what is it?Also known as 'solar thermal', a domestic solar hot water system is one which absorbs the sun’s energy and transfers it to a storage cylinder. It is different from photovoltaics; solar hot water panels do not produce electricity, they heat water directly.
In the UK it will not be the sole provider of hot water; it will complement a conventional system using gas, oil, electricity or solid fuel, but it will pre-heat water so that bills are drastically reduced. During summer months the system can provide all the hot water needed by a household.
Panels can either transfer heat to a separate pre-heat cylinder, or heat a twin-coil cylinder via the bottom coil.
In a direct system, the water that passes through the panels is the water that eventually comes out of the hot tap. In this type of system, there are issues around the water in the panels freezing in winter (so they need to be drained) and lime-scale build-up; in an indirect system, the water in the panels passes through a heat exchanger (coil) in the cylinder and then back to the panels in a continuous loop. Anti-freeze can be added, and there is no problem with lime-scale build-up.
The two main types of collectors are flat-plate and evacuated tube. Flat-plate collectors heat the water directly, evacuated tubes contain a fluid which evaporates at low temperatures, and the resulting gas rises and condenses on a manifold, transferring its heat as it does so.
what are the benefits?
hot water, along with photovoltaics, wind power, hydro, wave and
tidal power and geothermal energy are renewable energy sources which
don’t involve the burning of fossil fuels, and its associated
Solar hot water is probably the most cost-effective renewable energy
technology that you can install in a domestic situation in this
country, with the shortest payback time. A DTI investigation into
solar hot water systems in the UK from 1970-2000 found that a typical
system will provide 72% of a household’s hot water over the
course of a year (c. 15% in winter and 100% in summer). This is
assuming that the roof is south-facing – although if it faces
south-east or south-west there will only be a 5% loss of efficiency.
There are (cheaper) special systems for swimming pools, consisting of a large area of black tubing.
NB: if you're thinking of getting solar hot water at some point in the future, then don't install a combination (combi) boiler - it's not impossible, but it's very difficult and expensive to combine solar hot water with a combi. Condensing boilers are fine.
See here for a detailed, step-by-step guide to getting and using a solar hot water system.
a typical indirect solar hot water system: the gas boiler will kick in if the solar coil in the cylinder doesn’t raise the temperature of the water enough
an entire solar hot water kit, comprising panels, twin-coil cylinder, pump & control set, expansion vessel, air release set, filling bottle, anti-freeze, high-temperature pipe insulation and manual