What is Geothermal Energy?

What is Geothermal Energy and where does it come from?

- geothermal energy is contained in reservoirs of steam, hot water, and hot dry rocks underground

- the earth's crust is broken into plates where magma comes close to the earth's surface near the edges

- underground, rocks and water absorb heat from magma

- geothermal energy occasionally rises to the earth's surface in the form of volcanoes, hot springs, and geysers

- plate boundaries in the earth's crust contain some of the most active geothermal resources; the Ring of Fire, located around the rim of the Pacific Ocean, is a hotbed of geothermal activity

- ground water trapped in porous rocks or water running along fractured rock surfaces and faults is heated by magma when it comes close to the surface

- geothermal reservoirs are large areas of hydrothermal resources

- outside of areas with obvious geothermal activity, it is still possible to trap the heat from the earth (See geothermal heat pumps)

Direct use of geothermal energy

Using hot water from springs or reservoirs near the surface as an energy source

District heating

Water is pumped through a heat exchanger where its heat is transferred to clean city water that is then piped to buildings where a second heat exchanger transfers the heat to the building

- the geothermal water is injected back into a reservoir in order to be heated and used once again

- this is a common practice in Reykjavik, Iceland

Geothermal heat pumps

Geothermal heat pumps can be used to heat and cool individual buildings in almost any region in the world because they are not reliant on plate activity, springs or geysers

- also known as GeoExchange Systems and ground-source heat pumps

- the upper 3 meters of the earth's surface is stable at roughly 45-58 deg F, 7-14 deg C

- one of the most common systems is a closed-loop system

- pipes are buried in the ground in either a horizontal or vertical loop field

- water or another liquid circulates through pipes buried in a continuous loop; the water has a heating or cooling effect, depending on the season

Heating - the Earth's heat is transferred through pipes into the water and than transferred again into the building; the Earth is used as a heat source

Cooling - water continually circulating in the pipes "picks up" heat from the building and then transfers it into the Earth, cooling the building; the Earth is used as a heat sink

Geothermal power plants

While some regions of the world have geysers and hot springs that provide geothermal energy close to the surface, drilling deep underground can also tap into the Earth's reserves of geothermal energy, in the form of hot water or hot dry rocks.

Hydrothermal resources

- power plants require hydrothermal resources that are high temperature (300-700 deg F, 149 – 371 deg C)

- wells a few kilometers deep are drilled into the earth and heat or hot water from the geothermal reservoir provides enough force to spin turbine generators to produce electricity

- used geothermal water is returned back into the reservoir by an injection well in order to be reheated, to maintain pressure, and sustain the reservoir

Hot dry rock technology

- Engineers drill deep (a few kilometers) into the earth's crust to tap into its heat

- Drilling equipment is similar to that used for oil, but drilling goes much further

- High-pressure water is pumped down borehole or injection well into a heat zone

- Water travels in cracks in the rock until it is forced out of a second borehole as extremely hot water

- The steam from the extremely hot water drive generators, creating energy

- The cool water is injected back into the ground to be heated up again

Geothermal power plants:

Dry Steam Plants – dry steam reservoirs produce steam but very little water; steam is piped directly from the reservoir and into the plant to turn generator turbines; largest dry steam reservoir is in The Geysers region of California

Flash Steam Plants (the most popular plants)– high-pressure water ranging from 300 to 700 deg F, 149 – 371 deg C deep inside the earth is brought to the surface using production wells and is converted to steam to drive the turbines; the steam cools and condenses to water that is injected back into the ground to be used again

Binary Power Plants –reservoir with temperatures of 250-360 deg F, 121-182 deg C; geothermal water is passed through a "heat exchanger" which transfers heat from geothermal hot water to another liquid, such as isopentane; heat causes the second liquid to steam which drives the turbines

Geothermal Disadvantages

- drilling for geothermal energy can be expensive: a geothermal well costs $7-10 million US

- rocks could lose heat after a few decades of drilling and new wells would have to be installed at new sites

- could potentially trigger earthquakes; in Basel, Switzerland, an earthquake occurred near the drilling site
- technology is relatively new: has trouble attracting large-scale funding

Geothermal Advantages

- mostly emissions and pollution-free

- though initial drilling for power plants may be expensive, operation costs are relatively inexpensive

- has the potential to be the cheapest and most consistent/predictable type of renewable energy

- massive and well-distributed resource

- baseload resource: can produce power 24 hours a day, unlike other renewable energy resources such as solar and wind energy

What is Geothermal Energy?