Here’s how to build one yourself.
(Photo 1) Solar furnace mounted on an external door
Warm air exits from the output port on the top
(Photo 2) The fan pushes cold air into the input port on the bottom.
(Figure 2) Schematic
The solar furnace can be assembled from materials you have laying around your garage, and can heat up a small room, but can be scaled up to any size, and can even add up to heating savings during winter.
However, the heater only works well during the day, because at night it will cause some losses due to the downward movement of cold air through it. To counter it, all you have to do is bring the input port to the level of the output port. You can use right-angle ducts for that matter. If you use a U-shaped duct, cold air will become trapped at the bottom of the box and won’t enter your home at night.
The solar furnace’s fan has a nominal operating voltage of 12 volts DC at a current of 200ma, while the heat output is the equivalent of a few hundred watts when the box is in full sun.
For example, on a sunny day with a 10 degree C outdoor temperature, the furnace was able to increase the interior garage air from 12 degrees C to 29 degrees C. The fan’s air flow rating is 30 cubic feet per minute at full speed.
How the Solar Furnace Works
The initial creator of the solar furnace designed it as a self-sufficient device. The heat collector is a black metal plate that is placed in full sun. A photovoltaic solar cell powers the fan, pushing cold air from outside into the bottom of the box. Then, as it is pushed upwards by the fan, the air starts to warm from the contact with the back of the black plate, and exits at the top of the box, in your room. Simple, isn’t it?
The electronic part isn’t difficult to understand, either. It only consists of a solar cell powering a fan – even a kid can do the setup. Still, there’s a capacitor that smooths out the electrical supply to the fan (you can put a battery as well), so the motor rotation speed doesn’t vary with sunlight intensity and protects the fan’s bearings.
The box has to be insulated at its back, to prevent heat loss to the cold side. And because it’s solar powered, when there’s enough sun outside to power the fan, there’s enough power to heat your room – it’s a self-regulating device, the simplest possible.
Of course, one could put up a digital thermometer and an Arduino to measure the actual usefulness of work being done in the solar furnace, so it doesn’t continue to blow when inside is warmer than outside.
How to Build the Solar Furnace
A longer and detailed explanation you can find here. However, here’s the basic schematic of how it should look on the inside and a picture of a finished outside part.
In the end, wire the solar panels in parallel with each other, the fan, and the capacitor. All positive leads connect together, all negative leads connect together. Fasten the wire junctions with the two wire nuts.
Point the solar furnace to the South if you live in a Northern climate. If you can, mount the box at an angle so that it faces directly toward the sun at noon during the winter months. In some cases, mounting the box vertically is the only option, useful heat will be produced as long as the box is in full sun. In my installation, the box is mounted vertically and it is pointed toward the South West. The furnace starts up later in the day than it would if it were pointed due South, but it warms up quickly and provides more heat later in the day.
If the sun shines, the box will run all day, producing warm air out of the top duct. The solar powered heater box can pay for itself over time by lowering your electricity and gas bills. When it’s hot outside, the glass front of the solar furnace should be covered with a reflective surface such as a piece of plywood or metal and the cover painted white to reflect sunlight. The the fan can be disconnected to prolong its life.
BOM (Bill of Materials)
2x 12V/150ma amorphous or polysilicon solar panels1x 12VDC 4" computer fan, approximately 200ma operating current1x fan finger guard (optional)1x 1000uF 25V electrolytic capacitor2x wire nuts 10 feet of #20 stranded hookup wire 1x clear glass pane, 3/8" tempered glass is recommended 2x6 lumber for the box frame 2x2 lumber for the internal spacer frame plywood or press-board for the back Corrugated steel roofing material 2 pieces of 1/8" x 1/2" soft steel bar flat black paint for the metal collector panel, high temperature paint is best deck type water seal for the wood pieces 1 tube of silicone caulk fiberglass or high temperature solid insulation 2x 8" lengths of 4" diameter metal ducting miscellaneous screws
The mechanical parts in the solar furnace can be purchased from a local hardware/building supply store. Companies selling PV panels can be found on eBay.