NOVELTY - The controller has a processor for measuring incremental changes in temperature of heat transferring fluid entering into a ground loop heat exchanging (GLRE) subsystem, and measuring heat transfer rate (RTR) across the GLRE subsystem. The processor automatically generates flow rate control signals for use by a set of electrically controlled pumps to control flow rate of the heat transferring fluid flowing through the GLRE subsystem in response to the monitored incremental changes so that the GLRE subsystem satisfies the RTR requirements of a geothermal equipment e.g. chiller.    USE - Controller for controlling enthalpy-driven ground loop volumetric flow rate of heat transferring fluid i.e. water, flowing through a ground loop heat exchanging (GLRE) subsystem installed in a deep earth environment outside of a building.    ADVANTAGE - The controller measures capacity of ground heat exchanger (GHE) installations to transfer heat energy with the surrounding deep earth environment. The controller produces a controlled flow of water at a controlled output temperature when connected to a test GHE installation during an enthalpy-based method of measuring the heat transfer rate of the GHE installation in a deep earth environment, thus guaranteeing geothermal system performance of the GLHE subsystem. The controller measures the heat transfer rate (HTR), flow work rate (FWR) and energy efficiency ratio (EER) and heat transfer efficiency (HTE) of GHE installations, so that engineers can make rational decisions to use a particular class, type or design of GHE to construct the GLHE subsystem for a particular geothermal system project, planned for construction at a specific ground loop site. The controller provides a wireless portable GHE performance testing and monitoring system and network capable of acquiring performance data on GHE installations in diverse geological environments and operating environments, so that empirical heat transfer rate (HTR) surveys can be produced using a standardized high-efficiency closed-loop type concentric-type GHE. The controller does not use or require control or timing signals to control the operation of refrigerant compressors associated with the geothermal equipment. The controller automatically increases or decreases the volumetric flow rate of water flowing through into and out of the GLRE subsystem, so as to minimize electrical energy consumption of electronically-controlled ground loop pumps employed to pump water through the GLRE subsystem.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a process for controlling flow rate of heat transferring fluid flowing through a ground loop heat exchanging subsystem.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of an enthalpy-based ground loop heat exchanging performance test instrumentation system with an electrical power generator in a non-contact state.    Enthalpy-based ground loop heat exchanging performance test instrumentation system (1)    Ground heat exchanger (2)