ASHRAE Journal - June 2024 - 8

LETTERS
Marginal Carbon Factors Beyond Day One
" Understanding Marginal Carbon Factors and
Why They Matter " by William Ryan, Ph.D., P.E.,
Fellow ASHRAE, in the January 2024 issue of ASHRAE
Journal is helpful in discussing the most appropriate
carbon metrics to apply to the global warming
impacts of equipment selection decisions in building
design. Dr. Ryan describes in detail how the operating
configurations of the electric grid directly impact what
additional increments of load have on the emission of
the existing electric system.
Unfortunately, a key factor is omitted in evaluating
the emissions impact of a piece of equipment and
its efficiency of energy use-it has a lifetime of its
own. Day one should not be the only day of operation
of concern. The article focuses on only past or current
grid marginal emissions, though those pieces of
installed equipment may be in operation for 15, 20 or
30 years to come.
As the article asserts, the Environmental Protection
Agency's (EPA) AVoided Emissions and geneRation
Tool (AVERT) data is based on how the grid is actually
operating. But decision makers, whether they are
building designers or policy officials, will want to
know how the grid will operate over the lifetime
of the equipment, when actual marginal emission
impacts will occur. Total climate forcing is the key
impact metric, and that is why we need to understand
the marginal emissions profile over a lifetime of
equipment operation.
Fortunately, the National Renewable Energy
Laboratory (NREL) compiles and regularly updates the
Cambium emissions factor database for estimating
hourly emission factors in grid regions across the
U.S. two or three decades into the future based on a
range of scenarios for renewable penetration over
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ASHRAE JOURNAL ashrae.org J U N E 2024
those periods. The scenarios for future grid emission
profiles do not agree with the conclusions drawn
from the backwards-looking data in the article's
Figure 5. NREL and Cambium provide data that are
being incorporated into ASHRAE Standards 189.1-2023,
90.2 and 240P and others to derive tables of emission
values to measure lifetime greenhouse gas impacts of
design decisions.
Dr. Ryan could not be more correct in saying how
important it is to get carbon metrics " right. " But
this means selecting appropriate timeframes and
geographies of electricity generation to evaluate
lifetime impacts, not just evaluating day one.
Jim Edelson
Associate Member ASHRAE
Portland, Ore.
RYAN RESPONDS
Thank you for your comments. I find the Cambium
system highly interesting. NREL has clearly done
extensive work here.
My article focused on explaining aspects of electric
grid behavior, leading to the importance of marginal
carbon factors in engineering evaluations. The
Cambium system referred to in your letter also seems
to focus on marginal carbon evaluation.
These marginal factors are clearly shown to be
highly dependent on the dispatchable generation
(generation that can be freely varied in power output
at any time) that is available to the grid to achieve grid
stability when variable renewable energy (VRE) are not
available. My motivation for including backward-facing
marginal carbon data was to illustrate that even though
substantial VRE capacity has been added to the grid
since 2010, VREs rarely participate at the top of the " bid
stack " and are less relevant in reduction in the marginal
carbon factors as indicated by the historic data. I suspect
that the moderate reductions in marginal carbon factors
since 2007 shown in my article have more to do with the
ongoing coal to gas substitution, as these fossil plants
are what we currently depend on for dispatchable
generation, but this needs more detailed historic study.
Cambium 2023 Scenario Descriptions and Documentation
from NREL describes the Cambium system and eight
different evaluation scenarios. To quote from the
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FJSHSS/index.php#/p/Intro https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FJSHSS/index.php#/p/Intro https://tinyurl.com/JournalSubmit http://technologyportal.ashrae.org http://www.ashrae.org
ASHRAE Journal - June 2024

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