ASHRAE Journal - October 2021 - 20

TECHNICAL FEATURE
valve upstream of the heat recovery chiller evaporator
inlets (shown and boxed in yellow) prevents
water warmer than 80°F (27°C) from entering the
heating heat recovery chiller's evaporator barrel
as is required by many chiller manufacturers
for continuous operation. Since the day is warm,
morning heating loads were small, meaning the
tank is already fully charged by early afternoon.
Therefore, all excess heat is rejected through the
cooling towers, which are isolated with a heat
exchanger to prevent dirty tower water from
entering the tank or the chilled or hot water loops.
These three schematics illustrate three of many
FIGURE 7 Tank discharge profile, TIER design with two 1,765 kBtu/h ASHPs.
8,000
6,000
4,000
2,000
-2,000
-4,000
-6,000
-8,000
-10,000
-12,000
-14,000
possible modes of operation for this plant. For
instance, it is possible to index valves such that
heat recovery chillers reject heat from the chilled
water loop directly to the hot water loop. Such a
configuration might be optimal when loads are
fairly balanced between the chilled water and
hot water loops; otherwise, it will result in serving the
cooling load at higher lift than necessary (e.g., if there
are 200 tons (703 kW) of cooling load but only 50 tons
(176 kW) of heating load), in which case operating in
one of the other aforementioned modes is preferable.
Reviewing all operating modes is beyond the scope of
this article, but this discussion hopefully conveys the
flexibility this plant configuration provides to optimize
operational efficiency under a variety of heating and
cooling load conditions.
The Benefits of TIER
Spatial Requirements
While TES designs are often thought of as space
intensive, the TIER solution is a space saver relative to
a conventional ASHP plant. This is because load shifting
allows the TIER design to reduce ASHP capacity
dramatically.
A traditional TES tank is used for cooling peak shifting,
not for heat recovery, and is typically sized to either ride
through the utility peak period without running chillers
or trim some fraction of chiller capacity throughout
that period. A TIER TES tank is sized to ensure that on a
design heating day, heating loads can be met during all
hours of the day using the available heat recovered from
the building(s) and trim heat source energy added to the
tank.
20
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
Tank Charge (kBtu)
1 2 3 4 5 6 7 8 9 10 11 12 13 1415 16 1718 19 20 21 22 23 24
Tank Energy Draw (kBtu/h)
Building Cooling Load and Cooling Chiller Compressor Heat (kBtu/h)
Note that the peak tank energy draw is less than the design heating load because heat recovery chiller
compressor energy makes up the difference.
Designers can manipulate tank size by providing more
or less trim heat source capacity. The more trim heat
source capacity is available, the smaller the tank can be
while riding through sustained heating peaks. But as
the tank gets smaller, some of the opportunities for heat
recovery are lost and plant efficiency gets worse. Since
providing more tank capacity generally reduces overall
project costs and improves plant efficiency, tank capacity
should be maximized to the extent that spatial and
project aesthetic constraints allow.
At a certain point, however, there is no value in
increasing tank size further since doing so will not yield
additional reductions in trim heat source size. This is so
because the amount of heat required to warm a building
over a 24-hour peak heating day does not change irrespective
of the amount of load shifting the tank can provide-heat
recovery and trim heat must meet that load
over a 24-hour period.
In the real example below, two ASHPs totaling
3,530 kBtu/h (1 MW) of capacity at near design ambient
dry bulb of 36°F (2.2°C) were proposed along with a
condenser water tank providing 31,200 kBtu (9.1 MWh)
of storage for a 1.1 million ft2 (102 000 m2) office/dry
computer lab complex with a design heating load of
approximately 16,000 kBtu/h (4.7 MW). A 110,000 gallon,
50 ft tall, 20 ft diameter (416 000 L, 15 m tall, 6 m diameter)
tank was selected for the project. Figure 7 shows a
35,000
30,000
25,000
20,000
15,000
10,000
5,000
ASHP Trim Heat to Tank (kBtu/h)
kBtu/h
kBtu
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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
ASHRAE Journal - October 2021 - 23
ASHRAE Journal - October 2021 - 24
ASHRAE Journal - October 2021 - 25
ASHRAE Journal - October 2021 - 26
ASHRAE Journal - October 2021 - 27
ASHRAE Journal - October 2021 - 28
ASHRAE Journal - October 2021 - 29
ASHRAE Journal - October 2021 - 30
ASHRAE Journal - October 2021 - 31
ASHRAE Journal - October 2021 - 32
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
ASHRAE Journal - October 2021 - 41
ASHRAE Journal - October 2021 - 42
ASHRAE Journal - October 2021 - 43
ASHRAE Journal - October 2021 - 44
ASHRAE Journal - October 2021 - 45
ASHRAE Journal - October 2021 - 46
ASHRAE Journal - October 2021 - 47
ASHRAE Journal - October 2021 - 48
ASHRAE Journal - October 2021 - 49
ASHRAE Journal - October 2021 - 50
ASHRAE Journal - October 2021 - 51
ASHRAE Journal - October 2021 - 52
ASHRAE Journal - October 2021 - 53
ASHRAE Journal - October 2021 - 54
ASHRAE Journal - October 2021 - 55
ASHRAE Journal - October 2021 - 56
ASHRAE Journal - October 2021 - 57
ASHRAE Journal - October 2021 - 58
ASHRAE Journal - October 2021 - 59
ASHRAE Journal - October 2021 - 60
ASHRAE Journal - October 2021 - 61
ASHRAE Journal - October 2021 - 62
ASHRAE Journal - October 2021 - 63
ASHRAE Journal - October 2021 - 64
ASHRAE Journal - October 2021 - 65
ASHRAE Journal - October 2021 - 66
ASHRAE Journal - October 2021 - 67
ASHRAE Journal - October 2021 - 68
ASHRAE Journal - October 2021 - 69
ASHRAE Journal - October 2021 - 70
ASHRAE Journal - October 2021 - 71
ASHRAE Journal - October 2021 - 72
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
ASHRAE Journal - October 2021 - 73
ASHRAE Journal - October 2021 - 74
ASHRAE Journal - October 2021 - 75
ASHRAE Journal - October 2021 - 76
ASHRAE Journal - October 2021 - 77
ASHRAE Journal - October 2021 - 78
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ASHRAE Journal - October 2021 - 81
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ASHRAE Journal - October 2021 - 92
ASHRAE Journal - October 2021 - 93
ASHRAE Journal - October 2021 - 94
ASHRAE Journal - October 2021 - 95
ASHRAE Journal - October 2021 - 96
ASHRAE Journal - October 2021 - Cover3
ASHRAE Journal - October 2021 - Cover4
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