Summer sun can provide winter heat

RELYING on solar heating might sound like a bad idea if you live in a country that sees little daylight for much of the winter. Yet that's exactly what 50 households in Anneburg, Sweden, have been doing for the past two years.

During the summer, water is pumped through rooftop solar heaters to warm it, before being stored in pipes embedded in granite 65 metres below ground. The water is kept hot by the rock, and can then be pumped back up to heat homes in winter.

So far the system has reduced reliance on conventional heating by nearly 25 per cent. This could rise to 70 per cent over the next few years as the storage rocks get hotter, according to an evaluation by a team at Uppsala University. "During the winter season there isn't that much sun, but we can still use solar," says team member Magdalena Lundh.
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