Solar water heaters

Increasingly, solar powered water heaters are being used. Their solar collectors are installed outside dwellings, typically on the roof or walls or nearby. Many models are the direct-gain type, consisting of flat panels in which water circulates. Heating water itself directly is inherently more efficient than heating it indirectly via antifreeze and heat exchangers. However with hard water supplies, direct solar heaters may need limescale control.

Another type of solar collector is the evacuated tube. It has a row of glass tubes containing heat conducting rods, typically copper which as heating elements in a circulating loop of antifreeze. The captured heat is transferred into the domestic hot water system via a heat exchanger. Usefully, this design is smaller and more efficient than traditional flat plate collectors, and works well in very cold climates. The evacuated description refers to air having been removed from the glass tubes to create a vacuum. This results in very low heat loss, once the inside coating has absorbed solar radiation. So the antifreeze, if pressurised, can be heated to well over 100C if required. Vacuum tubes can be deployed successfully in homes where suitable roof space is a limiting factor: where there is typically less than 1 sqm of sunny roof per person. Other types of solar collector may use solar concentrator dish or trough mirrors to concentrate sunlight on a collector tube filled with water, brine or other heat transfer fluid.

A storage vessel/container is placed indoors or out. Circulation is ideally zero carbon, caused by either natural convection thermosyphon or by a small solar electric pump. However it can also be low carbon circulation, typically, when higher power mains electric powered pumps are used, for example to cope with viscous antifreeze based circuits in cold climates. At night, or when insufficient sunlight is present, circulation through the panel can be stopped by closing a motorised valve and/or stopping the circulating pump, to keep hot water in the storage tank from cooling. Depending on local climate, freeze protection (e.g. via freeze-tolerant silicone rubber water channels, draining the system down or the use of antifreeze), as well as prevention of overheating, must be addressed in their design, installation, and operation.