NREL{fP-257-4492 • UC Category: 261 • DE92001216

SERI Advance ind Turbine

Blades

J. Tangier
B. Smith
D. Jager

National Renewable Energy Laboratory

(formerly the Solar Energy Research Institute)
1617 Cole Boulevard
Golden, Colorado 80401-3393
A Division of Midwest Research Institute
Operated for the U.S. Department of Energy
under Contract No. DE-AC02-83CH10093

February 1992
On September 16, 1991 the Solar Energy Institute was designated a national laboratory, and its name was changed
to the National Renewable Energy Laboratory.

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 SEAl ADVANCED WIND TURBINE BLADES

J. Tangier, B. Smith, and D. Jager

Solar Energy Research Institute

1617 Cole Blvd., Golden, CO 80401

ABSTRACT

The primary goal of the Solar Energy Research Institute's (SERI) advanced wind turbine blades is to
convert the kinetic energy in the wind into mechanical energy in an inexpensive and efficient
manner. To accomplish this goal, advanced wind turbine blades have been developed by SERI that
utilize unique airfoil technology. Performance characteristics of the advanced blades were verified
through atmospheric testing on ftxed-pitch, stall-regulated horizontal-axis wind turbines (HAWTs).
Of the various wind turbine configurations, the stall-regulated HAWT dominates the market because
of its simplicity and low cost. Results of the atmospheric tests show that the SERI advanced blades
produce 10% to 30% more energy annually than conventional blades.

KEYWORDS

Airfoils; wind turbine blades; stall-regulated; airfoil roughness; horizontal-axis wind turbines.

INTRODUCTION

Conventional aircraft airfoils have created problems for wind turbines. These problems include
excessive power in high winds, which leads to burned-out generators; inadequate energy output when
the blade becomes soiled with insect accumulation and airborne pollutants, which reduces revenues;
and poor rotor power-to-thrust ratios, which result in high array losses for wind farms. The key to
the advanced blades (Fig. 1) is the use of specially designed airfoils that govern the airflow around
the blade in a manner substantially different from conventional airfoils.

Fig. 1. SEAl Advanced Wind Turbine Blade.

1
To eliminate the performance problems resulting from conventional airfoils, SERI (Tangier, 1987)
designed new thin- and thick- airfoil families (Fig. 2) that have the performance characteristics
needed to satisfy the requirements of stall-regulated HAWfs. The low drag, thin-airfoil family lends
itself to fiberglass rotors that are 10 to 20 meters in diameter. The thick-airfoil family, having slightly
more drag, addresses the more demanding structural requirements of either fiberglass or wood
composite rotors that are 20 to 30 meters in diameter. Both the thin- and thick- airfoil families have
performance characteristics that are tailored to change in a prescribed manner from the blade tip
(95% radius) to the blade root (30% radius).

To control peak rotor power in high winds, the airfoil in the tip region of the blade must have a
maximum lift coefficient (qmax) that is about 25% lower than typical aircraft airfoils. Conversely, the
airfoil in the root region of the blade must have a high C1max to aid rotor start-up and energy
produCtion at medium wind speeds. Unlike previous wind turbine blades, use of the new airfoil
families results in a blade whose C1max increases in a continuous manner from blade tip to blade root
for effective peak power control. In addition, controlling peak power in this way permits the use of
15% greater swept-disc area for a given generator size and results in increased energy production.

Also, designing the airfoil's C1max to be less sensitive to roughness effects minimizes energy losses
resulting from dirty blades. Airfoil roughness insensitivity is achieved through geometric tailoring of
the airfoil's shape to force a transition from laminar to turbulent flow on both the upper and lower
surfaces of the airfoil as qmax is approached.

