ASHRAE Journal - June 2013 - 62

emerging technologies
Saves Energy, Money

Thermal Energy Storage
By Michael Rutberg; Mildred Hastbacka; Alissa Cooperman; Antonio Bouza, Associate Member ASHRAE

hermal energy storage (TES) has been widely used in the com- glycol) through coils submerged in a
mercial sector since the 1980s to shift HVAC cooling load out of

the peak demand period of the day. By charging a chilled water or
ice-based TES system overnight, a facility can take advantage of
off-peak electricity prices and reduce any charges based on peak
demand. Adding TES can also increase cooling capacity without installing more or larger chillers. TES is often viewed as a cost-savings
measure, rather than as an energy-savings technology.

However, TES can save energy overall,
sometimes at the site of use and usually
at the electricity generation source. Current advances in TES include progressively tighter integration of distributed
TES systems with regional power grids
to reduce the cost of generation. This
integration is facilitated by new control
technologies, communication channels,
and business models.

Technology Overview
The simplest form of cool TES uses
chilled water as the storage medium.
The water is typically stored in a large
concrete or welded steel tank, with
warm and cool layers stratified by the
density difference between them. During
system charge, warm water is removed
from the top layer, chilled, and returned
to the bottom layer through specialized
diffusers that minimize mixing. Chilled
water TES is simple, efficient and costeffective, particularly at large scale as
the cost per gallon decreases with tank
size. Existing system capacities range
from about 500 to more than 100,000
ton·hours.
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ASHRAE Journal

The downside to chilled water TES
is a large footprint; for a tank with a
typical temperature differential (~15°F
[8.3°C]), the storage volume is around
15 ft3/ton·hour. The footprint of chilled
water TES can be decreased by increasing
the temperature differential. However,
water reaches its maximum density at
39°F (4°C), so if the temperature of
the cool layer drops below that point,
it will disrupt the stratification. This
destratification can be avoided by using
commercially available salt additives that
depress the temperature of maximum
density, allowing a colder cool layer and
thus a larger temperature differential.
Aggressive temperature differentials can
reduce the volume footprint by a factor
of two or more.
Ice-based systems have a smaller
footprint than chilled water TES systems,
thanks to the high latent heat of fusion
of water, with effective storage volumes
usually in the range of 2 to 4 ft3/ton·hour.
In the most common configuration, ice is
built around chilled coils; this is known as
“static ice” or “ice-on-coil.” The system
is charged by passing cold fluid (usually
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tank of water, causing ice to accumulate
around them. Static ice TES modules are
offered by several well-established manufacturers, with per-module capacities
generally in the high tens to low hundreds
of ton·hours; larger capacities can be
achieved by combining multiple modules
in a central or distributed system.
One relatively recent development is
the emergence of a 30 ton·hour, ~175 ft3
(5 m3) unitary static ice system, incorporating both a storage tank and an ice-making condenser, and designed for use with
rooftop and other direct expansion (DX)
systems (Figure 1). This unitary system
uses gravity-fed liquid overfeed refrigerant loops to build ice (charge mode) and
to pass cooling to the associated packaged
DX system (discharge mode).1

Energy Savings
There are three main mechanisms
through which cool thermal energy storage can contribute to site energy savings.
First, cool storage is charged overnight,
when ambient outdoor temperatures are
lower. Particularly in the case of aircooled chillers, this increases efficiency
substantially compared to daytime operation. In addition, charging runs the
chillers in an efficient mode, i.e., at or
near full load, with reduced start/stop
and off-design operation time. Finally,
in new construction ice-based TES may
suggest the use of low-temperature air
distribution, with resulting reductions in
air handler size and power.2,3
Using TES can result in system efficiency penalties, however. There is
inevitably some degree of heat leakage
into the cool storage. Also, in the case
of ice-based TES, the chiller output
June 2013

ASHRAE Journal - June 2013

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