8 Geothermal Energy Uses Fully Explained

Geothermal energy uses are; space heating, water heating, agriculture, industrial processes, infrastructural maintenance, cooling, wastewater treatment, and electricity generation. 

This article discusses the uses of geothermal energy as follows;

-What are the 3 Main Uses of Geothermal Energy?

-Uses of Geothermal Energy

-Conclusion

What are the 3 Main Uses of Geothermal Energy?

The three main uses of geothermal energy are;

1). Electricity-generation

2). Cooling

3). Heating

While there are various direct and indirect applications of geothermal energy, these three main uses represent a broad categorization of all the possible applications.

Electricity-generation using geothermal energy, involves elaborate mechanisms whereby steam produced by geothermal heat (or derived directly from geothermal reservoirs) is used to spin a turbine, which in turn rotates a generator [5].

The architectural and structural configuration of the turbine and its integrated/associated components, may vary widely depending on specific conditions and power needs. These components together make up what is known as a geothermal power plant.

Both geothermal heating and cooling involve the use of geothermal heat pumps [3]. In these heat pumps, water (or any other suitable fluid) is pumped and circulated through a system of conduits, at high temperature.

The basic difference between the mechanism of geothermal heating and cooling in heat pump systems, is the direction of flow of the heated fluid. This is elaborated in the discussion below.

Uses of Geothermal Energy

1). Geothermal Energy for Space Heating

Heating, is currently the most common use of geothermal energy.

It is especially used in buildings, where it often replaces the fossil fuel-powered space heating systems.

There are some advantages of using geothermal energy for space heating operations. One of these is the fact that geothermal energy is renewable [10], and provides an attribute of sustainability when used in such systems, compared to fossil fuels.

Economically, geothermal energy is also a helpful option for space heating. Although the capital cost of geothermal heat pumps is generally higher than that of fossil fuel-driven space heaters, the operational cost of geothermal heat pumps is less.

Geothermal energy also offers flexibility when used for space heating, compared to fossil fuels. This is because geothermal heat pumps can be deployed either centrally or locally, using a fairly simple configuration.

Space heating applications using geothermal energy, may be employed in different scenarios, including commercial, industrial and residential buildings.

The heat provided by geothermal energy is directly derived from the Earth. The primary source of this heat is radioactive decay of isotopes like Uranium in the Earth’s subsurface [2]. Geothermal heating is possible due to heat transfer mechanisms that include conduction and radiation.

The mechanisms mentioned above, enable heat from geothermal fluid to be transmitted to the environment. Geothermal fluid (which is essentially groundwater that is derived from a geothermal reservoir) may possess temperatures of up to 200°C (20°C-150°C on average) [6]. When the geothermal fluid is pumped out from a geothermal reservoir and circulated through a conduit, this heat is exchanged with the surroundings through conduction and radiation.

In general, both conduction and radiation work simultaneously to bring about geothermal heating. Conduction causes the heat to be transmitted to the enclosing material (the circulating pipe or conduit) or to a secondary fluid, while radiation transmits the heat from the enclosing material or secondary fluid, to the surrounding.

As we will see in this article, the mechanism of geothermal space heating using a heat pump, is very similar to the mechanism of geothermal cooling. However, it does not require that the geothermal water is pumped out of the reservoir, but rather, that the heat from the geothermal water is absorbed and transferred effectively.

To simplify our understanding of geothermal space heating, the basic procedure is outlined below;

Step 1: Fluid Circulation

This first stage of operation in a geothermal heat pump, is basically the most important; as it is the stage at which geothermal energy is brought in contact with the fluid in the heat pump.

To make this possible, the heat pump circulates this fluid through pipes (usually configured in loops) that have been passed through the ground, and which are imbedded in the geothermal reservoir. By circulating the fluid, mobilization of geothermal energy (in the form of heat) by the system, is made possible.

Step 2: Heat Absorption

Heat absorption is a fairly spontaneous process.

In order for heat (geothermal energy) from the reservoir to be effectively absorbed at this stage, a fluid with good conductive properties is usually employed. This fluid may be water, ethanol, ethylene glycol, or methanol, among others [4].

Conduction is usually the dominant mechanism by which heat is absorbed from the geothermal reservoir. When the geothermal fluid comes in contact with the walls of the pipes, its heat is absorbed by the fluid which is being circulated in the pipes. The circulating fluid then transports this heat from the underground geothermal reservoir, toward the surface.

Step 3: Heat Transfer

Another suitable term for what occurs at this stage is ‘heat exchange’. This is because it involves the exchange of heat between the geothermal heat pump, and the environment.

Using a heat-exchanger, the geothermal heat pump usually transfers energy in the form of heat, to the surrounding, at this stage. The mechanism of heat exchange and heat transfer which is used in geothermal space heating, is fairly similar to that which is used in other geothermal energy applications.

Step 4: Recirculation

This is the stage which makes the overall process of space heating using geothermal energy, to be sustainable.

After the heat has been transferred to (and exchanged with) the surroundings, the circulating fluid in the heat pump flows back toward the geothermal reservoir. This return-flow enables the fluid to be recharged with geothermal energy, which it absorbs in the form of heat from the geothermal reservoir, thereby repeating the entire process.

