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
a The decision to specify chiller efficiency using full load or IPLV should depend on the application.
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.


Additional chiller configurations can be applied to improve efficiency:
  • Multiple chiller operations may be more efficient by using unequally sized units and staging them. A smaller chiller can more efficiently meet light loads or operate only during peak load. A larger chiller may cover the "base load" for most of the remaining time operating closer to full load, which is generally a more efficient way. 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 can be used in facilities in cold climates with substantial hours of simultaneous cooling and heating demands.
  • Instead of using electricity, absorption chillers use mixtures of water/lithium-bromide or ammonia/water that are heated with steam, natural gas or hot water to provide the driving force for this special chiller's refrigeration cycle. Double-effect absorption chillers are significantly more efficient than single-effect machines. Operating absorption chillers, when steam, gas or hot water rates are lower than electric rates, may save substantial operating costs. A careful economic analysis is necessary to determine if this technology is appropriate.
  • Using an electric chiller in combination with a natural gas chiller — a hybrid chiller system — may take advantage of pricing opportunities in the volatile energy market. However, equipment and maintenance costs must be considered. The first-time costs of the gas cooling equipment are higher than an electric chiller, so expected savings in energy costs with these hybrid systems must outweigh the first-time and maintenance costs.
  • 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 or load shifting.

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:

  • Part loads exist for many hours per year
  • The climate does not have a constant high wet-bulb (humid) temperature
  • The condenser water temperature can be reset higher under low part-load conditions

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:

  • Air economizer cycles cannot be used
  • Cooling loads during 55°F or less outdoor conditions do not exceed 35 to 50% of full design cooling loads. The frequency of this condition must be economically analyzed to determine the value of bypassing chillers (using water side economizers)

Other conservation measures to consider when looking at the chiller system upgrades include:

  • Higher-efficiency, two-speed or variable-speed pump and motor systems
  • Operation with low condenser water temperatures
  • Low-pressure-drop evaporators and condensers (oversized chiller "barrels")
  • Interconnecting multiple chillers into a single system
  • Upgrading cooling towers:
    Consider two-speed motors or VFD for the cooling tower
    Consider oversizing the cooling tower to decrease the chiller size
  • Upgrading control systems (e.g., temperature reset)

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
* A lower number indicates a more efficient chiller.
** 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
* The efficiency of the base model is just sufficient to meet ASHRAE Standard 90.1-99.

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.

Tools to help you select the appropriate chiller:
  • Pisces; This Windows-based software program sizes hot- or chilled-water circuits. You can use PISCES to size systems from scratch, or evaluate an existing system, when pipe sizes and flow rates are specified.
  • Energy Cost Calculator for Air-Cooled Electric Chillers — An online calculator from the Federal Energy Management Program (FEMP)
  • American Council for an Energy Efficient Economy (ACEEE) publishes the Guide to Energy-Efficient Commercial Equipment, which includes a chapter on HVAC systems, as well as a listing of energy-efficient chiller models.
  • ASHRAE publishes the Cooling and Heating Load Calculation Manual.

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.