Heat pumps of the future will be the key to effective use of "low-grade heat".
-- They will be included in the exhaust of HRV ventilation systems to extract that last little bit of usable heat before throwing the stale air out. The exhaust air will probably leave the house colder than the winter air outside.
-- They will eventually be used with solar heating systems, allowing the solar systems to function at lower, more efficient, temperatures. More heat could be drawn out of the rock storage, effectively increasing the storage capacity without increasing its size.
Heat pumps are also being considered as a means of moderating either high or low temperature heat sources into even heat -- permitting warm (not hot) air ducting. This will permit using cheap and easy-to-install flexible tubing for the entire duct system, and lower heat losses through the duct.
Ground-source heat pumps are already on the market. They bury pipes in the ground and take the heat out of the earth itself. They can be used only in very special soil conditions. The problems of ice block formation in the soil around the pipes limits these installations to experienced professionals.