About 4% of total water reserves on our planet are concentrated underground – in the rock mass.
Waters which temperature exceeds 20 оС are called thermal (Greek “therme” – “heat”).
Underground rivers and lakes are heated due to the radioactive processes and chemical reactions proceeding in the Earth's interior. In the areas of volcanic activity at a depth of 500-1000 m there are basins with temperatures up to 150-250 °С; water doesn’t boil due to high pressure. In mountain areas thermal waters often crop up in the form of heat sources which temperature is up to 90 оС.
People learned how to use the heat of Earth's interior in economic purposes. In countries were thermal waters come close to the surface thermal electric power station are constructed. They transform heat energy of underground sources into electrical one.
In Russia the first geothermal station was built in 1966 year on the south of Kamchatka in the valley of Paugetka river near volcanoes named after Koshelev and Kambalniy.
Its capacity was 11 Mw in 1980 year. 11 similar stations with 384 Mw total capacity operate near Landerello, Monte-Amiata and Travele, Italy. Geothermal stations also operate in the USA (in California, in the Valley of big geysers), Island (near lake Mivatn), New Zealand (near Uayrakei), Mexicoa and Japan.
As fuel for geothermal stations is free net cost of produced energy is several times less. There are a few schemes of electrical energy production by geothermal station. Direct scheme: nature steam is destined through a pipe for turbines, connected with electrical generators. Indirect scheme: before the steam gets into the turbines it is cleared from gases damaging the pipes. Mixed scheme: uncleared steam gets into turbines and then the gases that didn’t melt are removed from the water appeared as a result of condensation.
It is the mixed scheme on the base of which Puagetskaya electrical generating station works. Water-steam mixture containing heat amount of 840 kilojoule/kg is output through a bore hole 350 m depth to the surface and is destined for separating device. Here the steam is separated from the water under the pressure of 225 kilopaskal (more than 2 atm) and goes to the turbines through the pipes. The turbines are turning and activate electrical generators. The steam developed in the pipes gets into a mixing condenser where it is cooled and transformed into water. The gases that were produced in this process (nitrogen and oxygen) are removed by a pump.
Hot water (120 °С) is used for heating the communities. Water for steam cooling is flowed by gravity through a pipe line 600 m long from Paugetsky River. In Russia, Bulgaria, Hungary, Georgia, Island, New Zealand, USA, and Japan and in other countries buildings, greenhouses, garden frames, swimming pools. For example Reykjavik, the capital of New Zealand receives the heat only from hot sources.