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How Does Geothermal Engery Work?

How does geothermal energy work? - Geothermal energy is harnessed in many differnt ways from dry steam passing through turbines to ground source heat pumps.

By, Gavin Whale | Date: 27/06/2017 | Updated: 08/07/17

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What is Geothermal?

geo energy, geothermal power coming out of the ground in the form of geysers

The term geothermal comes from the word "geo" which means from the earth and "thermal" which means heat. So simply geothermal is defined as heat from the earth. Geothermal energy is energy that has been taken from the heat of the earth.

The importance of geothermal energy - As supplies of fossil fuels dwindle, mankind needs to find new renewable sources of energy. The answer to mankind's problem could be right under our feet in the form of geothermal energy. Generally the closer to the core of the Earth you get the hotter it becomes, the core of the Earth is about 4000 miles deep and can reach temperatures of around 4200ºC. This heat melts rock creating magma that heats up rocks and water surrounding it. This heat is geothermal energy and it will last for billions of years making it an ideal renewable energy source.

Geothermal energy sources

molten rock, geothermic energy, geo energy

Deep underground, beneath the Earth's crust there is a layer of magma, which is molten rock. This layer is where geothermal energy is created. There are 3 main sources of geothermal energy;

  1. The geothermal energy that remains from when Earth was formed.

  2. The geothermal energy that is created from the friction between tectonic plates in the Earth's mantle, which move around and rub against each other.

  3. The geothermal energy that is released from the decaying of radioactive elements.

10km underground the amount of geothermal energy amounts to 50,000 times more than all the energy that can be released from all the oil and gas reserves on the planet. So there is huge potential for mankind to harness this energy to meet future demands, especially as fossil fuels become scarce and as we take better care of the environment.

The regions with the highest underground temperatures tend to be at the boundaries between tectonic plates. These areas also tend to have active volcanoes, hot springs and geysers. This is due to the seismic activity of the plates often resulting in earthquakes and magma movement.

Water in these seismically active areas can reach temperatures of over 200 °C due to the geothermal energy heating up the water. A well known example of one of these regions being the "Ring of Fire" located in the Pacific Rim. Famous for chains of volcanoes which are a direct result of the masses of geothermal energy in these areas.

map of the ring of fire

Naturally occurring displays of geothermal energy

Geysers

geyser erupting

Geysers generally occur in volcanic areas, they are a hot spring that occasionally boils. When the geyser erupts it sends a column of steam and water shooting into the sky. The word geyser comes from the Icelandic word to gush.

Half the worlds geysers occur in Yellowstone National Park

The water underground is heated by magma sitting beneath it keeping the water high about boiling point.

When the heat and pressure of the water become unbalanced the water explodes to the surface

Hot Springs

morning glory pool Yellowstone National Park, Wyoming

Hot springs are pools of hot water that are heated by geothermal energy, some of these hop springs are the perfect temperature to bath in. Though be warned, if you come across a steamy naturally occurring hot spring some can be too hot and cause serious burns.

Fumaroles

fumaroles in new zealand

Fumaroles are naturally occurring vents where steam from beneath the surface of the Earth, heated up by geothermal energy, erupts on the surface. They tend to appear along cracks, fissures and lava flows. They can continue to spout steam on the surface for anything between a few weeks of a few hundred years. The longer lasting fumaroles tend to be situated above a constant geothermal energy source such as active magma

How geothermal energy works as a resource

Briefly there are types of rock that contain radioactive substances, when the substances within these rocks decay they produce heat energy, geothermal energy. The geothermal energy is harnessed to heat up water, in turn creating steam. Steam is then used to turn turbines that are connected to generators. Completing the process of converting the geothermal energy into electricity. There are geothermal energy power stations of this nature in existence within California, Iceland and Italy.

At this moment in time the most wide spread method used for capturing geothermal energy is the use of naturally occurring geothermal springs. In these locations due to hydrothermal convection water penetrates into areas of the Earth's crust that have a high temperature due to the geothermal energy and the water and steam once heated rises up to the surface. This can be directly harnessed to drive generators and product electricity. To improve efficiency geothermal energy power stations of this nature normally drill holes of their own for the water and steam to re-emerge to the surface through.

Types of Geothermal energy power stations

geothermal plant, geothermal powerplant

Dry Steam

Hot water and steam is pulled from the ground, directly passes through the turbine then is injected back into the ground.

Used when most of the water erupting from the ground is steam.

Flash Steam

Hot water and steam is pulled from the ground, the hot water is depressurised so that is turns into steam then passes through the turbine then is injected back into the ground.

Used when most of the water erupting from the ground is hot enough to be depressurised into steam. (148-371°C)

Binary Cycle

Hot water and steam is pulled from the ground, it then passes through a heat exchanger where it heats a liquid with a lower boiling temperature in a closed loop so that the substance is not lost. This substance then turns a turbine. With the water used to heat it being injected back into the ground once the heat has been extracted from it. Butane or pentane hydrocarbon tend to be the liquids normally used in these systems, because of their low boiling points.

Used when there is mainly hot water erupting from the ground but it is not hot enough to be depressurised into steam.

Open Loop Systems

The open loop systems such as dry steam and flash steam geothermal energy power stations can emit polluting gasses such as hydrogen sulphide and arsenic within the steam that is released. In areas that have salt water seeping into the ground there can also be considerable amounts of salt that can clog pipes and be released into the surrounding areas.

Closed Loop Systems

Closed loop systems such as binary cycle systems are much better for the environment with no emissions being released as all the water is returned directly back into the ground. Closed loop systems are to play more of a part in future geothermal energy projects.

