Why to heat up air with solar power?
We have an average of 230 days of sunshine in Uruguay and in the winter time there are plenty of beautiful sunny days too. If you assume that on a global average about 1300 watts per square meter power from the sun reaches the earth, we should try to use this free energy. Some ways to use solar power are used already successfully in Uruguay for many years, but at the moment only for heating up water and generating electricity. The easiest way of use, the heating of air, is unkown here.
Collector for roof mounting
Suitable materials from Uruguay
Solar hot air collectors are already used in Europe and the US and there are plenty of providers of complete modules and also many DIY-instructions for building your own. In Germany, the effort seemed to be too high for the expected earning. But in Uruguay it looks completely different.
The best and most promising guidance we found on some sustainable energy sites in the United States. The goal is simple. You have to build a box that collects as much solar heat as possible and transport it to where it is needed. The problem in Uruguay starts with the construction of the boxes. The materials provided by the instructions for the most parts are not available here or do not meet the requirements. So we had to experiment with the materials we get here, until we found what we need.
This was the first prototype built in spring 2015, using old plans from Germany (picture above). Unfortunately, it was not very efficient. Also the available fans and vents did not comply with the requirements. The first practical prototype was designed as a device for wall mounting, in order to be able to test new features faster.
Fans and controllers from Germany
The wall mounting type is the simplest form of solar air heating because the air does not have to be transported. Through the existing thermo dynamic convection of warm air, it would even work without a fan, when using large collector areas. But when a fan is used, a suitable control is necessary . We weren't successful here in Uruguay neither finding efficient fans nor appropriate controls, both now come from Germany.
The controls ensure that the fan is only running with a positive temperature difference of collector, so additional energy is only needed when additional heating is available. A variant of the control even slows down the speed of the fan if the differences are getting smaller. The fan used from Germany reached a maximum air volume of 190m³ per hour with only 29 watts of power consumption.
In case of a vertical solar radiation onto the collector, without cloud cover and shading from trees etc., we can reach an output of up to 500 watts per square meter and more. A collector with 1,5-2 m² will be enough to hold a bathroom or bedroom at a comfortable temperature. Depending on the design and insulation of the room, around 100 watts per m² living space are needed to keep it at a temperature of 20 degrees celsius. But all these values will vary depending on the locality.
What a solar-air-heater is not built for
These panels are only able to heat up the air and no any other media, such as water, oil or others. Therefore no storage of energy is possible. This type of heating is only suitable in addition to an existing heat source, such as air conditioning, wood stove, etc.
Where solar-hot-air definitely makes sense
One of the biggest problems in Uruguay is the humidity in rooms and therefore the growing mold. This usually results from not heated rooms with high humidity and the moisture condenses on the cold walls which runs down as water. But if you can keep the walls at comfortable temperatures, the mold problem will be minimized.
We practice this in our house (constructed 1970) for more than 2 years now and are virtually free of mold. Our neighbors who don't live here during the winter time, open their homes with respirators and have a lot of work every year to get it "visually" free of mold. A suitable solar-hot-air-collector would minimize this problem.
So far, the collectors are not able to remove moisture like a dehumidifier or an air condition system. But we are working on a solution to remove at least some humidity.