NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

THE WESTERN 

WIND AND 

SOLAR 

INTEGRATION

STUDY PHASE 2:

Executive Summary

i

THE WESTERN WIND AND SOLAR 

INTEGRATION STUDY PHASE 2:

Executive Summary

Debra Lew and Greg Brinkman

National Renewable Energy Laboratory

Prepared under Task Nos. OE10.3020, SS12.2720, SM12.2010, and WE11.0810 

Technical Report

NREL/TP-5500-58798

September 2013
ii

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iii

Acknowledgments

The National Renewable Energy Laboratory (NREL) thanks the U.S. Department of Energy’s (DOE’s) Office of 

Energy Efficiency and Renewable Energy and Office of Electricity Delivery and Energy Reliability for sponsoring 

Phase 2 of the Western Wind and Solar Integration Study. Contributors include the following:

Project Manager

NREL Debra Lew

Technical Team Leader

NREL Greg Brinkman 

Study Team

NREL Eduardo Ibanez, Anthony Florita, Marissa Hummon,

Bri-Mathias Hodge, and Greg Stark

RePPAE Jack King

Intertek-APTECH Steven A. Lefton, Nikhil Kumar, and Dwight Agan

GE Energy Sundar Venkataraman and Gary Jordan

Technical Review Committee

Arizona Public Service Ron Flood

Bonneville Power Administration Eric King

California Independent System Operator Shucheng Liu

DOE Venkat Banunarayanan, Charlton Clark, Richard Gilker,

Sikander Khan, Kevin Lynn, Seungwook Ma, and

Lawrence Mansueti

Electric Power Research Institute Daniel Brooks and Aidan Tuohy 

Electric Power System Consulting Brendan Kirby

Electricity Research Centre/ Mark O’Malley

University College Dublin

Energy Exemplar Tao Guo
iv

GE Energy Richard Piwko 

Lawrence Berkeley National Laboratory Andrew Mills

National Energy Technology Laboratory Joel Theis

New England Independent System Operator Jonathan Black

NextEra Energy Resources Mark Ahlstrom

Northern Arizona University Tom Acker

NREL Lori Bird, Kara Clark, Paul Denholm, and Brian Parsons

NV Energy John Lescenski and Mike Smart

PacifiCorp Jamie Austin

Public Service Company of New Mexico Thomas Duane and Derek Stout

Renewable Energy Consulting Services Edgar DeMeo

Salt River Project Robert Hess and Brandon Quijada

Sandia National Laboratories Abe Ellis and Clifford Hansen

Tri-State Generation and Kevin Cox, Ray LaPanse, Art Mander, and Curtis Miller

Transmission Association

Tucson Electric Power Ron Belval and Gary Trent

Utility Variable Generation Charles Smith

Integration Group

WestConnect Charles Reinhold

Western Area Power Administration Bob Easton and James Hirning

Western Electricity Coordinating Council Matt Hunsaker, Bradley Nickell, and Heidi Pacini

Western Governors’ Association Tom Carr

Wind Wear Eugene Danneman

Xcel Energy Stephen Beuning, Justin Briggs, Sean Connolly, and James Schetter

Finally, we thank Aaron Bloom, Erik Ela, Neal Kruis, Noel Merket, Michael Milligan, and Yih-Huei Wan for their technical 

contributions and René Howard, Amy Vaughn, and Katie Wensuc for their editing.
v

List of Acronyms

4HA 4-hour-ahead

BA balancing authority 

C&M capital and maintenance

CC combined-cycle

CEM continuous emissions monitoring

CF capacity factor

CO2 carbon dioxide

CSP concentrating solar power

CT combustion turbine

CY calendar year

DA day-ahead

DC direct current

DCOE delivered cost of energy

DOE U.S. Department of Energy

EPA Environmental Protection Agency

GT gas turbine

GW gigawatt

GWh gigawatt hour

HRSG heat recovery steam generator

MMBtu million British thermal units

MW megawatt

MWh megawatt hour, energy

MW-h megawatt hour, reserves

NOX nitrogen oxides

NREL National Renewable Energy Laboratory

O&M operations and maintenance

PPA power purchase agreement

PV photovoltaic

ReEDS Regional Energy Deployment System

SO2 sulfur dioxide

ST steam turbine

TEPPC Transmission Expansion Planning

Policy Committee

TRC Technical Review Committee 

TW terawatt

TWh terawatt hour, energy

TW-h terawatt hour, reserves

VOM variable operations and maintenance 

WECC Western Electricity Coordinating Council

WWSIS-1 Western Wind and Solar Integration

Study Phase 1

WWSIS-2 Western Wind and Solar Integration

Study Phase 2

vii

Table of Contents

1 Overview

1 Motivation

3 Background

5 Study Approach

5 Wear-and-Tear Costs and Impacts Data

6 CO2, NOX, and SO2 Emissions Data

6 Wind and Solar Power Output Data

7 Production Simulations and Scenarios

10 Key Findings

10 Wind and Solar Displace Primarily Gas Generation and Increase

Coal Ramping

16 Wind- and Solar-Induced Cycling Affects Fossil-Fueled Plant Operations and Maintenance Costs

18 Cycling Increases Production Costs Slightly

19 CO2, NOX, and SO2 Emissions Reductions Are Significantly Greater Than Cycling Emissions

24 Gas Price Has a Greater Impact on Cycling Costs than Wind and Solar Penetration

25 Solar Dominates Variability and Wind Dominates Uncertainty

28 Conclusions

29 Future Work

1

Overview

The electric grid is a highly complex, interconnected machine, and changing one part of the grid can have consequences 

elsewhere. Adding wind and solar affects the operation of the other power plants and adding high penetrations can induce 

cycling of fossil-fueled generators. Cycling leads to wear-and-tear costs and changes in emissions. Phase 2 of the Western 

Wind and Solar Integration Study (WWSIS-2) evaluated these costs and emissions and simulated grid operations for a year to 

investigate the detailed impact of wind and solar on the fossil-fueled fleet. This built on Phase 1, one of the largest wind and 

solar integration studies ever conducted, which examined operational impacts of high wind and solar penetrations in the West 

(GE Energy 2010).

Frequent cycling of fossil-fueled generators can cause thermal and pressure stresses. Over time, these can result in premature 

component failure and increased maintenance and repair. Starting a generator or increasing its output can increase emissions 

compared to noncyclic operation. And operating a generator at part-load can affect emissions rates. Utilities are concerned 

that cycling impacts can significantly negate the benefits that wind and solar bring to the system. And to plan accordingly, 

power plant owners need to understand the magnitude of cycling impacts. 

In WWSIS-2, we calculated these wear-and-tear costs and emissions impacts. These data were incorporated into commercial 

software that simulates operations of the western grid (which includes the United States, Canada, and Mexico) on a subhourly 

basis, because wind and solar output can change within the hour. We designed five hypothetical scenarios to examine up to 

33% wind and solar energy penetration in the Western U.S. and to compare the impacts of wind and solar. We then examined 

how wind and solar affected operation, costs, and emissions from fossil-fueled generators. This work was overseen by a 

Technical Review Committee (TRC) to ensure that assumptions, methodologies, and analyses were realistic and credible. 

Our results are based on the specific characteristics of the western grid and key assumptions, including an average gas price 

of $4.60/MMBtu, significant balancing authority cooperation, and least-cost economic dispatch and transmission usage that 

does not model bilateral transactions. The goal of WWSIS-2 is to quantify the cycling impacts that are induced by wind and 

solar. It does not address whether wind and solar should be built, but rather what happens if they are built.

In this study, we found that up to 33% of wind and solar energy penetration increases annual cycling costs by $35–$157 

million in the West. From the perspective of the average fossil-fueled plant, 33% wind and solar penetration causes cycling 

costs to increase by $0.47–$1.28/MWh, compared to total fuel and variable operations and maintenance (VOM) costs of 

$27–$28/MWh. The impact of 33% wind and solar penetration on system operations is to increase cycling costs but also to 

displace annual fuel costs by approximately $7 billion. WWSIS-2 simulates production or operational costs, which do not 

include plant or transmission construction costs. From the perspective of wind and solar, these additional cycling costs are 

$0.14–0.67 per MWh of wind and solar generated compared to fuel cost reductions of $28–$29/MWh, based on the generator 

characteristics and modeling assumptions described in this report. 

This study finds that up to 33% wind and solar energy penetration in the United States’ portion of the Western grid (which is 

equivalent to 24%–26% throughout the western grid) avoids 29%–34% carbon dioxide (CO2) emissions, 16%–22% nitrogen 

oxides (NOx) emissions, and 14%–24% sulfur dioxide (SO2) emissions throughout the western grid. Cycling had very little 

(<5%) impact on the CO2, NOX, and SO2 emissions reductions from wind and solar. For the average fossil-fueled plant, we 

found that wind- and solar-induced cycling can have a positive or negative impact on CO2, NOx, and SO2 emissions rates, 

depending on the mix and penetrations of wind and solar. 

