Conference PaperPDF Available

Performance of AeroMINEs for Distributed Wind Energy

Authors:
  • Westergaard Solutions, inc.
1
Performance of AeroMINEs for
Distributed Wind Energy
Suhas Pol1
Texas Tech University, Lubbock, Texas, 79409, USA
Brent C. Houchens2 and David V. Marian3
Sandia National Laboratories, Livermore, California, 94550, USA
Carsten H. Westergaard4
Westergaard Solutions Inc., Houston, Texas, 77006, USA
AeroMINE (Motionless, INtegrated Extraction) wind harvesters provide distributed
power generation with no external moving parts. The patent-protected design easily
integrates into buildings and can operate stand-alone or in conjunction with rooftop solar
photovoltaics. Here, the AeroMINE configuration of a single-pair of opposing foils is
investigated in wind tunnel tests. Through various geometric optimizations (foil spacing,
angle-of-attack and air-jet configuration) a mechanical efficiency of approximately 1/3 of the
Betz limit is achieved at a Reynolds number corresponding to the low-end wind speed for
operation at full-scale. Intermittent operation at significantly higher efficiency approaching
½ of the Betz limit is demonstrated for higher angles-of-attack, but steady operation is
impeded by an aerodynamic instability. In addition to pressure and anemometry, particle
image velocimetry is utilized to characterize the flow around and through the AeroMINE pair.
I. Nomenclature
Aduct
= cross-sectional area of AeroMINE inlet duct
Aexit
= exit area of AeroMINE defined by a rectangle enclosing the trailing edges of the foils
Ajet
= total area of air-jets (orifices) in foil surface
AoA = angle-of-attack of each foil (or half-angle between foils)
Cp
= power coefficient for the device
Pjet
= pressure-drop across the jets from inside to outside the foils
Pchoke
= pressure-drop across choke in the inlet duct
L = chord length of foils
LCOE = levelized-cost-of-electricity
Power
= mechanical power of the AeroMINE unit
PIV = particle image velocimetry
PV = photovoltaic
ρ
= air density
Re = Reynolds number based on chord
duct
= average flow velocity in the inlet duct
jet
= flow velocity at the air-jets
U
= freestream velocity
Vjet
= total volume flow through all air-jets
1 Research Assistant Professor, Department of Mechanical Engineering.
2 Principal Mechanical Engineer, Department of Thermal-Fluids Sciences and Engineering, AIAA Senior Member.
3 Product Design Engineer, Department of Mechanical Design and Additive Manufacturing.
4 President, AeroMINE Power.
2
II. Introduction
AeroMINE (Aero Motionless, INtegrated Extraction) distributed wind power generators have no external moving
parts and easily integrate into buildings (Fig. 1a). By sweeping a large area of wind with a reliable design, AeroMINEs
overcome the challenges that have plagued other distributed wind solutions and have hindered distributed wind from
playing a significant role in energy markets [1].
Incident wind creates low-pressure regions between the mirrored airfoil-pairs, and this suction pulls air from
orifices (air-jets) in the skins of the foils, from the hollow airfoil interiors, supplied by a manifold which incorporates
a turbine-generator (Fig. 1b). The turbine-generator is located inside the building, away from extreme weather
conditions and protected from people and animals. The patented-protected design of AeroMINEs are modular and
scalable [2, 3]. They complement rooftop solar photovoltaics (PV) as shown in Fig. 1a. Additional details on
AeroMINE operation, including the performance optimization, are available in Houchens et al. [4].
Fig. 1 Renderings of a) 14 AeroMINE pairs on remote buildings coupled with a 180 solar PV panels make up
a distributed energy system for inclement settings, and b) schematic of operation of an AeroMINE pair.
