How Does Geothermal Energy Work? Is It Renewable? Facts, Pros and Cons

Geothermal energy has become popular over the years and you may wonder,⁹ what is it and how does Geothermal energy work?

Simply speaking, geothermal energy is heat obtained from within the earth and it’s one of the renewable energy sources that humans harvest for use.

The following guide explains facts about Geothermal energy including its pros and cons, and answers the question, how does geothermal energy work?

What Is Geothermal Energy?

First, you must answer the question, what is Geothermal energy? This is a type of renewable energy resource that is derived from the core of the earth.

The word geothermal comes from two Greek words ‘therme’ which means heat and ‘geo’ which means earth. This energy resource is derived from the continuous production of heat inside the earth’s core.

You can use geothermal resources with both small scale and large scale units. For example, a utility can use steam and hot water from the reservoir to power a generator and create electricity for its users.

(Image: Jan Van Bizar²⁰)

You can also decide to apply the Geothermal heat directly to various industrial plants, roads, buildings, and agriculture. Some people use the heat from the ground to regulate the temperatures in their homes or other buildings.

How Is Geothermal Energy Created?

Once you know what Geothermal energy is, it’s easy to answer the question; where does Geothermal energy come from? This is heat generating from the earth.

The hottest part of the earth is found about 1,800 miles below the surface or crust of the earth. You can call this part of the earth, the core.

A small part of the heat from the core generated from the gravitational pull and friction formed during the earth’s creation which occurred 4 billion years ago.¹ You should note that this is only a small part.

The vast majority of the heat in the core is created when radioactive isotopes decay. These radioactive isotopes (Nuclear energy) include Thorium-232 and Potassium-40.

If you remember your high school science, you will know that isotopes are types of elements that have different neutron numbers than regular atoms of the same element. For example, in the nucleus of a regular potassium atom, there are 20 neutrons, however, in the nucleus of a Potassium 40 atom, there are 21 neutrons.

When Potassium-40 atoms decay, their nucleus changes and emits high energy amounts in the form of radiation. These atoms will decay to isotopes of argon (Argon-40) and calcium (Calcium-40).

This process explains the continuous radioactive decay in the earth’s core. This is the reason for the core’s high temperatures of 5,000 degrees Celsius or 9,000 degrees Fahrenheit.

This heat generated from the earth’s core constantly radiates outwards heating up water, rocks, gas, as well as other geological material. The temperature of the earth increases with the depth of the crust to the core.

This gradual temperature change is what is called a Geothermal gradient. The Geothermal gradient in most areas in the world is 25 degrees Celsius for one kilometer depth or one degrees Fahrenheit for 77 feet of depth.

The rocks underground are heated at about 1,300 degrees Celsius or 2,400 degrees Fahrenheit, they turn into magma. This magma consists of a mixture of liquid rock and gas bubbles.

It exists in the lower crust and comes to the surface as lava. This magma is what heats up the surrounding geological features creating Geothermal energy.

Where Does Geothermal Heat Come From?

Basically, Geothermal heat is created from magma from the earth core heating up geological features, but how does this apply to the Geothermal heat used by humans? Practically speaking, humans cannot mine the heat from the high temperature magma because the current technology lacks the means to obtain and control it.

(Image: mcfisher²¹)

If so, where does Geothermal heat come from? You should know that the magma heats up underground aquifers and rocks that are nearby.

As a result, hot water is released through:

• Mud pots
• Steam vents
• Hot springs
• Underwater hydrothermal vents¹⁰ and
• Geysers

These five are the real geothermal energy resources. The heat from these sources is either used directly or used as steam in generators to create electricity.

The heat from geothermal energy is used for structures such as sidewalks, parking lots and buildings.

Besides these five sources there is another way to obtain geothermal energy. Most of the energy from the earth’s core does not come to the surface in the form of steam, water, or magma.