Motivation

Phase 1 of the Western Wind and Solar Integration Study (WWSIS-1) was a landmark analysis of the operational impacts of 

high penetrations of wind and solar power on the Western Interconnection (GE Energy 2010). The study found no technical 

barriers to accommodating the integration of 35% wind and solar energy on a subregional basis if adequate transmission 

was available and certain operational changes could be made. The two most important of the operational changes were 

increased balancing authority (BA) cooperation and increased use of subhourly scheduling between BAs for generation and 

interchanges.
2

The variability and uncertainty of wind and solar can have profound impacts on grid operations. Figure ES-1 shows the most 

challenging week of the 3 years of data studied in WWSIS-1, when high penetrations of wind and solar caused fossil-fueled 

plants to cycle more frequently. In this report, cycling is a broad term that means shutting down and restarting, ramping up 

and down, and operating at part-load.

50,000

40,000

30,000

20,000

10,000

0

MW

10-Apr 11-Apr 12-Apr 13-Apr 14-Apr 15-Apr 16-Apr 

-1

Wind

PV

CSP

Combined cycle

Hydro

Coal

Nuclear

Pumped storage hydro

Gas combustion turbine

Figure ES-1. WWSIS-1 dispatch for the most challenging week of 3 years of data analyzed 

Notes: PV, photovoltaic; CSP, concentrating solar power

Utilities were concerned about this type of operation and its impacts on repair and maintenance costs and component 

lifetimes. In addition, some analysts asserted that the emissions imposed by cycling could be a significant fraction of—

or even larger than—the emissions reduced by wind and solar (Bentek Energy 2010; Katzenstein and Apt 2009).

WWSIS-2 was initiated in 2011 to determine the wear-and-tear costs and emissions impacts of cycling and to simulate grid 

operations to investigate the detailed impact of wind and solar on the fossil-fueled fleet. WWSIS-1 focused on whether high 

penetrations were technically feasible. In WWSIS-2, we analyzed the cycling impacts in detail and with a higher degree of 

fidelity. WWSIS-2 simulates operation of the entire Western Interconnection but wind and solar is only added to the U.S. 

portion of the Western Interconnection because data from outside the United States are lacking.

In WWSIS-2, we dove deep into the impacts of cycling on the operation of fossil-fueled plants. We created new data sets and 

simulated subhourly grid operations to answer questions such as the following:

• What are the increased costs because of wear and tear on fossil-fueled plants? 

• Do these wear-and-tear costs significantly reduce the benefits of wind and solar?

• Will incorporating these costs into optimization of grid operation reduce cycling? 

• What are the emissions impacts of cycling? 

• How do wind impacts compare to solar impacts on cycling and grid operations?

This study focused on simulating grid operations on a subhourly basis. The results discussed here are specific to the Western 

Interconnection and the characteristics of the generation and transmission in the West. Adapting these results to other regions 

would require simulating the characteristics of those regions.
3

WWSIS-2 was one piece in a larger puzzle of understanding the impacts of wind and solar on the electric power grid. 

Although WWSIS-2 needed hypothetical scenarios of renewable energy siting and transmission expansion, these were not

the main focus. System reliability and stability issues were not the focus of this study either, but are being examined in

Phase 3 of WWSIS.

Background

Impacts of cycling induced by wind and solar additions can be investigated in different ways. The first is from the perspective 

of a fossil-fueled plant. If that plant is required to cycle more frequently, this can affect wear-and-tear costs and emission 

rates, which in turn affect that plant’s marginal costs and emissions requirements. The second way to frame these impacts is 

from a system perspective. Wind and solar can impact grid operations by displacing fossil-fueled generation (and the costs 

and emissions associated with fossil fuels) but also increasing cycling (and the costs and emissions impacts associated with 

it). This study examines whether these cycling impacts significantly reduce the benefits of displacing fossil-fueled generation.

From the perspective of a power plant owner or a resource planner, the delivered cost of energy (DCOE) from a specific plant 

is important. From the perspective of the overall system or in terms of societal impacts, costs across the entire system are 

important. This report attempts to examine cycling impacts from all these perspectives.

The DCOE for a specific plant looks very different for a fossil-fueled plant than for a wind/solar plant, as shown in Figure 

ES-2. The DCOE for a fossil-fueled plant is a mix of fixed costs and production costs. The DCOE for a wind/solar plant 

is nearly all fixed capital costs. Fixed costs are those costs that do not change based on how much the plant is run, such as 

power plant and transmission construction costs and fixed operations and maintenance (O&M) costs. Production costs are 

the variable costs that increase as the plant produces more electricity and consist of fuel and VOM. VOM, in turn, comprises 

cycling O&M (which consists of start fuel plus wear and tear from starts and ramps) and noncyclic O&M (which are the 

routine overhauls and maintenance costs from the plant running at some steady-state output). The only capital costs included 

in production costs are capitalized maintenance (e.g., more frequent boiler tube replacements) because cycling and steadystate operation reduces the lifetimes of those components. Production simulation tools, such as the one used in this study, 

model operations of the power system. Production costs are key outputs of these tools.

Fossil-fueled generator Wind/solar

Production 

costs

O&M from cycling/ramping

O&M of steady state operation

Fuel

Fixed O&M

Capital cost of transmission

Capital cost of generation

Production costs Fixed costs

Fixed costs

Figure ES-2. Illustrative DCOE for a fossil-fueled plant and a wind/solar plant
4

Adding any new generation to the power system will change the way existing plants operate. Studies show that adding wind 

and solar can cause existing fossil-fueled plants to cycle more and have lower capacity factors (EnerNex 2011; GE Energy 

2010). Adding new, low-priced baseload generation can also cause the incumbent fossil-fueled plants to cycle more and have 

lower capacity factors (Milligan et al. 2011). An incumbent fossil-fueled plant that now has a lower capacity factor (and 

likely reduced revenue) and a higher O&M cost (because of cycling) might have a hard time remaining viable. This raises 

questions about who should pay for the cycling costs of incumbent plants or what happens in the marketplace to address the 

viability of a plant that might be needed for reliability but might no longer be profitable. These questions are not addressed in 

this technical report. 

When O&M from cycling increases, the cost of energy component circled in red in Figure ES-2 also increases. Before this 

study, little wear-and-tear data for different types of cycling operation were publicly available. WWSIS-2 investigates this 

cost in depth. It explores the magnitude of that cost, how that cost changes when wind and solar are added to the system, 

how that cost changes the fuel savings that wind and solar bring to the system, and how increased wind and solar penetration 

affects that cost. 

From a system perspective, utility planning decisions have resulted in a given portfolio of plants. Those fixed capital costs 

(or power purchase agreements [PPAs] if the utility is buying from an independent power producer) are now sunk costs. 

The system operator’s job is to manage operations of that portfolio to supply reliable power at low cost to consumers. The 

operators do not see the fixed costs, only the production costs. If we consider the what-if scenario of this same system with a 

new wind/solar plant (for simplicity, ignore the bilateral transactions and incentives such as production tax credits), we can 

see that the near-zero production cost of wind/solar will lead the operator to dispatch the wind/solar instead of fossil-fueled 

generation, as long as it is within all the constraints of transmission and operating limits. This displaces the fossil-fueled 

generators’ production costs (fuel and O&M). The change in production cost with and without wind/solar is shown in Figure 

ES-3. From the perspective of the system operators, this reduction in fuel cost is the benefit that wind and solar bring to the 

system. WWSIS-2 addresses how that benefit is affected when cycling costs are modeled in detail. The cost of wind/solar is 

the difference in fixed costs (capital costs of the wind/solar plants and transmission). WWSIS-2 did not conduct a cost-benefit 

analysis of wind and solar to determine profitability. Instead, it posited that if wind/solar is present, what benefit does it bring 

to the system and how much is that value reduced by cycling?

O&M from cycling/ramping

O&M of steady state operation

Fuel

Fixed O&M

Capital cost of transmission

Capital cost of generation

Production costs

Production

cost savings

of wind

& solar

Before renewables After renewables Production costs

Figure ES-3. Illustrative system-wide costs before and after wind/solar 

Notes: The wind/solar requires additional fixed costs but offsets production costs. 

The change in production cost from wind/solar is shown by the green arrow.
5

Study Approach

WWSIS-2 examined the impact of up to 33% wind and solar energy penetration on the U.S. portion of the Western 

Interconnection. We explicitly calculated values for various types of wear-and-tear costs resulting from cycling. We used 

the wear-and-tear start costs to optimize detailed operations of the grid and included ramping costs in the total cost impact. 

