The University of Minnesota, which was founded in 1851, is an imposing institution that comprises four campuses as well as six agricultural experiment stations, two biological stations, one forestry station, and regional extension services throughout the state. At any given time, up to 60,000 students are learning one of the university's 370 fields of study.
Considering the university's age, it can be expected that some of the buildings' mechanical systems might be in need of repair. When it was determined that many of the 41 independent chillers, which cooled 33 buildings on the St. Paul campus of the University of Minnesota Twin Cities (UMTC), were in peril, a bold move was made. Instead of simply replacing the older, mainly single-effect absorption chillers as they failed, it was decided to centralize operations with a new chilled water plant.
Given the costs involved, the project was broken up into several phases. Phase I, which was completed in November 2005, involved installing the district piping needed to eventually supply 19 buildings with chilled water, as well as the construction of a central chilled water plant. The St. Paul Chiller Plant, as it is now called, wasn't built from the ground up, though; instead, it was constructed in a semi-historic building that sat unused for many years.
Today, 15 buildings on UMTC's St. Paul campus are connected to the new chiller plant, and others will follow as soon as the second chiller plant, which is part of Phase II, comes online within the next year or two.
Plenty of PlanningThe concept of a central chilled water plant for the St. Paul campus of UMTC was discussed as early as 1999. At that time, concerns arose about the aging independent chillers all over campus. Maintenance costs for the chillers were skyrocketing and equipment efficiencies were plummeting. In addition, some of the older equipment was only capable of producing relatively warm chilled water, so dehumidification performance was sometimes less than desirable in certain buildings.
As a result of these issues, an initial master plan suggested that a 12,000-ton central chilled water plant be installed. The original proposal recommended that the chilled water system be housed in the existing heating plant, but the project did not proceed, and the concept was idled for the time being.
Fast forward to 2001, when the engineering firm of Sebesta Blomberg (Roseville, MN) was brought in to conduct a formal study of the cooling systems at UMTC. The firm calculated the cooling loads for all the buildings on campus, then it looked at the first cost as well as the life-cycle costs associated with upgrading the campus's cooling systems.
"We looked at the most economical way to tackle the project. Do we replace the old individual chillers with new chillers in the buildings where they are, or do we build one central plant that has fewer, larger chillers but can be run in a much more efficient way?" said Dave Boyles, project manager, Sebesta Blomberg.
The analysis showed that there was a significant savings associated with a central chilled water plant, so Sebesta Blomberg developed a model that included the distribution system, the size of the proposed central plant, and the electrical requirements. A hydraulic model was also created to show how the water would flow between the various buildings on campus and the new central plant.
Once the study was approved, the state legislature appropriated $20 million for Phase I of the project. The project had to be performed in phases due to funding, as well as electrical supply. The original design called for only one central chilled water plant; however, there were limitations in terms of how much electrical capacity was available at UMTC.
"By studying the best place to put the chiller plants and looking at the capacity of the campus distribution system to accept the additional electrical load, we came up with a phased approach," said Boyles. "This would allow us to put in several different chiller plants, and we'd still be able to operate under the restrictions of the electrical capacity of the campus."
With that decided, the chillers were assessed to determine which ones would be decommissioned as part of Phase I. Many chillers were 35 to 40 years old and way past their useful lives. The buildings with the oldest chillers were put online first. Chillers that were in better shape will be kept until the buildings are put on the chilled water system in the second phase.
Phase I also involved the installation of new underground distribution piping, which runs from the new chiller plant to the mechanical rooms of the 19 buildings being upgraded around campus. As can be expected, this required temporarily closing major roads on campus to allow for excavation and construction of the piping system.
"Obviously, this was a big challenge," said Boyles. "We had to schedule it so as to minimize disruption on the campus, because in effect, we were putting in all this chilled water piping under roads and sidewalks while the campus was still in operation. We had to re-route traffic in such a way as to keep access to parking lots and buildings without disrupting functions that were going on at the campus."
The piping was installed one section at a time, with traffic re-routed accordingly. Crews would dig up the street, lay the pipe, test the pipe to make sure it held pressure, re-pave the street, then go onto the next section. It was a tedious process, which started in May 2004 and finished in September of that same year.
Where Should It Go?One of the more exciting parts of the project involved deciding where the new chiller plant was to be located. It was decided early on that the placement of the chilled water system in the existing heating facility didn't make sense, but instead of building a new structure, an older, unused building on campus was renovated to house the new chiller plant.
Jay Denny, energy engineer for UMTC, explained that the interior of the existing building was completely demolished and rebuilt with all new structural elements. "The building was built in 1939. It's a nice looking building, and it's in a prominent part of the campus. It faces a neighborhood, so care was taken to make sure it looked like it did before, as opposed to looking like a chiller plant."
