Geothermal Power Plants Explained

Geothermal energy is a renewable resource produced by the inner core of our planet, and is mainly used to generate electricity in geothermal power plants, but also for residential heating and cooling.

The heat produced by the inner core of the planet heats the underground reservoirs of water, which become this way the geothermal resource used by geothermal power plants to produce energy.

These power plants require a high-temperature geothermal resource (between 225°F to 700°F or 107°C to 371°C), which comes from the underground reservoir under the form of dry steam or very hot water.

To access the geothermal resource, deep wells are drilled into the earth, and using a piping system, the hydrothermal resource is pumped to the surface and used to spin a turbine and produce clean electricity using a generator.

What Are Geothermal Power Plants?

Short History

Geothermal energy was first tested by Prince Piero Ginori Conti that built the first geothermal power generator on 4th of July 1904, in Larderello, Tuscany, Italy.
His generator produced enough energy to power four light bulbs.

Seven years later (1911), Prince Piero managed to build the world’s first commercial geothermal power station in Larderello, Tuscany, Italy.

In the 1920s, experimental geothermal generators were built in Beppu, Japan, and the Geysers, California.

By 1958, Italy was the world’s only producer of geothermal electricity at industrial level.

However, in 1958, New Zealand has become the world’s second major producer of geothermal electricity after commissioning its first flash steam power plant (Wairakei station).

United States built its first geothermal electric power station in 1960, at the Geysers in California.

The binary cycle power station technology was created for the first time in the former Soviet Union in 1967, and was introduced in the United States in 1981.

After the 1970s energy crisis, the regulatory policies allowed the use of much lower temperature resources, which created the opportunity to harvest even more geothermal resources.

Geothermal Power Station in Alaska

This way, in 2006, a binary cycle geothermal power station in Chena Hot Springs, Alaska, managed to produce electricity using a record low fluid temperature of only 135°F (57°C).

Today, we are using different types of geothermal power plants, which can successfully harvest the broad range of geothermal resources available on the planet.

Types of Geothermal Power Plants

After the type of geothermal resource available in the underground reservoir (dry steam, very hot water, or water at lower temperatures), there are three different types of geothermal power plants.

1. Dry Steam Power Plant

This is the first geothermal technology used to turn the underground resource into electricity by Prince Piero Ginori Conti in 1904, in Larderello, Tuscany, Italy.

A Dry Steam Geothermal Power Station uses two pipes inserted into two wells drilled to reach the underground reservoir of hot water and steam.

The wells used by this type of geothermal power plant can be as much as two miles deep.

The pipe used to extract the geothermal resource (production well) from the underground reservoir has a series of holes, which allows the hot steam to enter the pipe and rise to the surface.

Once reaching the surface, the hot dry steam (with a temperature between 356°F and 662°F or 180°C and 350°C) goes through a valve in the pipe and from there is sent to the steam turbine.

The high pressure hot steam is used to rotate the turbine and produce electricity with the help of a generator.

Once the hot steam ended its work (to spin the turbine), goes to the cooling tower (heat exchanger) where is condensed and turned back into water, and from there is sent back into the underground reservoir using the second pipe (injection well), to prolong the life of the geothermal reservoir.

Geothermal power plants using this technology are quite rare today, and they can be found in Italy (Tuscany) and in the United States (California).

2. Flash Steam Power Plant

This type of geothermal power plant also uses two wells (the production well and the injection well) drilled to reach the underground reservoir.

The wells used by this type of geothermal power plant can reach a depth between 1.2 and 1.8 miles or 2 to 3 km.

Two pipes are introduced through these wells, and the pipe used to extract the hot water and steam (production well) has several holes at the end that is sunk in the underground reservoir.

Through these holes, the very hot water and steam (with a temperature exceeding 360°F or 182°C) reaches the surface and goes through a valve (production head) and from there is sent to several pressure vessels.

The pressure in these vessels is reduced, and this makes the hot water entered there to “flash” into vapor (it evaporates in a second because it boils instantly).

A single flash steam power plant flashes the hot water only once, while a double flash steam power plant flashes the hot water several times.

Hot Water and Steam a Great Source of Clean Electricity

The hot steam obtained in the pressure vessels is sent through the high pressure steam line to the turbine to generate electricity while rotating the turbine and the shaft of the generator.

