Geothermal Energy as a Potential Renewable Energy Source

Geothermal energy resources consist of energy from the Earth’s core stored in both rocks and trapped steam or liquid waters created by the radioactive decay of materials most probably when the planet was formed.

Effectiveness of geothermal energy utilization and its widespread application plays a significant role in mitigating pollution by reducing greenhouse gas emissions. In this study we will discuss geothermal energy as a potential renewable energy source which is replacing existing fossil fuel-fired power and heating plants globally.

Geothermal Energy – A Reliable Renewable Energy Source

Geothermal energy is a type of renewable energy in the form of heat that comes from beneath the Earth's surface. Found sometimes 3 to 10Km deep, it is stored in the form of hot water and steam, or as hot rock, and can be harnessed for various uses, such as electricity generation, heating and even cooling.

Geothermal energy provides long-term base-load energy and helps reduce greenhouse gas emissions. Extracted from the Earth’s interior, geothermal energy supplies heat for direct use and to generate electric energy. In electricity applications, the commercialization and use of integrated or enhanced geothermal systems can contribute to long-term GHG emissions reductions.

With appropriate resource management, heat from an active reservoir is continuously restored by natural heat production, conduction and convection from surrounding hotter regions, and the extracted geothermal fluids are replenished by natural recharge mostly rainwater and by injection of the depleted (cooled) fluids.

Geothermal energy is being used commercially for more than a century. Currently, geothermal energy is used for base-load electric generation in 26 countries where the total installed geothermal power generation capacity by the end of 2023 was 16,335 MW which is only 0.3% of total electricity generation globally.

United States has geothermal power plants in seven states, which are producing about 0.4% of total U.S. utility-scale electricity generation. Similarly, there are geothermal power plants all over the world, and geothermal power generation capacity for top 10 countries account for about 93% of the world's total generation.

Future geothermal deployment could meet more than 3% of global electricity demand and about 5% of the global demand for heat by 2050.

However, more countries are establishing their systems of harnessing geothermal energy due to its potential output. Overall, geothermal technologies are environmentally benefitting because there is no combustion process emitting carbon dioxide (CO2), with the only direct emissions coming from the underground fluids in the reservoir is water.

Where is Geothermal Energy Located?

Geothermal systems occur in a number of geological environments where the temperatures and depths of the reservoirs vary accordingly. Earth’s temperature gradually rises with depth from the surface to the core. This gradual change in temperature is known as the geothermal gradient.

In most parts of the world, the geothermal gradient is about 25° C per 1 kilometer of depth. However, in some particular areas where temperature increases with an average of 25 to 30 centigrade per kilometer, are considered as the most potential sites for harnessing geothermal energy.

The place where underground rock formations are heated to about 700-1,300° Centigrade, they can become magma. Magma is a molten (partly melted) rock permeated by gas and gas bubbles. Magma exists in the mantle and lower crust, and sometimes bubbles to the surface as lava. The Magma then heats nearby rocks and underground aquifers where hot water can be released through geysers, hot springs, steam vents, underwater hydrothermal vents, and mud pots.

These are the primary sources of geothermal energy. This heat can be captured and used directly for heat, or their steam can be used to generate electricity.

However, sometimes most of the Earth’s geothermal energy does not bubble out as magma, water, or steam and remains in the mantle, streaming outwards at a slow pace in the shape of pockets of high heat. This dry geothermal heat can also be accessed through Enhanced Geothermal Systems.

How is Geothermal Energy Collected?

There are different types of geothermal energy available in different parts of the world and different methods are used for tapping into geothermal energy in different countries.

For example in In Iceland, there are abundant sources of easily accessible underground hot water available for people as a safe, dependable, and inexpensive source of energy. Other countries, such as United States have to use drilling methods for harnessing geothermal energy and these methods are relatively costly.

In many countries geothermal energy is currently extracted by drilling wells or other means that produce hot fluids from hydrothermal reservoirs with naturally high permeability. The technology for electricity generation from hydrothermal reservoirs is mature and reliable being used for even 100 years.

In United States, The Geysers Geothermal Complex in California is the world’s largest geothermal power plant with its geysers spread over an area of 45 square miles having a capacity of producing 900 megawatts of electricity for around 1 million homes.

