In a typical commercial or industrial building, chillers consume more electricity than any other device, except for an extremely large fan. Thus, inefficient chillers can waste significant electricity, and even modest improvements in your chiller’s efficiency can be worth the investment.

However, increasing the efficiency of your system isn’t as simple as purchasing the most efficient unit possible. Choosing a chiller that’s most efficient when operating at full or part load might be counterproductive because the ratings don’t measure the efficiency of the overall cooling system. To maximize cost-effectiveness, we recommend analyzing the entire chilled-water system before selecting and specifying equipment—it may be wiser to buy a less efficient chiller and spend money on efficient auxiliary equipment and improved operating strategies.

Chiller terminology

AHRI This manufacturer trade organization certifies chillers using several efficiency metrics. AHRI tests equipment in standard conditions to determine its coefficient of performance (COP), energy-efficiency ratio (EER), and its full-load and part-load efficiency (given in kilowatts [kW] per ton). Use these conversion factors to relate COP, EER, and kW per ton:

COP = EER ÷ 3.412; EER = 3.412 COP

kW per ton = 12 ÷ EER; EER = 12 ÷ (kW per ton)

kW per ton = 3.517 ÷ COP; COP = [12 ÷ (kW per ton)] ÷ 3.412

AHRI standard conditions AHRI defines standard operating conditions in order to measure efficiency metrics consistently and accurately. For water-cooled chillers, AHRI defines standard conditions as constant flow rates of 2.4 gallons per minute (gpm) for 44° Fahrenheit (F) water leaving the evaporator and 3.0 gpm for 85°F water entering the condenser.

COP This metric is the ratio of the cooling output power to the total power input. Because cooling output and power input are both in watts, COP is a unitless number. The higher the COP, the more efficient the equipment is.

EER This metric is frequently used for smaller chillers—it’s the ratio of the cooling capacity to the total power input at any given set of rating conditions, expressed as Btu per watt-hour.

Full-load efficiency This metric, given in kW per ton, is the efficiency of the chiller when it’s operating at peak load and AHRI standard conditions. The lower the kW per ton rating is, the more efficient the equipment is.

Part-load efficiency This is the efficiency of the chiller when it’s operating at part load. The industry measures this metric in kW per ton using the equation for integrated-part-load value (IPLV) and nonstandard part load value (NPLV) (figure 1).

Figure 1: Equation for part-load efficiency

Part-load efficiency is given in either IPLV or NPLV. The two values use the same equation, but the IPLV represents part-load efficiency when the chiller water loop temperature is 44°F, and NPLV represents part-load efficiency when the chiller loop runs at a nonstandard temperature. Both give the efficiency of the chiller averaged over four operating loads according to this formula:
This is an image of the equation for part-load efficiency, which equals the inverse of the sum of four fractions: 0.01 divided by A, 0.42 divided by B, 0.45 divided by C, and 0.12 divided by D. A, B, C, and D are the kW per ton rating at 100%, 75%, 50%, and 25% load respectively.

Tons One ton of cooling is the amount of heat absorbed by 1 ton of ice melting in one day, which is equivalent to 12,000 Btu per hour, or 3.516 kW in thermal energy.

What are the options?

Air or water cooled

Most large commercial and industrial buildings use water-cooled chillers (figure 2). This equipment cools water with a vapor-compression refrigeration cycle and circulates it to provide air-conditioning. Central chiller plants consist of one or more chillers and their auxiliary systems, which include chilled-water pumps, condenser water pumps, and cooling towers. The chillers produce cold water, which the system pumps to one or more air handlers throughout the building. As the water passes through the building, it absorbs heat from warm indoor air. The system then distributes the cool air around the building through a network of ducts.

Figure 2: Components of a typical chilled-water system

The chiller, cooling tower, and pumps use cooled water to absorb heat throughout the building, while the outside-air intake, air handler, ducts, and diffusers provide fresh air. The terminal unit exchanges heat and the boiler provides heat through separate hot-water pipes.
This diagram shows the components of a chilled water system. The cooling tower and outside air intake are on the roof, and connect to a series of ducts that distribute air throughout the building. The chiller, pumps, boiler, and energy management system are in the basement.

Chillers can also use air to absorb and transfer heat, though this method is less efficient than using water. In air-cooled chillers, outside air flows across the condenser to remove heat.

Compressor type

There are several types of compressors in chillers. Screw and centrifugal compressors are the most efficient options. Reciprocating and scroll compressors are also available, but because they aren’t the most efficient options, we won’t cover them in this article.

Centrifugal compressors Centrifugal compressors, which are the most efficient option when the equipment runs most often at full load, use centrifugal force to compress refrigerant vapor (figure 3). Specifically, the machine uses a rotating wheel called an impeller to spin the refrigerant, forcing it outward. Some machines use multiple impellers to compress the refrigerant in stages. Because centrifugal compressors have few moving parts, they’re reliable and require minimal maintenance.

Figure 3: Centrifugal chiller cutaway

The wheel-shaped compressor uses centrifugal force to compress the refrigerant vapor.
In this cutaway diagram of a centrifugal chiller, there are evaporator and condenser tubes inside of the chiller.

