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Contract No. DE-AC36-08GO28308

Lessons from Large-Scale 

Renewable Energy Integration 

Studies

Preprint

L. Bird and M. Milligan

Presented at the 2012 World Renewable Energy Forum 

(WREF 2012)

Denver, Colorado

May 13-17, 2012

Conference Paper

NREL/CP-6A20-54666

June 2012
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1

WREF 2012: LESSONS FROM LARGE-SCALE RENEWABLE ENERGY INTEGRATION 

STUDIES

A number of large-scale studies have been conducted in 

the United States and Europe that examine the ability for 

electric grids to accommodate higher penetrations of 

variable renewable energy generation. Renewable sources, 

such as wind and solar, can pose difficulties because their 

output can vary according to seasonal and daily patterns 

and be uncertain due to changes in the weather. While the 

operational changes that are needed to accommodate 

higher penetrations of renewable energy vary depending on 

the characteristics of a particular grid, there are a number 

of common lessons from large integration studies. In 

general, large-scale integration studies in Europe and the 

United States find that high penetrations of renewable 

generation are technically feasible with operational 

changes and increased access to transmission. This paper 

describes other key findings such as the need for fast 

markets, large balancing areas, system flexibility, and the 

use of advanced forecasting.

ABSTRACT

1.

Renewable energy sources, such as wind and solar, are 

variable due to changes in the amount of available wind 

and sunlight and uncertain due to the vagaries of the 

weather. Integration of large amounts of variable 

renewable generation can pose challenges for the 

electricity grid. Electricity grids are designed to handle the 

frequent but difficult to predict changes in electric loads, 

but the presence of large amounts of wind and solar add to 

the variability and uncertainty.

INTRODUCTION

Several large scale integration studies have been conducted 

in Europe and the United States that aim to understand how 

higher penetrations of variable renewable energy 

generation will impact the grid. These studies also examine 

the need for grid improvements and operational changes 

that may be necessary to integrate higher penetrations of 

variable renewable generation.

These studies have modeled the electricity system using 

production cost modeling and simulations of higher 

penetrations of renewable energy generation in future 

years. In general, these studies have aimed to understand 

the need for transmission improvements, interconnections, 

balancing area cooperation, market design issues, and other 

operational changes needed to economically operate the 

grid under high penetrations of variable renewable energy 

generation.

This paper identifies key insights and common themes 

from large-scale studies that examine issues associated 

with integrating high penetrations of renewables in the 

electric grid. While many of the general findings from the 

studies are similar, the results vary based on specific 

characteristics of the grids, the location of the renewable 

resources and the need for transmission, differences in 

existing operations, and the presence or absence of 

organized wholesale power markets.

Lori Bird

Michael Milligan

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, CO 80401

lori.bird@nrel.gov

michael.milligan@nrel.gov
2

2.

In the United States, two major integration studies have 

examined the impact of higher penetrations of renewable 

generation on the Eastern and Western electricity grids. In 

Europe, two Europe-wide studies have examined the needs 

to economically integrate higher penetrations of wind 

throughout Europe. 

OVERVIEW OF LARGE-SCALE INTEGRATION 

STUDIES

Study

TABLE 1: LARGE-SCALE INTEGRATION STUDIES

Year 

Published

Location Grid Penetration 

Studied

Year of 

Completion

Western Wind and Solar 

Integration Study

2010 Western United 

States

11%–35% Study uses 

estimates for grid 

performance for 

2017, though 35% 

penetration is not 

expected by 2017

Eastern Wind Integration and 

Transmission Study

2011 Eastern United States 20%–30% 2024

TradeWind Study 2009 Europe 200 GW–350 GW

(up to 25%)

2030

European Wind Integration 

Study

2010 Europe 70 GW–185 GW

(up to 13%)*

2015

* This percentage calculation is based on the numbers direction above, from the TradeWind study.

