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Self Powered Solar Box Furnace

Self Powered Solar Box Furnace

This project involves the construction of a self contained solar box furnace.
With the constantly rising price of natural gas and heating oil, this project is becoming more and more attractive. I assembled the prototype with materials that I had laying around my workshop. It is suitable for heating up a small room or a detached shed. I use the furnace to warm up my garage, it takes the chill out of a fairly large space on cold sunny days. It is possible to scale this project up to any size. A larger version of this furnace could be used to add a lot of supplemental heat a house.

At night, the solar furnace will cause some heat loss due to downward convection of cold air through the box. This convection can be greatly reduced by bringing the input port up to the level of the output port with some right-angle ducting. A U shaped air passage will cause cold air to become trapped in the the bottom of the box at night.

This is a self-contained and self-powered device. Sun shining on black metal collector plate is absorbed and converted to heat. Sun shining on the solar panel array powers the fan, which pushes cold air into the bottom of the box (Photo 2). The cold air warms up as it passes over the collector plate, then the warmed air exits from the top of the box (Photo 3).

The electronics in this project are quite simple (Figure 2). The solar panel array provides enough current to run a 12V computer fan. The capacitor smooths out the electrical supply to the fan, so that interruptions in the light source don’t produce sudden motor speed changes. This should increase the life of the fan bearings.

The insulation at the back of the box prevents heat loss through the cold side. When there is enough sun to heat the box, there is also enough sun to run the fan. No electrical controls are necessary.

A differential temperature control could be added to the fan circuit to insure that the fan never runs when the furnace is not producing useful heat. One big advantage of such as system is that the control can be set to turn on when the box air temperature is a specified number of degrees warmer than the room air. This would allow the furnace to output air that is noticeably warmer than the inside air, and the fan would only intermittantly.

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