How does geothermal energy work when used directly

Geothermal energy is used to directly heat buildings and water. Other uses include, greenhouses, drying out food, improve oil recovery, pasteurising milk and hot water spas. Iceland lead the way with directly using the geothermal energy resources, with the majority of the buildings there being heated by geothermal energy. In fact half of all the primary energy used in Iceland is from geothermal sources, it is also used for hot tap water.

Ground-source heat pumps

Geothermal heating and cooling - Geothermal energy is often tapped into to directly heat and cool buildings. A meter or so underground the temperature is constant all year round at around 10°C. At this depth air or liquid is pumped through pipes around the building that is being heated or cooled then through the building itself and back out again. This heats the building in winter and cools it during the summer. Sometimes to increase the effectiveness of geothermal energy ground-source heating systems a number of compresses and pumps are used so heat transfer is improved.

These geothermal energy systems are most effective in areas with extreme temperatures, they are an environmentally clean way of heating and cooling buildings and can be more effective than electrical heating and cooling systems. In these areas they can save homeowners £100s a year and with the savings will pay for their own instalments in around 10 years.

Advantages and disadvantages of geothermal energy

Advantages of geothermal energy

  • Renewable Energy Source

  • No Fuel Costs

  • No polluting gas emissions (When using closed loop systems)

  • High potential capacity

  • Consistent output, low energy fluctuations

  • Small land footprint

  • Abundant supply

  • Effective for cost effective Heating and Cooling of buildings

Disadvantages of geothermal energy

  • Potentially releases toxic and greenhouse gasses

  • Can make the surface instable and potentially trigger earthquakes

  • High cost electricity

  • High initial cost for heating and cooling systems

  • If using a natural water source it can run out

  • There are few locations where geothermal energy in its current form can be harnessed

This disadvantage is only true at this moment in time. Geothermal energy can be harnessed anywhere on the Earth's surface by drilling deep  enough though this energy is milder in non seismically active areas. This geothermal energy can still be used to directly heat buildings, but there is not enough geothermal energy for electricity production.

However work is currently on going with a new technology called Enhanced Geothermal Systems, this new technology is being created to capture the geothermal energy that exists at depths of between 4-10km. The Enhanced Geothermal Systems aim to allow us to exploit this geothermal energy at these depths to produce electricity and in 2013 this was successfully demonstrated in Australia and the United States .

Once the Enhanced Geothermal Systems have improved a study has found that the hot dry rock areas that can support them would allow for a capacity of over 4 Million Megawatts which is more than the entire of the United States currently consumes. Unlike other forms of renewable energy sources this geothermal energy method of getting clean electricity can provide a consistent steady stream of reliable electricity, where as wind power relies on the wind and solar on the cloud coverage.

The future for geothermal energy

energy in the form of electricity

Geothermal energy has lots of potential and will most likely play a big part in the future of electrical energy production. As humanity strides to produce more electricity to meet increasing demand, as well as to create this electricity in a clean way with minimal damage to the environment. Geothermal energy provides this renewable energy solution. Geothermal energy power stations are one of the few renewable energy sources that can supply a constant rate of electricity and binary cycle systems even have flexible output which can be changed anywhere from 10% to 100% output, this means that they can work in harmony with other renewable energy sources that have variable output.

The cost of producing electricity by geothermal energy power stations is lowering as technology advances. For the new geothermal energy power stations planned to be completed and operational in 2019 the cost of electricity produced will forecast to be cheaper than that produced at natural gas and coal power stations.

Geothermal energy has two technologies currently under development that have yet to emerge. Enhanced geothermal systems and the production of geothermal energy electricity alongside oil and gas wells.

Enhanced geothermal systems

There is geothermal energy that can potentially be harnessed everywhere on Earth, currently the only places that are being exploited are the areas here water circulates back up to the surface. These locations account for under 10% of the Earth's land mass. In areas where this does not occur enhanced geothermal systems can be utilised. Deep underground there are hot dry rocks that can be accessed by drilling deep, once reached these rocks can be broken up with high pressure streams of water. Once the rocks are broken water is pumped through them and gets heated up until it turns into steam which rises to the surface. The steam is then used to turn turbines attached to an electric generator. The geothermal energy then successfully converted into electricity. Condensed water is then sent back down to the hot rocks. Closed loop binary cycles can also be used where the steam heats a separate fluid in a closed system so there are no emissions.

With enhanced geothermal systems there is a risk that needs to be taken into consideration. There is a risk of inducing seismic activity when drilling hot dry rock, similar to the threat posed by hydraulic fracturing. However the risk can be minimised with selecting sites for new geothermal energy power stations carefully. Enhanced geothermal systems should be located away from major fault lines and densely populated areas, with close monitoring of the drilling process.

Production of geothermal energy electricity alongside oil and gas

Present within some oil and gas wells is a geothermal fluid that currently has little use. It exists due to the high temperature and high pressure conditions within these wells. The geothermal fluid is sometimes used to heat buildings but it can be put to further use making electricity. This geothermal fluid can be found at temperatures of 150ºC and with new technology it can produce electricity when passed through a binary cycle system. The electricity made from this geothermal energy can even be used in the oil and gas fields themselves to increase the efficiency of the sites.

Geothermal energy companies

Below is a list of geothermal energy companies:

  • Calpine Corporation

  • Enel Green Power North America, Inc.

  • Cal Energy/Berkshire Hathaway Energy

  • Northern California Power Agency

  • Ormat Technologies, Inc.

  • Terra-Gen Power, LLC

  • ThermaSource, LLC

  • US Geothermal

  • Imperial Irrigation Disctrict

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