Technical Support Document (TSD) for 

New Source Performance Standards for Carbon Pollution from 

Electric Utility Generating Units (EGUs) 

Docket ID No. EPA-HQ-OAR-2013-0495 

Docket ID No. EPA-HQ-OAR-2013-0603 

Net-electric Sales Applicability Threshold for 

Combustion Turbines as Related to 

Integration of Intermittent Renewables 

U.S. Environmental Protection Agency 

Office of Air and Radiation 

August 2015 
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Purpose 

The purpose of this TSD is to provide an analysis assessing the claims made by respondents to the proposed 

rule that reliable use of intermittent renewables causes percentage of electric sales relative to the potential 

electric output of simple cycle turbines to increase. 

Overall Relationship 

This technical support document presents an analysis of the annual net generation provided by solar and 

wind electricity generators versus the annual net generation provided by simple cycle combustion turbines 

for the years 2010 through 2014 based on EIA1

 Form 923 Data. In order to minimize the effect of 

independent variables that may affect the percentage electric sales of combustion turbines but that are 

unrelated to the expanding use of intermittent renewables, this analysis focused on the western United States, 

the region of the U.S. that has seen the largest increase in output from renewable generating assets over the 

study period. The analysis relies on the EIA net generator output data from the following states that provide 

electricity to the Western Interconnection2

: 

Arizona California Colorado Idaho Montana New Mexico Wyoming 

Nevada Oregon South Dakota Texas Utah Washington 

In addition to limiting the analysis to the states that have seen the largest increases in output from 

intermittent renewables, the following additional steps were also taken minimize the interference of 

independent variables unrelated to increasing output from intermittent renewables: 

1. The output from simple cycle turbines serving combined heat and power functions were excluded 

from the total output attributed to simple cycle turbines because the reliability and availability 

(to WECC system operators) of industrial combined heat and power output is largely driven by 

independent factors specific to the associated industrial process. These generators are identified in 

the EIA Form 923 source data in column B (under the “Combined Heat & Power Plant” heading). 

2. Consistent with the preceding rationale, output from simple cycle turbines with industrial NAICS 

codes (e.g., NAICS codes beginning with “21”, “32”, and “33” were excluded from the total output 

attributed to simple cycle turbines during the study period because independent factors specific to the 

industrial process may limit the availability of these generating assets for balancing system load 

during periods of rapid fluctuations in intermittent renewable output. Only generators identified with 

NAICS codes beginning with “22” were included in the study because these assets are owned by 

utility companies and strictly operated to maintain reliable service to consumers regardless of 

variations in the net output available from industrial generators and intermittent renewables. 

The general approach was to evaluate if a correlation exists between increases in electrical output from 

intermittent renewable generators (i.e., solar and wind only) and increases in output from utility-owned 

simple cycle combustion turbines. The data used in the analysis are summarized in Tables 1.A. through 1.E., 

                                                            

1 “Page 1 Generation and Fuel Data” available for 2010‐2014 at http://www.eia.gov/electricity/data/eia923/. 2 The Western Interconnection is comprised of the electricity generators and transmission systems that serve most of

the western regions of the United States. This interconnection is operated by the Western Electricity Coordinating

Council (WECC). https://www.wecc.biz/Pages/home.aspx.

WECC
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and the complete data set is included in the docket with the filename 

“EIA_2010_TO_2014_Renewables_vs_Simple_Cycles.xlsx.”
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Discussion of Findings 

As shown in Figure 1, from 2010 - 2014 there was a strong correlation between increases in output from intermittent 

renewables and increases in output from simple cycle turbines within the 13 western states.3

 Comparatively Figure 2

shows that there was a weaker 

negative correlation for 

conventional (non-combined 

cycle) steam plants (e.g., 

nuclear and coal-fired EGUs); 

these units have more limited 

ramping4

 capabilities than 

simple cycle turbines. 

Additionally, Figure 3 shows 

that there was a weaker 

(compared to the simple cycle 

correlation) positive 

relationship between 

increasing output from 

renewables and output from 

combined cycle power blocks 

in the 13 interconnected 

western states. Figure 1. Solar + Wind Output vs. Simple Cycle Output in 13 Interconnected Western States.

