Can you imagine that our beautiful green earth hides unbelievable amounts of heat a few miles down from its surface?
You may have heard about how Greeks and Romans used the warmth from hot springs to heat public baths in ancient times. There are references in the history books of how in the Roman city of Pompeii geothermal energy was used to heat buildings.
The use of geothermal energy in North America dates back 10,000 years, as per archeological evidence. Paleo-Indians used hot springs for their warmth, cleansing, bathing, and cooking needs. They discovered that the mineral-rich water from the hot springs has healing properties.
Inspired by hot springs and geysers, scientists and entrepreneurs explored and experimented with the hot water from underground in numerous ways. All of this led to the installation of the first geothermal power plant at the Larderello dry steam field in Tuscany, Italy by Prince Piero Ginori Conti.
The first geothermal power plant began operation at the Geysers, near San Francisco, California in 1921. Though this plant fell into disuse soon as it was not efficient or profitable, in the 1950s, the Geysers was developed into the world’s largest geothermal complex. Now, it has 18 geothermal power plants and 350 wells in operation.
The United States is the leading producer of geothermal energy in the world with an installed capacity of 3,714MW and producing 16 billion kilowatt-hours (kWh) of geothermal energy throughout the year.
Geothermal energy is a sustainable and renewable energy source besides being clean and green energy. It is a perfect fit for the kind of energy we are searching for to replace harmful fossil fuels. This article digs deep into geothermal energy and comes up with interesting facts. Here you will find information about how geothermal energy is harnessed and converted into usable form at the power stations.
Before we go further ahead, let’s understand more about geothermal energy.
What is geothermal energy?
At the time of the formation of the earth, considerable amounts of energy generated by nuclear fusion got trapped in the earth’s core. This primordial heat, together with frictional heat produced when denser core material sinks to the center of the planet and heat generated by radioactive decay, results in considerable heat accumulating inside the earth.
The temperature may go up as high as 7,000°F (~3,900°C), causing the rocks to melt and form magma. This heat gets transmitted to the earth’s surface through rocks and water in the mantle. As this heat reaches the earth’s surface most of it will be lost. The rocks and water in the earth’s crust can reach temperatures of up to 700°F (~370°C).
This thermal energy present in the earth’s crust can be tapped by digging shallow to deep wells in the crust of the earth. This is known as geothermal energy.
The term ‘geothermal’ has Greek origins. Geo (γη)) meaning earth and thermos (θερμος) meaning hot.
How is geothermal energy harnessed?
The heat is present in the earth’s crust in both rocks and water. The heat can be extracted as hot water or steam. Geothermal energy can be extracted in power plants as well as directly on a smaller scale.
Geothermal power plants
Based on what is extracted and the diverse extraction techniques, there are three types of geothermal power plants.
This geothermal plant uses the heat from the steam present underground. Using pipes drilled into the rocks, the steam is brought up to the surface of the earth and used to turn the rotor of a generator. Thus electricity is produced.
In the United States, steam is available for extraction only in two underground reservoirs. The Geysers in northern California and Yellowstone National Park in Wyoming. As Yellowstone is a protected site with no prospects for developmental activities, the only dry steam power plant in the country is the one at the Geysers.
This is the most common among geothermal power plants. It uses hot water at temperatures above 360°F (182°C). As the temperature is quite high, the water will rise to the surface of the earth on its own. As it rises, the water will lose some of its heat and pressure. At boiling point, the water will get converted into steam.
The steam is collected separately and used to turn the rotors of a generator. Thus electricity is generated. The cooled-down water as well as the water obtained from condensation is pumped back into the underground reservoir, making this process sustainable.
The hot water pumped up from underground in these power plants is less hot at about 225-360°F (107-182°C). In this geothermal plant, the heat from the hot water is transferred to a working fluid in a heat exchanger. This working fluid is typically an organic compound chosen with care to have a lower boiling point than water.
