3.2.2 STORAGE OF THE COLLECTED HEAT

The majority of solar heating applications require a well insulated hot water storage tank, to store about 1^1/2 days worth of collected solar energy. Storage tanks are classified as pressurized or unpressurized.

Pressurized tanks are commonly available, as all conventional gas and electric water heaters are pressurized. Any water tank connected directly to a well or municipal water system must be built to withstand pressures up to 20 bar. Pressurized tanks are readily available in 20 to 450 litre sizes and are constructed of steel with a glass lining to prevent rusting.

Unpressurized tanks are used to store larger amounts of water in space heating systems. These tanks are built in sizes from 750 to 35,000 litres and larger, and are made from stainless steel, fibreglass or high temperature plastic. Hot liquid from the collectors flows through a coiled tube in the tank to transfer heat to the water.

There are two basic kinds of storage used in solar-heating systems. The first, well-mixed storage is most common with water storage in space-heating systems, while the second one, stratified storage, is virtually mandatory in air-heating systems and often used for domestic hot water systems. Solar domestic water heaters generally use a tank containing an electric immersion heater element, which serves as a back-up heater when solar energy is either not available or insufficient. Water is pumped from the lower portion of the storage tank to the collector or heat exchanger, where it is heated (usually by 1.5-5 °C) and returned to the top of the tank. A dip tube at the top or upper side of the tank feeds the heated water in below the heating element, ensuring proper hot water stratification.