THIN AIRFOIL FAMILY THICK AIRFOIL FAMILY

FOR MEDIUM BLADES FOR LARGE BLADES

c::== I SERI S806A I

� � I SERI S813 l �
TIP REGION AIRFOIL, 95% RADIUS TIP REGION AIRFOIL, 95% RADIUS

c::; I
~
SERI S805A �
PRIMARY OUTBOARD AIRFOIL, 75% RADIUS PRIMARY OUTBOARD AIRFOIL, 75% RADIUS

:s=====
E ~
( SE�

ROOT REGION AIRFOIL, 40% RADIUS O

RO T REGION AIRFOIL, 40% RADIUS

SPECIAL PURPOSE THIN AND THICK AIRFOIL FAMILY

Fig. 2. Special-Purpose Thin- and Thick-Airfoil Family.

Because of the aerodynamic improvements, the SERI advanced blades (Jackson, 1987) produce 10%
to 30% more energy annually than conventional blades. These energy gains were verified in a side­
by-side atmospheric test (Fig. 3; Tangier, 1989, 1990). SERI advanced blades were installed on one
turbine in a wind farm and original equipment blades manufactured by AeroStar were installed on
the othe(identical turbine.
2
 Fig. 3. Side-by-Side Wind Turbine Test.

Over 6700 HAWfs currently operating in California wind farms use 8- to 10-meter blades that
produce insufficient energy. Although most of these original equipment blades were made by the
now defunct AeroStar company, replacement blades are being supplied by other manufacturers.
Most of these replacement blades still use traditional aircraft airfoils. Use of aircraft airfoils on stall­
regulated machines results in either excessive peak power at efficient blade-pitch angles or reduced
energy production at inefficient blade pitch angles. Blades manufactured using SERI's advanced
thin- and thick-airfoil families can be operated at efficient blade-pitch angles to maximize energy
production and increase revenues. Replacement of conventional blades with SERI advanced blades
provides the following economic benefits:
•
Fewer burned-out generators: The excessive peak power produced by conventional blades in
high winds burns out generators. By using airfoil families that have a restrained C1max in the tip
region (Fig. 4), SERI's advanced blades control power output at high wind speeds more
effectively than conventional blades.
•
More consistent power output: By using airfoils whose qmax is relatively insensitive to
roughness effects, SERI's advanced wind turbine blades produce more consistent power output
than conventional blades.
•
Improved rotor efficiency: Compared to conventional blades, SERI's thin-airfoil advanced
blades produce 15% to 20% more power for clean blades and 27% to 35% more power for dirty
blades. These improvements are achieved through the use of greater swept-disc area, C1max
roughness insensitivity, and improved aerodynamics (Fig. 5).
•
Greater annual energy production: Compared to conventional blades, SERI's advanced thin­
airfoil blades are projected to produce 10% greater annual energy when clean and as much as
30% greater energy in the dirty condition (Fig. 6). The projections are based on measured
power curves and a standard Rayleigh wind speed distribution. Actual wind farm energy
measurements for a complete year resulted in a 30% energy improvement (Powers, 1990).
•
Lower array losses in wind farms: The SERI _advanced blades achieve this result by operating at
a more favorable blade-pitch angle for a given rotor size, which results in a greater rotor power­
to-thrust ratio (Fig. 7). The energy in the wind is used more efficiently so that each succeeding
row of wind turbines has more available wind energy, and total wind farm array losses are lower.
3
 90
CLEAN BLADE DIRTY BLADE
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WIND SPEED (M/S, MPH) WIND SPEED (M/S, MPH)

Fig. 4. Clean and Dirty Blade Generator Power.

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WIND SPEED (M/S, MPH) WIND SPEED (M/S,
_
, >
SWEPT AREA x
CLEAN BLADES
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ooooo DIRTY BLADES x � � AERODYNAMICS
'><""
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'X"'

Fig. 5. Generator Output Improvements and Components.

