Variable Capacity Rooftop Unit Evaluation of Field Performance
Traditional, single speed space conditioning equipment operates in one of two states: either on or off. When single speed equipment is operating, it provides 100% heating or cooling output at a fixed efficiency based on the ambient conditions. In recent years, manufacturers have incorporated variable speed compressors, blowers, and fans with more intelligent controls into space conditioning equipment. These variable speed systems have the ability to actually vary the heating or cooling output, which is termed “variable capacity.” A variable capacity system may offer several potential benefits to electric utilities in implementing energy efficiency, peak load reduction, and demand response.
This technical update summarizes findings from retrofitting a fixed speed, commercial, packaged air conditioning rooftop unit (RTU) (the baseline unit) with a variable capacity (variable speed) air conditioning RTU (the treatment unit). The site selected for this study is a single floor commercial building that houses a welcome lobby, guest services counter, office space, hallway, and restrooms at a resort in Southern California in the San Diego Gas & Electric (SDG&E) service territory. The baseline unit was a 10-ton air conditioning system (with gas heat) supplemented by a 2.5-ton split air conditioning system. A 12-ton air conditioning unit with gas heat replaced these two units.
The baseline system (only the 10-ton packaged RTU) was monitored for two months—from April 15, 2015, to June 14, 2015—for electrical characteristics and ambient conditions. Since the variable capacity treatment unit was a new install, the RTU was instrumented for electrical characteristics, thermal characteristics, and air flow measurements. A period of one month (October 2015) was chosen to compare treatment RTU performance with the two months of data from the baseline RTU system. The two periods were chosen based on the cooling degree days experienced by both systems. To account for the difference in size (tonnage) of these units and for the slightly different weather conditions encountered, the energy consumption was expressed in terms of kWh per cooling degree day per ton.
The results showed a 10.4% reduction in energy use by the variable capacity treatment RTU when compared with the fixed speed baseline RTU. The treatment RTU achieved significant demand reduction on the order of 2.8 kW (30% reduction) in 60°F–100°F ambient temperatures. This reduction in demand can be attributed to the modulating components of the variable capacity RTU.
One of the challenges to use of these variable capacity systems is the cost of equipment. The cost difference between the advanced variable capacity system and a code minimum system is very high, thus increasing the payback period significantly. It must be noted that the study and the results are based on one site; results can vary depending on the site selected, hours of operation, and system load.