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How Does Wind Affect Coal? 

Cycling, Emissions, and Costs

Debra Lew,

Greg Brinkman, 

Michael Milligan NREL

Steve Lefton, Intertek

APTECH

Dick Piwko, GE

WindPower 2011

Anaheim, California

May 25, 2011

NREL/PR-5500-51579

Composite photo created by NREL
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What is the Impact of Wind on Fossil Plants?

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• Does wind increase emissions?

• Bentek study (April 2010) claims coal cycling results in increases 

in total SO2, NOx and CO2 emissions in Xcel/PSCO and ERCOT.

• Xcel refutes (May 2010) this, saying they have added significant 

wind since 2007, resulting in declining total emissions.

• What is the impact of wind on ‘wear and tear’ on the fossil 

units?

• Numerous wind integration studies have been undertaken, with 

criticism that they do not account for the full impact of ‘wear and 

tear’ on fossil units.

• Studies of specific plants to determine ‘wear and tear’ costs are 

confidential.

• Can’t we upgrade fossil plants to better cycle/ramp?

• Ontario (IESO) claims their coal units are more flexible than their 

gas units
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Integration of 35% Wind and Solar in the West 

Western Wind and Solar 

Integration Study

To assess the operating impacts 

and economics of wind and solar 

on the WestConnect grid.

The worst week of 

three years:
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Can the Fossil Fleet do This?

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Confidential 5

Boiler Corrosion Fatigue

Source: Steve Lefton, Intertek APTECH, with permission.
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Confidential 6

Generation Unit Cycling Definitions

Load Cycling

– LL1: Lowest Load at Which Design SH/RH Temperatures can be maintained

– LL2: Current “Advertised” Low Load

– LL3: Lowest Load at Which the Unit can Remain On-Line

EHS

Source: Steve Lefton, Intertek APTECH, with permission.
NATIONAL RENEWABLE ENERGY LABORATORY

Confidential 7

Cycling Effects

Source: Steve Lefton, Intertek APTECH, with permission.
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Confidential 8

Typical Cycling Cost Breakdown for Two Large Units

Source: Steve Lefton, Intertek APTECH, with permission.
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Heat Rate and Emission Curves

• Local linear fit for every unit.

• Compile emissions at full load and 50% of full load.

• Residuals used for subsequent analysis.

• Eliminate units with obviously clustered data, caused by:

• Installation of pollution control equipment during year;

• Part-time operation of pollution control equipment;

• Combined cycle units in various modes of operation.

0 50 100 150 200 250 300 350 400

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7

8

9

10

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12

13

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Comanche Unit 2

Generation (MW)

Heat Rate (mmbtu/MWh)

0 50 100 150 200 250

6

7

8

9

10

11

12

13

14

Generation (MW)

Heat Rate (mmbtu/MWh)

Front Range CC Unit 2

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Time-Lagged Pollution Control Failures?

• Lagged cross-correlation between unit ramp rate and the change in 

emission residuals (see figure on left) ─

• Should identify if pollution control problems are impacted by ramping.

• If evidence of a correlation exists, sum the residuals for all hours 

impacted by ramping (within 10 hours of a 5% hourly ramp) and average 

to determine excess emissions caused by ramping (see figure on right).

0 2 4 6 8 10 12 14 16

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

Lag (hours)

Cross-correlation

Comanche Unit 2

0 10 20 30 40 50 60 70 80 90 100

-40

-30

-20

-10

0

10

20

30

40

50

60

Lag (hours)

Excess NOx per ramp (lbs)

Comanche Unit 2

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Startup Emissions

• Add up residuals 

from all hours prior 

to and following a 

startup until unit 

reaches its 

minimum generation 

level.

• Integral between the 

predicted and actual 

NOx curves.

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1760 1762 1764 1766 1768 1770 1772 1774

0

50

100

150

200

250

Generation (MW)

Ft St Vrain CC Unit 3

1760 1762 1764 1766 1768 1770 1772 1774

0

50

100

150

Hour of year

NOx emissions (lbs)

actual NOx

predicted NOx

Generation

Min generation
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Results (explanation)

All results are generation-weighted averages by type.

Part-load penalty:

• Percentage increase in emissions (lbs) per unit of generation 

(MWh) when the unit is operating at 50% of maximum 

generation (compared to maximum generation).

Ramping penalty:

• Ratio of the increased emissions due to a 5% hourly ramp to 

the emissions from the unit during one hour of full-load 

operation.

