ASHRAE Journal - October 2021 - 16

TECHNICAL FEATURE
FIGURE 2 Direction of heat transfer for a hot water TIER system.
FIGURE 3 Direction of heat transfer for a condenser water TIER system.
Hot Water
TES
Two-Pipe
Air-Source
Heat Pump
Water-Cooled
Heat Recovery
Screw
Chiller
Cooling
Tower
High
EEffi ciencyffi ciency
Centrifugal
CHW HW CW
High Effi ciency
Centrifugal
Water-Cooled Heat
Recovery
Screw
Chiller
Two-Pipe
Air-Source
Heat Pump
Condenser
Water TES
CHW HW CW
combined, these elements allow effi cient water-to-water
chillers to perform heat recovery even when heating
and cooling loads are not simultaneous, as is done with
a geothermal system, while avoiding the high costs
and temperature degradation inherent to geothermal
designs.
Though perhaps initially nonintuitive, each TES
approach can be used to store energy for heating irrespective
of whether the medium is 130°F (54°C) hot
water, 80°F (27°C) condenser water or 32°F (0°C) ice.
The fi rst two will be used to illustrate this concept. In a
design with a hot water storage tank, trim ASHPs (which
are only sized for a fraction of design heating load),
charge the hot water tank throughout a heating design
day. Heat recovery chillers also charge the tank by pulling
any available heat from the chilled water loop and
rejecting it to the tank. During winter mornings when
the building is heating dominated, the tank discharges;
in the afternoon when combined building heat recovery
and trim ASHP capacity exceeds heating load, the tank
charges. Figure 2 illustrates the energy fl ow paths for the
hot water storage system design.
In a condenser water storage design, trim air-source
heat pumps, which are again only sized for a fraction of
design heating load, charge the condenser water tank
throughout a cold day with tepid 80°F (27°C) water.
Heat rejection loads from the condenser side of chillers
in " cooling mode " also charge the tank with 80°F (27°C)
water. During winter mornings when the building is
heating dominated, the tank discharges as heat recovery
chillers extract more heat from the tank than the ASHPs,
and any chillers in " cooling mode " reject to the tank;
in the afternoon, when combined chilled water heat
rejection load and trim ASHP capacity exceeds building
heating load, the tank charges. In the summer, the heat
16
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2 0 2 1
recovery chillers can be indexed to the chilled water loop
to provide cooling. Figure 3 illustrates the energy fl ow
paths for the condenser water storage system design.
Understanding Condenser Water TIER
The remainder of this article focuses on condenser
water as the storage medium of choice to illustrate the
benefi ts of TIER since we believe condenser water is the
best option for many applications. Many of the subsequent
benefi ts also apply to other TIER TES schemes, but
all approaches are not equal. Pros and cons of alternative
TES strategies are discussed at the end of the article.
The condenser water TIER plants we have designed
take heat rejected from cooling loads via high-effi ciency,
low-lift, centrifugal chillers and store it in a TES tank
at tepid temperatures between 60°F (16°C) and 80°F
(27°C). Tank temperature excursions down to 40°F
(4.4°C) are allowed on peak heating days to minimize
tank size.
When energy is needed for building heating, heat
is extracted from the tank using water-to-water heat
recovery chillers. In effect, the cooling chillers and heat
recovery chillers are placed in a cascade confi guration:
the cooling chillers have a lift envelope of 40°F (4.4°C)
chilled water supply temperature to 80°F (27°C) condenser
water leaving temperature, while the heat recovery
chillers have a lift envelope of 60°F (16°C) evaporator
supply temperature to the active hot water supply temperature
setpoint, typically 110°F (43°C) to 140°F (60°C)
for all-electric designs.
During most days in California's mild climate zones
where the author practices, the energy recovered from
cooling loads alone can satisfy heating loads. During
the small fraction of the year when heat recovery alone
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ASHRAE Journal - October 2021

Table of Contents for the Digital Edition of ASHRAE Journal - October 2021

Contents
ASHRAE Journal - October 2021 - Intro
ASHRAE Journal - October 2021 - Cover1
ASHRAE Journal - October 2021 - Cover2
ASHRAE Journal - October 2021 - 1
ASHRAE Journal - October 2021 - Contents
ASHRAE Journal - October 2021 - 3
ASHRAE Journal - October 2021 - 4
ASHRAE Journal - October 2021 - 5
ASHRAE Journal - October 2021 - 6
ASHRAE Journal - October 2021 - 7
ASHRAE Journal - October 2021 - 8
ASHRAE Journal - October 2021 - 9
ASHRAE Journal - October 2021 - 10
ASHRAE Journal - October 2021 - 11
ASHRAE Journal - October 2021 - 12
ASHRAE Journal - October 2021 - 13
ASHRAE Journal - October 2021 - 14
ASHRAE Journal - October 2021 - 15
ASHRAE Journal - October 2021 - 16
ASHRAE Journal - October 2021 - 17
ASHRAE Journal - October 2021 - 18
ASHRAE Journal - October 2021 - 19
ASHRAE Journal - October 2021 - 20
ASHRAE Journal - October 2021 - 21
ASHRAE Journal - October 2021 - 22
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ASHRAE Journal - October 2021 - 24
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ASHRAE Journal - October 2021 - 26
ASHRAE Journal - October 2021 - 27
ASHRAE Journal - October 2021 - 28
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ASHRAE Journal - October 2021 - 30
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ASHRAE Journal - October 2021 - 33
ASHRAE Journal - October 2021 - 34
ASHRAE Journal - October 2021 - 35
ASHRAE Journal - October 2021 - 36
ASHRAE Journal - October 2021 - 37
ASHRAE Journal - October 2021 - 38
ASHRAE Journal - October 2021 - 39
ASHRAE Journal - October 2021 - 40
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ASHRAE Journal - October 2021 - 42
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ASHRAE Journal - October 2021 - 49
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ASHRAE Journal - October 2021 - HR1
ASHRAE Journal - October 2021 - HR2
ASHRAE Journal - October 2021 - HR3
ASHRAE Journal - October 2021 - HR4
ASHRAE Journal - October 2021 - HR5
ASHRAE Journal - October 2021 - HR6
ASHRAE Journal - October 2021 - HR7
ASHRAE Journal - October 2021 - HR8
ASHRAE Journal - October 2021 - HR9
ASHRAE Journal - October 2021 - HR10
ASHRAE Journal - October 2021 - HR11
ASHRAE Journal - October 2021 - HR12
ASHRAE Journal - October 2021 - HR13
ASHRAE Journal - October 2021 - HR14
ASHRAE Journal - October 2021 - HR15
ASHRAE Journal - October 2021 - HR16
ASHRAE Journal - October 2021 - HR17
ASHRAE Journal - October 2021 - HR18
ASHRAE Journal - October 2021 - HR19
ASHRAE Journal - October 2021 - HR20
ASHRAE Journal - October 2021 - HR21
ASHRAE Journal - October 2021 - HR22
ASHRAE Journal - October 2021 - HR23
ASHRAE Journal - October 2021 - HR24
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ASHRAE Journal - October 2021 - Cover3
ASHRAE Journal - October 2021 - Cover4
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