The term geothermal comes from the Greek words geo, meaning earth and therme, meaning heat. Therefore, geothermal literally means earth heat or heat from the earth. This heat originates at the Earth's core, where temperatures reach 7000 degrees Centigrade, and is continually conducted outwards to the surface. The heat at the Earth's core was first generated when our planet coalesced from dust and it is maintained through the constant decay of radioactive elements in the core.
There are five potential sources of geothermal energy: hydrothermal reservoirs, earth energy, geopressured brines, hot dry rock and magma. The first two sources are already in widespread use while the last three can only be accessed by advanced technologies and engineering techniques. These technologies are only experimental or theoretical at present.
Hydrothermal reservoirs are large pools of steam or hot water, trapped in porous rock, which have been heated by energy from the Earth’s core. They can only be found in certain areas of the world, for example Japan, New Zealand, and other countries located on the Pacific ‘Ring of Fire’. There are also geothermal ‘hotspots’ in places like Hawaii and Yellowstone National Park in the USA. Other significant hydrothermal reservoirs can be found in countries like Iceland, Italy or those along the Himalayan geothermal belt.
Earth energy can be found virtually anywhere in the world and refers to the ‘thermal mass’ of the shallow ground. This means that the soil and groundwater at a shallow depth, of around 10 feet beneath the surface, maintains a constant temperature of about 10 to 16 degrees Celsius all year round. We are able to use this to our advantage in providing cheap heating and cooling.
The direct use of geothermal energy dates back many thousands of years. For example, there is evidence that Japanese people used hot springs for bathing and cooking from 11,000 BC. It is also known that Native American Indians settled near hot springs in North America about 3,000 years later, and they used the springs for bathing and medicinal purposes.
Large ‘Roman baths’, utilising natural hot water, were built throughout the Roman Empire over 2000 years ago. As well as the bathing and medicinal aspects, the water was also used to provide heating.
From the ninth century AD, people in Iceland planted their crops in naturally heated ground. This had the effect of promoting plant growth and bringing an earlier harvest. A little later, in areas of significant geothermal activity in New Zealand, the Maori people used heated ground and steam resources for cooking.
The energy of the wind is captured and converted into electricity by devices called wind turbines. There are two major types of turbine: vertical axis or horizontal axis.
Nearly seven hundred years ago, hot water of up to 85 degrees Celsius from the Paris sedimentary basin in France, was used to heat buildings. Health resorts utilising mineral spas became extremely popular throughout Europe over the last three hundred years or so.
Geothermal energy was first used to generate electricity in 1904 in Italy, using what is known as a ‘dry steam’ plant. The geothermal field, Lardarello in Tuscany, is still in use today.
Direct use of geothermal energy
Hot water from underground but close to the Earths surface is piped directly to the facilities where it will be used. Common uses include spas, the heating of buildings, greenhouses, fish farms, roads and pathways. Other uses include the washing of wool, pasteurising milk, dehydrating fruit, production of paper and various industrial processes.
Sometimes, an extensive network of pipes may be used to deliver warm water to all the buildings in a suburb or town. This is called a ‘district heating scheme’ and the best current example is the city of Reykjavik, capital of Iceland, where virtually every resident receives piped hot water.
Earth energy is used directly by geothermal heat pumps (GHP's) to provide low cost heating or cooling for a building. As outlined above, the ground just below the Earth's surface maintains a constant temperature all year round which can be used to our advantage.
There are essentially three components to a GHP. First there is a heat exchanger, which is a system of pipes called a loop, buried in the shallow ground near the building that is to be heated or cooled. A mixture of water and antifreeze is circulated through the loop and this liquid either absorbs heat from or releases heat into the ground. The second component of a GHP is a system of ducts within the building, through which warm or cool air can be circulated. The third component is a heat pump, which transfers heat between the loop and the ducts.
