ASHRAE Journal - July 2020 - 35

TECHNICAL FEATURE

FIGURE 5A Winter cooling only mode (storing excess energy).
M

Cooling
Tower

M

T

95°F Supply
80°F Return

Chiller on or Off Depending
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On Load
Chiller Minimum
Flow Bypass
M

55°F Glycol
Building Bypass M

T

FIGURE 5B Heating with thermal storage (morning warm-up).
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Cooling
Tower

M

T

95°F Supply
M 80°F Return

Chiller in Heating Mode 25°F Glycol

Thermal
Storage
Tanks

Thermal
Storage
Tanks

M

Chiller Minimum
Flow Bypass

40°F

M

31°F Glycol

Building Bypass

M

M

M

Heat Exchanger
57°F Water
Cooling Coils
M

M

Heat Exchanger

Heating Coils

Heating Coils
Cooling Coils

M

42°F Water

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M

M

M
M

80°F Return Heat

energy moved to a usable temperature level with a heat
pump.
The thermal storage in the low-energy state (all ice) is
ready to absorb excess energy, changing ice into water
and storing the energy.
On winter afternoons, instead of releasing the building's waste heat to the atmosphere, it can be used to melt
ice; in this way the energy is saved for tomorrow's energy
needs. In the winter, the goal would be to have all the
thermal storage in the high-energy state (all water) by
the end of the business day. Each storage tank of water
now represents millions of Btus ready to be pumped into
the exterior zones of the building for morning warm-up
by a standard (water-cooled) chiller at a heating COP of
about 5.5 (Figure 5b). Removing the energy from the tank
of water for heating the building will change the phase
of the water to ice, which will be available for the following afternoon's cooling needs. Think of this system
in heating mode as a storage source heat pump (SSHP).
Your energy is extracted from water in the storage
device.
When the building's core has excess energy (it's too
warm) and the perimeter zones still need heat, simultaneous heating and cooling (Figure 6) by the heat pump
(chiller) satisﬁes the building's load requirements. In
smaller buildings, there may not be excess energy during this mode. But in many large buildings, by afternoon
cooling will dominate the load requirements, so the
winter cooling mode (melting ice and collecting excess
energy) starts again.
Also in Figure 6, the air side has been modiﬁed and is
now shown as a dedicated outdoor air system (DOAS),
with the latent loads being addressed in the DOAS coils
and the sensible loads in each zone's coils. The thermal

M

M

M

M

80°F Return Heat

FIGURE 6 Simultaneous heating and cooling mode.
M

Cooling
Tower

M

T

95°F Supply
M 80°F Return
M

60 - 55°F
Chiller in
Glycol
Heating Mode M

Chiller Minimum
Flow Bypass

M

M

65°F Building Bypass M
Glycol
M

T

Heat Exchanger
Sensible
Cooling Coils

M

M

55°F Glycol

T

Melting Ice
Makes 42°F T
Fluid for
42°F Glycol
Dehumidiﬁcation

57°F Water
M

M

Thermal
Storage
Tanks

M

M

DOAS
Cooling Coil

Heating Coils
M

M

M

M

80°F Return Heat

storage supplies the cold (42°F [5.6°C]) solution to the
DOAS coils to dehumidify the air. The chillers, handling
just the sensible building load, run at extremely efﬁcient
conditions with a 55°F (13°C) supply liquid.
In the summer, when the building has no need for
excess thermal energy, the chillers will run at night
pumping as much energy out of the thermal storage as
possible, rejecting the heat through the cooling tower, so
that in the morning they are 100% ice, ready to absorb as
much excess energy (heat) as possible during the peak
periods, cooling the space and lowering energy costs.

Beneﬁts
Many obvious and some not so obvious advantages
exist to having thermal storage integrated into the HVAC
systems. These include:
* Helps make electriﬁcation possible;
* Reduces carbon emissions;
* Reduces domestic water use;
* Reduces peak energy demand;
* Integrates vital energy storage into buildings;
J U LY 2 0 2 0

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ASHRAE Journal - July 2020

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