,000 people/day.
  • Uses automatic tracking parabolic dish concentrators.
  • System is supported by diesel boiler backup for reliability.

Dish Type Solar Cookers

• Usage: Outdoor cooking for 10-15 people; suitable for homes and small establishments.
• Design: Uses a parabolic dish (also called SK-14 type) to concentrate solar radiation.
• Performance:
  • Power output ≈ 0.6 kW on sunny days.
  • Can boil 2-3 liters of water in about 30 minutes.
  • Vessel bottom temperature: 350-500ºC.
• Cost: Rs. 5500 to Rs. 7000 depending on reflector type.
• Savings: Can save up to 10 LPG cylinders per year with full use.
• Incentives: MNRE subsidy up to 30% of cost (max Rs. 1500).

Concentrating Solar Cooker (Solar Oven)

• Used for quick cooking by concentrating solar radiation.
• Example calculation shows the required area of solar oven for 200W heat with 20% efficiency and solar insolation of 800 W/m² is 1.25 m².
• Doubling efficiency to 40% halves the required area to 0.625 m².

Community Solar Cookers (Scheffler Cooker)

• Larger parabolic reflector (7 to 12 m² aperture area).
• Can cook for 40-50 people, cooking time per dish 60-90 minutes.
• Has a mechanical clockwork system to track the sun automatically.
• Suitable for schools, hospitals, canteens, etc.
• Cost ~ Rs. 50,000; payback period of 4-5 years.

Material Problems

• Problems relate to outer/inner boxes, insulation, rubber strips, black paint.
• Need better transparent covers with high transmissivity and low heat loss.
• Using better insulation like polyurethane can improve performance.

Efficiency of Solar Cookers

• Ordinary black paint: 20-25% efficiency.
• Selective coating: ~30%.
• Nickel-black coating: 50-60%.
• Anti-reflective coatings can improve efficiency by 8-10%.

Heat Transfer in Solar Cookers

• Cooking involves conduction, convection, and sometimes radiation.
• Water is commonly used as a heat transfer medium (boiling point limit 100°C).
• Higher cooking temperature = faster cooking.
• Preheating cooker for 45 min in sunlight reduces cooking time.
• Pressure cookers with blackened aluminum bottom work better in solar cookers.

Power Ratings of Solar Cookers

• Solar cookers have the lowest power rating but can improve with better design.

Usage Tips

• Add exact water quantity; do not exceed half pot level.
• Cut vegetables into small pieces.
• Soak raw ingredients before cooking.
• Cover containers to retain nutrients and heat (except baking).
• Use gloves or cloth to handle hot containers.
• Clean cooker and pots regularly; maintain black surfaces.

Summary

• Solar cookers reduce firewood use and environmental degradation.
• Useful in rural and urban sectors, including institutions.
• Advantages: multiple dishes at once, nutrient preservation, no fire hazard, pollution-free.
• Disadvantages: longer cooking time, limited use on cloudy days.
• Types: box type (little/no concentration) and concentrating type.
• Hybrid designs (electricity + thermal storage) can enable night cooking.
• New designs for kitchens with south-facing windows may enhance usability.
• Solar steam cooking systems for community kitchens use solar concentration.