Experimenting with solar: Using waste heat from the roof to warm our house.
This is the first post in a series I’m working on about our attempts at using solar energy for heating and electricity generation. Later posts will cover construction of a sunroom, a solar hot water heater and a 3kw grid connect PV electricity system.
A cool climate
Unlike many other places in Australia, winter here is cool as we live up in the tablelands at about 1000m altitude. Many nights during winter the thermometer drops down well below zero (32°F) and we wake up to glistening frost. It’s not just that the nights end up cold, it starts to cool down as soon as the sun sets. Often cars outside have a layer of ice crystals on them by 8:00 or 9:00 at night.
Days are often clear but the average maximum winter temperature is only around 13.5°C (56°F). Without external heating houses here get very cool.
All things are relative so people from really cold places like Alaska and Siberia are probably rolling their eyes at this point and thinking what a wimp I am, while most other Australians are nodding their heads and remembering a freezing cold night they spent here camped in a tent at the local caravan park. The thing is, houses here are not built that differently to those in more tropical parts of the country – put simply we are not really set up for the climate.
Our house is fairly old – maybe 80 to 100 years old and used to be quite cool and I don’t mean cool in an “awesome” sense. It used to get down to quite unpleasant temperatures inside in winter. Like many old houses in the area it has small windows and a verandah; in our case facing north (that’s towards the sun here in the southern hemisphere) and also to the west so it blocked precious sunlight entering the house. Like many houses in Australia, the house also has a corrugated iron roof.
It’s hot up there
While doing some maintenance in the roof cavity I noticed how hot it often was up in there on a sunny day even though it was quite cool outside. A traditional way of dealing with this heat is to add vents to get it out. There is a layer of fiberglass wool insulation bats on top of the ceiling but there is no layer of reflective insulation under the roofing iron that would normally be there. That could be useful if I wanted to do some experiments as I would not need to damage anything that was already there. I started thinking about ways to use some of this heat to warm the house and made a series of measurements.
I bought some 25 x 50mm (2” x 1”) pine timber, some reflective insulation, closed cell foam, some 15cm diameter fruit tins and a lot of duct tape and set about sealing off a section of the north facing roof. I had an old DIY heat transfer system that was designed to move warm air from one room to another. This system consisted of some flexible 15cm (6″) tubing wrapped with insulation, vents and a low wattage fan. I setup a vent in one corner of the house and had the fan between that and the heat collector. I ran some more of the flexible tubing down to the coldest room at the back of the house.
It was while doing the construction that I realised just how hot ifT gets up there. It was quite uncomfortable in protective gear and I was reminded again how much tougher builders must be than I am.
Unfortunately, I don’t have any pictures of the system, but I do have a picture taken before the build started. The reflective insulation was fastened to the underside of the rafters and held up with the pine timber. The ends were sealed with MDF that had holes with old fruit tins fitted into them for intake and exiting.
I had planned to use a thermostat to control the fan so that it would automatically come on when it was warmer at the roof than it was in the house.
So how did it go? Well not quite as good as I had hoped. The corrugated roofing iron is silver and I expect that a reasonable amount of heat is reflected off it. Also, as the iron gets quite hot I expect quite a bit of that heat rises up from it into the air above, rather than down below. Air blowing into the room did feel warm but the rooms temperature only increased by a few degrees. While it was warm up there when the fan was on heat may have been extracted faster than it was being collected so temperatures in the system dropped off.
I took a lot of steps to isolate the system so that glass fibers did not enter the house. I had set up the fan so that it was blowing air into the heat collector rather than sucking are out of it so if there was an air leak air would escape into the roof rather than suck air from the roof. Even so, I was never comfortable using it because of those fiberglass insulation bats in the ceiling. After all, the bags this insulation came in had a warning on them “Possible cancer hazard by inhalation”.
In the end we decided to convert our verandah into a sunroom and that made such an enormous difference that the ceiling system was abandoned.
Was it a complete failure?
It did give me a lot to think about. There is a lot of heat falling on most roofs and lots of houses need heating. There is roughly 1KW of energy in ever square metre of sunlight at midday, so even allowing for lower levels of energy at other times of the day and inefficiencies in the system there is still a lot of energy to be harnessed – at least in an area that has sufficient sunshine at cooler times. Another way of thinking about it is to imagine if half the roof was glass and the sun shone directly into the house, the house would be unbearably hot during the day even in winter. I don’t recommend creating your own system like I tried. There may well be health risks not just from insulation and dust but also from fumes from heated materials.
One of the benefits of a well designed system could be that it can be automated to switch off when the roof is cold and the insulated rooms below should remain warmer for longer. A problem with our sunroom is lots of heat is lost overnight through the glass windows but there is ways around that.
It is possible to buy solar air heating systems that can be retrofitted to a house, but I have not seen where this has been designed into the house so that it does not look like another thing bolted onto the roof.
Our verandah sunroom conversion will be the next post in this series.
Posted on June 11, 2012, in Solar and tagged self sufficiency, solar, Solar energy, solar heating. Bookmark the permalink. Comments Off on Experimenting with solar: Using waste heat from the roof to warm our house..