Research conducted by the Geothermal Exchange Organization (GEO), Oklahoma State University (OSU) and Oak Ridge National Laboratory (ORNL) has shown that a geothermal heat pump (GHP) system offers more efficiency in a large commercial building than a variable refrigerant flow (VRF) system.
For the study, researchers collected data at the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) international headquarters building in Atlanta, Ga., for two years from July 1, 2011 – June 30, 2013.
ASHRAE’s two-story, 31,000 square foot headquarters includes a “living lab” to offer researchers the chance to evaluate commercial building energy and sustainability performance. Among other energy efficient features, the building uses three separate HVAC systems:
- A variable refrigerant flow system for spaces on the first floor.
- A geothermal heat pump system, primarily for spaces on the second floor.
- A dedicated outdoor air system (DOAS), which supplies fresh air to both floors.
To facilitate efficiency studies, the ASHRAE building also includes 1,600 sensors that monitor HVAC operations and conditions in each building zone.
The GEO study concentrated on actual heating and cooling provided by the GHP and VRF systems, data analysis, and computation of measured and experimental metrics. Data points included operating mode, zone temperature and discharge air temperature for each individual FCU or heat pump. The researchers gathered data on ground-loop water supply, return temperatures and flow rates for the geothermal system.
The team also collected data for metered energy used by each system.
Raw data show that the VRF system used twice as much energy as the geothermal system during the study period. When differences in floor space were normalized, energy use by the GHP system was an average 44 percent less than the VRF system. The GHP system used 29 percent less energy in the summer and 63 percent less energy in the winter/shoulder seasons than the VRF system, while maintaining similar zone temperatures.
Though the control strategy for the VRF system resulted in longer runtimes than the geothermal system, ground loop water supply temperatures were more favorable than ambient air temperatures for heat pump operation, which allows the geothermal equipment to operate more efficiently.
The researchers documented their findings in the report, “Final Report, Performance of the HVAC Systems at the ASHRAE Headquarters Building.”