In operation since August 1999, the new thermal storage system currently uses three low-temperature, 2,250-hp screw-compressor chillers utilizing R-717 ammonia refrigerant, each dedicated to building ice on banks of externally galvanized steel coils in an existing 4-million-gal underground storage tank, previously dedicated to storing only chilled water. The result is a solution that builds on the strengths of the existing cooling system while effectively tripling the storage capacity within the existing tank structure. As a result, the new ice-on-coil technology enables Stanford to produce up to 93,000-ton hours of thermal storage nightly and generate chilled water during the peak daytime cooling period.
The new ice-on-coil storage system, which uses coils designed and manufactured by Baltimore Air Coil Company (Baltimore), allows a greater proportion of electrical consumption to be shifted to off-peak hours. During the “ice-build” mode at night, fluid flows from the chillers’ evaporators to four banks of coils of externally galvanized steel tubing, 1 in. in diameter, located inside the 160- by 160- by 24- ft storage tank. “To our knowledge it is the largest ‘internal melt’ system in North America,” observes Steve Mischissin, manager of Stanford University utility operations.
One of the keys to the new thermal storage system is Dowtherm™ SR-1 industrially inhibited ethylene glycol-based heat transfer fluid. Premixed at approximately 26% solution with deionized water, Dowtherm SR-1 fluid lowers the freezing capacity of the solution to 10˚F. About 130,000 gallons of it circulate through a closed loop between the coils in the storage tank and a new 18,000-sq-ft aboveground facility that houses the low-temperature chillers and associated equipment.
To Ice and Back
In “ice-build mode,” which usually starts about 7 p.m. each evening, the solution of Dowtherm SR-1 fluid is chilled to 18˚F and pumped to the banks of coils inside the underground storage tank, where it freezes the water in the tank surrounding the coils. The solution flows at 28,000 gpm until full ice-build is reached. This mode could take as little as eight to 10 hours or as long as 14 hours, depending on the weather and cooling loads expected for the following day. The screw chillers are run from 7 p.m. to 11 a.m. to take advantage of the lower electrical rates and to minimize electrical use during the daytime peak period from noon to 6 p.m.In “ice-burn” mode, which runs from noon to about 6 or 7 p.m., glycol pumps are started and the heat transfer fluid flow is switched from the screw chillers to glycol-to-chilled-water plate-and-frame heat exchangers. The solution of Dowtherm SR-1 fluid enters the plate and frames at 34˚ and then returns to the storage tank at 53˚ to start melting the ice on the coils.
On the other side of the plate-and-frame heat exchanger is the campus chilled water, which is cooled from 58˚ to 41˚. When the load on the campus cooling system starts to level off in the early evening, the plate-and-frame heat-exchange system shuts down, and the system is then switched back to ice-build mode.
‘Squeezing’ Space
The difference between an ice storage system vs. chilled-water storage systems is the “heat of fusion” of water requires 144 Btu/lb of water of thermal energy transfer to go from a liquid to solid state and vice versa. This means that ice can “store” more thermal energy per pound than “liquid” water.According to Mischissin, this was a key point in the university’s decision to install a new thermal storage system as opposed to expanding the existing chilled-water system with conventional chillers. “We already had an underground facility with a parking lot above it, and the tank was in good shape. The ice-on-coil storage technology allows you to get more capacity in a smaller space.”
The thermal storage system uses the existing campus distribution system to pump chilled water to each building and back to the new chiller plant through 13 miles of underground piping.
The water is pumped out at 60 psi and returns at about 30 psi, with all pumps and valves monitored and controlled from the distributed control system. Stanford engineers designed the control logic.
The new thermal storage system operates in parallel with the existing chilled-water system, with the ice-chilled water blended with the water from the existing chiller plant. Both systems are controlled by the distributed control system, which can be controlled remotely either from the university’s central energy facility, which also supplies the campus with steam and electricity, or from the utility office. During the peak period, however, the new thermal storage system contributes 80% to 90% of the total cooling.
Future Perfect
As for any future expanded cooling needs, a third screw chiller was added to the original two, and has been in operation at the new thermal storage facility since May. In addition, space exists for two more chillers. There is enough space remaining in the storage tank for a fifth bank of steel coils capable of adding another 25,000-ton hours of ice storage.As for future maintenance concerns, “We’re not anticipating any major problems over the life of the new system,” Mischissin observes. “All the piping systems are carbon steel, and if we keep the right glycol concentration and check on the fluid inhibitors, the fluid should give us at least 20 years of service.
“We consider the ice-on-coil technology and the glycol system to be very low maintenance,” Mischissin adds. “We don’t plan on opening the glycol system up other than to take out fluid samples for annual analysis.” ES
