I am using a Swedish Ground-source Heat Pump for domestic water and heating. Many thousands of homes use them in Sweden, Germany USA etc but they are rare in the UK. Essentially it extracts heat from the ground as follows:
A continuous loop(s) of water filled pipe (2 x 200m in my case) is buried 1m
The loop passes into the house and passes through the heat pump giving its
heat to a loop of refrigerant
The refrigerant is compressed and reaches 55°C
The heat from the refrigerant is used to heat domestic hot water and the
1 Kw of electricity (the only energy source I need) produces 3.5Kw of heat
i.e. 350% efficient!