2.1 Western Wind and Solar Integration Study 

(WWSIS)1

Sponsored by the U.S. Department of Energy and managed 

by the National Renewable Energy Laboratory, the 

WWSIS examined the planning and operational 

implications of adding up to 35% wind and solar energy 

penetration to the Western Interconnect of the United 

States. The study specifically examined: 1) different levels 

of energy penetration for wind and solar generation, 

ranging from 11% to 35%, 2) different geographic 

locations for the wind and solar resources, and 3) a wide 

array of sensitivities to assess issues such as fuel costs, 

operating reserve levels, unit commitment strategies, 

storage alternatives, PHEV, and balancing area size. The 

study had a technical review committee comprised of 

representatives of the power system industry in the West.

2.2 Eastern Wind Integration and Transmission Study 

(EWITS)2

The companion report to the Western study, the EWITS 

examined the operational impact of up to 20% to 30% 

wind energy penetration on the bulk power system in the 

Eastern Interconnection of the United States by 2024. The 

study yielded detailed quantitative information on 1) 

transmission concepts for delivering energy economically 

for each wind energy penetration scenario, 2) economic

sensitivity simulations of the hourly operation of the power 

system defined by a wind generation forecast scenario and 

the associated transmission overlay, and 3) the contribution 

made by wind generation to resource adequacy and 

planning capacity margin. The study had a technical 

review committee comprised of representatives of the 

regional transmission organizations, utilities, and other 

stakeholders in the East.
3

2.3 TradeWind Study (Europe)3

Initiated by the European Wind Energy Association, the 

TradeWind study was the first Europe-wide study to 

examine the infrastructure and operational changes needed 

to manage higher penetrations of wind energy in Europe. It 

included wind penetration scenarios across Europe ranging 

from approximately 200 GW to 350 GW by 2030 (300 GW 

is approximately 25% of electricity demand) and examined 

the effects of improved grid interconnections and potential 

modifications to power market design on the integration of 

higher levels of renewable generation.

2.4 European Wind Integration Study (EWIS)4

Undertaken by 15 Transmission System Operators in 

Europe acting through the trade association, the European 

Network of Transmission System Operators for Electricity 

(ENTSO-E), the EWIS examined the technical, regulatory, 

market, and transmission system impacts of higher 

penetration scenarios of renewable energy in Europe. The 

study examined near term impacts of wind generation on 

the electric grid with low, medium, and high penetration 

scenarios of installed wind generation across Europe 

ranging from 70 GW to 185 GW by 2015. The modeling 

effort included annual market simulations to assess a range 

of grid conditions and costs as well as detailed power flow

modeling of grid pinch points.

3.

Collectively, large-scale integration studies shed light on 

the operational changes required to accommodate higher 

penetrations of renewable energy generation on electric 

grids. While some of the findings differed based on 

differences in grid operations, resources, transmission or 

other factors, a number of the findings are common among 

the various studies.

LESSONS FROM LARGE INTEGRATION 

STUDIES

3.1

How much renewable generation can the system handle? 

The large scale studies examined here found that high 

penetrations—up to 25% or 35%—of variable renewable 

generation are technically feasible with the addition of 

transmission infrastructure and with operational changes.

Higher Penetrations of Variable Renewable 

Generation are Manageable

The EWIS found that the fuel cost savings and avoided 

carbon dioxide emissions from wind energy substantially 

exceed the balancing and grid reinforcement costs that can 

be attributed to wind.

The Western study showed significant benefits to 

integrating renewable energy in the 10% case, with no 

negative effects on the system. In the 20% penetration 

scenarios, increased renewable generation led to increased 

stress on system operations within a portion of the study 

footprint, with some instances of insufficient reserves due 

to wind and solar forecast error. The study found that this 

additional need for reserves can be addressed, but the 

system has to work harder to handle the variability of the 

renewables. In the 30% renewable energy case, the 

variability is even more challenging for system operations 

and the load and contingency reserves are met only if the 

wind and solar forecasts are perfect.

The WWSIS and EWITS estimated the fuel savings and 

reduced emissions from increased levels of wind and solar 

energy result in lower annual operating costs. In the 

Western study, the high wind and solar case (35% on an 

energy basis) reduced annual operating costs by 40% of 

$20 billion in 2017 dollars ($17 billion in 2009 dollars) 

compared to the case without any wind and solar. In the 

EWITS, the high renewable energy case (30%) reduced the 

annual production costs by 10% to $63 billion in 2006, 

compared to the reference case.