Instead, it stays in the mantle and emanates outwards very slowly and causes a collection of high heat pockets.² These dry Geothermal heat pockets are accessible to humans by drilling and can be used when water is injected into them to create steam.

The methods of tapping into geothermal energy are numerous and vary from country to country. Additionally, different parts of the world have different kinds of geothermal energy.

For instance, Iceland has numerous sources of hot underground water that is easily accessible and allows the people there to rely on Geothermal energy.

These energy sources are inexpensive, dependable, and safe for the people of Iceland. Other countries such as the United States must incur drilling costs if they want to obtain geothermal energy.

How Is Geothermal Energy Harvested?

You need three elements to acquire power from geothermal systems. These elements include:

• Heat: There are various heat pockets hidden in rocks with varying geology, depth and geographic location.
• Fluid: Enough fluid to transport this heath from underground to the surface of the earth
• Permeability: Creating small routes to transport the fluid through the hot rocks

Low Temperature Geothermal Energy

In most places worldwide, Geothermal heat is easily accessible and often used immediately as a heat source. This process or heat energy is what is known as Low Temperature Geothermal energy.¹¹

The source of this heat is pockets of heat of about 302 degrees Fahrenheit. These pockets are found just a few feet below ground.

Some major uses of Low-Temperature Geothermal energy include heating:

• Homes
• Greenhouses
• Industrial processes and
• Fisheries

The most efficient use of this energy is heating though it can be used to generate electricity sometimes. Over the years, people have long used this type of energy for cooking, healing, comfort, and engineering.

There is archeological evidence that reveals that some Native Americans used natural occurring hot springs for various purposes about 10,000 years ago.

(Image: Mike Goad²²)

You may know one of the most famous hot spring spas which is found in the famous town of Bath, England. Construction of this spa started in 60 CE when Roman conquerors created an elaborate system of pools and steam rooms using the region’s Low-Temperature Geothermal energy found in shallow pockets.

The first geothermal district heating system in the US opened in Boise, Idaho in 1982 and it currently provides heat to about 500 homes.

Co-Produced Geothermal Energy

This type of technology is dependent on other sources of energy. Co-produced Geothermal energy uses heated water that results from a byproduct of gas and oil wells.

The US provides about 25 billion hot water barrels annually as a byproduct of other energy sources. In the past, the water was considered waste and discarded until it was discovered that people could use it as an energy source.

The steam can generate electricity which can be sold to the grid or used immediately. In the US, among the first co-produced Geothermal energy projects was the Rocky Mountain Oilfield Testing Center found in the state of Wyoming.

The current technology allows the transportation of co-produced Geothermal energy though it’s still at its infancy.

Geothermal Energy: Geothermal Heat Pump

A Geothermal Heat Pump (GHP),¹² also called water source, ground source, earth coupled or GeoExchange heat pump, has been around since the 1940s. Basically, they take advantage of the relatively constant earth temperature as a medium of exchange instead of the air temperature from outside.³

Most regions in the world experience fluctuating air temperatures in different seasons and weathers, from cold winters to scorching summer heat. However, just below the earth’s surface, a few feet down, the ground has a relatively constant temperature.

This temperature depends on the latitude and ranges from 45 degrees Fahrenheit to 75 degrees Fahrenheit.

(Image: U.S. Environmental Protection Agency²³)

GHPs use these favorable temperatures to increase efficiency by exchanging heat with the earth by a ground heat exchanger. GHPs serve the same function as regular heat pumps, they are able to cool, heat, and if so equipped, provide the household with hot water.

Some Geothermal system models have variable fans and two-speed compressors that save energy and promote comfort. Compared to air source heat pumps, they need little maintenance, are quieter and last longer.

Most importantly, they do not depend on outside air temperature.

Dual-Source Heat Pump

This is a combination of a Geothermal heat pump and an air-source heat pump. The appliance has the best of both systems.

They are more efficient than air source units but are less efficient than Geothermal units. Their main advantage is that they have a lesser installation cost than geothermal units and work almost as efficiently.