Applications of Geothermal Energy

Currently, direct heating methods using geothermal heat pumps for heating purposes are mainly in practice providing heating and cooling for districts and towns, buildings, fish ponds, greenhouses, household, swimming pools, water purification/ desalination and processing heat for agricultural products and mineral drying.

Electricity Production from Geothermal Energy

Geothermal power plants harness heat below the surface of earth in order to obtain enough energy to generate electricity. In some areas, this heat naturally exists underground as pocket steam or hot waters and in most areas enhanced techniques are used with injected water to create steam. There are currently 3 methods used for producing electricity from this source.

Dry-steam power plants obtain heat directly from natural underground sources, where steam is piped directly to a power plant, where it is used to fuel turbines and generate electricity.

Flash-steam power plants use natural underground hot water and steam where water is pumped into a low-pressure area where it evaporates rapidly into steam, and is funneled out to power a turbine and generate electricity.

Binary cycle power plants use a typical process to preserve water and generate heat. Water is heated underground to about 107°-182° centigrade and is contained in a pipe, which cycles above ground. The hot water heats a liquid organic compound having lower boiling point than water. This organic liquid creates steam, which flows through a turbine and powers a generator to create electricity. The water in the pipe is recycled back to the ground, to be reheated under the earth’s surface and provide heat for the organic compound again.

Advanced Methods of Producing Electricity from Geothermal Energy

Enhanced Geothermal Systems EGS are the most integrated systems currently used. Although the earth has endless amounts of energy and heat beneath its surface, many areas do not have the energy which can be easily harnessed. In such areas an enhanced geothermal system uses drilling, fracturing, and injection processes to provide fluid and permeability in areas which are potentially hot but with dry underground rock.

To develop an EGS, an injection well is drilled vertically into the ground. This can be as shallow as one kilometer as deep as 4.5 kilometers depending on the geological characteristics. Eventually, high-pressure cold water is injected into the drilled space, which forces the rock to create new fractures and sometimes expand the existing fractures. This creates a reservoir of underground fluid.

Water is pumped through the injection well which absorbs the rocks’ heat as it flows through the reservoir. This hot water, called “brine”, is then piped back up to Earth’s surface through a separate well.

The brine is contained in a pipe which warms a secondary fluid having low boiling point, which evaporates to steam and powers a turbine. The brine then cools off, and goes back through the injection well to absorb underground heat again. There are no gaseous emissions besides the water vapor from the evaporated liquid.

Enhanced Geothermal System market is expected to grow due to increasing demand and trends towards shifting to renewable sources. Currently, North America is the most emerging market for EG systems.

Full Stream Geothermal Systems

A new publication from the US Department of Energy, National Renewable Energy Laboratory uncovers the current most integrated state of geothermal energy use in the United States. It provides a future outlook where geothermal power and heat can play a key role in the national transition to a renewable, decarbonized energy system.

In this model, geothermal energy will reach its full potential where a carbon free electric grid of enhanced geothermal systems can be developed at different geographic locations. The system also supports carbon capture and storage facilities to reduce greenhouse gas emissions. Gradually, an intensive fossil fuel based heating can be reduced and building heating and cooling systems can be widely decarbonized.

Geothermal Energy - Future Perspective

Geothermal energy offers several promising advantages for the future, particularly as the world is shifting towards sustainable and low-carbon energy sources. Here are some significant benefits;

Renewable Energy Source: Geothermal energy is essentially abundant, as the Earth’s internal heat is being continuously replenished and thus considered a most potential renewable energy source.

Zero Carbon Emissions: Geothermal power plants produce zero carbon emissions thus contributing to climate safety.

Base-load Energy: Unlike solar or wind power, geothermal energy is available 24/7 making it a most consistent base-load energy source, ensuring a consistent supply of electricity.

Employment: Geothermal energy projects have good social impact by creating jobs in drilling, operations, and maintenance areas.

Availability of Smaller Systems: Integrated technologies are encouraging smaller geothermal plants also expanding the applicability of geothermal energy to a broader scale.

Direct Use Applications: Geothermal heat can be used directly for heating buildings, industrial processes, and greenhouses.

Low Operational and Maintenance Costs: Operational and maintenance costs are relatively low, which can result in long-term cost savings.