The further you go beneath the earth’s surface, the hotter it gets. The centre of the earth is composed of molten rock with temperatures of between four and six thousand degrees Celsius.
Hot substances transfer thermal energy to colder substances, so heat travels from the centre of the earth where it is hottest, to its surface. Once it reaches the surface, it can be released into the atmosphere. The temperature of the surface of the earth fluctuates according to the amount of solar energy reaching it. But at a depth of only around three metres, there is already sufficient insulation from the fluctuations in surface temperatures to maintain a constant 10 – 16 degrees Celsius all year round. So in fact we have a ready-made source of thermal energy not too far beneath our feet.
Accessing Geothermal Energy
The ease with which we can tap into geothermal energy varies from place to place, according to the local geology. In some locations, the geothermal energy is brought to the surface by hot springs, providing a convenient source of thermal energy that can be harnessed by man. Sometimes, less conveniently, the molten rock itself has an easy passage to the surface and geothermal energy erupts in the form of destructive volcanic activity.
There are certain locations where groundwater relatively close to the surface is found to contain an exceptional amount of thermal energy, because it happens to have been surrounded by layers of insulating rocks. The groundwater can be accessed by drilling a well, and this thermal energy can then be circulated through pipes. Around Paris, wells have been sunk to a depth of around two kilometres to draw up water with a temperature of between 50 and 70 degrees Celsius, for domestic heating. But this is not truly renewable energy, because the groundwater in that location will eventually run dry.
Nowadays we do not depend exclusively on natural flows of thermal water, as we can use electrically-operated pumps. Heat pumps circulate fluid through a system of pipes running both underground and above ground in a building. When cool fluid is pumped down through the warmer regions below ground level it gains thermal energy, which is released as heat as the fluid circulates around the building. In summer, when the temperature inside the building is higher than the below-ground temperature, this will operate in reverse to return thermal energy to the earth and produce a cooling effect in the building.
Using Geothermal Energy
In classical times, geothermal energy from geysers was used to heat Roman Baths. Today’s heat pumps can be used in small-scale installations that supply heat to a single building, or as part of much larger systems. District heating schemes, in which hot water or steam is produced at a central location and then piped to numerous residential buildings, can use geothermal energy as the power source, although biomass combustion is more usual.
Geothermal Power Stations
Another use of geothermal energy is to convert it into electricity. Geothermal power stations, like conventional power stations, use steam to drive turbines. There are three ways in which geothermal energy can be used to produce steam. One method is to drill into a geothermal source that is so hot that steam can be collected directly from the well; plants that use this system are called dry steam power stations.
Another method is to convert liquid containing geothermal energy into steam by reducing pressure and thus lowering its boiling point. The process of reducing the pressure is called ‘flashing’, and therefore these are called flash steam power stations. The third system, used in binary-cycle power plants, is to use the thermal energy of the geothermal fluid to heat another fluid that has a lower boiling point. It is this ‘secondary’ fluid which vaporises to produce steam.
Once commissioned, geothermal power stations work very efficiently and cleanly, and unlike solar and wind powered power stations, their energy source is permanently available.
Untapped Reserves of Geothermal Energy
Geothermal energy is not widely used. Recent figures from the International Energy Association show that less than half a per cent of total global energy consumption comes from geothermal sources. Germany, the USA, Iceland and the Philippines are amongst those countries currently showing an interest in geothermal energy. The biggest obstacle is finding a suitable location where the geology is right; there is sufficient thermal energy buried deep in the earth to supply us with power for tens of thousands of years, but for the foreseeable future most of it seems likely to remain untapped.