A source of enormous energy is located deep inside the Earth. The Earth stores this energy as heat, therefore, we call it geothermal energy. Sometimes it is also referred to as “environmental energy”. Both the Earth’s top layer – the soil – and the layers deep inside the Earth contain this environmental energy. Environmental energy has potential for both heating and cooling purposes. The Sun warms up Earth’s top layer, the soil. A device called heat pump can further increase this temperature, so that we can use it for heating. A couple of hundreds meters deeper inside the Earth a new source of energy already becomes more noticeable – the Earth’s inner heat.
Part of that geothermal heat heats up the ground water. In most countries the ground waters can only be used for heating when they lie at a depth of several hundreds meters. In some countries, however, geothermal heat is visible even on the surface, in the form of geysers.
Geothermal energy has one big advantage over the other forms of sustainable energy, like solar and wind energy, – it does not depend on the weather conditions. This is a big reason to develop techniques enabling the usage of this energy in the future.
The energy of the Earth
The natural decay of radioactive isotopes inside the Earth continuously produces energy in the form of heat. Only 30% of this heat is the so-called primordial heat – the heat our planet has kept since its formation. The Earth itself continues to produce 70% of its internal heat. The main difficulty with using this immense heat source is that the heat flow to the surface of the Earth is very small. That happens because the rocks forming the Earth are very poor thermal conductors. Compared with good heat conductors such as the majority of the metals, thermal conductivity of the rocks is lower by a factor of 500.
Therefore, the heat flow to the surface is far too low, it constitutes only about 1% of the heat the surface receives with the sunlight. If we want to make effective use of geothermal energy, we have to penetrate very deep inside the planet. For each kilometer of depth the temperature increases by roughly 30 degrees Celsius. Heat pumps provide the means to use geothermal energy stored deep inside the Earth
Using the geothermal energy directly
Direct use of geothermal energy is only possible if heat energy is the desired energy. That means, that we do not want to use it as, for example, electricity, but we really need to heat something up. Using the geothermal energy directly can be very helpful with heating up building or providing warm tap water.
In order to get the heat from the inside of the Earth to the surface, one usually implements a system of two heat reservoirs – a cold one and a hot one. First water is directed to move along a layer of hot rocks – the rocks are heated by Earth’s internal heat. Consequently, the pumps direct this heated water up to the surface, where it enters the heating pipes system of a building. The water then releases its heat at the desired location. The water that has cooled down again after releasing its heat travels back into the ground again where it reheats. Then the whole cycle repeats.
Similarly to using the hot water, one can directly use the cold superficial water close to Earth’s surface, for cooling purposes.