Variable Capacity Residential Heat Pumps
Variable capacity heat pumps (VCHPs) have the ability to deliver capacity below and above their nominal rated capacity by changing the speed of the compressor and modulating ancillary components like fans and expansion valves. This flexibility allows for similar component sizes to provide a wider range of delivered capacity. Variable capacity is a promising technology for addressing the imbalance between heating and cooling load demands, while maintaining proper cooling and dehumidifying ability under part-load.
This report examines variable capacity technology for a common U.S. residential application, the ducted, split configuration. The site selected for this demonstration of VCHP technology is a condominium building in Escondido, California. The selected site is in California climate zone 10 (CZ 10), which encompasses the interior hills and valleys of Southern California. The existing heating, ventilation, and air conditioning (HVAC) system, consisting of a single speed split heat pump system, was replaced in two of four units with VCHPs from Carrier’s Greenspeed line of heat pumps. Instrumentation was installed to monitor electrical and thermal characteristics of the baseline and VCHP systems. The baseline and treatment systems were monitored for a period of one full year, from April 2014 to March 2015.
The data shows a demand reduction on the order of 1.2 kW can be achieved by using a VCHP unit. This reduction aggregated over a large population can provide significant demand reduction. Estimating energy savings was difficult with the given data set since neither the occupancy data nor the set points on thermostats were available. To estimate occupancy an averaging methodology was used to determine heat pump operating hours. A conservative estimate of 30% energy savings over code minimum system was observed for the condos that were monitored. The energy savings and demand reduction demonstrated in this study can be achieved at other similar installations.