SOLAR TECHNOLOGIES

There is now quite a wide variety of techniques for capturing solar energy and they can be combined in several different ways. Since the terminology can be confusing, this page summarises the main alternatives and suggests some terms that may serve to distinguish them.

Somewhat confusingly, the expression 'solar thermal' is used to describe solar power plants that create heat but it is also used to describe small-scale solar water heaters. Given this ambiguity, it would probably be best if the expression was dropped. For much the same reason, it would be best if the term 'solar panel' was avoided because it is often not clear whether it means photovoltaic panels or solar water heaters.

Concentrating solar power (CSP)

The general idea is to use mirrors to concentrate sunlight to create heat. But the concentrating technique can be used in conjunction with PV (below).

CSP needs direct sunlight and works best in regions where there is plenty of it. In general, the mirrors of CSP plants track the sun throughout the day.

There are pictures of CSP plants on the pictures page.

The main variants on CSP are described briefly in the following subsections.

CSP (power tower)

Mirrors are arranged to focus sunlight onto a receiver at the top of a tower. Normally, the heat is used to raise steam and drive a turbine in the conventional way.

Solar heat may be stored in melted salt or other substance so that electricity generation may continue through the night or on cloudy days. Waste heat from electricity generation may be used for desalination of sea water.

CSP (parabolic trough)

Trough-shaped mirrors focus sunlight onto a tube containing oil or other liquid. The heated oil is used to raise steam and drive a turbine in the conventional way.

As with the power tower, solar heat may be stored so that electricity generation may continue at night or on cloudy days. Waste heat from electricity generation may be used for desalination of sea water.

CSP (dish/engine)

Dish-shaped mirrors focus sunlight onto a Stirling engine which drives a generator.

CSP (PV, large-scale)

Using the power tower arrangement or dish-shaped (or Fresnel) mirrors, sunlight is concentrated on photovoltaic (PV) panels which generate electricity. This means that a smaller area of the relatively expensive PV panels is needed.

CSP (PV, small-scale)

For use on roofs or walls of buildings, there are now systems available that combine the concentrating technique with PV. As before, this economises on the relatively expensive PV panels.

Some systems use Fresnel lenses to concentrate the sunlight but the Heliotube system uses small-scale trough-shaped mirrors that track the sun.

Photovoltaics (PV)

The classic photovoltaic panel is made from silicon but other substances can also be used. The technology has suffered from high costs and low efficiencies but there are several new developments in the pipeline and the cost for each unit of power is falling steadily.

As mentioned above, it is possible to economise on PV panels by using the concentrating technique.

Solar water heating

Even in temperate climates, water for domestic uses or for swimming pools can be heated very effectively with roof-mounted systems that capture heat from the sun.

An interesting variant on ordinary (non-concentrating) PV is PV-thermal (PVT) which uses water to keep the PV panels cool (which helps to maintain their efficiency) and yields warmed water as a bonus (see also Analysis: Israeli firm taps solar with PVT).

Back to Concentrating Solar Power