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Harvesting Wind Power from Tall Buildings

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Abstract and Figures

The incorporation of wind turbines into tall buildings is becoming increasingly common as a method of both reducing carbon footprint and making a very public statement about a building's green credentials. There are, however, a number of considerations that should be assessed in determining the long-term environmental benefits of these incorporations. This paper will discuss some of the practical aspects of assessing the benefits of incorporating wind turbines, methods of assessing efficiency and optimizing design, and a discussion of key issues in introducing wind turbines to the urban environment.
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Integrating Wind Energy into the Design of Tall Buildings –
A Case Study of the Houston Discovery Tower
WINDPOWER 2008
Brad C. Cochran, MS
Rick R. Damiani, PhD, P.E. (Europe)
CPP, Inc.
1415 Blue Spruce Drive
Fort Collins, CO 80524
Corresponding Author: Brad Cochran
970-221-3371
bcochran@cppwind.com
WindPower 2008, Houston, Texas June 2-4, 2008 1
Harvesting Wind Power from Tall Buildings
Brad C. Cochran
1
, Rick R. Damiani
1
1. CPP, Wind Engineering and Air Quality Consultants, 1415 Blue Spruce Drive, Fort Collins, CO,
80524, USA
Abstract
Integrating wind energy systems into building design is a small but growing trend, and high rises with
their elevated wind speeds seem particularly suited to the technology. Designs that incorporate wind
turbines are increasingly being seen on the drawing boards for skyscrapers across the globe. Leading
the development of this fledgling field, wind engineers at CPP have evaluated the potential for wind
turbine integrated buildings in both the U.S. and China. This paper will focus on a project conducted for
the Discovery Tower in downtown Houston, TX, which is slated to break ground in early 2008.
Keywords: Small wind turbines, tall buildings, wind resource assessment
Introduction
Tall building designers are showing an
increasing interest in reducing the environmental
impact of the construction and operation of their
buildings. One of the approaches currently used, is
the incorporation of on-site power generation.
This is primarily achieved by integrating solar and
wind devices into the design of the building. One
such example is the Discovery Tower, which is
currently under construction in Houston, TX.
The requirements for optimizing the
performance on wind generators in an urban
environment are quite different from those
pertaining to wind farms in open sites. This entails
the use of different design approaches to assess
the optimum placement of wind turbines within
the building envelope, the most suitable generator
types for the building environment, and to
estimate annual energy production for the wind
turbines.
This paper will examine aspects involved in
wind power generation on tall buildings, and how
those were specifically addressed for the
Discovery Tower.
Wind climate
When contemplating the incorporation of
wind power generation into a tall building design,
the first consideration must be the local wind
climate of the area. Bluntly, if there is not
sufficient wind resource in the area, then the
potential for successful use of turbines will be
very limited.
Wind conditions in urban environments tend
to be very different. The effect of urban
environments on a boundary-layer is shown in
Figure 1. This shows how buildings slow the wind
near the ground, and increases turbulence.
Turbines work most efficiently in low-turbulence
environments; therefore care needs to be taken in
specifying turbine types that will cope with both
existing turbulence levels and potential future
changes as a result of urban development.
Urban development is likely to pose one of
the greatest challenges to increasing use of
turbines on tall buildings. In city center locations,
height restrictions often mean that many tall
buildings are of similar heights. Even if a building
is very tall, if all the surrounding buildings are of
similar height then the potential for efficient
turbine installation is significantly reduced.
Unlike rural wind farms, where the nearest
anemometer may be located many miles away,
most cities have reasonable lengths of records
from nearby airports. This is not, however, to say
these are necessarily good records. It is not
uncommon to see anomalous directionality
characteristics due to poor anemometer siting
close to buildings. Wherever possible, records
from multiple stations should be used. A rule of
thumb is to use a minimum of 10 years of records
to ensure statistical robustness. Trends are also
sometimes apparent that don’t reflect climate
changes, but are more often indicative of changing
urban development close to the anemometer site.
WindPower 2008, Houston, Texas June 2-4, 2008 2
In all cases, the first stage is to correct the data
back to the equivalent of open-country exposure
to simulate the readings that would be experienced
in the absence of any development. There are a
number of methods for doing this, and the
approach codified by ESDU (1993a and 1993b)
and based on the work of Deaves and Harris is
among the most common.
In areas where available anemometer records
are suitable to describe the local wind
environment, a simple wind speed transfer
approach can be utilized, as illustrated in Figure 1.
In this approach, the wind speeds at the
anemometer are extended to a gradient height
(200 m to 600 m above grade), where the local
terrain has little or no impact on the wind speeds,
using a power law relationship. That same wind
speed is then assumed to exist at the gradient
height (which may not be at the same height
above grade) above the site. The wind speeds are
then transferred down to the site using either the
power law with a site specific exponent, or by
measuring the vertical velocity profile in an
atmospheric boundary layer wind tunnel.
When there are no reliable anemometer
records within a reasonable distance of the site,
meso-scale modeling can be used to determine the
wind climate of the area. This uses input from
historical meteorological records from, maybe,
hundreds of kilometers away to regenerate the
weather systems affecting the site. This is an
approach that is commonly used for rural turbine
locations.
