Currently the energy derived from geothermal resources ranks third amongst the renewable energies. It ranks higher than wind power and solar power, but lower than hydroelectricity and biomass ( I will be covering biomass next)

So what is Geothermal energy and how does the heating and cooling systems work?

Outdoor temperatures fluctuate with the changing seasons but underground temperatures don't. Four to six feet below the earth's surface, temperatures remain relatively constant year-round.
It is the thermal energy from the earth's core, which is stored in the rock in the earth's crust, and this can range from low temperature (less than 90°C), moderate temperature (90°C - 150°C), and high temperature (greater than 150°C). The higher temperatures are to be found in the core and generating outward to the lower temperatures.  A geothermal system utilises the low temperatures.

                                      

A geothermal system, which typically consists of an indoor unit and a buried earth loop, capitalizes on these constant temperatures to provide "free" energy. The heat pump, a device that moves heat from one place to another, transfers heat from the soil to the building in winter and from the building to the soil in summer.

                    

Unlike ordinary systems, geothermal systems do not burn fossil fuel to generate heat; they simply transfer heat to and from the earth to provide a more efficient, affordable and environmentally friendly method of heating and cooling. Typically, electric power is used only to operate the unit's , compressor and pump.

The three main parts consist of the heat-pump unit, the liquid heat-exchange medium, which comes in either the form of a looped coil or thermal antenna, and the air-delivery system which is normally in the form of wet radiators or under floor heating depending on the type of construction of your property. The vertical loops are buried between 50 to 60cm beneath the soil, whereas the vertical antenna is buried 70-100m deep. Within this sort of system heated towel rails could also be implemented, giving added comfort for your bathroom.

A geothermal system is three to four times more efficient than the most efficient ordinary system. Because geothermal systems do not burn fossil fuels to make heat, they provide three to four units of energy for every one unit used to power the system.

When installed properly, the buried loop will last for generations. And the other half of the operation—the unit's compressor and pump—is housed indoors, protected from the harsh weather conditions. Usually, periodic checks are the only required maintenance.

Geothermal systems work with nature, not against it. They emit no greenhouse gases, which have been linked to global warming, acid rain and other environmental hazards. Neither does this type of heating generate condensation, which is a major consideration for a cave house. Certainly a thought for not only future generations but also for the future of this planet of ours.

There are suppliers and installers of this type of heating present in Spain, but this technology seems to be slow catching on in Spain. However I believe there may be funding or grants available in the future. To date the only information I can find regarding the cost of this sort of system is that they are comparable with traditional oil or gas installations.