Holli

European Geothermal Energy Council

GEOTHERMAL ELECTRICITY AND COMBINED HEAT & POWER

European Geothermal Energy Council

Geothermal Electricity and Combined Heat & Power
Content Introduction Geothermal Power Systems Dry steam and Flash power plants Binary cycle power plants Enhanced Geothermal Systems Geothermal electricity history Geothermal electricity in Europe Geothermal electricityworldwide Combined Heat and Power Page 3 4 5 6 7 9 10 12 13

EGEC | Geothermal Electricity and Combined Heat & Power

Introduction

Geothermal manifestation

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eothermal energy is the heat from the Earth, or, more precisely, that part of the Earth’s heat that can be recovered and exploited by man. Evidence of terrestrial heat is given by volcanoes, hot springs, and other thermalmanifestations. Early mine excavations showed that the Earth’s temperature was increasing with depth, under a gradient of 2-3°C/100m. The total heat flux from the Earth’s interior amounts to ca 80 mWth/m2. It provides us with an abundant, non-polluting, almost infinite source of clean and renewable energy. The heat originates from the Earth core temperature (4,000°C at 6,000 km depth) and theradioactive decay of rocks, long life isotopes of Uranium, Thorium and Potassium. The total heat content of the Earth stands in the order of 12.6 x 1024 MJ, and that of the crust of 5.4 x 1021 MJ, indeed a huge figure when compared to the total world energy demand which amounts to ca 61013 MJ/yr i.e. a 100 million times lower. However, only a fraction of it can be utilised by man. Our utilisation of thisenergy has been limited to areas in which geological conditions allow a fluid (liquid water or steam) to “transfer” the heat from deep hot zones to, or near, the surface, thus giving rise to geothermal resources. The heat outflows from the Earth’s core, melting the rocks and forming the magma. Then, the magma rises toward the Earth’s crust, carrying the heat from below through convective motions. Itmay flow as lava, smoothly or explosively, at

the surface. In some areas it remains below the crust, heating the surrounding rocks and hosted waters. Some of this hot geothermal water migrates upwards, through faults and cracks, reaching the surface as hot springs or geysers, but most of it remains underground, trapped in cracks and porous rocks, forming the geothermal reservoirs. In suchlocations the geothermal heat flow can reach values ten times higher than normal. Under standard conditions 30 to 50°C temperatures would be expected at 1 to 1.5 km depths; in geothermal areas enjoying higher than normal heat flows, temperatures are likely to reach 100 to 150°C at similar depths. In areas close to lithospheric plate margins, geothermal resources would display a wider temperaturerange, from 150°C to very high values, ultimately culminating at 400°C and supercritical fluid state.

Utilization of geothermal fluid

Graph from Geothermal Education Office, California

EGEC | Geothermal Electricity and Combined Heat & Power

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Geothermal Power Systems

World Geothermal Power plant distribution - 2009

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chematically, a geothermal system may be described asconvective water in the Upper Earth crust, transferring heat, in a confined state, from a heat source to a heat sink (usually a free surface). Hence, a geothermal system includes three components, a heat source, a reservoir and a heat carrier fluid. The heat source can be either a magmatic intrusion at very high temperature at relatively shallow depths (5 to 10 km) or simply, in case of low temperaturesystems, hot rocks at depth. The reservoir consists of hot permeable rocks from which circulating fluids extract heat. The reservoir is generally overlain by impervious cap rocks and connected to a superficial outcropping area subject to meteoric recharge, which replaces, at least in part, the fluids abstracted through spring or/and well discharge as depicted in the attached sketch. The geothermal...