RTU Outside Air and Condenser Air Precooling – Laboratory Test

This report documents and discusses results of a detailed laboratory evaluation of an indirect evaporative cooler (IEC) coupled to a rooftop packaged air conditioner (RTU) that was retrofit with a condenser air pre-cooler. The scheme reduces air conditioner compressor energy use in two main ways: - Reducing condenser inlet temperature and improving efficiency for the vapor compression cycle - Meeting a substantial portion of the building cooling needs with indirect evaporative cooling The primary goal of this project was to characterize the energy efficiency of the retrofit package in all modes of operation and possible configurations, as well as across a full range of operating conditions. To this end, the project team conducted extensive laboratory tests and carefully analyzed results to consider the technical opportunities and challenges related to the equipment and identify opportunities for improving the technology. These findings provide the basis for recommendations for utility efficiency programs, design engineers, and customers on applying this technology for managing indoor environmental quality in commercial buildings while simultaneously reducing energy consumption and peak demand. The study showed that IEC coupled with condenser air pre-cooling has the potential to transform new or existing RTUs into climate-appropriate equipment for hot dry climates. The IEC delivers a very efficient means to cool outside air for ventilation or modest room cooling while the condenser air pre-cooler can significantly improve the performance of the RTU vapor compression cycle. The configuration tested demonstrated it could provide these benefits: - Reduce annual energy use for cooling and ventilation by in 68% in California commercial buildings - At peak cooling conditions (105°FDB/73°FWB), achieve equal capacity while reducing energy use for cooling by 26% - At milder conditions (65°FDB/ 52.8°FWB), offer savings at large as 86% Because the retrofit connects new loads, it increases electrical demand at every condition tested. On aggregate, the retrofit would decrease energy used during peak cooling conditions by reducing compressor operation. Nonetheless, the highest instantaneous electrical draw will be larger, potentially increasing customer demand charges. For regional and statewide grid management, the gains in system efficiency would reduce peak generation requirements. Moreover, the retrofit could reduce the total electrical demand for a building if the retrofit handled ventilation air for multiple units, thereby allowing the operation of existing units in recirculation mode or even disconnection of the existing units. The equipment expands the number of potential operating modes by allowing multiple combinations of IEC and compressor stages—driving the need to develop smart controls that can determine the most appropriate and beneficial operating mode for a particular situation. The project team recommends that the technology be adopted by utility energy efficiency programs. As is the case for most air conditioning technologies, some applications will be better suited than others, and programs that adopt this technology should be designed to avoid scenarios where overall energy performance is limited. A simulation study should be conducted to assess the savings potential for target customers and identify applications that ought to be avoided. A first-generation program that draws on customer smart meter data and limited field monitoring can help assess the actual savings achieved by installation of the equipment. Technology improvement efforts could focus on design guidance to facilitate proper physical application and optimized controls integration.