3.2

Integration studies have consistently found that expanding 

access to diverse resources aids in the integration of high 

penetrations of variable renewable generation.

Larger Balancing Areas and Geographic Diversity of 

Renewable Resources are Preferred to Minimize Impacts

5

The Tradewind study found that the variability of wind 

generation was reduced when aggregated across Europe. It 

also found that cross-border exchange of wind is important 

for the system to be able to capture the benefits of highlevels of wind energy production, when it coincides with 

peak load levels.

 This can 

be achieved in two ways—enlarging balancing areas and 

diversifying the location and types of the renewable energy 

facilities. By enlarging balancing areas, the relative 

variability and uncertainty in both the load and renewable 

energy generation will be lowered, smoothing out 

differences among individual loads and generators. Also, 

larger balancing areas reduce the costs of integrating wind 

and solar by reducing the need for reserves or enabling 

access to the most cost-effective reserves in a larger 

system. Larger balancing areas may also provide access to 

a greater amount of flexible generation. Greater geographic 

distribution of renewable resources reduces their variability 

because weather patterns are less correlated, reducing the 

magnitude of output changes, particularly the likelihood of 

a sudden drop in wind generation.
4

The WWSIS found that balancing area cooperation can 

lead to cost savings because reserves can be pooled. It 

found that savings from reserve sharing can be significant 

with or without renewable energy on the system. In the 

study, a sensitivity analysis was performed, comparing the 

operating costs for running the system with 106 zones 

versus five large regions. The analysis found $1.7 billion in 

2009 dollars in savings in operating costs from larger 

balancing areas in the scenario with 10% renewable energy 

penetration.

Larger balancing areas can be achieved through a variety 

of mechanisms, such as developing interconnections with 

other regions or expanding wholesale power markets. In 

the Western United States, where there are no wholesale 

power markets, efforts are underway to enable balancing 

areas to net their imbalances with neighboring balancing 

areas to reduce the need for regulation reserves. Also, a 

limited market for imbalance energy is under 

consideration, which would enable balancing areas to 

procure reserves from the most cost-effective resources in 

the region, if adopted.

3.3

Integration studies reinforce the concept that sub-hourly 

scheduling and dispatch is preferred with higher 

penetrations of variable renewable generation. Sub-hourly 

dispatch improves system efficiency, increases reliability, 

reduces the amount of reserves required to balance the 

system, and enables systems to integrate higher 

penetrations of variable renewable energy generation. The 

amount of imbalance energy is reduced because the 

dispatch is optimized more frequently. While sub-hourly 

scheduling and dispatch increases costs of managing the 

system because it is dispatched more frequently, the 

reserve requirements are reduced resulting in a net benefit.

Faster Markets are Preferred

6

Faster dispatch can enable the system to access reserves 

from existing units at little or no extra cost. It can also 

reduce the need for regulation reserves, which are the most 

expensive types of reserves, because there are fewer 

minute-to-minute deviations between load and generation 

when the system is re-dispatched frequently. In areas with 

hourly dispatch, generators often must follow a set 

schedule for each hour, with the level typically set an hour 

or more before.7

Sub-hourly scheduling between balancing areas, as 

opposed to within the balancing area, can lead to 

operational efficiencies and allow for more rapid balancing 

of generation changes in each region. The benefits of subhourly scheduling between interties are greater with higher 

levels of variable renewable generation. Particularly where 

transmission constraints exist, faster scheduling across 

interties would enable the variable generation to be more 

efficiently integrated through faster and coordinated 

dispatch with a neighboring market.

Thus, sub-hourly dispatch provides 

greater access to physically-available maneuverability of 

generation compared to hourly dispatch in which units can 

only change their output once an hour.

