9.6: Geothermal Energy

Geothermal energy is heat that originates within the Earth—which includes heat left over from the original formation of the Earth and heat produced by radioactive decay—and is either naturally present at surface or is accessed at depth by drilling. We can harness this energy where there is higher than average heat flow from depth, and this is most common in areas near to active volcanoes. About 50% of all geothermal energy use is for heating buildings and other infrastructure, about 33% for hot pools and spas (Figure \(\PageIndex{1}\)), while only about 17% is used to generate electricity. In many cases the left-over heat from electrical generation facilities is used for district heating or for swimming and bathing facilities.

Figure \(\PageIndex{1}\): The Geothermal Pool at Fludir, Iceland

A geo-exchange system (a.k.a. geothermal heat pump, or ground-source heat pump) is not based on geothermal energy at all. Instead, geo-exchange relies on the relatively constant temperature of the ground at depths between about 1 and 5 m, and that temperature is maintained by energy from the sun. Geo-exchange is used for either heating or cooling, or both.

In the context of generating electricity, geothermal heat is used to boil water or some other fluid to power a turbine. In the case of very hot sources, water piped to surface from depth will spontaneously convert to steam because of the reduction in pressure. If the water from depth is less than about 180° C it won’t spontaneously boil but can be used in a binary cycle system to heat a working fluid, such as pentene or toluene, that will boil at a lower temperature than water.

Figure \(\PageIndex{2}\): The Components of a Typical Binary Cycle Geothermal System

The components of a typical geothermal system are illustrated on Figure \(\PageIndex{2}\). The heat at depth (hundreds to a few thousand meters) is accessed via deep production wells. Water from within that rock (5) is pumped to surface and its heat is used to boil the working fluid to power the turbines (4). The original water is returned to the ground via an injection well (6) along with any additional surface water needed to maintain volume (1). Most geothermal plants have numerous production wells (dozens in some cases). It is common for wells to be cycled on and off to allow for recovery of the heat reservoir around them.

The Krafla power station in northern Iceland (Figure \(\PageIndex{3}\)) is a 60 MW flash steam system (the hot water from depth boils at surface). It is situated in a very active volcanic region on the mid-Atlantic spreading ridge. An eruption episode that lasted from 1975 to 1984 almost led to cancellation of the project while it was under construction.

Figure \(\PageIndex{3}\): The Krafla Geothermal Plant in Iceland

The contribution of geothermal to total electricity production is important in some countries, especially in Iceland, where 30% of electricity is generated by geothermal (with most of the rest coming from hydro), and the Philippines, at 27%, and El Salvador at 25%. In Costa Rica, Kenya and Nicaragua and New Zealand geothermal makes up 14, 11, 10 and 10% of electrical energy.^[1] These are all countries with active volcanoes and hence significantly high heat flow.

In the past 10 years global geothermal energy capacity has grown by an average of 3.75% per year. That is significant growth, although lower than for wind and solar, both of which have grown by an average of 20% per year in this century. But the potential for sustained growth is limited by the limited geothermal resource, and by the high capital cost of geothermal facilities.

Media Attributions

• Figure \(\PageIndex{1}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Modified by Steven Earle, CC BY SA 4.0, based on EGS Diagram by Fishx, 2009, CC-BY-SA-3.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...GS_diagram.svg
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0

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1. International Renewable Energy Agency, https://www.irena.org/ ↵