The directionality of the wind is also
important. The incorporation of turbines into tall
buildings tends to favor limited wind directions,
perhaps within a 45 degree sector, depending
upon the building configuration and the location
of the wind turbines on the building.
Figure 2 shows the wind frequency
distribution at the Hobby Airport in Houston, TX,
while Figure 3 shows the alignment of the
Discovery Tower. The wind turbines are located
along the southwest side of the roof. The wind
rose indicates that the predominant winds in
Houston are from the SSE through S. The
discovery tower is aligned such that the broad
sides of the structure are along a SE to NW axis.
As a result, the roof-top turbines are likely to be
somewhat sheltered from the predominant winds,
whereas the most favorable wind conditions for
the turbines (winds from the SW) are fairly
infrequent. As shown later on, this mismatch will
significantly impact the wind energy potential for
the roof top wind turbines.
Basic tall building
aerodynamics
As discussed in the previous sections, it is
desirable to locate turbines in regions of high
wind speed and low turbulence. Describing the
wind flow around a tall buildings can be quite
complex and has been studied in depth for many
years (Cermak, 1975 and 1976). A simplified
sketch of the mean flow is shown in Figure 4.
There will be positive pressure on the windward
face and negative pressure on the side and leeward
faces. As air naturally flows from areas of high
pressure to areas of low pressure, the most
effective locations for wind turbines will be either
in the accelerated shear layers around the edge
and top of the building, or in specially developed
passages linking the areas of positive and negative
pressure. Note that wind speeds close to the center
of a flat roof may be low as this area is often in a
region of separated flow. Whereas with a pitched
or tiered roof, the center may be the location of
the greatest wind resource.
Figure 1: Transfer of Wind Speed from Hobb
y
Airport to
the Discovery Tower site
WindPower 2008, Houston, Texas June 2-4, 2008 3
Shaping of tall buildings to increase
efficiency of wind turbines
Shaping of tall buildings can be used
effectively to enhance the performance of wind
turbines. Two examples of this are the Bahrain
World Trade Center (Figure 5) and the Pearl River
Tower in China (Figure 6). The Bahrain World
Trade Centre Tower is formed to create a Venturi
effect, placing the horizontal axis turbines
between two wings of the building. This approach
clearly works for only a limited number of wind
directions, but may be useful in a location with a
very dominant prevailing wind direction.
Restricting the orientation of horizontal axis
turbines, however, severely limits the efficiencies
gained from using this type of turbine. In the Pearl
River Tower, slots through the tower are used to
relieve the pressure between the front and rear
faces of the tower with these slots being
aerodynamically shaped to increase flow through
them. Again, this approach is most efficient for
only a few wind directions, but has the advantages
accelerating the flow while likely reducing the
turbulence approaching the turbines.
Wind Energy Potential
A physical model of the Discovery Tower
and surroundings within a 450 m radius, shown in
Figure 2: Wind frequenc
y
distribution at Hobb
y
Airport, Houston TX.
Figure 3: Ali
g
nment of the Discover
y
Tower i n
Houston, TX.
S
S
S
S
E
E
S
S
W
W
Figure 4: Wind flow around a tall building.
WindPower 2008, Houston, Texas June 2-4, 2008 4
Figure 3, was placed in an atmospheric boundary
layer wind tunnel. The wind tunnel was used to
characterize the wind environment on the tower
roof, including any shielding or acceleration. The
analysis included vertical wind profiles at four
locations along the southwest end of the tower for
16 approach wind directions. Each profile was
collected using a five-hole probe, shown in Figure
8, which is capable of measuring the mean wind
vector and turbulence intensity values. The mean
wind speeds were combined with the wind
frequency distributions to determine the wind
resource at each measurement location. The
velocity vector and turbulence intensity values
were used to determine the appropriateness of
various wind turbine designs.
Figure 7 Photograph of the 1:300 scale model installed in
an atmospheric boundary layer wind.
The mean velocity vector within each profile
was used as the transfer value between the
reference velocity at 152 m above grade (upwind
and outside of the effects of the local buildings)
and the local wind speed. For profiles with little or
no local shear, the average value accurately
represents the wind environment across the entire
turbine rotor. In areas with high shear, this value
may be less valid. A more accurate wind speed is
Figure 6: Pearl River Tower (Image courtesy
Figure 5: Bahrain World Train Center (Ima
g
e
courtesy of www.bahrainwtc.com
)
WindPower 2008, Houston, Texas June 2-4, 2008 5
obtained by integrating the wind speed vertical
distribution. However, since information on the
impact of the vertical shear is not available from
most turbine manufacturers, the average wind
speed was used to characterize each profile.
Figure 9 shows the distribution of the normalized
mean wind speeds (the ratio of the local wind
speed with (U
mean
) and without (U
ref
) the presence
of the building) at each of the four measurement
locations as a function of the approach wind
speed.
Figure 8: Close-up of the roof showing the 5-hole probe
used to measure local wind velocities.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW
Wind Dire ction
U
mean
/U
ref
Location 1 Location 2 Location 3 Location 4
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW
Wind Dire ction
U
mean
/U
ref
Location 1 Location 2 Location 3 Location 4
Figure 9: Mean longitudinal wind speed distribution vs.
approach wind direction
The results indicated that there is accelerated flow
(U
mean
/U
ref
greater than unity) on the roof of the
Discovery Tower at the proposed wind turbine
locations for wind directions from the SSW
through SW. For these wind directions the local
wind speeds are as much as 20 percent greater
than the reference wind speed. The results also
indicate that there is considerable sheltering of the
air flow for winds from E through SE and from W
through NW.
