Large Energy Savings Per Installation VFDs for Large Chillers

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1 This article was published in ASHRAE Journal, June Copyright 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Reprinted here by permission from ASHRAE at This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE s permission. For more information about ASHRAE, visit Large Energy Savings Per Installation VFDs for Large Chillers This third column in a five-part series on variable frequency drives will cover application of VFDs to large chillers in commercial building air-conditioning systems. By John Dieckmann, Member ASHRAE; Kurtis McKenney; Matthew Guernsey; and James Brodrick, Ph.D., Member ASHRAE U sing variable frequency drives (VFDs) for compressor motors in large chillers is a newer application than using VFDs with blowers (discussed in last month s column). VFDs are being used with centrifugal compressors and screw compressors, the two major compressor categories for large chillers. Applying a variable speed drive to the compressor provides energy-efficient capacity modulation. While the focus of this series has been on using VFDs for induction motors, high-speed brushless dc motors are an important class of motors. Such motors generally are used to directly drive high-speed centrifugal compressors. Large tonnage chillers generally are capable of capacity modulation, which enables the chiller to run continuously while delivering constant-temperature chilled water as the cooling load varies from maximum to minimum design load. The traditional methods of capacity modulation have been: For centrifugal compressors, prerotation vanes, also called inlet guide vanes, reduce capacity while avoiding surge, down to around 40%. Hot gas bypass (directing compressor discharge gas to the evaporator) is used to reduce capacity further. Screw compressors have been equipped with slide valves that simultaneously vary the inlet displacement and the discharge displacement, maintaining a relatively constant built-in volume compression ratio, typically modulating down to about 30% of full load before hot gas bypass is needed for further capacity reduction. Staged operation of multiple compressors (or multiple chillers) has been used with both types of chillers, as well as with chillers that use other compressor types, such as scroll or reciprocating. Two metrics commonly used to rate the efficiency of chillers are full-load steady state performance and the integrated part load value (IPLV). The IPLV metric is intended to approximate typical seasonal performance as the load on the chiller varies. Figure 1 shows the air or water temperature entering the condenser versus load that is used to determine the IPLV for air-cooled and water-cooled chillers. In each case, reduced load is assumed to correlate with lower outdoor wet- and dry-bulb temperature, resulting in lower temperature of the cooling medium entering the condenser. The IPLV is a widely accepted metric for comparing the seasonal average performance of chillers. However, in actual applications, chillers operate over a much wider range of loads and condenser temperatures than the IPLV rating load-condenser temperature curves. Reasons for operation at low load percentage and higher condenser temperatures include high ambient wet or dry-bulb temperature coupled with low building occupancy or low solar loading and the general tendency to oversize building cooling equipment. Reasons for operation at high load percentage and lower condenser temperatures include high load during chiller start-up after an unoccupied period and operation of one of a set of multiple chillers to handle the building part load. The combination of chiller plant oversizing and the variation of the factors driving the cooling load (some of which can be highly variable, namely weather) can result in almost any combination of chiller load level and condenser temperature. Estimates suggest that more than 90% of water-cooled chiller plant installations include multiple chillers, with two being the most common number. 1 In multiple chiller plants, operation of any chiller below 50% load is unusual, with a large percentage of operating time occurring around the 75% load point or higher. 1 For multiple chiller plants, equipping each compressor with VFD may provide the greatest energy savings and/or operational flexibility, but it may be more economical (or at least less capital intensive) to apply VFD to the base load chillers or chillers expected to operate at part-load ( swing chillers ). Chillers that operate the most hours with reduced condenser water temperatures (i.e., reduced lift) and/ or reduced load will offer the greatest energy cost savings potential. Several factors account for the improvement in the IPLV provided by variable speed drives. Variable speed operation significantly reduces, if not eliminates altogether, the use of hot gas bypass for continuous operation at low load. In centrifugal chillers, variable speed combined with prerotation vanes enables operation 5 8 A S H R A E J o u r n a l a s h r a e. o r g J u n e