The SERI advanced blades are unique in that they are the only blades using airfoil families
specifically designed for stall-regulated HAWTs. Current domestic and foreign wind turbine blades
utilize traditional aircraft airfoils. The peak power problem encountered using aircraft airfoils can
only be corrected at the expense of energy output and higher mean blade loads (Tangier, 1988).
Peak power can be reduced by operating the blades at non-optimum pitch angles (closer to stall).
However, this approach typically results in a reduction in rotor performance at low to medium wind
speeds along with higher rotor thrust loads for a given power output. Effectively, a larger portion of
the wind energy goes into destructive blade loads and the corresponding wake-induced losses rather
than useful power output. 4
The principal application of the SERI advanced blades is for replacement blades on over 6700
existing machines operating in California. Although the SERI advanced blades were designed for
the unique performance requirements of fiXed-pitch stall-regulated HAWfs, the associated airfoils
are also expected to become the airfoils of choice for more expensive variable-pitch wind turbines.
These turbines would also benefit from the airfoils' greater insensitivity to roughness, which results in
more consistent power output. The restrained C1max for the tip region of the blade might also help
mitigate the large peak power spikes, common to variable-pitch turbines, that result from coherent
turbulence.

The biggest future application for the SERI airfoil families will be for new advanced stall-regulated
HAWfs. With the use of this airfoil technology, these advanced wind turbines are expected to
generate electricity at $.04 to .05/kWh in 13-mph wind sites.

j4
�
so

0r
I G-B-S-8-El DIRTY BLADES
Ge-9-e-€) SMOOTH BLADES

30
20
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>-

08 10 12 6 14 16 818 20 2210
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AVERAGE WIND SPEED (M/S, MPH)

Fig. 6. Annual Energy Improvement: Rayleigh Distribution.

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Mean wind speed (m/s)

Fig. 7. Power{fhrust Ratio versus Wind Speed.

5
 CONCLUSIONS

In summary, comparative field testing of the advanced thin-airfoil blades on stall-regulated HAWfs
has demonstrated the following performance improvements relative to original equipment blades:

• Peak power regulation

• Minimal roughness sensitivity

• Greater annual energy output

• Higher power-to-thrust ratios for reduced mean turbine loads and wind farm array losses

These improvements are attributed to increased swept area, greater airfoil C1max tolerance to
roughness effects, and improved aerodynamics.

The SERI advanced blades directly benefit the wind industry by expanding the viability of wind
energy into regions of lower average wind speeds, which could more than double the potential land
area for wind farm development in the United States.

ACKNOWLEDGMENTS

The authors wish to acknowledge the contributions of key individuals who participated in the design
and development of the thin-airfoil blades. These individuals are D. Somers for airfoil design, M.
Zuteck and K. Jackson for structural design, and J. Frerotte ·(Phoenix Industries, Crookston,
Minnesota) for blade fabrication. Appreciation is also expressed to SeaWest Energy Group for
support in conducting the atmospheric test and E. McKenna of SERI for instrumentation support.
All this work was performed under U.S. Department of Energy Contract No. DE-AC02-83CH10093.

REFERENCES

Jackson, K., and P. Migliore (1987). Design of Wind Turbine Blades EmployingAdvanced Airfoils.
Windpower '87 Proceedings 106-111.
Powers, J. (1990). Promising Results from New Blades. WindStats, Summer 1990.
Tangier, J., and D. Somers (1987). Status ofthe Special Purpose Airfoil Families. Windpower '87
Proceedings 99-105.
Tangier, J., and Co-Workers (1989). Atmospheric Performance Testing ofthe Special-Purpose SERI
Thin-Airfoil Family - Preliminary Results. Windpower '89 Proceedings 115-120.
Tangier, J., and Co-Workers (1990). SERI Thin-Airfoil Blade Atmospheric Performance Test - Final
Results. Windpower '90 Proceedings 118-125.
Tangier, J., and P. Tu (1988). Peak Power and Blade Loads on Stall-Regulated Rotors as Influenced
by Different Airfoil Families. SERI!fP-217-3334.