Ground-source heat pump (GSHP) systems are perhaps the most widely used “green” HVAC system, with an estimated 1.1 million1 ground-source heat pumps installed worldwide. These systems may be closed-loop (“ground-coupled”) or open-loop. The frequency of use tends to vary regionally, depending to some degree on building load profile, geology, environmental regulations, and human infrastructure. This editorial reviews past research, from the 1940s to present, and looks forward to what advances may be expected in the future.

A Swiss patent issued in 1912 to Heinrich Zoelly is the first known reference to ground-source heat pump systems. 2 In the US, some ground-source heat pump systems were installed just prior to World War II3 and post-war, installations began to take off. At the same time, about a dozen research projects involving laboratory investigations and field monitoring were undertaken by US electric utilities. In addition, analytical investigations by Ingersoll et al. 4 provided some of the theoretical basis for later design programs. After some time, interest in further research waned—apparently, problems with drying around horizontal ground-loop heat exchangers, 5 leakage, 6 and undersizing7 led to the gradual cessation of new installations.