Solar hot air collectors (AKA solar heating systems for homes that use passive solar heating) can directly heat individual rooms or can potentially pre-heat the air passing into a heat recovery ventilator or through the air coil of an air-source heat pump. Air collectors produce heat earlier and later in the day than liquid systems, so they may produce more usable energy over a heating season than a liquid system of the same size.
Also, unlike liquid systems, air systems do not freeze, and minor leaks in the collector or distribution ducts will not cause significant problems, although they will degrade performance. However, air is a less efficient heat transfer medium than liquid, so solar air heater collectors operate at lower efficiencies than solar liquid collectors. Solar air collectors are often integrated into walls or roofs to hide their appearance.
Room Air Heaters
Air collectors can be installed on a roof or an exterior (south facing) wall for heating one or more rooms. Although factory-built collectors for on-site installation are available, do-it-yourselfers may choose to build and install their own air collector. A simple window air heater collector can be made for a few hundred dollars.
The collector has an airtight and insulated metal frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air in the collector. An electrically powered fan or blower pulls air from the room through the collector, and blows it back into the room.
Roof-mounted collectors require ducts to carry air between the room and the collector. Wall-mounted collectors are placed directly on a south-facing wall, and holes are cut through the wall for the collector air inlet and outlets.
Simple “window box collectors” fit in an existing window opening. They can be active (using a fan) or passive. In passive types, air enters the bottom of the collector, rises as it is heated, and enters the room.
A baffle or damper keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, since the collector area is relatively small.
Transpired Air Collectors
Transpired air collectors use a simple technology to capture the sun’s heat to warm buildings: The collectors consist of dark, perforated metal plates installed over a building’s south-facing wall. An air space is created between the old wall and the new facade.
The dark outer facade absorbs solar energy and rapidly heats up on sunny days—even when the outside air is cold.
A fan or blower draws ventilation air into the building through tiny holes in the collectors and up through the air space between the collectors and the south wall. The solar energy absorbed by the collectors warms the air flowing through them by as much as 40°F. Unlike other space heating technologies, transpired air collectors require no expensive glazing.
Transpired air collectors are most suitable for large buildings with high ventilation loads, a fact which makes them generally unsuitable for today’s tightly sealed homes. However, small transpired air collectors could be used to pre-heat the air passing into a heat recovery ventilator or could warm the air coil on an air source heat pump, improving its efficiency and comfort level on cold days. However, no information is currently available on the cost effectiveness of using a transpired air collector in this way.
DIY: Building a Solar Space Heater
This type of solar space heater works by drawing the air to be heated into the bottom can of a column of cans. The air is then heated inside the cans by the sun’s energy and the hot air within them rises upwards (thanks to convection) to be fed into a pipe which re-enters the building to be heated.
1° Building the Box – First of all make a box out of whatever scrap materials you have to hand. Set the dimensions to that the width of the interior of the box is exactly the same as the width of however many columns of cans you would like to use in your heater.
For increased efficiency, you may choose to insulate the box to prevent heat escaping (by conduction) through the plywood. If so, size your box so that the cans and insulation will fit snuggly.
2° Drilling the Cans – For the air to pass through a column of cans, holes must be drilled into them. Remember that there is already a hole at the top of each can out of which the drink is poured. That just leaves holes at the bottom of each can to be drilled.
In the bottom can of each column a 1/2 to 1 inch hole is drilled in the side. (see image below) – Drill a hole in the side of the bottom can of each column – The rest of the cans in the column have a similiarly sized hole drilled into the bottom. See image below)
3° Building the Can Columns and Painting – Then the cans of each column are glued together using caulk or silicon adhesive and painted using black paint to help them absorb the sun’s energy. Barbecue or fireplace/stove paint is excellent for this as it will not flake off, but any marine grade paint will do a similar job – just make sure it has a totally matt finish.
The inside of the box must also be painted with the same paint before the columns of cans are glued into position using caulk or silicon adhesive. The outside of the box should be treated with preservative, varnish, or paint to help it survive the elements for many years.
4° Sealing the Solar Heating Box – Ideally the whole unit will be sealed with a sheet of tempered glass – of the type used in car windscreens. This glass is very strong and resilient to heat. However, tempered glass (unless you can find and recycle a sheet) is also very expensive. Therefore plexi-glass (plastic) can be used, but it will degrade far more quickly and become opaque blocking out the sunlight.
A hole at the top of the box acts as the hot air outlet and can be connected to the building/room to be heated using an insulated pipe.
5° A PV Electric Solar Panel could be used to power a small fan (such as that used to cool the processor in a computer) to drive air through the snake. The final temperature achieved would be lower, but having a large quantity of 30 degree Celcius air entering a room is much better than a much smaller quantity of 50 degree Celcius air.