Even though Geothermal units cost several times higher than air source units to install, they save a lot of energy bringing a much higher return of investment. You can save on energy costs for 5 to 10 years depending on the available incentives and energy costs in your area.

The life of a Geothermal system is about 24 years for the inside components and 50 years for the ground loops. The US averages about 50,000 geothermal installations every year.

What Are the Types of Geothermal Heat Pump Systems?

Ground loop systems come in for basic types. Among the four, three are closed-loop systems they include: Vertical, Horizontal, and Pond/Lake systems. The fourth one is an Open Loop system.

To determine what system is the best in the area you will have to consider several factors such as:

• Climate
• Available land
• Soil conditions
• Local installation costs

All the systems can be used in either commercial or residential building applications.

Closed-Loop Systems

The circulation of almost all Closed-loop Geothermal heat pumps is an antifreeze solution that passes through a closed loop. The solution is made of plastic tubing of high density that is either submerged in water or buried in the ground.

The heat exchanger transports heat between the antifreeze solution in the closed loop and the refrigerant in the heat pump.

(Image: United States Department of Energy¹³)

A direct exchange Closed-loop System works without a heat exchanger, rather it pumps the refrigerant through a copper tubing that you bury on the ground in a vertical or horizontal configuration.⁴ These types of exchange systems need large compressors and are at their most efficient in moist soils.

Sometimes you will need to add additional irrigation to maintain the moist soil.

Direct Exchange Closed-loop Systems should not be placed in soils that can corrode the copper tubing. Additionally, some local environmental regulations may not allow these kinds of systems because they circulate refrigerant through the ground.

• Horizontal: This is the most cost effective installation type for residential settings especially if there’s sufficient land and it’s a new construction.
  It requires at least four feet of dip trenches. The most used layouts include either burying two pipes five feet side by side, or burying two pipes one at six feet and the other at four feet.
  The pipes are buried in a trench two feet wide. Thy pipes should be looped in a method that allows more pipe in a short trench to reduce installation costs and facilitate installation in areas that do not necessarily allow horizontal installation applications.
• Vertical: Vertical systems are often used in schools and other large commercial buildings.
  In this case, the land area required for horizontal loops is not feasible. It’s also best to use vertical loops when the soil cannot be trenched because it is too shallow.
  Vertical loops also do not affect existing landscapes. To create a vertical system, four inch holes are drilled about 100 to 400 feet deep and they are about 20 feet apart.
  A U-bend connects two pipes at the bottom to create a loop and the pipes are grouted to enhance performance. The vertical loops are joined together with horizontal pipes (such as, manifold), put in the trenches then connected to the building’s heat pump.
• Pond/Lake: If there is an adequate water body next to the site then this could be the option with the lowest cost.
  You run a supply line pipe underground from the water to the building. The line is coiled into circles and placed eight feet deep to prevent freezing.
  The water source should meet minimum quality, depth and volume requirements

Open-Loop Systems

This system utilizes the surface or well water body as the heat exchange fluid that directly circulates into the GHP system. After the circulation in the Open-loop System occurs,¹³ the water returns to the ground through surface discharge, a recharge well or the connected well.⁵

This option is feasible only when relatively clean water is in adequate supply. You must also meet all the required regulations and local codes regarding groundwater discharge.

Hybrid Systems

Hybrid systems are a combination of either different geothermal systems or outdoor air (i.e. cooling tower) combined with a geothermal resource. They are the best options if you have more cooling needs than heating needs.

Additionally, it’s more convenient to have a ‘standing column well’ if your local geology permits it. This is a variation of open-loop systems combined with standing column wells.

In this open-loop variation system, you will need to drill one or more vertical wells. The system draws water from a standing column’s bottom and returns into the top.

During peak cooling and heating periods, the system bleeds a portion of the return water instead of reinjecting it all and causes water inflow from the surrounding aquifer to the column. The function of the bleed cycle is to cool the column during heat rejection and heat it during heat extraction reducing the required bore depth.

