Adding a heat pump to a geothermal doublet increases the amount of heat that can be produced. Depending on the production temperature of the geothermal source, the heat pump can achieve this by:
- Increasing the production temperature, and/or
- Further reducing the injection temperature (i.e., the return temperature of the district heating network supplied by the geothermal source).
The first option may be applied, for example, when the production temperature is lower than the minimum allowable supply temperature of the heat network. By using a heat pump, the difference between the production and injection temperatures of the doublet increases, thereby increasing the amount of heat extracted from the produced water.
For the ThermoGIS basecase scenario, the return temperature of the heating network is fixed at 30°C. In the heat pump scenario, this changes as follows:
The calculations are done for a 70-30°C district heating network. The heat pump option in ThermoGIS uses a lower geothermal injection temperature than the network return temperature, Tnetwork return. It is assumed that the geothermal injection temperature, Tgeo inj, is selected as low as possible to maximize geothermal output, while being limited by a maximum temperature difference:
Tgeo inj = max(Tgeo prod − ΔTmax, Tgeo inj min)
where:
- Tgeo inj Aquifer injection temperature
- Tgeo prod Aquifer production temperature
- ΔTmax Maximum temperature difference between produced and injected water (100°C1)
- Tgeo inj min 20°C
The heat pump COP is calculated using a Carnot2-calculation which depends on the condenser and evaporator temperatures, as follows:
where:
- COPhp Carnot Coefficient of Performance of the heat pump
- η Heat pump system efficiency (0.63)
- Tnetwork supply Supply temperature to the district heating network / outlet temperature of the heat pump (70°C)
- Tgeo inj Aquifer injection temperature (calculated)
- Tloss Heat losses in the evaporator and condenser (3°C)
No distinction is made between flow rates on either side of the heat pump, nor is the heat pump type considered.
The ThermoGIS base-case scenario (without a heat pump) represents the solely the “pure” geothermal output, which corresponds to the product of the flow rate, ΔT, and the volumetric heat capacity of the produced water.
In the heat pump scenario, the reported thermal output consists of the geothermal output plus the additional heat produced by the heat pump. In this case, the calculation of the economic potential also includes the electricity consumption of both the electrical submersible pump (ESP) and the heat pump.
Footnotes
1 A large maximum cooling range has been selected in the current calculations to ensure that an injection temperature of 20°C is applied for deep geothermal systems in the heat pump scenario. The extent to which water can actually be cooled in practice depends on the specific location and project conditions. Therefore, a site-specific analysis is required for each project.
2 The Carnot efficiency represents the theoretical maximum efficiency of a heat pump.
3 The heat pump system efficiency reflects how efficiently the heat pump performs in practice relative to the theoretical maximum. Typical system efficiencies range between 50% and 70% (www.industrialheatpump.nl); ThermoGIS assumes a value of 60%. This accounts for practical factors that reduce efficiency and results in performance estimates that better reflect real-world installations.