Project profile: Chiller retrofit at office building
A prepackaged chiller retrofit cuts energy costs by 30%, reduces required maintenance, and extends operating life at Duke Realty Corp.’s 19-story office building in Clayton, Mo.
Project name: Duke Realty Corp.
Project type: Existing building retrofit
Engineering firm: Air Masters Inc. of Fenton, Mo.
Building type: Office building
Location: Clayton, Mo.
Timeline: April 2013–June 2013
The Duke Realty Corp. building uses three 360-ton chillers to provide cooling for its 19-story office building. In the spring of 2013, Air Masters Inc. of St. Louis and Yaskawa America Inc. provided chiller optimization packages (COP) to Duke Realty Corp. The reason for these COP-retrofit installations is that chillers typically represent the single largest consumer of power in buildings, therefore, they are a logical place to consider for reducing energy consumption/costs.
A chiller will use excess energy when its compressor operates at full-rated constant speed and demand does not require full cooling capacity. This is a wasteful and unnecessary operating condition, as most chillers operate at partial load or off-peak load 99% of the time. The compressor motor in a chiller is quite large in kilowatt capacity. Typically, the motors are in the approximate range of 150 to 600 hp. Money saved by operating the compressor motor more efficiently can add up quickly.
Three 360-ton chillers were operating at full speed, and wasting energy with the use of variable-discharge vane throttling. The challenge was to improve both the efficiency and the operational lifecycle of the facility’s chillers.
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) and chiller control automation. Rising energy costs and electric utility rebates for VFD upgrades coupled with falling prices for large-horsepower VFDs have reduced payback times to as little as 1 yr.
However, the chiller compressor motor VFD retrofits require some complexity in actual chiller control and condenser water temperature optimization. This may cause some owners/operators to postpone retrofits and forego savings. Details below provide clarity related to the opportunity cost of not retrofitting a chiller with a COP. Actual performance of the COP can be measured against a baseline of typical operating conditions.
Figure 1 illustrates the general system differences in chiller efficiency between a constant-speed chiller and a VFD-operated chiller. The Duke Realty Corp. building uses three 360-ton Trane Centravac chillers to provide cooling for its 19-story office building. Before the retrofit of the COP, the three chillers ran at constant speed.
They incorporated only mechanically actuated inlet guide vanes to modulate the chiller capacity and nominally reduce energy-consumption performance.
Condenser water control also impacts chiller efficiency. The condenser water temperature control is now set to a minimum of 65 F, at 7 F wet bulb.
All three of the chillers were designed to operate at constant speed and relied on inlet guide vanes to modulate the capacity of the chillers. The vanes improved the partial loading efficiency of chillers. VFDs were installed on two of the three chillers. The third chiller was kept for peak-demand operation.
Figure 2 is a screenshot image of the system PC chiller control and monitoring. Figure 3 is the HMI of the chiller optimization packages’ controller.
The condenser water reset temperature control further reduces the lift of the chiller, thereby favorably impacting chiller operation and efficiency.
An independent energy-management firm inspected the facility and used an eQuest (version 3-64) simulation model to collect data. This study was completed by the St. Louis-based energy utility.
The simulation was first built using as-built parameters, which included two of the three chillers being equipped with VFDs and condenser water reset controls. The model was then calibrated using billing data and 2014 weather data for the St. Louis area.
Actual chiller operation information was taken from the facility’s energy-management system.
The data included chiller tonnage, kilowatt demand, and condenser/chilled water temperatures.
Using the provided data in conjunction with corresponding local weather averages, a regression was developed allowing for the determination of chiller run-time hours.
The two COP-retrofitted chillers should operate approximately 5,815 and 1,725 hr/year. The model resulted in annual run-time hours of 5,695 and 1,721 hr. The model was very close to the actual run time.
The below model was created by removing the condenser water reset control and removing the VFDs from the two chillers. The annual savings is the difference between the annual consumption of the baseline and as-built model, which can be seen in Table 1.
Verified gross savings/realization rates
The model defined the outside air temperature (OAT) and compared the constant speed (CS) with variable speed drive (VSD) operations.
The measured results of the calibration effort are shown in Table 3.
As the study indicates, a much more efficient way to adjust to changing load characteristics or lower condenser water temperatures is to vary the speed of the chiller’s impeller. The COP retrofit accomplishes this by controlling the voltage and frequency of the electrical power consumed by the compressor motor. The magnum continually optimizes chiller speed to realize the energy savings of 30% annually.