(ii)Solar radiation collection with porous materials

Interesting results were found in an experimen measuring the temperature in cow dung. The results showed a high temperature retention as can be seen in following figures.

Temperature change in cow dung

Later on we did some similar experiments on the top of a building with a variety of porous material which were placed in double cardboard boxes, and covered with a transparent plastic sheet. All the boxes were arranged in a line with the same exposure to the sun. One of the purposes was to see if the high temperature in the cow dung was because of fermentation or solar radiation absorption. The material used were wool fleece, stainless wool, horse dung, cow dung, cut rice straw, fiber glass and so on. Wool fleece, stainless wool and a few others were painted black and put into different boxes. The temperatures near the surface(4cm deep) and in the middle(12cm deep) of each box were measured. A few results measured on a fine day and a cloudy day, as well as the respective solar radiation, can be seen in the following figures.

Each material shows its own interesting behavior and following items will be further investigated,
i)The fermentation heat of cow and horse dung produces high temperatures, However, this phenomenon lasts for only one week's time.
ii)The temperature near the surface of unpainted stainless wool was higher than that of the black painted one.
iii)Wool fleece has high solar radiation absorption and the temperature change from the surface to the bottom was gradual.

We measured the temperature distribution in stainless wool and wool fleece in more detail. The measured temperature distribution in the stainless wool is given in the next figure as well as the solar radiation. Using these results, we tried to make the water warm in a black pipe placed between a stainless wool layer and a wool fleece layer. The measured result is shown in the next two figures.

Measured temperature distribution in stainless wool.


Temperature change of water in a pipe placed between stainless wool and wool fleece.