Geothermal energy technology has been around for more than a century and is used extensively in countries such as Iceland, the USA, El Salvador, and New Zealand. Harnessing the heat emanating from the earth’s core, it is considered a clean, continuous, and reliable form of energy, and expanding its use would have great benefits for the clean energy revolution. But can geothermal plants be built anywhere?
Since geothermal plants rely on the heat from the earth’s core, these plants cannot be built anywhere and are restricted to places where this heat source is accessible such as along the edges of tectonic plates and areas with volcanic activity.
This renders geothermal energy the most location-specific energy source used by humans. Despite this, it has huge potential in areas where this energy source is accessible, with a bright and promising future. The section below details some of the more important factors and considerations related to harnessing geothermal energy.
Geothermal energy basics
Before we can get into more detail about how exactly these plants work and are constructed, we first need to understand what geothermal energy is. The nature and mechanics of this form of energy are vital to understanding the limitations and advantages of geothermal energy and its uses.
What exactly is geothermal energy?
Geothermal energy is heat that comes from the earth’s core and is contained in the rocks, fluids and even magma (or molten rock) found deep beneath the earth’s surface. This heat energy in the earth’s core is relatively stable and has three sources.
The first and largest source of the earth’s core energy is an ancient relic of heat leftover from the formation of the earth roughly 4 billion years ago. The chemical reaction responsible for the compacting of minerals to form the earth’s solid core released a whole lot of heat energy, which was trapped in the deep layers of the earth during compaction and has remained there ever since.
The second heat contribution comes from internal friction caused by the rotation of the earth. As the earth rotates, the solid core, fluid outer core and subsequent layers of the earth’s crust all rotate at slightly different speeds, causing friction that generates heat. This heat energy remains trapped in the deep layers of the earth along with its relic heat energy.
Finally, the slow and constant decay of radioactive isotopes found in the earth’s core and crust also releases heat. A recent study has found that the decay of radioactive isotopes uranium-238 and thorium-232 alone contributed 20 trillion watts of energy to the heat the earth radiates. This is more energy than the whole US consumes in an average year.
All three of these heat sources provide the earth with a relatively constant heat source, especially since the escape of heat from the earth’s core is so slow. This ultimately renders geothermal energy sustainable, renewable, and green.
In places where this heat energy has risen to the surface such as on the boundaries of tectonic plates, or near volcanoes and thermal vents – this energy can be harvested with the help of geothermal energy plants and turned into electricity or a heat source for indoor heating.
Types of geothermal plants
While all geothermal plants aim to access and use heat in the form of steam or hot water rising from the depths of the earth, this hot water or steam can be used in different ways which will determine the type of geothermal plant that has to be built.
Depending on the depth that the hot water source is found, it will be at slightly different temperatures which will also influence the type of geothermal plant needed to convert the hot water to energy. All of these are constructed by drilling wells into the earth roughly 1-2 miles deep, to access the heated water or steam. Geothermal plants can be classified into three main groups, with two additional systems after.
1. Dry steam plants
These plants access steam directly from geothermal reservoirs and use it to drive turbines that generate electricity in return. Naturally, these dry steam plants operate at very high temperatures and pressures. This is the oldest method of harvesting geothermal energy and releases small amounts of water vapor, carbon dioxide, and sulfur gas in the process, which some consider as potential pollutants.
In the US there are only two dry steam plants – The Geysers in northern California and the famous geyser “Old Faithful” in Yellowstone National Park in Wyoming.
2. Flash steam plants
Currently, most geothermal plants are flash steam plants and make use of hot water wells found deep inside the earth. This hot water (temperatures higher than 360°F or 182°C) is converted to steam as it rises, and the pressure drops. The steam is then used to drive turbines to produce electricity. The steam is then cooled and recycled as water to be pumped below ground and heated again.
These plants also release small amounts of gasses found in pockets beneath the earth’s crust, which can be considered pollution. It is important to remember though that the amounts of carbon dioxide and hydrogen sulfide released are very small compared to traditional fossil fuel plants.
