Chillers consume 25 to 50 percent of a laboratory's annual energy budget. Chillers come in several different mechanical arrangements: reciprocating, rotary-screw, and centrifugal and with practically unlimited combinations of features, capacities, and efficiencies. The chiller's energy savings potential depends on operating conditions, space constraints, and willingness of the building owner to pay for energy saving features. [Kruse, 1991]
Understanding the annual load distribution of a laboratory facility is crucial to efficient sizing of the chiller system. Chiller compressor loads range from 100 percent during the summer to as low as 10 percent in the winter, depending on energy-efficiency features of the air-handling system. As stated above, optimizing the energy-efficient operation of the chiller system requires that the chiller plant be modularized and the plant operation be matched to the cooling load as it varies annually. ["Screw Compressors Provide Energy and Cost Savings." 1989]
Chiller plants are not often modularized. An understanding of the relative merits of different chiller types -reciprocating, rotary-screw, and centrifugal - is a prerequisite to designing a modular chiller plant. In general, reciprocating chillers can serve the smallest loads efficiently, rotary-screw chillers are the most flexible, and centrifugal chillers are most efficient when fully loaded. Typical kW/ton profiles must be identified for the loads identified and temperatures required; consideration can then be given to using two chillers of unequal size instead of two chillers of equal size, which allows more flexibility in matching load. The energy-efficient modularizing approach can help designers match loads effectively by a rotary-screw compressor and a centrifugal compressor instead of two centrifugal compressors; a reciprocating chiller could also be part of the energy-efficient module, combined with a screw compressor or a centrifugal compressor. [Veneklase, 2001] [Beyene and Asfaw, 1994]
A laboratory-type facility's chiller operation will vary widely during the year because of the changing load profile. To satisfy this load profile, sensible heat removal and dehumidification requirements can be defined as two distinct thermal cooling levels. An energy-savings opportunity is presented when these cooling levels are evaluated and matched separately. [Brown, 1990]
To meet dehumidification requirements, a low-temperature fluid is required, typically 38°F (3.3°C) to 45°F (7.2°C). If a chiller efficiency, with auxiliaries, is 0.80 kW/ton (0.227 kW/kW) to produce water at 40°F (4.4°C), the efficiency to produce water at 60°F (15.6°C) could increase to about 0.65 kW/ton (0.185 kW/kW), saving nearly 20 percent. When cooling is required all year for the facility support spaces, water at 60°F (15.6°C) can be produced using a free-cooling heat exchanger system. Knebel (1999) reminds us that the often-used rule-of-thumb supply air temperature of 55°F is "rarely" the optimum supply air temperature. He asserts that when the entire HVAC system's power requirements are taken into consideration, which includes main supply and exhaust fans, pumps, compressors, condenser fans, cooling towers, etc., the optimum temperature may much lower. [Knebel, 1999] [Brown, 1990]
Approximately one-half of the HVAC energy consumed in cleanrooms is for chiller and associated pump operation. Therefore, incorporating two temperatures of chilled water works very well in cleanroom designs, yielding major energy savings. [Kruse, 1991]
Rotary-screw chillers provide significant operational energy savings because they can match full load or part load as small as 10 percent of the chiller's capacity. Rotary-screw compressors can vary their capacity infinitely. This continuous capacity modulation results in 25 percent energy savings in contrast to standard unloading techniques. ["Screw Compressors Provide Energy and Cost Savings," 1989]
In particular instances, chiller compressor or auxiliaries can be operated with VFDs to produce energy-efficient matching of the cooling load. The energy engineer who is considering adding variable flow through the compressor, evaporators, or condensers needs to be sure that a VFD is compatible with the particular chiller.
Installing oversized evaporators and condensers saves energy by reducing internal pressure losses and altering the temperature lift in the chiller. For instance, lowering condenser water entering temperature to 75 deg. F by using an oversized cooling tower can be cost effective within five years.
All-Variable Speed Chiller Plants
