SWH systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be sufficient solar heat gain to deliver sufficient hot water. In this case a gas or electric booster is used to heat the water.
In order to heat water using solar energy, a collector, often fastened to a roof or a wall facing the sun, heats working fluid that is either pumped (active system) or driven by natural convection (passive system) through it.
Thermosiphons are used in some liquid-based solar heating systems to heat a liquid such as water. The water is heated passively by solar energy and relies on heat energy being transferred from the sun to a solar collector. The heat from the collector can be transferred to water in two ways: directly where water circulates through the collector, or indirectly where ananti-freeze solution carries the heat from the collector and transfers it to water in the tank via a heat exchanger. Convection allows for the movement of the heated liquid out of the solar collector to be replaced by colder liquid which is in turn heated.
Evacuated tube collectors (ETC) incorporate rows of glass tube collectors. Inside each glass tube is an absorber sitting within a vacuum. The absorbers can be shaped like a long metal fin that can be rotated to optimize sunlight striking it, or as a tube within a tube. Half of all collectors used throughout the world are of the evacuated tube type. China is by far the world’s biggest user of ETCs.
