POWER June 2021 - 18

WIND TECHNOLOGY
2. ÉOLE, a 3.8-MW Darrieus turbine with a
total height of 110 meters, is the largest vertical-axis
wind turbine (VAWT) ever constructed.
The turbine, built in Cap-Chat, Quebec, had a
rotating mass of 880 metric tons. It operated
for 18,550 hours between 1987 and 1993. It
was shut down due to a failure of the bottom
bearing. Courtesy: Paul Gipe/Wind-Works.org
dian of 5.5 MW for land-based turbines,
and 17 MW for offshore turbines. That in
turn will drive substantial cost declines
of between 35% and 50% by 2050 for
both land-based and offshore wind installations,
it said.
However, in a recent paper published
in the journal Renewable Energy,
experts from Oxford Brookes
University in England, argue that traditional
propeller-type HAWT wind
farms have a major flaw. " As the wind
approaches the front row of turbines,
turbulence will be generated downstream.
The turbulence is detrimental
to the performance of the subsequent
rows, " said lead author of the report
Joachim Toftegaard Hansen. " In other
words, the front row will convert about
half the kinetic energy of the wind into
electricity, whereas for the back row,
that number is down to 25-30%. "
The research led by Professor Iakovos
Tzanakis at the public research university's
School of Engineering, Computing
and Mathematics (ECM) scrutinized
11,500 hours of computer simulation.
It showed, essentially, that if HAWTs
were replaced with large-scale verticalaxis
wind turbines (VAWTs) at a turbine
diameter (Reynolds number 1.35 · 107),
VAWTs exhibited a 15% increase in
power output compared to operating in
isolation, specifically when the second
rotor was spaced three turbine diameters
downstream and at an angle of 60
degrees to the wind direction. If positioned
in a series, the power output was
even greater, by about 3%.
The study is notable because though
several VAWT projects (mostly involving
Darrieus turbines in North America
and H-rotors in Europe) have been built
since the 1970s (Figure 2), many prototypes
had terminal failures or problems
with metal fatigue and durability.
18
But according to Erik Möllerström from
the Rydberg Laboratory for Applied Sciences
at Halmstad University, VAWTs
have also generally suffered a lack of
long-term interest from governmental
or private funders. Meanwhile, the Oxford
Brookes study acknowledges that
VAWTs offer an efficiency of 35% to
40% in isolation. A HAWT's, by comparison,
is close to 50%. Yet another
challenge is that some VAWT designs
with a low starting torque require external
power to accelerate the turbine at
small angular velocities to its optimal tip
speed ratio.
Another concept developed decades
ago that is seeing a moderate resurgence
in interest is the diffuser-augmented
wind turbine (DAWT), also known as
" Wind Lens. " Essentially HAWTs that
feature a diffuser-type funnel-like structure,
these concepts work by collecting
and concentrating approaching wind.
DAWT development is ongoing at Kyushu
University in Japan, which has
studied several configurations, including
multi-rotor systems with a maximum
tested power of 100 kW. The university
has also installed a multi-rotor version
on an 18-meter-wide concrete platform
in an offshore hybrid farm experiment at
Hakata Bay, Japan.
Going Bladeless
One newer concept, meanwhile, turns
entirely away from blades, seeking instead
to harvest energy through the air
flow-induced vibrations of mechanical
systems-through " aeroelastic phenomena. "
Vortex Bladeless, a Spanish
tech startup that has received funding
from the the European Union's Horizon
2020 research and innovation programme,
recently garnered Equinor's
backing to develop " aerogenerators "
for the residential market.
Vortex Bladeless technology (Figure
3) is a silent " vortex-induced vibration
resonant wind generator " that harnesses
wind energy from a phenomenon
of vorticity called " vortex shedding, " it
explained. " Basically, bladeless technology
consists of a cylinder fixed vertically
with an elastic rod. The cylinder
oscillates on a wind range, which then
generates electricity through an alternator
system. In other words, it is a wind
turbine which is not actually a turbine, "
it said. Explained in fluid mechanics
terms: " As the wind passes through
a blunt body, the flow is modified and
generates a cyclical pattern of vortices.
Once the frequency of these forces is
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close enough to the body's structural
frequency, the body starts to oscillate
and enters into resonance with the
wind. This is also known as Vortex Induced
Vibration (VIV), " it said.
Development on the turbine has
been ongoing for at least three years,
but the company has already installed
field tests of its 2.75-meter-high 100W
Vortex Tacoma device. While a commercialization
timeframe for that device
is still uncertain, the company in 2021
manufactured about 100 units of an
85-centimeter-high version, the Vortex
Nano, to distribute to universities, independent
laboratories, municipalities,
and parks around the world. " The aim is
to obtain measurements, study their behavior
in different locations and types of
installations, and generate metrics that
will allow the devices to be improved
and their long-term behavior to be understood
according to the variables of
the environment. "
Vortex, however, is optimistic the
larger Tacoma device will play a role in
residential applications, much as lowpower
solar photovoltaics does today.
It envisions installations of multiple
devices in " arrays or grids, " noting the
devices only need to be separated by a
distance that is equivalent to half their
height. The low-power market is " an
almost unexploited market for a wind
technology like ours; which can give you
little power but constant all year round
even with weak or turbulent and very
changeable winds, which requires almost
no maintenance, which is safe and
3. Vortex Bladeless is a vortex-induced
vibration resonant wind generator that harnesses
wind energy from a phenomenon of
vorticity called vortex shedding. Courtesy:
Vortex Bladeless
POWER | June 2021
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POWER June 2021

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