ASHRAE Journal - October 2021 - 25

TECHNICAL FEATURE
noted conditions, the effi ciency of a centrifugal chiller
might be ~0.30 kW/ton (~0.09 kW/kW) or 12.7 COPh; the
COPh of a heat recovery machine might be 4.2. The net
process heating COP, therefore, works out to 3.36 with
condenser water storage. The condenser water storage
approach also requires slightly more pumping energy,
but the overall effi ciency delta is minor.
Hot water (HW) storage allows nearly full heating peak
load shifting because most of the heating peak period
loads are met using energy stored in the tank, plus a
small amount of trim ASHP capacity. In contrast, a CW
storage design only shifts the portion of the energy
required to charge the TES outside of the heating peak
period. Heating peak load shifting is only of benefi t in
areas with higher utility rates during the morning peak
heating period.
HW TES also eliminates the potential for low heating
load heat recovery chiller cycling, which can be an issue
with CW TES designs if a HW buffer tank is not provided.
The benefi ts of HW storage are offset by several drawbacks
that must be given close consideration. First,
ASHPs must be able to generate hot water at the same
temperature as the heat recovery chillers feeding the
TES tank. This can be problematic since many ASHPs are
limited to a maximum HWST of approximately 120°F
(49°C). The ASHPs therefore dictate the maximum
design hot water supply temperature, lowering the hot
water delta T achievable by the plant. Lower hot water
delta Ts require larger tanks, bigger HW pipes and larger
pumps. Each of these factors contributes to higher fi rst
costs. This issue does not exist with CW storage because
the ASHPs reject heat at tepid conditions to a CW storage
tank.
Because ASHPs must generate design HW temperature
in the middle of winter, the HW TES design may not be
viable in very cold climates since many ASHPs cannot
produce 120°F (49°C) water at extreme ambient conditions
as noted previously. The cascade introduced by CW
storage eliminates this issue.
Hot water storage does not allow for demand-based
hot water temperature resets. Instead, the hot water
supply temperature needs to be fi xed at the tank charge
temperature throughout the day to maintain proper
stratifi cation and ensure the worst-case temperature
is always available as demand varies. In a CW storage
solution, HWST can be reset based on demand, which
FIGURE 10 Direction of heat transfer for a chilled water TIER system.
Water-Cooled Heat
Recovery Screw
Chiller
Two-Pipe
Air-Source
Heat
Pump
CHW HW CW
CHW
Water TES
Cooling
Tower
High Effi ciency
Centrifugal
should make up for the small full-load effi ciency penalty
discussed previously.
Last, hot water storage tanks are subject to signifi cantly
greater jacket losses than CW storage tanks, which spend
many more hours close to neutral relative to ambient in
all climates.
Chilled Water Storage
Another interesting TIER alternative is chilled water
(CHW) storage, for which the energy fl ows are illustrated
in Figure 10.
The most compelling reason to consider chilled water
TIER is that it integrates exceptionally well with conventional
peak shifting schemes. It therefore represents a
viable all-electric retrofi t strategy for existing campus
and district chilled water TES plants. CHW TES also
eliminates low-load cooling chiller cycling concerns; CW
TES requires a buffer tank to avoid this issue in systems
with insuffi cient base load.
A downside of chilled water TIER is that the storage
tank needs to be approximately twice as large as a
condenser water tank because the design range is on
the order of 20°F to 25°F (11°C to 14°C) instead of 40°F
(22°C). This is not an issue if the tank is also designed for
cooling peak shifting since that requirement will drive
the tank size in many applications; but, it is an issue in
non-campus designs where peak shifting is not a primary
driver.
Chilled water TIER also prohibits chilled water supply
temperature reset when charging the tank since operating
at design chilled water range is required to maximize
tank storage, maintain stratifi cation and ensure
the water stored in the tank is cold enough to serve
loads irrespective of varying temperature requirements
later in the day. This is in contrast to condenser storage,
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ASHRAE Journal - October 2021

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