We considered the impacts of both the variability and the uncertainty of wind and solar on starts, ramps, and operation of 

the power system. We modeled five scenarios that were designed to illuminate the impacts of increased wind and solar and 

compare the impacts of wind and solar on the power system.

To assess the cycling impacts on the fossil-fueled fleet induced by wind and solar, we needed the following information:

• Wear-and-tear costs and impacts for cycling

• Emissions impacts resulting from cycling

• Subhourly wind and solar plant output for future hypothetical plants

• A tool to model grid operations on a subhourly time frame.

This study was conducted by a team of researchers from the National Renewable Energy Laboratory (NREL), GE Energy, 

Intertek-APTECH (APTECH), and RePPAE. The TRC met every 2 months to discuss and review assumptions, data inputs, 

methodology, and results. TRC members included representatives from utilities, transmission planning groups, the Western 

Electricity Coordinating Council (WECC), and DOE and its laboratories, along with power system and fossil-fueled 

plant experts. As data sets or preliminary results were completed, they were vetted in public forums and peer-reviewed 

publications. This study has been thoroughly reviewed for technical rigor.

Wear-and-Tear Costs and Impacts Data

Cycling of thermal plants can create thermal and pressure stresses in power plant components. This leads to increased O&M 

costs, more frequent repairs, reduced component life, and more frequent forced outages. Power plants that were designed for 

baseloaded operation suffer much more wear-and-tear damage from cycling. In this report, a start is defined as “starting a unit 

that is offline.” Ramping is defined as “load-following operation in which a generating unit increases its production.” Cycling

includes both starts and ramping.

To address the lack of public data on the wear-and-tear costs and impacts from cycling of coal and gas generators, NREL and 

WECC jointly retained APTECH to create a data set. APTECH had previously investigated these costs for hundreds of plants 

around the world. For each plant, APTECH had determined a best fit and a lower-bound and an upper-bound fit for cycling 

costs, where the bounds reflected the uncertainty range for that plant. APTECH statistically analyzed those proprietary data 

to develop generic costs and impacts for seven categories of coal and gas generators (Kumar et al. 2012). Figure ES-4 shows 

the statistics for the lower-bound costs for cold starts for the seven plant types. The medians of these lower-bound costs were 

used in the operational optimization so that the wear and tear on fossil-fueled generators was considered in the decision to 

commit and dispatch units. Upper-bound start costs were then applied to this dispatch to estimate the range of start costs. 

This may yield a conservative estimate because using those upper-bound costs in the unit commitment process could reduce 

cycling. On the other hand, many plant operators do not consider wear-and-tear costs in their dispatch decisions, so this may 

reflect a realistic view of current operations. Unless otherwise specified, ranges of wear-and-tear costs in this report reflect the 

uncertainty range from the lower to the upper bound. High-impact, low-probability events such as a generator failure were 

not included in these wear-and-tear costs because there was not enough data to assess the impact of cycling on those events.
6

400

300

200

100

0

Coal–small, subcritical

Coal–large, subcritical

Coal–supercritical

Gas–CC [GT+HRSG+ST]

Gas–large-frame CT

Gas–aero-derivative CT

Gas–steam

Cold start C&M low

estimate ($/MW)

Figure ES-4. Lower-bound costs for one cold start

Notes: C&M, capital and maintenance; CC, combined cycle; GT, gas turbine; HRSG, heat recovery steam generator; ST, steam turbine; CT, combustion turbine. The range shows the 

25th to 75th percentile, with the median shown within that range. Nonoutlier extrema are depicted by the whiskers in the plots. Outliers are represented as dots.

CO2, NOX, and SO2 Emissions Data

Starts, ramping, and part-loading also have impacts on emissions. To address the lack of emissions data from cycling, NREL 

analyzed unit-specific measured emissions from the U.S. Environmental Protection Agency (EPA) Continuous Emissions 

Monitoring (CEM) data set (EPA 2009) to develop refined emissions rates for most units in the U.S. portion of the Western 

Interconnection for CO2, NOx, and SO2. Figure ES-5 shows an example of how heat rates and emissions rates were calculated 

for part-load. In addition, unit-specific incremental emissions from starts and ramps were calculated using these measured 

emissions data. Part-loading generally results in a higher emission rate overall, except for NOx emission rates, which decrease 

for coal and gas steam units. Compliance with existing or proposed emissions regulations was not analyzed.

Wind and Solar Power Output Data

In WWSIS-2, we updated the wind and solar plant output and forecast data sets from WWSIS-1 to best represent current 

technologies and methodologies. For example, we capitalized on recent advances in modeling of utility-scale PV plants 

on a subhourly timescale. The following types of plants were modeled in WWSIS-2: utility-scale wind, rooftop distributed 

generation PV, utility-scale PV, and CSP with 6 hours of thermal storage.

ches Word le.

Generation (MW)

Heat Rate (MMBtu/MWh)

0 50 100 150 200 250

14

13

12

11

10

9

8

7

6

Figure ES-5. Heat-rate curve for a typical gas CC unit 

Notes: The black dots show measured emission rates for every hour of the year. The green line shows a local linear fit. 
7

Production Simulations and Scenarios

Production simulations were used as the primary tool to examine operations of the power system. These simulations produce 

extensive data outputs including generator commitment and dispatch, emissions, costs, and transmission path flows for each 

time step. Production costs are a key output. Fixed capital costs and PPAs are not included in these simulations.

We simulated scenarios in 2020 using the WECC Transmission Expansion Planning Policy Committee’s (TEPPC’s) 2020 

Portfolio Case 1 as the basis for the production simulation modeling (WECC 2011). Because that case had a relatively high ($7.28/

MMBtu) average gas price, we used the gas price projections from WECC TEPPC 2022, which averages $4.60/MMBtu, for the 

base runs. Load and weather data from 2006 were used. The following five scenarios were created, with penetrations by energy: 

• No Renewables—0% wind, 0% solar

• TEPPC—9.4% wind, 3.6% solar

• High Wind—25% wind, 8% solar

• High Solar—25% solar, 8% wind

• High Mix—16.5% wind, 16.5% solar. 

Table ES-1 shows installed capacities. NREL’s Regional Energy Deployment System (ReEDS) model was used to select which 

regions were optimal locations for siting the wind and solar based on resources, load, and transmission (Short et al. 2011). We 

used the commercial production simulation tool PLEXOS to model unit commitment, dispatch, and power flow for the system 

for a year. The power flow was an optimal direct current (DC) power flow, respecting transmission constraints and using power 

transfer distribution factors, not a simplified pipeline model. We added capacity to interfaces with high shadow prices and 

iterated until all shadow prices were within a consistent cutoff. The shadow price is the marginal value of relaxing the interface 

limit constraint. It defines the potential value of new transmission along each interface (but not the cost). The nearly 40 BAs in 

the Western Interconnection were modeled using the 20 WECC Load and Resource Subcommittee zones, which were the most 

granular we could obtain from WECC. The production simulation was run zonally so that collector systems would not need 

to be designed for each plant. This means that we assumed that sufficient intrazonal transmission was built for each plant and 

ignored local congestion that could result in curtailment.

Table ES-1. Installed Solar and Wind Capacity and Average Capacity Factor for Each State for Each Scenario 