III. Experiments
Experiments were conducted in Texas Tech University’s National Wind Institute recirculating wind tunnel
depicted in Fig. 2 (shown with two 1/6-scale AeroMINEs mounted in parallel from a different study in Fig. 2b). The
cross-section of the tunnel test-section is 1.22 m high x 1.83 m wide (4-feet high x 6-feet wide). The wall on the
backside of the tunnel (Fig. 2a) hinges down to mount the AeroMINEs in a fixture which allows reproducible
variations of the angle-of-attack (AoA) of the foils, spacing between a pair of foils, and spacing between multiple
devices mounted in parallel. This fixture also allows mounting at off-axis orientations to investigate the impact on
efficiency of off-axis incident wind. The opposite side of the test section (Fig. 2b) is transparent, allowing particle
image velocimetry (PIV) using a laser mounted downstream of the test section and an external camera array. The
tunnel is seeded with oil droplets for this purpose.
Here 1/3-scale (based on chord) single-pair AeroMINE was tested. The foil dimensions were 0.5 m chord and 0.8
m height (mounted sideways as shown in Fig. 3). Small rib sections containing static pressure taps were inserted in
the middle of the device as shown in green in Fig. 3. Tubes were run from the taps to outside the wind tunnel for
pressure measurements, and a reference static tap was placed at the inner wall of the wind tunnel near the corner.
Pressures were measured at five locations along the low-pressure sides of both of the foils. The S1210-based design
was selected for its excellent lift characteristics over a wide range of Reynolds numbers [5]. However, as will be
shown, the airfoil performance changes dramatically due to the presences of the second mirrored foil, even when all
of the air-jets are covered.
a)
b)
internal
turbine-generator
internal
flows
air-jets
duct
flow
inlet
3
Fig. 2 Wide-angle photos of the National Wind Institute wind tunnel including a) the backside and b) the PIV
camera array on the transparent side, looking into a smaller array of two AeroMINE pairs.
Testing on the rapid prototyped foils was performed at freestream velocities of 5, 7.5, 10 and 12 m/s, corresponding
to chord-based Reynolds numbers (Re) between ~139,000 and ~333,000. Results are shown for either 10° or 15°
AoA, previously determined to be near optimum performance based on the measured inlet duct velocity [4]. The
velocity was measured forward of the AeroMINE and just off the wall of the wind tunnel with a vane anemometer.
This was taken as the freestream value
U
. The proximity of this measurement to the wall is expected to make this
representative of the average velocity seen across the height of the AeroMINE pair, which is zero at the base.
Fig. 3 a) 0.5 m chord AeroMINE pair mounted in the wind tunnel and b) a close-up showing three rows of
open air-jets and the green rib containing static pressure taps along the low-pressure side of the foil.
a)
b)
4
The foil pressure profiles were sampled along the low-pressure sides of the foils via static pressure taps in the
green insert shown in Fig. 3. A choke was placed at the inlet flow to the duct to simulate the load of the turbine-
generator (Fig. 4). The constriction of the choke was varied to produce a load curve.
Pressures were measured on either side of the choke near the duct inlet and the average velocity was measured
inside the duct downstream of the choke, allowing calculation of a mechanical power, as shown in Fig. 4. The choke
simulates the turbine-generator under various loading conditions. Additionally, PIV measurements of the flow field
were taken to better understand the acceleration between the foils and the wake structure.
Fig. 4 a) Duct inlet outside the wind tunnel with choke section and
b) close-up view down the inlet showing the choke and hot wire anemometer.
Blockage measurements showed acceptable impact of the device on the freestream velocity in the adjacent portion
of the tunnel. The speed in the tunnel adjacent to the device was measured to be between 10% and 13.5% higher than
the freestream
U
values taken just off the floor of the tunnel. This was true for both the full-length (0.8 m) and a
half-length (0.4 m) devices suggesting that the primary increase in this velocity over the assumed freestream
U
is
due to the freestream measurement location being closer to the wall.
IV. Results and Discussion
First the pressure profiles of just the foils with all air-jets closed are presented. In this configuration the AeroMINE
produces no power and there is no flow in the inlet. Strong asymmetries thought to result from a flow instability are
observed in this no-power configuration.
Then results with the air-jets open, a configuration in which the system produces power, are presented. The
reduction of asymmetries with a choke load are presented. Then the aero-mechanical efficiency of the system is
discussed. Together these provide an estimate of potential full-scale system efficiency and insights into sensitivities
to flow instabilities which limit the maximum viable AoA for steady performance.
