Direct solar energy Solar radiation can be converted into useful energy directly, using various technologies. Absorbed in solar ‘collectors’, it can provide hot water for washing or space heating. Buildings can also be designed with ‘passive solar’ features that enhance the Sun’s contribution to their space heating and lighting requirements. The Sun’s rays can also be concentrated by mirrors to provide high-temperature steam for generating electricity. Solar radiation can also be converted directly into electricity using photovoltaic (PV) panels, normally mounted on the roofs or facades of buildings.
Indirect solar energy Solar radiation can be converted to useful energy indirectly, via the other energy forms it causes. Bioenergy, powered by solar-powered photosynthesis in plants, is an indirect manifestation of solar energy. Solar radiation warms the oceans, adding water vapour to the air. This condenses as rain to feed rivers, into which dams and turbines can be located to extract hydropower from the flowing water.
Sunlight heats the tropics to a greater degree than the polar regions, resulting in massive heat flows towards the poles, carried by currents in the oceans and the atmosphere. The energy in such currents can be harnessed by wind turbines. Where winds blow over long stretches of ocean they create waves, and a variety of wave power devices are being developed to extract their energy.
There are also two other sources of renewable energy that do not depend on solar radiation: tidal energy and geothermal energy.