TEPPC

Rooftop PV Utility Scale PV CSP Wind Total

State Capacity 

(MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF

Arizona 1,171 22% 472 43% 3,681 30% 5,324 30%

California 3,545 25% 3,221 44% 7,299 30% 14,065 32%

Colorado 1,342 20% 169 37% 3,256 29% 4,767 27%

Idaho 523 27% 523 27%

Montana 838 34% 838 34%

Nevada 304 22% 334 42% 150 25% 788 31%

New Mexico 140 27% 156 39% 494 28% 790 30%

Oregon 4,903 26% 4,903 26%

South Dakota

Texas

Utah 571 20% 323 31% 894 24%

Washington 4,652 27% 4,652 27%

Wyoming 1,784 42% 1,784 42%

Total 7,074 23% 4,352 43% 27,900 29% 39,326 30%
8

High Solar

Rooftop PV Utility Scale PV CSP Wind Total

State Capacity 

(MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF

Arizona 4,498 19% 9,570 23% 9,644 42% 270 33% 23,982 30%

California 9,006 18% 14,258 23% 9,197 43% 5,203 33% 37,663 28%

Colorado 1,127 18% 4,437 22% 1,440 35% 3,617 31% 10,620 27%

Idaho 3 15% 2 16% 583 28% 588 28%

Montana 25 15% 34 17% 988 35% 1,047 34%

Nevada 772 19% 6,503 24% 672 40% 150 25% 8,098 25%

New Mexico 943 20% 2,874 24% 574 38% 644 32% 5,034 26%

Oregon 101 14% 126 21% 4,665 26% 4,892 26%

South Dakota 4 17% 6 19% 330 37% 340 37%

Texas 233 20% 335 23% 568 22%

Utah 2,132 17% 3,759 21% 323 31% 6,214 20%

Washington 405 13% 759 19% 4,952 27% 6,116 25%

Wyoming 10 18% 18 21% 1,634 43% 1,662 42%

Total 19,261 18% 42,680 23% 21,526 42% 23,357 31% 106,824 27%

High Wind

Rooftop PV Utility Scale PV CSP Wind Total

State Capacity 

(MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF

Arizona 1,975 19% 2,330 25% 3,303 43% 4,941 30% 12,548 31%

California 4,875 18% 5,372 25% 2,469 45% 11,109 30% 23,824 28%

Colorado 1,059 18% 1,128 22% 169 37% 6,226 35% 8,581 31%

Idaho 3 15% 2 16% 1,333 29% 1,338 29%

Montana 22 15% 34 17% 6,658 36% 6,714 36%

Nevada 398 19% 344 22% 439 42% 3,270 31% 4,452 30%

New Mexico 172 20% 209 27% 156 39% 4,784 38% 5,321 37%

Oregon 91 14% 101 22% 5,473 26% 5,665 26%

South Dakota 4 17% 6 19% 2,640 36% 2,650 36%

Texas 76 20% 122 27% 198 24%

Utah 361 17% 489 21% 1,343 32% 2,193 27%

Washington 371 13% 492 20% 5,882 27% 6,745 26%

Wyoming 9 18% 18 21% 10,184 43% 10,211 43%

Total 9,417 18% 10,647 24% 6,536 43% 63,840 34% 90,439 32%
9

High Mix

Rooftop PV Utility Scale PV CSP Wind Total

State Capacity 

(MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF Capacity (MW) CF

Arizona 3,655 19% 5,394 25% 9,374 42% 1,440 32% 19,863 33%

California 8,412 18% 9,592 23% 3,594 44% 6,157 31% 27,754 26%

Colorado 1,127 18% 1,653 22% 169 37% 4,396 33% 7,344 29%

Idaho 3 15% 2 16% 1,093 29% 1,098 29%

Montana 25 15% 34 17% 4,288 36% 4,347 36%

Nevada 772 19% 3,282 26% 562 40% 1,560 32% 6,177 28%

New Mexico 943 20% 1,280 27% 298 40% 3,134 38% 5,654 33%

Oregon 101 14% 126 21% 5,413 26% 5,640 26%

South Dakota 4 17% 6 19% 1,950 36% 1,960 36%

Texas 208 20% 193 25% 401 22%

Utah 1,204 17% 1,216 22% 683 33% 3,102 22%

Washington 405 13% 709 19% 5,762 27% 6,876 26%

Wyoming 10 18% 18 21% 7,244 44% 7,272 44%

Total 16,870 18% 23,504 24% 13,997 42% 43,118 34% 97,489 30%

Note: CF, capacity factor

Operations of the entire Western Interconnection were modeled in detail in PLEXOS. We ran a day-ahead (DA) unit 

commitment for all generation using DA wind and solar forecasts. Coal and nuclear units were committed during the DA 

market. We next ran a 4-hour-ahead (4HA) unit commitment to commit CC and gas steam units, using 4HA wind and solar 

forecasts. Finally, we ran a real-time economic dispatch on a 5-minute interval to dispatch all units (i.e., gas CT and internal 

combustion units were allowed to start during the real-time dispatch). 

Load forecasts were assumed to be perfect because we lacked a consistent set of load forecasts; as a result, all the uncertainty 

in operations came from wind and solar. This assumption may result in putting more of a burden on wind/solar than is 

realistic. Variability, on the other hand, came from both load and wind/solar.

Three types of operating reserves were held: contingency, regulating, and flexibility (or load-following). Contingency 

reserves were unchanged with wind and solar because no wind or solar plant was the single largest contingency. Regulating 

reserves covered 1% of load and 95% of the 10-minute forecast errors of wind and PV. Increases in regulation requirements 

were modest in the high-penetration scenarios: up to 10% greater than in the No Renewables Scenario. Finally, flexibility 

reserves, specifically to address load-following needs for wind and PV, were held to cover 70% of the 60-minute forecast 

errors of wind and PV. 

We conducted statistical analysis to examine the geographic diversity of wind, solar, and load. We investigated monthly, 

diurnal, hourly, and subhourly variability to determine increased ramping needs and correlations between load, wind, and PV. 

Extreme event analysis determined maximum ramping needs and tail events.

Production simulation models are not a perfectly accurate representation of operations. As much as possible, we used WECC 

TEPPC assumptions, data, and scenarios because they have been widely vetted. It is important to note the following:

• Most of the Western Interconnection (except California and Alberta) today operates on the basis of a combination of 

short-term and long-term bilateral contracts. This information is confidential and could not be used in this study. As a 

result, the grid was assumed to be operated on the basis of least-cost economic dispatch.
10

• Most of the Western Interconnection today primarily uses contractual obligations to schedule transmission. Transmission 

that is not accessible to other generation might be available. In this study, we did not model these contracts; instead, 

we assumed that existing available transmission capacity was used in a way that minimized production costs across the 

Western Interconnection. 

What are the impacts of these assumptions? If a bilateral contract results in operating a less economic plant, that increases 

production cost. It might also result in more wind/PV curtailment or less flexibility available to balance the system, which 

could increase cycling. If sufficient transmission capacity is not available, that might also result in more wind/PV curtailment.

Key Findings

Our analysis in WWSIS-2 yielded a tremendous amount of noteworthy results, which are detailed in the main report. All study 

results are in 2011 nominal dollars. Under the scenarios studied, we found the following for the Western Interconnection:

• High penetrations of wind and solar increase annual wear-and-tear costs from cycling by $35–$157 million1. This represents 

an additional $0.47–$1.28/MWh of cycling costs for the average fossil-fueled generator. Cycling diminishes the production 

cost reduction of wind and solar by $0.14–$0.67/MWh, based on the specific system and generator characteristics modeled. 

These costs are a small percentage of annual fuel displaced across the Western Interconnection (approximately $7 billion) 

and the reduction in fuel costs ($28–$29/MWh of wind and solar generated). The costs are, however, significant compared 

to the average steady-state VOM and cycling costs of fossil-fueled plants ($2.43–$4.68/MWh, depending on scenario). 

Production costs do not include the capital or PPA costs to construct power plants or transmission.

• CO2, NOX, and SO2 emissions impacts resulting from wind- and solar-induced cycling of fossil-fueled generators are a 

small percentage of emissions avoided by the wind and solar generation. Wind- and solar-induced cycling has a negligible 

impact on avoided CO2 emissions. Wind- and solar-induced cycling will cause SO2 emissions reductions from wind and 

solar to be 2%–5% less than expected and NOx emissions reductions to be 1%–2% larger than expected. From a fossilfueled generator perspective, this cycling can have a positive or negative impact on CO2, NOX, and SO2 emissions rates.

• Solar tends to dominate variability challenges for the grid; wind tends to dominate uncertainty challenges. Both of these 

challenges can be mitigated. Because we know the largest component of solar variability, the path of the sun through the 

sky, we can plan for this in the unit commitment. The DA wind forecast error can be mitigated with a 4HA commitment of 

gas units to take advantage of the improved forecasts.

• Although wind and solar affect the grid in very different ways, their impacts on system-wide production costs are 

remarkably similar.

Wind and Solar Displace Primarily Gas Generation and Increase

Coal Ramping

As the quantity of resources with zero or very low marginal cost (such as wind and solar, but also possibly hydropower 

[hydro] or nuclear) increases, the new resources displace higher-cost resources (such as gas). The new resources can, however, 

also start to displace more traditional low-cost resources (such as coal). Figure ES-6 shows the dispatch stacks in the summer, 

depicting the high loads that lead the increased wind/solar to displace mostly gas CC units. The significant solar output in the 

High Solar Scenario, though, resulted in some displacement of coal generation even in the summer.

The impacts on other resources were amplified in the spring, when loads are low and both wind and solar generation are high. 

Figure ES-7 shows the most challenging week, defined by the minimum net load condition (net load is load minus wind minus 

solar). In the High Wind Scenario, the significant wind on March 29 displaced nearly all the gas output and severely cut into 

the coal output. Some wind and PV was curtailed, as shown by the dashed line in the dispatch stack on March 29 and 30. The 

curtailment occurred when the other types of generation hit their minimum generation levels. Coal was cycled, but without any 

periodicity and relatively slowly over days. The High Solar Scenario had a very different impact. Solar generation was high 

enough at midday to lead to significant curtailment of wind/PV and ramping of coal up and down on a daily basis. Impacts 

from wind- and solar-induced cycling are likely to be greater during the spring than during the summer.

