Through the roofing glass
Home heating becomes crystal clear with roof concept
Solar panels are not the only way to capture clean energy from the sun. Glass roofs that draw on technology developed at KTH have emerged as an aesthetically pleasing way to self-generate household heat.
KTH researcher Peter Kjaerboes is one of those behind the concept being marketed by a Swedish company, which enables buildings to capture solar heat through specially-designed glass roof tiles.
With a mix of traditional roofing and solar science, the company offers technology that can be integrated with most central heating systems, including water borne heating via a storage tank.
Identical to the double S-shaped pan tiles commonly used in northern Europe, the roofing is composed of highly transparent glass with low iron oxide content. Laid in between the roof battens is a network of specially developed, liquid-based absorber modules that harvest the sun’s energy.
“When light passes through the glass and hits the fabric, it is converted into heat. The heated air can then either heat the house directly or be transferred to liquid heat,” Kjaerboe says.
The idea is based on an invention called the Light Absorber, the result of a long process of development, that began with Kjaerboes’ degree project at KTH 1976-1977.
“I tried first to build a solar panel that contained thin, lightweight glass tubes instead of heavy glass,” Kjaerboes says. “The pipes were too fragile, so maybe one could use easy-to-manage roof tiles instead?”
The first glass tiles were produced in collaboration with OrreforsKosta Boda AB. Kjaerboe’s research team was assisted by an employee at the plant who assembled crystal waste from the factory floor and poured a couple of roof tiles as a test, all in crystal.
The project resulted in today's glass pan tiles. The system requires no installation of photovoltaic or solar panels. Glass roof tiles have the same function as traditional roof tiles, with the difference that the glass allows light to the substrate.
Kjaerboe points out that the roof of a typical residence annually receives about five times more energy from the sun that the house’s total annual energy consumption.
“The sun is a safe source of energy, and the amount of solar energy varies only by about 10 percent from year to year,” Kjaerboe says.
“Nowadays, most of us who work indoors don’t think of the sunlight as an engine. But the sun drives just about everything that happens on the Earth’s service. Farmers and other who work outdoors understand this.”
One of the issues the researchers and developers in the project had to address was the risk of condensation and moisture. But, on the contrary, Kjaerboe says that the glass tile roof system dries out the house.
“When the sun's heat circulates under glass tiles, it creates a window effect, and any and all moisture evaporates and disappears
quickly,” he says. “Several builders who used the glass tiles have actually pointed out the advantage that the system contributes to a dry indoor environment.”
The glass roof tiles can be made from any ordinary transparent glass, including recycled glass, but SolTech Energy uses a glass that is as ferrous and transparent as possible.
“The glass does not turn green, which would usually be the case with conventional clear glass,” Kjaerboe says.
He says however that homeowners and architects in the future are likely to be able to choose from different shades of coloured glass.
The glass tiles are designed to last well over 40 years, while the life of the heating system itself is at least as long.
The thermal system costs between 80,000 and 100,000. According to estimates, the investment pays off through reduced heating costs between 10 and 20 years.
“The cost becomes lower over time, since the glass is an inexpensive material and recycled glass also works well to cast glass boilers,” he says.
Katarina Ahlfort