Comparison of geothermal heat pump with LWW pump

And?
This is the case with all heat pumps that the efficiency decreases the colder it gets.

What is decisive for the subsidy is what the annual performance factor calculator outputs.

Yep. However, you can use heat sources that are not (so much) subject to seasonal fluctuations. And that are particularly inefficient precisely when they are especially needed. But this depends on the region and the winter.

And? This is the case with all heat pumps that the efficiency decreases the colder it gets.

Our assumption was that the issue with particularly cold days would likely decrease over the coming decades. When looking at the development of the global climate, the really cold winters here by the North Sea are more likely to become fewer rather than more, and temperatures around 0°C in the coldest months seem more probable than really deep subzero temperatures.

Not at all. The discussion here is about air-water heat pumps <> brine-water heat pumps and with the brine-water heat pump, the brine temperature is mostly independent of the outside air temperature.

COP and calculated annual performance factor are theoretical values. The fact is, however, they are always improving, at least in the laboratory. Practice still remains to be seen, the models are too new. Maybe it’s also time for BAFA to adjust its criteria. Yep. But you can use heat sources that are not (as strongly) subject to seasonal fluctuations. And which are particularly inefficient precisely when they are most needed. But that depends on the region and the winter. In the last heating season, January 2020 was the month where we used the most heat energy in the house. 1641 kWh heat including hot water. With a performance factor (including pumps and control) of 4.69 or 350 kWh electricity. When I think that a common air-water heat pump might have managed about 3.5 there, that would have been over €30 more electricity costs in that month to heat the house. And we had five such months this season... Last calendar year, I kept a four-person household including hot water warm over 200 sqm with 2200 kWh electricity. That corresponds here to about €572 per year, or €48 per month using household electricity (not heat pump tariff). Although that is not entirely complete, since part of the consumption comes from my own photovoltaic system, actual consumption costs are therefore a bit lower. Maintenance costs €0. I am therefore still very satisfied with a brine-water heat pump using drilling.