3. Binary cycle power plants
In these plants, hot water is used to heat a liquid with a lower boiling point than water, which then turns to steam and powers turbines. The water in these underground reservoirs are usually at a lower temperature between 225-360°F (107-182°C), which is why it is used to convert another liquid to steam, rather than trying to convert the water itself to steam by adding additional heat.
Since these plants run a closed-loop system, no carbon dioxide or sulfur gas is emitted, compared to the low levels emitted from flash steam and dry steam plants. These plants are thus the most ecologically conscious of the three.
4. Co-produced resources
The first of the two additional systems, co-produced hot water is often a byproduct of oil and natural gas plants in the US. This hot water is currently being investigated for its use in electricity production and will help lower the carbon footprint of these oil and gas plants. In addition, these co-produced resources can add more clean electricity to the grid with very little initial input, as most of the infrastructure needed is already there
Finally, geothermal energy can also be used for direct heating, without being converted into electricity first. This is done though the use of heat pumps. Countries like Iceland have relied on geothermal heating for a very long time, and currently supply nearly 90% of their heating through this renewable and green source.
Heat pumps are available for use in commercial buildings and residential homes, and simply make use of pipes, containing a liquid, buried beneath the ground. This liquid is pumped into the underground section of the piping system, heated by the earth, and then pumped into the house or building to heat the air or water. When the liquid has cooled again it is naturally cycled back underground to be heated again.
Geothermal plant construction
Planning the construction of a geothermal plant is a detailed and intricate affair. Since there are so many factors to consider, experts in geothermal energy harvesting are usually employed to assess the validity of a geothermal site. They also estimate the production costs, as well as the final energy output of such a plant before construction begins.
Basically, geothermal plant construction entails two main phases – exploration and exploitation.
The exploration phase is primarily concerned with underground hot water or steam reservoir identification as well as an investigation into the feasibility of the site for geothermal exploration. Experts employ volcanological, structural, and petrological methods to survey and explore a proposed area and to determine its feasibility as a geothermal energy source.
Morphological, stratigraphic, and radiometric methods are also used to evaluate the surrounding area for structural soundness, reservoir size, and longevity. Once the long term viability and safety of a source is determined as satisfactory, the following step in the exploratory phase can begin (Source).
This entails the drilling of deep exploratory wells to investigate the potential of a site for geothermal energy production. Extraction of the liquid or steam allows a more accurate estimation of the resource size, the appropriate design of the power plant, and finally the plant construction.
The costs of the exploration phase depend on the geological knowledge of the area. In areas where very little or no geological exploration has been done before, more extensive investigation and analyses are needed before exploratory wells are drilled, which will ultimately increase the initial costs.
This phase basically encompasses the fully operational state of the plant and its maintenance for long term operation. This includes the management of the geothermal fluid from extraction to exploitation in the form of electricity production or heat generation. In the early stages, exploitation also involves optimization of water and steam use and transportation through piping and pumping systems (Source).
Expansion of the plant to increase production is also included in this phase. Most geothermal plants have the ability to be seamlessly up of downgraded and even shut down if needed, which is a great advantage of this energy production system. With the advance of technology, the upgrading of exiting geothermal power plants is a very real possibility.
Advantages and disadvantages of geothermal energy
Although geothermal energy has a few disadvantages, its success comes from the fact that it has many more advantages.
Startup costs for geothermal plants are reported to be quite high, but with a constant yield of energy that is virtually inexhaustible and renewable, these high startup costs can be offset by high returns over a long period of time. Improvements made due to new technology have also dropped construction and operational costs considerably over the last two decades.
Geothermal pumps and pipe systems are also very durable and require very little maintenance. Most of these systems are guaranteed for between 20 and 25 years of operation when newly installed, which reduces overall operational costs.
Emitting as little as one-sixth of the carbon that normal power plants do, it is considered carbon-neutral, and a green energy source. Geothermal heat pumps serving homes and businesses have an added advantage in that the pumps are virtually silent and do not contribute to noise pollution. The use of geothermal energy to heat homes and businesses is thus completely green.
On the downside, startup concerns also include the possibility of drilling causing local earthquakes and surface instability, which need to be accounted for before construction. Despite this, geothermal plants do release small amounts of sulfur into the air in the form of hydrogen sulfide (H2S), which has some environmental concerns. The long term effects of these gas emissions are not yet known.
