Variable refrigerant flow (VRF), also known as variable refrigerant volume, was a concept developed by Daikin Industries in the 1980s. The technology is based on transferring heat through refrigerant lines from an outdoor compressor to multiple indoor fan coil units. VRF systems vary the amount of refrigerant delivered to each indoor unit based on demand, typically through variable speed drives (VFDs) and electronic expansion valves (EEVs). This technology differs from conventional HVAC systems in which airflow is varied based on changes in the thermal load of the space.
The two main VRF systems are heat pump systems that deliver either heating or cooling, or heat recovery systems that can provide simultaneous heating and cooling. These two applications, plus the inverter-driven technology of the outdoor compressors, allow for greater design flexibility and energy savings. In applications where heating and cooling are simultaneously called for in different zones, VRF heat recovery systems allow heat rejected from spaces that are being cooled to be used in spaces where heating is desired.
VRF systems offer increased energy savings and improved climate control. With typical HVAC systems, the compressor operates in either ‘ON’ or ‘OFF’ mode, resulting in temperature swings as the compressor stops and starts to maintain the thermostat setting. In VRF systems, the compressor operates continuously, supplying the required refrigerant flow and adjusting the flow once the desired temperature is achieved, minimizing temperature swings. By varying the compressor speed to match the load as closely as possible, VRF systems achieve greater energy savings than conventional HVAC systems. Typical energy losses are also avoided because VRF systems do not move conditioned air through ductwork.
VRFs in Action: Bryant Park Winter Village
Since 2002, the Bryant Park Winter Village has been a popular attraction in New York during the holiday season. The Winter Village draws around 4 million visitors each year, and includes a restaurant, a holiday market, and a skating rink. Keeping temporary structures comfortably warm in the NYC winter is no easy feat, but even temporary structures such as the tents, which house the skate rental and the restaurant, can be a great application for energy efficient technology.
In past years, electrical resistance heaters have been used to provide heating to the Winter Village’s two largest structures: the skate rental tent and the USE Tent, home to Danny Meyer’s Union Square Events-run restaurant, Public Fare, and the Rinkside event space. For the winter of 2016-2017, the Bryant Park Corporation collaborated with Fujitsu General America and Green Star Energy to test the benefits of using air-source heat pumps with variable refrigerant flow (VRF) as an alternative to electrical resistance heating. Fujitsu provided the VRF system, Green Star performed the installation, and SWA performed the measurement and verification services.
The USE Tent was heated by the Fujitsu VRF Heat Pumps with back-up electrical resistance heaters. Space temperature was measured in the event space, which had double-height walls and vaulted ceilings.
The skate tent served as the control group, and was heated using only electrical resistance heaters. Space temperatures were measured in four locations: The Overlook event space, the office, the skate rental area and the baggage counter. The Overlook and office had single-height walls with a vaulted ceiling while the skate rental area and baggage counter had double-height walls with a vaulted ceiling. All space temperature measurements were taken at a height of 5-7 feet above the ground, which ensures that the temperatures experienced by the tents occupants were captured accurately.
How Did the VRF System Perform?
Electrical consumption was measured for all Fujitsu indoor and outdoor units and for all electrical resistance heaters in five-minute intervals. SWA graphed the electrical consumption of the heating equipment against outdoor air temperatures, and the results were promising for the VRF system.
The graph above shows outdoor air temperature on the x-axis and electrical demand in kW on the y-axis. The orange line shows the power draw of the electric resistance heaters and the blue line shows the power draw of the VRF system plus its back-up electric heaters. The Fujitsu system required a maximum of 12 kW, while the electric resistance heaters required double the amount of energy. One thing to note is that the Fujitsu system showed a sharp decrease in energy consumption when outdoor air temperature was below 28°F. This is not typical, and was a result of a period of cold weather when the back-up electric heaters were not engaged.
While it’s clear that the VRF system saved energy when compared to the resistance heaters, it’s important to verify that the space temperature inside the tent with the VRF system remained comfortable. At first glance, the interior temperature of the tent seems to leave something to be desired – 40°F is cold.
However, looking at the interior temperature of the Skate Tent, which was our control group, shows that the problem was not unique to the VRF system:
Both tents experienced temperatures as low as 40°F, and for good reason – there are many challenges involved in keeping these tents at a comfortable temperature. Both tents are double-height, with vaulted ceilings. Warm air produced by the VRF system and the electrical resistance heaters rises to the ceilings, rather than staying at occupant level. The tents are not tightly sealed, and allow cold air to infiltrate the interior. The tents are also high-traffic areas: the constant opening and closing of the tent doors allows cold air into the space.
Although the comfort levels were not ideal in either tent, the VRF system was as successful at maintaining space temperature as the electric resistance heaters, but with significantly less energy consumption.