Though this type of compressor is particularly efficient when it’s operating at full load, it’s compatible with variable-frequency drives (VFDs). Using VFDs could increase efficiency at part-load operation but can cause surge, which is when refrigerant vapor doesn’t flow continuously. Because surge can cause noise, vibration, and serious damage to the compressor, have a third-party contractor with expertise determine if the technology will cause your equipment to malfunction.

Centrifugal compressors are the most common choice for equipment with capacities of 300 tons or larger—there are factory-assembled units with capacities up to several thousand tons and even larger field-assembled units. Although centrifugal compressors are less common in the under-300-ton market, manufacturer Danfoss designed the Turbocor centrifugal compressor specifically for smaller capacities. The Turbocor is most effective with water-cooled chillers, which can achieve one of the highest efficiencies in chillers under 300 tons when paired with the Turbocor compressor. Its unique compressor design uses magnetic fields to levitate the compressor shaft in midair, eliminating mechanical friction and the need for traditional oil-lubricated bearings (figure 4). Other benefits include variable-speed operation, dramatically lower maintenance costs, small size, flexibility, low noise levels, and external digital communications.

Figure 4: Floating on a magnetic field

The Turbocor’s shaft floats on front and rear radial magnetic bearings, eliminating all mechanical friction. The axial magnetic bearing keeps the shaft aligned.
This diagram shows how the shaft floats between bearings.

Screw compressors Screw compressors are positive displacement devices, which means the equipment compresses the refrigerant by reducing the volume of the refrigerant chamber. The compressors do this by squeezing refrigerant between two rotating helical rotors that interlock. There are two types of screw compressors: a single-screw compressor that consists of a cylindrical main rotor positioned between identical gate rotors (figure 5), and twin-screw compressors that use two mating twin-grooved rotors.

Figure 5: Compression process in a single-screw compressor

The top part of the diagram illustrates the three-step compression process. As the gate rotors turn, suction compresses the gas inside of the main screw rotor and pushes it to the discharge port. The bottom part of the diagram shows how the rotors fit into the compressor.
This diagram illustrates that two gate rotors sit on either side of the main rotor in the back of a compressor. On the front of the compressor there is an economizer and motor assembly.

Screw chillers are good options because they’re:

  • Rugged and small (up to 40% smaller and lighter than centrifugal machines)
  • Quiet when operating
  • Available in capacities up to about 1,000 tons

Chiller efficiencies

For current chiller efficiency standards, see ASHRAE’s Standard 90.1-2016—Energy Standard for Buildings Except Low-Rise Residential Buildings (figure 6). As you shop for efficient chillers, make sure that they meet or exceed these efficiency guidelines.

Figure 6: ASHRAE efficiency standards for water-cooled chillers

In this chart, path A indicates efficiency values for the equipment that will operate at full load more than part load. Path B indicates efficiency values for equipment that will spend more time at part-load operation.







ASHRAE 90.1-2007 standards as of January 1, 2010

ASHRAE 90.1-2016 standards as of April 22, 2019

Path A

Path B

Path A

Path B
Rated chiller capacity (tons) Full load (kW per ton) Part load (kW per ton) Full load (kW per ton) Part load (kW per ton) Full load (kW per ton) Part load (kW per ton) Full load (kW per ton) Part load (kW per ton)
Centrifugal chillers
Note: kW = kilowatt; NA = not applicable. © E Source; data from ASHRAE
<150 .634 .596 .639 .450 .610 .550 .695 .440
150 to 299 .634 .596 .639 .450 .610 .550 .635 .440
300 to 400 .576 .549 .600 .400 .560 .520 .595 .380
400 to 600 .576 .549 .600 .400 .560 .500 .585 .380
>600 .570 .539 .590 .400 .560 .500 .585 .380
Screw chillers
<75 .780 .630 .800 .600 .750 .600 .780 .500
75 to 149 .775 .615 .790 .586 .720 .500 .750 .490
150 to 300 .680 .580 .718 .540 .660 .540 .680 .440
>300 .620 .540 .639 .490 .610 .520 .625 .410
>600 NA NA NA NA .560 .500 .585 .380

How to make the best choice

Several factors determine the type of chiller compressor that’s best for your application. A major factor will be what size load you have—centrifugal compressors are the most common choice for compressors sized above 300 tons, and screw compressors and the Turbocor are the most common choices for compressors below 300 tons. Whether you run your chiller more often at part or full load will also influence your chiller selection. Centrifugal compressors have better full-load efficiency, and screw compressors can achieve comparable part-load efficiencies. Screw compressors are also better for part-load applications because they’re stable down to about 10% capacity, whereas centrifugal compressors can begin to surge at about 20% to 40% capacity. Screw compressors also work well in applications where there is a large difference between the evaporator and condenser temperatures, such as with ice-making for thermal storage. One potential downside of screw compressors is that some require large amounts of lubricant oil and an oil-and-gas separator that reduces efficiency.

