The principle of geothermal energy is the conversion of the earth’s thermal energy to electricity and heat. Heat is recovered from water pumped from underground reservoirs located usually 1 to 6 kilometers below the surface. That water is hot because the earth’s temperature rises by 17 to 30 degrees C per kilometer of depth. Examples of geothermal activity are volcanoes and geysers, such as those at Yellowstone Park and in Iceland. Although geothermal has a lower energy density than solar, it is not intermittent and is continuously available.

The source of energy is hot water under pressure or, sometimes, steam from geothermal reservoirs. The heat content of the underground water depends on temperature, pressure, and volume of the reservoir, which are classified according to temperature. High temperature reservoirs provide steam or pressured water at 182 degrees C or higher, whereas medium temperature reservoirs are between 100 and 182 degrees. Low temperature reservoirs provide water below 100 degrees. Geothermal power plants can be supported by deep reservoirs with temperatures of over 120 degrees. Shallow reservoirs with temperatures below 150 degrees are used for space heating and heat pumps.

Narration: How the temperature increases with depth is shown in these geothermal maps. At a depth of 4.5 kilometers very few locations have underground temperatures above 150 degrees. Going to a depth of 5.5 kilometers we see that many more locations fall in that category and could be used for power generation, as indicated by the orange color.

Just as for other renewable resources, maps are available to show the geothermal resources of the country. The dark red colors indicate the highest heat flows available and a stronger potential for geothermal power generation. Clearly, the vast majority of geothermal resources are located in the western part of the country.

This simplified schematic captures the essential parts of a geothermal energy operation. Hot water under pressure is pumped from the underground reservoir and turns into steam when pressure is released at the surface of the well. The steam turns a turbine, which drives a generator to produce electricity. The steam condenses to water in a cooling tower and is recycled through injection back to the underground reservoir.

The thermal power contained in geothermal hot water is described by this equation, where P is thermal power, m is the flow rate of the hot water, and HSL is the enthalpy of saturated liquid water. Enthalpy is a measure of energy content and its values can be found in thermodynamic steam tables.

Using that formula we will calculate the thermal power of geothermal water flowing at the rate of 40 kilograms per second at a power plant where the entry temperature is 95 degrees C and the exit temperature is 55 degrees. From steam tables we find that the enthalpy of saturated water at 95 degrees is 398 kilojoules per kilogram, whereas at 55 degrees it is 230 kilojoules per kilogram. Plugging the numbers into the formula we find that the thermal power of such a geothermal operation is 6.7 MW.

Geothermal energy is renewable and continuously available, therefore it can serve as base load electricity. It is widely available around the world along tectonic plate boundaries and at magma plumes. Large geothermal reservoirs around the world could satisfy global energy requirements, but the economics are not always attractive, although the fuel is free.

Challenges include the long start-up time for geothermal power plants and technical challenges in the process and also with seismic side effects. The high mineral content of the hot water makes it corrosive. Also, it may contain toxic elements, such as gases and heavy metals, and may cause pollution if the used water is not injected back into the ground. Finally, geothermal energy is often far from population centers necessitating costly transmission lines.

There are several kinds of geothermal systems. In convective ones, the reservoirs have enough permeability and porosity to allow water to flow, as indicated by the presence of hot springs and geysers. In enhanced geothermal systems, or EGS, that is not the case, but instead the rock conducts heat, so the rock is fractured and injection wells are built in a fashion similar to oil and gas operations. Conductive sedimentary systems have high-heat flows, but are costly because of the considerable depth.

Coproduction of geothermal energy is also possible at existing oil and gas operations or at depleted oil fields. Geopressure systems are characterized by significantly high pressures, meaning that the kinetic energy of the hot water can also be used. Finally, magma systems aim at recovering the heat contained in magma, but are technically challenging.

The schematics depict two of the most common geothermal systems. On the left is a convective hydrothermal system, where rock permeability allows for the flow of hot water to the surface. On the right is an enhanced geothermal system, where the rock is fractured to access and pump hot water for power and heat generation.

Source: “Renewable Energy Sources and Climate Change Mitigation”, IPCC (2012)

There is an abundance of geothermal energy potential in the US that resides in depths of up to 10 kilometers. EGS is by far the most common form followed by conductive and geopressure. To what extent they can be utilized depends on a techno-economic analysis of each location.

