The geothermal power of a doublet can be enhanced in several ways. Two options were calculated in ThermoGIS: by stimulating both wells and by adding a heat pump. A third scenario combines the two options. This results in four scenarios:
- Base case
- Well stimulation
- Heat pump
- Well stimulation and heat pump
Well stimulation
The goal of stimulating a well is to decrease the resistance the water encounters while being produced from or injected into the reservoir. Technically, this means that the skin factor of the well is reduced. For the well stimulation scenario in ThermoGIS, the skin reduction was set to a value of 3. In combination with the skin due to the well deviation, this results in a skin factor of -4 for both wells. The CAPEX, which is the cost of stimulation, was then increased by €0.5 million.
Heat pump
The heat pump option in ThermoGIS allows for a lower injection temperature than specified by the return temperature of the greenhouse or district heat network (i.e. the inlet temperature of the heat exchanger / heat pump Tdh_in. It is assumed that the injection temperature Tinj is chosen as low as possible to maximize geothermal power, however limited to a maximum offset:
with:
- Tinj = injection temperature
- Taq = aquifer (reservoir) temperature
- ΔT = temperature difference between produced and injected water
Many options exist for implementing a heat pump, relating for isntance to the in- en output temperature, the capacity of the heat pump, etc. For the scenario available online, the heat pump was added to the end of the surface installation. The return temperature was decreased from the default 30 °C to 20 °C. In this way, the additional heat qualifies for SDE+. The electricity that is consumed by running the pump and for extracting more heat from the brine before re-injection does not qualify. The way in which the heat is used in the process is not described here - the application can be very diverse. Instead of calculating a COP for the heat pump, a constant value of 5 was adopted. TNO is currently working on a more detailed implementation of the heat pump option. Some information can already be read here. The heat pump CAPEX is assumed to be €200 per kW-power, and the yearly OPEX €20 per kW. The heat produced by the electrical pump of the heat pump is not added to the power of the doublet because it does not qualify for the SDE+ subsidy. It has been given a value of 2 €ct/kWh, which represents the cost price of an alternative hot water installation. This value is subtracted from the OPEX.
The result of applying this heat pump scenario can be viewed in the Map Viewer's 'Detailed' tab once the power or economic potential map of an aquifer is shown, by clicking one or both of the Stimulation and Heat Pump buttons.