ASHRAE Journal - February 2023 - 8

LETTERS
The Role of
Cogeneration:
Reducing
Carbon
Emissions
The article " The Role of
Cogeneration: Reducing Carbon
Emissions " by Richard Sweetser,
Life Member ASHRAE and Bruce
Hedman, Ph.D., in the November
issue of ASHRAE Journal presents an
unrealistic scale and incomplete
picture of the role that combined
heat and power (CHP) will have in
the use of natural gas as a transition
fuel. As we continue to transition to
lower carbon energy sources, one
thing is clear: natural gas will not
disappear overnight.
The article makes a good point
that cogeneration is a very efficient
way of using natural gas. However,
most cogeneration plants last for
40 years or more. While we need to
make sure that our use of natural
gas is as efficient as possible,
cogeneration looks less like a
transition and more like a way of
locking in the use of fossil fuels.
The author touts renewable natural
gas (RNG) as a fundamental way
of creating low-carbon CHP operation
without noting any of the key
concerns surrounding RNG. At
best, renewable gas, if all available
sources are tapped, will only
cover ~15% of the current total U.S.
natural gas consumption. This limited
supply is reflected in the price
of RNG, which is already between
two to five times more expensive
than conventional natural gas. Why
would we use this limited supply
for applications such as heat and
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electricity generation, which already
have carbon-free and more efficient
alternatives, instead of in industry
applications which are notoriously
hard to decarbonize?
Finally, RNG is chemically identical
to natural gas, meaning that it
is largely methane. Methane leakage
roughly doubles the impact of
natural gas use. As RNG is distributed
through the same leaky pipe as
natural gas, it will emit methane to
the environment and substantially
reduce the " low carbon " reality of
the fuel source.
Additional key issues the author
does not address include:
* The cost-effectiveness of renewable
generated electricity compared
to cogeneration plants powered with
RNG when full maintenance costs of
the cogeneration are included.
* In many campus settings,
cogeneration plants are sized at or
above the campus energy needs,
which discourages energy efficiency
because energy savings have little
monetary value.
* Hydrogen was noted as the key
zero carbon fuel source. In addition
to the fact that hydrogen is not
largely available today, the hydrogen
that is available is largely produced
from natural gas or other nonrenewable
sources.
We feel that the advantages of
renewably generated electricity powering
efficient, all-electric buildings
far outweigh the long-term carbon
emissions of CHP systems, however
efficient they might be.
Peter Rumsey, P.E., Fellow ASHRAE, San Francisco;
Jorlyn Le Garrec,Associate Member ASHRAE, San Francisco
The Authors Respond
This letter provides an opportunity
to further describe the important
F E B R U A RY 2023
role of cogeneration, or combined
heat and power, in decarbonization.
The respondents recognize
that " natural gas will not disappear
overnight " and that " cogeneration is
a very efficient way of using natural
gas, " resulting in enormous amounts
of GHG emissions savings today.
However, they do not acknowledge
ongoing developments in expanding
renewable and net-zero carbon fuels
including green and blue hydrogen
which, we believe, will play an
important role in a future net-zero
carbon economy.
Renewable and hydrogen-fueled
CHP can decarbonize thermal
end-uses in industrial and commercial
facilities that are difficult
or too costly to electrify, as well as
critical operations that need dispatchable
on-site power for longduration
resilience and reliability.
At the same time, CHP's inherent
efficiency advantage can serve to
further extend the resource base of
these emerging renewable and netzero
fuels.
Installing natural gas CHP today
by no means locks one into the longterm
use of fossil fuels. CHP systems
currently operate on a variety of
renewable and low- to no-carbon
fuels, including renewable natural
gas (RNG), hydrogen and biogas.
RNG is biogas that has been
upgraded to commercial natural
gas specifications for injection into
the existing natural gas pipeline
infrastructure and is produced at
landfills, through anaerobic digestion
at wastewater treatment plants,
agricultural operations, food processors
and animal feed lots, and
from gasification of biomass. RNG
can have negative GHG emissions
on a life-cycle basis depending on
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ASHRAE Journal - February 2023

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