Validating the COOLNOMIX AC and Refrigeration Compressor Control Retrofit
Bryan Urban | Fraunhofer USA, Inc.
Objectives of the Demonstration
Cooling and refrigeration equipment can waste energy when fixed-capacity compressors run longer than necessary during part-load conditions. This demonstration evaluated a relay-based compressor control retrofit designed to modulate part-load capacity in six single-zone ducted (packaged RTU and split-system, 5-7.5 ton) air conditioning systems and six single-room walk-in cooler refrigeration applications. On/off testing was performed in hot and dry climates. The goal was to demonstrate predictable, cost-effective, and replicable cooling energy savings of at least 15% with a <2 year payback; ability to maintain or improve space temperature control; ease of installation; reliable operation; and end-user satisfaction. Space cooling and refrigeration accounts for about 15% of commercial building site energy use. This represents an addressable savings opportunity of about $60M per year or a 1.5% reduction in total DoD facility energy utilization.
COOLNOMIX is a standalone control retrofit for vapor-compression air conditioners and refrigerators that modulates compressor runtime using a relay to save energy while maintaining temperature setpoints. A patented algorithm uses signals from wired supply and return air temperature sensors to intelligently cycle the compressor, aiming to supply just enough liquid refrigerant to meet space cooling loads. Evaporator fans continue running between cycles to provide residual cooling while the compressors remain off. The controller is installed in-line with the existing thermostat or control system and requires no special hardware or software integration.
Energy savings results were mixed. Air conditioning systems performed as designed for five of six cases, but meaningful savings were found in only two special cases whose ventilation fans were programmed to run all the time: (1) 17% net savings (4,030 kWh/yr) and (2) 8% net savings (1,960 kWh/yr). In other cases, savings were either not significant or caused total energy use to increase. The single-zone ducted cooling systems, common in the U.S., had high-powered ventilation fans that were caused to run about twice as much, incurring steep fan energy penalties and higher ventilation loads. All six refrigeration systems performed as designed, though none met the 15% savings target (best-case was about 5% savings, or 100 kWh/yr). Installation cost per system ranged from about $560-880, including hardware ($500) plus labor and materials ($60-380). Further technical development is underway to address the excess fan runtime issue.
COOLNOMIX was compatible with most existing single-zone equipment, functioned as designed, and caused no significant equipment problems. Aside from causing excess fan runtime in most air conditioning applications, implementation issues were relatively minor. In some cases, zone temperature and humidity were affected slightly, but not enough to seriously impact thermal comfort. In this study, several recommended improvements to the installation procedures were identified related to sensor placement, temperature setpoint selection, and commissioning.