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Geothermal energy

posted 14 Feb 2010, 00:48 by Toby Roscoe   [ updated 14 Feb 2010, 00:54 ]

Geothermal energy is an energy source that has the potential to gather ample energy from the Earth’s hot interior for the millennia to come. At the moment it is only hotspots (focussed intrusions of magma) close to the surface that are economic to harvest energy from commercially. Huge capital investment is required, but economic payback is sure to be immense in the coming decades given the life-span of these projects. The other plus side is that geothermal plants have very few ongoing emissions or maintenance.

The first geothermal plant was conceived over a hundred years ago, the theory behind this source of energy is very simple.  Water is pumped into a shaft drilled in the ground, to a point where the rocks are super-heated,  the water filters through the fractured rocks, is heated, and eventually returns to the surface through a second shaft, re-emerging as steam. The steam is then used to drive a turbine as is a conventional power station.  

Two of the more modern types of geothermal power plants are explained below.

Flash Steam Power Plants

Hydrothermal fluids above 182°C (360°F) can be used in flash plants to make electricity. Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator. If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy. A contra-flow heat system is used so that the warmest production steam in the system is exposes to the strongest geothermal heat, and thus maximising the heat transfer (the rate of heat transfer is proportional to the difference in temperatures and the level ofresistance to the flow of heat). 

Most geothermal areas contain moderate-temperature water (below 210°C). Energy is extracted from these fluids in binary-cycle power plants. Hot geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. 

Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines. Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.

Fig.1. Binary cycle plant.