W
IND POWER DENSITY
In order to estimate how much energy a
specific turbine will be expected to produce at a
given location, the wind resource at that location
must be identified. A wind turbine works by
extracting kinetic energy out of the wind and
converting it to mechanical and then electrical
energy. The power that is available in the wind to
be converted to electrical energy is defined in the
following relationships:
AUP
w
3
2
1
ρ
= Equation 1
and
APPD
w
/
=
Equation 2
or
3
2
1
UPD
ρ
= Equation 3
Where:
PD -Power density, W/m
2
;
P
W
- Power available in the wind, W;
ρ - Air density, kg/m
3
;
U - Wind speed approaching the wind
turbine, m/s; and
A- Projected area of the turbine
perpendicular to the approaching wind,
m
2
.
Since the potential power production is
proportional to the wind speed cubed, the annual
average wind power density cannot be defined by
strictly using the mean annual wind speed. Rather
some knowledge of the distribution of wind
speeds must be known to accurately estimate the
annual average wind power density (PD). This
can be achieved by applying Equation 3 to the
hourly recorded wind speed measurements
obtained throughout the year and then taking the
average of the hourly values, where U is the local
mean wind speed derived from the airport
anemometer and the normalized wind speed
distributions for each measurement location, using
the techniques described above.
WindPower 2008, Houston, Texas June 2-4, 2008 6
The results of this analysis are shown below in
Table 1. The table indicates that the wind power
density at the site in the absence of the Discovery
Tower and surrounding buildings is 472 W/m
2
.
When the impact of the Discovery Tower and
surrounding buildings is taken into account, the
wind power density values are reduced by
approximately 1/3
rd
and range from 114 W/m
2
to
153 W/m
2
.
Table 1
Average wind speed and wind power density
To determine the feasibility of wind power at a
site, the wind power density is often compared to
various classifications developed to describe a
site’s wind power potential. Table 2 lists the
classifications as a function of the predicted wind
power density and mean annual wind speed.
The Discovery site falls into Class 4. The
resource potential in a Class 4 environment is
considered “Good”. In the U.S. much effort has
been undertaken in the last few years to develop
wind turbines that are economically feasible in a
Class 3 environment, since this is the most
common wind class found in the U.S. Therefore,
these results suggest that the wind resource at the
Discovery Tower site is sufficient to be
considered feasible for modern wind turbines.
However, as currently configured, the wind
turbines on the roof of the Discovery Tower will
only experience a Class 1 to Class 2 wind
environment. In this environment it may be
economically difficult to justify wind turbine
installations.
Table 2
Wind Power Classifications
Wind
Power Resource
Class Potential
2 Marginal 200 - 300 5.6 - 6.40
3 Fair 300 - 400 6.4 - 7.00
4 Good 400 - 500 7.0 - 7.50
5 Excellent 500 - 600 7.5 - 8.00
6 Outstanding 600 - 800 8.0 - 8.80
7 Superb 800 - 1600 8.8 - 11.1
* - Wind classifications are typically base on the wind power density at 50m above grade
Wind Power
Density
*
(W/m
2
)
Annual Average
Mean Wind Speed
(m/s)
Wind Environment Characterization
In order to determine the appropriateness of
installing wind turbines at the site, it is not only
important to evaluate the wind power density, but
also to characterize the wind environment in terms
of flow vectors, gradients, and turbulence
intensities. The local wind speeds on the roof of
the Discovery Tower were measured using the
Aeroprobe 5-hole probe, described above. The 5-
hole probe provides measurements of wind speed
in each of the three coordinates, longitudinal (U),
lateral (V), and vertical (W) and can provide both
mean and fluctuating wind speeds. For this
analysis, each of the three components of the local
wind velocity were normalized by the mean
reference wind speed measured upwind of the
turntable in unobstructed flow at a full-scale
height of 152 m above the local grade. This results
in normalized wind speed values of U
mean
/U
ref
;
V
mean
/U
ref
; and W
mean
/U
ref
. The local turbulence
intensity values were calculated by normalizing
the fluctuating component of the wind speed by
the mean longitudinal wind speed (U
rms
/U
mean
;
V
rms
/U
mean
; and W
rms
/U
mean
). The lateral (θ
Y
) and
vertical (θ
z
) angles of attack were calculated as the
inverse tangent of the lateral to longitudinal
(V
mean
/U
mean
) and vertical to longitudinal
(W
mean
/U
mean
) velocity ratios. Finally, the
magnitude of the local velocity vector was
Annual Average Annual Wind
Wind Speed Power Density
Location (m/s)
(W/m
2
)
Reference
1
6.89 472
1 4.26 153
2 3.93 114
3 4.11 141
4 4.18 148
1) 152 m above local grade, outside of the influence of
nearby structures.
WindPower 2008, Houston, Texas June 2-4, 2008 7
calculated as the square root of the sum of the
squares of the three velocity components.
