(ii)Cooking equipment to use solar energy directly

Cooking equipment that can convert solar energy directly into heat are explained in this section. The experimental house uses a solar oven and a solar cooker for cooking1) as well. Although their use is strongly affected by prevailing weather conditions, they are great devices. When it is fine, we are also able to produce enough heat to cook preserved food and melt plastic to reform.

1)B & D Halacy;"Cooking with the sun", Morning Sun Press, 1978.

(a)Solar oven

The mouth of the reflector aimed at the sun collects solar radiation that goes to the oven in the bottom. The inside the oven is painted black and the walls are highly thermally insulated. The inside of the reflector is coated with very highly reflecting sheets on cardboard. At Kaiwaka, wool fleece is used as a thermal insulator for the walls. The calculated results(dotted line) can be compared with the measured ones (continuous line). The calculation used a reflection coefficient of the reflecting surface at 0.7, the transmission coefficient of a glass plate at 0.8, the radiation absorption of a black surface at 0.8, the overall coefficient of heat transfer at 0.64kcal/‡uhr�Ž, and heat capacity at 0.352kcal/‡u�Ž. Direct solar radiation on the the day of experiment is also shown, there it changed from 75 to 380kcal/‡uhr)
The measured result is lower than the calculated one in the later stage because of air leaks.
Temperature calculations in the oven were made by using measured direct solar radiation from two different days as

    (a)Measured direct solar radiation on a partly cloudy day with a change 270�`440kcal/‡uhr

    (b)Measured direct solar radiation on a fine day with a change 590�`640kcal/‡uhr

When the area of the reflector's mouth was in the same position as in the previously used reflector, the overall thermal coefficient K, the heat capacity CAP and the oven volume V, changed as the above figures show. The calculated results are compared in the three figures above.

A device to reduce heat loss is shown in the figure. When the cooking is finished, taking the dish out allows the hot air to dissipate. If the dish is moved into an adjacent compartment as shown in the following figure, the acquired energy can be reused.

(b)Solar cooker

The reflection of solar radiation on the parabolic surface is focused to the bottom of a pot as shown in the figure. The reflector's mouth has a dimension of 100.6cm x 100.6cm. The distance to the forcus is 50cm. The pot is made of aluminium and has a depth of 6.5cm, a diameter of 14cm and a thickness of 0.5mm. The outside is painted black. There is a heat-resistant glass case of 6mm thickness surrounding the pot with a 4cm air gap. The specially designed pot is placed between two parallel iron angles supported by 12mm thick wooden plates, the total weight is about 14kg.

We made an experiment with a similar cooker in our Lab in Japan. A temperature change of the plant oil (300cc) in the pot was measured and a thermal calculation of the oil was done. They are compared in the next figure.

Oil temperature was raised 260�Ž on a fine day. Frozen croquettes were well cooked. Outside air temperature, direct solar radiation, and temperature of the plant oil at that time are shown in the figures on the left.

It can be seen that it is not difficult to produce high enough temperatures for frying.

Cooking of croquettes

When material with a large heat capacity, like a steak, is cooked, a pot with its own heat capacity should be used. The thin pot was replaced with one made of 3mm thick stainless steel. It was possible to produce enough heat to well-cook the steak.