The power, loads, and service life of a wind turbine can be improved when vortex generators are precisely attached to turbine blades. This case history report on the outcomes of one study.
Jean-Paul Cane / Rope Access Product Line Leader, Upwind Solutions Inc. / www.upwindsolutions.com
Georgios Pechlivanoglou /SMART BLADE GmbH / www.smart-blade.com
The blades of large, pitch regulated wind turbines often have poor aerodynamic properties at the root due to form and operation limitations. The required structural blade shape is expensive to produce, so there is always potential to improve their performance. Furthermore surface roughness and leading-edge erosion have a significant impact on blade aerodynamics because they induce local flow separation. SMART BLADE and the Institute of Fluid Dynamics and Technical Acoustics of TU Berlin investigated and developed several solutions for aerodynamics improvements in this field[6, 7, 5].
Vortex generators (VGs) which look like small fins, are designed for each blade geometry. They improve blade performance by reducing flow separation, thereby improving lift, and hence turbine power output. This research describes the new technology and procedures that resulted in a safe, efficient, and repeatable installation with an average one day turnaround per turbine.
After analyzing the turbine data, post-VG installations, we draw these conclusions:
• The mean Annual Energy Production (AEP) increased within the period. Considering the good condition of the blades and simple topography of the site, this is a good outcome.
• Turbine performance without VGs (a control group) is characterized by significant power scatter. This is most likely the result of considerable aerodynamic stall of a large part or even the entire blade during storms and gusty winds.
• The overall effectiveness of VGs on this site is quite positive and able to increase the revenue of the operator-owner with a high ROI. These elements combine to yield one of the highest ROI vortex generators on the market. Considering the degradation of the aerodynamic performance caused by surface roughness and leading-edge erosion, older wind turbines will benefit even more from VGs, thus recovering most if not all the lost energy.
WHAT WE LEARNED
Custom VG designs can maximize energy yield for every technology on which they are installed. SMART BLADE engineers designed the VGs in collaboration with the Institute of Fluid Dynamics and Technical Acoustics of TU Berlin. The VG design was evaluated by wind tunnel measurements on wind-turbine airfoils and extensive flow simulations. A custom V-shape installation line on a turbine blade optimizes the stall delay versus drag penalty. Once the VG pattern is developed, precise installation processes are formulated to ensure an accurate and repeatable placement. Templates for position also reduce the turbine’s down-time.
To install the VGs, rope-access technicians used a durable, proprietary rapid-curing adhesive tape. The self-adhesive, cost-effective tape reduces room for error and assures a firm bond of VG to blade even in extreme climates.
Safe and certified rope access technicians manage high-quality installations with minimum down-time for the turbines. The custom templates, processes, adhesive, and skilled rope-access technicians, allow an average of less than one day of downtime per turbine.
The expected result for pitch regulated wind turbines shows the largest increases to performance at wind speeds lower than nominal. At high wind speeds, the pitch control system takes over and the blades begin to pitch out of the wind to prevent generator over-power. Thus, the performance benefit of the VGs is removed by the turbine power control system. In certain cases however, wind turbine blades exhibit a flow separation at high wind speeds due to gusts and possibly slow-speed variability of the rotor. As a result, the performance of a turbine in high wind speeds is reduced which lets the VG provide beneficial results in this wind-speed region as well.
BLADE ANALYSIS Every wind-turbine manufacturer settles on a different blade geometry for a particular model. Thus, for best performance improvements, the VG design and installation must be customized for each model blade. To identify the best VG positions, it is necessary to investigate the blade’s aerodynamic performance. For this reason, SMART BLADE developed a proprietary flow analysis that involves flow visualization and advanced image processing. A team from the company will conduct an aerodynamic analysis for each new wind-turbine model.
Custom equipment is installed on site and the aerodynamic performance of the test turbines is carefully analyzed. Flow analysis data are combined with parametric wind tunnel tests. The combined field and lab research make it possible to identify the best placement and orientation of the VG units on each blade design.
A FEW INSTALLATION SPECIFICS For the test, the flow analysis was performed on site. VGs were installed close to the blade root and angled to the leading edge. This orientation was determined to decrease flow separation without further increasing drag.
Placing the VGs near the center of rotation contributes little to loads as they might on outboard region of the blade. The purpose of the VGs is to eliminate the flow separation at the inner part of the blade and return the blade to an “as-designed” condition. Pitch blades are designed NOT to stall. The fact that there is flow separation is merely an operation issue. By restoring the flow, the turbine comes close to its ideal operation. Turbines with dirty blades (high surface roughness) and leading-edge erosion face periodic stall events at every rotation. These cause high-load fluctuations which increase blade fatigue. As it turns out, vortex generators reduce load fluctuations, so they actually work to counter blade fatigue.
