Your choices:
1 Comfortable indoor climate; 2 Geothermal; 3 District heating
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 1 Comfortable indoor climate |
| Biomass (fermentable sludge) | 3 District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| 2 Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
This is a well-established technology and several examples can be found throughout Europe.
Extracting energy from a volume in the bedrock will successively cool down the rock, unless the energy is replenished at – at the very least – the same rate it is extracted. Since the heat fluxes are limited, this replenishment is mainly supplied by water flowing in the bedrock and in many cases water-carrying layers in the bedrock, aquifers, are used as sources for geothermal energy. These aquifers may be of two different kinds, namely stationary (confined), where the water is actually not flowing through but only serves to even out the temperature in the horizontal direction and dynamic (u-confined), in which case warm water flow through the layer and thus supplies new energy continuously.
At about 700 m depth below Lund in southern Sweden, there is a stationary aquifer originally at 22-25 °C which has been used to provide district heating for 15-20 years and is expected to last another 15 years before it has been exhausted, i.e. has been cooled down to about 15 °C. So even stationary aquifers may well serve as energy sources.