“The ability to use electricity or steam gives me the flexibility I need to manage energy costs. Having occupied the building last September, we are beginning to understand the real load profile. I can change the operating parameters of my chillers based on my real energy costs,” said John I. Vucci, assistant director for HVAC systems, in the university’s facilities management division.
18,000 warm bodies coming and going
Vucci is responsible for operation and maintenance of HVAC systems in over 260 buildings comprising 12 million sq ft of conditioned space. Situated on 1,500 acres, the campus serves nearly 35,000 students and 12,500 employees — plus thousands of visitors annually for events like Terrapin basketball games, which bring up to 18,000 fans to the Comcast Center. Vucci said the total cooling requirement for the campus approaches 30,000 tr. Because the university has gone through a number of growth and construction cycles, there is no central chilled water plant. Rather, the cooling load is served by 175 chillers and myriad smaller unitary systems.In addition to the main arena, the Comcast Center houses athletics administration offices and the 7,000-sq-ft Academic Support Center for the school’s 700 student athletes on 25 teams. An auxiliary, 1,500-seat gym serves as home to the Terps’ volleyball, gymnastics, and wrestling teams. A multipurpose room (capacity 400) is equipped to host banquets, press conferences, large meetings, and serve as a pregame restaurant suite overlooking the competition arena.
Major events, including basketball games, occur in the arena about 100 times a year, mostly during the traditional school year (September through May). It is this variable, diversified load that engineers had to consider when designing the HVAC system. According to Vucci, TES was considered but ruled out due to anticipated first cost, together with the university’s architectural sensitivity to large thermal storage tanks.
Because he already had experience with both steam- and gas-driven chillers, and because steam was available, he encouraged consideration of the hybrid plant design. Vucci’s analysis also projected a life-cycle operating cost advantage with the hybrid plant (vs. an all-electric plant), based on an annual energy cost savings of almost $70,000.
The university buys its energy from TriGen, which provides electricity, gas, and steam as well as cogeneration capability. Electricity from the cogeneration plant is used to base load the campus’s power requirement — 18 to 19 MW — and reduce the purchase of supplemental power during times of higher demand (the campus’s peak load is 35 MW).
Plotting a solid strategy
Originally, Vucci planned to operate the York MaxE YST steam turbine drive centrifugal chiller as the base load machine, and then use the York MaxE YK electric drive centrifugal chiller to meet peak loads. However, that operating strategy could shift depending on cost of steam vs. electricity at those times of peak demand (notably, not in the hottest part of summer).Chilled water is supplied at 44¿F to a total of 29 York AHUs, all equipped with electronic VSDs. Eight main AHUs serve the basketball arena, each with a capacity of 45,000 cfm. Air distribution follows a demand-control ventilation strategy, taking advantage of cool outdoor air during the basketball season and allowing the AHU fans to coast down to slower speeds. The arena was designed to maintain air changes at 7.5 cfm per person per hour. This complies with ASHRAE guidelines because of the short duration — up to three hours — of a basketball game. Overriding this, the entire ventilation system can open to 100% outdoor air supply automatically if CO2 levels reach 1,200 ppm in the arena.
Air temperature at the floor level is maintained at 73 degrees during basketball games. Operators must also maintain between 35% and 55% rh at all times to protect the permanent hardwood floor in the basketball arena. So, even on the majority of days when no major events occur, humidity control is critical.
With the significant swings in cooling load from hour to hour and day by day coupled with the uncertainties in the cost of energy, operating the Comcast Center in the most economical way is a challenge to Vucci and his team. However, with the adoption of a steam-electric hybrid chiller plant the team at Comcast Center has the flexibility to meet these challenges, now, and in the future. ES