A. All Air-jets Closed: No-Power Configuration
With all air-jets closed (taped over) the pressure at five locations on the low-pressure side of both foils was
measured. For convenience of explanation the foils are referenced here as “top” and “bottom” based on their relative
position on the wall. The pressure values for two realizations are shown in Fig. 5. Both realizations showed strong
a)
b)
inlet flare
inlet duct with
area 
choke section
hot wire
measuring 
5
asymmetry, with one foil maintaining a much higher pressure far from the leading edge. Initially there was concern
that the system was not aligned parallel to the wind direction in the tunnel, which would explain the asymmetry.
However, after multiple realizations it was observed that the asymmetry switched side (second realization shown on
the same plot in Fig. 5a), suggesting this is the result of a flow instability.
Fig. 5 a) Pressure tap measurements for two realizations for the “top” and “bottom” foils with all air-jets
closed (no-power) at AoA = 15° and
U
= 10.4 m/s. For reference, two pressure tap locations are shown in b)
with arrows pointing to their related measurements. Note that the taps do not extend to the trailing edge.
This asymmetry was observed even for AoA = 10° with all air-jets closed as shown by PIV data in in Fig. 6. Note
the zero-velocity portion below the “lower” airfoil in the PIV data is due to the support which can be seen in Fig. 3.
Some artifacts from the mounts are also visible near the foil surfaces.
Fig. 6 PIV sampling in a plane aligned with the chord with all air-jets closed
(no-power) at AoA = 10° and
U
= 10.4 m/s.
B. Air-jets Open: Pressures, PIV and Aero-Mechanical Efficiency
With the air-jets open, various chokes were explored. When the choke is fully open (no-choke), air is pulled from
outside the wind tunnel through the inlet (shown in Fig. 4), moves through the foil internal ducts, exits through the
air-jets and mixes into the freestream flow. Within the measurement error, no significant pressure difference was
observed between the inside and outside of the tunnel, indicating air-jet outflow results from AeroMINE operation.
Though the air-jets are operational, without a choke there is no load on system. The pressures along the foils are
shown in Fig. 7a for AoA = 15° and Fig. 7b for AoA = 10°. In general the asymmetry is reduced with the air-jets
operational as compared to the all air-jets closed scenario (Fig 5a). Some asymmetry still occurs in the AoA = 15°
case, with the realization shown here considered a typical case. The asymmetry is reduced by reducing the AoA to
10°, but this comes at a price of sweeping less area of the incident wind and thus reducing the potential power available.
a)
b)
6
Fig. 7 Pressure tap measurements on the suction side for the “top” and “bottom” foils with air-jets
operational and no-choke (open inlet), for
U
= 10.4 m/s and a) AoA = 15° and b) AoA = 10°.
When a choke, representative of the internal turbine-generator, is included as shown in Fig. 4b, symmetry is greatly
improved. This can be seen from the PIV data in Fig. 8 for AoA = 10° where the a) no-choke and b) choked flows
are shown. As the choked scenario represents the most realistic operating condition, this improvement in symmetry
is highly beneficial to the practical design.
Fig. 8 PIV data for
U
= 10.4 m/s and AoA = 10° showing the a) no-choke scenario and b) improved
symmetry when a load is applied via a choke at the inlet of the AeroMINE.
Finally, the aero-mechanical efficiency provides a measure of the maximum power extraction. By varying the size
of the choke it is possible to produce a load curve for the system. The pneumatically transmitted mechanical power
was calculated using the measured pressure drop across the choke, multiplied by the duct area and the average duct
velocity
 =   .
The swept wind is taken to be the largest area of the AeroMINE,
Aexit
, defined by a rectangle enclosing both trailing
edges of the foils. This exit area includes freestream wind mixed with air from the air-jets. The power available for
extraction from the wind in the swept area of the AeroMINE in the wind tunnel represents the maximum possible
power of the system, given as
   = 0.5 
Dividing the
Power
by the maximum    in the swept wind gives the aero-efficiency of the
system. The peak measured efficiency at AoA = 10° was determined to be 18%, which equates to roughly 1/3 of the
Betz limit. The full load curve, measured by varying the choke, is shown in Fig. 9.
a)
b)
a)
b)
7
Fig. 9 Load curve for 1/3-scale AeroMINE at AoA = 10° for four freestream velocities spanning 5 to 12 m/s.
