I know exactly what and how a heat pump works and does, thank you. I'm
concerned you are morphing into a troll.
Back to your original question about efficiency...
IFF you have an "ample" ground water supply, you can go open loop, draw
ground temp water from one well, run it through the heat exchanger then
back out. Where it goes varies, but for now, just assume that you're
pouring it back into the aquifer.
Water temperature in this case will be pretty much constant throughout the
year. Say 50F for the mid atlantic state. At this temperature, you'll get
the heat pump's rated output which is typically given at around this
temperature.
If you don't have enough ground water to waste, you can use a "standing
column well". this is a deep well where you get water from the bottom of
the well and pour the "used" water back in the top. If the well is deep
enough, then the water drawn out will be a fairly constant temperature.
Downside is the depth of the well will cost you.
If you don't have an ample ground water supply, you can go vertical closed
loop. Ground temperature down below ~15' is essentially constant. However,
as you draw heat from the ground, it will cool. Depending on how fast the
heat can get back in to where the loop is, the cooling of the ground will
vary. If the loop field is poorly designed, it can drop to significantly
and your heat pump output and efficiency will diminish.
Vertical loops will require lots of drilling in most situations to "do it
right". Typical ~250ft/ton but this varies on ground conditions.
If you can't go vertical and go horizontal, you have seasonal ground temp
variations to consider as well as temperature drop from loop use. But
excavation costs will likely be far lower than drilling. And, keep in mind
that a good horizontal loop will be far better than a poorly designed
system of other sorts.
Finally, you have air source. As other posters indicated, if the air
temperature is higher than the ground temperature, all else being equal,
then the air source system will be more efficient but during the cold of
the winter, this is unlikely to be the case. If you look at the "bin data"
for your region, you can find the amount of the year that is at each
temperature.
Beware however!!! As soon as you start running a ground source system, the
ground temperature will start dropping. The longer you run it, the more it
will drop. When you stop running it, the ground temperature will recover.
The problem is, if you have a stretch of cold weather where you run the
system many hours a day, the ground may never have a chance to recover and
will just drop in temperature until the energy extracted balances the
energy that can return. As such, the system will perform worse than
expected. Moreover, it may take some time for the temperature to recover,
so you might find that from December through February, your system is
running at, say, 20F rather than the stated ground temperature of 50F.
Then you'll be getting less than 70% of the rated heat output and far less
efficiency.
