When we collect solar radiation, current methods tend to use direct sunlight. However, it is important to know how much energy we can obtain even when it is cloudy. To test this, an experiment was done with a black hemisphere filled with water to find out how much the temperature was raised. The hemisphere was covered with a transparent plastic sheet with a 2cm air gap. For the calculation, we used the measured sky radiation on and the cloud surface radiation was calculated backward. The hemisphere was divided into rings. The solid angle configuration factor of a ring from the cloud surface was calculated and the sky radiation to the hemisphere was obtained by summing up the radiation of the rings. The reflection coefficient of the concrete ground was assumed at 0.1. The water temperature when the hemisphere received direct solar radiation only, has been calculated as well. These calculated results are compared with measured results in the following figure. This type of solar energy collection gets more energy from the sky radiation. The water temperature change on a winter day is shown in the successive figure.
Solar radiation collection with a black hemisphere.
Water temperature change in the hemisphere on a cloudy winter day.
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