Plan of chilled and heated water loops

Cal Poly SLO Variable Chilled Water Pumping and Central Plant Optimization

While improvements to central heating and cooling plants may offer significant energy saving opportunities, the ultimate performance may be limited by the systems in the buildings served by the plant. At Cal Poly San Luis Obispo, a team including the campus energy manager and consultants successfully optimized the campus central plant by implementing a coordinated systems retrofit in 14 older buildings.

Using a tested project delivery approach, and supported by creative financing, the campus modernized older buildings in coordination with optimized operations at the central plant.

Initially constructed in 1997, the central plant at Cal Poly San Luis Obispo houses five chillers, three boilers, and a 1.6M gallon chilled water thermal energy storage tank. The plant provides cooling for 21 buildings with a total area of 1.1M ft2, and heating for 54 buildings totaling 3.3M ft2.

Veiw of chillers in central plant

Some of the five chillers housed in Cal Poly's central plant.

As is common on established campuses, the diversity of building ages resulted in non-optimal performance both at the central plant and in individual buildings. Newer buildings included variable speed water pumps, two-way control valves, and were operated with high temperature differentials (delta-T) between supply and return flows, allowing for efficient operation. However, many older buildings had constant speed pumps, three-way valves, and were operated with low and less efficient delta-T values. These older systems reduced the options available to central plan operators, wasting pumping energy and reducing the effective capacity of the thermal energy storage.

In 2015 the campus energy managers devised a comprehensive retrofit and controls upgrade to contend with this problem. The overarching goals of the project were to maximize energy and cost savings, modernize chilled water pumping systems, optimize chiller plant operations, upgrade boiler controls, and to pilot test wireless pneumatic thermostats. The campus engaged the PG&E Sustainable Solutions Turnkey program as the prime contractor, AECOM provided engineering and management, and local subcontractors provided installation.

The team took advantage of $3M in creative financing, including PG&E On Bill Finance (OBF) that provided an interest-free loan of $1M upon closeout and verification of savings, to be repaid via utility bills. Funding also came from a $2 million, one-percent interest loan through the California Energy Commission’s Energy Conservation Assistance Act (ECAA) program.

Thermal storage diagram

Schematic diagram shows charging of thermal storage during off-peak (left) and peak-rate operation.

In the central plant, the team implemented several optimization measures. They improved the sequence of operations of the five chillers, and expanded the use of a second cooling tower in order to increase the effective evaporative area and reduce pumping energy, while driving condenser water temperatures as low as possible. The central plant boiler combustion controls were also upgraded with digital fuel-air controls and variable frequency drives for the combustion air blowers, improving combustion efficiency and reducing emissions.

In addition to these central plant changes, the project installed equipment upgrades in 14 buildings. This work eliminated hydraulic decouplers and provided chilled water pump bypasses, two way control valves, and variable frequency drives, combined with new operational strategies to maximize the delta-T and thereby reduce pumping energy.

Upon completion, all buildings on the central chiller loop had variable flow with two-way valves, allowing for proper operation of the chiller plant and thermal energy storage. This has yielded significant savings in both building and central plant pumping energy, with improvement in system delta-T, now reported to be at an efficient value of 20 degrees F, which might yet be increased through ongoing commissioning and experimentation. This higher temperature differential also increases the effective capacity of the thermal energy storage system, and reduces the time required for its recharge. Chillers are now prevented from operating during peak periods, reducing utility demand charges.

The project has also served educational purposes, with tours of the central plant and building systems being given to mechanical engineering students while the project was on going. Also, a mechanical engineering student conducted energy simulations to determine the optimal operational sequences for the chillers and thermal storage, work that was included in the student's master’s thesis.

Finally, project team members note that the genesis of the project came from a presentation on a similar project completed at UC San Bernardino. Such an approach to the integrated retrofit of building systems and optimization of the central plant may be applicable other campuses with a central plant and a diversity of building ages and systems.

Images courtesy of Cal Poly SLO.

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Key Strategies
  • Optimization of chiller plan operation
  • Installed chilled water pump bypasses, two-way valves, VFDs, and modified pumping systems in older buildings
  • Upgraded boilers with digital fuel-air controls and VFDs for the combustion air blowers
  • Cypress wireless digital/pneumatic thermostats with multiple setpoints
  • Increased system delta-T to improve overall performance and expand thermal storage effectiveness
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