POWER January 2020 - 40

POWER TRANSITIONS
fleet was 54.8%, however, the hourly
capacity factors ranged from 37.1% to
72.7%. Also, the percentage of energy
generation from coal on an hourly basis
remained relatively constant during this
period. The gas fleet average capacity
factor was 46.3% and hourly data ranged
from 29.3% to 64.3%. These wide-ranging
capacity factors not only impact heat
rates, but also significantly increase plant
operation and maintenance costs.
During winter months, there was a
2. U.S. electric generation energy source profile on Jan. 30, 2019. There are generally two
demand peaks observed in the profile during winter days. Source: EIA
different profile (Figure 5) related to
coal and gas generation. During the
period of Jan. 25-31, 2019, for example,
there were two load peaks per
day. Coal and gas equally provided the
bulk of the generation capacity during
these periods. Nuclear remained constant
and hydro generation peaked at
high-demand times. Solar generation
was different during the winter period;
it peaked during low-demand times.
Wind generation was more random
during this period.
During January, the price of natural
3. U.S. electric generation energy source profile on July 19, 2019. Summer days generally have
a single, larger demand peak each day. Source: EIA
Longer-Term Snapshots
To get a different perspective on the U.S.
generation profile, consider a week's
worth of data during a high-temperature
period. The data for the period of Aug.
14-20, 2019, is presented in Figure
4. During this period, significant load
swings for both gas and coal-fired generation
are shown. Nuclear generation
was baseloaded as usual. Solar energy
peaked during periods of high demand.
Hydro operators were swinging load
to match demand and to optimize revenue,
while wind generation peaked at
low-demand times. There was a single
peak demand period every day during
this period.
The large swings in generation for the
gas and coal-fired fleets have a detrimental
impact on the heat rates (efficiency)
of these units. Heat rate is directly related
to economics and the generation
of CO2
(Btu/kWh) and CO2
40
. Cycling load increases heat rate
emissions. For example,
as the coal fleet annual capacity
factor decreased from 64% to 54%,
the heat rate increased from 10,100 Btu/
kWh to 10,200 Btu/kWh. Poor heat rates
mean less-efficient operation, that is,
more fuel usage per MWh, higher maintenance
costs, and more CO2
emissions
per MWh.
The Department of Energy's Dr. Robert
K. Smith has reported the impact
coal plant cycling has had on costs in
the Western region. The analysis shows
costs increase from $0.14/MWh to $0.67/
MWh over the system, representing 2%
to 7% of total production costs. As the
capacity factor for coal has decreased,
the capacity factor for simple cycle gas
turbines has significantly increased (see
Table 2 above). Simple cycle gas plants
are being used for rapid ramping to fill
the generation gaps due to the variability
of wind and solar.
During this August 2019 period, the
capacity
average
factor
for
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the
coal
Regional Impacts
Another important aspect of the U.S.
power grid is regional impacts. In some
regions of the U.S., wind and solar energy
generation are not as technically/
economically viable as in other areas.
Wind energy is limited in application
in the Southeastern U.S. and Florida,
while Texas, North Dakota, and California
have excellent wind potential. Coal
generation can make up some of the
generation gaps in areas where wind
and solar are limited.
On July 19, for example, the Electric
Reliability Council of Texas (ERCOT)
sourced 27.8% of its power from wind,
while the state of Florida generated 0%
POWER | January 2020
gas for power generation was high and
led to increased coal generation because
it was more economical. Also, natural
gas was restricted for power generation
in certain regions due to limited pipeline
capacity and residential heating use. For
example, in the Consolidated Edison service
area of New York, gas-fired plants
switch to ultra-low-sulfur diesel when
the temperature falls below 20F. During
this period, the average capacity factor
for the coal fleet was 60.0%, however,
the hourly capacity factors ranged from
48.2% to 73.5%. The gas fleet average
capacity factor was 32.8% and hourly
data ranged from 23.4% to 45.0%.
When gas turbines switch from gas to
oil, there is an increase in heat rate and
air emissions.

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POWER January 2020

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