Your choices so far:
1 Geothermal; 2 District heating
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | 2 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) |
Except for a few areas, the geothermal resources found in the European continent are low-temperature. To produce district heating from such sources, heat pumps are used.
A heat pump is a device that can take (just as an example) 2 kWh of energy from a source at say 15 °C, it adds 1 kWh of pure exergy (i.e. electricity) and then delivers 3 kWh of energy at an elevated temperature, say 100 °C. A heat pump is not just any ordinary electrical heater but is in itself a sophisticated piece of equipment that performs an intricate thermodynamic operation – but heat pumps are today commercially available at a variety of scales and the life-spans are typically more than 10 years even for those that are manufactured in household scale.
Heat pumps are characterised by their COP (coefficient of performance). In the above example: input of 1 kWh of electricity plus 2 kWh of low-temperature energy produces 3 kWh of useful heat, the COP is 3. If instead the input had been 1 kWh of electricity plus 4 kWh of low-temperature heat and the output – consequently – had been 5 kWh, then the COP would have been 5. So the COP is the output divided by the electricity input. Hence, the COP is a measure of efficiency.
The bigger the temperature difference between the product medium (i.e. the district heating water) and the input medium (i.e. the temperature in the geothermal well), the lower the COP.
In district heating applications, where the forward temperature must typically exceed 70 °C, the return water may not fall below 55 °C and supposing the geothermal well to deliver at 20 °C, the COP is a bit more than 4.2. If the forward temperature could be reduced to 60 °C the COP would exceed 4.6. Saying then that the heat power delivered through the district heating system is 100 MW, a COP of 4.2 means that the heat pumps demand 23.8 MW of electricity while a COP of 4.6 reduced the need of electricity to 100/4.6 = 21.7 MW. The saving amounts to almost 10 %.
Hence, the layout of the district heating system and the temperature levels will have a pronounced effect of the operational cost.
Opposed to when district heating is produced from fuel firing, electricity cannot be co-produced if geothermal energy is used as the source.