ASHRAE Journal - January 2024 - 38

TECHNICAL FEATURE
FIGURE 5 (a) Marginal carbon emission factors over time7 and
(b) changes in the fuel sources used for utility electric generation in the U.S.8
Note that AVERT region boundaries were altered in 2017. Marginal emission
factors may be called " Avoided Emission Factors " when evaluating electric use
reductions.
A
2,000
1,900
1,800
1,700
1,600
1,500
1,400
1,300
1,200
1,100
1,000
B
2,000,000
1,800,000
1,600,000
1,400,000
1,200,000
1,000,000
800,000
600,000
400,000
200,000
Generation at Utility Scale Facilities (GWh/Yr)
Uniform EE Marginal Emission Factors (lb/MWh)
National Average
Midwest
Mid-Atlantic
New England
New York
Northeast
Great Lakes/
Mid-Atlantic
Texas
by electric load changes, as taken from eGRID data.
AVERT also produces evaluations of NO2, PM2.5 and
SO2. Therefore, all numbers are for CO2 and not CO2e.
Fugitive methane emissions upstream of the power
plants are not included.
Also, the eGRID data is limited to the United States;
therefore, similar calculations cannot be run for other
locations by the AVERT system. However, power plant
dispatch by marginal cost is a microeconomic principle
that is universally applicable.
Coal
Natural Gas
Nuclear
Hydro
Wind
Solar
Conclusions
Readers of ASHRAE Journal are chiefly involved in
the design of systems that either consume or produce
electricity at the individual building level in systems that
are very small in comparison to the size of the national
or regional grid. These additions or subtractions to the
overall load are therefore marginal changes to regional
electric consumption. Proper calculation of the carbon
emission implications of these marginal changes must
use the marginal carbon emission factors that have been
made available by the EPA for this purpose.
On the current grid, marginal carbon emission factors
are significantly higher than carbon average factors that
are too often mistakenly used. Using marginal carbon
emission factors, rather than average factors, will
correctly show a significantly higher carbon reduction
for efficiency improvements, a core focus of many
ASHRAE members.
The variety of efficiency and carbon reduction
Marginal Carbon Factors Over Time
Due to the multiyear lifetimes of new efficient
equipment installations, the degree to which marginal
carbon factors are changing over time is also of interest.
In Figure 5a, changes in the marginal carbon emission
rates are shown, taken from EPA calculations.7 Although,
as shown in Figure 5b, there has been significant growth
in wind and solar generating sources in recent years,
the changes in the marginal carbon emissions rates
have not been large, as many of the new sources such as
wind and solar are not used at the margin, as shown on
Figure 3. This has been noted by others including a recent
publication by the National Academy of Sciences.2
AVERT produces calculation of CO2 emissions changes
directly from the power generation units impacted
38
ASHRAE JOURNAL ashrae.org J A N U ARY 2024
strategies being promoted today range from those
that are economically desirable for the user to others
that can only make economic sense when reinforced
by substantial government incentives. Given that
such incentives must be, ultimately, limited in
scale suggests that technologies that provide the
greatest carbon reductions at the lowest cost should
be a focus. This requires proper carbon emission
reduction evaluations.
More unfortunately, some suggestions for carbon
reduction technologies that depend on replacing
on-site fossil fuel use with electric technologies may
even increase global carbon emissions. A proper
understanding of the carbon emissions due to added
electric generation must be featured in the analysis
of new electric technologies. Engineers must also
communicate to the public that just because a new
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ASHRAE Journal - January 2024

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