Green Energy

Renewable Energy: Geothermal

Not all renewable resources draw their power from the sun. There is also energy available in the latent heat of the earth's core that causes earthquakes and drives volcanic activity. This energy is called geothermal energy.

Geothermal energy has been in use for centuries when natural hot springs were first used for cooking and bathing. Some innovative bathers in ancient Rome realized that the thermal waters welling up in their bathhouses could be used not only for bathing but to heat the bathhouses, as well. Following the lead of their ancestors in the bathhouses, Italians in Tuscany were the first to generate electricity from geothermal water in 1904. 

Geothermal energy can be found in several forms: 

.   in hydrothermal reservoirs of steam or hot water trapped in rock. These reservoirs are concentrated in particular regions as a result of geologic processes.

.   in the heat of the shallow ground. Called "earth energy," this geothermal source  is found everywhere and is the normal temperature of the ground at shallow depths. Earth energy is not "enhanced by" geologic process and therefore is not as hot as other geothermal sources.

.   in the hot dry rock found everywhere between 5-10 miles beneath the earth's surface and at even shallower depths in areas of geologic activity.

.   in magma, molten or partially molten rock, that can reach temperatures of up to 1200 C or 2192 F.  While some magma is found at accessible depths, much of it lies too deep to be reached by current technology.

.   in geopressurized brines - hot, pressurized waters containing dissolved methane that are found 10,000 to 20,000 feet below the surface.

Currently, only hydrothermal reservoirs and earth energy sources supply geothermal energy on a large scale - accessing the other forms is more difficult and will require improvements in the technology for them to be economically viable sources of energy.

Hydrothermal reservoirs are tapped by existing well-drilling and energy-conversion technologies to generate electricity or to produce hot water for direct use. Earth energy is converted for use by geothermal heat pumps.

In order for geothermal energy to be applied usefully, the heat must be conveyed by a "carrier fluid" such as water or gas.  In hydrothermal reservoirs, this fluid is found naturally in the form of groundwater.

A carrier fluid can also be artificially added to create a geothermal system. For example, geothermal heat pumps, that use "earth energy" sources to provide heating and cooling for buildings circulate a water or antifreeze solution through a plastic tube. This solution removes heat from or transfers heat to the ground. The ground water is not used up in any way nor is there any direct contact between the solution and the earth or ground water.

The temperature of the carrier fluid determines how the geothermal energy can be used - the hotter the fluid, the greater the range of possible applications. 

Thermal fluids in the steam phase, at temperatures above 100 degrees Celsius, can be used for water distillation or "industrial-scale" evaporation such as drying timber. Lower temperature thermal heat - at less than 100 Celsius in the form of hot water - can be used to heat homes, power district heating systems, or for smaller scale evaporation processes such as food drying. Typically, for such applications, the geothermal water is actually used to heat water through a heat exchanger and is then injected back into the earth to maintain the system. The heated water then provides the energy for the heating function.

The geothermal heat pumps that use earth energy sources to supply direct heat to homes are the most efficient technology currently available for heating and cooling.  They are actually net producers of energy, delivering 3 to 4 times more energy than they consume. They can reduce the peak generating capacity for residential installations by 1-5 kW and can be used effectively even with a wide range of ground temperatures.

Electricity generation usually requires higher temperature fluids, above 140 degrees Celsius, although in California electricity is currently being generated using geothermal water resources that are as low as 100 degrees Celsius.

Geothermal power plants use wells to draw water from depths of 1 to 3 kilometers and then produce electricity in one of two types of plants.

1) Steam turbine plants release the pressure on the water at the surface of the well in a flash tank where some of the water "flashes" or explosively boils to steam.  The steam then turns a turbine engine which drives a generator to produce electricity. The water that does not boil to steam is injected back into the ground to maintain the pressure of the reservoir.

2) The second type of plant is called a binary plant. Instead of being flashed to steam, the water actually heats a secondary working fluid such as isobutane or isopentane through a heat exchanger. This secondary fluid is then vaporized and sent through a turbine to turn a generator after which it is cooled and condensed into a liquid again. It is then sent back through the heat exchanger to be vaporized again - it is not consumed in the process. The water is injected back into the reservoir to recharge the system.

Because the working fluids vaporize at lower temperatures than water, binary plants can produce electricity from lower temperature geothermal resources. Although binary plants are more expensive to build than steam-turbine plants, they are becoming more common. 

Globally, geothermal power plants supply about 8,000 megawatts of electricity and are operating in several countries, including China, Costa Rica, El Salvador, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, the Philippines, Romania, Russia, Turkey, and the US.

Geothermal energy has many advantages over conventional energy sources. They are located throughout the world and are particularly abundant in many developing countries where there is growing demand for energy services.

Geothermal power plants are extremely reliable and flexible. Whereas coal-based power plants are on line about 75% of the time, hydrothermal electric plants are on line about 97% of the time. And geothermal systems can be installed modularly, increasing power levels incrementally to fit demand. Construction of smaller geothermal plants in the .5 to 10 MW range can take as little as 6 months - larger plants supplying up to 250 MW or more, may take only about 2 years to construct.

Geothermal power plants use only a fraction of the land that is needed for plants that supply power from other energy sources - and that same land can be used simultaneously for other purposes (such as agriculture) with little interference or chance of accidents. In the Imperial Valley of Southern California, which is one of the most productive agricultural areas in the US, 15 individual geothermal plants currently produce 400 MW of electrical power.

Geothermal power has environmental benefits, as well.  Geothermal power plants have very low emissions of the sulfur oxide and nitrogen oxide that cause acid rain and the carbon dioxide that contributes to climate change. 

While geothermal energy may not be able to supply all the world's clean energy needs, the large geothermal systems operating now are certainly considered "world-class" energy resources, and there is potential for significant expansion. 

Even though geothermal energy is technically a finite resource, the typical lifetime for geothermal activity around magmatic centers - from 5,000 years to 1,000,000 years - is so long that it is considered a renewable resource. 

Using a geothermal source for commercial purposes, however, does affect its lifetime. While geothermal reservoirs do recharge naturally at a rate of anywhere from a few to over 1,000 thermal megawatts, in order for it to be economically feasible to use the heat commercially, it must be drawn at a faster rate. However, after a particular system is no longer able to supply heat at temperatures hot enough for electric power, using it for direct-heat applications, which can use lower-temperature heat, can extend its lifetime as a useful source of energy.

While it is difficult initially to know how long a particular system will be productive as an energy source, it is easier to project as a production record is established. It is also possible to gauge how long a given reservoir might be tapped by estimating the thermal resource found in the rocks where most of the heat is concentrated.