How a Solar Water Heater Works

A In many sunny countries, heating water accounts for a large share of the energy a household uses. A solar water heater uses the sun's energy instead of gas or electricity, and a simple design can be built from widely available materials.

B The most visible part is the collector, a flat, shallow box placed on the roof and angled towards the sun. Inside the box, a black metal plate absorbs sunlight and turns it into heat. Thin copper pipes are fixed to this plate, and a sheet of glass covers the top of the box. The glass lets sunlight in but stops much of the heat from escaping, in the same way that a closed car becomes hot on a summer day. The sides and bottom of the box are lined with insulation.

C Cold water enters the bottom of the collector, warms up as it flows through the pipes, and leaves from the top. Because warm water is lighter than cold water, it rises naturally into a storage tank placed above the collector. This movement, known as thermosiphon circulation, means that the simplest systems need no pump at all. Cooler water from the bottom of the tank flows back down to the collector to be heated again.

D The storage tank is wrapped in a thick layer of insulation so that the water stays hot overnight. Hot water is drawn from the top of the tank, where it is warmest, and the tank is refilled with cold water from the mains supply. In cloudy weather, an electric element inside the tank can provide backup heating.

E A well-made system can supply most of a family's hot water for fifteen years or more. The main jobs for owners are to keep the glass clean and to check the pipes for leaks once a year.