How solar air heaters work:
The above diagram indicates the basic concept of a solar air heater, and while there are many designs, the basic principle is the same - a small fan feeds interior air into a south-facing wall-mounted panel. Air is heated as it passes behind a black surface and is then returned to the conditioned space at a much higher temperature.
DIY solar air heater videos are a big hit on YouTube, with a couple of main staple ideas - recycled pop can solar collectors, downspout solar collectors, screen or sheet metal solar collectors. If you don't have it in you to make one yourself they are available for purchase with a bit of internet digging.
Aside from large commercial installations, the most common application for solar air heaters seems to be supplemental heat for individual rooms, like an addition, a workshop, garage or any other small outbuilding.
The reason we say 'supplemental' is that while while a modicum of heat can be collected on cloudy days, you're mostly going to feel it when the sun is shinning. And without some additional feature to store heat and later distribute heat (thermal mass), it's unlikely that anything but the most well-insulated buildings will maintain a comfortable room temperature from sundown to sunup on a cold winter night.
If you're looking for a solar air heater to warm a building with no power, you can gain heat simply through natural convection as warm air rises, but you will get much more heat by forcing air through it with a fan. Fans don't require a lot of energy to operate, so a small dedicated PV panel would do the job when there is no other available power.
Pop-can solar panels: This is nothing short of brilliant and could be the only solid reason to justify drinking pop. It is however a rather time consuming process - cans need to be cleaned, have holes cut in the bottom, tabs removed, then they need to be glued together in a stack and finally painted black.
Air is blown into a chamber at the bottom of the panel and forced up through the stacks of cans into a top chamber that collects the heated air and directs it back indoors.
Downspout solar collectors: Just as it sounds, this design replaces the stack of pop cans with a standard eaves trough downspout. The same principles apply to this as the pop can solar collector, and although you will spend more on materials, you'll save a lot on labour. The end result is the same; air is heated as it passes through black tubes.
Solar screen or sheet metal absorber: The designs we found involved 3 layers of screen to ensure a unified black surface. Screen collectors typically do not separate air into individual chambers like the previous two designs; air rises up a single chamber behind the screen or flat metal surface.
Of the two, it would seem to me the screen design sounds like a bit more work compared to using sheet metal (as seen above), which could be made using old metal roofing and painting it black. Labour aside, testing between a screen collector and a pop can collector showed that the screen collector did provide more heat, read more here.
How much heat can solar air heaters provide?
That depends on a lot of variables:
Panel size: This will determine the volume of air you can condition and the output temperature. Choosing what size to build or buy will depend on your needs and how much exterior wall space you can dedicate to a panel.
Solar absorption: Panels are limited in how much heat they can collect depending on how reflective the black surface is, and you will do better with a matt paint than glossy. Glazing will instantly reflect about 10% on its own, so really the best you can hope for in overall performance is about 80% absorption of available light.
Conductivity: Materials with higher conductivity will improve performance. A black PVC pipe for example, would not provide as much heat as a black metal pipe. Even different metals will have different rates of conductivity. Copper is one of the better conductors but it is very expensive, so the advantage of increased conductivity would likely not be worth the added cost.
To choose the downspout panel option, be sure to use metal and not plastic, and if it has a glossy finish it would be worth it to paint it matt black.
Home performance: How much heat a house needs is determined by how much it loses. A solar heater will provide a greater percentage of the required heat if that heat demand is lower, so how well insulated and airtight a house is will be a deciding factor.
Cloud coverage: In areas that are regularly overcast, like Vancouver for example, it may not be worth the cost and hassle to either buy or build one. Certainly the payback period for labour and money invested would be much longer.
Latitude: The further north you go the less sun hours you will have in a day, so the cost or effort required to make a panel will stop being worthwhile at a certain higher latitude.
The downsides of solar air heaters:
The Achilles heel of most renewable energy generators is reliability, but also energy storage. The wind doesn't always blow and the sun doesn't always shine (or more accurately we can't always see it). So the drawback of solar heaters is that you only get heat when the sun is shining.
Short winter days and unpredictable cloud coverage make it difficult to rely on solar air heaters as a primary heat source because you will gain all your heat during sunny hours but then have to go an easy 16 hours at times with no heat input. And shorter days means that they generate the least amount of heat when you need it most. In all but the most heavily insulated homes in more gentle climates, with thermal mass included for heat storage, you will likely need an additional heat source.
Storing solar heat (thermal batteries):
If you incorporate thermal mass into the equation you can distribute the heat collected over a longer period of time, and there are any number of creative ways to do this. Sticking with the DIY theme for say - sheds, garages or greenhouses, you could run the heated air through tubes embedded in sand, bricks, masonry etc before venting it directly into the conditioned space. Rather than simply heating the air, dense materials will absorb some of that heat and slowly release it over time after the sun goes down.
There is nothing to say you couldn't do this to an addition on your home, it's just that we are generally a bit more picky with the final appearance in our houses. So a slightly more aesthetic design might be required in a house than in a workshop or garage.
Specifically with greenhouses, they have a tendency to overheat in the day but sometimes get too cool at night for young plants. Keep in mind that it is more important to keep the roots warm than the plant itself, assuming of course the air stays above zero. If you include a solar air heater in a greenhouse design and transfer some of the heat to a platform with thermal mass on which your soil boxes can sit, you could start a growing season earlier.
It's also a good idea to include a bypass vent of some kind that can release air in summer to prevent overheating when the panel isn't actively in use.
You could also apply the principles of passive heating and cooling by placing the panel under the eave where it will have full exposure with the low winter sun but be shaded when the sun is high overhead and you have no need for heat.
How to build your own solar air heater:
If you do a web search you will find an endless list of designs and assembly techniques, the same holds true for DIY videos on YouTube. Different designs will resonate differently with different people, so pick the one that best suits your skill set, tool collection and attention span. If you come up with any brilliant design modifications in the process, please share them in the comment section below.
Check out the DIY video below to give you a better idea of how easy they can be to build.