San Jose State University Chiller Plant Improvements

With several new projects scheduled to come online, and more planned for the future, peak cooing loads were expected to soon exceed the capacity of San Jose State University’s central plant. Staff at Facilities Development and Operations took advantage of the required improvements to implement a set of major improvements: replacing chillers, changing piping configurations, and optimizing sequences to enable efficient partial-load operation and provide overall cost savings.

The optimization of the central plant will meet the demands of future campus growth, with improved efficiency and stability that enhances individual building performance.

Prior to the retrofit, the plant included two constant-speed electric chillers, two steam absorption chillers, and an ice thermal storage system with a dedicated glycol chiller. During the retrofit, both electric chillers were replaced with larger capacity units having variable frequency drives to provide better energy performance during partial-load conditions, which represent the majority of operating conditions. The new chillers’ increased size will provide the needed capacity as new campus buildings come online.

The piping for both the chilled water and the condenser water was redesigned to allow for in-series operation, and to optimize the operation of chillers by "stepping" the water temperature as it flows through the system. Electric chillers are most efficient with very cold condenser water, and in turn can produce very cold chilled water. The steam absorption chillers, on the other hand, can operate with warmer condenser water and produce warmer chiller water. By piping in series, water from campus is pre-cooled as it passes though the absorption chiller evaporators before entering the electric chiller evaporators. On the condenser side, cold water from the cooling towers enters the electric chillers first before going into absorption chiller condensers. In addition to the piping redesign, the primary chilled water system was converted from constant to variable flow, saving energy during low-load conditions. The resulting system efficiency has improved from approximately 0.8 kWh per ton of refrigeration to 0.4-0.5 kWh per ton. In addition, the more stable chilled water temperature is saving energy within individual buildings, as the cooler chilled water reduces pumping energy needed within buildings.

In addition to physical retrofits, a number of operational changes were engineered and implemented. First, a simulation study was conducted to develop operational strategies that will reduce costs, taking into consideration electricity pricing tiers, operation of the two types of chillers, and use of the ice bath thermal storage system. During peak hours the campus load is served by the thermal storage system, supplemented by absorption chillers, and finally served through the use of variable speed electrical chillers. The project team selected the industrial-grade CIMPLICITY control system from General Electric to integrate controls with the cogeneration plant, using predictive control based on weather predictions and historic load data, and enabling robust trend data collection.

Project Manager Chris Nordby notes that the project team was disciplined and made decisions based on a long-term outlook that would meet future demands as new buildings come online. A challenge was fitting such a significant scope within standard 12 or 24-month utility incentive cycles. However the project team managed this challenge by identifying and setting aside aspects of the project that could be completed as part of future energy efficiency investments.