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

Dr. Debra Lew, Dr. Greg Brinkman, and Dr. Eduardo Ibanez, NREL

Nikhil Kumar and Steven Lefton, Intertek-APTECH

Gary Jordan and Sundar Venkataraman, GE

Jack King, RePPAE

May 6–8, 2013

Funded by the U.S. DOE Wind, Solar, and Office of Electricity

NREL/PR-5500-57922
2

The Impacts of Wind- and Solar-Induced Cycling
3

Subhourly Modeling of Grid Operations

• Needed wear-and-tear costs and emissions for 

start-ups and ramps for fossil-fueled plants

o APTECH developed a wear-and-tear cost and impact 

data set based on studies of 170 plants

o NREL developed an emissions database based on 

measured emissions from every power plant

• Used commercial software PLEXOS to model grid 

operations on a 5-minute basis for the year 2020

• 50 utility and power plant experts on the 

technical review committee reviewed the data, 

methodology, and results
4

Scope of WWSIS-2 

• We modeled the western grid based on 

transmission planning models and 

methodologies of the Western Electricity 

Coordinating Council (TEPPC 2020)

o Results are specific to the grid and generator 

characteristics of the West

• We examined grid operations

o This was not a transmission planning study

o Reliability and stability are being examined in 

WWSIS-3 

• Wind and solar was sited in the United States
5

Scenarios Compared Wind and Solar

5
6

Spring Is Most Challenging for Operations

No Renewables

16.5% wind and

16.5% solar 

energy penetration
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Spring: Wind Leads to Coal Shutdowns; 

Solar Leads to Coal Ramp-downs
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Renewables Displace Gas and Some Coal

Gas prices average $4.60/mmbtu
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How are wear-and-tear costs 

impacted by cycling?
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Electricity Costs Include Capital 

and Production Costs
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Production Costs Include Cycling Costs
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33% Wind and Solar Induce 

$35–157 M of Cycling Costs

• Fossil-fueled plant perspective

o No Renewables had $300–650M of cycling costs

o Cycling costs increased by 13%–24% 

o This represents an increase of $0.5–1.3/MWh of O&M

• System perspective

o Wind/solar avoided $7–8B in production costs

o Cycling costs reduced that production cost savings by 

0.5%–2.2% 

o This represents a reduction in production cost savings 

of $0.14–0.67/MWh
13

33% High Mix Had Lower Cycling Costs 

Than 13% TEPCC Scenario
14

Gas Combustion Turbines Bear Brunt 

of Cycling Costs

Cycling costs 

sometimes decrease

Gas CCs have a 

significant increase in 

cycling costs

Note: These are the lower bound cycling costs
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Increase in Cycling and Ramping Costs

Increase in Cycling and Ramping Costs:

Scenario As a fraction of 

production cost 

savings from 

renewable generation

Per MWh renewable 

generation

Per MWh of fossilfueled generation

TEPPC 1.2% – 3.2% $0.41 – 1.05 / MWh $0.18 – 0.44 / MWh

High Wind 0.7% – 1.7% $0.20 – 0.50 / MWh $0.52 – 1.24 / MWh

High Mix 0.5% – 1.3% $0.14 – 0.38 / MWh $0.47 – 1.14 / MWh

High Solar 0.7% – 2.2% $0.22 – 0.67 / MWh $0.50 – 1.28 / MWh
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How are emissions impacted by cycling?
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Wind- and Solar-Induced Cycling Can Have 

a Positive or Negative Impact on Emissions

• 24%–26% wind and solar 

energy across the 

western grid reduces:

o CO2 by 29%–34%

o NOX by 16%–22%

o SO2 by 14%–24%

• System-wide impacts of 

cycling:

o Negligible impact (<0.2%) 

on CO2 benefit

o Improves NOX benefit by 

1%–2%

o Lessens SO2 benefit by 2%–

5%

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.4 0.9

Average NO

X emissions rate (lbs/MWh)

Fraction of maximum 

generation

Coal CC CT GasSteam
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Average CO2 Emissions Rates 

From Coal Do Not Change
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Changes in NOX Emissions Rates 

Depend on Wind/Solar Mix 
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Changes in SO2 Rates Depend on Wind/Solar Mix
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Balanced Mix of Wind and Solar 

Reduces Curtailment
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Solar Dominates Variability Extremes
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Wind Dominates Uncertainty Extremes
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4-Hour-Ahead Unit Commitment 

Can Mitigate Wind Forecast Error 
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Conclusions

• Wind and solar increase cycling costs

o From the fossil-fueled perspective, cycling O&M increases 

by $0.5–1.3/MWh 

o From the system perspective, cycling reduces production 

cost savings by $0.14–0.67/MWh 

• Emissions induced by cycling are much smaller than 

benefits

o Wind- and solar-induced cycling can help or hurt emissions 

from a fossil-fueled plant, depending on plant type, 

wind/solar mix, and penetration

• Wind and solar impact fossil-fueled plants differently, 

but production cost savings are similar

• As with any analysis, conclusions are specific to only 

grid footprint studied
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Next Steps

• U.S. DOE is reviewing final report with 

anticipated publication in February

• Examining cost-benefit analysis of retrofitting 

coal/gas plants for increased flexibility

• Starting reliability and stability study in 

Western Interconnection

o How do wind and solar provide grid-friendly 

support for frequency response and transient 

stability?
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For More Details

• Preliminary results: 

www.nrel.gov/docs/fy12osti/56171.pdf

www.nrel.gov/docs/fy12osti/56217.pdf 

• Emissions and wear-and-tear summary: 

www.nrel.gov/docs/fy12osti/53504.pdf

• Wear-and-tear costs and impacts: 

www.nrel.gov/docs/fy12osti/55433.pdf

• Cycling cost analysis: 

www.nrel.gov/docs/fy12osti/54864.pdf

• Forecasts: www.nrel.gov/docs/fy12osti/54384.pdf

• Reserves: www.nrel.gov/docs/fy12osti/56169.pdf

• Contact Debbie Lew at debra.lew@nrel.gov or 303- 384-7037