HEAT GAIN & HEAT LOSS
DIRECT GAIN
Sometimes it's desirable to store heat inside a
greenhouse even though the heat losses at night may be great. Temperature
swings are moderated with thermal mass.
This is a simple solar greenhouse made with a 2x3 frame held together with
plywood. It would also be a good idea to glue solid insulation on the
inside and paint everything black before installing the glazing. You
should be able to remove a side panel to have access to the inside.
Place the greenhouse in bright sunlight on a clear, cold day. Take temperature readings over a period of several hours of the inside of the greenhouse. Also record the outside temperature. Toward sunset take readings every minuet.
Place a row of blackened, glass bottles, filled with water in the back of the solar greenhouse and repeat this experiment on a similar bright sunny cold, clear day. The more temperature readings you take the better. You may repeat this experiment as many times as you like with different glazings and different kinds of thermal mass. You may decide to use bricks instead of bottles of water. Be sure to keep an accurate log of all your temperature readings. Also record weather conditions and ambient temperatures.
Actually the best way to run this kind of experiment would be to have two solar greenhouses side by side. One greenhouse would have nothing in it and the other would have thermal mass, like bottles of water or perhaps some bricks. It would also be nice to have a data logger capable of sampling the inside temperature of both greenhouses and the ambient (outside temperature) simultaneously. We don't always have the best tools to work with, but if we're careful and persistent we should notice a pattern of temperature moderation in the greenhouse with thermal mass.
The previous experiment has so many variables that it might have been difficult to get accurate results. This experiment will be done under a more controlled environment so it should be easier to collect accurate data. We will be comparing the temperature drops of two different cups of boiling water. One cup will be ceramic and the other will be made of Styrofoam.
You may use an oven thermometer for this
experiment because an ordinary household thermometer is only good up to 130 F.
Take readings every minute for 30 minutes and plot the results. If we placed the
Styrofoam cup of water inside another Styrofoam cup how long would the
water stay above 150 F? Where is most of the heat loss from the Styrofoam cup of
water? How important is insulation in preventing heat loss?
Why are direct gain systems not very practical in cold climates?
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