That was easier said than done, as Boyles stated that one of the more difficult aspects of this project was figuring out how to modify the building to accept all the new equipment, because it was not originally set up to operate as a utility plant. "In addition, it did not meet current codes, so we had to essentially determine how to structurally change the building to allow it to support all this equipment and piping that the building wasn't originally designed to accept."
The three-story brick building was totally gutted in order to make way for the four 1,000-ton Trane CenTraVac® centrifugal chillers and four modular Marley cooling towers. The towers are fed from a common header, and the single fans on each cross-flow cell have VFDs. The control system maximizes the number of towers that receive flow to minimize the fan power required.
Due to height restrictions, as well as preserving the building façade, the cooling towers were essentially built into the walls. "We built a steel structure that those cooling towers could be mounted on, but in effect, the building became the enclosure for the cooling towers, so that from the outside, it looks exactly like it did before, but on the other side, it's the cooling towers," said Boyles.
The plant is a conventional primary-secondary chiller plant with 4,160V motors and a medium voltage switchgear room that includes four 13.8kV to 4,160V transformers. Primary and condenser water pumps are piped into common headers, and the chillers have modulating valves and flow meters to maintain balanced flow through each machine. The secondary (district) pumps have VFDs. All building pumps were removed or bypassed as part of this project, and the district pumps provide flow through all the chilled water coils.
The chiller plant is controlled by a Trane "Tracer Summit" BAS, which also controls the HVAC that serves the plant. In addition, UMTC installed Btu meters in each building served by the district system. These meters are connected to the campus DDC network and are read remotely.
Denny explained that monthly billing and 15-min time-of-day usage data is collected for each building. "In addition, the temperature differential across each building is monitored. Buildings with DDC systems also monitor the differential pressure available at each building, and this information is available to the chiller plant DDC via ‘BACnet®' integration."
All this activity now takes place in a building that doesn't look any different from the street than before, other than now its windows are new, and its previously cracked masonry is repaired. It also has fiber optic lighting in its windows to simulate occupancy.
"It certainly would have been easier to tear down the old building and start from scratch, but this way we were able to preserve a part of the campus history," said Denny. "In hindsight, I think the decision to preserve the building shell was the right choice; however, doing so did raise the degree of difficulty on the project."
The Next Phase BeginsPhase II, which began as soon as Phase I ended last summer, includes the construction of a new utility building on the southeast side of campus (the Phase I plant is on the northwest side). The utility building will house a new electrical switch station and up to 6,000 tons of additional cooling capacity, although only about 3,000 tons will be installed initially. The new plant will also provide redundancy, so it will be possible to finally remove the individual chillers, which were left in place as back-up in case the district system should ever fail.
The first chiller(s) installed in the new Phase II plant will probably be electric centrifugal, but provisions are being made to allow for steam-driven chillers. Construction on the utility building will start this year, with the switch station equipment being installed first, followed by the chiller(s). The schedule will depend on funding and other factors, but Denny anticipates the second plant could be online as early as the end of the 2008 cooling season.
Phase II also involves working on several of the issues not included in Phase I, such as replacement of some of the existing cooling coils in the individual buildings. A few coils were changed out over the winter, but UMTC is monitoring the performance of the remaining coils this season.
"We're basically monitoring coil performance, then in the fall, we're going to change out coils that need to be replaced either due to thermal performance or because we want to increase our ΔT," said Denny. "We recognized there was little benefit in rushing, so we decided to live with what we had last year. It will be better because we'll have control of the flow, and we'll be able to take a detailed look at performance and pick the right coils for the application."
The system has also been changed from a constant volume system to a variable-volume system. Last year, the new chilled water plant ran as a constant volume plant, and most of the 15 buildings that were connected to the plant utilized three-way valves. The secondary pumps ran at full speed, and the only time flow changed was if a building was added or removed from the chilled water loop. Due to the lack of flow control, the chilled water plant could only produce about 3,000 tons of cooling.
"The number of buildings we could serve was limited by pump capacity, not chiller capacity," said Denny. "During this past winter, as part of Phase II, all the buildings were converted to variable-volume systems with the addition of pressure-independent two-way control valves. A few buildings also had bypass valves installed to allow the existing chillers to be used in case the district plant was not available."
The new variable-volume system should help UMTC achieve better efficiencies, although considering that the plant ran in a constant volume mode (with low ΔT) last season, Denny is still pleased. "The total plant efficiency approached 0.7 kW/ton near the end of last season (August and September) when we were serving 12 buildings. With the new pressure-independent valves, we expect the efficiency to improve along with our capacity utilization."
Phase III is likely to follow Phase II, although that's far off in the future. Part of that distant phase could include the installation of a cogeneration plant, especially if fuel costs keep rising. If that occurs, then the Phase II switch station could incorporate the ability for UMTC to self-generate with a turbine.
"We are definitely looking at getting that flexibility back," said Denny. "But for now, it's hard to beat the economics, the ease, and peace of mind of dealing with a single fuel and four chillers instead of 25 that we had to babysit 24/7."