1

 The low and high ends of this range give an uncertainty range for cycling costs and represent application of the lower-bound and 

upper-bound cycling costs, respectively. The high end of the uncertainty range is an overestimate because of the method used.
11

Curtailment

Wind

PV

CSP

Storage

Other

Gas CT

Gas CC

Hydro

Geothermal

Coal

Nuclear

0

50

100

150

Jul 24 Jul 25 Jul 26 Jul 27 Jul 28 Jul 29 Jul 30 Jul 31

Generation (GW)

Curtailment

Wind

PV

CSP

Storage

Other

Gas CT

Gas CC

Hydro

Geothermal

Coal

Nuclear

0

50

100

150

Jul 24 Jul 25 Jul 26 Jul 27 Jul 28 Jul 29 Jul 30 Jul 31

Generation (GW)

0

50

100

150

Jul 24 Jul 25 Jul 26 Jul 27 Jul 28 Jul 29 Jul 30 Jul 31

Generation (GW)

Curtailment

Wind

PV

CSP

Storage

Other

Gas CT

Gas CC

Hydro

Geothermal

Coal

Nuclear

Figure ES-6. Five-minute dispatch stacks for the (top) TEPPC, (middle) High Wind,

and (bottom) High Solar Scenarios for a week in July
12

Mar 25 Mar 26 Mar 27 Mar 28 Mar 29 Mar 30 Mar 31 Apr 01

0

25

50

75

100

Generation (GW)

Curtailment

Wind

PV

CSP

Storage

Other

Gas CT

Gas CC

Hydro

Geothermal

Coal

Nuclear

0

25

50

75

100

Mar 25 Mar 26 Mar 27 Mar 28 Mar 29 Mar 30 Mar 31 Apr 01

Generation (GW)

Curtailment

Wind

PV

CSP

Storage

Other

Gas CT

Gas CC

Hydro

Geothermal

Coal

Nuclear

es Word le.

0

25

50

75

100

Mar 25 Mar 26 Mar 27 Mar 28 Mar 29 Mar 30 Mar 31 Apr 01

Generation (GW)

Curtailment

Wind

PV

CSP

Storage

Other

Gas CT

Gas CC

Hydro

Geothermal

Coal

Nuclear

Figure ES-7. Five-minute dispatch stacks for the (top) TEPPC, (middle)

High Wind, and (bottom) High Solar Scenarios for a week in March 

Note: This week represented the minimum net load condition.
13

Despite these challenges, the 5-minute production simulation results showed that the system can operate and balance load and 

generation. Operational results for contingency, regulating, and flexibility reserves were examined, and issues were minimal. 

There were no regulating reserve violations and very few contingency reserve violations. Figure ES-8 shows that wind and 

solar mostly displace gas CC generation. Displacement of coal increased with increasing penetrations of wind, because gas 

already tends to be decommitted or backed down at night when there are high levels of wind. 

The dispatch stacks showed that the system used the least expensive methods for flexibility from various types of generators 

to serve load and reserves. In the summer, capacity was required more than flexibility. In the spring, balancing the load with 

high instantaneous wind/solar penetrations required a lot of flexibility. Ramping hydro within its constraints was one source 

for flexibility; wind/PV curtailment was another. Cycling of fossil-fueled plants was a third, and we delve into that here. The 

High Solar Scenario ramped coal up and down on a daily basis. In the High Wind Scenario, coal was shut down and restarted 

on a weekly or longer frequency, especially during the low net load event on March 29. In all scenarios, CTs are shut down 

and restarted frequently, running for only several hours per start.

Over the course of 1 year, Figure ES-9 shows the cycling impact by plant type by scenario. Coal starts do not change 

appreciably but the High Wind Scenario decreased the average coal runtime per start by a third and the High Solar Scenario 

increased the number of ramps by an order of magnitude compared to the No Renewables Scenario. The High Wind Scenario 

required somewhat less ramping of coal units compared to the High Solar Scenario.

Wind and solar generation displaced primarily gas generation, based on an average gas price of $4.60/MMBtu.

HiSolar

HiMix

HiWind

TEPPC

0 25 50 75 100

Generation (%)

Scenario

Nuclear

Coal

Geothermal

Hydro

Gas CC

Gas CT

Other

Steam

Figure ES-8. Generation displaced by wind/solar, compared to the No Renewables Scenario

Increasing wind/solar first increased and then decreased the number of CC starts. Even moderate penetrations halved the

CC runtime per start, where it basically remained even at high penetrations. CC ramps actually decreased in the highpenetration scenarios.

Wind causes a significant reduction in CT cycling (and generation). The High Solar Scenario, however, shows more CT 

capacity started compared to the No Renewables Scenario, partly because of the correlation of evening peak load with 

decreased PV output at sunset.
14

0

30,000

60,000

90,000

Number of ramps

0

100

200

300

Hours online per start

0

500

1,000

Coal Gas CC Gas CT

Coal Gas CC Gas CT

Coal

(a) (b)

(c)

Gas CC Gas CT

Capacity started (GW)

Scenario

NoRenew

TEPPC

HiWind

HiMix

HiSolar

Figure ES-9. (a) Capacity started, (b) average number of hours online per start (must-run CTs 

have been excluded), and (c) total number of ramps for each plant type by scenario for 1 year

To determine the importance of considering wear-and-tear start costs during optimization, we ran the High Wind Scenario 

without including wear-and-tear start costs (but including start fuel costs) to compare to the original High Wind Scenario. 

Although this had almost no impact on annual generation from different unit types, it had a very significant impact on the 

number of starts at CC and CTs, which have very low start fuel costs. This demonstrates that it is important to consider wearand-tear start costs during optimization.

Figure ES-10 gives a more detailed look into the starts and ramps. The solid line shows the committed coal capacity and the 

shaded area shows the dispatched capacity. The white area between the solid line and the shaded area illustrates how far the 

coal capacity has been backed down. In the No Renewables and TEPPC Scenarios, there is little change in coal commitments 

and the coal plants are typically running at or near full output, with an exception during the minimum net load day of March 

29. In the high-penetration scenarios in the spring, coal capacity is shut down approximately each week, and the coal is 

ramped up and down each day, especially with high penetrations of solar. In the summer, coal is ramped very little except for 

some ramping in the High Solar Scenario during the day.
15

0

10

20

30

0

10

20

30

0

10

20

30

0

10

20

30

0

10

20

30

NoRenew TEPPC HiWind HiMix HiSolar

03/25 03/26 03/27 03/28 03/29 03/30 03/31 04/01 07/24 07/25 07/26 07/27 07/28 07/29 07/30 07/31

Generation (GW)

Spring Summer

Figure ES-10. (Solid line) capacity committed and (shaded area)

dispatched for coal units during March and July sample weeks
16

Wind- and Solar-Induced Cycling Affects Fossil-Fueled Plant Operations 

and Maintenance Costs

Figure ES-11 shows the production cost (operational cost of meeting load in the Western Interconnection in 2020) of each 

scenario. Production costs do not include any capital costs, except capitalized maintenance caused by cycling or noncyclic 

operation. The production cost was dominated by fuel costs, assuming an average natural gas price of $4.60/MMBtu and 

a zero carbon price. Noncyclic VOM costs comprise about a tenth of the total production cost. Cycling VOM costs (starts, 

start fuel, and ramping costs) were all a small percentage of the total production cost. They range from 1.5% of the total 

production cost in the No Renewables Scenario using lower-bound cycling costs to 7% in the High Solar Scenario using 

upper-bound cycling costs.

NoRenew TEPPC HiWind HiMix HiSolar NoRenew TEPPC HiWind HiMix HiSolar

0

5

10

15

Production cost (billion $)

Component

Ramping VOM

Start-up fuel

Start-up VOM

Noncyclic VOM

Fuel

Lower Bound Upper Bound

Figure ES-11. Production cost for each scenario showing the (left) lower and (right) upper bound for the cycling costs

0

200

400

600

800

NoRenew TEPPC HiWind HiMix HiSolar NoRenew TEPPC HiWind HiMix HiSolar

Cycling cost (million $)

Component

Ramping VOM

Start-up fuel

Start-up VOM

Lower Bound Upper Bound

Figure ES-12. Production cost components resulting from cycling, showing

the (left) lower and (right) upper-bound wear-and-tear costs for each scenario 

Note: Cost components have been broken down into starts, start fuel, and ramping costs.
17

Figure ES-12 shows only the cycling portion of these same costs. The cycling costs range from about $270 million in the 

No Renewables Scenario using the lower-bound cycling costs to about $800 million in the High Solar Scenario using the 

upper-bound cycling costs. When wind and solar are added to the system, cycling costs increase by $35–$157 million, or 

13%–24%. Interestingly, the High Mix Scenario has a higher wind/solar penetration but lower cycling costs than the TEPPC 

Scenario. There is not necessarily a monotonic increase in cycling costs with wind/solar penetration. In terms of cycling 

costs, there may be a big step in going from 0% to 13% wind/solar, but a much smaller step in going from 13% to 33%.

