ASHRAE Journal - July 2020 - 34

TECHNICAL FEATURE

FIGURE 4 A. "Free" cooling with no coincident heating load. B. Simultaneous
heating with boiler and "free cooling." C. Simultaneous heating and cooling with
heat pump.

FIGURE 3 Thermal energy storage tank capacity.

Cooling Tower

A.
Boiler Chiller

Boiler Chiller

Cooling Tower
Interior and
Perimeter
Zones
Perimeter
Zones in the
Building's
Shell

Interior and
Perimeter
Zones

Perimeter
Zones in the
Building's
Shell

Boiler Chiller

below ground or back. Because of the relatively low sensible heat capacity of the ground, this requires using a
large mass of ground and having heat exchange tubing
embedded throughout the ground mass. Another option
is to put the energy in water, raising its temperature.
Because no phase change is involved, either much more
water or a very large temperature change would be
needed, compared to using the phase change of H2O as
the storage approach.

thermal energy to the atmosphere, you are burning
fossil fuels to add energy (heat) to the perimeter zones
for portions of the day (Figure 4b). When this is evaluated considering the goal of electrification with big heat
pumps in buildings to eliminate fossil fuel use, the practice seems illogical.
To meet simultaneous heating and cooling needs in
a building, a heat pump can be used to simply pump
the energy from one area to another. Figure 4c shows a
simplified schematic of a building that has simultaneous heating and cooling needs and a standard chiller
being used as an energy pump to move excess energy
from the core zones to the perimeter zones. This is
a very efficient cycle with high heating and cooling
COPs, is all electric and is reducing fossil fuel use at the
building.

Free Cooling is Wasting Energy (To the Atmosphere)

Storing Yesterday's Waste Energy for Tomorrow's Heating

During the fall, winter and spring, most large buildings in northern climates use "free cooling" or "waterside economizer cycles" to cool the building's internal
zones (Figure 4a), which have approximately the same
cooling loads no matter the season. (And if you think
about it, that makes perfect sense. If you are keeping
the people in the perimeter zones comfortable, the
internal zones do not experience a seasonal change.)
So, this process of "free cooling" takes energy that is
being emitted by people and electric devices inside the
building and removes it from the air in the space by
rejecting (wasting) it to the atmosphere via a cooling
tower.
The energy logic of this process being "free" quickly
breaks down when, on the same day you are rejecting

Using a free cooling cycle, by definition, is wasting
energy by rejecting it to the atmosphere. This occurs
when the daily heating and cooling loads are not simultaneous and balanced. Adding thermal storage to the
system can minimize or eliminate this waste of energy in
certain types of buildings, having a potential impact on
its energy intensity. Thermal storage allows a system to
save today's waste energy for tomorrow morning's heating needs.
Figure 5a depicts thermal storage integrated into a
building's chiller system. To some it may look like standard piping of an ice storage system, but this system is
capable of heating by making ice, and it may be hard
to conceptualize. One way to think of the devices is
as cells that store thermal energy that will have their

One Tank 8 ft 6 in. Tall by
7 ft 6 in. Diameter
1,655 Gal of Water = 13,786 lb
13,786 lb × 144 Btu/lb ~2,000,000 Btu
2,000,000 Btu = ~14 Gal of Fuel Oil
~20 Therms of Natural Gas
~2,000 lb of Steam
~160 Ton-Hours

ASHRAE JOURNAL

One New York City Project has 44 Tanks
88,000,000 Btu
616 Gal of Fuel Oil ($1,835)
880 Therms ($1,056)
88 Mlb of Steam ($3,520)
~7,000 Ton-Hours

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ASHRAE Journal - July 2020

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