By using ASHRAE Standard 90.1-2013 as a baseline, a marine research lab at the University of North Carolina Wilmington (UNCW) reduced energy consumption at its chilled water plant by nearly 40 percent.
SCHOOL SMARTS: The 69,000-square-foot MARBIONC Building, a research lab at the University of North Carolina Wilmington, employs Smardt chillers with Danfoss Turbocor® compressors to exceed ASHRAE Standard 90.1 for baseline energy efficiency.
Located just 100 yards from the Atlantic Intercoastal Waterway and 500 yards from the Atlantic Ocean itself, UNCW takes marine science seriously. UNCW CREST Research Park includes three world-class marine laboratories, including the MARBIONC Building — a new 69,000-square-foot interdisciplinary research facility for marine biotechnology. A LEED Silver facility, the MARBIONC Building leases labs to commercial enterprises that require reliable, energy-efficient 24/7/365 cooling. That’s why Steve Sharpe, energy manager for UNCW, selected Smardt chillers using Danfoss Turbocor® compressors to handle humidity that ranges from Amazon-rainforest highs to Seattle lows.
“Summers can get pretty swampy on the North Carolina coast,” said Sharpe. “But it’s a mid-Atlantic region, so we experience climate variations throughout the year. Consequently, we decided to build a chiller plant that could take advantage of that wide range of operating conditions and still run reliably. If we lose cooling, we can lose research — which is very bad. Smardt centrifugal chillers with Danfoss Turbocor magnetic-bearing variable-speed compressors could handle that range of conditions with 100 percent reliability.”
Sharpe aimed to build the chilled water plant to exceed ASHRAE Standard 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Buildings, which requires selecting chillers optimized for part-load conditions as gauged by the Integrated Part Load Value (IPLV) metric. IPLV measures chiller efficiency over a range of operating conditions — precisely the situation Sharpe was facing.
“Our climate extremes are 97.5°F dry bulb and 88.3° wet bulb temperatures,” Sharpe explained. “The MARBIONC Building uses nearly 100 percent outside air. So, when you have high wet bulb temperatures, you have to wring the moisture out of the outside air being supplied inside. Outdoors, the high humidity hampers evaporation, making it tough for cooling towers to reject heat into the atmosphere.”
Fortunately, less than 150 hours a year occur at the highest dry bulb and wet bulb temperatures — creating potential to save energy during lower operating conditions. Based on ASHRAE recommendations at that time, the most efficient type of plant was determined to be a single, primary-flow, fully variable-volume chiller plant. To take maximum advantage of the 8,610 hours operating below full load, Sharpe selected two 750-ton Smardt WA240 water-cooled variable-speed chillers. Each chiller uses five Danfoss Turbocor TT400 oil-free magnetic-bearing centrifugal compressors, each with a nomical rating of 150 tons.
“The key is to use multiple compressors that can throttle back or ‘turn down’ capacity to match the reduced load,” Sharpe said — for example, when outdoor conditions are cooler, or on weekends, when the building has fewer occupants. When entering condenser water temperatures (ECWT) can be lowered, the compressor work and energy use decreases. For example, every 1° drop in ECWT below the full-load design point can increase chiller efficiency by 1 to 2 percent.
Often, when load decreases, so does a factor called “lift:” the difference between refrigerant pressure in the condenser and refrigerant pressure in the evaporator. It’s affected by both weather and the cooling load of the chiller. Through constant system monitoring, the Turbocor compressor modulates refrigerant flow and impeller speed to counteract conditions that could create surge or choke. Turbocor also mandates that a stop/check valve be installed to prevent reverse gas flow, which could cause an internal mechanical issue.
Along with a variable-speed, variable-flow chiller, the system also uses a Tower Tech forced-updraft, counter-flow cooling tower. In this case, one 100-hp Danfoss VLT FC 102 drive was used to operate 10 10-hp direct-drive fan motors. The drive slows fan motor RPMs and cuts electricity consumption exponentially. For example, reducing speed by 20 percent results in nearly 50 percent energy savings.
To optimize all the variables of the chiller plant, Sharpe used a Central Plant Energy Control System (CPECS) from Kiltech, a Smardt company. The system runs on a model-based analysis method that analyzes the chiller plant’s actual load and outside air conditions, uses equipment models to predict the most efficient operating point, then controls the plant to meet this operating point. At the same time, it runs a model ASHRAE 90.1 plant and compares the two to determine the savings realized over the ASHRAE 90.1 baseline.
The CPECS report from August 2016 to 2017 shows the plant ran 6,687 hours using 1.1 million kWh, for an average annual plant efficiency of 0.576 kW per ton. A baseline ASHRAE Standard 90.1 plant modeled on those same hours would have had an efficiency of 1.034 kW per ton, costing an additional $67,855.
With a conventional chiller plant design, those numbers would never be possible, Sharpe noted.
“It’s smarter to use chillers that can handle temperature reset and not work as hard,” he said. “If you have a plant that can unload efficiently, run at those part-load conditions, and take advantage of cooler outside conditions, the savings cascade hour by hour, day by day. We’re saving a lot of money, with a payback well under five years, with less maintenance and 100 percent uptime — all while shrinking our facility’s carbon footprint.”