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3 100 F Temperature Entering Condenser 90 F 80 F 70 F 60 F 50 F Water-Cooled Air-Cooled IPLV Weighting: 12% 45% 42% 1% 40 F 0% 25% 50% 75% 100% Load Figure 1: Temperature of cooling air or water entering the condenser versus load and weighting factors for IPLV determination. Advertisement formerly in this space. closer to maximum compressor efficiency across a wide range of operating conditions. With screw compressors, it is possible to eliminate the slide valve, using speed variation to cover the full operating range at higher efficiency. There are many variables involved in determining seasonal energy performance. However, based on informal conversations with industry engineers, the typical energy savings over a cooling season provided by applying a VFD to a chiller is approximately 25%. 2 Additional benefits include improved power factor and soft starting of the chiller motor, reducing both impulse loading on the motor and large in-rush currents. Energy Saving Potential Space cooling in U.S. commercial buildings consumed 2.3 quadrillion Btus (quads) of primary electric energy in Of this, an estimated 0.3 quads are attributable to centrifugal and screw chillers in commercial and institutional buildings. If we assume, for the sake of simplicity, that VFDs are not widely implemented in large chillers, and that VFDs result in 25% average energy savings over conventional single-speed chillers, then VFD offers a technical energy savings potential for large chillers of approximately 0.1 quads. However, one manufacturer claims that VFDs are more prevalent now, indicating that 80% of centrifugal chillers with a VFD option available ship with a VFD. 2 This is a relatively low national energy savings potential relative to other VFD applications. However, the number of buildings with central chiller plants is only on the order of 100,000, 4 indicating there is a large amount of energy savings potential per installation. In large commercial buildings, the central chiller plant is often one of the main energy end uses, and therefore 25% savings on the chiller energy can represent a significant reduction in the building s overall operating costs. The 25% savings should not be assumed for any given application. For the reasons mentioned, the actual energy savings from using VFDs in chillers will vary widely from one installation to the next. Some applications with significant off-design operation may see energy reductions greater than 6 0 A S H R A E J o u r n a l J u n e

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5 35%, while applications with multiple chillers operating near design conditions, may experience energy savings of less than 15%. 2 Energy savings estimates for a single installation should rely on locationspecific factors, particularly weather. Chiller energy savings should only be modeled with respect to total chiller plant energy savings, which would also consider plant ancillary equipment. In a typical water-cooled chiller plant, the chiller accounts for approximately 85% of the system energy. 5 Market Factors Variable speed drives can offer significant energy savings when applied to the right chiller applications. Also, chillers with VFDs can exhibit improved reliability and life as a result of an improved power factor and soft starting of the chiller motor. However, real energy savings tend to be application-specific. VFDs can give the perception of energy savings when the IPLV formula is applied, particularly in buildings with multiple chillers, which is generally the case. 1 High-load operating points are noticeably more important than the IPLV metric may indicate. 1 Therefore, a more involved modeling process is necessary to determine the cost effectiveness of installing one or more VFDs in a chiller plant. The cost of VFD technology has dropped substantially over the past 30 years, and in many new designs, VFD technology is included in the base-unit design. 2 Several geographic regions offer rebates for the application of VFD technology. 2 Advertisement formerly in this space. References 1. Geister, R. and M. Thompson A closer look at chiller ratings. ASHRAE Journal 51(12): Communication with Johnson Controls. 3. Energy Efficiency and Renewable Energy Buildings Energy Data Book. U.S. Department of Energy. buildingsdatabook.eere.energy.gov/. 4. EIA Commercial Building Energy Consumption Survey, Table B40. Energy Information Administration, U.S. Department of Energy Furlong, J. and F. Morrison. Optimization of water-cooled chiller cooling tower combinations, CTI Journal 26(1): AHRI Standard 550/ , 2003 Standard for Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle ASHRAE Handbook HVAC Systems and Equipment. Chapter 44, Motors, Motor Controls, and Variable-Speed Drives. John Dieckmann is a director, Kurtis McKenney is an associate principal, and Matthew Guernsey is a senior technologist in the Mechanical Systems Group of TIAX LLC, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C. 6 2 A S H R A E J o u r n a l J u n e