Thermoactive geostructures Thermal and mechanical operation

The heat present in soils and rocks has been known and exploited for a very long time.

Over the course of the 20th century, the exploitation of thermal resources in the ground has been widely developed, giving rise to geothermal energy. There are several types of geothermal energy, depending on the depth at which the ground's thermal resources are exploited. Very low-energy geothermal energy covers the first few hundred meters of ground, exploiting the thermal resource to heat or cool buildings. The simplest application is the Canadian well. Air is circulated through a pipe a few meters below the surface, leading into a dwelling. In summer, it cools the house, because it cools as it flows through the pipe, and in winter, it heats the house, at least partially, because it heats as it flows through the pipe. Although the temperature of the ground increases with depth due to the effect of the natural thermal gradient, this technique shows that the ground temperature remains more or less constant from the surface of the ground to a depth of several dozen metres.

Other techniques were subsequently developed to take advantage of this observation. These include geothermal doublets, geothermal probes and thermal geostructures. The idea is to use the ground as a hot source (i.e., a medium for extracting heat) or a cold source (i.e., a medium for injecting heat) to produce heat or cold.

In a geothermal doublet, groundwater circulating in the ground serves as the hot or cold source. Water is pumped into the ground at one point from an extraction well and discharged at another point from an injection well. Groundwater flow in the vicinity of the two wells plays a major role, and various issues relating to interactions between the two wells need to be considered.

With a geothermal probe, the principle is to circulate a heat transfer fluid through a borehole inside a heat exchanger tube, then through a heat pump. In summer, for example, the fluid injected has a temperature of around 30°C and is extracted at a temperature of 25°C. The cold is then produced by the heat pump. In winter, the fluid is injected, for example, at a temperature of around 4°C and extracted at a temperature of 8°C, and the heat is still produced by a heat pump. The coefficients of performance achieved (ratio between the power extracted and the power enabling the heat pump to operate) are of the order of 3 to 5.

At certain times of the year, such as spring and autumn, the heat pump can be dispensed with. For example, the injected fluid can have a temperature of around 19°C and be extracted at a temperature of 14°C, known as geo-cooling or free-cooling.

For thermal geostructures, also known as "energy geostructures"...