 One observation is that while current "best practice" heat pumps (ground source system, operating between 0 °C and 35 °C) have a typical COP around 4, no better than 5, the maximum achievable is 8.8 because of fundamental Carnot cycle limits.This means that in the coming decades, the energy efficiency of top-end heat pumps could roughly double.Cranking up efficiency requires the development of a better gas compressor, fitting HVAC machines with larger heat exchangers with slower gas flows, and solving internal lubrication problems resulting from slower gas flow.