Heating and – increasingly – cooling buildings is one of the biggest sources of energy demand, making up as much as a third of energy use in some parts of the world. But there are ways to cut the energy required for temperature control dramatically, and one of the simplest heating and cooling technologies is also one of the most familiar.

The fridge is a standard feature of the modern home, but few people realise that the technology upon which it is based can be harnessed to remove the need for most of their heating energy. The underlying principle is that of heat exchange – swapping hot air for cold.