SOMETIMES, the simplest ideas are the best. To absorb heat from the sun efficiently—to use it, for example, to heat water—you need large, flat, black surfaces. One way to do that is to construct 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 asphalted roads?

This was the thought that occurred ten years ago to Arian de Bondt, an engineer who works for a Dutch building company called Ooms. Eventually, Dr de Bondt persuaded his employers to follow it up. The result is that their headquarters in Scharwoude is now heated in winter—and also cooled in summer—by a system that relies on the surface of the road outside.

The heat-collector itself is a circuit of connected water pipes. Most of them run from one side of the street to the other, just under the asphalt layer. Some, however, dive deep into the ground.

In summer, when the surface of the street gets hot, water pumped through the pipes picks up this heat and takes it underground through one of the diving pipes. At a depth of about 100 metres lies a natural aquifer into which a series of heat exchangers have been built. The hot water from the street runs through these exchangers, warming the groundwater, before returning to the surface via another pipe. The aquifer is thus used as a heat store.

In winter, the circuit is changed slightly. Water is pumped through the heat exchangers to pick up the heat that was stored during summer. This water goes into the Ooms building and is used to warm the place up. After performing that task, it is pumped under the asphalt and the residual heat it carries helps to keep the road free of snow and ice.

That is not the end of the story. By now the water has been cooled to near freezing point, and it is once again sent underground—this time through a different pipe to a second aquifer. Here, another set of heat exchangers is used to cool the groundwater. This store of cold water is then used in summer to keep the Ooms building cool in exactly the same way that the store of heat is used in winter to keep it warm.

The result is cheap heating in winter and cheap cooling in summer. And there is a bonus. Summer heating softens asphalt. That makes it easier for heavy traffic to damage the road surface. Dr de Bondt's system not only saves on electricity, but also saves the road. Expect to see more examples of it, in other countries, soon.