Solar collector systems

Although solar collectors are less well known than photovoltaic solar cells, they are a very effective method of providing space heating and/or hot water for domestic properties. This is an important factor when considering energy efficient housing as water and space heating are usually the largest single consumer of energy in domestic households. These systems work by directly capturing the sun’s energy and converting it into a more usable form (heat). Most parts of the world have abundant solar energy available (averaging about 1000 watts per square meter in clear conditions).

Solar collector systems consist of the collector itself (usually roof mounted) along with a heat transfer circuit using fluid such as water/glycol, a pump or other method of circulating the the fluid and in some systems a heat exchanger. Heat from the collector is transferred to a reservoir (such as a well insulated hot water cylinder) where the heat can be utilised by providing hot water for heating (using radiators or underfloor heating) and/or hot water for showers and bathing in a domestic environment. Commercial uses include warming swimming pools, heating industrial dryers and various manufacturing processes requiring heat.

There are two main types of solar collector heating systems, thermosyphon systems and pumped systems.

In the thermosyphon system, the storage tank for the fluid has to be placed above the collector as there is no pump in these systems. Water in the collector rises as it is heated naturally circulating around the tank. This system is requires no moving parts or external energy but placing the tank higher than the collectors is not always physically possible.

A pumped system uses a pump to circulate the water, so the tank does not have to be located above the collector. The disadvantage of this system is that it requires external energy to run the pump and an electronics system to control the pump properly.

Both the pumped and thermosyphon type systems can be further divided into three distinct types of collector systems

Flat plate collector - consists of a thin, thermally stable absorber sheet with the tubing containing fluid arranged in a coil or grid pattern underneath encased in a glass or polycarbonate cover.

Parabolic trough concentrating collector - This type of collector uses a trough-shaped parabolic reflector to concentrate sunlight on a pipe or tubing which contains the coolant which is used to transfer the heat. There are large scale versions of these types of systems in use today in the form of solar towers surrounded by parabolic mirror reflectors which track the sun (heliostats) constantly focussing the energy on the tower.

Evacuated tube - similar to the flat plate collector but using multiple vacuum tubes to reduce convection heat losses. This type of collector is especially useful in milder climates where it can reach much higher temperatures than a conventional collector.