Overview
As part of an Energy Services Company (ESCO) performance contract involving an overall campus energy efficiency upgrade, P2S was tasked to design and commission California State University San Bernardino’s central chiller plant and its14-building pumping system.
The chiller plant upgrade required increasing chiller capacity from 3,145 tons to 4,200 tons of more efficient capacity and increasing the campus chilled water thermal storage capacity from 15,000 ton-hours to 30,000 ton-hours. The system required new DDC controls as well an improved and automated operational controls sequence.
To complement the plant operation, the building pumping project required adding or upgrading variable speed drives on existing chilled water pumps, reworking piping around the pumps and reprogramming the building controls sequences for more efficient pumping.
Challenge
The objective of the project was twofold. First, P2S had to deliver a more efficient chilled water plant and distribution system. Second, we had to increase the capacity of the upgraded plant. Since the campus master plan projected a need for increased cooling capacity to serve both new and existing clients, it was possible that overall energy consumption could increase. However, P2S and the ESCO team had to design a more energy efficient plant that would produce cooling at a per ton rate measurably more efficiently than before.
Action
Chiller Plant
The P2S design team was fastidious in their selection of the best chiller and tower selection and mode of operation. The team’s strategy was to operate the new chillers and support equipment as base-loaded equipment and operate them at night. This would produce 38°F chilled water and deliver it to well-stratified storage tanks. This meant that the only plant equipment that would run on peak or mid peak utility time-of-use periods would be the secondary chilled water pumps for distribution to the buildings.
When the design of the plant was complete the condenser and chilled water systems were thoroughly tested. After commissioning was complete, the plant was monitored for eight months. During this period, it was discovered that energy consumed per ton produced the required improvement. Also, the time-of-use period had shifted midstream during the construction period, requiring further fine-tuning of the controls. P2S rewrote and refined the initial control sequence and the plant operators monitored and tested it over the next several months and verified that it improved plant performance.
Chilled Water Distribution
P2S addressed the chilled water distribution system by converting the water-pumping scheme in 14 buildings from primary-secondary-tertiary to primary-secondary-boosted secondary. Piping around the building pumps had to be modified so they could be either bypassed if plant pressure was sufficient or operated to boost secondary chiller plant flow if the secondary pumps failed to provide enough head to pump the building. New drives were fitted for the existing pumps so the boosted secondary pumps could modulate and address building load. Plus, P2S programmed a new sequence of operation for operating the boosted secondary pumps. The sequence looked at the position of the building cooling coil control valves and modulated the building pump speed based on valve position. We also tuned the building chilled water control valves so they modulated with good control in response to the load without hunting.
Results
After intense monitoring, testing of new strategies and refining operations on the chilled and condenser water equipment, the campus plant now produces chilled water at an overall lower kW per ton rate during all hours of operation. Building customers are more comfortable than before the upgrade.
With ample tank capacity, good tank stratification, and optimized building pumping and control valve operation, the new system achieves as high a Delta T as possible. During the more mild seasons, the campus discovered that a nightly charge of the TES tanks is not required.
The automated plant control sequence has freed the operators from micromanaging the equipment operation which allows the staff more time to focus on improving campus operations and efficiency rather than babysitting the equipment.
