A brine heat pump can be realized with flat collectors or with a borehole. Since no boreholes are approved in my area (karst region), only flat collectors were possible, and even those only under conditions with geological supervision, etc..
Unfortunately, I cannot report any experiences to you yet, as we will only pour the base slab next week. However, I have dealt with it intensively. Sharp subzero temperatures are often mentioned, but they do not pose a problem. I will take my pump as an example, the Logatherm WPL 8 AR. It operates down to -20 degrees ambient temperature and still has an output of almost 6 kW at that point. In addition, there is the electric heater with 9 kW (bivalent but only 6 kW), which means a real heating capacity of 12 kW @ -20 degrees. Temperatures colder than -20 degrees are extremely rare. I have analyzed historical weather data from the DWD regarding this. In the last 67 years, the daily low temperature was below -20 on 59 days. However, this is still not a problem just because it is colder for about 3 hours at night and the heat pump’s capacity drops from 12 kW to 9 kW. And only on 2 days in 67 years was the daily mean below -20. We are at 550 m and in an area described as cold and snowy.
Additionally, at such cold temperatures there is generally no cloud cover. That means there are solar gains during the day which should not be neglected. These are not taken into account in a heating load calculation, but in reality they are there.