ASHRAE Journal - September 2020 - 8

LETTERS

Electrification,
Heat Pumps, and
Thermal Storage
Referring to "Electrification,
Heat Pumps and Thermal Storage"
by Mark M. MacCracken, P.E., in
the July 2020 ASHRAE Journal, it is
agreeable that electrification with
renewables is viable for sustained
decarbonization.
However, the second law of thermodynamics, which we often ignore
in practice, tells us that tracing the
downstream path along which and
how the so-called green electricity
is used is as important as where and
how it is generated. The second law
also tells us that once electric power
is generated from renewables (or
not), it is important to keep using it
as electricity as far as possible downstream along the power demand
chain.
The key is to question whether
any particular power demand has
options other than electricity. For
example, comfort cooling with electrically driven chillers has options
like absorption and or adsorption
cooling with waste heat, but electric
lighting and electric public transport do not. Indeed, industry and
transport above about 400°C have
no option other than electricity.
Therefore, electric power should be
directed first to the demand points
that have no option except electricity.
For example, in the Netherlands
state trains have the privilege, by
all means, to run on green electricity generated from wind energy
because they do not have any other
option. In this respect, the author
did a fine eye-opening job in his
article. Yet revisiting the article
seems necessary.
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For example, his data about a COP
of 5.5 seems quite impressive in
terms of the first law. But looking
at the same example in terms of
the second law, we see a different
picture: electric power is a "highquality" energy source of which
95% may be used in useful works
(0.95 kWh/kWh).
Compare such a high quality of
supplied energy with the quality
of temperature elevation process
from 26.7°C (299.85K, 80°F) to 35°C
(308.15K, 95°F) in their design case
where the chiller acts as a heat pump
in winter for morning warm-up.
According to the ideal
Carnot cycle, the energy quality of this temperature jump
is only 0.027 kWh/kWh
(1 − 299.85K/308.15K). This means
that the supplied unit exergy of
0.95 kWh/kWh is destroyed by
0.92 kWh/kWh.
Moreover, until the entire globe is
electrified by renewables, an indirect fossil fuel-equivalent carbon
footprint will always exist in proportion to the destroyed exergy.
This footprint is expressed as 0.31
times the destroyed exergy, as referenced in the natural gas boiler data
given in the article (the net thermal
efficiency is 0.75), where natural
gas has about 0.2 kg CO2/kWh in
combustion and has unit exergy
of 0.87 kWh/kWh. This calculation comes from 0.2/(0.75 × 0.87)
and means that the heat pump will
be responsible for additional CO2
emissions of 0.285 kW = h/kWh
(0.31 × 0.92) even though the electricity comes from renewables,
because the electricity is used only
one step away from the plug by the
heat pump and then converted to a
heat of very low quality.

SEPTEM BER 2020

This responsibility would be zero
only if the COP of the heat pump
was 35 (0.95/0.027), which is practically impossible. Conversely speaking, the exergy-based COP (COPEX)
of the heat pump is 5.5 × 0.027/0.95,
which is much less than one
(COPEX = 0.156).
Of course, the author is absolutely
right in looking for ways to save
energy in buildings, but it would
probably be better to compare
CO2 emissions savings of storing
energy in the ice tanks with the
COPEX value of the heating process by the chillers. The second
law may have also guided us to
another option, which is using a
trigeneration system even if it runs
on natural gas because a typical
trigeneration system destroys only
about 0.15 kWh/kWh (therefore,
0.31 × 0.15 = 0.046 kg CO2/kWh)
and provides on-site heat, electricity (without transmission losses)
and cold together using absorption
chillers.
Thermal storage (but not in the
form of ice) is indispensable in this
alternative, too. The total CO2 emissions responsibility will include
the exhaust emissions of about
0.307 kg CO2/kWh at a partial thermal efficiency of 0.65, and the total
CO2 emissions responsibility will
be 0.353 kg CO2/kWh. This responsibility rate is not too far from the
design case presented in the article,
even if we ignore the ice-making/
ice-melting cycle losses, grid power
losses, etc. If all these are included,
these two cases will become even in
terms of decarbonization.
By the way, this letter is not
intended to criticize the design of
the authors, but only to underscore
that the second law is an important

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