ASHRAE Journal - February 2015 - 40

COLUMN ENGINEER'S NOTEBOOK
Kent W. Peterson

Chilled Water TES Hydraulics
BY KENT W. PETERSON, P.E., PRESIDENTIAL MEMBER/ FELLOW ASHRAE

Thermal energy storage (TES) is an effective means of shifting cooling electrical load
from peak to off-peak electrical rates. Chilled water is the most common form of TES,
using concrete or steel tanks to store chilled water at 39°F (4°C), which is the temperature at which water density is highest, encouraging stratification within the tank.
Under normal conditions, a chilled water TES tank is always filled with water. During
discharge, cold water is pumped from the bottom of the tank, while an equal amount
of warm return water is returned to the top of the tank. Due to the increased density
of colder water, a stable stratification of layers of water can be obtained.
Placing an open chilled water thermal energy storage tank in a chilled water system has several ramifications on the hydraulic performance of the system.
This month, I intend to point out engineering issues
that must be addressed under various scenarios. These
are all real examples from actual facilities that I have
designed, peer reviews I performed of designs by others,
or retrofits of designs by others.

System Considerations
TES tanks are seldom ASME-rated pressure vessels due
to the high cost, so they must be vented to atmosphere.
This establishes the reference pressure for the system;
essentially the TES tank is serving as a vented expansion
tank. Heat exchangers could be used to physically isolate
the TES tank from the distribution system, but this is
typically not preferred since chilled water ∆T will suffer
with heat exchangers and additional pumps would be
required. This column will focus on how to control for
pressure in chilled water systems with atmospheric TES
tanks without heat exchangers.
A chilled water system with an atmospheric TES tank
must always maintain a positive gauge pressure in all
parts of the system to prevent air from leaking into the
system. Ideally, the TES tank would be located at a vertical high point in the system or constructed tall enough
that the system's static pressure requirements are met
by the tank water height. If the water level in the TES
tank is lower than parts of the chilled water piping system, some means of sustaining positive pressure when
the system is operating will be required in the system
40

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FEBRUARY 2015

design. This means of control will create an additional
pressure drop in the chilled water return system to
maintain positive pressure at the highest points of the
chilled water system. Even a small air leak when the
chilled water system is off can lead to the atmospheric
TES tank overflowing when the tank is located below the
highest point in the piping system.
Chilled water systems with multiple TES tanks pose
additional considerations since flow can be motivated
by the static head differences between the open tanks.
A very small difference in static head pressure would
result in water removed from one tank possibly overflowing to the other tank if both tanks are open to the
system at the same time.1 This month's column will focus
on single atmospheric TES tanks.

Location of Chilled Water TES Tank
The locations of chilled water TES tanks are sometimes
located based solely on aesthetic considerations without
understanding the engineering ramifications of the tank
location. Chilled water TES tanks in large chilled water systems are often located at or near the central chiller plant or
at or near one or more satellite chiller plants. However, in
some cases it is not practical to locate the TES tank at or near
the plant and will need to be located near the chilled water
distribution network preferably near the highest point.
Once a TES tank location has been sited within a plot
plan of a chilled water distribution network, there is still
the choice of locating the tank above ground or below
Kent W. Peterson, P.E., is chief engineer/COO at P2S Engineering in Long Beach, Calif.
He is former chair of Standard 189.1.
ASHRAE Journal - February 2015

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