The ABCs of geothermal energy

Geothermal energy is one source of renewable energy that has developed alongside mankind from the Roman times to the 21st century. Today, there is a total installed capacity of over 14,000 MW and a power generation of more than 88,000 GWh worldwide [1]. So how does this millenary energy really work and what are its common uses?

Geothermal energy is essentially thermal energy contained within the Earth’s layers. This is produced due to the slow cooling of primordial heat, which is accumulated heat from the planet’s first years of evolution that has been dissipating over time. The other origin is the decay of long-living radioactive isotopes, which are chemical elements with an unstable nucleus that causes them to dissipate excess energy through radiation. The thermal energy is extracted in the form of heat from a maximum depth of 10 km below the surface using water as a carrier. This is done through the extraction from wells, which makes it an efficient way to use convection as a mediator between the heat and water [2].

The two main uses for the extracted heat are to produce electricity and for heating.

1.       Electricity generation: The steam from thermal fluids (e.g., water) activates a generator and produces electricity. The common systems used for this purpose are:

a.       Dry steam power plants that use geothermal fluids in a vapor state while reaching the surface with a temperature range of 180⁰C-300⁰C [2,3].

b.       Flash power plants that use a single or double flash generator and takes advantage of the pressure drop to change water from liquid to steam when moving it towards the surface at a temperature above 180⁰C2,3. With a single generator, the flash output passes through a separator where the vapor activates the turbine, and the water is injected back into the well3. A double generator however reuses the water in a cycle [3].

c.       Binary power plants that use the heat from water to boil other fluids (mainly organic fluids with low boiling points and high vapor pressure) and vaporizes them to run a turbine [2].

2.       Heating: Geothermal heat pumps are used to transfer heat from one space to another. These devices are more energy-efficient than the conventional air-source-based heat pumps because of the relatively consistent ground temperature[i]. They can be classified into closed-loop and open-loop systems.

a.       Closed-loop system can be further subclassified into 3 types:

1. Vertical systems that are often used when the floor area is limited (e.g., high-rise and commercial buildings) using U-pipes.

2. Horizontal systems that are used where there is sufficient floor area (e.g., residential buildings).

3. Pond systems that utilize water bodies through an underground pipeline that coils into circles under the surface to prevent from freezing.

b.       Open-loop system extracts water from a well or open body source that circulates through the system and returns it to its source afterwards[4].

Apart from the above-mentioned uses, geothermal energy is utilized in thermal baths, soil heating in agriculture, breeding in aquaculture, greenhouse heating, industry applications, among others[2].

Open-loop system (top) and closed-loop system (bottom) Adopted from [2]

Geothermal energy has many versatile uses for every sector regardless of where it is implemented. Its potential can be tapped into anywhere in the world, unlike other renewable resources where location is key for the construction of power plants. Therefore, countries should analyze the potential of this energy source that could not only benefit the economy but also to meet the climate goals.

References:

[1] IRENA. 2021. Statistics Time Series. https://www.irena.org/Statistics/View-Data-by-Topic/Capacity-and-Generation/Statistics-Time-Series

[2] Manzella, Adele. 2017. "Geothermal energy." The European Physical Journal Conferences 1-26

[3] Mulyana, Cukup, Reza Adiprana, Aswad H Saad, H Ridwan, and Fajar Muhammad. 2016. "The Thermodynamic Cycle Models for Geothermal Power Plants by Considering the Working Fluid Characteristic." AIP Conference Proceedings 1712. 1-7.

[4] EERE. 2011. Guide to geothermal heat pumps. U.S. Department of Energy.