Comparison of Figures 1, 2 and 3 confirms that a uniquely-strong positive correlation exists between increasing output 

from intermittent renewables 

and output from simple cycle 

turbines. The correlation 

equation suggests that for 

approximately every 11 

megawatt hours of additional 

carbon-free electrical output 

capacity that was added in the 

13 interconnected western 

states from 2010 to 2014, about 

1 additional megawatt hour of 

simple cycle operations was 

necessary to ensure reliable 

transitions from nondispatchable to dispatchable 

generators. Figure 2. Solar + Wind Output vs. Steam Plant Output in 13 Interconnected Western States

                                                            

3 This analysis does not attempt to account for the impact of increased electric demand. 4 The term “ramping capabilities” refers to a generating unit’s ability to rapidly increase or decrease output in situations where the system operator

must rapidly replace a sudden loss of output such as when clouds associated with a thunderstorm suddenly decrease the output from a large bank

of photovoltaic cells or when the wind suddenly stops blowing.
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Figures 4 and 5 provide a comparison of the percentage electric sales of utility owned simple cycle turbines 

within the 13 western, interconnected states to the percentage electric sales of utility owned combined cycle 

turbines within the 13 states. 

Figure 4. Percentage of Electric Sales of Simple Cycle Combustion Turbines Serving in the 13 Western States.

 

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

0 20 40 60 80 100 120

SIMPLE CYCLE CAPACITY FACTORS

IN 13 INTERCONNECTED WESTERN STATES

SC CAP. FACTORS, 2010

SC CAP. FACTORS, 2011

SC CAP. FACTORS, 2012

SC CAP. FACTORS, 2013

SC CAP. FACTORS, 2014

Figure 3. Wind + Solar Output vs. Combined Cycle Output in 13 Western
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Figure 5. Capacity Factors of Combined Cycle Combustion Turbines Serving in the 13 Western States. 

Figures 4 and 5 demonstrate that the percentage of electric sales of combined cycle turbines and simple 

cycle turbines can have significant overlap. 

Figure 4 demonstrates that even in states with significant intermittent renewable capacity, simple cycle 

operating profiles are consistent with the national statistic that simple cycle units continue to operate 

primarily as peaking units. 

Impact on Percentage Electric Sales 

While the overall generation data suggests a positive relationship between the absolute amount of electricity 

generated by simple cycle turbines and intermittent renewable generation, the percentage electric sales for 

individual simple cycle units is the relevant threshold for this final rule. For the preamble discussion, we used 

EIA data from 2008 to 2014 and individual generation data the EPA’s Clean Air Markets Division to 

determine the impact on individual units. As discussed in the preamble to this final rule, we reviewed the 

historical generation from simple cycle generation in states with increasing and significant amounts of 

generation (e.g., California and Texas) to determine the relationship between simple cycle turbines and 

generation from intermittent renewable generation (i.e., wind and solar). To date, increased generation from 

renewable generation has had minimal impact on the way the existing simple cycle fleet operates. These 

units continue to be used primarily to satisfy periods of peak electric demand. The primary relationship 

appears to be the addition of large capacity of new simple cycle units to provide firm backup power. These 

units would be subject to the 111(b) requirements. However, these units are operating well below the 

percentage electric sales threshold for base load units in this final rule. See the preamble for a detailed 

discussion. 

Base Load Percentage Electric Sales Threshold 

The rationale for the sliding scale percentage electric sales threshold is described in detail in the preamble to 

this final rule. The following is a brief discussion on why the EPA concluded that a single value percentage 

electric threshold below one-third electric sales is not appropriate at this time. For example, 88 percent of 

existing simple cycle units did not have annual electric sales that exceeded 10 percent of their potential 

electric output between 2005 and 2014. However, we have concluded that implementing a limitation this low 

could constrain further expansion of renewables. Specifically, commenters from the utility sector 

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0 10 20 30 40 50 60 70 80

COMBINED CYCLE CAPACITY FACTORS

IN 13 INTERCONNECTED WESTERN STATES

CC CAP. FACTORS, 2010

CC CAP. FACTORS, 2011

CC CAP. FACTORS, 2012

CC CAP. FACTORS, 2013

CC CAP. FACTORS, 2014
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consistently objected to the proposed one-third electric sales threshold, so few, if any, of these commenters 

would consider a 10 percent threshold sufficiently flexible for new non-base load units. In addition, our cost 

models show that it would not be cost-effective to install a NGCC combustion turbine that would operate at a 

capacity factor near 10 percent. 

