Geothermal power
Geothermal Heating
09/10/2010
Low-energy geothermal power is used to supply heating systems with hot underground water or the heat of the earth’s subsurface. To use these heat sources, a well is drilled and pumps or geothermal collectors are installed. These heating techniques have a wide variety of applications in housing, agriculture, and industry.
© Keblow
Producing Heat- Geothermal Techniques Adapted to Every Situation
When its temperature is between 20°C and 90°C, geothermal water is used to heat buildings and facilities. This is called low-energy geothermal power. The heat contained in the rocky layers of the earth’s subsurface can also be used directly for heating purposes.
Depending on the thermal gradient (the average rate at which the temperature rises with depth, which depends on the physical and geographical conditions of each region), the groundwater reservoirs used for this type of geothermal power are generally at depths of 1000-2500 meters.
Regardless of the equipment to be installed, geothermal experts start with drilling. They drill one or more wells to reach the areas or aquifers whose heat they want to harness.
They then insert a long, hollow cylinder (or casing) into each well to support the walls and prevent the hole from collapsing.
For a geothermal facility to recover hot underground water, there needs to be enough pressure to ensure a regular, sufficient flow of water, both in the well and in the related heating system. Depending on the location, there are a number of scenarios. For instance:
• When pressure in the rocky reservoir is higher than that in the atmosphere, water is forced out through the well. This is known as an artesian well. If the water is the right temperature for heating (approx. 60°C), and pressure is constant and not too strong, it can be sent directly to radiators, as in Iceland.
• If water pressure is not high enough to force it out of the well, a hydraulic motor pump is installed. This pump sucks up the hot water from underground and sends it to the heating system.
To ensure that the heating component works properly, geothermal water must be pure enough. If there is too much salt or gas in the water, this can lead to corrosion in the pipes (abnormal and accelerated wear and tear) and even cause holes to form in them. In addition, water containing mineral salts and corrosive gas cannot be used or drained off without harming the environment. To deal with these problems geothermal experts have developed a number of technical solutions.
• If the water is not too corrosive, corrosion inhibitors (chemical products that slow down corrosion) are injected at the bottom of the well. Moreover, when the geothermal circuit is installed, pipes made of composite materials are used instead of traditional steel pipes, because they are less prone to corrosion.
• However, if the water is more corrosive, it cannot be used directly in the heating system. A heat exchanger is installed between the geothermal circuit and the heating pipes. Here, the geothermal circuit is in contact with pipes filled with municipal water that are linked to radiators. With this system, geothermal water heats up the water circulating in the heating pipes without coming into contact with them. This limits the risks of corroding the heating system.
Finally, if the underground water has high salt and gas content, it cannot be drained off with waste water. In this case, it is discharged into its original reservoir using a well doublet or twin-well system. This system includes a pumping well linked to a heat exchanger and a reinjection well. After transferring its heat to the heating system, the water goes back to its reservoir via the second well to be reused again once heated underground. A twin-well system with a flow of 200 m3 of water per hour can heat 2,000-3,000 homes.
Uses for Heat Pumps and Geothermal Collectors
Geothermal heating systems and heat exchangers work with a water temperature of at least 40°C. However, other techniques can be used to produce heat in locations where there is no access to this type of water.
• Heat pumps can recover the heat from water contained in water tables less than 100 m underground with temperatures of 20-40°C (this is sometimes referred to as very low-energy geothermal power). These systems, if correctly installed, can produce water at a higher temperature. Inside the pumps, low-temperature water is used to boil a liquid with a boiling point of less than 0°C (e.g. ammonia, which has a boiling point of -33°C). This liquid is converted to gas and compressed using an electric compressor. This heats the gas and transfers this heat to a piping system linked to radiators. Once it loses its heat, the gas liquefies and returns to the start of the circuit.
• Geothermal collectors buried underground can also be used. These take heat from the ground and are used in conjunction with heat pumps. Some collectors are buried horizontally at shallow depths (0.60-1.20 m) and require a surface area of 1.5 to 2 times the surface area of the building to be heated. In built-up areas where available land area is often restricted, the collectors or geothermal probes are buried vertically at depths of 30 to 150 m. In both cases, the geothermal collectors have a system of tubes in which a mix of water and antifreeze is circulated. After being heated underground, the water rises to the surface and drives a heat pump. The heat recovered in this way is generally used to supply an underfloor heating system.
Coefficients of performance have been developed to inform consumers about the amount of energy produced by a heat pump. This is a figure that shows how many kilowatt-hours of electricity the pump supplies and uses. Most heat pumps sold retail have a coefficient of performance of 3, i.e. for each kWh of electricity it uses, it supplies 3 kWh.
Heat pumps can also be used to recover heat from warm sea water. Since 2007, the French town of La Seyne-sur-Mer (in the Southern department of Var) has been heating some of its buildings (theatre, hotel, conference center, and government housing) using heat pumps with water from the Mediterranean. This geothermal program has allowed the town to reduce its annual carbon dioxide emissions by 750 tons1.
All buildings heated with this system are less than 600 m from the coast, which is a prerequisite for economic viability. There are also heat pumps that recover heat from the air. These are known as aerothermal heat pumps.
The Various Uses of Geothermal Heating
Most low-energy geothermal techniques are used in public and residential buildings where they supply heat to radiators and underfloor heating systems. There are other applications, including:
• Heating swimming pools, spas and saunas, and amusement parks with tropical plants or wildlife
• Producing domestic hot water
• Heating fish-farming or aquaculture basins, mushroom beds, and greenhouses
• Drying wood or fish
• Freeze protection for industrial facilities (maintaining the temperature to prevent frost formation in industrial premises).
Furthermore, while the industrial sector uses heat energy, it may be called on to supply it as well in the future. This is because industrial facilities such as power plants generate significant amounts of heat. This energy could be recovered using technologies similar to those used in low-energy geothermal power.
[1] Source ADEME, 2007