The limitations of geothermal energy do lie in its restricted availability, resulting in some countries being able to make use of this power while others cannot. This may be countered by new developments such as Enhanced Geothermal Systems (EGS), which we discuss in more detail later in the post.
The cost of geothermal energy
Geothermal plants are expensive to build, but they can run at between 90 and 95% capacity on a permanent basis, generating a lot of electricity which helps offset the initial cost. Running at more than 97% capacity does however increase maintenance costs, thereby decreasing profits.
For consumers, geothermal energy will cost roughly $0.037 per kW-hr. This is low compared to other green energy sources such as wind energy ($0.106/ kW-hr); solar energy ($0.165/kW-hr); hydro energy ($0.039/ kW-hr). It is also very low compared to more conventional energy sources such as coal or nuclear ($0.102 and $0.093/ kW-hr respectively) (Source).
Case studies – countries using geothermal energy
The pioneer – Iceland
Currently, Iceland has the highest geothermal potential in the world, and they have been harnessing its power since the early 1900’s for heating and electricity production. Its largest geothermal power plant and heating system, Reykjavik, is one of the worlds most sophisticated geothermal heating systems.
This plant has employed natural hot water to heat buildings and homes since 1930, and today this plant powers the entire city with electricity and heat. Because of this, they have reduced their dependency on fossil fuels, thereby reducing carbon emissions by up to 110 000 000 tons since the 1940’s. Reykjavik is currently one of the cleanest cities in the world, and its people enjoy a very high standard of living.
The plant works on dual system whereby hot water from lower temperature fields (colder than 150°C at 1000 m depth), is used to directly heat homes without heat exchangers or treatment, through an extensive piping system.
Energy derived from warm temperature fields (warmer than 200°C at 1000 m depth) is used for generating electricity, while some of this warm water can be recycled and reheated to be used for indoor heating purposes (Source).
The potential – Turkey
In terms of geothermal energy potential, Turkey is snapping at the heels of Iceland. It is considered the country with the second most geothermal potential in Europe, and one of the first to use it to directly heat homes and buildings.
Despite this, its electricity production from geothermal energy is relatively low. At the moment only one geothermal plant is installed and operational with a capacity of 20.4 MWe, located in the Denizli–Kizildere geothermal field.
A second geothermal plant is under construction in the Aydin–Germencik field and studies examining the potential of a third plant in the Kutahya–Simav region have found that the high temperatures of the water there is suitable for a binary-cycle electricity generating system, that could generate up to 2.9 MWe energy (Source).
The leader – USA
Currently, the US has 64 functioning geothermal plants that together produce the largest amount of geothermal electricity in the world, accounting for nearly 0.4% or 3.5 gigawatts of the US’s energy needs.
Most of the US’s geothermal energy plants are located in the west and Hawaii, due to the close proximity of geothermal energy to the earth’s surface in these areas. The state of California is the top producer of geothermal derived electricity, while the Geysers dry steam reservoir in Northern California, which has been active since 1960, is the largest known dry steam field in the world (Source).
Future prospects include the upgrading of many of these systems with the use of EGS, to increase the geothermal electricity output in the US to between 10 and 50% in the next century.
The future of geothermal energy
The main objective of new advancements in geothermal energy mining is to increase energy production from these sites in order to produce more electricity and heat. Some speculate that geothermal energy will become a major contributor to the world’s electricity supply by as soon as 2050 (Source).
There is a very real need for this to happen not only because of increasing power demands but mostly because many of the fossil fuel-based power plants in the world will need to be retired due to the environmental stress it has as a result. This will create a drop in energy supply that can potentially be filled with geothermal power.
The future of geothermal energy can be discussed in terms of three main aspects – improved and new technology; the re-use of old oil and gas mines for geothermal prospects and Enhanced Geothermal Systems (EGS).
Improved and new technology
New and improved technology in terms of geothermal energy has two advantages. Firstly, it allows for more efficient extraction of hot water or steam and a more efficient conversion of this heat energy to electricity. Secondly, as this technology improves, the price of running these plants drops, and in turn, so does the user price of geothermal energy (Source).
