Performance Evaluation of Rooftop Air Conditioning Units At High Ambient Temperatures

Ramin Faramarzi, Bruce Coburn, Rafik Sarhadian, Scott Mitchell, and R. Anthony Pierce, Southern California Edison

Keywords

Abstract

The cooling capacity of packaged, air-cooled, rooftop air conditioning units (RTUs) declines as the ambient temperatures increases. Rooftop air conditioning units’ performance parameters such as power consumption and efficiency rating are much less understood at temperatures exceeding 115oF. These temperatures are typically beyond those at which the RTU manufacturers test their products. Many climate regions within California achieve these high ambient temperatures on a consistent basis.

The purpose of this research is to determine the impact of high ambient temperatures on the electric demand and cooling efficiency of five-ton RTUs. This study seeks to better understand efficiency degradations and electric demand implications of high efficiency and standard efficiency units for three leading RTU manufacturers under realistic peak summer cooling conditions seen in California.

A series of laboratory tests were performed to quantify the impacts of high ambient temperatures on the energy efficiency and power usage of three standard and three high efficiency, five-ton RTUs. Performance of these units was evaluated and compared at 85oF, 95oF, 105oF, 115oF, 120oF, 125oF and 130oF air temperatures measured at the condenser air inlet. Test results revealed that in some instances the compressor of the high efficiency model experienced a more drastic increase in electric demand than the standard efficiency model, requiring up to 74% more power per degree Fahrenheit of change in ambient temperature. At an ambient temperature of 130oF, the power demand of one of the high efficiency units surpassed that of the standard model by 120 watts. The high efficiency unit from a second manufacturer would have consumed more power than its standard model had it not failed to operate beyond an ambient temperature of 127oF. Despite these instances of a high efficiency unit having a higher electric demand, all three high efficiency units proved more efficient than the standard models. Overall, the high efficiency units maintained superior efficiency and cooling output than the standard efficiency units.

Each RTU’s cooling capacity deteriorated as the ambient temperature increased. The standard efficiency units used more power than the high efficiency units under almost all conditions. The rate of cooling capacity deterioration, however, varied between individual units. Generally, the high efficiency units provided superior cooling capacity to that of their standard efficiency counterparts. The superior capacity was due to improvements such as larger evaporator and condenser surface areas. Coupling the higher capacities with efficient compressors in high efficiency units also contributed to higher energy efficiency ratios (EER).