Reliability and performance of direct-expansion ground-coupled heat pump systems: Issues and possible solutions

The literature on Direct-expansion ground-coupled heat pumps (DX-GCHP) is reviewed. Despite being potentially the most efficient vapor compression-based air conditioning system, DX-GCHPs are not very popular due to the numerous operational and design issues they are faced with: shortage of field studies, oil return to the compressor, expansion valve hunting, refrigerant flow mal-distribution in parallel loops, high pressure drops and gains in the ground heat exchanger, variable speed operation of the compressor, lack of reliable numerical models of the ground heat exchanger and of the whole system, and lack of generalized design guidelines. This paper addresses these issues and discusses possible solutions to them.