Solar thermal heating and cooling

HEATING and cooling churns through nearly 40 per cent of Australian household energy use. Energy prices are set to hit ever-higher levels, so the more efficient a temperature control system is, the more attractive it looks – almost day-by-day.

There’s a growing industry selling units that use the sun’s free energy to ventilate, warm and even cool your home. They can also improve indoor air quality by reducing humidity, moisture build up and mould. Broadly, the systems fall under the banner of ‘solar thermal air heating and cooling’.

So what’s commercially available and how well do they work?

Roof space systems

These units work by moving the air in our roof cavities. When the sun strikes the roof, it warms the air below – just as cars get stuffy on clear days even if it’s chilly outside. For heating in wintry weather, roof space systems pump that warmer air inside the home. To help with cooling in summer, some units can extract the hot air from the roof during the day, and also, blow cooler outside air into the house overnight – useful if you’re worried about security or the noise of open windows.

There are two types of roof space solar collectors. One kind, sold here by businesses such as HRV, Solectair and Smart Roof, uses the whole roof cavity as a collector. The intake is near the top of the space, where the warm air rises. A fan sucks the air through a filter and ducts it into the home.

The second kind of technology, offered by Smart Roof in its Smart Breeze system, is suitable for metal roofs. It captures heat in the gap between the roof sheet and a layer of insulating foil or sarking underneath. The warm air rises to the apex of the roof cavity, where it’s nabbed and whisked into the home (using a solar-powered fan). “The ribs act as ducts and we seal the ridge so the air can’t escape,” says Smart Roof CEO Robert Semmel.

He says that the system can significantly reduce bills, but won’t do away with other sources of temperature control. “Our aim is to reduce heating and cooling by up to 40 per cent, not to replace it. If you’ve got a miserable, wet day there’s no radiant heat and on really hot nights there won’t be any cool air to bring in. But those are extremes.

“If the sun is out on a 15 degree day, we get temperatures up to 40 degrees [in the roof]. When cloud cover comes over, we get 30 degrees. There’s a lot of wasted energy we can use,” Semmel says.

According to energy efficiency expert, Adjunct Professor Alan Pears from RMIT University, the catch is that our roofs aren’t very good solar heat collectors. “A roof space can be described as an unglazed, very leaky solar collector, with a substantial proportion of its area facing the wrong direction,” he says. “So in that sense it’s not very efficient.”

But all is not lost. “On the other hand, it’s a very large area, so you can collect quite a lot of useful heat, especially in mild seasons,” Pears says. Light-coloured roofs with insulation directly underneath will collect much less warmth than a dark roof without insulation. Similarly, the roofing material will influence the way the systems function. Metal roofs heat up (and cool down) quickly, whereas the thermal mass in tile roofs means they take longer to warm, but stay hotter for longer in the evening.

If you’ve ever crawled into your roof space, the idea of pumping that air into your home might make you splutter. But Pears argues the air near the top of the roof space is likely to be no worse than other sources of ventilation in our homes. “The air coming through windows or under doors is unfiltered and often highly polluted,” he says. “In any case, most roof space systems offer a filter. The filtration systems available are very impressive, as long as you buy a good one and maintain it properly.”

Solar air collectors

A solar air collector consists of a clear plastic or glass-fronted panel that uses solar radiation to heat air, like the way a solar hot water unit heats water. The panel is mounted outside the house, either on the roof or a wall. A fan (usually solar-powered) blows the warmed air into the house. For cooling at night during summer, it may also blow cooler outside air into the home.

In Australia, a variety of solar air collectors are available, including products from SolaMate, Solar Breeze, Sun Lizard and Solar Venti.

Arne Hachmann’s business, Global Eco and Environmental Solutions, sells Solar Venti units. If you want to ventilate your home, he advises that a 3 square-metre panel will suffice for an energy-efficient 250 square-metre home. If heat gain is your priority, he recommends one unit of that size per 100 square metres of floor space.

He says the Solar Venti warms air to about 35 degrees above the outside ambient temperature. “If you have a sunny 15-degree day, you would expect to get 50-degree air ducted into your house for free, at a rate of about 200 cubic metres per hour.”

Pears has a SolaMate prototype solar air collector installed on his Melbourne house. The panel is three square metres and feeds into his hallway and living room. “In reasonably sunny winter weather, it’s enough to make a significant difference to the temperature of my fairly small and well-insulated home,” he says. “It makes a bigger difference in spring and autumn when the house cools down a lot overnight but the weather outside is pleasant.”

He warns that householders should buy big units if they want to collect a lot of heat. “Most systems are not very large, so they may not collect a lot of energy, especially on cloudy days. Also, if they’re single glazed, their efficiency may not be very good in colder weather.”

He says that solar air heaters work better in winter if they’re steeply sloped to face the low sun. “It may also be preferable to angle them slightly east of north, so they warm up the house more in the morning when air temperatures are lower and the building interior is colder.”

Cost and benefit

According to the Solar Thermal Air Heating and Cooling Association, both kinds of systems start from about $2,500 installed, with an average price of $4,500 installed. It estimates that the return on investment will vary from two to ten years, depending on the house and the habits of the householders.

Dr Bob Fuller, a low-temperature solar thermal researcher from Deakin University, is not yet convinced of the effectiveness of the systems. When it comes to heating, “there’s a big mismatch – the resource is low in winter when the demand is high,” he says.

Another problem is that there is no Australian Standard for this technology. “There needs to be some independent modelling and testing,” Fuller says. He maintains that as a first step, householders are best served by improving energy efficiency in other ways. “I would spend the money on conservation – on better insulation, curtains, and shading in summer.”

Certainly, these products should not be considered as a replacement for appropriate solar passive design, either when building from scratch or retrofitting. Semmel, from Smart Roof, says that if sun is pouring through a bank of west-facing windows, a ventilation system can only do so much. “In that situation, stopping heat getting in from the ceiling is not your major concern.”

For heating and cooling, the systems are most useful in a well-designed and insulated building. Where a home has thermal mass, the heavy materials will store the extra warmth and ‘coolth’ provided by the units. In a well-sealed house, the systems not only provide fresh air securely and noiselessly, but also, less of the airflow will escape through gaps.

Both Fuller and Pears note that there is a trade-off between the temperature and the volume of the air coming into the home. That is, the systems take time to heat the air, but if the fan runs too slowly, then not enough will enter the home to make any difference. “In some cases, because of other heat losses or gains, to maintain a temperature you might need enough volume for up to ten air changes per hour,” Fuller says.

Pears says it’s vital that solar air collectors include a high-capacity, variable-speed fan that can adjust flow rates according to the heat of the air in the panel. Likewise, the ducting on both kinds of systems should be good quality, and as short and straight as possible, so as not to reduce airflow.

Some systems, complete with clever thermostats, can adjust and make the tricky decisions for you. Set to function automatically, they will sense the temperatures in the collector, as well as inside and outside the home, and heat or ventilate as required.

But even the best designed systems, says Fuller, will not always be able to provide enough warmth and airflow to maintain the temperature inside the home. So it’s important not to expect miracles. These are supplementary systems – they won’t do the job all by themselves. 

Published in Sanctuary Magazine