Chillers have been significantly reengineered in recent years to use new HCFC and hydrofluorocarbon (HFC) refrigerants. New machines have full-load efficiencies down to 0.50 kW/ton in the 170- to 2,300-ton range. Some have built-in refrigerant containment, are designed to leak no more than 0.1% refrigerant per year, and do not require purging.
Other important energy efficiency improvements in new chillers include larger heat transfer surfaces, microprocessor controls for chiller optimization, high-efficiency motors, variable-frequency drives, and optional automatic tube-cleaning systems. To facilitate replacement, new equipment is available from all manufacturers that can be unbolted for passage through conventional doors into equipment rooms. Many positive-pressure chillers are approximately one-third smaller than negative-pressure chillers of similar capacity.
Thermal energy storage may be added when replacing chillers and may enable the use of smaller chillers. Although this strategy does not save energy per se, operating costs may be reduced by lowering electrical demand charges and by using cheaper, off-peak electricity. Thermal storage systems commonly use one of three thermal storage media: water, eutectic salts, or ice. Volumes of these materials required for storage of 1 ton-hour of cooling are approximately 11.4, 2.5, and 1.5 cu ft. (0.33, 0.07, and 0.04 cu m), respectively.
Multiple chiller operations may be made more efficient by using unequally sized units. With this configuration, the smallest chiller can efficiently meet light loads. The other chillers are staged to meet higher loads after the lead chiller is operating close to full capacity. If an existing chiller operates frequently at part-load conditions, it may be cost-effective to replace it with multiple chillers staged to meet varying loads.
Double-bundle chillers have two possible pathways for rejecting condenser heat. One pathway is a conventional cooling tower. The other pathway is heat recovery for space heating or service-water heating. Candidates for these chillers are facilities in cold climates with substantial hours of simultaneous cooling and heating demands. Retrofitting existing water heating may be difficult, because of the low temperature rise available from the heat-recovery loop.
Steam or hot water absorption chillers use mixtures of water/lithium-bromide or ammonia/water that are heated with steam or hot water to provide the driving force for cooling. This eliminates global environmental concerns about refrigerants used in vapor-compression chillers. Double-effect absorption chillers are significantly more efficient than single-effect machines. See section Absorption Cooling.
Specifying and procuring chillers should include load-reduction efforts, careful equipment sizing, and good engineering. Proper sizing is important in order to save on both initial costs and operating costs. Building loads often decrease over time as a result of conservation measures, so replacing a chiller should be accomplished only after recalculating building loads.