Wind- and solar-induced cycling increases average fossil-fueled plant O&M costs by $0.47–$1.28/MWh

in the high-penetration scenarios. Gas CTs bear the highest wear-and-tear cycling costs.

We first examine these costs from the perspective of the fossil-fueled plants. Figure ES-13 divides the cycling costs shown 

in Figure ES-12 by each MWh of fossil-fueled generation. These cycling O&M costs increase from $0.45–$1.07/MWh in 

the No Renewables Scenario to $0.63–$1.51/MWh in the TEPPC Scenario, where the ranges reflect the uncertainty in the 

wear-and-tear costs. This cycling wear and tear increases to $0.92–$2.36/MWh in the high-penetration scenarios. Table ES-2 

shows the cycling cost impacts of wind and solar for each scenario.

Table ES-2. Increase in Cycling Cost (Compared to No Renewables Scenario)

Scenario Total (Million $) Per MWh Wind/Solar Generation Per MWh of Fossil-Fueled Generation

TEPPC 42–108 $0.41–$1.05/MWh $0.18–0.44/MWh

High Wind 50–127 $0.20–$0.50/MWh $0.52–1.24/MWh

High Mix 35–95 $0.14–$0.38/MWh $0.47–1.14/MWh

High Solar 52–157 $0.22–$0.67/MWh $0.50–1.28/MWh

NoRenew TEPPC HiWind HiMix HiSolar NoRenew TEPPC HiWind HiMix HiSolar

0.0

0.5

1.0

1.5

2.0

2.5

Cycling cost ($/MWh)

Lower Bound Upper Bound

Component

Ramping VOM

Start-up fuel

Start-up VOM

Figure ES-13. Cycling cost, showing the (left) lower- and (right) upper-bound wear-and-tear costs for each scenario

Note: These cycling costs are defined as the total system-wide cycling costs per MWh of fossil-fueled generation.
18

Figure ES-14 further disaggregates the cycling cost by plant type. For confidentiality reasons, only the lower bounds can 

be shown. Note, however, that although the absolute magnitudes of costs are higher with the upper bounds, the relative 

comparisons discussed here also hold true for the upper bounds. CTs (must-run CTs were excluded to delve into these 

impacts) bear the brunt of the wear-and-tear costs (Figure ES-14, right). Notably, these cycling costs actually decrease at low 

wind/solar penetrations (TEPPC Scenario) and do not change in the High Wind Scenario from the No Renewables Scenario. 

For the coal plants (Figure ES-14, left), cycling costs are only slightly affected. For the CC plants (Figure ES-14, center), 

cycling costs increase with increasing wind/solar penetrations.

0

5

10

NoRenew

TEPPC

HiWind

HiMix

HiSolar

NoRenew

TEPPC

HiWind

HiMix

HiSolar

NoRenew

TEPPC

HiWind

HiMix

HiSolar

Lower-bound cycling cost ($/MWh)

Coal Gas CC Gas CT

Component

Ramping VOM

Start-up fuel

Start-up VOM

Figure ES-14. Lower-bound cycling cost for (left) coal,

(center) gas CC units, and (right) gas CTs (excluding the must-run CTs) 

Note: Total, system-wide, lower-bound cycling costs were disaggregated by plant type and divided by MWh of generation of that plant type.

Cycling Increases Production Costs Slightly

We next examine these costs from a system perspective. When we compared the production cost of each scenario to the No 

Renewables Scenario, we saw a decrease of $3.34–$3.43 billion at low penetrations (TEPPC Scenario) and $7.12–$7.65 

billion in the high-penetration scenarios (see Section 6). This change in production cost is dominated by displaced fuel costs. 

Dividing this production cost reduction by the amount of wind and solar energy delivered yielded a production cost reduction 

of $32.6–$33.2/MWh in the TEPPC Scenario and $29.4–$30.6/MWh in the high-penetration scenarios (see Table ES-3 for 

details). Figure ES-15 breaks down the production cost reduction into cost components. Cycling costs (shown by the positive 

values) offset $0.14–$0.67 of the fuel and VOM reduction per MWh of wind and solar generated in the high-penetration 

scenarios. This production cost reduction does not reflect fixed capital costs or PPA costs. Utility planners conducting a costbenefit analysis of wind and solar might want to weigh such fixed capital costs against production costs, but that analysis is 

not conducted here.

Cycling costs increase by $0.14–$0.67 per MWh of wind and solar generated in the high-penetration

scenarios, based on the specific system characteristics of the Western Interconnection.
19

TEPPC HiWind HiMix HiSolar

−30

−20

−10

0

Production cost change ($/MWh)

Component

Ramping VOM

Start-up fuel

Start-up VOM

Noncyclic VOM

Fuel

Lower Bound Upper Bound

TEPPC HiWind HiMix HiSolar

Figure ES-15. The change in production cost for each scenario relative to the No Renewables Scenario,

per MWh of wind and solar generation, for the (left) lower-bound and (right) upper-bound wear-and-tear costs

Note: Production costs do not include any fixed capital or PPA costs.

Table ES-3. Change in Production Cost, Compared to No Renewables Scenario

Scenario Production Cost Reduction

(Billion $)

Production Cost Reduction

($ per MWh of Wind and Solar Generated)

TEPPC 3.34–3.43 32.6–33.2

High Wind 7.48–7.56 29.4–29.7

High Mix 7.59–7.65 30.2–30.4

High Solar 7.12–7.23 30.2–30.6

Note: Production costs do not include any fixed capital or PPA costs.

CO2, NOX, and SO2 Emissions Reductions Are Significantly Greater Than 

Cycling Emissions

Figure ES-16 (left) shows the total CO2 emissions for each scenario. Ramping had no significant impact on CO2 emissions, 

so those estimates are not shown. The start-up CO2 emissions (shown by the thin, dark green line at the top of each bar) 

were negligible in all cases. Figure ES-16 (right) shows the CO2 emissions saved by each MWh of wind/solar. Avoided 

CO2—considering part-load, ramping, and starts—was 1,100 lb/MWh to 1,190 lb/MWh of wind and solar produced in the 

high-penetration scenarios (see Table ES-4). CO2 emissions from starts were negligible. We also calculated the part-load 

penalty—which was the incremental CO2 emissions from part-loading—as negligible. This emissions analysis reflects 

aggregate emissions across the Western Interconnection. Any specific plant might have lower or higher emissions than shown 

here. Because wind tended to displace more coal compared to solar, and because coal emission rates of CO2, NOx, and SO2

are higher than those of gas, higher penetrations of wind resulted in higher levels of avoided emissions.

Starts, ramps, and part-loading had a negligible impact on CO2 emissions reductions of wind and solar. 
20

0

200

400

600

800

TEPPC HiWind HiMix HiSolar

 C

O2 emissions (billion lb)

0

500

1,000

1,500

HiWind HiMix HiSolar

C

O2 emission savings (lb/MWh)

Full−load rate

Part−load adjustment

Start

Start

Operation

Figure ES-16. CO2 emissions by scenario: (left) absolute CO2 emissions for operation and starts

and (right) CO2 emission reductions compared to the No Renewables Scenario, separated into the

constant emissions rate assumption and adjustments for part-load and starts

Note: Ramping emissions are excluded because they have no significant impact on CO2 emissions.

Table ES-4. Emissions Avoided per MWh of Wind and Solar—Considering Part-Load, Ramping, and Start Impacts

Scenario Avoided CO2 (lb/MWh) Avoided NOx (lb/MWh) Avoided SO2 (lb/MWh)

High Wind 1,190 0.92 0.56

High Mix 1,150 0.80 0.44

High Solar 1,100 0.72 0.35

Note: Part-load impacts were not studied for SO2 because of inadequate data.

From the fossil-fueled plant perspective, average CO2 emission rates of coal, CCs, or CTs change only slightly with wind and 

solar as shown in Figure ES-17 (top). Figure ES-17 (bottom) shows that adding wind and solar can positively or negatively 

affect emissions rates, depending on plant type and scenario. Generally for coal and CCs, wind/solar improves emissions 

rates by up to 1%. The largest negative impact of wind- and solar-induced cycling is in the High Wind Scenario on the CTs 

where the emissions rate increases by 2%. This is on average; individual units might be more or less affected.

Wind- and solar-induced cycling can have a small positive or negative impact on

NOx and SO2 emissions rates, and it depends on the pollutant and mix of wind and solar.