C. Flow Instabilities
Operation at higher AoA than reported in the above section has the potential to increase the energy extraction of
the system by producing a lower pressure between the foils. For an individual S1210 foil alone and for Re at and
above 200,000 the lift continues to increase dramatically up to and past AoA of 20° [5]. Thus, operating at a higher
AoA is expected to increase the power of the AeroMINE system.
Note that, in some cases, this increased energy extraction may not offset the increase in swept area, such that while
the power produced might increase, the efficiency could decrease due to a more than proportionally larger increase in
swept area. Nevertheless, in building installations, the optimum design may correspond to a non-optimum efficiency,
driven by capital expenditure and geometric constraints. Thus, higher AoAs were investigated to better understand
subtleties of this optimization.
At higher AoA it is observed that AeroMINEs experiences a flow instability not observed for a single S1210
airfoil. Efficiencies at these higher AoAs can reach as high as 27%, but are subject to instabilities which significantly
reduce the power production and result in efficiencies around 10%, well below the stable value of 18% observed at
AoA = 1 as shown above. The flow mode associated with the higher-AoA, lower-efficiency operation of 10%
efficiency appears to be associated with a stable mode, as recovery to the higher efficiency has not been observed.
This instability also appears to be unique from the symmetry-breaking instability observed when all air-jets are closed.
V. Conclusions
At the optimum stable geometric configuration, AeroMINE has achieved an 18% mechanical efficiency based on
wind tunnel experiments conducted at the National Wind Institute at Texas Tech University. Asymmetries apparently
due to flow instabilities are present both for scenarios with no air-jets (two mirrored foils alone) and with air-jets
operational but no load applied. Applying a load from a choke improves the symmetry of the flow.
A poorly understood second instability exists at higher AoA. In this configuration up to 27% efficiency can be
achieved, but the stable flow mode is associated with a very low efficiency. Understanding and controlling this
instability is the focus of future research efforts.
Acknowledgments
This work was funded in part by awards from the 2017 Sandia Pitch Competition and subsequent DOE-wide
National Lab Accelerator Pitch Event to aid in commercialization of promising technologies, as well as a grant from
the GLEAMM Spark Fund to accelerate the commercialization of energy technologies through gap funding.
Laboratory Directed Research and Development (LDRD) funds have also been provided by Sandia National
Laboratories through the 2018 Innovation Challenge initiative of the Environment and Homeland Security Investment
Area.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology &
Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S.
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
00.1 0.2 0.3 0.4 0.5 0.6 0.7 0. 8 0.9 1
Power coefficient Cp
Orifice diameter ratio
Re=138,889 Re=208,333 Re=277,778 Re=333,333
8
Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. The views
expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States
Government. This paper is referenced internally to Sandia National Laboratories by number SAND2019-14842 C.
References
[1] Orrell, A. C., Foster, N. F., Morris, S. L. and Homer, J. S. “2016 Distributed Wind Market Report,Prepared for the U.S.
Department of Energy Office of Energy Efficiency and Renewable Energy by the Pacific Northwest National Laboratory, 2017.
[2] Westergaard, C.H., Fluid flow energy extraction system and method related thereto, US2017298900, US Patent Office.
[3] Houchens, B.C., Blaylock, M.L., Marian, D.V. Maniaci, D.C. and Westergaard, C.H., Methods, systems, and devices to
optimize a fluid harvester, priority app US 16/182,488, 2018.
[4] Houchens, B. C., Marian, D. V., Pol, S. and Westergaard, C. H. A Novel Energy Conversion Device for Wind and Hydro-
kinetic Applications,” Proceedings of the ASME-JSME-KSME 2019 Joint Fluids Engineering Conference, AJKFLUIDS2019-
5542, ASME, San Francisco, CA, 2019.
[5] Selig, M.S., Guglielmo, J.J., Broeren, A.P. and Giguere, P. Summary of Low-Speed Airfoil Data,Vol. 1, 1995.