Startup penalty:

• Ratio of the increased emissions due to a startup to the 

emissions from the unit during one hour of full-load 

operation.

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Emissions Analysis

• Part-load and full-load 

properties;

• Data from continuous 

emission monitors;

• Run PLEXOS with unitspecific heat-rate curves, 

emission cost sensitivities.

50 100 150 200 250 300

6

8

10

12

14

16

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Average heatrate (mmbtu/MWh)

Gen (MWh)

Coal (Martin Drake)

0 100 200 300

6

8

10

12

14

16

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Average heatrate (mmbtu/MWh)

Gen (MWh)

Combined Cycle (Front Range)

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0 10 20 30 40 50

6

8

10

12

14

16

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Average heatrate (mmbtu/MWh)

Gen (MWh)

Combustion Turbine (Fountain Valley)
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Results (heat input or CO2)

Unit type Part-load 

penalty

Ramping 

penalty

Startup 

penalty

Coal 5.1% 0.4% 110%

Gas CC 15.6% 0.3% 32%

Gas CT 12.4% 0.3% 32%

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Nox Emissions vs. Heat Input, Ramp Rate

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0 1000 2000 3000 4000 5000

0

200

400

600

800

1000

NOx (lbs)

HtIn (mmbtu)

Comanche (470), unit 2 (Coal Dry bottom wall-fired boiler)

-200 -100 0 100 200

-200

-100

0

100

200

300

NOx residual (lbs)

RampRate (mw/hr)

Correlation coefficient is 0.00

0 500 1000 1500 2000

0

50

100

150

200

NOx (lbs)

HtIn (mmbtu)

Fort St. Vrain, unit 2 (Pipeline Natural Gas Combined cycle)

-150 -100 -50 0 50 100 150

-40

-20

0

20

40

60

80

NOx residual (lbs)

RampRate (mw/hr)

Correlation coefficient is -0.02

NOx curves 

vary widely 

between 

generators.

Little evidence 

that dynamic 

ramping 

impacts 

emissions.
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Results (NOx,SO2)

Unit type Part-load 

penalty

Ramping 

penalty

Startup 

penalty

Coal (NOx) 1.2% 2.8% 290%

Gas CC 30%** 0.7% 950%**

Gas CT 19% 0.8% 670%**

Coal (SO2) 5.4% 13.4% 270%

**These numbers are highly sensitive to input assumptions (percent loading) 

and/or a small number of extreme outliers (some are bad fits).

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WWSIS Phase 2

1. Obtain better data for wear and tear costs of thermal 

units during cycling and ramping;

2. Examine emission impacts of thermal generation 

cycling and ramping in greater detail;

3. Optimize unit commitment and economic dispatch with 

these inputs and examine impact of increasing 

penetrations of wind and solar on thermal units;

4. Examine mitigation options to reduce costs of thermal 

unit cycling and ramping.

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Wear and Tear Cost Data

• Intertek APTECH has analyzed some 400 thermal units to 

determine wear and tear costs due to ramping and 

cycling. They have also developed a Cycling Advisor 

model to optimize commitment and dispatch of thermal 

generation with these costs taken into account.

• Split thermal units into 7 categories by size and type. 

• Costs to include:

• Hot, warm, and cold start;

• Cost for normal ramp rate from min. to max. and for fast ramp rate;

• Cost for different min output levels.

• Apply cost data to WWSIS-1 results to determine ‘ceiling’ 

on costs.

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PLEXOS Modeling of WECC

• As with WWSIS-1, model all of WECC because 

renewables in WECC impact WestConnect.

• Benchmark PLEXOS model with WECC-TEPPC model.

• Build base scenario:

• Opportunity to build more realistic base scenario;

• May include refining solar datasets to model centralized PV;

• Incorporate new wear and tear cost data.

• Cases:

• Run with and without wear and tear cost data;

• Run at different renewable energy penetration levels;

• Run with different mitigation options.

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Mitigation Options

• Work with GE power plant experts on emissions, 

combined cycle, steam turbines.

• Examine initial modeling results: 

• What are the parameters that have the biggest impact on 

production cost? Mingen, downtime, ramp rates;

• What are the impacts that are most important to mitigate? 

Efficiency, emissions, equipment lifetime.

• Propose and rank mitigation options:

• E.g., Cycling specific coal units off in spring, upgrade units to 

better cycle/ramp.

• When does it make sense to upgrade a unit and what 

kind of upgrades are needed?

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Contact Information

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Debbie Lew

NREL

303-384-7037

debra.lew@nrel.gov

http://www.nrel.gov/wwsis