Geothermal heating system

Geothermal heating uses low energy geothermal water. It is exploited by pumping from bore wells sunk into the reservoir of water. The number of wells sunk in a given zone must not be too high. A very precise calculation must be made in order to allow renewal of the hot water as it is exploited. If the water is sufficiently pure and neither too hot nor at too high a pressure, it can be dispatched directly into radiators, as in Iceland. Otherwise a heat exchanger must be used: the water extracted from the earth transfers its heat by means of plates or tubes, to a secondary circuit of pure water, which will then be dispatched into the radiators.
If the water from the earth is really too heavily laden with salts, hydrogen sulphide (H₂S), or dissolved gas, it cannot be discharged into the natural surroundings. It must be re-injected into its original reservoir. To achieve this, a technique of double wells – a pumping well and a re-injection well – is used. The re-injection well is a deviated well: that is to say the cold water is re-injected sufficiently far from the pumping wells.

A double well system with a flow rate of 200m³ per hour is capable of heating 2 to 3000 dwellings. Besides the heating of buildings and dwellings, geothermal energy is used to heat greenhouses and fish farms.

Corrosion from salts and dissolved gases is the number one enemy in geothermal energy applications. It can be fought by injecting certain chemical products (corrosion inhibitors) into the bottom of the wells, and by using tubes and pipe work in a composite material rather than in steel.

Heat pumps: if the temperature of the water is too low to use in a heat exchanger, that is to say it is below 40°C, an alternative solution exists: heat pumps, which can be used with water between 20 and 40°C. They require energy to stimulate the system, but in the end, more energy is recovered than is consumed.
The system of energy recovery from low-temperature water using a heat pump is not restricted to geothermal energy applications. It can also be applied to relatively warm sea water, such as that of the Mediterranean. As a result, following the 30-year-old example of Monaco, the town of La Seyne-sur-Mer, in the Var department, decided in 2007 to launch the use of sea water for heating public buildings, with the help of a heat pump installation ( 500-seat theatre complex , Town Hall… and, as a first phase, 500 new accommodation units. For the project to be profitable, the buildings to be heated must be situated within 600m of the coast.

How does it work? :

-water at a low temperature is used to boil a liquid whose boiling point is slightly below 0° C (for example ammonia, boiling point -33° C) ;
-the gas obtained is compressed by means of an electric compressor.This operation gives out heat. The gas heated by compression passes its heat to a secondary water circuit feeding the radiators. This exchange causes the gas to liquefy and it is returned to the start of the process. In fact, a heat pump is a refrigerator in reverse!

The performance coefficient of a heat pump measures the quantity of energy obtained, compared to that supplied to drive the compressor. Its value lies between 3 and 4.

Heat pumps have also found an application in the recovery of heat from the earth (heated by the rays of sunlight) to heat a house. It is what is called "domestic geothermics", using geothermal heat pumps. A network of horizontal captors, just below the surface of the ground, transmits the heat of the ground to a floor-heating system in a house, using a heat pump as intermediary. The schema below outlines the working of the system.

Source : ADEME.

The installation of this type of system is quite expensive, but it can pay for itself in a few years and such installations are robust.