Geothermal power accounts for 0.4% of the US and 0.3% of world electricity generation. The US actually leads the world in installed geothermal capacity and power generation. Growth has been rather slow at an average annual rate of 1.6% between 2000 and 2012. However, much faster growth at a rate of 5.4% is projected for the next few years. It will be second in growth rate only to solar energy.

Given their rich geothermal resources, western states lead the US in capacity, which in 2012 amounted to 3.4 gigawatts. Interestingly, both Alaska and Hawaii are among the top 7 states.

Worldwide, as mentioned earlier, the US leads by far. It is followed by the Philippines at 2 gigawatts, Indonesia at 1.3, and Mexico at 1. Interestingly, small Central American countries rank among the top 10 in world capacity taking advantage of the strong geothermal resources in the region.

In the longer run, geothermal will expand significantly despite the fact that it presently represents just a small fraction of the US renewable power portfolio. By 2040, capacity in the country is expected to triple compared to the present level of 3.4 gigawatts.

As a result of future capacity expansion, geothermal power generation will grow by 5.4% per year, a much faster rate than so far. Hence, through 2040 geothermal will be the second fastest growing form of renewable energy behind only solar, which will grow at 7.5% per year.

There are multiple ways to use geothermal energy. Besides power generation via thermal energy conversion to electricity, there is direct use for space heating and recreational use of the water, district heating for entire neighborhoods and cities, and heat pumps for heating and cooling of residential and commercial facilities.

High- and medium-temperature geothermal reservoirs provide constant flow of hot water that generates power for both base and peak needs. Power generation is conducted following a variety of methods: dry steam, flash, and binary plants. It is also coupled with heat generation in a process called combined heat and power, or CHP. Flash plants are the most popular ones, followed by dry steam.

In the dry steam process, steam from the underground reservoir is directly fed to a turbine, which drives an electric generator. A cooling tower is used to create a vacuum that pulls the steam through the turbine. There are concerns about air pollutants accompanying the steam, such as hydrogen sulfide and heavy metals.

In the flash process hot water at temperatures higher than 182 degrees C is pumped under high pressure from an underground reservoir through a large number of wells. The water evaporates to steam when it reaches the surface of the well, and steam generates electricity. It is the most common method in geothermal power generation.

In the binary process hot water or steam from a reservoir does not go through the turbine. Instead, it runs though a heat exchanger, where it transfers heat to a working fluid, which boils at a lower temperature than water. The advantage of this method is its ability to make use of medium temperature reservoirs as well. In addition, it generates no air emissions.

In addition to power, a combined heat and power, or CHP, process generates heat as well. First, power is generated using hot water under pressure. Then, heat is recovered from the condensate, whose temperature is rather low. That heat is used directly for heating purposes. This process is the same as conventional CHP except it is sustainable as it uses geothermal energy instead of fossil energy.

Besides power generation and CHP, geothermal energy is also used directly for space heating and recreational use, such as swimming pools and spas. The hot water comes under pressure from a hot spring or pumped from a reservoir. Leading this application are China, Sweden, the US, and Iceland. In Iceland, in particular, 90% of homes are heated by geothermal hot water.

In addition to residences, geothermal is also used to heat large residential and commercial areas. This application is called district heating. Hot water is pumped even from low-temperature reservoirs. To avoid using geothermal water as a heating fluid because of its often high content in gases and minerals, freshwater can be used instead as the heating fluid. In that case, heat is transferred from the geothermal to the freshwater in a heat exchanger. Interestingly, the entire city of Reykjavik, the capital of Iceland, is heated by geothermal energy. An example in the US is the downtown part of Boise in Idaho.

Finally, geothermal energy can be recovered and used via heat pumps. Underground temperature is fairly constant, for example 10 degrees C at a 5-meter depth. Hence, the ground can serve as an energy reservoir for heating and cooling. The pump takes heat out of the ground in winter and injects heat into the ground in summer. Over a million heat pumps are used in the US and over 750,000 in Europe. Such heat pumps help conserve energy because they reduce electricity needed for conventional heating and cooling by 25-50%.