Buying a more efficient chiller will reduce energy costs, but investing in improvements to the system as a whole could be more cost-effective. Annual operating costs for a chiller may amount to as much as a third of the equipment price, so even a modest improvement in operation efficiency can yield substantial energy savings and attractive paybacks. For example, if you have a 500-ton chiller and you increase the efficiency from 0.60 kW per ton to 0.56 kW per ton, you might pay an extra $7 per ton for each 0.01 kW per ton increase in efficiency. That would increase your chiller’s first cost by $14,000, but that change might reduce operating costs by as much as $3,600 per year—assuming 1,500 equivalent full-load hours and electricity at an average price of $0.12 per kilowatt-hour (kWh). In this case, the upgrade’s simple payback would take less than four years. Though you can conduct your own financial calculation for efficiency improvements on the equipment, only an analysis of the entire chiller plant, preferably using simulations, will indicate whether buying a more efficient chiller is a better option than upgrading the auxiliary equipment.

We have a few tips for choosing the best chiller for your facility.

Buy as small a chiller as possible First reduce building loads and improve air-side distribution at your facility (see the Energy Star Building Upgrade Manual for more details), then size the chiller. Buying a chiller with a higher capacity than you need not only increases equipment costs, it also increases monthly utility bills.

Know what equipment is available Chillers are available in a range of technologies and efficiencies. Compare what manufacturers are offering to guidelines established by various organizations such as ASHRAE.

Select different-size machines for multiple-chiller installations Select one machine small enough to meet light loads efficiently and others to meet larger loads efficiently. Start additional chillers only when the chillers that are already running are near full capacity. This strategy ensures that chillers operate near their most efficient capacities and avoids excess pumping power. It also allows for more stages of chiller sequencing. For example, two chillers set to operate at 50% and 100% load can operate only in these two stages, whereas two chillers sized for 30% and 60% offer three stages (30%, 60%, and 100% load).

Consider a chiller with a VFD to maximize part-load efficiency All major manufacturers offer variable-frequency centrifugal chillers, and Carrier offers a VFD screw chiller. All of those available offer energy performance that’s superior to traditional constant-speed chillers under most conditions, but particularly part-load operation. Chillers equipped with VFDs typically have part-load efficiencies between 0.35 and 0.45 kW per ton, which is considerably better than constant-speed equipment. Some manufacturers are fairly new to the VFD chiller market, however, so it pays to research the track record of specific products before you make a purchase.

Use simulations to maximize system efficiency The interaction between the chiller, its auxiliary equipment, your climate, and your internal cooling loads will significantly affect overall chiller plant efficiency—but these system interactions are difficult to analyze without computer simulations. Using simulations not only helps you select efficient components, it also helps you determine efficient operating strategies. For example, chiller efficiency increases with higher-temperature chilled water and lower-temperature condenser water. However, these operational changes will require pumping more chilled water and blowing more air, which increases pump and fan power. Simulations can help you identify optimal temperatures for balancing these opposing trends and minimizing energy consumption.

Use zero-tolerance bids Manufacturers design chillers to perform at a range of specification tolerances, meaning that they can claim that the equipment performs better than it does in reality. They might use generous specification tolerances for water pressure, cooling capacity, and energy efficiency at particular loads. For example, the AHRI tolerances allow a manufacturer to claim that a chiller has a capacity of 550 tons at 40% load when its actual capacity is only 500 tons. By using zero-tolerance language in your bids, you force manufacturers to provide the realistic chiller performance.

Include a liquidated-damages clause with bids This clause ensures that you can hold the manufacturer financially responsible if a purchased chiller doesn’t measure up to the bid specification.

What’s on the horizon?

Due to environmental concerns, manufacturers have been producing fewer chillers that use hydrofluorocarbon (HFC) refrigerants. Additionally, starting in 2020, manufacturers won’t make any hydrochlorofluorocarbon (HCFC) refrigerants. As the industry phases out HFCs and HCFCs, it will be looking to other refrigerants to fill the gap.

For centrifugal chillers, and chillers that use refrigerants at low pressure, the industry will move on to more-efficient hydrofluoroolefins (HFOs) and blends of HFCs and HFOs.

For chillers that use refrigerants at high pressure, the dominant refrigerant is HFC-134a. It’s likely to be phased down as well, but the timeframe is unclear. An amendment to the Montreal Protocol, known as the Kigali Amendment, proposes a timetable of the phase-down of refrigerants with high global-warming potential, but it doesn’t specify a timeframe for HFC-134a in particular. Although the amendment has gone into effect in Canada and 196 other countries, the US Senate has yet to ratify it. To learn more about the Kigali Amendment and how it will affect the industry, see the E Source blog Don’t Assume Trump Will Stop the HFC Phasedown.

It’s also likely that the chillers charged only with HFOs will be more efficient than current models by a few percentage points. However, they’ll also be more expensive, as HFO refrigerants cost about 10 times as much as current HFC refrigerants. And since HFOs are slightly flammable, introducing them into the market will require code changes and new installation techniques.

Who are the manufacturers?

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by the authors, nor does any content in the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of the technology-related programs mentioned herein.

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