How Does Geothermal Energy Work?

The US generates the highest Geothermal electricity amount in the world with more than 3.7 gigawatts which is enough to power 2.8 million homes.¹⁴ How does geothermal energy work?

First you need three elements to generate power from Geothermal systems; permeability, fluid, and heat.

(Image: U.S. Environmental Protection Agency²³)

Below the earth’s surface, there are natural Geothermal systems because of the presence of fluid, hot rocks, and permeability. The fluids are conducted through the hot rocks through small underground pathways (i.e., fractures).

To generate geothermal electricity, the fluid is drawn as energy to the earth surface in the form of heat through wells. Once the heat reaches the surface, it’s used to create steam which turns turbines that then produce electricity.

In most cases, the Hydrothermal resources will contain all the three elements. However, there are some areas where these three conditions do not exist naturally.

In these areas, you will find that the rocks are hot but lack sufficient fluid flow and permeability. In this case, you will have to use Enhanced Geothermal Systems (EGS).

EGS are man-made reservoirs that create proper conditions for geothermal systems by injecting fluids on hot rocks and creating permeability. The fractures are reopened enhancing the connectivity and size of fluid pathways.

EGS once created will function the same as natural geothermal systems. That is, the now available fluids will transport energy to the surface through wells and drive the turbines to create electricity.

EGS overcomes natural limitations in the subsurface expanding geothermal energy worldwide.

Geothermal Energy: Geothermal Power Plant

A Geothermal power plant draws fluid from an underground reservoir bringing it to the surface to create steam.^6,15 This steam turns the turbines generating electricity.

(Image: Dietmar Schäffer²⁴)

Geothermal power plant technology has three main types: Binary Cycle, Flash Steam, and Dry Steam. The conversion type, which is a part of the design of the power plant, usually depends on the state and temperature of the subsurface fluid (water or steam).

Dry Steam Power Plant

This kind of plant uses a hydrothermal fluid that is already mostly steam. It’s important to note that naturally occurring steam is quite rare.

The steam is transported directly to the turbine which then drives a generator producing electricity. Once the steam condenses, it is reinjected into the reservoir for the process to occur again.

This Dry Steam power plant is the oldest type of geothermal plant and was first used in 1904, in Lardarello, Italy. These systems are still relevant today and can be found in Northern California at The Geysers which is the largest single geothermal power source world-wide.

Flash Steam Power Plant

These are the most common types of Geothermal power plants that run today. The Flash Steam power plant systems pump fluids at 360 degrees Fahrenheit or greater temperatures, from deep underground.

The fluid travels at a high pressure into a low pressure tank found on the surface of the earth. The pressure change causes some fluid to rapidly transform or ‘flash’ creating vapor.

The vapor is then used to turn the turbines which drive the generator creating electricity. The remaining liquid in the low-pressure tank is flashed again in a second tank to get even more energy.

Binary-Cycle Power Plant

These types of power plants are able to use lower temperature geothermal resources. Binary-cycle power plants are important because they deploy geothermal electricity productions in more locations than the previous two.

This power plant differs from the previous two in that the reservoir fluids in the system do not reach the turbines in the plant. Geothermal fluids that have a temperature of 360 degrees Fahrenheit or lower are driven through a heat exchanger with the ‘binary’ or secondary fluid.⁶

The boiling point of this binary fluid is much lower than water. Therefore, the heat from the Geothermal fluid makes it flash to vapor which then drives the turbines spinning the generator and creating electricity.

Is Geothermal Energy Renewable?

Geothermal energy is a renewable resource because earth retained high heat energy amounts during the planet’s formation. Additionally, the radioactive elements in the core of the earth continue to produce heat.

The heat amount within the earth and the heat lost through natural processes, such as volcanic activity are higher than the heat lost through the production of Geothermal energy.

