Performance Measurement and Verification of Variable-Speed Packaged Rooftop Units

Abstract

Packaged rooftop units (RTUs) are widely used in commercial buildings. Most RTUs in the field are equipped with constant-speed fans and compressors. Many efforts have been made to increase the operational efficiency of RTUs with advanced design and controls. This paper presents a field study that evaluates the cooling efficiency and energy performance of RTUs with all variable-speed components, including the supply fan, the compressor, and the condenser fan. The field evaluation was performed at two sites: a supermarket in New Smyrna Beach, FL and an office building in Fort Worth, TX. On each site, a conventional RTU with constant-speed fans and compressors was selected as the reference. A set of sensors were used to measure the dry-bulb temperature and the relative humidity for the outdoor air, the return air, the mixed air, and the supply air. The RTU total power consumption was also measured using a power transducer. These sensor measurements, together with a number of control signals were monitored at 1-minute intervals. Based on the monitored data, the energy efficiency ratios (EERs) was computed for each unit daily and over the entire monitoring period. On the Florida site, the advanced RTU had an overall EER of 10.9, which was about 31% higher than the reference unit. In contrast, on the Texas site, the advanced RTU had an overall EER of 12.2, which was about 16% higher than the reference unit. The economic analysis showed that relative to an alternative scenario that either the units were not replaced or like-to-like replacements were made, the advanced RTU could at least reduce the electricity bill by $1670/yr and $4082/yr on the two sites.