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.

Western Wind and Solar 

Integration Study 

Phase 2

EPRI Cycling Webinar

Debbie Lew, Greg Brinkman, Eduardo Ibanez – NREL 

Steve Lefton, Nikhil Kumar – Intertek APTECH

Sundar Venkataraman, Gary Jordan – GE Energy 

September 11, 2013

NREL/PR-5D00-60249
2

wind

coal

nuclear

hydro

Combined cycle

Gas turbine

PV 

CSP

WWSIS1: The worst week of three years

Can we integrate high 

penetrations of wind and 

solar into the Western 

Interconnection? 

What do we need to do to 

accommodate this?

Western Wind and Solar Integration Study Phase 1
3

Impacts of Solar-/Wind-Induced Cycling
4

Study Scope

Goal - Examine costs of cycling, emissions impacts of 

cycling, and compare wind and solar impacts

• We model 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 examine grid operations

o This is not a transmission planning study

o Reliability and stability are being examined in WWSIS-3 

• Used commercial software PLEXOS to model grid 

operations on 5 minute basis for year 2020 
5

Ensured Technical Rigor

• Technical Review Committee met every 2 

months

o Western Electricity Coordinating Council, Western 

Governor’s Association

o DOE and other labs (Sandia, LBNL, NETL)

o EPRI 

o Utilities (SRP, APS, PNM, TriState, WAPA, Xcel, 

Pacificorp, CAISO, BPA, NV Energy, ISO-NE, etc) 

o Other industry (NextEra, Energy Exemplar)

• Working groups

o Hydro (input from BPA, WAPA)

o Reserves (input from reserve sharing groups)

o Others 

• Nevertheless, any grid modeling has limitations
6

Developed Emissions Data Set

• Measured hourly 

emissions from each 

fossil-fueled plant in 

the U.S. for 2008

• CO2, NOX, SO2

• For each plant:

• Emissions rate as a 

function of 

generation 

o Additional emissions 

due to starts and 

ramps

0

20

40

60

80

100

120

1760 1765 1770 1775 1780

NOx emissions (lbs/hr)

Hour of year

Actual NOx Predicted NOx
7

Developed Wear-and-Tear Cost Data Set

Upper and lower bounds developed for costs

to reflect uncertainty ranges

Source: Kumar 2012, www.nrel.gov/docs/fy12osti/55433.pdf

. 
8

Scenarios Compare Wind and Solar

High Solar

8% wind

25% solar

High Mix

16.5% wind

16.5% solar

High Wind

25% wind

8% solar

TEPPC

9.5% wind

3.5% solar CSP Plant Capacity (MW)

150 to 200

100 to 125

75 to 100

50 to 75

PV Plant Capacity (MW)

75 to 200

50 to 75

25 to 50

10 to 25

Wind Plant Capacity (MW)

710 to 1,650

250 to 710

140 to 250

70 to 140

10 to 70
9

Adding ~100,000 MW of Wind/Solar

PV (MW) CSP (MW) Wind (MW) Total (MW)

TEPPC

(9.5% wind, 3.5% solar)

7,074 4,352 27,900 39,326 

High Wind 

(25% wind, 8% solar)

20,064 6,536 63,840 90,439 

High Mix 

(16.5% wind, 16.5% solar)

40,374 13,997 43,118 97,489 

High Solar 

(8% wind, 25% solar)

61,941 21,526 23,357 106,824 

• All wind and solar is sited in US portion of Western Interconnection. 

2020 peak WECC load is 171 GW, of which 147 GW is in the US. TEPPC 

case uses same MW as TEPPC but all sited in US, giving 13% total VG 

penetration 

• CSP has 6 hours storage

• WECC TEPPC 2020 PC1 case
10

Spring Is Most Challenging for Operations
11

No Renewables (0% Solar, 0% Wind)
12

TEPPC (3.5% Solar, 9.5% Wind)

12
13

High Wind (8% Solar, 25% Wind)
14

High Mix (16.5% Solar, 16.5% Wind)
15

High Solar (25% Solar, 8% Wind)

Solar is 60% PV and 40% Concentrating Solar Power with 6 hours thermal storage
16