In winter, because the ground is warmer than the air, heat from the ground is transferred to the building, and this process is reversed in summer. As electricity is being used only to move the heat rather than generate it, the GHP is more efficient and cost effective than traditional methods of temperature control.
Generating electricity from geothermal energy
Power plants draw steam or very hot water from wells drilled into geothermal reservoirs that are at least a mile below the surface. There are presently three different types of geothermal power plants in commercial operation.
1. Dry Steam Plants
Steam is piped directly from a geothermal reservoir to power a turbine and generator on the surface. It should be noted that dry steam is a high grade but relatively rare resource. For example, in the USA there is only one area where dry steam is available for commercial use, this being the "Geysers" field in northern California.
2. Flash Steam Plants
These are the most common type of geothermal power plant. They take superheated water, of at least 182 deg C (360 deg F), from a deep reservoir where it is under high pressure. The pressure prevents the water from turning into steam, even though it is hotter than the normal boiling point.
3. Binary cycle plants
These use geothermal water in the temperature range of 107 to 182 deg C (or 225 to 360 deg F). This isn’t hot enough for the water to flash to steam itself but the heat can be used to boil a secondary fluid. The secondary fluid has a much lower boiling point than water and is usually an organic compound such as isopentane or isobutane. The geothermal water is piped alongside the secondary fluid, in a heat exchanger, warming the secondary fluid and causing it to flash to steam which is then used to drive the turbines.
The water is injected back into the ground to be reheated while the secondary fluid is recondensed to a liquid for re-use. There are no emissions causing air pollution from this type of power plant.
Usage of geothermal energy
In 1999, the worldwide total installed capacity of electricity generated from geothermal energy was just over 8,000 Megawatts (MW). This is about a quarter of one per cent of the total installed electricity generating capacity of the world.
The USA accounts for slightly more than a third of the worldwide installed geothermal capacity. As of 1998, the USA had a capacity of almost 3,000 MW. Put another way, approximately 0.4% of electricity produced in the USA annually comes from geothermal sources. It would take burning 60 million barrels of oil to produce a similar amount of electricity.
The largest geothermal power plants in the world are located in the Geysers area in northern California, USA. Production started here in 1960 and while steam pressure and consequently electricity production have declined since 1989, the fields still had a capacity of nearly 1,100 Megawatts in 1998.
Of the renewable energy technologies, geothermal is the third most popular, after hydroelectricity and biomass. While the potential of geothermal energy is virtually limitless, it will require significant advances in engineering methods and technology before we are able to gain full access to it.
To give you an idea of the main players in the geothermal energy stakes, the following table on the left details the worldwide installed capacity as of 1999.
The direct use of geothermal energy for heating and other purposes provided the equivalent of almost 10,000 thermal megawatts, worldwide in 1998. The table on the above right outlines usage of geothermal energy in units of Thermal Megawatts by region:
The benefits of geothermal energy
The potential drawbacks of geothermal energy
The cost of generating electricity from geothermal energy
Currently the cost of generating electricity ranges from US 5 to 8 cents per kilowatt hour (kWh); this compares to US 1.5 cents per kWh for conventional power. Newer geothermal power plants are often more expensive because they are forced to utilise geothermal reservoirs that are deeper and/or cooler than the resources tapped by earlier plants. More and deeper wells have to be drilled in order to produce an equivalent amount of electricity to the earlier power plants.
The future for geothermal power
There are hot dry rock resources everywhere throughout the world at depths of 3 to 5 miles. The future for geothermal power lies in being able to tap into these resources. This would be accomplished by drilling wells into the rock in two places, injecting cold water down one well before circulating it through the rock and drawing the now heated water off from the second well. This would also require a process for making the rock itself more porous, i.e. by fracturing it. An experimental power plant utilising hot dry rock now exists in England.
There are also suggestions of one day drawing energy directly from the molten rock which exists beneath the Earth's crust. Further research and development is obviously needed before this can become a reality, but when it does, we will have access to virtually unlimited energy.