8

Regional or local integration cost studies have found that 

the costs of integrating renewables to be less in areas with 

faster scheduling and dispatch. Integration studies 

conducted in RTO/ISO areas with 5- or 10-minute dispatch 

had integration costs of $0-$4/MWh, while areas with 

hourly scheduling and dispatch had integration costs of 

about $8-$9/MWh or higher. The lower integration costs in 

RTO/ISO markets are in part due to the faster scheduling 

intervals, but also because of the larger size of the 

balancing areas, which enable the ISO/RTO markets to 

access more cost-effective resources in the region for 

balancing. 9

The Western study found sub-hourly scheduling to be 

important for minimizing regulation requirements on the 

system. By scheduling resources every 5 or 15 minutes, 

rather than every hour, the study found that the need to 

ramp units providing load following can be substantially 

reduced. The Western study case of high wind and solar 

penetration (30% energy) with sub-hourly scheduling 

required half the amount of quick maneuvering of 

combined cycle plants from what would be required with 

hourly scheduling. The amount of fast maneuvering of 

combined cycle plants was about the same in the 20% 

renewable energy penetration scenario with hourly 

scheduling and the 30% renewable energy penetration case 

with sub-hourly scheduling. This finding demonstrates that 

the use of sub-hourly scheduling could help accommodate 

higher penetrations of renewable energy on the system. 

Also, through minute-to-minute simulations, the study 

found that hourly scheduling has a greater impact on 

regulation requirements of the system than the variability 

introduced from the wind and solar.

 

The TradeWind study found that intraday markets for cross 

border trade are important for facilitating higher levels of 

wind penetration. The introduction of intraday trading 

resulted in savings of 1-2 billion euros annually compared 

to a scenario with only day ahead cross border trade.

A survey of grid operators from a variety of countries also 

supports the notion that faster scheduling and dispatch 

leads to more efficient system operations and helps to 

manage variable wind generation. Respondents that work 

in areas with and without wholesale electric power markets 

indicated that frequent generation scheduling and dispatch 
5

are efficient methods of managing variable renewable 

generation on the grid. The survey also noted that a 

number of grid operators are working to increase the 

scheduling frequency between balancing areas.10

3.4

The use of forecasts in grid operations can help predict the 

amount of wind energy available and reduce the 

uncertainty in the amount of generation that will be 

available to the system. Renewable energy generation can 

be variable, changing with the time of day and weather 

patterns, and uncertain because of the inability to predict 

the weather with perfect accuracy. Integrating forecasts in 

system operations and unit commitment practices can 

reduce the cost of integrating renewables and reduce the 

need for reserves.

Incorporate Forecasts into Unit Commitment and 

Grid Operations

With higher penetrations of variable generation, the use of 

forecasting is important to operating efficient energy 

markets. Integrating wind and solar day-ahead forecasts 

into the process of committing power generation units to 

meet loads in coming days or hours is essential to help 

mitigate the uncertainty of wind and solar generation. Even 

though forecast errors sometimes result in reserve 

shortfalls, it is still beneficial to incorporate forecasts into 

the day-ahead scheduling process to reduce the amount of 

shortfalls.

The WWSIS found that using advanced wind and solar 

forecasts in the day-ahead unit commitment process would 

reduce annual system operating costs by up to $4 billion or 

$10-$17/MWh of renewable energy, compared to not 

considering renewables in the unit commitment process. 

The study further found that if forecasts were perfectly 

accurate they would reduce annual operating costs by 

another $425 million or $0.9-$1.7/MWh of renewable 

energy. All values in 2009 dollars.

The TradeWind study found that rescheduling generation 

intraday and incorporating wind power forecasts up to 

three hours before dispatch resulted in cost savings of 

about 260 million euros annually. The cost savings were 

primarily due to a reduction in reserves by incorporating 

more accurate forecasts into scheduling and dispatch.

3.5

Higher penetrations of variable renewable generation 

require increased flexibility from the power system to 

manage the added variability and uncertainty. Flexibility 

can be achieved through increased transmission, or the 

addition of flexible resources to the system, such as more 

flexible generating units, storage, and demand response.

