Field Test of Hybrid RTU Retrofit - Big Box Application

Packaged rooftop air conditioning units (RTUs), the predominant equipment for space conditioning of small and medium-size commercial buildings, are used in an estimated 70% of space conditioning in commercial buildings. Because RTUs can last for roughly 15-20 years in California, many companies are entering the market with retrofit products designed to improve the energy efficiency of existing RTUs. To evaluate one of those retrofit products, this project gathered field data to demonstrate and understand the performance improvement associated with adding an evaporative precooler for condenser inlet air and ventilation air to conventional RTUs. The tested retrofit was expected to save energy in two ways: -By cooling the outdoor air being delivered to the RTU indoor coil, thereby reducing how much cooling it needs to perform -By reducing the air temperature seen by the RTU condenser coil, thereby decreasing refrigerant pressure and the work needed from the compressor In addition, the project measured the on-site water consumption associated with achieving these performance improvements and calculated key intermediate parameters, such as the sensible cooling capacity delivered to the ventilation air, and the maximum observable wet-bulb effectiveness of the precooling system. In a field test conducted between October 2012 and October 2013, the technology was installed on 13 RTUs serving a big-box retail store in Palmdale, California. The testing collected minute-by-minute data on four of the RTUs, before and after they were retrofit, and on two unmodified RTUs. Most of the evaluation compared the performance of individual RTUs before and after their retrofits, focusing on the following key performance parameters: sensible coefficient of performance (COP), sensible cooling capacity, and electric power draw. Sensible COP and sensible cooling capacity are based upon the sensible cooling provided by the RTU, not considering latent cooling. In addition, this study investigated the internal workings of the retrofit, including isolating the performance of the water-to-air heat exchanger for indirect evaporative cooling of ventilation air and monitoring the performance of the sump-pump control, as well as monitoring total water consumption and estimating the fraction of total water use associated with evaporation.