Solar water heating (SWH) also known as solar hot water (SHW) systems consist of various innovative and trusted renewable energy technologies that have been used for many, many years. Solar water heating is very popular in countries such as China, Australia, Turkey, Greece, Israel, Austria and Japan.
There are two types of solar water heating systems:
The ‘close-coupled’ system - where the storage tank is mounted horizontally above the solar collectors on the roof of a building and because the hot water rises naturally in the tank through a thermosiphon flow there is no need for a pump.
And the ‘pump-circulated’ system – where the storage tank is mounted to the ground or floor and is below the solar collectors, a pump circulates water or heat fluid between the collectors and tank.
Solar water heating systems are devised to produce hot water all year round. However, during the winter months there may be insufficient sunlight to produce the required amount of hot water. In this instance a electric or gas booster is used to heat up the water.
The water that is heated by the sun is used in many numerous ways. Although the best known ways are for the provision of a domestic water supply, solar water heating can also be put to work in an industrial setting, i.e. to produce electricity.
Systems suitable for very hot climates can be constructed in a much simpler and cheaper way, and are considered an appropriate technology for such places. The solar water heating market is dominated by India, China, Japan and Europe.
The systems heat water by using solar energy in the following way; a solar collector is fastened to a sun facing wall or roof and fluid is either pumped (active) or driven by natural convection (passive) around it. The collectors are often basic glass mounted insulation boxes with a metallic solar absorber attached to it with copper pipes, these are then painted black. Or they could be made from a set of metallic tubes which are encased by a glass cylinder. In Industrial systems a parabolic mirror concentrates sunlight onto a tube and the heat is stored in a hot water storage tank. The size of the tank needs to be much larger with solar heating systems in order to cater for bad weather. The heat transfer fluid used in the absorber can be the hot water from the storage tank, but more often than not is a fluid consisting anti-freeze and a corrosive inhibitor that passes heat through to the tank via a heat exchanger usually a copper coil tube inserted into the tank.
Another low maintenance solar water heating system is the ‘drain-back’ solution, in this instance no anti-freeze is needed and instead all the piping is angled to allow the water to flow back into the tank. The tank is not pressurised and is exposed to atmospheric pressure. Whenever the pump shuts down, the flow is reversed and the pipes are emptied before freezing can occur.
Residential solar water heating installations fall into two different camps; active and passive. Both methods typically include an alternative energy source (such as an electric heating element or an oil or gas central heating system) that is triggered to kick in if the water stored in the tank falls below a minimum temperature setting ensuring that hot water is always available.
Solar water heating used in conjunction with a back up heat source from a wood stove chimney will maintain a hot water system all year round in countries with a colder climate, without the necessity of fossil burning fuels or electricity.
In cases where solar water heating and central heating systems are used together, the solar heat will be either concentrated in a pre-heating tank that is linked to a central heated storage tank or the lower heating element will be replaced by a solar heat exchanger.
Central heating is mainly required at night and in the winter when solar gain is at its lowest, therefore, solar water heating for showering, bathing and washing is a much better choice because supply and demand is much better matched. In many climates a solar water heating system can provide up to 85% of domestic hot water, whilst in northern European countries, solar combi-systems (combined hot water and home heating systems) are capable of providing up to 25% of central heating energy.