Increasing the Flexibility of the System Helps 

Integrate Renewables

The TradeWind study identified transmission system 

upgrades to alleviate congestion and prevent high future 

system costs. It identified 42 connectors that would enable 

the European power system to more effectively integrate 

high penetrations of wind and avoid bottlenecks. It 

estimated that the benefits of the system upgrades in terms 

of operational savings would be approximately 1.5 billion 

euros annually, compared to the 22 billion euros in 

transmission investments.

Demand response can also provide systems with 

operational flexibility and can help address extreme events. 

Typically wind forecast errors are small but there are some 

hours when errors can be quite substantial, resulting in 

either contingency reserve shortfalls (over-forecasts) or 

wind curtailment (under forecasts). To address contingency 

reserve shortfalls, the WWSIS found that it is more costeffective to use demand response for the 89 hours of overforecasts rather than increase spinning reserves for all 

8,760 hours of the year. Compared to committing 

additional spinning reserves, the use of demand response 

saved up to $510 million per year.

3.6

Wind and solar energy increase the variability and 

uncertainty of output in the system, which results in a need 

for additional reserves. While wind and solar typically do 

not have an impact on contingency reserves that are needed 

to ensure system reliability in the case of unexpected 

system outages, they do generally require additional 

regulation and load following reserves needed to balance 

the system under normal operating conditions.

Additional Reserves may be Needed with Higher 

Penetrations of Renewables

11 In some 

hours, the presence of variable generation can lead to 

steeper ramps, necessitating greater system flexibility and 

the use of units with faster ramp rates. However, the 

variability can be reduced if the renewable energy 

generators are geographically dispersed and if forecasting 

is used in system operations. Wind generation primarily 

increases the need for load following reserves because it 

adds variability and uncertainty in the minutes-to-hours 

timeframe. Minute-to-minute fluctuations generally 

smooth out with a larger number of wind turbines and if 

they are spread out geographically. 12 Solar generation has 

greater minute-to-minute variability due to the effects of 

cloud cover on generation. However, the daily and 

seasonal patterns of the sun are highly predictable and 

coincident with system peaks.
6

While variable generation requires additional reserves, it 

typically frees up thermal capacity on the system to 

provide additional reserves. The WWSIS found that the 

average variability reserve requirement doubles in the 30% 

case. However, when wind and solar are added to the 

system, thermal units are backed down because it can be 

more cost-effective to ramp down a unit rather than take it 

offline. As a result, the Western study cases with wind and 

solar had more up-reserves available to the system. Thus, 

the study found that there was not a need to commit 

additional reserves to cover variability resulting from 

increased wind and solar in the study footprint.

3.7

While it is very difficult to calculate the costs of 

integrating renewables, estimates indicate that these costs 

are manageable. When considering the question of 

integration costs, it is also important to keep in mind that 

all generation sources, including nuclear and fossil plants, 

have costs associated with managing them on the grid.

Integration Costs are Manageable

13

The EWIS found that the cost of managing the variability 

of wind on the European grid to be relatively small 

compared to the benefits. The study estimated that the 

costs of integrating the wind to be about 2.1 euros/MWh 

under the best estimate scenario and 2.6 euros/MWh in the 

optimistic wind energy penetration scenario. These figures 

represent less than 5% of the calculated benefits of the 

wind energy in terms of fuel cost savings and reduced 

carbon dioxide emissions.

Despite these challenges, the EWITS found that 

interconnection-wide costs for integrating large amounts of 

wind generation are manageable with large regional 

operating pools and significant market and operational 

changes. For all scenarios in EWITS, the integration cost 

was estimated to be less than $5 per megawatt-hour 

(MWh) of wind, or less than $0.005 per kilowatt-hour 

(kWh) of electricity used by customers, assuming large 

balancing areas and fully developed regional electricity 

markets. 

4.

Large-scale integration studies play an important role in 

helping to understand how to economically integrate 

variable generation. These studies have found that 

integrating penetrations of variable renewable energy 

generation on the order of 20%–35% of energy on the 

electric grid are technically feasible, but that operational 

changes and improved transmission access are necessary. 

