Cut Chiller Energy Costs by 30%
A chiller is often the single largest consumer of power in a building, so it’s a logical place to look for reduced energy costs. Chillers expend excess energy when their compressors run at full-rated constant speed, a wasteful and unnecessary operating condition. Most chillers operate at partial load or are off 99% of the time. A chiller’s compressor motor is typically quite large, in the range of 150 hp to 600 hp. Money saved by operating the compressor motor more efficiently can add up quickly.
The best way to improve chiller compressor motor efficiency is by upgrading motor control from constant- to variable-speed using a variable frequency drive (VFD). Rising energy costs and electric utility rebates for VFD upgrades, coupled with falling prices for large horsepower VFDs, can reduce investment payback times to as little as one year. However, chiller compressor motor VFD retrofits can be complex. Many companies postpone retrofits and forego savings.
Prepackaged solutions can be economical to purchase, easy to install, and simple to operate. Such a prepackaged retrofit can be optimized to existing chillers regardless of brand or size.
This type of package is most effective retrofitted to centrifugal compressor chillers, but can also be applied to other chiller types, including those with reciprocating, rotary-scroll, or screw compressors. Centrifugal compressors make up the bulk of the chiller market with about 70% market share. To understand how the retrofit package works, it’s important to first examine chiller operating details.
Chilling out with the refrigerant cycle
A chiller transfers heat from an area where it is unwanted to a place where it is unobjectionable. This is accomplished using a refrigerant exchange medium and a compressor to perform the required heat transfer. This process is known as the refrigerant cycle, and is depicted in the diagram.
The refrigerant cycle employs four main components: a liquid metering device, evaporator, compressor, and condenser. The cycle begins with the refrigerant in the condenser, in a liquid state and at high pressure. From there, the liquid refrigerant flows to the evaporator. Flow is regulated by a metering device. The evaporator is at a lower pressure than the condenser, so flow is naturally induced from the higher to the lower pressure area.
In the evaporator, the refrigerant changes state from liquid to gas by absorbing heat from the area where it is unwanted, typically called the load. The refrigerant gas is then discharged from the evaporator to the compressor, which raises its pressure.
Next, the high-pressure gas discharges the unwanted heat to the unobjectionable place, typically the atmosphere, changing state from a gas back to a liquid in the condenser. The refrigerant then flows from the condenser to the evaporator, and the cycle begins again.
With a basic understanding of the refrigerant cycle in hand, we can now look at how a retrofit improves refrigerant cycle and overall chiller efficiency.
Solve the crime of wasted energy
To understand how a retrofitted chiller saves energy, consider the two factors that most affect chiller energy consumption. The first is the load, and the second is the temperature of water entering the condenser. A reduction in either load or water temperature saves energy. However, “load” is not a controlled parameter. Rather, it is a function of building demand. So the chiller retrofit focuses on efficient reduction of entering condenser water temperature.
A chiller with a constant-speed compressor motor reacts to a reduced load or lower entering condenser water temperature by closing its prerotational vanes, which reduces refrigerant flow and saves some energy. However, as the vanes close, they create frictional losses, reducing chiller efficiency and limiting energy savings.
On the other hand, the chiller retrofit sequences the vanes to a closed position before the compressor motor starts. The unloaded chiller is then given a command to make a controlled acceleration to full speed, usually 60 Hz or about 3600 rpm. This control sequence keeps starting torque low by providing gradual and controlled compressor acceleration, thereby increasing mechanical drive and chiller system longevity while cutting required maintenance. The compressor motor soft-start also reduces instantaneous power draw from the electric utility, often reducing demand charges.
The chiller retrofit controller measures the difference between compressor suction and discharge pressure while dialing the vanes open. The lift temperature—that is, the saturated compressor discharge temperature minus the saturated evaporator temperature—is measured as the vanes are simultaneously controlled to maintain the required chilled water setpoint for the condenser. A chiller has to maintain a certain minimum required lift temperature at all times.
Chiller compressor motor speed is then regulated to keep refrigerant velocity slightly higher than that needed to maintain the required lift temperature. For example, if the exiting condenser water temperature rises as load increases, the required lift will go up, thereby increasing chiller compressor motor drive speed.
Starting torque must be considered in selecting a drive, as refrigerant density is often much higher at start-up than at operating conditions. Typically, 160% of rated starting torque is provided by a standard motor starter or controller. The VFD’s inherent soft start eliminates this torque transient, so standard normal duty-rated ac drives can be used with the centrifugal compressor motor in the chiller, reducing VFD initial investment.
Variable speed wins
In summary, a much more efficient way to adjust to changing load characteristics or lower condenser water temperatures is to vary the speed of the chiller compressor motor, as opposed to adjusting vanes or other mechanical flow-control devices.
The chiller retrofit is not only the most efficient way to operate a chiller, it’s also simple to install. A retrofit can consist of a VFD, sensors, controller, and touchscreen operator-interface panel. Installation consists of supplying power to the VFD, connecting it to the compressor motor, installing and connecting the sensors, and connecting the vanes to the retrofit package.
As the photo shows, the retrofit front panel touchscreen shows relevant operating parameters. Building maintenance personnel easily can monitor and control chiller operation from the touchscreen using graphic icons and simple commands. A solid-state design doesn’t require maintenance and will typically operate trouble-free, usually for a service life exceeding that of the associated chiller.
Retrofitting an existing chiller with a predesigned package delivers many advantages as summarized in the table and as detailed above. A chiller retrofit is economical to purchase, easy to install, and simple to operate. Rising energy prices and more widespread electric utility rebates promise to further increase the attractiveness of a retrofit, spreading the solution to an ever greater number of users.
Advantages of chiller optimization
- Cuts electrical energy use by 30%
- Cuts peak power demand
- May be eligible for electric utility rebate
- Cuts required chiller maintenance
- Extends chiller operating life
– Michael Grant is regional drive specialist with Yaskawa America Inc., Waukegan, Ill. Edited by Kevin Parker, contributing content specialist, CFE Media.
The Chiller Optimization Package (COP) was created by Drives Systems division of Yaskawa America and Air Masters. COP is said to be economical to purchase, easy to install, simple to operate, and can be retrofitted to existing chillers regardless of brand or size, Yaskawa America noted.