Western Cooling Challenge Field Test - Restaurant Application (Evaporative pre cooling of supply and condenser air)

Packaged rooftop air conditioning units (RTUs) are the predominant equipment used for space conditioning of small and medium-size commercial buildings. It is estimated that roughly 70% of space conditioning in commercial buildings is provided by RTUs. These RTUs are generally mass produced to meet federal efficiency standards, and then sold throughout the country. After having identified significant savings opportunities for equipment optimized for the California climate, the Western Cooling Efficiency Center created an RTU energy efficiency challenge. This efficiency target for this challenge, called the Western Cooling Challenge (WCC), was designed based upon adding evaporative pre-cooling to air entering the RTU condenser, and indirect evaporative pre-cooling to the outdoor air intake of the RTU. The results of these additions were sensible-cooling efficiency targets for annual energy consumption and peak electricity demand roughly 40% better than the performance of 2010 minimum-federal-standard equipment. The purpose of this project was to gather field data to demonstrate and understand the performance of a hybrid rooftop air conditioner that uses dual-evaporative pre-cooling, essentially an RTU that employs the technology used to define the Western Cooling Challenge. The technology is expected to save energy and demand two ways: a) by cooling the outdoor air being delivered to the RTU indoor coil, and thereby reducing how much cooling it needs to perform, and b) by reducing the air temperature seen by the RTU condenser coil, thereby decreasing refrigerant pressure and the work that needs to be done by the compressor. The key metrics used to characterize the performance of the retrofit include: c) sensible Coefficient of Performance (COP) d) sensible cooling capacity, and e) electric power draw Each performance metric is evaluated as a function of outdoor weather conditions. In addition, the project was designed to measure the on-site water consumption associated with achieving those performance improvements, as well as to calculate a key intermediate parameter, the evaporative effectiveness of the precooling system. Evaporative effectiveness measures the ability of the evaporative media to cool the air entering the condenser toward the wet-bulb temperature (WBT) of the outside air, and is the key parameter used to characterize the performance of evaporative coolers for condenser air in the laboratory. The approach chosen was a field test conducted on three new identical packaged rooftop units (RTUs) installed in Ontario, California. Two of these RTUs serve interior office spaces, and the third unit serves the kitchen of a restaurant and bakery. Monitoring involved minute-by-minute data collection on the RTUs with and without the evaporative media installed and operational. In addition to the key performance metrics, the internal workings were also investigated, including isolating the performance of the water-to-air heat exchanger for indirect evaporative cooling of ventilation air, 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 The energy use signature for each system with and without the dual-evaporative pre-cooler indicates energy savings between 13%-66% when outdoor-temperature conditions were between 100-105 °F, and average savings of 20%-64% for all operating hours above 70°F. For an installed cost of $350 per ton, the increase in efficiency demonstrated by this equipment would equate to a simple payback between 5-15 years. The payback period depends on application – it is most appealing in hot climates, in scenarios with longer runtime hours, for units with larger outside air fractions, and for customers with high peak demand charges. For the project evaluated here, we estimate a simple payback of 7.5 years (see Appendix A).