Evacuated tube heat collectors, close-up. The tubes are designed to absorb heat from sunlight. Each evacuated (vacuum-containing) glass tube has an absorber plate fused to a heat pipe. A small amount of vaporizable fluid in the heat pipe evaporates and releases energy when it condenses at the upper end of the tube. The condensed liquid returns down the tube under gravity. The heat is then transferred, usually into a domestic heating system. This saves energy and reduces heating costs. It also avoids the use of power-generation methods that produce pollution, such as burning fossil fuels.