by Jerry Alverson
The University of Connecticut’s main campus in Storrs includes some 400 buildings on 450 acres. Chilled water generated at the central utility plant (CUP) is distributed to about 30% of the campus. However, the 1997 chillers were underperforming, even as demand grew. Working closely with UConn’s plant operations staff, BVH designed a streamlined system that draws on multiple fuel sources for maximum operating efficiency and flexibility, then devised a phased approach to keep service active while the major system modifications were implemented.
The plant was built in 1997 with two 1,100-ton electric centrifugal chillers plus two 940-ton natural gas engine-driven chillers, which could no longer perform to specifications while still meeting emissions requirements. An adjacent cogeneration facility built in 2006 houses three gas-turbine generators and the associated heat recovery steam generators (HRSG).
Each turbine/HRSG produces 20,000 pounds per hour of high-pressure steam, while generating 7,500 kW of electrical power for campus consumption. Steam from each HRSG is paired with four 2,100- ton steam turbine-driven centrifugal chillers to provide cooling to the campus from the “waste” heat, to minimize the cost of energy production.
The 1997 plant was configured with constant-speed, primary-chilled water pumps associated with each chiller, and variable-speed secondary pumps for campus distribution. The cogeneration facility chillers use variable speed primary pumping and are tied to the same campus distribution pumping. Campus cooling loads on this plant have grown to 8,400 tons in recent years and are projected to reach 10,000 tons. The revised combined plant provides N+1 redundancy of 10,000 tons.
Upgrades to the cooling plant include four new 400-ton gas-engine-driven chillers, new cooling towers, and reconfiguration of the plant into a common piping and pumping system that results in a seamless integration of CUP and CoGen chilled water systems. Removing the primary-secondary pumps in the CUP and extending the 30-inch-diameter chilled water headers from the CoGen plant into the CUP allow the CUP chillers to be connected in the same manner as the CoGen chillers, creating a more efficient variable-primary pumping arrangement.
New taps off the 30-inch supply and return headers feed each of the 20- and 18‑inch campus distribution loops, eliminating the need for subfeeding the smaller loop from the larger one.
New plate-frame heat exchangers provide up to 2,000 tons of “free cooling” capacity during the winter using only the cooling towers. By reconfiguring the condenser water piping to allow two of the CUP cooling towers to feed the heat exchangers independent of the other two, one condenser water loop can operate at 40°F for free cooling, while the other pair of cooling tower cells operate at 65°F for stable chiller operation. This eliminates the troublesome issue of switching from free cooling to running chillers while condenser water temperatures are being reset.
Another equipment upgrade is designed to achieve more operating hours at optimized temperatures. A plate-frame heat exchanger with a separate glycol loop obtains free cooling from the gas turbine inlet coolers. The turbine inlet coolers lower air inlet temperature during hot weather, making the turbines more efficient and capable of higher electrical output. This new heat exchanger allows the inlet coolers to operate as another free cooling source when it is below freezing, as well as raising the turbine inlet temperature for improved operation of the gas turbine generator.
The multiphase implementation plan relies on seasonal fluctuations, starting with the two winter months (Jan./Feb.) with low enough loads to supply the campus with temporary cooling. Connecting two rental chillers to the campus distribution piping at each end allowed us to shut down the cooling plant for modifications to the main headers. Phase 2 focused on installation of the new gas-engine chillers, which allowed the CUP chillers to service campus loads during the cogeneration plant’s annual maintenance shutdown. The final phase — replacing the CUP cooling towers — will be completed once the campus load is reduced in the fall. The new gas engine chillers will be operational in May 2019, and the overall project will be completed in early 2020.
Team: BVH Integrated Services (civil, structural, MEP/FP engineer); Bond Brothers District Energy Group, construction manager; Tucker Mechanical, McPhee Electrical, and Array Systems (instrumentation & controls)
Jerry Alverson, PE, is a project manager at BVH Integrated Services, P.C.