Figure 10 and Figure 11 show the vertical
distributions above roof level of mean velocity
and turbulence intensity at Location 2 for
approach wind direction of 135 degrees (the
prominent wind direction) and 202.5
(perpendicular to the broad side of the Discovery
Tower). The plot on the right side of each figure
shows the normalized mean velocity as a function
of the height above the local roof. The plot on the
left side shows the distributions of longitudinal
turbulence intensity as a function of the height
above the local roof.
Turbulence Intensity. Turbulence intensity
is the ratio of the fluctuating velocity to the mean
velocity and is a measurement of the gustiness of
the wind. Although all three components of
turbulence intensity are calculated, typically only
the longitudinal turbulence is relevant because it is
most readily available from field measurements.
Therefore, for the purpose of this analysis, the
focus was placed on the longitudinal turbulence
intensity, even though the lateral and vertical
turbulence intensity values can be significant
In an unobstructed open field environment
turbulence intensity values are typically in the
range of 10% to 15% at 30 m above grade and
decrease at higher elevations. Utility scale wind
turbines are typically designed for maximum
turbulence intensity values around 17% to 18%.
Smaller turbines, particularly those designed to be
integrated into buildings, must be able to
withstand much higher turbulence intensities.
Figure 10 indicates that the turbulence
intensity values at Location 2 for a southeast wind
direction are consistently above 60% throughout
the entire profile. This is likely due to vortex
shedding occurring at the upwind corner of the
Discovery Tower for this wind direction. The
turbulence intensity values for a southwest wind
direction, shown in Figure 11 are substantially
lower, particularly at 5 m above the roof and
higher. In this region, the turbulence intensity
values are within the range that one would expect
to find in an open field environment.
Wind Shear. The wind shear is a description of
the rate of change in wind speed along the vertical
profile. It is defined by the exponent, n, in the
power law equation. Wind shear is important,
particularly on large turbines, because it can
create unequal wind loading along the vertical
axis of the wind turbine. In an unobstructed open
environment the power law exponent typically
ranges between 0.1 and 0.2. Utility scale wind
turbines are typically limited to operating in
environments with wind shear values less than 0.2
to 0.23. Once again, building integrated wind
turbines must be designed to handle the higher
wind shear values that are commonly present on
and around physical structures.
WindPower 2008, Houston, Texas June 2-4, 2008 8
Figure 10: Wind shear and turbulence intensity ratios (in the plane of the rotor) for a wind direction of 135 degrees
(SE).
Figure 11: Wind shear and turbulence intensity ratios (in the plane of the rotor) for a wind direction of 202.5 degrees
(SW).
Wind shear values on the Discovery Tower roof
tend to follow the same trends as the turbulence
intensity values. They are often low in areas of
accelerated flow, whereas, they are typically
higher in areas exposed to either vortex
shedding or flow separation near the roof top.
In areas where sheltering exists, the wind shear
values tend of be fairly low throughout the
profile.
Angle of Attack. The angle of attack of the
local wind vector was defined in two
components, the lateral angle of attack, θ
y
, and
the vertical angle of attack, θ
z
. The lateral angle
of attack is only important if it varies
significantly with height. If the angle is
consistent over the height of the entire rotor, the
wind turbine will respond to it as a change in
the mean wind direction. A lateral angle of
attack that varies within the profile can be more
troublesome, particularly to horizontal axis
wind turbines (HAWT). If a HAWT rotor
experiences different wind directions across its
span, some portion of the rotor will always be
exposed to wind forces that are not
perpendicular to its plane. This will, at best,
WindPower 2008, Houston, Texas June 2-4, 2008 9
result in inefficient power production because
portions of the blades will not be creating lift.
At worst, it could destroy the blade due to
excessive loading. Because most vertical axis
wind turbines (VAWT) are omni-directional,
the lateral angle of attack will likely have little
or no influence on the turbine behavior, even if
the angle of attack is varying along the axis of
the turbine. It may alter the torque profile of the
turbine through the rotation as different
portions of the rotor enter and exit the
maximum power production at different
segments of the rotation. A large vertical angle
of attack can also be responsible for decrease in
turbine performance due to less than optimum
lift on the blades and can create destructive
loads if the turbines are not designed to
sufficiently handle the vertical component of
the velocity.
Wind Turbine Selection
Process
The wind power density calculations
described above were combined with
manufacturer published power production
curves for four different wind turbines to
determine annual energy production (AEP)
values. All four of the turbine evaluated can be
described as vertical axis Darrieus wind
turbines. VAWTs are expected to have a better
chance of withstanding the strong wind shear
predicted to exist on the roof of the Discovery
Tower.
The first wind turbine evaluated, shown in
Figure 12, is the UK Quiet Revolution, QR5.
The QR5 turbine is 5 m tall and 3.1 m in
diameter, and it is rated at 10kW at 11 m/s.
Based on the manufacturer’s supplied power
curve, ten of these turbines placed on the
Discovery Tower roof are predicted to produce
approximately 77 MWh per year.
Figure 12: Quiet Revolution twisted Darrieus wind
turbine
The second turbine evaluated is the PAC
Wind Delta II 10kW H-Darrieus, shown in
Figure 13. Eight of the 10 kW units are
expected to have an AEP of approximately 70
MWh. (It is the Author’s opinion that this value
is inflated due to the manufacturer overstating
their turbine performance, which indicates
efficiencies in excess of 40%.)
