The sun is a perennial, silent, free, and nonpolluting source of energy and is responsible for all lifeforms on the planet. Its use for energy generation can be direct or indirect. Indirect solar energy is related primarily to wind power, hydropower, photosynthesis, sea tidal energy, and to the microbiological conversion of organic matter into liquid fuels. Direct solar energy is used to heat water (domestic, industrial, or commercial uses), to cool and air condition, to dry agricultural products, for distillation (mainly for the production of salt or brine by evaporation for seawater), and for electric power generation. Thermal solar energy is most appropriate for areas of the planet that form the solar belt (about 30° to the north or south of the equator), where direct solar radiation is very high throughout the year.
Solar energy is a resource that is not available during the night and is only intermittently available on cloudy or overcast days. The solar thermal power systems collect the thermal energy in solar radiation and use it at high or low temperature. The low temperature applications include water and space heating for commercial and residential buildings. Producing electricity using the steam-turbine-driven electrical generator is a high temperature application. Many solar systems incorporate thermal storage to compensate for the intermittent nature of the solar resource. Thermal energy is normally stored as sensible heat, latent heat, or the heat of chemical reaction.
For solar energy systems, if the insolation is absorbed and utilised without significant mechanical pumping and blowing, the solar system is said to be passive. If the solar heat is collected in a fluid, usually water or air, which is then moved by pumps or fans for use, the solar system is said to be active.