Solar cooking.pdf

THE TRACKING SOLAR COOKER

C. Alan Nichols P.E., C.E.M.

Tucson, Arizona

INTRODUCTION

The concept of solar cooking began over 220 years ago and was used

by the French Foreign Legion starting in the 1870's. All solar cookers work on

the principle of concentrating the direct solar rays to raise food or water to

cooking temperatures. Cooking temperatures begin at about 150 degrees F.

although temperatures of 250 to 400 degrees F. are preferred.

Open reflector type solar cookers focus the sun's rays on open cooking

pots or pans. Solar ovens trap the sun's heat inside insulated boxes with transparent lids. Most solar

ovens are variations of the bread box type developed by two Arizona women, Barbara Kerr and Sherry

Cole.

These simple box cookers created in the 1970's are now being used world

wide to overcome food shortages in developing countries. In places like Eastern

Africa native women travel 20 to 30 miles to gather a two day supply of cooking

fuel. In other parts of the world increasingly high rates of malnutrition are caused

by a lack of fuel. Basic grain foods cannot be cooked without cooking fuel and

water infected with chronic bacteria must be heated before drinking. Continued gathering of wood

cooking fuel by chopping down trees has resulted in eroded hill sides with loss of precious topsoil thus

reducing their ability to grow food.

In America we concern ourselves with such things as

operating costs, air pollution, acid rain and the green house

effect. Let's say it takes one hour to cook a pot of beans on an

electric stove using one kilowatt. The coal fired power plant

that supplied the electricity consumed one pound of coal and

released 17.5 cubic feet or two pounds of CO2. The power plant also consumed 0.7 gallons of ground

water and released traces of SO2 as acid.

Suppose you are cooking outside. Five pounds of steaks on a grill will use a ten pound bag of

charcoal and five ounces of lighter fluid. This fire will produce approximately 160 cubic feet or fifteen

pounds of CO2 and untold air pollution. Wood fires are even worse.

Cooking in the home averages over 100 hours a year consuming approximately 1,175 kilowatt

hours. At a cost of $0.10 a kilowatt hour this amounts to $117 a year. During the summer cooking adds

$50 to the air conditioning bill bringing the total to $167 a year. In Arizona, solar cooking can replace

70% of the cost of cooking. This will save 1,675 pounds of coal and 3,000 pounds of CO2 generation

from coal fired electric utilities.

Clearly solar cooking has come of age.

HOW SOLAR OVENS WORK

A solar oven's cooking temperature is

reached when the solar gain equals the heat

losses. Thus an oven's cooking temperature is a

balance between solar gain and heat losses.

Heat losses fall into five categories:

1. Reflective losses

2. Absorption Losses

3. Transmitted Losses

4. Leakage Losses

5. Food losses (Heat Lost To Cooking)

Solar gain is a factor of the total area

exposed to the sun and the effectiveness of

collection.

For the reflectors this property is

acceptable but for the cover glass, light must enter

the cooking chamber, or the mirrors are of no use. As a stone skipping across water, sunlight will skip

off the surface of glass without penetrating if the angle is too shallow.

An angle of about 30 degrees from the

vertical for the mirrors or reflectors yield good

penetration with a healthy spread for an optimum

use of materials. As it turns out a reflector equal

to the width of the cooking chamber produces an

optimum design for use of materials.

With such a fortunate set of circumstance

the reflectors can be folded to cover the cooking

chamber making the cooker portable!

Mirrors have a reflection of about 99% but lose

5% each time the light passes through the glass.

Polished metal surfaces have about 90% reflection but

no losses due to glass. The result is that each behaves

about the same.

Light striking the far edge of the reflector

should enter the cooking chamber at the far edge

opposite the reflector to be beneficial. An angle of 30

degrees from the vertical (or 60 degrees from the horizontal) with each reflector equal to the width of the

cooking chamber will allow all the light striking the reflectors to enter the cooking chamber. With this

geometry you can still fold the mirrors!

An optimum sized cooker will cook just about anything you put in it. Extending the reflectors to

make the collector larger will produce more heat and will cook food faster. If you extend the

reflectors the cooker becomes less portable. Doubling the length of the reflectors increases its width by

only 20% because the angle must be steeper. The shallow sun angle on the reflectors makes them more

reflective. The penetration into the cover glass improves somewhat because of the more vertical angle.

Of the five factors affecting a cooker's

cooking temperature three of them were thermal

loss. Transmitted losses are controlled with

insulation. Double glass covers have twice as

much insulation value as single glass covers.

Leakage losses are controlled with good tight

construction. The chamber should be well fitted

and protected from moisture on the inside.

Solar heat is transferred into the food by

three mechanisms. First by direct solar rays, that

is sunlight striking the food directly. This is

somewhat like a broiler. Second by convection,

that is by the hot air surrounding the food inside

the chamber. Third by conduction of heat from

the tray upon which the food rests. If the tray is a

heavy metal conductor such as steel or aluminum the sun's rays will heat the tray and conduct the heat

under the food like a stove. All three mechanisms combine to make the food cooking process very

efficient.

Finally, as the sun's energy is transferred into the food the thermal loss drops the oven's

temperature 25 to 50 degrees initially. This loss is only temporary.

As the food approaches the oven's temperature the aroma begins to fill the air, that's the whole

idea behind solar cooking.

INTRODUCTION TO THE TRACKING SOLAR OVEN

This modified box cooker has been labeled

high tech, because of its ability to follow the sun

under its own power. The design described here

is not that of a portable oven. Materials used are

durable and meant for many years of outdoors

use.

The main features are:

- Pedestal mounted where the oven is at

counter height.

- Fixed tilt such that the collector does not

have to be adjusted for the time of year.

- Uses both mirrors and flat plate collector

to concentrate the sun's heat into the

cooking chamber.

- Natural air movement increases

convection cooking and warms the

cooking tray.

- Rotation about a fixed vertical axis

allows tracking of the sun during the day

by manual or automatic means.

- Capable of heavy food load.

Energy saving is best achieved when the solar device is used to its fullest extent, thus the design

of this oven is targeted for widespread use by those who demand convenience. This design requires no

attention during the day when cooking a large meal.

These instructions will provide a basic summery of the construction techniques for building a tracking

cooker of your own. There are many construction techniques that will produce the same results.

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