Coal Ramping Increases with Wind/Solar

Excludes must-run gas 

combustion turbines 

which would distort 

statistics
17

How much coal is 

committed

How much coal is dispatched

Little Impact on Coal Operation in Summer
18

Spring: Wind Leads to Coal Shutdowns

Solar Leads to Coal Rampdowns
19

Renewables Displace Gas and Some Coal

Gas prices average $4.60/mmbtu
20

How are emissions impacted by cycling?
21

Emission Impacts of Cycling Are Relatively Small
22

Wind- and Solar-Induced Cycling Can Have 

a Positive or Negative Impact on Emissions

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.4 0.5 0.6 0.7 0.8 0.9 1.0

Average NO

X emissions rate (lbs/MWh)

Fraction of maximum generation

Coal CC CT GasSteam
23

Average CO2 Emission Rates 

From Coal Don’t Change
24

Changes in NOX Emission Rates 

Depend on Wind/Solar Mix
25

Changes in SO2 Rates 

Depend on Wind/Solar Mix
26

How are wear-and-tear costs 

impacted by cycling?
27

Electricity Costs Include 

Capital and Production Costs
28

Production Costs Include Cycling Costs
29

Cycling costs in these scenarios range from $271-800 million

Cycling Costs are 1% to 7% of 

Overall Production Cost
30

33% High Mix has 

lower cycling costs 

than 13% TEPPC case

Cycling costs 

increase by 13-

24% 

Cycling cost (million $)

33% Wind/Solar Induces 

$35 to $157 M/yr of Cycling Costs
31

Starts, not ramps, drive wear and tear costs

Cycling cost ($/MWh)

The average fossil-fueled plant sees an increase in O&M of $0.47-

1.28 per MWH generation

The Generator Perspective
32

Cycling costs 

sometimes decrease

Gas CCs have a 

significant increase in 

cycling costs

Note: These are the lower bound cycling costs

Cycling cost ($/MWh)

Gas Combustion Turbines Bear Brunt 

of Cycling Costs
33

From a System Perspective, 

Cycling Costs Are Relatively Small
34

Production cost change of wind/solar ranges from $29.4-$30.6 

per MWh in the high penetration scenarios. 

Cycling costs reduce this by $0.14-$0.67 per MWh of wind/solar

The System Perspective

Note: These do not include capital costs of construction of generation or transmission
35

How do wind and solar impacts compare?
36

Aggregation Reduces Variability
37

PV Dominates Variability Extremes
38

Removing the known path of the 

sun through the sky leaves us with 

weather/cloud variability which is 

significantly less

PV Variability Is Dominated by Sun Movement
39

High Solar High Wind

High Solar Results in Less Transmission Use 
40

Solar Requires Less Reserves Than Wind

Scenario Contingency (MW) Regulation (MW) Flexibility (MW)

No Renewables 3,361 1,120 0 

TEPPC 3,361 1,158 1,193 

High Wind 3,361 1,236 2,599 

High Mix 3,361 1,211 2,035 

High Solar 3,361 1,207 1,545 

Contingency reserves are unchanged with wind/solar.

High Wind/Solar requires up to 10% more regulating reserves than the No 

Renewables Scenario.

High Wind Scenario requires Flexibility reserves of 3% of installed Wind/Solar 

capacity. This is reduced to 1% for the High Solar Scenario.
41

Balanced Mix of Wind and Solar Reduces Curtailment
42

Conclusions

• CO2, SO2, and NOx emissions induced by cycling are a 

fraction of the overall reductions in emissions

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

from a fossil plant, depending on plant type, wind/solar 

mix and penetration

• Wind and solar increase cycling costs by $35-157M/yr 

o From fossil perspective, cycling O&M increases by $0.47-

1.28 per MWh of fossil generation

o From system perspective, cycling reduces production cost 

savings by $0.14-0.67 per MWh of wind/solar 

• Wind and solar impact fossil plants differently but 

production cost savings is similar

• As with any analysis, conclusions specific to only to 

grid footprint studied
43

Contact Info.

• Debbie Lew

• 303-384-7037

• debra.lew@nrel.gov

• http://www.nrel.gov/wwsis
44

Extra Slides
45

What if gas prices change?
46

Gas Price Has Greater Impact on Cycling Costs 

Than Wind and Solar Do
47

With Low Gas Prices, Wind/Solar Still Displace 

Mostly Gas
48

Adding Wind/Soar to $2.30 and $9.60 Gas 

Scenarios Results in Less Systemwide Cycling
49

Adding Wind/Solar to $2.30 and $9.60 Gas 

Scenarios Results in Less Systemwide Cycling