My Beer Collector
By Matthew Driver
I started researching solar air heaters last year. My determination was to reduce the amount of gas we burn through our ducted gas system. There are a few commercially available solar heaters but I was so impressed with all the people building their own systems that I had to give it a go.
In general, a solar air heater uses a fan to duct air from inside the house, through a solar thermal panel to heat the air, and then back into the house. Some systems duct air from the outside to provide fresh air to the house. Systems using outside air are more practical in milder climates as more energy is lost heating air from an initially lower outside temperature than using inside air.
My heater is based on a design refined by the Hungarians and originally designed in Canada. The Hungarians affectionately call it the ‘Beer Collector’.
The basic principle is to use aluminium cans to create light weight pipes housed inside an insulated box with a clear lid. The columns of cans are painted black and absorb energy from the sun and heat up, the heat in the cans is passed to air flowing through them.
You might well wonder ‘What’s the point in a heater that only works when the sun is shining?’ A solar heater is not designed to be a primary heater for a house but it does have some great benefits:
- When the sun is shining it is possible to reduce your consumption of non-renewable energy.
- A room that does not have a north facing window can be heated during the day.
- If you are home during the day then it makes sense to utilise the free energy.
- If you have a high thermal mass, well insulated house then the heat added during the day will be retained to reduce your heating requirements at night.
In my situation I run an IT consultation business from home with myself and one employee. Our office is downstairs and at the south of the house and has always been the coldest room in the house. The office is normally heated by our ducted gas system. In order to heat the office it meant heating the whole house.
Solar heaters built for Europe are often mounted horizontally on the south wall. This makes sense when the winter sun is only rising about 15 degrees above the horizon at midday. Since the mid winter sun in Canberra reaches 32 degrees above the horizon, I decided to angle my panel about 30 degrees off the wall so that it would be facing directly at the midday winter sun. I’m also running out of usable roof space due to existing hot water and PV panel, so in the end I decided to mount the panel above my front door which has an unshaded section of north facing wall.
Building the system
The solar panel box
I chose to build the frame of the solar panel out of 150mm wide extruded aluminium ‘sign box section’. It’s designed for making marketing signs and has a 5mm slots on both sides for glass. It’s durable and strong. The box is insulated with 25mm Foilboard at the side and back, the reflective aluminium foil reflects heat back into the box and the polystyrene provides bulk insulation. The front of the panel is covered with 4.5mm Perspex and the back is covered by 5mm Corflute (the stuff used to make election signs) which happens to be very cheap, light weight and insulating.
The solar core
The core is made from 15 columns of 14 aluminium cans (210 in total). The cans are joined together with roofing silicon. Everything inside the box needs to be very heat resistant and roofing silicon can handle 160°C temperatures. The cans are sprayed with potty belly black paint which can handle 300°C temperatures. The foilboard used to insulate the box is my biggest concern with the expected high temperatures. There’s the possible chance of it melting in summer.
The cans are joined on either end to the duct through a device called a divider. I’ve built my divider out of 0.5mm sheet aluminium. The divider’s job is to evenly distribute the air from the duct down all the columns of cans. There is a lot of hot debate regarding the best way to design the divider. In the end I chose to use the method used by many commercial heat exchangers with the intake at opposite corners to the outtake.
The size of the fan is important. Too small a fan and you can’t extract sufficient heat out of the panel. Too large a fan and you expend too much electrical energy running the fan. I chose an axial fan that was part of an IXL extraction kit, designed for moving air from one room in the house to another via 6m of 150mm duct. It runs at 43 Watts and without any ducts installed blows at 200m3/hour. With the duct and panel in place this air flow drops to just 75m3/hour. I’m considering a more powerful radial fan.
To match the fan, I built my system using 150mm duct. Many systems are built using just 125mm duct which is fine. My total duct length is about 14m long to get from the panel on the north side of the house, through the roof cavity and down an external wall. On the external wall I’ve double insulated the duct with a 200mm duct slipped over the 150mm duct to give it added protection. A 14m duct length is pretty long and should be avoided if possible. Most of the Hungarian panels have a duct length just as thick as the wall, which means less heat loss in the duct and higher air volume flow.
To control when the fan turns on and off I bought a differential temperature controller kit (I had to solder it together). Using two temperature sensors (one in the panel and the other in the room) it turns on and off a 240V relay that powers the fan. When ever the panel is hotter than the room the fan is turned on. I then have a room thermostat connected in series to the differential controller to turn off the fan if the room gets too hot.
The result of the controller design is that it is completely set-and-forget. The controller draws only 0.4W of standby power and automatically turns the fan on when the room needs heating and the sun is shining.
The solar heater has been in operation now for one month (the last month of autumn) and I’m very happy with the results. Most days I don’t need any additional heating for my office. On a clear day the heater turns on about 9:15am and turns off at 4:30pm. It will raise the room temperature from 15 degrees to 20 degrees by about midday when the heat output is highest. The air temperature from the room vent ranges from 30 degrees in the morning to 50 degrees at midday.
The accumulated heat output for a sunny day adds up to about 5kWh. This doesn’t sound like much when compared to the output of a gas heater but for a well insulated space it does the job. Comparing it to Solar Photo Voltaics (PV), you would require about 2kW of PV on your roof in winter to provide the same energy (not that I’d recommend running an electric heater off PV generated electricity).
1. Building the columns.
2. Painting the columns.
3. The divider for the columns.
4. The panel core.
5. The frame join.
6. The fan mounted.
7. The wall brackets.
8. Installation of the panel.
9. The finished panel.
10. Differential temperature controller.
11. The controller and overheat thermostat.
12. The result!
After multiple cloudy cold days in a row I eventually turn on the gas heater. Those days are pretty rare, probably about one in ten, today was one of them. But on sunny days I bake in lovely solar heated air being pumped into my office. It’s a nice feeling.
I’m keen to see other people build solar heaters in Canberra and I’m happy to provide information, assistance and even cans if you are will to try.
I hope to have our house open for viewing on Sustainable House Day this year to show off my solar heater.
If you are not up to building your solar heater then there are some commercial products available. The latest being the ‘Sola mate’ that was recently shown on ABC’s New Inventors, it looks excellent.
Beer collector – English site: solaroregon.org/news/beer-bottle-solar-collector
Beer Collector – Hungarian site (via Google translator): http://translate.google.com/translate?prev=_t&hl=en&ie= UTF8&u=http://www.sorkollektor.hu&sl=hu&tl=en&history_state0=
Sola Mate: www.sola-mate.com