GRETA

Near-surface Geothermal Resources in the Territory of the Alpine Space

Each day, the ground and the underground of our planet is heated by solar energy and magma chambers, which are stored inside as calories in the ground. This geothermal heat is stored underground and can be reached and transformed to heat individual and collective housing, neighbourhoods and urban complexes, or used into industrial processes.

Open loop systems collect geothermal heat with two wells, from underground lake or groundwater. This system uses one or several drilling holes to pump and forward ground water with a heat interchange system, to a heat pump. Calories are then recovered, and reinjected in the aquifer through the second or several drilling holes. Good to know that reversing this system is applied for geocooling.

Closed loop systems capture surface or deep ground heat, depending on whether we use horizontal, vertical, or pond loops. Geothermal loop fields flow heat-carying liquid (usually glycoled water) on several underground loops. This field is made up of a polyethylene pipe network, exchanging heat energy through a simple heat transfert, and forwarding it to the final heat pump

Producing heat and electricity

Depending on the quantity of calories captured, and following their use, several geothermal systems are possible:

  • For very low temperatures (up to 30°C), geothermal energy is drawn with heat pumps from 50 meters depth. This system allows individual and tertiary uses, such as heating and providing hot water.
  • With low temperatures (30 to 150°C), the same mechanism is used for districts heating applied to agriculture, industrial processes and many other uses.
  • For geothermal plants and other industrial uses, high temperatures are needed (up to 200°C). Under these conditions, cogeneration systems are possible to produce heat and electricity at the same time.

Geocooling

Refreshing buildings is possible through the direct use of geothermal heat and does not require the use of heat pumps, which is really energy-efficient. In fact, this system converts heating into refreshing with a bypass, then spreads it through the floor or the ceiling. This system is accurate for summer time refreshing with winter time heating, 

Energy storage

Energy storage intends to gather geothermal energy into the underground without any further restriction than managing energy needs or availiablity of the surplus resources. Storing energy as heat or cold, aims at optimizing the sizing of the installations and operationnal costs facing a time variation of the demande (daily or seasonal). To ensure this, several systems are used...

  • The Aquifer Thermal Energy Storage system (ATES) uses at least 2 hot and cold areas where, at given times, groundwater will flow in one direction and then in the other passing through the surface installations where heat exchanges achieve.
  • The Borehole Thermal Energy Storage system (BTES) is characterized by the exploitation of underground rocks. During the cycles, which are maily seasonal, temperature of those rocks will oscillate between hot and cold. Buried exchangers from the closed loop are filled with clear or glycoled water (to avoid freezing) that will fluctuate as well and transfer the heat.
  • Diffusive storage in foundation and geostructures use areas under the buildings to store the heat, instead of using groundwaters. This system fulfils geotechnical functions, mainly as the foundation of the building. In fact, the temperature should not approach the 0°C to avoid any freezing risk that could permanently unstructure the ground.