Performance of Wind Turbines in a Closely Spaced Array

The concept of operating wind turbines in a closely spaced array has potential advantages in reducing capital costs associated with design and construction, improving wind turbine system reliability and optimizing wind turbine operating efficiency.

The wind turbine array concept has long been championed by Capt. William E. Heronemus (USN Ret.). Heronemus came to the University of Massachusetts-Amherst after a distinguished 26-year career in the U.S. Navy, serving among other things as the superintendant of construction and later the shipbuilding superintendent for the U.S. nuclear power submarine fleet. During his early studies at MIT, he became enamored of Putnam, Claude, Thomas, and Edward William Golding’s work in alternative, renewable energy sources and developed a passion to conduct further research in this field. Until his death in 2002, he dedicated his post-Navy years to this pursuit.

The development of Heronemus’ wind turbine array concept continues through the efforts of OWES, LLC and Marcia Heronemus-Pate. This includes initial design studies for 2 MW and 5 MW multi-rotor systems and due-diligence studies regarding the economic benefits of the concept. During this design process, specific design concerns surfaced with regard to the possible interactions between the turbines in the array. If the turbines are operated at a close enough spacing, it is reasonable to suspect that the flow coming off of the blade tips could interact to undermine the power performance of the array as a whole. To address this concern, Southwest Research Institute (SwRI) was contracted to perform a combined analytical and experimental program to study the potential blade tip interactions.

In the Wind Tunnel

The experimental portion of the project includes the design of a multi-rotor array using commercially available wind turbines and then subjecting the array to performance testing in a controlled environment. The testing is performed in the Langley Full Scale Tunnel (LFST), which is an open-wall, low-speed tunnel 30 feet tall and 60 feet wide. The tunnel has a long history in the study of aerodynamics including aircraft, spacecraft and land craft (NASCAR). The turbines used for the array are 400 W turbines with a diameter of 43 inches. In all, seven turbines are included in the array, with the center of the pentagon array 15 feet from the floor. As shown in the photo on page 32, an aluminum frame supports all seven turbines and allows for the translation of the outer six turbines along the radial lines of the pentagon pattern. This translation creates a variation in spacing between each of the turbines and the central turbine which is fixed.

The center turbine is instrumented for power performance measurement as well as blade dynamic strain and various flow visualization features. The power performance measurement is made possible by using a variable resistance load bank, which allows the test operator to control the rotational speed of the central wind turbine for any fixed wind speed. The remaining six turbines are operated in their original battery charging configuration. For each wind speed tested (13, 20 and 27 mph), the central turbine speed is varied from the cut-in speed to the max speed, resulting in a power vs. tip speed ratio plot which characterizes the turbine performance as a curve instead of a single point.

Several parameters are varied in the study, including wind speed, turbine spacing, number of active turbines and array yaw angle. The initial test is performed with only the central turbine to create a baseline performance curve at each of the test conditions (wind speed and yaw angle). Another series of tests are performed with two turbines to allow for a direct comparison to a two-rotor computational fluid dynamics (CFD) model of the same turbines. Finally, all seven rotors are activated for the main parametric study of blade tip clearance.

During the seven rotor tests, the blade tip clearance is varied from 2 percent (1 inch) to 16 percent (7 inches). At each spacing interval, a full set of tests characterize the performance curve of the central turbine at all three wind speeds. This provides a more thorough comparison of wind turbine performance as the entire curve is compared between runs instead of single point values.

In addition, two sets of performance runs are made with the turbine array at a “+” and “-” 10-degree yaw relative to the wind. This test is performed to evaluate the sensitivity of the array spacing to shadowing effects. Inevitably, the array will not always be perfectly aligned with the wind such that some turbines will be farther upstream than others. It is important to determine the relationship between turbine spacing and potential shadowing, if any.

Overall, the performance results show no variation that can be attributed to the spacing of the turbines in the array. For the normal yaw condition, the power performance curves for all of the spacing values fall right on top of the baseline power curve with only a single rotor in operation. This is demonstrated to be true within the measurement uncertainty bands (4 percent). For the yawed configurations, similar behavior is observed, although the performance of the array is clearly degraded due to the yawed configuration.