2). Water Heating

The principle which makes it possible to heat water using geothermal energy, is the same principle which is used in geothermal space heating. Both applications only differ slightly in the configuration of the geothermal system, and the mode of heat exchange.

Very often, water heating using geothermal energy is carried out in residential buildings, and is facilitated by a central or local geothermal heat pump system.

Three main components make up the geothermal heat pump system which is used for water heating. They are as follows;

-Conduit and Loop framework

This component is important for providing the equipment with which the entire system can access and absorb geothermal energy.

As the name implies, it comprises of a series of interconnected pipes (conduits) which are configured in the form of a loop that is imbedded in the geothermal reservoir at depth. This loop is the geometrical structure that enables the circulating fluid to be recycled after it has absorbed and transferred geothermal energy.

The conduit-and-loop framework is the part of the system which is in contact with the geothermal reservoir, and therefore is often fortified to withstand high temperatures.

-Heat Pump

Although the entire system is often referred to as a ‘Geothermal Heat Pump,’ the heat pump is only a component of the system.

It plays a key role of fluid-mobilization, by ensuring that the conductive fluid in the geothermal system is effectively circulated within the conduit subsystem. In addition to being the mechanism by which geothermal energy is transferred from one point to another in the system; the heat pump serves to guide the operation of the system.

-Heat Circulation Subsystem

The heat-circulation subsystem functions alongside the heat pump, and ensures that the fluid in the conduits is circulated as required.

This is done by controlling the direction in which the fluid is circulated. The direction of fluid circulation in turn determines if the system will function as a cooling or heating infrastructure, at any given time.

3). Geothermal Energy in Agriculture

In agriculture, geothermal energy is used primarily for heating.

However, this heating is carried out in various forms. One of the variants of geothermal heating in the agriculture industry is the drying of food produce and other forms of agricultural biomass.

Recent years have seen a rise in the prevalence of geothermal energy usage in agriculture. It has the advantage of being more reliable than solar energy, which has been used notably as a renewable alternative.

The agricultural uses of geothermal energy range from heating and cooling of greenhouses to open ground heating, soil thermal treatment, water heating and preservation/processing of agricultural produce. Greenhouse heating using geothermal energy is a fairly common practice, whereas drying is a more simplistic and versatile application.

It may be carried out on various scales including small and large-scale applications, and is effective for a wide range of crops including vegetables and grains, as well as animal produce like fish.

Drying helps to preserve food materials, while greenhouse heating enables the growing of various crops under optimal conditions. Geothermal energy can be used to warm irrigation water to be used in cold climatic regions (or under any other requisite condition), and is also useful for water heating in the aquacultural sector, where warm water is needed by some aquatic species for optimum growth and survival. 

4). Geothermal Energy in Industry

Geothermal energy finds application across a wide range of industries.

These include the food industry, where it is used in dehydration systems to facilitate preservation. Other industries where non-extreme heat is required, make use of geothermal energy as well. Examples of such industries include cement, wood-processing, and paper industries [7]. The main role of geothermal energy in these industries is that of drying.

In mining operations, geothermal energy may find application as well. This is especially the case for mines operating in remote, geothermally-active zones. The use of geothermal energy within this context is generally for electricity-generation, although it may be used where high-temperature ore-processing is required.

5). Infrastructure Maintenance

For infrastructure maintenance, geothermal energy is most commonly used to control icing and freezing on roads in winter [9]. This function is important, as it prolongs the life of infrastructure, and prevents accident occurrence.

De-icing using geothermal energy is most feasible and effective in regions with active geothermal reservoirs and abundant geothermal resources.

6). Geothermal Energy for Cooling

Geothermal represents one of the most sustainable, green energy options for cooling applications.

This is because, the temperature of the subsurface is generally not affected by climatic conditions on the surface, and therefore remains fairly stable throughout the year.

To understand the mechanism of geothermal heating, it might be necessary to consider a heat-source; heat-sink relationship which exists between the Earth’s surface and the subsurface. Due to the relative stability of subsurface temperatures, as we approach the Earth’s surface, these temperatures tend to be slightly lower than surface temperatures (which have increased significantly) in summer, and slightly higher than surface temperatures in winter.

The above description implies that the subsurface can function as a heat source in winter, and a heat sink in summer [12].

In order to achieve space cooling (or any other type of cooling) using geothermal energy, the heat pump is still applicable. However, the direction of fluid circulation which is employed for cooling, is usually the reverse of that which is used for heating.

The conductive fluid which flows in the conduit subsystem absorbs heat from (or exchanges heat with) the interior of the building or any other facility, then transfers this heat to the geothermal reservoir in the subsurface, which again absorbs this heat from the conductive fluid, exchanging it for a lower temperature.

A brief outline of the stages involved in geothermal cooling is given below;

Step 1: Heat Exchange (Absorption)

At this stage, heat is exchanged between the system and its surrounding (which usually is the interior of the building).

