Is a brine-water heat pump still worthwhile with KfW 40+?

That directly gives you an answer. ;-)
But there are other reasons as well.
For example, my impression is that flat plate collectors are only offered by a few companies.
Doing it yourself is also not an option for everyone.
Basically, however, it is a good option that should be considered more often.

What, no extra price or half extra price?
How was the collector implemented for you?

For us, no extra price. Normally in advance, about 50% of the deep drilling.

In advance, in KW 2 2022 the groundwork for collectors will begin. The collectors are stacked on top of each other. As far as I remember, the dimensions are 25m length x 4m width. They are buried about 80 cm deep. Above that, a drainage system is laid to increase efficiency. The pump is the latest from Bosch (Compress 7800i LMW). There is also a tempering system.

But there are few reasons why electricity should become significantly more expensive.
Electricity from wind or sun costs only about 3-8 cents/kWh to produce.

That's exactly what I think too. Generation is becoming cheaper, storage as well (and simultaneously more effective).
Why it will be more expensive at first lies in the mechanics of the Renewable Energy Act surcharge and the massive exemptions from it (the surcharge is simply paid by too few consumers and is therefore far too high).
In the long term, energy will become cheaper again. The transition period now is simply expensive.
(Please do not generalize from energy to other resources – some things are simply becoming scarce and expensive...)

We have a ground-source heat pump for aesthetic reasons... presumably one is not allowed to recalculate :oops:

Even if the "heat" coefficient of performance of a ground-source heat pump is systemically clearly advantageous at very cold temperatures, it is the case that a relatively large proportion of electricity is needed for pumps & co in the overall system – and that then influences the overall coefficient of performance again in favor of the air-to-water heat pump. The ratio of auxiliary electricity to heat output is all the less favorable the lower the heating/hot water load is... and 2x 95sqm KfW40 will certainly be very low (is there already a calculation?). And then there is the question of what actually creates the greatest heating demand on an annual average: the few days at -10°C outside temperature, or the several months around 0°C?

The additional costs of a ground-source heat pump would certainly be more economically invested in clean planning and execution as well as a possibly large photovoltaic system. Unless one has the same outdoor unit allergy as we do ;)

The calculation is very simplified though. You assume that electricity costs remain constant. How likely is that? The higher the electricity costs, the greater the savings...

Of course simplified. But the difference will not increase massively, since both heat pumps run on electricity. And whether I recoup the additional investment in 35 years or 60 years does not matter in the end. The bigger the house and the lower the surcharge or if there can be a ground collector, the more interesting the brine-water heat pump becomes. But in a normal house with a surcharge of €5k, you are already far from an economically sensible decision. As I said, I also had a brine-water heat pump with deep drilling. Without an outdoor unit and with a strong selling point when we sold the place. It was also 300 sqm to heat...

We have a brine-water heat pump for aesthetic reasons... I suppose you’re not supposed to do the math :oops:

There should be a laughing face here. Joyfully ignoring economic efficiency is part of everyday human life.

[*]Ever eaten out?
[*]Bought a piece of clothing that serves not only to warm and cover? (Or shoes...)
[*]Bought the most economical vehicle?
[*]Taken a vacation?
[*]Given someone a gift?

Enjoy the missing outdoor unit – I find exactly such decisions practical for life – at least in private life.

Very interesting. I think that if we decide on a brine-water heat pump, then trench collectors (or possibly baskets, for which I have not yet found anything useful) would be an option. We could do the excavation work ourselves, which would already be somewhat cheaper than drilling. Unfortunately, there is no precise calculation for the heating load/DHW load yet.