Figure 13: PAC Wind H-Darrieus wind turbine
The third arrangement evaluated consisted
of fifteen 2.5 kW Turby twisted Darrieus wind
turbines (Figure 14). The calculated AEP for
the fifteen units is 35 MWh per year.
WindPower 2008, Houston, Texas June 2-4, 2008 10
Figure 14: Turby twisted Darrieus wind turbine
The last turbine evaluated is the 3 kW
Eurowind H-Darrieus. Twelve of the 2.2 m tall
by 2.5 m wide turbines are expected to have an
AEP of 52 MWh per year.
Figure 15: Eurowind H-Darrieus Wind Turbine
It should be noted that at the time of this
publication, none of these turbines have
undergone testing to show their performance
and/or reliability in a wind environment similar
to that found on the roof of the Discovery
Tower.
Conclusions
When assessing the merit of building
integrated wind turbines, it is important to
consider that wind conditions near the building
surface will be very different from the general
wind conditions in the region, due to both the
influence of neighboring structures and the
effects of the building itself. The winds will
typically be more gusty (turbulence intensity)
and uneven across the turbine blades (wind
shear), which can significantly affect the
turbine’s performance. Improperly located, a
wind turbine in this environment may be
subjected to an inadequate wind resource,
resulting in less than optimum power
production, and/or an environment that the
turbine is not designed to withstand.
Through the use of an atmospheric
boundary layer wind tunnel, the building design
team is able to identify the wind resource and
wind flow characteristics at the proposed
turbine location(s) during the design process so
that an accurate assessment can be made of the
potential power performance and survivability
of the wind turbine before the building is
constructed.
WindPower 2008, Houston, Texas June 2-4, 2008 11
Table 3
Estimated Annual Average Energy Production
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Potential
Wind Height Width Area Number of
Turbine (m) (m)
(m
2
)
Turbines Location 1 Location 2 Location 3 Location 4 Total
Pacwind Delta II 4.3 4.0 7.4 8.0 9,196 8,013 8,770 8,925 69,808
Turby 2.9 2.0 5.7 15.0 2,509 2,130 2,383 2,419 35,404
QR5
5.0 3.1 15.5 10.0 8,189 6,976 7,782 7,973
77,300
Eurowind 3 kW 2.2 2.5 5.5 12.0 4,645 3,881 4,389 4,511 52,278
Rotor
Estimated Annual Average Energy Production
(kW hrs/yr)
(assumes 100% availablity)
... When wind gusts over urban areas the mean wind speed decreases and the fl ow becomes unpredictable in terms of direction and velocity due to the complexity and the variety of elements forming urban areas such as buildings, concrete sealed grounds, and vegetated gardens (Lei et al., 2006;Syngellakis & Traylor, 2007;Yuen et al., 2004). According to Denoon et al. (2008), turbines work most effi ciently in low-turbulence environments, so care needs to be taken in specifying turbine types that will cope with both existing turbulence and likely future changes in turbulence as a result of urban development. This is why it is mandatory to undergo a complete assessment of wind fl ow within particular urban areas before integrating wind turbines. ...
... On the other hand, Denoon et al. (2008) found it diffi cult to accurately model the effect of turbulence in a wind tunnel because the wind tunnel is limited by its size. This is why a completely accurate simulation of wind fl ow is not yet possible, which means that the results obtained from wind tunnel testing will have errors that should be considered. ...
... However, Denoon et al. (2008) claimed that CFD simulation is weak in the fi eld of assessing wind fl ow in very dense urban environments; this is attributed to the insuffi cient computational power available to accurately model the effects of turbulence in the built environment. In light of this and since wind fl ow in urban areas is unpredictable, CFD is considered a relevant tool for comparison between different designs and not a relevant tool for accurately assessing wind speeds in urban areas for energy yield calculations. ...
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With the growing interest among architects, planners and developers in integrating wind turbines within the built environment to reduce the reliance on grid supplied electricity for buildings to be self sufficient in terms of energy, the importance of assessing wind fl ow within the built environment becomes more important. This paper reviews existing researches on wind assessment tools. The main tools for assessing wind fl ow within the built environment are Mathematical Models, In Situ Measurements, Wind Tunnel Tests and Simulation Tools or Computational Fluid Dynamics (CFD) Calculations. The pros and cons of each tool revealed that researchers favour CFD and wind tunnel tests over the other tools. In addition, CFD has proven to be reliable in terms of obtaining data consistent with the results from wind tunnel tests. However, it should be noted that CFD modelling is not a straightforward process and it requires high levels of training and fluid mechanics knowledge for architects to efficiently use the tool. This knowledge would assist architects and planners to confidently specify the right simulation parameters in CFD. One of these parameters is the domain grid which plays an important role in the accuracy of the simulation. Wind fl ow around a building was simulated with different grid spacing and the simulation proved the discrepancies in the results due to refining the grid. This proves that CFD when used for assessing wind fl ow within the built environment should be validated using other wind assessment tools. Many studies have proven the reliability of CFD results if the right parameters and conditions were implemented and that makes CFD a good and economic tool for comparing design alternatives. However, this paper argues for the current need to train architects and planners on using CFD software efficiently in order to understand wind fl ow around and within buildings which would positively affect their design decisions regarding natural ventilation, thermal comfort or integrating wind turbines within the built environment.