Heat absorption is facilitated by the conductive fluid which is being circulated in the conduits. Whereas the reverse is usually the case for heating; in cooling operations, the fluid typically absorbs heat from its surrounding.

Step 2: Fluid Circulation and Heat Transmission

This stage directly follows the heat exchange/absorption stage. It is often driven by pressure from the pump components of the system, which mobilize and circulate the fluid through the conduits.

Through fluid circulation, it is ensured that the heat which has been absorbed is transmitted away from the interior of the building/facility, and this is an essential step in the cooling process.

Step 3: Heat Exchange (Discharge)

This stage is possible due to the heat-sink effect which has been mentioned earlier in this article.

As the conductive fluid (having absorbed heat) is circulated in the conduits, into the subsurface, it comes in contact which soil and water which is at a relatively low temperature. This contact results in a second heat-exchange, this time involving the loss or discharge of heat from the fluid to the geothermal reservoir in the subsurface.

Step 4: Recirculation

Recirculation ensures that the cooling process is repetitive, consistent and sustainable.

It is driven by pressure from the pump subsystem, which transmits the fluid through the conduits toward the surface again, where it absorbs heat from its surroundings again.

When compared with the process outlined for geothermal heating (under ‘Water Heating’), it is obvious that the cooling process is simply a reversal of the mechanism of geothermal heating.

In several geothermal systems, a refrigerant is used in place of water, to improve the effectiveness of heat transfer. Also, geothermal heating and cooling systems are often equipped with a mechanism which stops or adjusts fluid circulation when the desired temperature is achieved.

7). Geothermal Energy for Wastewater Treatment

Geothermal energy also finds application in the treatment of sludge and wastewater [1].

Studies have shown that by consistently heating wastewater to about 30°C using geothermal energy, the performance of treatment plants may be increased by 10-15% [11].

There have also been studies which have assessed the economic implications of using geothermal energy to heat wastewater. These studies have generally reveled that while the integration of a geothermal system can constitute 33-50% of the cost of constructing the wastewater treatment plant, over the long-term, geothermal energy is a fairly economical option for optimizing wastewater treatment.

8). Electricity-Generation

While it is not the most common application of geothermal energy, electricity-generation represents the most elaborate approach to the utilization of geothermal energy, so far.

In order to generate electricity from geothermal energy, a steam turbine is usually utilized. The steam is either directly derived from the geothermal reservoir (in cases where abundant geothermal resources are available), or it is produced by bringing water (or any other, suitable conductive fluid) in contact with the geothermal reservoir.

When steam has been provided, it is used to rotate a series of turbines, which in turn rotate a generator to yield electricity.

Because of the significant heat required to vaporize water, geothermal electricity-generation requires the drilling of relatively deep wells of between one (1) and two (2) miles in depth [8]. For this reason, geothermal power plants thrive specifically in areas where geothermal energy can be accessed in fairly large amounts. Such areas include regions of intensive volcanic activity, with natural geysers and hot springs.

Conclusion

The following table presents a summary of the applications of geothermal energy, which have been discussed in this article. They are outlined in terms of the sector of relevance, and the underlying mechanism.

References

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2). Duffield, W. A.; and Sass, J. H. (2003). “Geothermal Energy—Clean Power From the Earth’s Heat.” Available at: https://pubs.usgs.gov/circ/2004/c1249/. (Accessed 12 February 2022).

3). EPA (2021). “Geothermal Heating and Cooling Technologies.” Available at: https://www.epa.gov/rhc/geothermal-heating-and-cooling-technologies. (Accessed 12 February 2022).

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7). Rubio-Maya, C.; Diaz, A. V. M.; Martinez, E. P.; Belman-Flores, J. M. (2015). “Cascade utilization of low and medium enthalpy geothermal resources: A review.” Renewable and Sustainable Energy Reviews 52 (2015) 689–716. Available at: http://dx.doi.org/10.1016/j.rser.2015.07.162. (Accessed 12 February 2022).

8). Shakya, K. (2021). “What Is Geothermal Energy? Types and Advantages. Why should we care about Geothermal Power Plant?” Available at: https://greenesa.com/blog/geothermal-energy-types-uses-advantages. (Accessed 12 February 2022).

9). Tan, Y.; Song, J.; Chen, Z.; Xiao, H. (2021). “Experimental Study on the Influence of Buried Geothermal Pipes on the Temperature Field of Concrete Roads”, Advances in Civil Engineering, vol. 2021, Article ID 9977400. Available at: https://doi.org/10.1155/2021/9977400. (Accessed 12 February 2022).

10). Unwin, J. (2019). “What is geothermal energy?” Available at: https://www.power-technology.com/features/what-is-geothermal-energy/. (Accessed 12 February 2022).

11). Vigrass, L. W.; Viraraghavan, T; Curtis, F. A. (1989). “Geothermal Energy Applications in Wastewater Treatment.” p. 55-72, Journal of Cold Regions Engineering, vol. 3; p. 55-72. Abstract Available at: https://ascelibrary.org/doi/pdf/10.1061/%28ASCE%290887-381X%281989%293%3A2%2855%29. (Accessed 12 February 2022).

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