What about using a pool for water source HPs
Ok, new house here with HE HPs. Live in NC where heat costs about 3x a/c - hard to believe but energy star thinks its true. It is at least 2x just to give an idea.
Thinking about putting in a pool - imagine 20,000 gallons (nice even number).
Now - could you use a water based outdoor unit to use the more constant temp of the water as your heat source? In the summer, I wouldn't expect much of a difference except on hot days but it would have the side effect of heating the pool.
In the winter, the water with a solar cover would probably average 50 degrees as opposed to the air which might be 35 degrees. That probably represents a 20% improvement in efficiency assuming that water and air are equal. Obviously water needs to get pumped and also has a lot more heat density but I don't know what these differences would do to the efficiency of the system. The winter water temp could be supplemented with solar if it made sense but it is pretty sunny and I suspect a solar cover will heat it up a decent amount during the day.
On a really cold night, I calced a 5 degree drop in water temp from the heat pump alone. In a cloudy and cold situation, the water temp could get down to 32 degrees. People don't drain their pools around here. Certainly if you were driving the water temp to near freezing, you could switch back to air source.
In case you are wondering, no real room for geothermal except vertical loops and there is a real problem getting drills onto the property.