... However, DeMeo & Parsons (2003) warn that wind turbines can't be relied on as the single means of generating electricity due to their intermittent operation and that they have to be backed up by other sources of grid electricity supply and renewable energy generation. Denoon et al. (2008), Eriksson et al. (2008) and Yuen et al. (2004) stated that the results of assessing wind flow in the built environment underpin decisions on the method of integration and kind of wind turbine to be integrated in the built environment. Aguiló et al. (2009) classified three methods of integrating wind turbines into the built environment; the first is the building integrated wind turbines, where a separate wind turbine is located on a free-standing tower away from the building itself; the second is the building mounted wind turbines, where the wind FORUM Ejournal turbine is installed on to the building structure and the third is the building augmented wind turbines where the building form is shaped to concentrate wind flow and is shaped towards the wind turbine. ...
... The building design may require some modifications based on wind flow assessment using wind tunnel test or CFD simulations (Dutton et al., 2005). Denoon et al. (2008) illustrated a number of new developments that were based on the principle of aerodynamic building form to enhance the performance of the integrated wind turbines. For example Figure 3 includes the Bahrain World Trade Centre, Pearl River Tower in China, Strata SE1 project in London, all implemented BAWTs (Figure 3) (Peel & Lloyd, 2007;Cochran & Damiani, 2008). ...
... Sharp edged buildings are extensively investigated in research (Kubota & Ahmad, 2005;Lim et al., 2009;Lien et al., 2004;Sun & Huang, 2001;Tutar & Oguz, 2002). Blocken & Carmeliet (2004) and Denoon et al. (2008) found that wind speed increases at the edges of the building and wind turbines could be implemented to take advantage of the speeding effect of wind hitting the sharp edges. However, these turbines should be designed to operate within urban areas where air flow is unpredictable. ...
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In response to various international agreements and national initiatives towards tackling climate change researchers, architects and engineers continue to investigate the possibilities of reducing reliance on grid supplied energy to integrate micro-generated power from renewable resources including in-situ integrated wind turbines in buildings. This paper reviews existing research on the integration of wind turbines within the built environment. Three distinct strands of research, which represent the main three stages for integrating wind turbines in the built environment, are identified and reviewed: the first strand reviews types of integrating wind turbines within the built environment, this involves the role of the form of the building in harnessing wind power. The second strand is related to assessing wind flow within the built environment using different tools such as mathematical models, in situ measurements, wind tunnel tests, and CFD simulation software. The third strand assesses the feasibility of integrating wind turbines within the built environment in terms of environmental, economic and social aspects. Research across these strands presents key issues that challenge the design team when considering the integration of wind turbines in the built environment. The potential of integrating wind turbines in the built environment and manipulating building form to harness wind power is a multidisciplinary team work that requires involvement of architects and their consultants at an early stage. However, this paper argues for the need to widen the impact assessment of integrating wind turbines in the built environment to more than its economic, environmental and technical impacts to include their social acceptance.
... The efficient placement of smaller scale wind turbines in the urban environment is a largely unexplored area. This implies a waste of an important energy resource [2,3]. One of the potential outcomes of optimized wind farm layouts in an urban environment is distributed energy generation (generation at the consumption site), which offers significant benefits in terms of high energy efficiency, lower emissions of pollutants, reduced energy dependence and stimulation of the economy [4]. ...
... The lower T I threshold is reached for the spherical roof, at z/H = 1.12. Below this T I threshold, vertical axis wind turbines (VAWT) may be considered due to their better behavior under skewed and highly turbulent flow conditions [3,6,42,43]. Figure 11dshows a comparison of the speed-up (U/U ref ) and the T I threshold height for the state-of-the-art cases at the roof positions described above. The most interesting cases from the wind energy exploitation point of view are at the low-right position in Figure 11d. ...
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The European program HORIZON2020 aims to have 20% of electricity produced by renewable sources. The building sector represents 40% of the European Union energy consumption. Reducing energy consumption in buildings is therefore a priority for energy efficiency. The present investigation explores the most adequate roof shapes compatible with the placement of different types of small wind energy generators on high-rise buildings for urban wind energy exploitation. The wind flow around traditional state-of-the-art roof shapes is considered. In addition, the influence of the roof edge on the wind flow on high-rise buildings is analyzed. These geometries are investigated, both qualitatively and quantitatively, and the turbulence intensity threshold for horizontal axis wind turbines is considered. The most adequate shapes for wind energy exploitation are identified, studying vertical profiles of velocity, turbulent kinetic energy and turbulence intensity. Curved shapes are the most interesting building roof shapes from the wind energy exploitation point of view, leading to the highest speed-up and the lowest turbulence intensity.
... Recently, there has been interest in on-site energy generation using building integrated wind turbines, especially in taller commercial buildings. Power output from a wind turbine is proportional to the cube of wind speed, so a small increase in speed can result in greater quantities of energy generation (Denoon et al., 2008). Since wind speeds increase with height, the top of a tall commercial building is a promising place for wind energy harvesting. ...
Book
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This guide acts as a practical reference for the design of high-performance, low-carbon commercial buildings. The target audience is everyone involved in the creation of new commercial buildings, from architects, consultants and designers, to developers and owners. The guide delves into the technical details of many issues but is suitable for anyone wanting to explore low-carbon building design.