The technology used in geothermal energy plants can be divided into two categories – improved source location and extraction methods and improved electricity generation.
New developments in the identification of hidden geothermal systems include rapid reconnaissance tools such as satellite-based hyperspectral, thermal infrared, high-resolution panchromatic, and radar sensors, which would make source identification and analyses much faster and more accurate.
Faster and more accurate source identification will allow for the faster erection of geothermal plants, decreasing the turnaround time from exploration to production significantly.
In addition, once geothermal sources have been identified, more dedicated research is needed to improve the rate of penetration when drilling hard rock and to develop advanced slim-hole technologies and large-diameter drilling through ductile, creeping, or swelling formations.
Improvements in drilling can minimize costs and increase output. New-generation geothermal drilling is aimed at minimizing damage while drilling, through the optimization of the interaction between the drill, the rock, the drilling fluid, and the water reservoir (Source).
Re-use of oil and gas mines
Although this is not a new technology, it is definitely a feasible option for increasing geothermal output while minimizing costs in the future. To a certain extent, surveys are already being conducted in some countries, such as the US and Turkey, for the conversion of abandoned oils and gas mines to geothermal plants.
In short, abandoned oil and gas wells can be converted to geothermal plants by simply retrofitting the wells with geothermal wells and sealing off the bottom of the well. This creates a ring-shaped channel that allows the circulating fluid to be heated by the surrounding rocks and then used to generate electricity by steam, similar to a binary cycle geothermal plant.
Finally, the true future of geothermal energy lies in the use of enhanced geothermal systems, as discussed in the section below. Geothermal energy production is severely limited in its current state due to its reliance on the natural occurrence of hot water pockets close to the earth’s surface.
Very deep and hidden geothermal sources can only be accessed through additional drilling and fracking, and if the use of geothermal power is to be expanded, this will be necessary. Even though this technology seems like a vast improvement, the drawbacks of fracking involved in EGS and the long term consequences of these practices should be considered closely.
Geothermal plants, EGS, and fracking
While fracking or hydraulic fracturing has made huge uproars in the press over recent years, it is not the same as harvesting geothermal energy.
Fracking is a method of pumping liquid under high pressure into holes drilled into rocks under the earth’s surface in order to release existing fissures and pockets of natural oil or gas. This gas and oil are then used for various energy supplies that include electricity, petrol, and diesel.
The first and most important disadvantage is that it uses a non-renewable energy source, whose use contributes to the production of greenhouse gasses. It is therefore not a green form of energy. In addition, fracking itself has numerous side effects that include the potential polluting of groundwater, and the triggering of earthquakes.
Even though the process of extraction and the mode of operation is very different, some reports have labeled geothermal energy the new fracking (Source). The reason for this is what is known as Enhanced Geothermal Systems or EGS, which aims to increase energy production from existing and novel geothermal power plants with the use of new technology to locate and access hot water and steam wells deep within the earth’s crust.
In short, where current geothermal energy plants harness existing and naturally formed fissures, EGS will artificially create additional fissures where more water can be pumped in to be heated and used to drive turbines or heat homes. So, although current geothermal energy plants do not use fracking, the future of these plants may entail fracking in the form of EGS. This is supposed to increase the geothermal output of US plants to provide for nearly 10% of the countries energy needs.
In addition, EGS also involves extremely deep drilling to be able to access geothermal energy in places where this energy is currently difficult to reach due to the nature of the rock or the depth of the hot water or steam wells. This can increase green energy production in the world and help countries reduce their carbon footprint (Source) if it can be done reliably and sustainably.
To conclude, current geothermal plants are highly reliant on location, where this energy can only be harvested in hot spots near the edges of tectonic plates, hydrothermal vents, or areas with volcanic activity. Depending on the heat of the geothermal water source, these power plants can either be classified as dry steam, flash steam, or binary cycle power plants.
Although many countries, including Iceland, the US, Mexico, Turkey, and New Zealand are currently using geothermal energy for both electricity production and indoor heating, the technology is underdeveloped and underexploited.
With advances in technology to better locate and assess potential geothermal wells, along with improved drilling systems and EGS, geothermal power is said to become the world’s leader in green energy production in the next 50 years.