However, it’s important to note that in each Geothermal field,¹⁶ the reservoir temperature, fluid levels, and fluid pressure will decrease overtime due to constant use. While this fluid can be returned back to the reservoir, if you do not take care, it may reduce the reservoir’s overall temperature.

As a result, it will be necessary to drill additional wells over time in order to keep energy production consistent as temperatures of the fluid and reservoirs decline.

All in all, geothermal energy is a renewable resource created from heat generated from the earth’s core and is available 24/7. As long as the earth is not destroyed, the core will continue to create intense heat amounts that can be used for various purposes such as producing electricity.

(Image: Mark Ingraham²⁵)

Most Geothermal power plants use reservoirs to store hot water that is turned into steam to drive turbines and create electricity. When the reservoir water depletes, the water used in the turbine is reinjected into the pool to enhance a high function system at all times.

Though Geothermal power plants need routine shutdowns for maintenance, they can generate electricity 90% of the time every year. Compared to coal power plants that have 75% availability in the year, it’s easy to see that Geothermal resources are dependable, clean, and renewable.

Is Geothermal Energy Good for the Environment?

There are various ways to extract Geothermal energy, some do not affect the environment at all, while others may have an environmental impact to a degree. Therefore, there’s no one answer to the question; is Geothermal energy good for the environment?

Most of the widely developed Geothermal power plants, also called Hydrothermal plants, are found near geological hot spots. In these regions, molten rocks are close to the crust of the earth and produce hot water.

In other regions, Geothermal systems that are enhanced (i.e. Hot dry rock geothermal), involves drilling on the earth’s crust to reach deeper resources allowing broader access to Geothermal energy.

Additionally, Geothermal power plants employ different technology to convert the resources into electricity (Binary, Flash, Direct steam) and the cooling technology type they use (Air-cooled and Water-cooled).⁷ All these features affect the environmental impact of the geothermal energy resource for example, the conversion and cooling technology will definitely impact the surrounding areas.

Water Quality and Use

Geothermal power plants affect both water consumption and quality. The hot water derived from underground reservoirs is often mixed with high levels of salt, sulfur, heavy metals, and other minerals.

Therefore, releasing this water to the surrounding areas can be quite hazardous.

Fortunately, most Geothermal facilities use closed-loop water systems. In this case, the water extracted from underground is pumped back into the geothermal reservoir after it has served its purpose.

These systems contain the water within steel well casings that are cemented to surrounding rock. In the US there are no reported water contamination cases from geothermal sites.

Geothermal plants also use water for cooling and reinjection. All Geothermal power plants in the US use cooling towers with wet-recirculation technology.

This cooling technology determines how many gallons of water (between 1,700 and 4,000) per megawatt-hour a plant needs to cool down.

These plants use either fresh water or Geothermal fluid for cooling. Fresh water will increase the plant’s overall water impact while using geothermal fluid will not affect the environment.

Many geothermal plants prefer re-injecting the water into the reservoir to reduce contamination and land subsidence.

In some cases, not all water that is extracted from the reservoir is re-injected because some quantity is lost as steam. To maintain the appropriate water volume, some outside water is used.

(Image: gilangmadiantara²⁶)

The size of the plant and technology used determines the amount of water needed. This can have an impact on water sources around the area.

Fortunately, since reservoir water is often not clean, then you do not have to add clean water to the reservoir. For example, the California Geysers Geothermal site injects into its Geothermal reservoir, non-potable treated waste water.

Air Emissions

Air emissions are different for open- and closed-loop systems. In closed-loop systems, the gasses obtained from the well are not released to the atmosphere.

Instead, once they give up their heat, they are injected back into the ground. As such for these systems, air emissions are very little.

In contrast, open-loop systems release the following gasses into the atmosphere:

• Hydrogen sulfide¹⁷
• Carbon dioxide
• Ammonia
• Methane
• Boron

The most common emission is hydrogen sulfide which has a distinct rotten egg smell. This gas once released into the atmosphere turns into sulfur dioxide (SO₂) that contributes to acidic particulates which lead to heart and lung disease once absorbed into the bloodstream.⁸

Sulfur dioxide also contributes to acid rain which acidifies streams and lakes, and damages soils, forests, and crops. Even so, sulfur dioxide emissions from geothermal power plants are 30 times lower per megawatt-hour than emissions released from coal plants.