SUPPORT FOR KEY FINDINGS
Data was collected on a 10-minute average basis from before the date of installation to the present. (All historical performance data were provided by the wind park operator and owner, and covered a period of roughly two calendar years prior to VG installation.)
At the same time, nearby wind turbines were selected as a control group for a performance comparison. To date, there are more than three months of post-installation data. Careful analysis identified the performance trend of the test turbines (on which VGs were later installed) and control turbines (no VGs, but nearby). Analysis also assessed the influence of the optimization procedure with VGs.
The power curves were computed by means of the standard IEC 61400-12 binning process with data integration of the entire wind speed range. The computation of the AEP was based on the wind distribution at the site. This distribution was extracted from the normalized data of the measuring mast provided by the customer.
TURBINE PERFORMANCE BEFORE VGs
Within the analysis of historical data, significant effort was made to identify the potential of additional performance optimization opportunities. A simple study of the historical data reveals a large amount of power-performance scattering especially for the high wind band. A large amount of the scatter data seems to have stochastic behavior. However, power bands near the 1MW and 1.2MW regions occur at a high frequency.
A temperature correlation was to find whether or not the scatter is related to icing. Results showed that icing is not the cause of scatter. A wind direction analysis showed a slight wake interference, identified in control group D and test group B cannot be responsible for the extensive power scatter of these turbines. Finally, the correlation of the power performance with the atmospheric air pressure and weather report time series provided from the airport shows a correlation of the scattering data and local storms.4 Data was provided through www.wundrerground.com.
It appears that high turbulence due to storms as well as intense gusts cause stall on large parts of the blades thus forcing the pitch controller to wander between several pitch positions. Pitch controller “confusion” and aerodynamic stall cause significant power loss and revenue reduction for the operator. The stabilizing effect on the aerodynamics produced by the VGs will improve the blade performance during gusts and high turbulence thus reducing the power and load fluctuations.
ANNUAL ENERGY PRODUCTION
The turbine’s AEP was calculated based on the actual wind distribution of the site and the measured power curves of the test and control turbines. In all cases the test turbine had a slightly lower performance than the control turbine before VG installation. However the VGs fully reversed this trend and increased the energy yield of the test turbines. The AEP increased for both test groups.
Considering that test turbines are relatively new and their blades in good condition without indication of erosion or surface roughness, the AEP increases are quite significant. The additional revenue from the turbines (about $7,000 per turbine per year5 ) due to the VG installation can pay-back for the VG installation investment in a short time. The calculation is based on the difference of the mean AEP before and after the VG installation with an average feed-in price of $0.073/kWh. WPE
FOR FURTHER READING
 Normatmosphaere. Deutsches Institut Fuer Normung, DIN ISO 2533, 1979.
 IEC 61400: Wind turbines, part 12, power performance measurements of electricity producing wind turbines. International
Electrotechnical Commission, IEC 61400-12-1 Ed.2005-12, 2005.
 H.Mueller-Vahl, G. Pechlivanoglou, C.N. Nayeri, and C.O. Paschereit. Vortex generators for wind turbine blades: A combined wind tunnel and wind turbine parametric study. In Proceedings of ASME IGTI Turbo Expo 2012 ASME/IGTI June 11 -15, 2012, Copenhagen, Denmark. ASME, 2012.
 G. Pechlivanoglou, S. Fuehr, C.N. Nayeri, and C.O. Paschereit. The effect of distributed roughness on the power performance of wind turbines. In Proceedings of ASME IGTI Turbo Expo 2010 ASME/IGTI June 14 -18, 2010, Glasgow, Scotland, UK. ASME, 2010.
 G. Pechlivanoglou, C.N. Nayeri, and C.O. Paschereit. Fixed leading edge auxiliary wing as a performance increasing device for hawt blades. In DEWEK 2011. DEWI, 2010.
 G. Pechlivanoglou, C.N. Nayeri, and C.O. Paschereit. Performance optimization of wind turbine rotors with active flow control. In Proceedings of ASME IGTI
Turbo Expo 2011 ASME/IGTI June 6 -10, 2011, Vancouver, Canada. ASME, 2010.
 G. Weinzierl, G. Pechlivanoglou, C.N. Nayeri, and C.O. Paschereit. Performance optimization of wind turbine rotors with active ow control, part 2: Active aeroelastic simulations. In Proceedings of ASME IGTI Turbo Expo 2012 ASME/IGTI June 11 -15, 2012, Copenhagen, Denmark. ASME, 2012.