The measured static strain on the central turbine clearly demonstrates the pressure load acting on the blade as the turbine generates power, with static strain values around 400 to 600 με. At large turbine separation values (that is, 16 percent), little evidence exists of the other six turbines in the vibration spectrum. However, as the spacing reaches 2 percent, there are clear indications of new peaks in the vibration spectra, especially at turbine running speed (1X). However, these increases in dynamic strain are very small in comparison to the static strain (around 6 με) and do not pose a threat to the fatigue resistance of the blades.

Flow visualization techniques are also applied to the array during the wind tunnel testing, including a smoke rake to visualize the tunnel streamlines, reflective mini-tufts to indicate local velocity vector orientation trailing the turbine array and surface oil visualization to characterize the flow path of the wind on the blade surface. Consistent with the power performance and dynamic strain results, the flow visualization results provide no indication of interaction between the turbines in the array. The velocity vectors indicated by the mini-tufts are consistent with the expected flow for a normally operating turbine. The smoke streams failed to produce meaningful results due to significant turbulence experienced in the wind tunnel, while the surface oil measurements clearly indicate the radial flow pattern along the turbine blade, as well as a small region of flow separation on the suction side of the blade. This behavior is independent of the test configuration.

Computer Simulation

While the primary objective of the experimental work is to quantify the effect, if any, of closely spaced turbines on the power performance, the secondary objective is to collect data for comparison to a computer simulation of the array configuration. Using commercially available CFD software, a model of the turbine array is created with two closely spaced turbines.

Two rotor spacing values are simulated, one at 2 percent spacing and one at 5 percent. In addition, a single rotor solution is performed to provide results for infinite rotor spacing. The CFD simulations result in a power performance curve similar to the experimental results, although some of the simulation assumptions result in a mismatch of peak tip speed ratio. Comparison of the various rotor spacing solutions demonstrates a variation of about 4 percent between each of the simulation conditions, with the closely spaced solution producing the least power for a given tip speed ratio.

Although the simulation predicts a slight power drop due to the closely spaced operation, this is not realized in the experimental effort demonstrating that if there is a power reduction, it is within the measurement uncertainty. Most likely, this difference is due to the fact that the blades are modeled with smooth surfaces and undeformed shape. Including these features may provide a more accurate representation of real performance.

As a result of this work, the operation of wind turbines in a closely spaced array is successfully demonstrated with no negative impact of power performance within the 4 percent measurement uncertainty. Operation in a 10-degree yaw orientation produces less power (as expected), but is found also to be independent of rotor spacing. The use of CFD can provide insight into the flow interactions between turbines, although it is important to consider the influence of typical model assumptions when comparing to experimental results.

Future activities regarding this wind turbine array concept include detailed design of a prototype array structure and development of the control system necessary for maximizing power performance of the entire array.

Initial design efforts focused on the use of commercially available 600 kW turbines, which proved too massive for the envisioned support structure. Because the underlying engineering approach of keeping the weight out of the mast with multiple lightweight turbines is core to the Heronemus work and patents, the OWES focus is now on developing a lightweight turbine (patent pending) more suited to the array concept. The newly designed multi-rotor tower system projects to produce an annual energy product of 15 MWhr per tower at a cost of energy of $0.032/kWh in Class 4 winds.

Soon there will be multi-rotors on the horizon!

David L. Ransom is a principal engineer at Southwest Research Institute in San Antonio, Texas. His professional experience includes engineering and management responsibilities at Boeing, Turbocare and Rocketdyne. His research interests include rotordynamics, structural dynamics, seals and bearings, finite element analysis and root cause failure analysis. He received his M.S. (Mechanical Engineering, 1997) and his B.S. (Engineering Technology, 1995) from Texas A&M University. He is a licensed Professional Engineer in Texas.

Dr. Jeffrey Moore is a program manager at Southwest Research Institute. He holds B.S., M.S. and Ph.D. degrees in Mechanical Engineering from Texas A&M University. His professional experience includes engineering and management responsibilities related to centrifugal compressors and gas turbines at Solar Turbines Inc. in San Diego, Dresser-Rand in Olean, N.Y. and Southwest Research Institute.

Marcia Heronemus-Pate is Trustee for the Heronemus family interest in OWES as well as a personal equity owner. She has actively worked to secure the Heronemus patents; engage third-party engineering studies for proof of concept, prototyping, innovation of a new light-weight turbine; and secured funding for angel investment. She continues to establish the global roots for OWES’ technology manufacturing, sales and installation, while seeking continued funding. She holds a B.A. from the University of Massachusetts, Amherst.

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