... AeroMINE has been experimentally tested at Texas Tech University's National Wind Institute recirculating wind tunnel [2]. These wind tunnel experiments demonstrated that the mirrored airfoil pair creates a sufficient pressure gradient to drive flow in the duct. ...
... The AeroMINE was tested in a wind tunnel and data on the pressure gradient and flow velocity in the duct were recorded [2]. This data was used to find the optimal flow conditions for the FOM. ...
Conference Paper
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View Video Presentation: https://doi.org/10.2514/6.2021-3368.vid Multivariate designs using three optimization procedures were performed on a low Reynolds number (order 100,000) turbine blade that maximized lift over drag. The turbine blade was created to interface to AeroMINE, a novel wind energy harvester that has no external moving parts. To speed up the optimization process, an interpolation-based procedure using the Proper Orthogonal Decomposition (POD) method was used. This method was used in two ways: by itself (POD-i) and as an initial guess to a full-order model (FOM) solution that is truncated before it reaches full convergence (POD-i with truncated FOM). To compare the result of these methods and their efficiency, optimization using a FOM was also conducted. It was found that there exists a trade off between efficiency and optimal result. The FOM found the highest L/D of 28.87 while POD-i found a L/D of 16.19 and POD-i with truncated FOM found a L/D of 19.11. Nonetheless, POD-i and POD-i with truncated FOM were 32,302 and 697 times faster than the FOM, respectively.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-2203.vid A novel wind energy harvesting system has recently been designed to capture distributed wind energy. This design abandons the traditional turbine design that is standard for wind energy, and instead adopts two mirrored airfoils and extracts energy from the low pressure region between the foils. During experimental testing, the system showed high potential, with an increase in harvesting capacity as the angle-of-attack of the airfoils increased. This benefit, however, disappeared for sufficiently high angle-of-attack, commensurate with a shift from a symmetric to a directed wake. To investigate this detrimental transition, we present here simulations for the mirrored airfoil configuration at a Reynolds Number of 1000. Using this data, we provide a regime map that conveys the system dynamics for a sweep of the angle-ofattack and spacing between the mirrored airfoils. Our results indicate that the transition to the asymmetric wake is due to a linear instability—absent in the single airfoil case—that is due to the aerodynamic coupling of the foils. We explore this instability mechanism using detailed analysis of the time-varying flow-field and comment on potential implications on harvesting potential for this wind energy system.
Conference Paper
In its simplest implementation, patent-protected AeroMINE consists of two opposing foils, where a low-pressure zone is generated between them. The low pressure draws fluid through orifices in the foil surfaces from plenums inside the foils. The inner plenums are connected to ambient pressure. If an internal turbine-generator is placed in the path of the flow to the plenums, energy can be extracted. The fluid transports the energy through the plenums, and the turbine-generator can be located at ground level, inside a controlled environment for easy access and to avoid inclement weather conditions or harsh environments. This contained internal turbine-generator has the only moving parts in the system, isolated from people, birds and other wildlife. AeroMINEs could be used in distributed-wind energy settings, where the stationary foil pairs are located on warehouse rooftops, for example. Flow created by several such foil pairs could be combined to drive a common turbine-generator.
Distributed Wind Market Report
  • A C Orrell
  • N F Foster
  • S L Morris
  • J Homer
Orrell, A. C., Foster, N. F., Morris, S. L. and Homer, J. S. "2016 Distributed Wind Market Report," Prepared for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by the Pacific Northwest National Laboratory, 2017.
Fluid flow energy extraction system and method related thereto, US2017298900, US Patent Office
  • C H Westergaard
Westergaard, C.H., Fluid flow energy extraction system and method related thereto, US2017298900, US Patent Office.
Methods, systems, and devices to optimize a fluid harvester
  • B C Houchens
  • M L Blaylock
  • D V Marian
  • D C Maniaci
  • C H Westergaard
Houchens, B.C., Blaylock, M.L., Marian, D.V. Maniaci, D.C. and Westergaard, C.H., Methods, systems, and devices to optimize a fluid harvester, priority app US 16/182,488, 2018.