Geothermal energy refers to the power generated using heat from the Earth's core. It is considered renewable energy because this energy is essentially capturing the nearly infinite heat which is generated by the ongoing process at the molten core of our planet. According to the Geothermal Energy Association, the heat continuously flowing from the core of the Earth is estimated to be equivalent to 42,000 gigawatts (GW) of power (20+ times today's global electricity generation). If harnessed properly, geothermal could become a material contributor to global electricity generation. The Earth's natural heat produces molten rock (magma) which heats/creates reservoirs of superheated fluids (hot water or brine) at some locations within relatively shallow distances of the Earth's surface. Geothermal electricity generation is possible by drilling wells to bring to the surface these superheated fluids or steam to drive turbines.
Geothermal Power Plant Technology
Dry Steam Power Plants
Steam plants are the most cost effective technology when the geothermal resource produces dry steam. In these plants, steam is passed directly through a turbine to generate electricity. They are commonly used at areas such as the Geysers in Northern California.
The Dry Steam technology allows for the steam from a geothermal production well to be fed directly to a steam turbine without a secondary heat exchanger. The turbine then coverts the change in steam pressure to mechanical rotational energy, which is converted to electrical energy by a generator.
Although dry steam geothermal power plants may emit marginal quantities of hydrogen sulphide (H2S), nitric oxide (N2O) and carbon dioxide (CO2), these emissions are much lower per energy unit than those of fossil fuels, and the incorporation of gas mitigation systems to remove these small amounts make geothermal power an emission-free source of electricity. As a result, geothermal power has the potential to help mitigate global warming if widely deployed in place of fossil fuels.
Flash Steam Power Plants
It is more common for very high temperature geothermal fluids (above 350o F) to be produced from a geothermal resource. This high temperature pressurized fluid is passed through a low temperature tank which allows a portion of the flow to "flash" off as steam, which is then directed to a turbine to generate electricity. The remaining spent geothermal fluid is either returned for reinjection or, in some cases, may be used for additional energy generation in either a dual flash cycle or in a "binary bottoming cycle" power unit where a second flash tank is used to separate the fluid at a lower pressure to drive the turbine and produce more power.
Binary-Cycle Power Plants
For lower resource temperatures (300 to 350o F), it is more efficient to transfer heat from the geothermal fluid to a secondary fluid (with a lower boiling point -- typically a hydrocarbon such as isobutane or isopentane) that vaporizes. These vapors will then drive the turbine, which generates electricity. Such plants are called "binary" since a secondary fluid is used in the actual power cycle.