Your choices:
1 Geothermal; 2 Comfortable indoor climate; 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 | 2 Comfortable indoor climate |
| Biomass (fermentable sludge) | 3 District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| 1 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) |
Since the COP of a heat pump depends strongly on the product temperature, it is advantageous to use as low temperatures as possible in connection with heat pumps and, hence, with geothermal energy resources.
In district heating systems, the lower temperature limit is set by the fact that the last customer on the grid must always be provided with district heating water warm enough so so that this last customer may heat their tap water to more than 50 °C.
To save electricity it may then be advantageous to dimension the system so that the geothermal heat is used only to raise the district heating water to – say – 40 or 50 °C and then use fuel firing to "top up" the temperature to the levels desired for the system. This type of combined systems are common in the Scandinavian countries where the forward temperatures during winter may exceed 100 °C. The top-up boilers will also provide for a fast system response to weather changes of changes in the demand while the heat pumps will work at a constant rate throughout the year.