Geothermal energy is heat derived within the sub-surface of the earth. Water and/or steam carry the geothermal energy to the Earth’s surface. Depending on its characteristics, geothermal energy can be used for heating and cooling purposes or be harnessed to generate clean electricity. However, for electricity, generation high or medium temperature resources are needed, which are usually located close to tectonically active regions.
This key renewable resource covers a significant share of electricity demand in countries like Iceland, El Salvador, New Zealand, Kenya, and Philippines and more than 90% of heating demand in Iceland.
The main advantages are that it is not depending on weather conditions and has very high capacity factors; for these reasons, geothermal power plants are capable of supplying base load electricity, as well as providing ancillary services for short and long-term flexibility in some cases.
There are different geothermal technologies with distinct levels of maturity. Technologies for direct uses like district heating, geothermal heat pumps, greenhouses, and for other applications are widely used and can be considered mature. The technology for electricity generation from hydrothermal reservoirs with naturally high permeability is also mature and reliable and has been operating since 1913. Many of the power plants in operation today are dry steam plants or flash plants (single, double and triple) harnessing temperatures of more than 180°C. However, medium temperature fields are more and more used for electricity generation or for combined heat and power thanks to the development of binary cycle technology, in which geothermal fluid is used via heat exchangers to heat a process fluid in a closed loop. Additionally, new technologies are being developed like Enhanced Geothermal Systems (EGS), which are in the demonstration stage.
Ocean Technologies, although still at the research and development stage and not yet commercially available, promising ocean technologies include:
Wave Energy, whereby converters capture the energy contained in ocean waves and use it to generate electricity. Converters include oscillating water columns that trap air pockets to drive a turbine; oscillating body converters that use wave motion; and overtopping converters that make use of height differences.
Tidal Energy produced either by tidal-range technologies using a barrage (a dam or other barrier) to harness power between high and low tide; tidal-current or tidal-stream technologies; or hybrid applications.
Salinity Gradient Energy, arising from differing salt concentrations, as occurs where a river empties into an ocean. Demonstration projects use "pressure retarded osmosis", with freshwater flowing through a membrane to increase the pressure in a tank of saltwater; and "reverse electro dialysis" with ions of salt passing through alternating tanks of salt- and freshwater.
Ocean Thermal Energy Conversion (OTEC), which generates power from the temperature difference between warm surface seawater and cold seawater at 800 - 1,000 metres depth.
