Central Cooling System Upgrades
What is the Central Cooling System?
The Central Cooling System provides cooling to 32 buildings on the San Bernardino campus, utilizing chilled water technology. This system is made up of industrial water chillers, cooling towers, and chilled water storage tanks (otherwise known as Thermal Energy Storage or TES tanks), all housed at the Central Plant, plus a closed loop system of piping and pumps to transport chilled water to each building.
Why was it upgraded?
With campus growth, it became evident that the chilled water system needed to be expanded to accommodate the cooling load from new buildings. In warmer months, campus cooling load was depleting, and, during extreme conditions, exceeded the capacity of the existing TES tank, requiring chillers to be run during costly On-Peak rate periods at inefficient pump speeds. During lower demand periods, the system was not able to completely utilize the heat transfer from buildings on the system.
What was upgraded?
The components of this project range from new equipment and additional chilled water storage to reconfigured piping and automated control program changes. The resulting combination is complementary, further enhancing energy efficiencies with improved controls for occupant comfort:
Two of the existing water chillers were replaced with two new 1,250 ton high-efficiency units with variable speed drive pumps, increasing the plant's cooling capacity by 1,100 tons and increasing pumping efficiencies by approximately 45%!
With the relatively dry climate of San Bernardino, cooling towers are an economical means to provide the system with base-load cooling through evaporative heat rejection. This project added a new 3,000 ton cooling tower as well as new variable speed drive (VSD) condenser water pumps and VSD fans on both the existing and the newly installed cooling towers. This allows cooling flow and draft to be adjusted according to load for greater energy efficiency.
Thermal Energy Storage (TES) Tank
A second Thermal Energy Storage (TES) tank was constructed, doubling the campus' chilled water storage capacity with an added 30,000 ton-hours of cooling. These TES tanks allow chilled water to be generated more slowly during lower Off-Peak electricity cost periods and stored for use during higher On-Peak energy cost periods. Being able to produce chilled water during low or no-demand periods also allows increased energy efficiencies by slowing chillers down to optimal charging pump speeds. Conversely, pumping chilled water from the TES tanks to campus buildings can be slowed to optimal supply pump speeds (even lower than initial production) to maximize heat transfer to the chilled water.
System Piping & Pumps
System piping and pumps were reconfigured to accommodate variable speed pumping of chilled water through the campus with a ‘most open valve’ controls strategy at the building. This allows for reduced flow resistance, operating pumps at optimal efficiencies, and maximizing the heat transfer to the chilled water – all in order to reduce energy usage without compromising occupant comfort.
- Added 1,250 feet of 16-inch pipe with the construction of the College of Education building to complete the campus-wide chilled water piping loop to the Central Plant, and added 250 feet of 4-inch pipe to connect the building to the chilled water loop. These upgrades allow the building to utilize the efficiencies of the Central Plant rather than have a stand-alone system and results in a system-wide reduction of pumping resistance – lower pumping resistance equals less energy wasted.
- Repiped the Central Plant chilled water discharge from a blended return to a direct supply.
- Replaced the primary and secondary chilled water circulation pumps with lower head VSD pumps to be better sized for the newly configured system.
- Reconfigured piping to by-pass the tertiary (or booster) pumping stations in all existing buildings unless the building load is not satisfied. This reduces pumping resistance to the circulating chilled water loop, saving energy.
- Installed variable speed drives to existing tertiary pumping stations campus-wide.
With variable speed drives on pumps and fans, chilled water flow can be more efficiently set to demand, so tertiary pump controls were reprogrammed to utilize a 'most-open valve' strategy. This concept replaces the traditional pressure differential control strategy by using the correlation between cooling load at the air handler (represented by chilled water valve position) and the building pump speed.
With this strategy, a Delta T (temperature difference between the supply and the return chilled water) of 30 degrees Fahrenheit is typical with cooling loads satisfied and pumps running at optimal speeds!