In general, the studies find that the following operational or 

infrastructure changes can help facilitate the integration of 

higher renewable energy penetrations: faster scheduling 

and dispatch of generation, using advanced forecasting in 

fast market operations, greater system interconnections and 

balancing area cooperation, greater access to transmission, 

increased flexibility of dispatchable generation, and the use 

of demand response. A number of these changes are 

already underway in both Europe and the United States. 

Additional study is required to continue to refine economic 

options for managing the system and maintaining system 

reliability with higher penetrations of renewable energy.

CONCLUSIONS

 

1 Western Wind and Solar Integration Study, prepared by 

GE Energy for the National Renewable Energy Laboratory, 

Golden: CO, May 2010 

http://www.nrel.gov/wind/systemsintegration/pdfs/2010/w

wsis_final_report.pdf. 2 Eastern Wind Integration and Transmission Study, 

prepared by EnerNex Corporation for the National 

Renewable Energy Laboratory, Golden: CO, revised 

February 2011 

http://www.nrel.gov/wind/systemsintegration/pdfs/2010/e

wits_final_report.pdf

3 TradeWind 2009, Integrating Wind: Developing Europe’s 

Power Market for the Large-Scale Integration of Wind 

Power. http://www.tradewind.eu/fileadmin/documents/publications/Final_Report.p

df

4

 Winter, W. 2010 European Wind Integration Study: 

Towards a Successful Integration of Large Scale Wind 

Power into European Electricity Grids, March. 

http://www.wind-integration.eu/ 5 See for example, Corbus, D., D. Lew, G. Jordan, W. 

Winters, F. Van Hull, J. Manobianco, and R. Zavadil. 

November/December 2009. “Up with Wind: Studying the 

Integration and Transmission of Higher Levels of Wind 

Power.” IEEE Power and Energy, 7(6): 36–46. 6 Milligan and Kirby. Market Characteristics for Efficient 

Integration of Variable Generation in the Western 

Interconnection. .; NREL Report. August 2010. 

http://www.nrel.gov/docs/fy10osti/48192.pdf

7 DeCesaro, J. and K. Porter. 2009. Wind Energy and 

Power System Operations: A Review of Wind Integration 

Studies to Date. NREL/SR-550-47256. December. 

Integration costs of RTO/ISO vs. non-RTO/ISO. (p. 3) 

http://www.nrel.gov/docs/fy10osti/47256.pdf

8 ISO/RTO Council 2011 Briefing Paper: Variable Energy 

Resources, System Operations and Wholesale Markets, 

August http://www.isorto.org/atf/cf/%7b5B4E85C6-

7EAC-40A0-8DC3-003829518EBD%7d/IRC_VERBRIEFING_PAPER-AUGUST_2011.PDF
7

 

9 ISO/RTO Council 2007 

http://www.isorto.org/atf/cf/%7B5B4E85C6-7EAC-40A0-

8DC3-

003829518EBD%7D/IRC_Renewables_Report_101607_fi

nal.pdf

10 Jones, L. 2011, Strategies and Decision Support Systems 

for Integrating Variable Renewable Resources in Control 

Centers for Reliable Grid Operations: Global Best 

Practices, Examples of Excellence and Lessons Learned, 

Prepared by Alstom Grid Inc. for the U.S. Department of 

Energy. 

http://www1.eere.energy.gov/wind/pdfs/doe_wind_integrat

ion_report.pdf

11 Regulation reserves are used to balance load and 

generation on a momentary basis, while load following 

reserves are used to balance the system as load levels 

change over the course of the day, on a scale of minutes to 

hours. Contingency reserves are used to ensure reliability 

of the system in the case of the loss of a large power plant 

or contingency on the system. 12 North Amercian Electric Reliability Corporation, 2009 

Special Report: Accommodating High Levels of Variable 

Generation, April. 

http://www.nerc.com/files/IVGTF_Report_041609.pdf 13 Milligan, M.; Ela, E.; Hodge, B. M.; Kirby, B.; Lew, D.; 

Clark, C.; DeCesaro, J.; Lynn, K. (2011). Cost-Causation 

and Integration Cost Analysis for Variable Generation.

Electricity Journal, 24(9), November