Not all the water in the pressure vessels is flashed, so the remaining water is sent to a second evaporator (pressure vessel) where due to the lower pressure inside is turned into steam (hot vapor).

The hot water remained after this process is sent to a third pressure vessel to be flashed and turned into steam.

The steam produced by the second and third pressure vessels is sent through the medium pressure steam line to the turbine to generate electricity.

The steam used to rotate the turbine and produce electricity is then sent to cooling cells to lower its temperature, and from there reaches the condenser where is turned back into water.

The water produced by the condenser is called geofluid and is stored into a large storage tank.

The heat produced when cooling the steam is sent into the atmosphere, and the water from the storage tank is sent back into the underground reservoir through the injection well.

3. Binary Cycle Power Plant

This type of geothermal power plant uses an underground water reservoir with a lower temperature between 225°F and 360°F or 107°C to 182°C.

We have again two wells that are drilled from the surface down to the underground reservoir and a system of pipes used to extract the hot water and also to inject back the used water into the underground reservoir.

Binary cycle geothermal power plants can use geothermal fluid with a lower temperature because they rely on another fluid (working fluid) to generate steam and spin the turbine.

This way, the hot water pumped from the underground reservoir reaches the production well head located at the surface.

The valve will allow the geofluid (hot water) to be transferred through pipes to a heat exchanger, where it will exchange heat with the working fluid.

Geofluid is Turned into Hot Vapors

The working fluid is a liquid with low boiling point (propane, isobutane or a mixture of ammonia and water), and when is heated by the geofluid (hot water) it is turned into vapors.

The vapor comes out of the heat exchanger and is used to rotate the turbine and produce electricity with the help of a generator.

After being used to spin the turbine, the working fluid vapors are sent to a condenser to become liquid again.

Before being sent to the condenser, the hot vapors are cooled in the cooling tower.

After being turned into liquid form, the working fluid will be used again to produce vapors that will rotate the turbine.

After exchanging heat with the working fluid, the geofluid (hot water) is sent to the injection well head to be injected back into the underground reservoir.

Advantages of Geothermal Power Plants

Geothermal power plants have many advantages over other types of power plants, mostly because the underground resource used to produce energy is clean, renewable and free.

1. Geothermal Power Plants Don’t Use Fuels to Produce Energy

The underground resource used by geothermal power plants for clean electricity generation (hot water and steam) is free because is provided by the natural resources of the planet, consisting of underground water reservoirs that are heated by the inner core of the planet (also called ‘the inner Sun’).

2. Geothermal Power Plants Do Not Pollute the Environment

Compared to thermal power plants (power stations burning fossil fuels or using nuclear fuel for the nuclear fission reaction), geothermal power plants use only hot water and steam provided for free by the underground geothermal reservoirs.

Using the geothermal resource in a closed-loop system, the geothermal power plant does not release any harmful emissions into the atmosphere.

3. Geothermal Power Plants Cover Less Land Compared to other Power Plants

A geothermal power plant uses a long system of pipes only in the underground rock.

However, at the surface, a geothermal power plant will cover a smaller area of land compared to other types of power plants because it uses a system of pipes that only connects different devices (heat exchangers, cooling towers, pressure vessels, condensers, etc.) with the area where the turbine and the generator are located.

Disadvantages of Geothermal Power Plants

There are also a few drawbacks produced by the geothermal technology.

1. The Geothermal Resource if Pretty Rare

Geothermal resources are mostly concentrated today in a number of 24 countries such as: the United States, Philippines, Indonesia, Mexico, New Zealand, Italy, Iceland, Kenya, Japan, Turkey, Costa Rica, El Salvador, Nicaragua, Papua New Guinea, Guatemala, Portugal, Russia, China, Germany, France, Ethiopia, Austria, Australia and Thailand.

Geothermal energy has a share only in the energy mix of these countries.

2. Upfront Costs are Very High

Once installed, a geothermal power plant produces energy for free, but building the power station requires pretty high upfront costs with the infrastructure and the drilling process.

Conclusion

Today, mankind uses the geothermal resource for clean energy generation in only 24 countries.
However, other countries could have geothermal resources that are located much deeper in the underground rock.

When technology will evolve to a new level, our civilization will be able to generate even more clean power using only the geothermal resource available on the planet.