The benefit is increased COP of the heat pump as compared to geothermal. Here
in Michigan, a geothermal heat pump closed loop system drops to about 30 deg. F
in the middle of winter, which lowers the COP of the heat pump to less than 2.0.
If the inlet water could be raised to 60 deg. F, the COP would be closer to 5.0.
A solar heating system can easily create 60 degree water. The efficiency of a
solar collector goes up if it can be designed to work at lower temperatures.
Seems like the two technologies fit very well together.
Just about all refrigerant compressors are cooled by the refrigerant, so
the heat *does* go to the condenser.
The fan circulating air passed the evaporator would be a loss, but
rather small compared to the whole unit (what's a common ratio of fan
power versus compressor power?) With conventional heat pumps, the fan
circulating air passed the condenser is not a loss since it's energy is
delivered to the house air.
As far as running a compressor with it's valves shut, yes well the
idiotic can't be helped. Same would be true for a seized compressor
that doesn't have overload protection, or no refrigerant, or any number
of other idiotic scenarios.
In any realistic situation, the COP can't go 'much below 1.0' (guess it
depends how you define 'much below 1.0'.)
The bigger issue is that most home heating heat pumps are only designed
for a few tens of degrees rise (say about 40F) and heating water to
120F+ using 60F or less outside air is just not within its design. So a
solar 'boost' to raise the evaporator end to 80F+ has some merit. Of
course there are common heat pumps that can run with 70F+ temperature
difference (one in most homes is the food freezer, although they are
much lower capacity).
An issue comes up if the heat pump capacity exceeds the solar input.
Then it naturally cools the collector down and this reduces heat pump
capacity and reduces ambient losses of the collector. So its somewhat
A simple thermostat could be used to shut off the heat pump when the
collector temperature drops too low. Where 'too low' is not very
critical unless the collector might be damaged by freezing/frost.