The EPA also considered a percentage electric sales threshold of 25 percent for non-base load units. 

Nationally, ninety-eight percent of existing simple cycle turbines did not exceed this level of sales between 

2005 and 2014. In addition, at this level of electric sales, our cost models indicate that it would be cost 

effective to install an NGCC combustion turbine. This level is further supported by the permit restriction for 

the CPV Sentinel facility in Desert Hot Springs, California. This facility consists of eight high-efficiency 

intercooled simple cycle turbines intended to support wind generation in California. Each unit at this facility 

is restricted to 2,803 hours of operation annually.5

 Assuming an average duty cycle of 90 percent, this is 

equivalent to an annual percentage electric sales restriction of approximately 29 percent. 

Similar to the 10 percent threshold, however, the EPA acknowledges that a 29 percentage electric sales 

threshold to distinguish non-base load and base load units is not appropriate for multiple reasons. First, many 

commenters stated that existing simple cycle turbines are currently permitted to operate 2,920 hours or more 

annually to respond to variable annual demands and to provide capacity reserve (and the associated 

payments). Commenters stated that, due to the changing nature of the grid (e.g., increased generation from 

intermittent renewable generation), these needs will increase in the future. The cited permit from California 

is only for a single location, and a 29 percent electric sales constraint may not be representative of units in 

other areas. 

In the proposal, the EPA estimated the number of existing simple cycle turbines that would have exceeded 

the proposed one-third percentage electric sales threshold by determining if operated over 2,920 hours in any 

given year. However, this method overestimated the number of simple cycle turbines that would have 

exceeded the threshold in two ways. First, it did not account for hours in which simple cycle turbines were 

operating at part load. The typical duty cycle (i.e., average capacity factor while operating) of simple cycle 

turbines is approximately 70 percent. Therefore, simple cycle turbines can operate in excess of 2,920 hours 

and still not sell one-third of their potential electric output to the grid. Historically, simple cycle turbines sell 

approximately 3 percent of their potential electric output, and only 10 of the 1,939 existing simple cycle 

turbines have individual years where the percentage of electric sales exceeded one-third of their potential 

electric output. Second, we proposed to calculate the percentage electric sales criterion on a 3-year rolling 

average basis. Using a 3-year rolling average, only two existing simple cycle turbines historically sold 

enough electricity to exceed the proposed one-third percentage electric sales threshold. While this threshold 

appears to sufficient flexibility, as described in the preamble to this final rule the EPA has concluded that due 

to the changing nature of the electric grid it is prudent at this time to offer greater flexibility. 

Conclusion 

The final rule acknowledges the relationship between the use of flexible generating technologies (e.g., simple 

cycle turbines and fast-start combined cycle units) and expanded integration of intermittent renewables by 

providing a subcategory for non-base load combustion turbines. Non-base load turbines supply no more than 

their design efficiency times their potential electric output as net-electric sales on both a 12-operating month 

and 3 year rolling average basis. We refer to “design efficiency times the unit’s potential electric output” as 

“the sliding applicability scale.” The EPA has determined that this form of the applicability threshold is 

appropriate because it encourages capital investments in the most efficient fast-start technologies and 

accounts for the requirements necessary for integrating intermittent renewables. 

                                                            

5 http://docketpublic.energy.ca.gov/PublicDocuments/Regulatory/Non%20Active%20AFC's/07‐AFC‐

3%20Sentinel/2010/DECEMBER/TN%2059190%2012‐8‐10%20Final%20Commission%20Decision.pdf