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Raising the Efficiency Ceiling of Rooftop Units

Oct. 25, 2016
For cooling requirements of up to 100-plus tons, rooftop units often represent a low-first-cost solution, based on equipment cost and relative ease of installation. Until recently, however, they have not been a particularly energy-efficient option.

According to estimates, the majority of commercial floor space in the United States is cooled with self-contained air-cooled, packaged air-conditioning and heat-pump units. Most of these are roof-mounted and generally referred to as rooftop units, or RTUs. For cooling requirements of up to 100-plus tons, RTUs often represent a low-first-cost solution, based on equipment cost and relative ease of installation. Until recently, however, they have not been a particularly energy-efficient option, especially when compared with air-cooled chillers. That is improving, however; since the oil embargo of 1973, the efficiency of small packaged unitary air conditioners—specifically, RTUs of less than 7.5 tons—has almost doubled.

Contributing to that improvement has been technology known as Advanced RTU Control (ARC). For the past decade or so, ARC has emerged in an effort to improve RTU efficiency by adding such features as variable-speed fan control, carbon-dioxide-based demand-controlled ventilation (DCV), and—in suitable climates—integrated air-side economizer controls. A recent whitepaper by a manufacturer of this technology points out that 60 percent of the energy savings resulting from ARC comes from the installation of a variable-frequency drive for the supply-fan motor. Though that may seem counterintuitive, Srinivas Katipamula, a staff scientist at Pacific Northwest National Laboratory, explains, “Because the supply fan runs continuously during occupied periods, the fan energy consumption can be greater than the compressor energy consumption in many locations in the U.S.” With ARC, fan speed is modulated as required by the load, rather than kept constant. DCV also is a significant component of ARC, as it ventilates at a rate based on actual indoor-air quality—a function of real-time occupancy and occupant activity levels—rather than the occupancy for which the system was designed.

The cooling part-load energy efficiency of larger (greater than 65,000 Btuh) commercial air-conditioning and heat-pump equipment, based on weighted operation at various load capacities, is expressed in integrated energy-efficiency ratio (IEER). ANSI/ASHRAE/IES 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, establishes minimum IEER values for HVAC equipment. Around the time the 2013 version of ANSI/ASHRAE/IES 90.1 was being developed, the U.S. Department of Energy (DOE) joined industry partners to release a design specification for 10-ton RTUs. That specification was part of the High Performance Rooftop Unit Challenge, aimed at urging U.S. HVAC manufacturers to build and deliver competitively priced, energy-saving RTUs.

Units built according to the High Performance Rooftop Unit specification were expected to reduce energy use by as much as 50 percent compared with ANSI/ASHRAE/IES 90.1 requirements. In several cases, that goal was exceeded. For example, a 10-ton unit required to meet an efficiency of 18.0 IEER achieved a 20.8 IEER, a nearly 16-percent improvement. It was estimated commercial users could save $1 billion in energy costs per year by replacing 10- to 20-ton RTUs with equipment meeting the Challenge specification. Another DOE program, through the Better Buildings Alliance, is the Advanced RTU Campaign. This initiative encourages the replacement of older RTUs with more efficient ones and promotes retrofits with features such as ARC. Where should you look for those opportunities? Perhaps the 2016 (non-government) Campaign award winners will get you pointed in the right direction.

About the Author

Larry Clark

A member of HPAC Engineering’s Editorial Advisory Board, Lawrence (Larry) Clark, QCxP, GGP, LEED AP+, is principal of Sustainable Performance Solutions LLC, a South Florida-based engineering firm focused on energy and sustainability consulting. He has more than two dozen published articles on HVAC- and energy-related topics to his credit and frequently lectures on green-building best practices, central-energy-plant optimization, and demand-controlled ventilation.