Feature
Story
Thermal Energy Storage:
Optimizing district energy systems
in more ways than one
John S. Andrepont, President, The Cool Solutions Company
Thermal energy storage (TES) has been widely and increasingly inte- grated into the design and operation of district energy (and other HVAC)
systems around the world. TES can be,
and is, used with both district heating
and district cooling systems. While traditionally understood to provide peak
electric load management and associated
reductions in operating energy costs, TES
in fact often provides many additional
significant benefits to district energy system owners and operators.
Hot TES, generally employed as
hot water TES, has been fairly widely
utilized in Northern Europe as well as
in Canada, where district heating is
generally a hot water-based technology.
In the U.S., where most district heat-
ing systems have used steam-based
technology, hot water TES is less often
seen. Nevertheless, limited examples
exist of hot water TES used to store
preheated condensate return in steam
district heating systems; and there are
also examples of TES tanks designed for
dual service, as hot water TES in winter
and chilled-water TES in summer. By
and large, however, the most prevalent
examples of TES used in district energy
are cool TES applied to district cooling
systems.
Cool TES Technologies
Mature cool TES technologies,
each applied successfully for decades
in many district cooling applications,
include ice TES, chilled-water TES and
low-temperature fluid (LTF) TES. (One
example of LTF is SoCool®, a patented
aqueous solution of chemical additives
in water, which depresses the freezing
point and the temperature at which
maximum density occurs, allowing for
thermally stratified TES operation below
the plain water limit of approximately
39 degrees F.) These technologies fit
into two families of energy storage:
latent heat TES, in which cooling is
stored as the phase change of a storage medium, and sensible heat TES, in
which cooling is stored as the temperature change of a storage medium. See
table 1 for a comparison of these technologies, including how they work and
their respective benefits and drawbacks.
Each of these cool TES technologies,
as applied in district cooling, exhibits
characteristics ranging from excellent to
poor in various areas of performance or
capabilities, such as energy efficiency,
modularity or simplicity and reliability.
Table 2 provides some widely accepted
generalizations that can be used as a
rough guide to these inherent characteristics of the technologies. Accordingly,
when considering and selecting a TES
technology, it is critical to understand
which characteristics are the most
important for any particular district
cooling application.
Wide Range of Benefits
TES applications in district cooling
systems are anything but rare or new.
There are hundreds of cool TES installations in both thermal utility and campus district cooling systems, primarily
in the U.S. but also elsewhere in the
Americas, Asia, Australia and Europe
(see sidebar).
The case studies to follow of four
such installations demonstrate the
economic and operational benefits of
