|Fig. 1: A geothermal facility near Grindavik. (Source: Wikimedia Commons)|
Geothermal energy is clean and sustainable heat from the earth.  The upper ten feet of the earth's surface remains at a nearly constant temperature between 50 degrees and 60 degrees. Geothermal energy is gathered by geothermal heat pumps that tap into this natural resource for energy that can be used for heating and cooling. There are many reservoirs of geothermal energy across the world, including in the United States. In the United States most of the reservoirs of hot water are located in Alaska and Hawaii. These hot water resources are often used directly for heat. Hot dry rock resources are also found all over. There is a very different process for this type of geothermal energy though. Accessing these resources involves injecting cold water down one location, circulating it through hot fractured rock, and drawing off the heated water from another location.
Space heating is by far the most utilized sector of geothermal energy in Iceland (see figure for geothermal energy facility in Iceland). Using geothermal energy for space heating took off in 1930 with the Reykjavik District Heating.  Space heating through geothermal energy has enabled Iceland to import less fossil fuel, as well as allowed a low heating cost. In addition, this renewable energy source for space heating has also benefited the environment.  Space heating is not the only geothermal utilization sector. Electricity generation, swimming pools, melting snow, greenhouses, fish farming, and industrial process heat are additional geothermal utilization sectors (see Fig. 1).  Iceland annually uses 26,700 TJ (7,417 GWh) of Geothermal Energy, which makes up 29% of the total electricity generation in the country. 
Over the 20th century, Iceland went from one of the poorest European countries, highly relying on importing coal as its main source of energy, to becoming a much wealthier country with most of its energy coming from renewable resources (i.e. the majority from geothermal). 
© Justin Stein. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. Gando et al, "Partial Radiogenic Heat Model for Earth Revealed by Geoneutrino Measurements," Nat. Geosci. 4, 647 (2011).
 A. Ragnarsson, Geothermal Development in Iceland 2010-2014," Iceland Geosurvey, 19 Apr 15.
 C. Powicki et al., "Geothermal Power," Electric Power Research Institute, February 2010.