Variable Capacity Space Conditioning Systems for Residential Customers

Central air conditioning (AC) is the primary heating, ventilation, and air conditioning (HVAC) configuration throughout California residences. In California Climate Zone 10, located within southern California, central air-conditioning accounts for approximately 17% of the total energy consumption of a residence, at 1.1 kWh per square foot of cooled space [1, 2]. Utility programs aimed at reducing air conditioning energy use in California Climate Zone 10 include efficiency programs for air-conditioning equipment rated at 15 seasonal energy efficiency ratio (SEER) or higher and demand response (DR) programs, which cycle air-conditioning equipment at predetermined intervals. Because residential variable capacity (VC) AC may be a potential enhancement or additional resource for utility programs, this research study investigated VCAC for energy efficiency and DR programs for residential space conditioning. The investigation included a technical survey of available equipment, a laboratory assessment of three VC air conditioners of varying SEER, and a field study of the three VC systems over a California cooling season. The study demonstrates the potential efficiency and DR capabilities of the VC equipment for California residences and utility programs. The team explored three research areas in the laboratory and in the field: potential energy and demand savings of VCAC, correlation of VC equipment with SEER, and system effects of VC equipment. Based on survey findings and lab capabilities, the team selected three 2-ton VC systems for inclusion in the laboratory testing. In the laboratory evaluation, VCACs of 18, 19.5, and 22 SEER were evaluated under field design operating conditions for the California climate. All three systems demonstrated improved efficiency at part-load operation for a specific indoor and outdoor condition. When compared to a baseline 14 SEER air conditioner, an energy model for California Climate Zone 10 determined energy savings of 12%, 26%, and 30% for the 18, 19.5, and 22 SEER equipment, respectively. Compared to the current utility program level of 15 SEER, VC equipment of 18, 19.5, and 22 SEER demonstrated potential energy savings of 7%, 22%, and 26% respectively for California Climate Zone 10. The VCto-baseline comparisons demonstrated increasing energy savings with increasing SEER, as well as a potential efficiency enhancement for utility programs. The 18, 19.5, and 22 SEER VC equipment consisted of 11, 13, and 15.5 EER respectively. At 105°F outdoor temperature, the 13 and 15.5 EER VC equipment decrease power demand by approximately 5%, and 10% respectively over a baseline 12.2 EER air conditioner. In contrast, the 11 EER VC unit had a 10% increase in power demand over a baseline 12.2 EER air conditioner at 105°F outdoor temperature. In the field, VC equipment was installed with SEERs of 20, 17.5, and 21 at three occupied residences within California Climate Zone 10; the field-selected VC equipment were similar to the lab-selected equipment, but were larger in capacity, sized to the needs of each field site. The 20 SEER system was oversized for the monitored cooling season and predominately operated at the unit’s minimum cooling level. Based on the laboratory data for each of the three selected VC systems and each field site’s operational characteristics, an expected seasonal cooling efficiency was determined for each field site. For both units appropriately sized for their field sites, the expected seasonal cooling efficiency compared well to the measured seasonal cooling efficiency based on field data. The baseline air conditioning equipment previously installed was estimated to be 12, 11, and 10 SEER for Sites 1, 2, and 3, respectively. This VC retrofit was accompanied by enhanced quality installation practices that improved field conditions: the duct leakage, external static pressure, and duct insulation. Based on utility billing data and field monitoring data, the VC equipment and quality installation retrofit resulted in cooling season energy savings of approximately 30%, 18%, and 30% for the 20, 17.5, and 21 SEER equipment, respectively. Field monitoring also included a light investigation of sensible duct losses with variable airflow in VC equipment. At one of the VC field sites that contained supply ductwork in the attic, 4 supply air temperature sensors were placed in 10-foot increments along the primary supply air duct of the distribution system. The estimated sensible duct loss along the supply duct was determined by assessing the difference between the supply air temperature reading at the unit outlet and the sensor located approximately 30 feet down the supply duct. Over the range of recorded outdoor conditions, the estimated sensible duct loss approximately doubled from high airflow to low airflow. For example at an outdoor temperature of 95°F, the measured sensible duct loss at the site was approximately 12% for maximum airflow and approximately 21% for minimum output. To consider VC equipment as a resource for utility DR programs, the project team examined DR unit setup and DR operation of selected VC equipment in laboratory and field evaluations. DR operation consisted of OFF-cycle controls. VC system 2 claimed to have OFF-cycle and low stage capacity modulation controls. To enable DR operation, the VC equipment in this investigation required manual adjustments of dry-contacts in the outdoor units. VC systems 2 and 3 also required further adjustment of controls/thermostat settings in combination with dry-contact adjustment to enable DR operation. None of the equipment investigated was readily able to receive and act upon utility DR event signals. During the laboratory evaluation, it was discovered that the low stage DR capacity modulation controls of VC system 2 were not functioning. The manufacturer identified that a software update would have to be developed, and the issue was not resolved at the time of the investigation. During several DR tests of the VC equipment, the indoor blower continued to operate during the active DR time frame. In a field situation, continued operation of the indoor blower may circulate warm air throughout the air. Continued investigation into certain aspects of residential VC space conditioning could better define and quantify their potential performance for California residences and utility programs. Ongoing industry research is further investigating the impact of duct losses for VC systems. Integrating the expected duct performance of a variable speed blower would assist in quantifying the overall VC HVAC system performance with respect to a baseline system. Also, utility billing data was used to quantify the energy usage of the baseline field equipment in this study. Simple utility billing data is limited in the information provided, and a more robust baseline-to-VC monitoring package could potentially provide better quantification of energy savings with VC equipment.