You aren’t really extracting heat from the ground, it is the heat achieved from the change of state of the coolant. The cold is sent to the outside earth in the winter and the heat is retained inside. It really is just a giant refrigerator. That hot coil on the back of the refrigerator is indoors in winter and is switched to the outdoors in summer by changing the direction of coolant flow, making the evaporator inside during the winter and outside during the summer. If the ground were to freeze around the “evaporator” tubes the system will cease functioning. Here in Wisconsin the outside evaporator is 10’ underground and hundreds of feet long. A well designed system is very frugal...many older systems are under-designed from what I hear...
Not to be too picky, but yes, ground source heat pumps DO extract heat from the ground in the winter and then put it back into the ground in the summer. The phase change of the working fluid is a way of storing more energy in the transaction, but heating and cooling can be obtained without a phase change in certain systems.
See the International Ground Source Heat Pump Association at http://www.igshpa.okstate.edu/
for more details and some good diagrams.
And, in tech-speak, there really is not any such thing as “cold”. Everything is heat. “Cold” is just less thermal energy than “hot” when compared to absolute zero.
BS in Mechanical Engineering, 1979
Thermodynamics/Energy concentration
NC State University
Moo.