These coal plants are the largest SO₂ emitters in the US.

Related Reading: Carbon Footprint Calculator: Find YOUR Eco Footprint in Real Time

A few Geothermal sites also produce mercury emissions in small amounts. These emissions must be mitigated using mercury filter technology.

Though scrubbers reduce the air emissions they create water sludge that contains captured minerals such as;

• Nickel
• Mercury
• Arsenic
• Chlorides
• Silica compounds
• Vanadium
• Sulfur and
• Other heavy metals

This sludge is very toxic and must be disposed of at hazardous waste sites.

Land Use

The land amount needed for Geothermal plants varies depending on;

• Properties of resource reservoirs
• Amount of power capacity
• Type of energy conversion system
• Type of cooling system
• Arrangement of piping systems and wells
• Substation an auxiliary building needs

The Geysers in California is the largest geothermal power plant worldwide and has a capacity of 1,517 megawatts.¹⁸ It covers an area of about 80 square kilometers which is about 13 acres per megawatt.

Most Geothermal sites, like the Geysers, are found in sensitive and remote ecological areas. Therefore, if you are a project developer you must take this into account during the planning process.

Land use for Geothermal power plants can cause land subsidence. This is a situation where the land sinks and is sometimes caused by removing water from Geothermal reservoirs.

This issue can be easily addressed by re-injecting water back into the reservoir.

The geological hotspots where most Hydrothermal plants are built tend to have higher earthquake level risks. Hydrothermal plants can cause even greater earthquake frequencies.

This risk can be mitigated by reducing sitting plants an appropriate distance away from fault lines.

Pros and Cons of Geothermal Energy

Geothermal energy is renewable, and reliable. It will be around for a long time to come as long as the core continues to emit heat.

While these are advantages, there are some drawbacks to the practical usage of this type of renewable energy source.

(Image: Dietmar Schäffer²⁷)

However, there are some pros and cons of geothermal energy that you should definitely know about. They include:

Advantages of Geothermal Energy

You can obtain advantages of Geothermal energy whether you use it directly or indirectly.¹⁹ These benefits include:

1. It’s renewable: Geothermal energy is not like fossil fuels that will be used up eventually.
   The earth is currently and will continue to radiate heat for billions of years
2. It’s accessible: People everywhere can access some form or another of geothermal energy.
3. Its clean energy: Geothermal energy is relatively clean as most plans only emit water although some also emit small particles of sulfur dioxide and other harmful chemicals.
4. Geothermal power plants will last for decades and even centuries.
5. Geothermal systems are baseload: This means they can work in winter and summer and do not depend on changing factors like other systems such as solar and wind energy.
6. Takes less space: Compared to other power plants, geothermal electricity takes less space to build.
7. Geothermal power plants are adaptable to different conditions.
8. You can use them to cool, heat, or power individual homes, industrial processes or whole districts.

Disadvantages of Geothermal Energy

There are several disadvantages of renewable energy that should be mitigated before this type of energy source can be truly effective in implementation:

1. Geothermal energy extraction can result in small earthquakes or minor seismic activities.
2. It leads to subsidence or slow sinking of land.
3. It releases small amounts of greenhouse gasses.
4. Water passing through the ground picks up trace amounts of toxic substances such as mercury.
5. High installation costs

Related Reading: Why Is Solar Energy Bad? Solar Panels’ Environmental Impact (Toxic, Dangerous)

To summarize, Geothermal energy works by drawing energy from underground in the form of heat and converting it into electricity by driving turbines which then powers generators.

Geothermal energy is a renewable, dependable, relatively clean energy resource that can help reduce emissions.

Frequently Asked Questions About Geothermal Energy