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Ashok Mathur2

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February 9, 2010
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What's Wrong?Edit

What is wrong with the existing Solar cookers?
Plenty in one short word.
(For a more formal analysis of what is wrong with solar cookers see subsequent pages.)
They are made for the wrong audience.

Wrong for poor families:Edit

For a poor family, they cannot cook food for the members who leave the home in the morning, say by 8 AM or 9 AM.
In poor families where all the members go out for work, there is not much that a solar cooker can do.
They can cook lunch for about 180 to 200 days in a year for stay behind members.
If the Solar cookers are combined with hay-cookers, they can serve at least one hot dish for dinner.
This requires that a cooking member should be home after 3 PM.

They are too expensive for poor:Edit

A poor family that might benefit from not paying for fuel uses stoves of three bricks/five bricks and a pot on the top.
Often the pot is a tin that held 15 Kgs of oil and is sold in the second hand market at a throw away price.
It uses wood (sticks, broken wooden crates, branches etc) as its fuel.
The cooking arrangement of bricks and pot costs no more than Rs 25 (about US 50 cents)and wood is almost free.
It works on all days and gives a hot meal by 8AM.
At this price point it is not possible to design a solar cooker that can compete with the alternative.
At least I am not making any effort to compete in this market.

Uneconomical for Middle Class families:Edit

A middle class family where mother stays at home (and does not earn an income) uses a gas stove(LPG stove)that cooks all the food whenever its required.
Its capital cost is between Rs 600 to Rs 900 (US $12 to $16)
The monthly fuel bill is between Rs 150 to Rs 200 a month. (US $3 to $4)
A solar cooker cooks about one out of three meals on 200 out of 360 days.
It needs to be designed to cost between Rs 250 to Rs 3000. (US $5 to $60)
The cost of Rs 250 represents an ideal value for money as it is just below the proportionate capital cost of the device that is already with the house wife.
Paying more than Rs 3,000 for a device means that the house hold is purchasing something costing 5 times the existing device.
At Rs 3,000 capital cost the monthly interest locked in the cooker comes to Rs 30 per month.
The cost of fuel saved balances the interest locked up in the device.
For this market, we will try to make a solar cooker trying to keep the price as close to Rs 250 as possible.
NB The present parabolic solar cookers in the Indian market cost anywhere from Rs 8,500 to Rs 10,000.
Their high costs spring from
1. the costly plastic (Mylar equivalent) reflecting surfaces that they use and
2. the precision in manufacturing that is required.

Wrong TechnologyEdit

They use the wrong technology.
There are two underlying technologies of solar cooking.
One is of trapping the heat from the sun in a box by using the using a sheet of glass or plastic at the entrance. Solar box cookers use this technology.
The second technology is that of focusing the sunlight to achieve a very high temperature at the point focus. Parabolic solar cookers use this technology.
Solar Box cookers give satisfactory results with unattended cooking. ( At the very outset, I would like to clarify the meaning of the term unattended cooking. The term refers to the attention or attendance that is required to be given to the solar cooker to cook a satisfactory meal. It does not refer to the attendance or attention that the meal itself may require during cooking.) Yet they cannot be made very large as their reflectors then tend to tilt the whole cooker in wind. Also often the trapping glass is the most costly component of the cooker and increase in its size increases costs and the weight of the cooker. This renders it difficult to be carried in and out of the house. In some sense, Solar Box Cookers are an ideal means to enjoy a sun -cooked meal in a winter after noon and wonder why the rest of world does not adopt it?
Parabolic cookers are excellent at collecting sun light from large parabolic surfaces.
Unfortunately they focus the collected light at a single focus heating it very high. Here the Stefan-Boltzmann Law that states the heat loss is proportional to the fourth power of the <a external text href="http://www.engineeringtoolbox.com/temperature-d_291.html" rel="nofollow" title="http://www.engineeringtoolbox.com/temperature-d_291.html">absolute temperature </a>steps in and ensures that a very large chunk of the focused heat is lost as radiated heat.
Most design discussions of Solar cooker never measure/estimate these losses.
I suspect that more than half of focused heat never goes into cooking the food.
The sharp single point focus very quickly drifts away from the cooking vessel.
As a rough estimate the parabolic solar cooker needs adjustment every 15 minutes or its cost must be burdened with the cost of the two axis tracking system.

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