Geothermal

How does it work?

"Direct use": geothermally heated water from the ground used for cooking and heating. Needs temperatures of 50-300F Saunders, 16
Below the surface of the earth (everywhere, not just at special sites), the temperature is about 50F year round. Heat pumps can be used to heat in winter and cool in summer Saunders, 19

Conventional geothermal: drill a borehole "injection borehole" for injection, inject the fluid, drill other boreholes "production boreholes" to harvest the steam Hug, 4
Geothermal electricity harvested as "dry steam" (just a little bit of water), "flashed", "binary", and "hybrid". Saunders, 24
Flashed is the most common: water below ground is hotter than boiling but pressurized, so flashes (vaporizes) when it reaches the surface. Steam drives a turbine; the steam is condensed and returned below ground Saunders, 24
Binary geothermal is for sites where the water belowground is not boiling hot and therefore doesn't exit the surface as steam. A different fluid with a lower boiling point (e.g. methylbutane) is heated by the geothermally heated water; steam from this secondary fluid is used Saunders, 24
Hybrid is a combination of flashed and binary Saunders, 24
Hot dry-rock: add your own fluid to create steam where there is no natural steam Saunders, 28

3 necessary factors for a geothermal site: heat, fluid, and permeability Chao
Geothermal: requires hot rock and water (so e.g. island volcanic areas). Porous rock sponges up rainfall, volcano or other hot part of earth's crust heats this. Dig a hole to create a vent, then hook up a turbine to get power. Lovins, 75

Iceland uses geothermal for 25% of its electricity and 90% of its heating/hot water needs Lovins, 75
Half of the world's geothermal energy use is electric Saunders, 20
The world's top geothermal electricity producers are the United States, the Philippines, Italy, and Mexico Saunders, 20
The United States produces 1/3 of the world's geothermal electricity, 88% of which is in California Saunders, 20
The largest geothermal electric plant in the world is The Geysers in Sonoma, California, producing 750MW with dry steam Saunders, 23

EGS (enhanced geothermal): find an area with high temperature gradient but low permeability and artificially create a fracture network in the rocks (a "geothermal reservoir") Chao
Lawrence Berkeley National Laboratory + 7 other labs are partnering on a $9 million project to develop enhanced geothermal energy (EGS), particularly improving understanding of underground rock fractures Chao
Geothermal's necessary technology is the drilling of boreholes. This can use the same equipment as existing oil/natural gas drilling Saunders, 28

Lawrence Berkeley National Laboratory is leading studies on EGS Chao
DOE's geothermal project: Frontier Observatory for Research in Geothermal Energy (FORGE) Chao
Stanford Underground Research Facility (SURF) will be the site to study induced fracture networks for use as geothermal reservoirs Chao

OTEC was studied for feasibility by Japan– installations called Mini-OTEC and OTEC 1 were installed in 1979 and 1980, respectively. A 31.5 kW/h installation went into operation in 1981 Charlier, 13
Closed-circuit OTEC was installed at Keahole Point in Hawaii in 1993, produced 50 kWh Charlier, 14
No full-scale utility OTEC has been built or put into service Charlier, 14

One nuclear power plant produces 1GW. Geothermal across the States produces 2GW Lovins
In the continental United States, high geothermal gradients exist in the Western states, particularly Nevada Chao
Capacity of the United States for geothermal is 3.5GW. With EGS, it could theoretically be over 100GW Chao

[aggarwal]: https://www.sciencedirect.com/science/article/pii/S1040619013001917 "Aggarwal, Sonia and Harvey, Hal. 'Rethinking Energy Policy to Deliver a Clean Energy Future.' Energy Innovation, 2013."