Geothermal energy can be used in a direct or indirect way. The choice is determined by the available temperature, the presence of a reservoir, the intended purpose and the economic context.
Direct use of geothermal energy
Direct use involves the application of hot water to heat or cool buildings. The temperature level of the water primarily determines the exact purposes for which it can be used.
In most cases, direct use is based on the following simple and solid concept:
- Hot water is pumped from the subsurface via one or more boreholes.
- The heat from the pumped water is transferred to the application via a heat exchanger.
- The cooled water is then pumped back into the subsurface via one or more boreholes. This allows creating a loop and maintaining a steady underground water level.
The amount of energy that can be recuperated through such an 'open' hydrothermal system is determined by the heat flux in the subsurface, the pumped quantity of water (flow rate) and the temperature difference one is able to deploy effectively.
If no suitable aquifers are available, a heat conducting liquid can be used, which is circulated through a pipe system in the subsurface. Heat probes are 'closed' hydrothermal systems. During its circulation, the liquid gradually warms up and extracts heat from the surrounding rock layers with which it is at no moment in direct contact. Two versions exist according to the orientation of [scheme geothermal energy] the heat probes: horizontal and vertical. Water circulation generally occurs through a co-axial system. Via an in-hole tube, cold liquid flows downwards and is then sent back upwards along the borehole casing, while it is heated by the surrounding rock.
A geological exploration study allows to select areas and layers with suitable heat transfer properties to place the probes. The underground water flow determines the heat supply. Compared to an open system, the disadvantage of a closed system of geothermal probes is that less heat transfer takes place due to a smaller contact surface of the fluid with the heat source (i.e. the surrounding rock).
Shallow cold-heat storage
Cold-heat storage (CHS) is a distinct application sometimes grouped in 'direct geothermy'. Excess heat or cold originating from building or industrial applications is stored in the subsurface. CHS can be implemented at locations where suitable aquifers are present in the subsurface. The application normally involves the installation of two wells to a depth of 50 to 150 m and between 100 to 150 m apart. Because CHS uses groundwater to store the energy via a conventional well, CHS is regarded as an 'open system'.
Excessive heat can be stored during the summer, and can be extracted for heating purposes during the winter. In reversed mode, cold groundwater can be pumped from one of the wells (the cold source) during summer for cooling. A heat exchanger is used to transfer the cold to the building. The heated groundwater is subsequently pumped back via the other well (the heat sink). This stored hot groundwater can be pumped up again during the winter, using the same heat exchanger to transfer the heat to the buildings. The cooled groundwater is then pumped back again through the well connected to the 'cold source'. This creates a cycle for energy storage throughout the seasons.
Indirect use of geothermal energy
The Indirect use of geothermal energy classically involves converting geothermal energy into electricity. This normally takes place by using heat present in the deeper subsurface at depths of 3 to 5 km. However, in specific areas, one does not need to drill that deep (e.g. Island). Such geothermal applications have been implemented in various places in Europe (see EGEC Deep Geothermal Market report 2011). Until now, no such applications are in operation in Flanders, but scientific research into further optimising the technology is being carried out at VITO and potential pilot projects are under study.
Heat pumps represent another indirect use of geothermal heat. Geothermal heat pumps are used to further increase the temperature of pumped water or warm liquids. They allow to work with relatively low temperatures (< 25 °C) and hence, can be operated in cases where only shallow wells can be placed. A heat pump is a device that extracts heat from one medium and passes this heat on to another medium at a higher temperature. The heat pump thus transfers or "pumps" thermal energy from a low temperature level to a higher one. Often liquids are used as heat-bearing medium for the heat transfer. One generally uses a liquid that, under low pressure, has a boiling point lower than the heat source. This means the liquid can already vaporise at a low temperature and extract heat from the heat source. A compressor then puts the created vapour under higher pressure. This increases the boiling point. In other words, the vapour will condensate at a higher temperature and can thus provide heat at a higher temperature.
In practice, geothermal projects, direct or indirect, involve at least one well being drilled to the desired depth and set up in accordance with specific requirements (flow rate, power). Because the salt content in groundwater generally increases with depth, it is not always possible to discharge pumped water at the surface. That is why commonly at least two wells are drilled: a production well and an injection well. [geothermal doublet] Reinjection commonly is recommended also for maintaining the reservoir pressure at the long-term. If both wells target the same geological layer, one refers to this system as a 'geothermal doublet'. If both wells end in different layers, it is referred to as a 'semi-doublet'. The distance between the two wells is determined in function of the intended life-span of the geothermal system. After many years of extracting hot water from one well and injecting cooled water into the other, a slowly advancing cold front is created from the injection well towards the production well. Dynamic computer models allow the evolution of the reservoir to be predicted and, bearing in mind the specific geology, permit the best well locations to be selected, whereby optimising the operation and the life-span of the system.
Besides the traditional method of producing hot water from an aquifer, there are also ways to extract heat from rocks with low permeability. The 'Hot Dry Rock' (HDR) or 'Enhanced Geothermal System' (EGS) concept is an open system. It involves the artificial creation of an underground reservoir through hydraulic fracturing of the rock, followed by the injection of cold water and allowing it to circulate from injection to production well. The controlled 'fracturing' ('fracking') of rock is realised by gradually increasing the pore fluid pressure until the rocks' fracturing pressure is exceeded. The result is a fine network of cracks and crevices, which must permit sufficient volumes of water to be pumped up and to realise effective thermal exchange between the rock and the water.