Page 354 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Princeton University District Ener gy System 327

A 2.6-million-gallon chilled water thermal storage system was installed in 2005. It
has a design capacity of 40,000 ton-h with a 24° differential temperature. The system
was designed for “fast discharge.” Four 2500-ton plate-and-frame heat exchangers were
included to provide chemical and hydraulic separation from the campus and to allow
the system to deliver up to 10,000 tons of cooling. This makes the system extremely
responsive to changes in economic dispatch and campus emergencies. To maximize
thermal storage capacity and improve the campus temperature differential, the chilled
water (CHW) storage temperature on the plant side of the heat exchanger is 31°F, result-
ing in supply water as cold as 34°F available to the campus. Low storage temperatures
are achieved without the risk of freezing by using a density depressant additive. Low
distribution supply temperatures improve dehumidification capability, reduce pumping
energy requirements, and increase the distribution system capacity by approximately
20 percent.

Chilled Water Distribution
Princeton’s chilled water distribution piping network consists of a combination of tun-
nels and direct buried piping. Chilled water is normally distributed to the campus at
41°F with a typical return temperature, at higher loads, of 54°F. By specifying high
delta-T coils (typically 20° temperature rise) and pressure-independent control valves
for all new construction and renovation projects, the chilled water temperature differ-
ential and system capacity have steadily improved each year.

Water Systems Quality Management
By carefully monitoring and managing the water quality in all energy systems,
Princeton maintains high water-side efficiencies and long equipment life in chillers,
boilers, cooling towers, and air-handling units. This also minimizes health and
safety risks, and prevents corrosion and biological fouling in piping and control
equipment. Princeton runs a three-tiered water quality management program: Plant
personnel sample and test water systems at least once each shift. A water treatment
company representative repeats these tests and performs additional analysis and
makes recommendations on a weekly basis. Every 3 months, an independent water
treatment consultant samples and performs tests. Then, a water systems meeting is
held involving plant operators, campus maintenance personnel, water treatment
company representatives, and the independent water chemist. All results are com-
pared and discrepancies, concerns, and opportunities for improvement are dis-
cussed. The following systems are included in this program: chilled water, boiler
feed water, returning condensate, city water, well water, cooling tower water, and
thermal storage water.

Plant Controls
The energy plant has separate control rooms for the cogeneration plant and the chilled
water and thermal storage facilities. The controls are fully integrated in one system so
operators in both areas have complete plant indication and can respond to alarms and
troubleshooting in a consistent and straightforward way.
The plant control system is based on the Allen Bradley PLC hardware and Intellution
iFix 32 human–machine interface. The control system provides all supervisory, control,