... local accelerations, effects of obstacles, etc.) are very important in determining the real characteristics of the flow which invests the rotor (e.g. [24]). As a result, a turbine optimized only for one (generally quite high) wind speed could provide poor performance during the largest part of its operating time, with a remarkable reduction of the energy produced and, consequently, of the convenience of the investment [19]; this effect is maximized whenever the shape parameter of the distribution is low, i.e. when the higher wind velocities give a remarkable contribution to the global energy harvesting, although the mean velocity is quite low. ...
Chapter
Recent experiments and theoretical models showed that the aerodynamic performance of H-Darrieus wind turbines can even be enhanced in case of moderate skew angles, which are typical of installations in the urban environment. In this study, a design procedure oriented to the maximization of the annual energy yield in skewed flow, instead of the maximum rated power, was carried out. 14400 test cases of H-Darrieus rotors were simulated with a numerical code based on a Blade Element Momentum approach, including an in-house model to account for the skewed flow, and compared on the basis of their energy-yield capabilities for different annual wind distributions. The analysis highlighted that the optimal design configurations in skewed flow significantly differ from the corresponding ones in case of aligned flow and also that a design oriented to the maximum energy-yield in skewed flow can make H-Darrieus rotors competitive for urban installations in comparison to HAWTs.
... 건물일체형 풍력발전기(building-augmented wind turbine)는 높은 위치에서 지상보다 월등한 풍력자원 을 이용하기 위하여 풍력발전기를 건물 외벽 또는 옥상에 설치하여 전력을 생산하는 것이다 (1) (2) . 면진장치 고유주파수의    배 이상의 주파수 대 역에서 면진이 가능하므로 (6) , 풍력발전기의 1차 고 유주파수 1.65 Hz 이상에서 진동을 저감하기 위하여 ...
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Vibration issue of a building structure due to a wind turbine should be resolved for the application of building-augmented wind turbine. In this study, a dynamic analysis for an horizontal-axis upwind wind turbine is carried out to calculate vibration excited to an example building structure. Characteristics of vertical vibration transfer of the building structure are analytically studied and compared with a criteria. Then, a method to isolate the vibration is presented by analyzing the vibration characteristics of the wind turbine, and verified by applying to the building structure.
... [7][8][9][10][11][12]). The real feasibility of this scenario has, however, yet to be proved, both in terms of real energy harvesting and of compatibility of the machines with a populated area [9,10,13]. In particular, Vertical-Axis Wind Turbines (VAWTs), both drag [14][15][16]and lift-driven [17][18][19][20], are gaining popularity in the wind energy scenario, especially in medium and small-size installations, where they can work effectively even in presence of low-speed and unstructured flows with low noise emissions and high reliability. ...
Article
Abstract H-Darrieus wind turbines are gaining popularity in the wind energy market, particularly as they are thought to represent a suitable solution even in unconventional installation areas. To promote the diffusion of this technology, industrial manufacturers are continuously proposing new and appealing exterior solutions, coupled with tempting rated-power offers. The actual operating conditions of a rotor over a year can be, however, very different from the nominal one and strictly dependent on the features of the installation site. Based on these considerations, a turbine optimization oriented to maximize the annual energy yield, instead of the maximum power, is thought to represent a more interesting solution. With this goal in mind, 21,600 test cases of H-Darrieus rotors were compared on the basis of their energy-yield capabilities for different annual wind distributions in terms of average speed. The wind distributions were combined with the predicted performance maps of the rotors obtained with a specifically developed numerical code based on a Blade Element Momentum (BEM) approach. The influence on turbine performance of the cut-in speed was accounted for, as well as the limitations due to structural loads (i.e. maximum rotational speed and maximum wind velocity). The analysis, carried out in terms of dimensionless parameters, highlighted the aerodynamic configurations able to ensure the largest annual energy yield for each wind distribution and set of aerodynamic constraints.
... In particular, some perplexities are still connected to the influence on the available wind velocity of the complex geometry of the city (e.g., Refs. [4] and [8]). Moreover, in the rooftop of high buildings, the boundary layer separates at the windward roof edge of the building itself, generating a separation bubble on the roof below the streamlines [2,5]; the velocity vector of the flow above the separation bubble makes an inclination angle with the horizontal roof (Fig. 1). ...
Conference Paper
Increasing interest is being paid by architects, project developers and local governments to understand where small wind turbines can effectively be exploited to provide delocalized power in the built environment. The wind conditions in the rooftop area of buildings in urban locations are, however, very complex and the real adaptability of wind turbines to these environments is not yet tested both in terms of real producibility and of structural compatibility with the building themselves. In these installations, in particular, the flow which incomes on the rotors is often inclined with respect to the horizontal direction due to the interaction with the building façade and the roof. A correct estimation of the impact of an inclined flow on the performance of horizontal-axis wind turbines therefore becomes a very relevant issue to correctly predict the potential energy yield of a machine. To this purpose, a simulation code based on a Blade Element Momentum (BEM) approach was developed and validated by means of experimental data found in the literature. The code was then used to evaluate the energetic suitability of a small-size wind turbine installation in the rooftop of a building in a conventional European city. A numerical CFD analysis was carried out to characterize the flow field in the rooftop area of different buildings. The flow velocity modulus and direction were calculated for several oncoming wind profiles: the results were projected into an available wind power curve in the rooftop of the building. The effective energy-yield capabilities were then corrected using the model for the flow inclination as a function of the specific flow conditions in the rooftop area. The results were finally exploited to analyze the energy-oriented feasibility of an installation in a similar context.
