RETROFITTING

Home built air conditioner: waste thermal mass as cooling

What do you do with a big pile of old tiles left over from re-roofing your home? Turn them into an air conditioner, of course! Alan Leenaerts explains how.

AS PART of my recent conversion to a low- energy house, I replaced my tiles with Colorbond roofing. The tiles were 50 years old, so the sub-frame required replacing and the tiles required resurfacing. A corrugated iron roof is cheaper, easier and quicker to install than tiles, and has less embodied carbon. It also facilitates the installation of a home roof heater system, which I have done (to be covered in a future issue of ReNew), and which has reduced my heating costs by 75%.

After the re-roofing I had 8m3 of surplus concrete roof tiles. Rather than cart them away to land fill, I decided to use these to make a rockpile air conditioner.

Thermal mass for air conditioning has been used for thousands of years, as is evident when you visit such sites as the ancient city of Petra. There are also some examples from pre-Columbus America, complete with solar chimneys to create air ow.

I originally tried just pumping the cool air from under the house, but on a 35 °C day the performance wasn’t up to scratch. Initially the air came out at 20 °C but after about an hour it warmed up to an uncomfortable level.

My rockpile system uses the tiles as mass, with wooden spacers between the tiles to maximise the air-to-tile surface area, combined with a 30 watt air-transfer fan. The thermal mass is ‘pegged’ at ground temperature (about 10 °C in Melbourne) and provides an easy way of using this cooling resource.

This 30 watt low-energy solution easily matches the performance of the 2000 watt portable air conditioner I borrowed during the drought, before I modi ed my house to be energy e cient.

Like all air conditioning,you are best to use it sparingly and for cooling a small space only. You must also have a thermally efficient house envelope. My house has R3 insulation in the external walls, R1.5 in the internal walls and R5.5 in the ceiling, but nothing immediately under the roof. The windows are double- glazed and I’ve maximised draught proofing.

The rockpile air conditioner will easily cool my main bedroom, which is 3.5 x 4.5 metres in size, in less than half an hour, and will happily supply cool air at around 11 to 15 °C for 10 hours. It gradually gets warmer with time, but usually the cool change arrives before this and I open the windows.

It is much more comfortable than an evaporative system as it generates no humidity, and is much quieter than a split system air conditioner as the fan is 10 metres away under the house.

Materials used

• 5 cubic metres (approx.) of masonry
• a 30 watt air transfer fan and two ducts ($70)
• spacers to put between the tiles to maximise surface area. Mine are 20 x 10 x 50mm pine
• tarpaulin/envelope to make the pile airtight. I used a standard blue tarpaulin
• thermostat and timer — not essential, but I like the system to self-operate when I‘m asleep
• builders foil insulation—also not essential, but it improves efficiency ($25 a roll )
• duct tape ($5)
• inlet and outlet with gauze filter to prevent bugs and dust ($30)
• closeable vents to prevents cool draughts in winter. You need one for the floor and one for the ceiling ($20).

Building the rockpile

I stacked the tiles inside the tarpaulin, using the spacers to provide air gaps between the tiles. The resulting ‘rockpile’ is about 1.5 m wide by 1 m high and 4 m long.

The shape and size of the rockpile is important. You need to ensure that there is as much contact with the ground as possible, so that the tiles can discharge their heat into the earth when not in use. The pile also needs a shape about a square metre in cross- section and as long as you can make it. This ensures the air is in contact with the tiles for as long as possible.

I sealed up the tarpaulin with duct tape and taped in a section of duct at each end. I then wrapped re ective insulation around the ve sides that are not in contact with the ground. This ensures the 20 °C sub-floor air will not warm up the mass.

I then had my source of cool air — all I had to do was to duct it where I wanted it.

What I’ve done is to draw the cold air into the house through the oor, via a closeable oor duct. Warm air from the room escapes through a closeable vent in the ceiling, and then travels down through a cupboard and back through the oor to be recooled by the mass. Thus, it’s a closed loop system. You can also run it as an open loop, venting straight into the ceiling, but it will be less efficient.

It is also possible to build such a system using water as the thermal mass. But although water has a greater thermal mass (see Table 1), it is harder to put under your house and the heat exchanger mechanism is much more expensive. Brick or rock gabions will work just as well.

More info:
en.wikipedia.org/wiki/Speci c_heat_capacit
en.wikipedia.org/wiki/Gabion

CAN’T DIY?

If making your own thermal mass air conditioner is not for you, there are companies that can create the thermal mass pile for you. Using techniques similar to making rock retaining walls, Ground Tech Geo supplies & services can build you a rock pile to suit your needs, such as the one pictured here, which was used under a classroom at the Roberts McCubbin Primary school in Victoria.
www. groundtechgeo.com.au



Reprinted from ReNew issue 122 (renew.org.au) with permission of the author