Source: Western's Energy Services Bulletin

Ground-source heat pump (GSHP) systems that use variable refrigerant flow (VRF) are among the most efficient and cost-effective heating, ventilating and air conditioning (HVAC) systems available. By using ground water loops, multi-speed fans and variable speed compressors, ground-source VRF heat pump systems achieve savings over either GSHP or VRF alone.

GSHPs vs. air-to-air heat pumps

A typical air-to-air heat pump, which exchanges heat with outside air to provide heating or cooling, loses efficiency as the outside air temperature deviates from ideal. To compensate for decreased performance in extreme conditions, air-to-air systems are often over-sized. If ambient conditions could be moderated, this over-sizing would not be necessary. One way to do this is to bury pipes underground, where the temperature is relatively constant year round. 

GSHPs, also known as geothermal heat pumps, capitalize on this idea by tapping a large reservoir of nearly constant-temperature heat—the ground. Because heat is exchanged with the ground, not air, GSHPs remain a very efficient source of heating and cooling all year.

GSHPs typically use refrigerants to draw heat from or reject heat to water, which is the heat-exchange fluid. Water transfers heat much more efficiently than air, so GSHPs gain additional efficiency.

Benefits of VRF

VRF technology uses multi-speed fans and variable-speed compressors to enhance the efficiency of exchanging heat with water. VRF systems are gaining universal acceptance because:

• Outdoor heat exchanger units are connected to indoor units with small refrigerant piping.  This requires less indoor space, resulting in more design flexibility and rentable space.

• Some units may be in heating mode when others are in cooling mode. This allows heat rejected from rooms that are being cooled to be used in rooms that are being heated. 

• VRF systems minimize duct losses because most or all ductwork is eliminated. This may allow buildings to be designed with lower floor-to-floor heights, reducing construction costs. 

• VRF systems use multi-speed indoor air handler fans, multi-speed outdoor condensing unit fans, variable speed compressors and linear expansion valves, features that all contribute to the high part-load efficiency.

Energy savings from ground-source VRF heat pumps

To compare the energy use of various HVAC systems, an assisted living facility was modeled in various cities using Energy Pro building modeling software. Ground-source VRF heat pump systems were found to use, on average, 37 percent less energy than air-source heat pump systems:

Annual energy use and percent savings compared to air-source heat pumps

Little Deschutes Lodge case study

The Little Deschutes Lodge, an affordable housing project for seniors in La Pine, Ore., has a ground-source VRF heat pump system and the facility is nearly net-zero energy. Over a 12-month period, the gas and electric bills in this 26-unit complex averaged out to about $37 per month, per apartment! The Lodge's energy use index (EUI) is only 29 kBtu/square foot/year, compared to about 70 kBtu/square foot/year for an average dorm unit as calculated using Energy Star's Portfolio Manager, making it more efficient than 95 percent of similar units. Installing a ground-source VRF heat pump system at Little Deschutes Lodge has saved the owners and residents a lot of money. 

Conclusions

For affordable housing to be viable, operating and maintenance costs need to be as low as possible so property owners can hedge against rising costs for utilities, operation and maintenance. A highly energy-efficient HVAC system can go a long way toward helping to achieve these goals. Based on modeling of an assisted living facility using Energy Pro software, the ground-source VRF heat pump systems can reduce energy consumption for heating and cooling by 32 to 40 percent. 

However, ground-source systems are invariably more expensive than air-source systems, so the engineer will need to do a cost-effectiveness analysis to determine if the extra cost can be justified. For comparison, air-source VRF installation costs about $14-18/square foot. Ground-source system installation costs approximately $30/square foot. Systems combining ground-source with VRF will cost slightly more to install. 

More Information from Washington State University Extension Energy Program