Geothermal energy is extracted from inside the Earth to heat homes or spin turbines to make electricity. The world's first geothermal electric plant, in Italy, is more than 100 years old; and Iceland gets about 30% of its electricity from geothermal. But, because of limited surface access to geothermal sites, until quite recently, geothermal was not thought to be a viable alternative to carbon-based fuels.
However, recent improvements in geothermal energy extraction, called EGS (for Enhanced Geothermal Systems; also called Hot Dry Rock systems), promise to substantially increase the conribution of geothermal energy to the world's total energy pool. EGS typically reaches 10 km down into hard rock. At a typical site two holes are drilled, and the deep rock between them is hydraulically fractured. Water is pumped down one hole, and steam comes up the other. Drilling at such depths is now possible, but it is expensive: a typical pair of extraction and injection wells, in Nevada, for example, can produce about 4.5 megawatts, and they (the wells only) would cost about $10 million, with a 20% failure rate. However, the total cost, for electrical plant construction and well drilling, typically ranges from $3 million to $7 million dollars per megawatt of electrical generating capacity. So using an average of $5 million/mW, the Nevada plant would probably total about 22.5 million. The technological challenges are to drill holes at low cost and fracture larger volumes of deep rock.
Untapped world-wide potential of geothermal is thought at best to be 2 Terawatts.
Total world energy production rate from carbon-based fuels in 2008 was about
12.9 Terawatts (12.9 x 10^12 watts).
12.9 Terawatts ÷ 2 Terawatts = 6.45
Thus, total energy from carbon-based fuel in 2008 was 6.45 times as much as the total potential energy projected to be available from geothermal. Using these statistics, it would seem that the development of geothermal could make a significant contribution to the replacement of energy produced from the burning of carbon-based fuels.
How many 4,500,000 watt (4.5 mW) geothermal power plants would be required to utilize the 2,000,000,000,000 watts (2 TW) thought to be available from possible geothermal sites every hour here on Earth, assuming that the geothermal power plants are operating at 90% (0.9) efficiency? And how much would it cost for that many geothermal power plants ?
2,000,000,000,000 watts/(4,500,000 watts per plant x 0.9) = 494,000 plants
Each plant costs about $22,500,000 installed.
$22,500,000/plant x 494,000 plants = $11,100,000,000,000
Excuse me? $11.1 TRILLION dollars????
It's a pretty scary number, but just for future comparison purposes, to eventually put this number into perspective, let's calculate the cost per Terawatt:
$11.1 Trillion/2 Terawatts = $5.6 Trillion/Terawatt (remember this number!)
However, as with solar and wind, some locations are better than others (see map left).
Also, an added complication is the necessity to avoid rock with fault lines. Fault lines will "leak" the injected water away from the site, and drilling into fault lines tends to precipitate earthquakes. An earthquake, brought about in just that manner, already happened, Dec 2006, in Basel Switzerland.
In addition, there has -in the past- been some tendency for geothermal extraction sites to pollute the environment. Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3). These pollutants can contribute to global warming, acid rain, and noxious smells if they are released. Existing geothermal power plants emit an average of 122 kilograms (269 lb) of CO2 per megawatt-hour (MW·h) of electricity, which is a small fraction of the emission from coal or gas-fired power plants. In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic chemicals such as mercury, arsenic, boron, and antimony. These chemicals precipitate as the water cools, and can cause environmental damage if released. The modern practice of injecting cooled geothermal fluids back into the Earth to stimulate production has the side benefit of reducing these environmental risks. The new binary-cycle power plants, are closed loop systems: that is, the liquid from deep inside the earth is pumped through a heat-exchanger, and back into the earth again - it never sees the light of day (and neither do its pollutants!). The pressurized liquid in a second loop, is a pure liquid that flashes into steam and spins the turbine to make electricity.
Also, one particular aspect of geothermal energy extraction needs to be singled out for condemnation: the so-called heat pump, which allegedly pumps heat from the earth into one's home or business, but in fact serves only to use electricity from the power company. Such heating systems contain pumps and compressors, which consume electricity that is often produced by the burning of coal or gas. When this electrical source is included, the net emissions of such geothermal heating are ofter comparable to directly burning the fuel for heat.
For example, a geothermal heat pump powered by electricity from a natural gas power plant would produce about as much pollution as a natural gas furnace with the same heating capacity as the heat pump. Therefore the environmental value of a heat pump is highly dependent on the polluting emissions of the electric grid to which is is connected.