The modules, in turn, can be combined to create arrays that produce larger amounts of power. These arrays can be sized to meet the power requirements of the particular application. Solar energy can be used to heat both water and air. Solar water heaters use the sun to heat either water or a heat-transfer fluid, such as an antifreeze mixture, in collectors usually mounted on the roof. The hot water, produced directly in the collector or via heat from the transfer fluid, is then stored in a standard insulated water tank.
Some systems use an electric pump to circulate the fluid through the collectors.
These environmentally friendly systems are increasingly cost competitive for providing domestic hot water and for heating swimming pools. Solar space heating offsets building heating loads by either heating recirculated building air or preheating outside or ventilation air. For heating recirculated air, solar energy panels are normally required to receive direct sunlight and provide air temperatures higher than the interior temperature to be effective. The transpired collector, which uses a dark collection surface with perforated metal plates to heat the air just behind the plate surface, is very effective in preheating outside or ventilation air. One additional benefit of the transpired collector is that it can also serve as a component of the building shell.