How it works
Geothermal means 'Earth's heat'. The center of the Earth is extremely hot; current estimates are 5,500C at the core just over six and a half thousand kilometres below the crust. This is about as hot as the surface of the sun. It is not surprising therefore, that even the upper 3 metres of the Earth's surface stay at a nearly constant 10-16C throughout the year.
There are three main ways of tapping geothermal energy:
Geothermal heat found near the surface of the Earth can be used directly for heating buildings.
Geothermal heat pumps
The relatively constant temperature of the top 15 metres of the Earth's surface (or ground water) can be used to heat or cool buildings indirectly. The pump uses a series of pipes to circulate fluid through the warm ground. In the winter when the ground is warmer than the buildings above, the liquid absorbs heat from the ground, which is then concentrated and transferred to the buildings. This can also be used to heat domestic water. In the summer, when the ground is cooler, the pump transfers heat from the buildings back into the ground.
A fine example of this is the Trinity Church in Copley Square, Boston which was renovated in 2002. The landmark building, built in 1877, needed a new heating and ventilation system. With no place on the roof, and surrounded by public squares and other buildings, the church had no place to hide equipment. After investigating a variety of technologies, the congregation decided to go with a geothermal heat pump (GHP).
Instead of running many pipes through the soil about 200 to 300 feet deep, the church runs six pipes 1,500 feet straight down to provide enough cooling and heating water to keep the building well tempered. Water in these tubes will stay between 50 and 55 degrees—the constant temperature of the earth at this depth—providing a moderate source for heating during the winter and cooling during the summer (see diagram, right). According to principal Jean Carroon, AIA, the first costs of the geothermal system was comparable to that of a conventional system in this sensitive renovation project, but over time it will provide an operational payback in terms of lower energy costs.
A power plant can convert geothermal energy to electricity by using a turbine driven by steam, which then drives a generator to produce electricity.
People have used geothermal energy for thousands of years, using hot springs initially for cooking and building reservoirs around springs to create shrines and bathing complexes such as those built at Bath, England by the Romans.
The world's first District Heating system was built in 1892 in Idaho, USA and piped hot water from springs to town buildings. The first geothermal power plant was built in Ladarello in Italy in 1904.
By using geothermal energy, no fossil fuel burning is required. Geothermal power plants emit only excess steam and very few trace gases (1000-2000 times less carbon dioxide than fossil fuel power plants), they take up very little land compared to traditional fossil-fuel plants and advanced drilling techniques minimize the impact of drilling wells.
The electricity produced is also more 'available', as fossil-fuelled power plants produce electricity 65-75% of the time compared to 90% from geothermal power plants. While geothermal resources are not spread uniformly, geothermal heat pumps can be used nearly anywhere.
When a heat pump is used to provide domestic heating, the savings on electricity can outweigh the cost of installing and running the system. Where geothermal energy is used in agriculture (such as to heat greenhouses) heating costs can be cut by up to 80%. The cost of electricity from geothermal power plants is slowly becoming competitive with that from traditional power plants.