The transferred heat will make this working fluid boil and vaporize. This hot vapor is used to turn the rotor of the turbine and generate electricity. Once the heat is transferred to the working fluid, the water is injected back into the ground, where it will get reheated and circulated back.
During the entire process, the working fluid is not allowed to get mixed up with water. This means there are no reactions, harmful by-products, or emissions.
At present, binary cycle power plants use two kinds of geothermal resources to produce electricity – enhanced geothermal systems (EGS) and low-temperature or co-produced resources.
Enhanced Geothermal Systems (EGS)
This is useful when natural convective hydrothermal resources are absent. This means the hot rock is there but there is insufficient or little natural permeability or fluid saturation. This system creates a man-made reservoir by injecting a fluid under controlled conditions into the underground. This triggers pre-existing fractures to open up, creating permeability.
The US Geological Survey estimates that there are prospects for 500,000MW of EGS resources in the western United States.
Low-Temperature and Co-Produced Resources
As the name indicates, these geothermal resources are found at lower temperatures of 300°F (150°C) or even less. These resources can be put to use using a binary cycle system to produce electricity. A by-product of the oil and gas well, the co-produced hot water has the potential to generate electricity. Making full use of the potential of oil and gas wells can help in lowering their carbon footprint as well as stretch the resources for a longer period.
Direct small-scale geothermal applications
Besides the elaborate methods and infrastructure used in geothermal power plants, thermal energy can also be tapped with relative ease on smaller scales when hot springs or geothermal reservoirs are located near the earth’s surface. Such as direct application using a heat exchanger or using a geothermal heat pump.
Using a heat exchanger
The presence of a hot spring can be made use of by pumping the hot water directly to heat homes and other buildings. When the hot water passes through a heat exchanger, its heat is transferred to the heating system of the building. The water is injected back into the well for reheating and recirculation.
Geothermal heat pump
The soil, rock, or water lying a few feet under the earth’s surface maintains a steady temperature of 50-60°F (10-15°C) year-round. This warmth or coolness, as the case may be, can be exploited to heat as well as cool indoors.
In a geothermal heat pump, fluid is circulated through a series of loops under the ground and then brought up to the surface. A heat exchanger transfers the heat from the fluid to the heating system of the building. The cooled-down fluid goes back underground for reheating.
This process is reversed during summer for cooling the interiors. The fluid transfers the heat from inside the building to the water or ground outside, where it will get absorbed.
Geothermal heat pumps come in four varieties to suit the soil, lay of the land, and climatic conditions. Three of them are closed-loop systems and one is an open-loop system.
Closed-loop heat pumps
Closed-loop systems are of three types – horizontal, vertical, and lake/pond. In this system, a water-antifreeze mixture is circulated through the loop underground or under the water in a lake/pond. This fluid absorbs the warmth and transfers it to the building’s heating system using a heat exchanger.
In summer, the same system can be used to cool interiors. The underground temperature remains the same during winter and summer. As the air above the ground becomes warmer during summer, the fluid will carry this heat and transfer it to the ground.
The horizontal closed-loop systems are cost-effective and best suited for residential needs. For bigger buildings, the vertical systems are more effective. However, the closed-loop system built under a lake or pond comes the cheapest.
Open-loop heat pumps
In this, warm water from underground is pumped directly into the heat exchanger to harness the thermal energy. Once the water loses its heat, it can either be pumped back into the same or another source. This comes cheaper as long as there is the availability of enough water for circulation.
Geothermal energy has great potential in places where it is available close to the surface. Newer geothermal technologies like Enhanced Geothermal System (EGS) are being developed to better harness this hidden resource. Ideally, it is the most economical to use this resource in the areas where it is available. Currently, geothermal energy provides almost 60% of power along the coast of Northern California.
Geothermal energy systems help us harness the vast amounts of heat that got trapped inside the earth billions of years ago at the time of its formation. It is as if nature foresaw our need for renewable energy and maintained the heat all these years for us to use now. The wonders of nature are truly amazing!