ASHRAE Journal - October 2019 - 50

ASHRAE - CELEBRATING 125 YEARS

the diaphragm quickly jammed and failed, resulting
in the tank being converted to thermally stratified
TES, as it has performed for the past 34 years.

Thermally Stratified TES

Thermal stratification relies on the density difference
between the more dense, cool supply water and the less
dense, warm return water to create and maintain separation of the temperature zones with no physical barrier;
internal flow diffusers are required to slow the inlet and
outlet flows to avoid mixing. Seminal laboratory testing
was conducted by Professor Maurice "Bud" Wildin of
the University of New Mexico in the late 1970s and early
1980s, while Professor William Bahnfleth,Ph.D., P.E.,
of the Pennsylvania State University subsequently conducted full-scale field testing and analysis, all of which
contributed to past and current information in the
thermal storage chapter of the ASHRAE Handbook. Just as
the was case for the other configurations of CHW TES,
thermally stratified CHW TES was occasionally seen in
the 1970s and early 1980s as "one-off" designs by consultants for particular applications; common among these
were automotive plants where firewater storage tanks
were required, and for which insulation and flow diffusers were designed and specified by Gordon Holness
at Albert Kahn Associates or William Harrison at Giffels
Associates among others, to place these water tanks
into dual-service as TES. At the start of the 1980s, CB&I
(Chicago Bridge & Iron, now McDermott), which had
built a number of the one-off tanks, developed "turnkey" design-built CHW TES tanks, for which owners and
engineers could merely specify TES capacity, operating
temperatures, and peak flow rates, with the complete
design and thermal performance guarantees provided
by the tank supplier. This procurement approach (similar to that used for chillers, cooling towers, or other key
CHW system components) led to a dramatic increase
in the use of CHW TES, with CB&I having subsequently
executed hundreds of such installations with steel tanks,
and other tank builders following suit over time, including Natgun (now DN Tanks) being an early entrant with
concrete tanks, preferred especially for in-ground tanks.
Beyond the sensible heat systems in which water is the
dominant choice for the storage medium, some have
employed other low temperature fluids (LTFs). Aqueous
calcium chloride brines were occasionally used for
low temperature applications; however, chlorides are

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notoriously corrosive, and once environmental issues
eliminated use of the only viable corrosion inhibitor in
the early 1980s, this option became economically much
less practical. D-Limonene is a hydrocarbon (C10H16) with
a freezing point of -102°F (-74°C) and can thus be used as
a fluid storage medium for very low temperature (generally specialized and rare) applications. One example
dating to the 1960s or 1970s was a large aeronautical test
facility at Eglin Air Force Base in Florida; a hangar-sized
test chamber required that a massive make-up ambient
airflow be cooled to about -40°F (-40°C) but for only a
short test period. The solution was a staged TES system
of a two-tank "empty tank" method of calcium chloride
brine, in series with a two-tank "empty tank" method of
D-Limonene in two spherical pressure vessels.

Aqueous Sodium Nitrite/Nitrate TES

One particular version of low temperature fluid (LTF)
TES uses a patented, thermally stratified solution of
sodium nitrite and sodium nitrate in water, marketed as
SoCool fluid, developed by Trigen Energy Corporation,
with the patents later transferred to CB&I. The chemicals not only lower the freezing point versus that of pure
water, they also lower the temperature at which maximum density occurs. This allows thermal stratification
below the limit for pure water which occurs at about
39.4°F (4.1°C). It also increases the supply-to-return
Delta T in TES, thus reducing the volume per ton-hr
versus that of conventional CHW TES. First employed in
1994, over the next 15 years there were a dozen installations totaling nearly 300,000 ton-hrs (1055 MWh), with
supply temperatures ranging from 29°F to 36°F (-2°C to
+2°C). Applications include Chicago's McCormick Place
convention facilities (1994), DFW International Airport
(2002), and Princeton University (2005).

Pioneering Utility Programs

TU Electric was experiencing incredible growth in and
around Dallas in the late 1980s, and needed to manage its
peak demand (Photo 3). It offered customer incentives of
$250/kW for the first 500 kW shifted to off peak and $125/
kW thereafter. Texas Instruments (TI) installed a 2.7 million gallon (10,200 m3) stratified CHW TES tank in 1990 to
supplement its 4,200-ton (14 771 kW) chiller plant at its 1.1
million square foot (100 000 m2) electronics manufacturing facility. TI invested $1.6 million for 24,500 ton-hours
(86 MWh). Two more CHW TES installations followed at
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ASHRAE Journal - October 2019

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