Sometimes, the simplest ideas are the best．For example, to absorb heat from the sun to heat water, you need large, flat, black surfaces．One way to do that is to build those surfaces specially, on the roofs of buildings．But why go to all that trouble when cities are full of black surfaces already, in the form of asphalt （柏油） roads?
Ten years ago, this thought came into the mind of Arian de Bondt, a Dutch engineer．He finally persuaded his boss to follow it up．The result is that their building is now heated in winter and cooled in summer by a system that relies on the surface of the road outside．
