Chillers
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Introduction
A chiller is the most important component and largest energy user in a large commercial facility's heating, ventilation and air conditioning (HVAC) system. Industrial facilities also use chilled water systems for process cooling. A chiller uses a common refrigeration cycle to produce chilled water that circulates though an HVAC system's air handlers to extract heat from a building. In large facilities, the equipment used to produce chilled water for HVAC systems can account for up to 35% of a facility's electrical energy use. New chillers can be 30 to 40% more efficient than existing equipment.
Technology Options
Chillers have been significantly reengineered in recent years to have full-load efficiencies of 0.47 kilowatts (kW)/ton in the 170- to 2,300-ton range. Chiller efficiency is expressed in kW/ton for full load or integrated part load value (IPLV). The IPLV captures the part load efficiency and is important because chillers spend most of their operating hours at part load. Both ratios express the electrical energy input over the cooling output. The lower the value, the more efficient the chiller. New chillers may have larger heat transfer surfaces, microprocessor controls for chiller optimization, high-efficiency motors, variable-frequency drives and optional automatic tube-cleaning systems. Chillers are available with rotary, centrifugal or screw compressors. Below are the U.S. Department of Energy's (DOE) Federal Energy Management Program (FEMP) recommendations for a variety of chillers:
Efficiency Recommendationa | ||||
Product Type | Recommendedb | Best Available | ||
Full Load kW/tonc |
IPLV kW/tond |
Full Load kW/tonc |
IPLV kW/tond |
|
Centrifugal 150 - 299 tons |
0.59 or less | 0.52 or less | 0.50 | 0.47 |
Centrifugal 300 - 2,000 tons |
0.56 or less< | 0.44 or less | 0.47 | 0.38 |
Rotary Screw = 150 tons |
0.64 or less | 0.49 or less | 0.58 | 0.46 |
b Values are based on standard reference conditions, as specified in ARI standard 550/590.
cFull load efficiency is measured at peak load conditions as described in ARI Standard 550/590-98.
dIntegrated part load value (IPLV) is a weighted average of efficiency measurements at various part-load conditions, as described in ARI Standard 550/590-98. These weightings have changed substantially from the previous standard, ARI 550-92, lowering IPLV ratings by 10 to 15% for the same equipment.
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Additional chiller configurations can be applied to improve efficiency: | ||
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There are several possible ways to make an existing chiller system more energy efficient. However, note that careful engineering is needed to determine the practicality and economic feasibility of these measures before implementation.
Variable-frequency drives (VFD) provide an efficient method for reducing the capacity of centrifugal chillers, and thus saves energy. Savings can be significant, provided that:
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In reduced-load conditions, chillers may be bypassed enabling waterside economizers to provide primary cooling. In this scenario, the cooling tower provides chilled water either directly with filtering or indirectly with a heat exchanger. This scenario is applicable when:
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Other conservation measures to consider when looking at the chiller system upgrades include:
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Efficiency Benefits
Simply replacing an older, CFC-based chiller with a newer, more efficient chiller saves energy and reduces utility bills.
Efficiency Comparison for Different Chiller Types | ||||
Reciprocating (KW/ton*) | Centrifugal(KW/ton) | Rotary(KW/ton) | Absorption** (KW/ton) | |
Full Load | 0.84-1.00 | 0.48-0.65 | 0.70-0.80 | 3.2-5.6 |
Part Load | 0.84-1.00 | 0.55-1.00 | 0.75-0.90 | slightly higher |
** Because the input energy is not electricity, the units are converted to kW of heat energy demand.
Chiller Cost-Effectiveness Example, 100-ton Screw Chiller | ||||
Performance | Base Model * | Recommended Level | Best Available | |
IPLV Efficiency (kW/ton) | 1.25 | 0.98 | 0.83 | |
Annual Energy Use | 250,000 kWh | 196,000 kWh | 166,000 kWh | |
Annual Energy Cost | $15,000 | $11,800 | $10,000 | |
Lifetime Energy Cost | $219,000 | $172,000 | $145,000 | |
Lifetime Energy Cost Savings | - | $47,000 | $74,000 |
Manufacturers
The Air-Conditioning & Refrigeration Institute (ARI) is a good resource for finding a high efficiency chiller manufacturer. ARI's Online Directories of Certified Equipment, "Prime Net" lists manufacturers' catalogues and software with ARI-certified capacity and efficiency ratings.
Purchasing Tips
Chiller work is generally one of the most expensive measures you can undertake. Consider chiller replacement when existing equipment is more than 10 years old and analysis confirms that replacement is worthwhile. When specifying and procuring chillers, you need to include load-reduction efforts, careful equipment sizing and good engineering. And, proper sizing is important in order to save on both initial costs and operating costs. Ensure other energy conservation measures have been considered first such as lighting upgrades, air handler upgrades and building automated controls. Consideration of other energy conservation measures first may reduce the capacity of a new chiller, saving first cost and operating expense.
If you choose to install only one large chiller, it should have high part-load efficiency since it will rarely operate at full load. Variable frequency drives can dramatically improve low-load efficiency for centrifugal and rotary chillers. Some of the newest centrifugal chillers have markedly improved part-load performance. Absorption chillers are gaining some market share because they do not use electricity for cooling. Where waste heat or inexpensive heat is available, absorption units may be a worthwhile economic choice. In regions with high peak electrical demand costs, the most flexible scenario is to use both types of cooling systems, reducing fuel price risk.
When deciding on a chilled water system, you can choose between using air- or water-cooled chillers. Air-cooled systems eliminate the need for a cooling tower and its associated water pumps, piping and fans, which reduces installation and maintenance costs. As system tonnage rises, the merits of choosing a water-cooled system increase. When selecting a chiller, pay careful attention to appropriate sizing: an oversized chiller not only costs more to purchase, but also leads to substantial energy losses from poor low-load performance and excessive cycling.
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Tools to help you select the appropriate chiller: | ||
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Emerging Technology
Today the best efficiencies in chillers are less than 0.47 kW/ton nearly a 40% improvement since the 1970s. Chillers are likely to become even more efficient over the next decade because of improved microprocessor control, superior flow and temperature measurement and the introduction of larger heat exchangers.
Advanced desiccants and absorption chillers are another efficient cooling method on the horizon.
Absorption chillers provide cooling to buildings by using a building's waste heat or another heat source. This technology is most cost-effective in large commercial facilities with access to natural gas, waste heat or steam.
A desiccant is a drying agent that removes moisture from the air and can also be driven or regenerated using the building's waste heat. Desiccants can work in concert with absorption chillers or conventional air conditioning systems to significantly increase energy system efficiency by removing moisture from the conditioned air and allowing chillers to cool this low-moisture air, which takes less cooling energy.
The DOE has assumed part of the initial cost and risk of developing marketable advanced desiccants and absorption chillers, as well as providing technical assistance to the industry.