Engineering Challenges of 

Airborne Wind Technology

Dr. Fort Felker

Director, National Wind Technology Center

Airborne Wind Energy Conference

September 28, 2010 NREL/PR-5000-49409

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.
Innovation for Our Energy Future

Potential Benefits of Airborne Wind 

Energy Systems

• Excellent wind resource

• High capacity factors and reduced variability

• Can extract wind energy over instead of on

difficult terrain

• Reduced environmental impact

• May eliminate or mitigate land use conflicts

2
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Having a Better Mousetrap is Not Enough!

3

• Potential benefits must be so compelling that 

hundreds of millions of dollars can be put at risk

• Technology must be sufficiently mature that it can 

pass a critical Due Diligence examination

• Technology must be sufficiently mature that 

insurance can be acquired at acceptable rates

– Must insure the system against accidental loss

– Must protect the owners from accident liability

• Increased cost of project financing for immature 

technologies can be fatal

• Engineering challenge to sufficiently advance 

Technology Readiness Level so that financial needs 

are met
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Unique Niche for Airborne Wind Systems

Airborne wind needs:

• Low cost of wind turbines

• High reliability of aircraft

4

$5/lb

$500/lb
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Reliability Requirements

5

• Reliability affects

– Safety

– Public perception

– System development nonrecurring costs

– Operations and maintenance costs

– Power production (revenue!)

• Need a comprehensive approach to system 

reliability to address these diverse needs

• Need clear definition of all reliability requirements

• Reliability is expensive!
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Risk Management

6

• Development, deployment and operation of 

airborne wind energy systems have unique risks

Examples:

– If you exclude certain weather from your design 

environment, how do you know it’s coming?

– Who decides to recover? 

– How much time is required to recover the system? 

• Must follow a rigorous risk reduction process as 

these systems are developed

• Different set of issues for system operations, but 

need for risk management remains
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Design and Verification Standards

7

• Complete set of standards for design, verification, 

manufacture and operations of aircraft – Title 14 

of the Code of Federal Regulations

• Modest set of standards for wind turbines –

International Electrotechnical Commission 61400

• Nothing exists for airborne wind energy systems

• No basis for Certification or 3rd party assessment 

of designs (who is Certifying body?)

• Serious bar to rigorous Due Diligence process

• Must get started defining standards immediately 

– it takes time!
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Resource Assessment

8

Need much more than 

average wind speed!

• Wind speed probability 

distribution

• Turbulence

• Vertical shear

• Extreme winds

• Extreme coherent gusts

• Temperatures

• Probability of 

thunderstorms, hail, 

icing, tornados, 

hurricanes

To start, mesoscale models developed for wind turbines 

can be “queried” at higher elevations 
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Design Environment

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• Different issue than resource assessment!

• Design environment involves choices:

Will the system be operated in

– Thunderstorms and lightning

– Hail

– Icing

– Tornados

– Hurricanes

– Severe storms (how severe?)

– High winds (how high?)

• What is the maximum operating wind speed? What 

is the minimum?

• What is the design envelope for turbulence, gusts, 

etc?
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Design Load Cases

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• Definition of comprehensive set of Design Loads 

Cases is critical

• Must encompass sufficient range of conditions to 

adequately address entire system lifetime

– Operational cases, static and dynamic

– Launch and recovery process

– Adverse operational environments (e.g. flutter with 

icing present)

– Extreme events

– Partial failure cases

– Electrical and control system faults 

• Must constantly recalculate Design Loads Cases 

are system configuration evolves
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Margins of Safety

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• Must establish structural margins of safety

• Aircraft use 1.5. Wind turbines use different factors 

on loads and on structure

• Must define approach for fatigue analysis

– Design life

– Fail safe

– On condition

– Damage tolerant

• What is required margin for flutter? 

• Want most likely system failures to be handled 

routinely without loss of entire flight vehicle. Must 

define these failure cases.

• How much damping is required to be present for 

normal operational aero-servo-elastic modes?
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Simulation and Analysis

12

• Impossible to test all design conditions

• Must have a complete set of simulation and 

analysis tools to predict

– Steady and dynamic loads

– Aero-servo-elastic stability

– Stability and control (flight dynamics)

– Dynamics of launch and recovery process

– Dynamic response to faults and failures

• These tools must be validated with test data for 

wide range of conditions for the actual system 

configuration

• Must validate the tools and the input data
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Control Systems

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• Control system attributes:

– Autonomous with ground override

– Redundant and fault tolerant

– Must automate launch and recovery

– Will need to sense operational environment and 

adapt

• Development of control systems for aircraft has 

become long expensive process

• Need for autonomy complicates control system 

development for airborne wind systems

• Having a system that appears to work correctly 

is only half the battle – must then document, 

validate, and demonstrate to 3rd party
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Aero-servo-elastic Stability

14

• Systems will inevitably grow in size, and become 

more flexible

• Aeroelastic stability will eventually become a design 

driver

• Airborne wind energy systems have relatively low 

Mach numbers

• Linear potential flow analysis, with Prandtl-Glauert

corrections, should be sufficient

• Commercial versions of NASTRAN available with 

required aerostructural modeling capability

• Still requires lengthy and difficult process to build 

and verify models for novel configurations

• Little knowledge of modes of instability of novel 

configurations
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Verification and Validation

15

• No substitute for testing

• Companies that have a robust program of 

component and field testing are more likely to 

succeed

• Consider establishment of 1-year and 5-year 

field tests as important milestones

• Need to complete comprehensive set of 

component Highly Accelerated Life Tests (HALT)

• Sufficient instrumentation required to validate 

simulation results, and predict fatigue life

• Should partner with 3rd-party test facilities to 

provide independence and credibility
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Comprehensive Testing Required

16
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Environmental Impacts

17

• Environmental impacts cannot be ignored – to 

produce 1% of US electricity requires 15,000 

1MW systems

• Many lessons to be learned from wind industry

• Form and fund direct partnerships with 

environmental advocacy groups to guide 

approach (e.g. Audubon Society)

• Be sure to address human impacts as well, such 

as change in “Viewscape”

• Environmental impact assessments are data 

driven – go get the needed data!

• Common issues across companies and 

technologies – should collaborate
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Lifecycle Cost Modeling

18

• With all these details settled, can now assess 

overall costs

– Development cost

– Manufacturing and deployment cost

– Cost of financing

– Operations and maintenance cost

– Replacement cost

– Decommissioning cost

• Use systems engineering model to optimize

• Use model to inform directions for future R&D 

investment
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Summary and Recommendations

19

• Must explicitly define all system reliability 

requirements

• Use an integrated risk management approach to 

meet these requirements

• Certification Standards urgently needed to

– Define design environment

– Define design loads cases

– Define margins of safety

– Permit 3rd party assessment and Certification

• Major effort required to develop and validate 

simulation tools

• Test, test, test, test, test

• Test some more
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.

Dr. Fort Felker

Center Director

National Wind Technology Center

Contact Information:

Phone: 303-384-6905

Email: fort.felker@nrel.gov

Thank you!

Questions?