| NSF Org: |
CBET Division of Chemical, Bioengineering, Environmental, and Transport Systems |
| Recipient: |
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| Initial Amendment Date: | August 1, 2011 |
| Latest Amendment Date: | August 1, 2011 |
| Award Number: | 1133800 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Gregory Rorrer
grorrer@nsf.gov (703)292-7470 CBET Division of Chemical, Bioengineering, Environmental, and Transport Systems ENG Directorate for Engineering |
| Start Date: | January 1, 2012 |
| End Date: | December 31, 2015 (Estimated) |
| Total Intended Award Amount: | $294,795.00 |
| Total Awarded Amount to Date: | $294,795.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
3400 N CHARLES ST BALTIMORE MD US 21218-2608 (443)997-1898 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
MD US 21218-2683 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | EchemS-Electrochemical Systems |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
1133800 PI Meneveau/1133993 PI Castillo
The objective of this project is to develop and apply experimental and computational tools for predicting and improving wind farm performance by placing particular attention on large scales of turbulence and vertical fluxes of kinetic energy that are of great significance for large arrays of wind turbines. Much effort has been devoted in recent years to increasing the efficiency of individual wind turbines, assuming a given inflow in front of the turbine. Also, understanding how wakes affect the performance of downstream turbines and modeling superpositions of multiple such wakes has received considerable attention; however, there has been relatively little fundamental understanding of how a large array of wind turbines interacts with the turbulent atmospheric boundary layer at larger scales in the wind turbine array boundary layer (WTABL). Recent research has demonstrated that an important performance-limiting factor for large wind farms is the rate at which kinetic energy can be entrained into the array from the flow aloft, above the wind turbines. No matter how efficient an individual wind turbine is, or how well it can adapt to an upstream wind turbine, ultimately it is the vertical flux of kinetic energy into the overall array that largely determines how much power can be extracted from the atmospheric flow. The questions addressed in this project aim at better understanding the limiting factors and the effects of different scales of turbulence on vertical entrainment processes. The resulting models should guide wind turbine placement strategies and possible flow modifications so that vertical entrainment rates can be increased. Specifically, wind tunnel experiments coupled with large-eddy simulations (LES) will be employed to address the following research questions: (a) What are the essential differences between the developing and the fully developed WTABL? (b) What is the relative contribution from streamwise large-scale coherent vortices to vertical entrainment of kinetic energy? (c) What are the space-time correlations of hub-height velocity and power output between different wind turbines in the array? (d) Are there particular arrangements of wind turbines in the array that increase, on average, the entrainment? and (e) Can large-scale flow structures be affected through rotor modifications to increase such entrainment? Addressing such questions requires the ability to experiment under the highly controlled and reproducible conditions that can be afforded in the wind tunnel experiments and computer simulations. The data will be supplemented with comparisons with relevant new field data from a large wind farm.
Broader impacts: The robust growth of wind energy implies the possibility that large portions of the land and near-shore surface of the US and the world may ultimately be used for large wind farms. Predicting and better understanding the physical processes coupling the modified surface and atmosphere under such conditions is a timely and critical area of research. Through project activities the PIs will help train the next generation of engineers and scientists with the necessary tools and insights to help reach the US goal of 20% wind energy by 2030. Graduate education/mentoring will stress the interplay between wind tunnel experimentation, computer simulation and field data analysis. International (Switzerland, Spain) and industrial experiences (General Electric) will also be emphasized in this project. Recruiting and outreach will leverage both PIs' ongoing efforts to recruit US Hispanic graduate students through contacts in Puerto Rico (NSF-AGEP and LSAMP), as well as an IGERT at JHU on modeling complex systems. A GK-12 at RPI on energy and environment will leverage NSF resources in training teachers on wind energy issues. The PI's ongoing outreach to a Baltimore high school will be continued, providing research experiences for high-school juniors and seniors.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The primary goal of this collaborative project involving researchers at Johns Hopkins University (JHU) and Texas Tech University (TTU) was to explore the roles played by the large-scale motions of turbulent flows occurring in the atmospheric boundary layer in transferring energy vertically into a wind turbine array, and to quantify how this energy transfer develops in the stream-wise direction. The mechanism of turbulent transport of kinetic energy into the wind turbine array becomes crucial especially as wind farms are becoming more extended spatially. For small wind farms, it is simple horizontal advection of kinetic energy that determines the overall power available at each wind turbine. For larger wind farms, however, it has become clear (and this project’s outcomes have further cemented this view), that it is transport in the vertical direction that becomes dominant.
Using analysis of data obtained from computer models of wind farms performed at JHU, we have found that most of the vertical transport is caused by “streamwise-roller” modes of the velocity field. Also using simulation data, we were able to show that by applying vertical “deflection forces” at the rotor location timed to coincide with faster or slower wind fluctuations, one could affect the vertical entrainment and hence the power extraction. While such initial results are based on highly idealized numerical simulations, they serve to show that such flow-control of wind turbine array boundary layers is possible, at least in principle. Another set of simulation studies was devoted to finding the optimal staggering of turbines in an array. It was found that full staggering is in fact not necessarily optimal and that the best arrangement for a given wind direction is when wakes “just miss” the downstream ones. Figure 1 shows a 3D volume rendering of low-velocity regions in the wakes of the turbines in the case of a fully aligned wind farm. Furthermore, the project led to new concepts (generalized transport tubes to supplement the traditional streamtube analysis) and tools (a new concurrent-precursor inflow method for Large Eddy Simulations).
In real wind farms, entrance effects dominate the first several rows of turbines in an array. Using data obtained in an experiment with a model array of turbines in a low-speed wind tunnel, we quantified how quickly the flow develops from entrance to fully developed conditions. Surprisingly, even at the second turbine, vertical fluxes became comparable or larger than the horizontal ones and the flow development was rather fast (for aligned conditions). In order to study very large wind farms in a wind tunnel, a “micro-wind farm model” was constructed with 100 model wind turbines modeled as porous, 3D printed, disks. Figure 2 shows a photograph of the wind farm in a staggered configuration placed in the wind tunnel. Each turbine was instrumented with a strain-gage and thus we were able to measure temporal fluctuations in the (experimental surrogate) power production. The measurements show complex correlation structure between the power fluctuations of downstream turbines, which confirm numerical simulation results and can help understand power-smoothing when power signals from various turbines are aggregated in a wind farm.
At TTU, we have fielded a meter scale turbine on a 4-acre plot of land outside of Lubbock, TX. This installation has allowed us to address issues about turbulence such as inflow characterization, development of turbulence statistics, wake characteristics, and how well wind tunnel experiments re-create field conditions. The array is instrumented with sonic anemometers, hot-wire anemometers, and instruments to measure turbine power output and rotational speed. Moreover, energy entrainment into wind farms has been considered in the context of so-called low-level jets ...
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