Figure ES-18 shows the analysis for NOx emissions. There was a negligible impact of starts on NOx. Ramping reduced the 

avoided NOx by 2% to 4%. This is shown in Figure ES-18 (right) as a small negative contribution. Part-loading impacts, on 

the other hand, increased avoided NOx by 4% to 6%. On average, coal units in the West emit less NOx per MWh of generation 

at part-load. The net impact of considering cycling improved avoided NOx emissions from wind/solar by 1% to 2%.
21

−1

0

1

2

 C

O2 emissions rate change (%)

Coal Gas CC Gas CT

0

500

1,000

1,500

2,000

 C

O2 emission rate (lb/MWh)

Coal Gas CC Gas CT

NoRenew

TEPPC

HiWind

HiMix

HiSolar

Figure ES-17. (Top) average CO2 emission rates by plant type (defined as CO2 emissions divided by MWh of coal, CC, 

or CT generation) for each scenario and (bottom) change in emissions rate compared to the No Renewables Scenario

0

300

600

900

TEPPC HiWind HiMix HiSolar

N

Ox emissions (million lb)

0.00

0.25

0.50

0.75

1.00

HiWind HiMix HiSolar

 N

Ox emission savings (lb/MWh)

Full−load rate

Part−load adjustment

Ramp

Start

Start

Ramp

Operation

Figure ES-18. NOX emissions by scenario: (left) absolute NOX emissions for operation, ramps,

and starts, and (right) NOX emission reductions compared to the No Renewables Scenario, separated

into the constant emissions rate assumption and adjustments for part-load, ramps, and starts

Figure ES-19 shows that average NOx emission rates for different plants can also be positively or negatively affected by 

wind/solar. Wind- and solar-induced cycling impacts on NOx emissions rates are relatively small. Impacts on coal units are 

negligible, but high-penetration scenarios increase overall CC NOx emission rates by approximately 5%. CTs show the largest 

impacts. The scenarios with a high wind-to-solar ratio show reductions in CT emissions rates by approximately 10% and the 

scenario with a high solar-to-wind ratio shows increases in CT emissions rates by approximately 10%. This is on average; 

individual units might be more or less affected.
22

Figure ES-20 shows the emissions analysis for SO2. Because there were inadequate data to create SO2 emission part-load 

curves, part-load impacts were not studied for SO2. Ramping impacts on avoided SO2 were modest for the high-penetration 

scenarios, reducing avoided SO2 by 2% to 5%. Start-up emissions affected the avoided emissions rates by significantly less 

than 1%. The net impact of considering starts and ramps lessened avoided SO2 from wind/solar by 2% to 5%.

Figure ES-21 shows the SO2 emissions rates for coal plants. The High Wind Scenario improves the SO2 emission rate by 1%; 

the High Solar Scenario increases the SO2 emission rate by 2%. N

Ox emission rate change (%)

N

Ox emission rate (lb/MWh)

NoRenew

TEPPC

HiWind

HiMix

HiSolar

0

1

2

3

0.00

0.02

0.04

0.06

0.08

0.00

0.05

0.10

0.15

0.20

−10

−5

0

5

10

Coal Gas CC Gas CT

Coal Gas CC Gas CT

Figure ES-19. (Top) average NOX emissions rates by plant type for each scenario

and (bottom) change in NOX emissions rate from the No Renewables Scenario

Note: Observe the difference in y-axes.

0.0

0.2

0.4

HiWind HiMix HiSolar

 SO2 emission savings (lb/MWh)

0

200

400

600

TEPPC HiWind HiMix HiSolar

 SO2 emissions (million lb)

Start

Ramp

Operation Start

Ramp

Operation

Figure ES-20. SO2 emissions by scenario: (left) absolute SO2 emissions for operation, ramping,

and starts, and (right) SO2 emission reductions compared to the No Renewables Scenario, separated

into the constant emissions rate assumption and adjustments for ramps and starts.
23

 SO2 emission rate change (%)

SO2 emission rate (lb/MWh)

NoRenew

TEPPC

HiWind

HiMix

HiSolar

0.0

0.5

1.0

1.5

2.0

−1

0

1

2

Coal Coal

Figure ES-21. (Left) average SO2 emission rate for each scenario and

(right) change in SO2 emission rate from No Renewables Scenario

Sometimes, transmission congestion or minimum generation levels of the thermal plants result in a need for curtailment. We 

curtailed wind and solar in these situations. Wind/solar curtailment was highest in the High Wind and High Solar Scenarios, 

and much reduced (to below 2%) in the High Mix Scenario (see Figure ES-22). High solar penetrations resulted in the highest 

curtailment, but curtailment was still modest (below 5%). High solar penetrations resulted in curtailment midday; high wind 

penetrations more frequently resulted in curtailment at night. We did not model take-or-pay contracts or production tax 

credits, which would result in a cost for wind/solar curtailment, and possibly reduced wind/solar curtailment at the expense of 

increased fossil-fueled plant cycling. Because wind/solar curtailment was low, however, we do not think a cost for wind/solar 

curtailment would change our results significantly.

0

1

2

3

4

5

TEPPC HiWind HiMix HiSolar

Curtailment (%)

Scenario

TEPPC

HiWind

HiMix

HiSolar

Figure ES-22. Curtailment as a percentage of potential wind and solar generation

The high-penetration scenarios saw the least curtailment with a balanced mix of wind and solar.
24

Gas Price Has a Greater Impact on Cycling Costs than Wind and

Solar Penetration

To understand the impacts of gas prices on the results, we modeled the High Mix and No Renewables Scenarios with 

gas prices averaging $2.30/MMBtu, $4.60/MMBtu (the core assumption), and $9.20/MMBtu. In the $2.30 case, system 

operations changed significantly because gas CC units often became cheaper than coal units. As a result, the gas CC units 

were often operated as baseload and cycled less. Adding wind and solar in all cases, however, displaced approximately onequarter coal and three-quarters gas CC generation. Figure ES-23 shows the annual generation for all unit types. 

0

100

200

300

Nuclear Coal Geothermal Hydro Gas CC Gas CT Other Steam CSP PV Wind

Generation (TWh)

Scenario

NoRenew ($2.30 gas)

NoRenew ($4.60 gas)

NoRenew ($9.20 gas)

HiMix ($2.30 gas)

HiMix ($4.60 gas)

HiMix ($9.20 gas)

Figure ES-23. Annual generation by type in the gas price sensitivities

Figure ES-24 shows the capacity started in the gas price sensitivities. This plot also illustrates that gas CC units are operated 

as baseload units in the $2.30 No Renewables Scenario, and as “peakers” (meaning that they are run for a relatively short 

period each time they are turned on) in the $9.20 cases. Coal units are started less often (and generate less power) in the $2.30 

cases because gas CC units are cheaper. 

0

1,000

2,000

3,000

Coal Gas CC Gas CT

Capacity started (GW)

Scenario

NoRenew ($2.30 gas)

NoRenew ($4.60 gas)

NoRenew ($9.20 gas)

HiMix ($2.30 gas)

HiMix ($4.60 gas)

HiMix ($9.20 gas)

Figure ES-24. Capacity started in the gas price sensitivities
25

Figure ES-25 shows that cycling costs are affected much more by gas price assumptions than by wind and solar penetration. 

In the $2.30 and $9.20 gas price sensitivities, adding wind and solar actually reduces the overall cycling cost slightly because 

some of the starts are displaced at various unit types. Because fossil-fueled generation is displaced, though, adding wind and 

solar increases the cycling cost per MWh of fossil-fueled generation by $0.30–$1.16, a range that is relatively consistent 

regardless of gas price. Cycling costs increase at fossil-fuel units despite the reduction in overall cycling costs because fossilfuel unit operation is significantly reduced in the High Mix Scenario. 

0

400

800

1,200

NoRenew ($2.30 gas)

NoRenew ($4.60 gas)

NoRenew ($9.20 gas)

HiMix ($2.30 gas)

HiMix ($4.60 gas)

HiMix ($9.20 gas)

NoRenew ($2.30 gas)

NoRenew ($4.60 gas)

NoRenew ($9.20 gas)

HiMix ($2.30 gas)

HiMix ($4.60 gas)

HiMix ($9.20 gas)

Cycling cost (million $)

Lower Bound Upper Bound

Component

Ramping VOM

Start-up fuel

Start-up VOM

Figure ES-25. Cycling cost in the gas price sensitivities showing (left) lower and (right) upper bounds

The price of gas has a much greater impact on system-wide cycling costs than

the addition of wind and solar. Adding wind and solar affects the cycling cost

per MWh at thermal units similarly under various gas price assumptions. 

Solar Dominates Variability and Wind Dominates Uncertainty

Many integration studies have investigated high wind penetrations (EnerNex 2011; Charles River Associates 2010; New York 

Independent System Operator 2010; Intelligent Energy 2009; GE Energy 2008; United Kingdom Department of Enterprise, 

Trade, and Investment 2008; EnerNex 2006). Fewer studies have examined high penetrations of solar—in part because high 

solar penetrations have only recently become a concern and in part because of lack of data to model solar well (Orwig et al. 

