ASHRAE Journal - October 2019 - 48

ASHRAE - CELEBRATING 125 YEARS

Eutectic Salts
Also, in the 1980s, a eutectic salt PCM was marketed
commercially by Transphase with a freezing point of
47 °F (8°C). It promised key benefits of Ice TES (having
high energy density) and CHW TES (being charged with
conventional CHW supply temperatures of 40°F to 42°F
or 4°C to 6°C); however, during the discharge (melt)
cycle of the encapsulated PCM, CHW supply temperatures quickly rose well above the phase change temperature, thus limiting the applications to partial shift
TES with chillers necessarily running downstream of
the TES. This, combined with system costs and material
environmental issues, ended the commercial applications after a number of years.

return water could still always be kept separate, as once
a given "empty tank" had been filled, another tank had
become empty and available to receive the subsequent
flow of water. The larger the number of tanks, the smaller
the required excess tank volume; however, smaller
tanks have a higher unit capital cost; and each additional
tank requires full size inlet/outlet piping and valves to
handle the peak charge and discharge flows. Installations
occurred periodically in the 1970s and 1980s; but perhaps
the last significant example was in 1990 at Arizona State
University, where five (plus one) underground tank compartments contained 5.5 million gallons (20,800 m3) for
54,000 ton-hrs (190 MWh) of TES.

Labyrinth Tank TES
In the late 1980s, CB&I and Liquid Carbonic developed and demonstrated a very low-temperature TES
phase change technology using carbon dioxide at its
triple point (i.e., where all three phases, solid, liquid
and vapor, are in equilibrium). Deemed SECO2 (for
Stored Energy in Carbon Dioxide) it used CO2 vapor as
its refrigerant to achieve a liquid-solid phase change at
-70°F (- 57°C) and 60 psig (4.1 atmg).

Labyrinth tanks use a multitude of compartments in a
horizontal layout with connections such that water flows
in a long circuitous path from cell to cell, in one direction
during charging and reversed during discharging. Dating
to at least the 1970s, this technique was seen primarily in
Japan in the sub-basements of high-rise buildings where
the foundations already employed an "egg-crate" (horizontal grid) construction for reasons of seismic design.
However, some temperature mixing would occur.

Sensible Heat TES

Baffle-and-Weir TES

Triple Point Carbon Dioxide

Most sensible heat TES systems employ water as the
storage medium, though a minority of others have used
other low temperature fluids (LTFs). Primary benefits are
simplicity, energy efficiency, and high economy-of-scale,
while drawbacks include low energy density (high volume
per stored ton-hr). Chilled water (CHW) TES has taken the
form of a variety of configurations, each discussed below,
with different means for maintaining necessary separation between cool supply water and warm return water.

Empty Tank Method TES
In its simplest configuration, the "empty tank" method
employs just two tanks: one to hold the cool supply water
and one to hold the warm return water; this keeps the
two temperature zones separate, but requires a 100%
increase in tank volume versus the water volume. To
minimize this excess tank volume and cost, systems were
sometimes configured with more than two tanks, sometimes as many as six, ten, or more tanks, always with all
but one of the tanks being adequate to hold the full water
volume; in this manner cool supply water and warm
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Also dating to the 1970s, baffle-and-weir tanks were
occasionally used, where internal walls separated compartments within a tank, with water flowing over a wall
to one compartment then under a wall to the next, and
so on, with the flow reversed between charging and discharging. However, again, temperature mixing would
occur.

Membrane or Diaphragm Separation TES
During the 1980s, primarily in Ontario, often based
on designs by Robert (Bob) Tamblyn of Engineering
Interface, tanks used internal flexible membranes to
separate the upper warm zone from the lower cool
zone; however, operational problems such as failures of the membranes, blocked pump suctions, and
convection causing temperature mixing, were common issues. The first district cooling utility, begun
by Hartford Steam in Connecticut in 1964, added a
20,000 ton-hr (70 MWh) CHW TES tank in 1985 using
a rigid horizontal diaphragm designed to move up
and down during charging and discharging; however,
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