The Underground

Figure 6.15 gives an overview of possibilities to use the underground in the provision of energy, water and other resources. There are various opportunities for heat exchange with the underground:

• Heat and cold from open water, ground water, or soil to approximately 50 m of depth

• Heat and cold storage in aquifers in the shallow underground from 50 to 250 m

• Geothermal heat (20-60°) from aquifers in the intermediate underground (between 100 m and 1 km)

• Energy and CO2 storage in salt cavities in the same intermediate underground

• Geothermal heat (60-140°) from aquifers, gas and salt cavities in the deep underground ondergrond (1 km and deeper)

• Gas and CO2 storage in empty gas fields in these deep layers

The potentials of a specific region depend on local geophysical properties. 6.3.8.1 The Deep Underground

Heat can be won in all places with good potentials for geothermal heat from the deep underground, but in particular where emptying, in-watering gas fields (permeable to aquifers) are present. At the depth of gas fields, the water can reach 100-140°C, a practical temperature for all domestic and many industrial purposes. Lower temperatures (from shallowers) are still useful for heat-demanding functions (washing, showering, heating, swimming pools etc.).

Fig. 6.14 Heat balance of different municipalities in the province of Groningen, Netherlands, for manure fermentation applications (Source: KNN Milieu, 2006)

6.3.8.2 The Intermediate Underground

The potential for storage of heat and cold in shallow aquifers down to a depth of 250 m is partly dependent on the thickness of the aquifer. Storage of this heat and cold is particularly suited for functions with a changeable pattern of heat and cold demand and supply, necessitating seasonal storage. Instances of this demand are dwellings, holiday homes, recreational facilities and season-bound industrial activities. Unsuitability for heat and cold storage, in reverse, indicates that patterns of supply and demand should be tuned optimally through a well-deliberated mix of functions. A second solution would transportation of heat and cold to areas with a reversible demand pattern. The third solution is storage of heat and cold in the underground without the use of aquifers, so in closed storage systems.

Fig. 6.15 Possibilities of the underground (Source: TNO/Built Environment and Geosciences)

6.3.8.3 The Shallow Underground

By means of heat pumps and heat exchangers, heat and cold can be exchanged with the underground, ground water, openwater and outside air. In the shallow underground the potential to exchange heat and cold is many restricted by other concessions. Figure 6.16 is a map of the suitability of the soil for the application of vertical heat exchangers, with a conversion to energy potential.

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