Solar Air Heater

I don’t know about other people, but generally I find a pattern with new things that I learn which goes something like this: 1) conceive the general idea; 2) do some basic research; 3) try something out; 4) find out 2 or 3 things you really wish you had known about at step 1.  I’m not sure that this pattern is inevitable. For example, if you have somebody experienced there to teach you, then it shouldn’t happen but that relies of having a good teacher. All too often, teachers don’t teach first principles and so you inevitably have to end up learning them yourself anyway.

So it went with the house renovation and specifically with the idea of energy efficiency. I had thought about that at the time, installed very good insulation, knew the place would be nice and tight at the end etc. Indeed, I had even thought about the attached greenhouse option as a way to add solar heating to the mix. Anyway, as winter started to kick in this year, the issue of heating came up. I should say at this point that winter is my favourite season in Melbourne and I like the cold. Left to my own devices, I don’t use heating. Nevertheless, I realise I’m weird in that respect and so I wanted to see what could be done for sustainable heating for the “average person”.

And so I got on to solar heating and, to cut a long story short, solar air heaters.  Air heaters have a few advantages. They are cheap and relatively easy to make with only basic DIY skills. They generate quite a lot of heat quickly because air heats up fast. And, when built correctly, can last decades and are therefore very cost effective.

There is heaps of info for solar air heaters here and here and I used these sites extensively when planning my build.  For the solar air heater, colder air will come in through the bottom and be pulled to the top via convection current. As it rises it will have to flow through a solar absorber of some kind. In this case, simple flywire screen is highly effective.  The hot air flows out the top and into the space you are trying to heat up.  Very simple idea. Here’s some pics of the build:-

air heater 1I picked up some old flywire screens from the tip shop for a couple of bucks. I thought they would make the job easier but I’m pretty sure just a roll of flywire would be just as easy. You have a strip of wood on either side that the flywire gets attached to. It slopes from front to back so that the air coming in the bottom must go through the flywire. Two or three layers of flywire is recommended. I went with two.

air heater 2You attach the wood strip to the frame and then the flywire to the wood strip.

air heater 3The dimensions are 1200 x 2400.  Glazing goes on the front to allow the sunlight through. I went with some polycarbonate greenhouse panels. The same type I had used to construct my greenhouse and which worked very well for that. On the back, I just put some panels of MDF. Some people like to put rigid fibreglass insulation there but most of those guys seem to come from northern USA where you probably want every drop of heat you can get in mid-winter.

air heater 4An old computer fan to pull the air out. This is one thing I still haven’t fully solved.  The heat gain you get is a function of the temperature rise in the air multiplied by the air flow. Thus, the size of the fan and the size of the outlet become important. With full sun, I get air coming out the top of the heater at about 45 degrees which is not optimal efficiency and means I could get more heat gain if I could increase the airflow.  There are passive designs for the solar air heaters but they require a lot of vent area at the top an the bottom which becomes problematic for attaching to a house.  This fan has a nominal CFM of about 100 so it might be the case that I need to adjust vent size or fan position. Need to look at that more.

air heater 5Speaking of attaching to the house, here it is. Drilling a hole in the wall of a newly renovated house took a leap of faith. In the end it went fairly smoothly.

air heater 6The heater was installed on the winter solstice, but we had beautiful clear skies all day. This was the result at 4.07pm. The heater faces about 15 degrees west of true north and on a clear day it definitely makes a real difference. Even with the right gear, it’s hard to get exact measurements of heat gain but I’m guessing I get about 3 degrees of heat gain to the front of the house.  Of course, that’s on a perfectly clear day and it just so happens that those are quite rare in a typical Melbourne winter. The heater will work in partly cloudy conditions but on heavily overcast and rainy days, it generates no heat.

In summary, this was a fun little project and I’m glad I did it. Since installing the heater and seeing the results, I’ve read up a lot more and done the maths on solar gain versus heat loss and how house orientation and other factors influence the possibility of solar heating of a house.  It turns out that given Melbourne’s solar radiation in winter, the orientation of the house, the shape of the house etc. etc. about the best I could hope for would be maybe 50% solar heating in mid-winter and that would involve having a couple of solar air heaters in addition to solar gain through windows. Ideally, I would have thought about this before doing the renovation as there were a couple of things I could have done to improve this.  But shit happens.  It serves yet again to the prove the truth of that T.S.Elliot line about returning to the place where you started and knowing it for the first time.