Post 15 – It’s So Cold – What About Solar Air Heaters?

It’s another amazingly beautiful morning in the central highlands of Victoria. Dawn is just breaking and the sky above the tree line is stunning orange, streaked with yellow and deep grey, fading to shimmering white. It is changing by the minute as the sun continues to rise. I can see the white ground emerging and the intense colors of a few minutes ago are fading; we have another heavy frost and the inevitable cold that goes with it. Everything is so still as though it is snow rather than frost.

Over the last week it has been really cold and grey; there has been no visible sun but yesterday and, probably today, there has been, and will be, sun. When there is sun, my thoughts turn to the capacity of the sun to provide us with energy. In this instance, my thoughts turn to the solar air heaters which we installed on our roof about five years ago in the fervent hope that they would help with winter heating. Over the last week, when the sky has been grey, they have not worked at all. Yesterday, they helped a little but not a lot, even though the sun was out for most of the day. As far as I could tell, watching the indoor monitor carefully, the outside air remained cold and the panels just could heat the air enough and pump it into the house fast enough, to make much of a difference. The following is some back ground research I did on solar air heaters for the building design course I am doing.

Overview

Solar air heaters are usually roof mounted panels that look like photo voltaic panels (PV). Some solar air heaters can now be wall mounted. Panels range in size but are usually about 3 sqm. The materials used in these panels is generally some form of glass over a darken surface that absorbs radiant heat from the sun. Cold air is drawn into the panel as a result of thermo-siphoning, heated via convection and then a fan usually moves the heated air into the house via ducts. A thermostat is usually located in the system with a wall controller in the house that allows maximum temperatures to be set. Installation requires sufficient north facing roof space that is not shaded in winter. They can be installed by a skilled home owner if required but this isn’t something I would do.

Most brands and models have more than one vent per panel so vents can be spread between more than one room so we chose to have two panels each with three ducts. One of the panels has ducts that lead to ceiling vents in each of the three bedrooms and the other panel is linked to three ducts that have ceiling vents in the open plan living space.

Advantages/Disadvantages

The advantages and disadvantages of solar air panels depend on the size of your installation; the type of conventional heating you have; your local climate; the behavior of the occupants; and, the thermal properties of your house. So, keeping these things in mind, this is what I have found:

They are most effective in spring and autumn months when sun shines and we need heating here in central Victoria.
They will never fulfill all heating requirements in cool climates but they do reduce the daytime need for central heating especially in spring and autumn; it is likely that you may need an additional heat source in the evenings and definitely when the sun doesn’t shine.
When they panels are operating they provide warm, fresh air to the house. Usually I find the air to be drier than the air that builds up in our well sealed house.
The heaters would probably work even better if we had some sort of thermal mass in the living room that could be heated while the unit is working, then return heat later to the room.
The heaters use very little electricity to run and some models now have their own PV supply; it’s really only the fan, that moves the air from the panel to the room, that needs power and it uses very little.
The panels need to be cleaned regularly. I try to get this done annually but it involves getting up onto the roof which is not for me. (Cleaning solar panels and air heaters and hot water systems would be a great business for the right person. I see there are a few in the major cities but not around here yet).
Having the vent system can assist with night purging in summer as well as ventilation and filtration of indoor air. Our night purging system, to get rid of summer heat works well without using the vents and duct system but for other houses, this system could be good.
Some brands are manufactured in Australia.
Most systems are thermostatically controlled via a control panel in the house so you can set the thermostat to heat the house to, say 22º Celsius and it will automatically shut off the system when that temperature is reached.
Unfortunately I have found that if I leave the system on, even with the thermostat set to say 22º, and there is no heat being generated, the vents let cold air leak in so the whole system has to be turned off it is not working.

Cost/Payback

We purchased Solamate solar air heaters at a cost of cost approximately $6,650 (installed but excluding delivery) for two panels. Running costs are almost $0 so I have only considered the capital cost. Payback depends on local climate, thermal properties of building and behavior of occupants so everyone probably needs to make their own calculations but here are ours. There are a couple of sites where you can work out how much sun shines on which days and I found our location has about 219 days/year with some sun but the critical issue is how much sun is some sun. I did a quick and dirty calculation recently and thought that it would take the best part of 15 years to recoup our costs but even then I made an error in assuming ‘some’ sun meant about four hours a day. Had I looked a bit further, and had available the resources that I would use today, I would have found the weather bureau provides useful data about how many hours of sunshine per day by location. So, if you want to get down and dirty with the calculations you can but, bottom line, I wouldn’t buy solar air heaters again in this location. I think I wasted our money on them, but on a positive note, when they do work, the heat is really nice. In my defense of this poor decision, I was excited at the time by their possibilities, as they had been featured on a TV show that was on at the time called ‘The New Inventors’. There was also a lot of local support for a bulk buying initiative on the heaters. Bye for now and keep warm, Jane