2012; Navigant Consulting et al. 2011). 

Utilities have concerns about whether fast-moving clouds over PV plants might result in high variability. PV has two 

characteristics that affect this variability: (1) the size of the plant and (2) the number of plants. A small plant, such as a 

rooftop PV system, might see high variability from clouds, but the impact of a small system’s variability on the bulk power 

system is minimal. Impacts could be seen on a distribution level, but WWSIS-2 focuses only on impacts at the transmission 

level. A large plant can have a higher impact on the bulk power system, but its larger area helps to smooth out the variability. 

With additional PV plants, the geographic diversity of the plants and the improbability of cloud fronts obscuring all PV plants 

at the same time result in further smoothing of this variability, as shown in Figure ES-26.
26

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

6:00 8:00 10:00 12:00 14:00 16:00 18:00

Normalized output

One Plant

Two plants

Six Plants

25 plants

So. Calif.

Figure ES-26. Normalized power output for increasing aggregation of PV in Southern California for a partly cloudy day

Solar dominates variability extreme events. At a system level, however, most of this variability

comes from the known path of the sun through the sky, instead of from fast-moving clouds.

The sunrise and sunset do, however, affect variability significantly with high penetrations of solar. High penetrations of solar 

dominate variability on a 5-minute and an hourly basis, and extreme events are because of sunrise and sunset (see Figure 

ES-27). Although extreme variability events increase, they can also be relatively easily mitigated because we know when the 

sun sets and rises every day. In fact, because we know the path of the sun through the sky for every hour of the year, system 

operators can accommodate much of this diurnal variability. We removed this known diurnal variability when we calculated 

reserves for solar (see Section 5).

5-minute dierence in power (MW)

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0

Density

-3,000 -2,000 -1,000 0 1,000 2,000 3,000 -3,000 -2,000 -1,000 0 1,000 2,000 3,000

0.01

0.008

0.006

0.004

0.002

0

Density

5-minute dierence in power (MW)

Load only

HiWind

HiSolar

Load only

HiWind

HiSolar

Figure ES-27. (Left) distributions of 5-minute changes in power output for load only and the net load

for the High Wind and High Solar Scenarios and (right) an enlargement of the tails of the distribution
27

Wind, on the other hand, led to greater uncertainty. The high penetrations of wind led to greater extremes in the DA 

forecast error, as shown in Figure ES-28. Because the 4HA wind forecasts are much more accurate, shown by the tighter 

distribution in Figure ES-29, this uncertainty in the DA time frame can be mitigated with a 4HA unit commitment of CCs 

and CTs. Similarly, higher penetrations of wind led to higher reserve requirements (Ibanez et al. 2013) than those with high 

penetrations of solar because reserve requirements for wind/solar are driven by short-term uncertainty.

Wind dominated the uncertainty extreme events in the DA forecast. We can mitigate this

by committing gas CCs and CTs in the 4HA time frame, in which forecasts are more accurate.

0

0.05

0.1

0.15

0.2

0.25

Density

TEPPC

HiWind

HiMix

HiSolar

Wind & PV day-ahead forecast error (MW)

-12,000 -9,000 -6,000 -3,000 0 3,000 6,000 9,000 12,000

Figure ES-28. DA wind and PV forecast error for the TEPPC, High Wind, High Mix, and High Solar Scenarios

-12,000 -9,000 -6,000 -3,000 0 3,000 6,000 9,000 12,000

Density

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

Wind & PV 4 hour-ahead forecast error (MW)

TEPPC

HiWind

HiMix

HiSolar

Figure ES-29. 4HA wind and PV forecast error for the TEPPC, High Wind, High Mix, and High Solar Scenarios
28

Conclusions

We conducted a detailed operational analysis of the Western Interconnection, focusing on the wear-and-tear costs and 

emissions impacts from cycling of fossil-fueled plants. Detailed wear-and-tear costs and forced outage rate impacts were 

determined for seven categories of plants for starts, ramps, and noncyclic operation. Emissions impacts were obtained for 

every power plant for starts, ramps, and part-load operation. Subhourly impacts were examined using a unit commitment and 

an economic dispatch model with 5-minute dispatch resolution.

In this study, we found that wind and solar increase annual cycling costs by $35–$157 million, or 13%–24%, across the 

Western Interconnection. Cycling costs for the average fossil-fueled plant increase from $0.45–$1.07/MWh to $0.63–$2.36/

MWh, compared to total fuel and VOM costs of $27–$28/MWh. Any specific unit could see more or less cycling and 

associated costs. Starts, not ramps, drive total cycling costs. CTs bear the brunt of the cycling costs, although CT cycling 

costs do not increase in the High Wind Scenario and are actually decreased in the TEPPC Scenario. Wind and solar lead to 

markedly increased ramping for coal generators, and coal runs fewer hours per start with high wind penetrations. Coal units 

ramp daily instead of weekly as wind/solar, especially solar, penetrations increase. Wind and solar have a relatively small 

impact on the number of starts for coal units. Wind and solar mostly displace gas CC generation and cut CC unit runtime 

per start in half. Gas CTs start and ramp less often in scenarios with high ratios of wind to solar penetration. High solar 

penetrations, on the other hand, lead to more starts, shorter run times, more ramping, and more generation for CTs. 

From a system perspective, the $35–$157 million cycling cost increase is a small percentage of the annual fuel displaced by 

wind and solar of approximately $7 billion. Each MWh of wind and solar generation displaces $29.90–$33.60 of fuel and 

VOM costs. Wind- and solar-induced cycling offsets $0.14–$0.67/MWh of this reduction in the high-penetration scenarios 

and $0.41–$1.05/MWh in the low-penetration scenario, based on the specific generator and system characteristics modeled for 

the Western Interconnection.

We found that cycling impacts on CO2 emissions are negligible. Emissions reductions of NOx are 1%–2% more than expected 

when considering cycling and part-load in detail because, on average, coal plants in the West have lower NOx emissions rates 

at part-load. Emissions reductions of SO2 are 2%–5% less than expected because of cycling.

We also compared the impacts of wind and solar, using new data sets that illuminated the subhourly variability of utility-scale 

PV. Wind and solar generation affect the system in different ways. They both mostly displace gas CC generation, but wind 

also tends to displace more coal. Solar tends to dominate variability extremes, but it can be mitigated because most of this 

variability is known and can be anticipated in the unit commitment. Wind tends to dominate uncertainty extremes because 

of tail events in the DA wind forecast error. This can be mitigated by committing gas CC units in the 4HA time frame and 

gas CTs in shorter time frames. High wind/solar penetrations result in modest curtailment—up to 5%. WWSIS-2 finds that a 

balanced mix of wind and solar reduces curtailment to less than 2%. 
29

Future Work

Even though system-wide impacts of cycling are modest, an individual unit could suffer higher than average cycling. Plant 

owners in this situation will want to know whether they should retrofit their unit or change their operations to better manage 

cycling at a lower overall cost. Ongoing work includes research on potential retrofits or operational strategies to increase the 

flexibility of fossil-fueled generators. This includes analysis of the costs and benefits of retrofitting existing plants for options 

such as lower minimum generation levels or faster ramp rates.

Additional analysis work that would illuminate the impacts of cycling and further compare wind and solar includes

the following:

• Market impacts on fossil-fueled plants: How do increased O&M costs and reduced capacity factors affect cost recovery 

for fossil-fueled plants? What market structures might need revision in a high wind and solar paradigm? How do the 

economics look for those plants that were most affected?

• Fuel-price sensitivities: How are operations and results affected by different fuel prices for coal and gas?

• Different retirement scenarios: How are operations and results affected if significant coal capacity is retired or if the 

balance of plants is flexible versus inflexible?

• Storage: Does storage mitigate cycling and is it cost effective?

• Impacts of dispersed versus centralized PV: How does rooftop versus utility-scale PV affect the grid?

• Reserves requirement testing to fine tune flexibility reserves: What confidence levels of flexibility reserves are most cost 

effective and still retain reliable grid operation?

• Scenarios with constrained transmission build-outs: If transmission is constrained, what is grid performance and how is 

cycling affected?

• Reserve-sharing options: How do different reserve-sharing options affect grid operations?

• Increased hydro flexibility and modeling assumptions: How does flexibility in the hydro fleet affect grid operations and 

what is the impact on cycling?

• Hurdle rates to represent market friction: With higher hurdle rates to mimic less BA cooperation, how are grid operations 

and cycling affected?

• Comparison of the detailed 5-minute production simulation modeling with cycling costs to hourly production simulation 

modeling without cycling costs: How much more accurate is the detailed modeling?

• Gas supply: Is additional gas storage needed? How does increased wind/solar affect gas scheduling and supply issues? 

• Market sequence: How much does the system benefit from the 4HA market?