ASHRAE Journal - August 2013 - 93

Column EmErging TEchnologiEs

Heat Flux (W/m2)

temperatures. One problem with
Figure 1 Passive heating/cooling effect arising from delay and attenuation of temperature variation (simulation of a constant-temperature space with 0.3 m- (1 ft.) thick PCM-enhanced fiber insulation).3
organic materials is their inherent flam0.63
2
mability. Current efforts have emphaReverse Heat Flow During the Day
sized formulas and packaging tech0.32
1
niques that result in PCM-based building components that meet applicable
0
0
4
8
12
16
20
24
28
32
36
40
44
48
code requirements for fire resistance.
Among inorganic materials, hydrated
0.32
–1
No PCM
salts have shown the most promise for
30% PCM
–2
–0.63
building PCMs. Hydrated salts were
Reverse Heat Flow During the Night
the first PCMs to be used for building
–3
–0.95
applications. While comparatively low
Time (h)
(Btu/ft2·h)
cost and nonflammable, they can be
difficult to encapsulate and often have large phase change and hydropower, and/or if oil (which, in addition to being
hysteresis. Recent development efforts have focused
more expensive than gas, has a higher carbon footprint per
on additives to mitigate these weaknesses, with some
Btu) is the only available heating fuel, the sustainability case
success.4
for electric heat becomes somewhat more attractive. With
To be used in buildings, PCMs must be packaged, or
very high penetrations of wind power, ETS is appealing in
encapsulated, in a form that does not leak and is easy to
that it could be used to absorb intermittent surpluses of
handle and install. “Macro-encapsulation” commonly
wind-generated power overnight.
involves bulk PCM contained in pouches or packets. One
In addition to saving energy at the source through timecompany has developed sheets or mats of macro-encapsu- of-use delay, PCMs also save energy on site. A 2013 report4
lated bio-based PCM for use behind wallboard and under published by the DOE Office of Energy Efficiency and
roofs.5 “Micro-encapsulation” traps tiny quantities of PCM Renewable Energy (EERE) reviewed experimental studies
in polymer shells, which are then incorporated into mate- of cooling load reduction from PCM applications. Cooling
rials such as thin laminated panels,6 foam, fiberglass, or
load savings ranged from 9% to 25%, with in-wall PCM
cellulose insulation,3 or gypsum wallboard.7
loadings of 10 to 45 Btu/ft2 (0.11 to 0.51 MJ/m2).
The energy savings achieved by using PCMs, however,
Energy Savings
are highly dependent on the climate and season, as well as
ETS does not save energy at the site of use, although,
on the specifics of wall design, internal temperature setcompared to an electric heating system without ETS,
point, and phase transition temperature. For example, to
it may save energy at the electricity generation source.
function as thermal storage, a PCM must experience temCharging periods for residential ETS systems can be man- perature cycles that span the phase transition point (near
aged remotely by the utility, enabling the lowest possible
room temperature). A hot summer night may never allow
electricity cost to homeowners. Electricity generated offthe PCM to solidify, thus rendering it ineffective. There
peak usually consumes less primary energy per kWh as
are other, more subtle relationships at work as well. A 2012
compared to on-peak electricity.
simulation study by Oak Ridge National Labs8 found that
From a sustainability standpoint, however, the merits of an effective PCM phase transition temperature could be
selected for either the cooling season or the heating seaETS (and electric heat in general) depend strongly on the
regional electricity mix. If regional generation is predomi- son, but not both.
nantly fossil-based, the carbon footprint of electric heat
Market Potential
(with or without ETS) is larger than that of gas heat.
Because of the primary energy efficiency advantages of
The efficiency of a state-of-the-art natural gas fueled
natural gas heat (which translates approximately into cost
combined cycle power plant (~50%) is still far less than
advantage), ETS is unlikely to displace gas heat at scale
the efficiency of an in-home gas boiler or furnace (~90%).
in the near term. Neither is ETS likely to be attractive in
However, if the regional grid is weighted toward nuclear
Au g ust 2013

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