High Efficiency RTU Development - CRADA final report (Trane)

Between September 2012 and September 2015, Oak Ridge National Laboratory (ORNL) and Trane Commercial Systems, Ingersoll Rand Inc. engaged in a Cooperative Research and Development Agreement (CRADA) to develop a high efficiency, unitary rooftop air conditioning system (RTU) for the US commercial market. We went through an exhaustive technologies survey to select energy efficient and cost-effective components. We conducted in-depth engineering design and optimization, based on the ORNL Heat Pump Design Model (HPDM), by which we located the final choices. Starting from a Trane baseline unit, having a rated cooling capacity of 13 tons and a 17.9 integrated energy efficiency ratio (IEER), we made major modifications to improve the efficiency. We re-configured the vapor compression system by combining two refrigerant circuits into one; innovatively applied a combination of two Copeland UltraTech 2-stage compressors and single-speed compressor, which was proved to be the most efficient compressor combination in the targeted capacity range. The original indoor blower and condenser fans were replaced using high efficiency fans from the Ebm-Papst company. We also replaced the original one indoor blower with three parallel indoor blowers. This strategy facilitated blower power reduction and improved indoor air flow distribution. We also added a submerged subcooler to recover free cooling capacity from the condensate water without adding power consumption. The submerged subcooler also serves as a charge buffer to prevent two-phase refrigerant exiting the condenser at part-load conditions. Furthermore, a low GWP refrigerant (DR-55) as a drop-in replacement of R-410A was evaluated in the RTU prototype.