SOLAR
GREENHOUSESolar Greenhouse & Trombe Wall
SOLAR
GREENHOUSE
Visible light from the sun turns into heat when it strikes the dark interior
wall of the greenhouse. This heated air tends to rise and concentrate near the
top of the greenhouse. It then pushes open the top plastic flap and enters the
living quarters. The raised pressure of air in the house causes cold air near
the floor to push the bottom flap open into the greenhouse.
The solar greenhouse will continue to bring heat into the house as long as there's sun. without sunlight to power this solar chimney convection currents inside the greenhouse reverse. Air cooled by the glazing drops and the hot air from the living space will rise and try to enter the opening at the top of the greenhouse. Without these one way flaps all the heat gained during the day would soon be lost by this cooling process, but with these flaps daytime heat is trapped inside the house. The flaps turn this ordinary greenhouse into a solar greenhouse.
OK! Now you understand how a solar greenhouse works. Do you think you can build one. The larger you make it the better it works. My solar greenhouse is 10 feet tall and reaches a temperature of 180 F. in mid winter with an outside temperature of 0 F. You might not be able to build one this big for your science fair project, but you should be able to construct one about 2 feet high. It would be nice if you could frame it with wood and install real glass for glazing. If wood and glass are out of the question you could always make it out of cardboard and plastic. Better yet you could make a set of posters to explain how the solar greenhouse works. One poster should demonstrate what happens in bright sunlight and the other should demonstrate what happens when the sun goes down.
An
alternative to the solar greenhouse is the Trombe Wall that gathers almost as
much heat and takes up less space. As winter approaches in the northern
hemisphere sunlight striking a vertical surface is more direct. This is why a
Trombe Wall actually gets hotter in the winter than in the summer. Flaps not
shown in this picture are still needed to prevent heat loss at night.
A Trombe Wall is
fairly easy to build with common construction materials such as plywood and
1x4's. Just frame out the box as shown in the picture. This Trombe Wall is made
with a 4x8 sheet of quarter inch plywood and two 1x4x8's. Notice the top and
bottom vents that allow cold room air to rise and heat up inside. The back of
the Trombe Wall is of course painted black to change visible sunlight into heat.
Don't forget the glazing. The best glazing for a project like this is Kalwall
available from:
www.solar-components.com . A
4x8 sheet will run you about $100. Clear plastic works, but not as well.
To transform a passive solar greenhouse or passive Trombe
Wall into an active system simply replace the top or bottom flaps with fans. The
increase in heat transfer efficiency more than compensates for the power used to
run the fans.
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