A while ago my sister, Cate, told me that she was getting quotes for a Solar Air Heater to add on to her house in Canberra. The basic idea is to use a flat-plate solar collector to heat air which is then pumped into the house during sunny winter days to a) provide fresh air during winter when the house is closed-up most of the time, b) reduce the humidity inside the house which helps prevent condensation and mould, and c) help to heat the interior with ‘free’ heat from the sun. There are a number of commercially available systems and I remember that one of these Cate was looking at was the SolarVenti system. I also remember that the installed cost seemed very high at the time. I don’t remember the exact figure but it was around $4,500 or so which I thought was a lot (and so much for ‘free heat’).
Still, I liked the concept so I had a look at other options, one of which is to buy this kit on ebay for $899 and install it yourself. What I like about this kit is that it comes with its own solar panel built in. The solar panel runs the fan directly and you have the option to use a thermostat to control when the fan runs. It comes in two sizes. The smaller one is rated at approximately 250W of heat output and the larger one at 500W. The larger panel is 950mm x 1400mm x 80mm and uses a 20W fan that can move as much as 120 cubic meters of air per hour.
This unit claims to be able to increase the air temperature by 10-20 degrees C. So if the outside air temp is 10degC the air coming into the house should be between 20 and 30degC. Alternatively, with this unit you can arrange it so that it takes air from inside the house and warms it further. So, if the air inside the house is already 20degC the air coming back into the house from the heater could be up to 40degC. But if you recirculate the air from inside the house you lose the stated benefits of a) providing fresh air and b) reducing humidity.
For us in the Greeny Flat these are important considerations. On our page about ventilation I talk about the vital part that good, controlled ventilation plays in maintaining a healthy interior in a well-sealed passive solar house like the Greeny Flat. In that discussion I show a picture of the heavy condensation that appears on our windows on cold mornings IF we don’t run our exhaust fans enough in winter. I also show an image of our under-slab fresh air tube and describe how we use this to pre-heat the air that comes in to replace the air we blow out through our exhaust fans. This system works well to maintain good indoor air quality and keep the humidity level down to a healthy level. The problem with it is that we lose the heat that’s in the warm air that we blow out or the house. On our ventilation page I describe the best option for controlled ventilation which is called a ‘Heat Recovery Ventilator’. These were common and readily available in Montana where I lived for a long time but I have yet to meet one in Australia.
The next best thing for us in this sunny country seems to be to the Solar Air Heater concept. So I did some more digging around and found a TON of information on the internet. A quick search for ‘DIY Solar Air Heater’ brought up over 400,000 results and YouTube has lots of videos on the subject including the following which I found quite informative.
Around the time I was investigating all of this I was visiting a builder friend’s place and I saw a couple of old flat-plate solar hot water collectors sitting in his yard. I immediately thought they would be perfect for a solar air heater. It turned out that he didn’t want them and gave them to me, and so a new project was born. The idea being to convert a flat-plate solar hot water collector to a solar air heater. I made a 3D model of the idea and came up with the following drawing to explain it to a couple of helpers.
Over the last couple of weekends we’ve proceeded to put the thing together and today we got it running. Here’s what the finished prototype looks like.
I have posted a bunch more photos of us building the prototype on our Gallery Page if you’re interested in how we put it together. The exciting thing is that it seems to work very well. As you can see from the photos below, when we first switched it on, the outside air temp was 19.6degC, the indoor air temp was 21.7degC and the temperature at the inlet (where the air enters the house from the solar collector) was 33.7degC. So it was adding at least 14degC to the fresh air coming in from outdoors. And that was with the fan running pretty fast.
The fan is currently connected directly to the solar PV panel which means that the speed of the fan (and the amount of air it moves) varies greatly with with the amount of sun hitting the solar PV panel. When there is full sun directly on the PV panel the fan is moving a LOT of air. I have no way of measuring exactly how much air but the fan is rated at 220 cubic meters per hour and was probably running at, or close to, full speed. Later in the day, as the sun got lower in the west and hit the PV panel less directly, the speed of the fan slowed dramatically which is exactly what we want. When there is less production from the PV panel there is also less heat being generated in the solar air collector so a slower fan speed means that the collector has more time to heat the air passing through it. In short I have to say that the first run of the prototype is a big success!
DIY Solar Air Heater Costs
I got the solar hot water heating panels for free and I already had the foam insulation, piping, wood, insect screen, and mounting hardware lying around. All I have paid for so far is the solar panel (which I got on ebay for $66 including delivery) and the fan (also on ebay for $49 inc del) making a total cost of $115.
Lessons Learned So Far and Next Steps
I took a wild guess at the size of the fan and solar panel that we would need. The cheap fan I bought is VERY noisy and probably moves a lot more air than is necessary. The next step will be to find a smaller, quieter fan that can handle variations in voltage and lengthy running times. A smaller fan will move less air but I expect that we’ll end up producing about the same amount of heat. Less air flow will mean that the air heats up more as it moves through the collector so we should see temperatures at the inlet of 40degC or more.
A smaller fan will also mean that we should be able to use a smaller solar panel which will take up less space. Ideally, the solar panel would be in a narrow strip across the bottom of the solar air collector (similar to the one in the photo at the top of the page but with the PV at the bottom to ensure that it is in full sun).
A further refinement will be to place three thermostats, one to measure the outside air temperature, one to measure the temperature at the top of the inside of the solar collector, and one to measure the air temperature inside the house. These can be used to monitor how much heat the unit produces and, more importantly, to control the fan so that it only comes on when the air inside the solar collector is warmer than the air inside the house.
This would also give us the ability to reverse the process in the summer time, i.e. to bring in fresh air when the outside temperature is cooler than the inside. By placing the solar collector panel vertically on the north wall of the Greeny Flat we have ensured that the roof overhang (which is designed to stop the sun from coming in the north windows in summer) will also shade the collector. On summer nights we could use the fan to help bring cool air into the house if we also added a battery to the system.
We might also look into filling the pipes inside the collector with a Phase Change Material (PCM) that would store some of the excess heat when the sun is hitting the panel full on, and release it again when the sun is not so strong. This could help to give us a more even temperature over a longer period of time.
Thanks to the Helpers
As you can tell, there are lots of ways we can improve on the basic prototype. First, we wanted to build it and make sure that the concept is viable before putting more time into refinements. At this point we can clearly say that the concept works and is worthy of further refinement.
Many thanks to those who have helped so far including Cate Lemann (who put the idea in my head), Colin McNeil (who donated the panels), Ben Leenders (who wired the electronics) George Lemann and Wadan Rainer (who helped put it all together).