ASHRAE Journal - October 2021 - 26

TECHNICAL FEATURE
which allows for demand-based chilled water supply
setpoint reset.
Finally, chilled water TES creates a less efficient cascade
than condenser water TES any time trim heat is
required. This is because the ASHPs end up doing a
small fraction of the total required lift (the difference
between the saturated suction temperature required
to extract heat from ambient air up to the saturated
condensing temperature required to reject heat to the
heating hot water loop) or create excess " lift overlap. "
For instance, instead of an air-source heat pump absorbing
heat at 32°F (0°C) and rejecting it as 80°F (27°C)
condenser water, followed by a heat recovery chiller
supplying 60°F (16°C) water from its evaporator barrel
and rejecting heat as 140°F (60°C) hot water, an airsource
heat pump ends up absorbing heat at 32°F (0°C)
and rejecting it as 60°F (16°C) chilled water, followed by
a heat recovery chiller supplying 40°F (4.4°C) chilled
water and rejecting heat as 140°F (60°C) hot water.
The latter cascade will be less efficient with most
equipment and is also problematic for some ASHPs on
the market. One market leader's product, for instance,
cannot supply water colder than 77°F (25°C). This in
turn creates 37°F (21°C) of " lift overlap " (77°F [25°C] on
the condenser leaving side of one machine, 40°F [4.4°C]
on the evaporator leaving side of the next) where only
20°F (11°C) of overlap needs to exist.
Ice Storage
Ice storage has many of the same pros and cons as
CHW storage and conceptually ties into a plant in the
same way, so CHW storage serves as a useful point of reference.
Ice tanks could replace the CHW tank in Figure 10
since the energy flows are otherwise identical. The primary
benefit of ice storage relative to CHW storage is
energy density and therefore space. Because ice storage
captures energy in the latent heat of fusion, only roughly
12% and 24% as much volume is required to store energy
in ice as in chilled water (20°F [11°C] delta T) and condenser
water (40°F [22°C] delta T), respectively. Total
floor area savings are, however, not as dramatic as these
figures would suggest because ice TES systems are typically
broken up into many smaller vessels instead of one
monolithic tank.
The primary downsides of ice storage are that it
requires adding glycol to the cooling loop to prevent
freezing, which reduces heat exchange efficiency and
26
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
adds a maintenance complication; it requires that
" chilled fluid " supply temperatures be below freezing
whenever the storage is being charged, thus creating
high lift conditions for cooling year-round; and it creates
an even less efficient cascade than the chilled water
design with more lift overlap.
Phase-Change Materials
Phase-change materials (PCMs) are like ice in that they
store energy in the latent heat of fusion. Conceptually,
they could replace HW storage, CW storage or CHW
storage in any of the schematics in Figure 2, Figure 3 or
Figure 10. The main benefit of PCMs is that, like ice, they
dramatically reduce the TES footprint. The key downside
of PCM solutions is that they are typically significantly
more expensive than any of the other TES options and
therefore may not be life-cycle cost-effective.
Conclusions
Further research is required to investigate the applications
and climates for which each of the above storage
options is the most life-cycle cost effective. In the
meantime, the author encourages designers to begin
exploring TIER, especially the novel concept of condenser
water TIER, as an option on their all-electric jobs.
It is likely that, regardless of the approach taken, TIER
will unlock the potential of all-electric solutions for big
buildings by improving energy efficiency while reducing
costs and spatial requirements relative to typical ASHP
designs.
References
1. Jadun, P, C. McMillan, D. Steinberg, M. Muratori, et al. 2017.
" Electrification Futures Study: End-Use Electric Technology Cost
and Performance Projections through 2050. " NREL/TP-6A20-70485.
National Renewable Energy Laboratory.
2. ASHRAE Standard 90.1-2019, Energy Standard for Buildings Except
Low-Rise Residential Buildings.
3. CEC. 2019. " 2019 Building Energy Efficiency Standards
for Residential and Nonresidential Buildings, Title 24, Part 6. "
California Energy Commission.
4. Raftery, P., A. Geronazzo, H. Cheng, G. Paliaga. 2018.
" Quantifying energy losses in hot water reheat systems. " Energy and
Buildings 179:183 - 199. https://doi.org/10.1016/j.enbuild.2018.09.020
5. Mai, T., et al. 2018. " Electrification Futures Study: Scenarios
of Electric Technology Adoption and Power Consumption for the
United States. " NREL/TP-6A20-71500. National Renewable Energy
Laboratory.
6. MacCracken, M. 2020. " Electrification, heat pumps and
thermal energy storage: using yesterday's waste energy for
tomorrow's heating. " ASHRAE Journal 62(7):32 - 39.
https://www.doi.org/10.1016/j.enbuild.2018.09.020 http://ashrae.org

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
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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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