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A preliminary study is carried out to investigate the aerodynamic characteristics of a square cylinder with Savonius wind turbines and to explain the reason why this kind of structure can suppress wind-induced vibrations. A series of computational fluid dynamics simulations are performed for the square cylinders with stationary and rotating wind turbines at the cylinder corners. The turbine orientation and the turbine rotation speed are two key factors that affect aerodynamic characteristics of the cylinder for the stationary and rotating turbine cases, respectively. The numerical simulation results show that the presence of either the stationary or rotating wind turbines has a significant effect on wind forces acting on the square cylinder. For the stationary wind turbine cases, the mean drag and fluctuating lift coefficients decrease by 37.7% and 90.7%, respectively, when the turbine orientation angle is 45°. For the rotating wind turbine cases, the mean drag and fluctuating lift coefficients decrease by 34.2% and 86.0%, respectively, when the rotation speed is 0.2 times of vortex shedding frequency. Wind turbines installed at the corners of the square cylinder not only enhance structural safety but also exploit wind energy simultaneously.
Thesis
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A newly emerging way to promote sustainability in the built environment is through the incorporation of wind power within buildings, resulting in minimum transmission losses (distributed generation). However, the effectiveness of the proposed solutions are seriously dependent on early integration with the architectural design process. Wind power is considered a potential renewable energy source in tall buildings due to the possibility of accessing greater wind velocities at higher altitudes. In addition, air-flow patterns around buildings are considerably influenced by a buildings’ geometric characteristics. Hypothetically, proper modification of building form can turn this unstructured phenomenon in to a massive concentrator effect, capable of boosting power production in tall buildings with an integrated wind turbine (BIWT). These aerodynamic modifications are typically evaluated via CFD simulation or wind tunnel testing. However, these methods are too expensive and time-consuming to analyze all annual fluctuations of local wind regimes (velocity, direction, and density) and is therefore inappropriate for use in early design stages when architectural concepts quickly evolve. As a result, existing wind analysis techniques are often used under simplified conditions (steady state analysis, single velocity, and angle). This approach simply disregards the wide variety of other criteria influencing “BIWT annual energy output” including fluctuations of local wind regimes, and surrounding urban terrain roughness. This research seeks to address the issues indicated above, and proposes a performance based parametric design tool, primarily for the early design stages when architectural concepts evolve rapidly. The automated output delivers real time assessment of BIWT potential energy enhancement for each alternation of the concept, as well as analysis of multiple BIWT typologies simultaneously. The parametric tool employs hourly weather data, different terrain condition mathematical models, and two databases of CFD measurements to approximate annual energy enhancement as result of BIWT geometrical transformations. The tool develops a decision mechanism to find the best BIWT typology and optimum angle, based on the long-term local climatic trends and adjacent terrain context. The outcome of this dissertation is an automated parametric tool which addresses all above indicated difficulties associated with incorporation of current wind analysis method and the architectural design process of BIWT.
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An atmospheric, multi-scale numerical model is combined with a boundary layer wind tunnel to form a powerful hybrid tool for evaluating wind power sites in complex terrain. The wind tunnel serves to guide the refinement, calibration, and validation of the numerical model for neutral thermal stability in a controlled, reproducible manner, and to improve wind power site assessments in the most severe terrain such as steep escarpments and cliffs where the numerical model loses accuracy. The numerical model treats thermally stratified flow and the Coriolis force generated by Earth's rotation, which are beyond the capability of the wind tunnel1, and can assess all but the most severe terrain with accuracy. Through parametric analysis, it also serves to guide wind tunnel testing protocols. The hybrid numerical/wind-tunnel tool provides unique insights and serves as a significant complement to other wind power assessment tools in common usage, but requires additional development to reach its full potential. Future efforts will include field measurements as a vital part of the validation process
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The objectives of this review are to establish an initial subject-matter base for wind engineering, to demonstrate current capabilities and deficiencies of this base for an engineering treatment of wind-effect problems, and to indicate areas of research needed to broaden and strengthen the subject-matter base. Focusing of subject matter for wind engineerng is accomplished through a historical summary of relevant scientific and technological material, and examination of information on wind characteristics, and a review of current capabilities for physical modeling of winds and wind effects in the laboratory. Current methods and capabilities in wind engineering are demonstrated by a review of problems related to atmospheric advection and dispersion of air pollutants, wind forces on buildings and structures, and control of winds.
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The high density, high rise city is explored as a 'green' option for urban settlements. A new kind of skyscraper is presented as requiring reduced land consumption, reduced overall energy use and reduced transport demand, Ken Yeang considers the planning and design considerations for creating the bioclimatic, lower energy skyscraper.
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Similarity criteria are given for micro-, small-, and meso-scale motion of the atmospheric boundary layer. Requirements for simulation of dispersion of passive contaminants in the atmosphere are discussed. The characteristic features of a unique meteorological wind tunnel having a capability for simulating thermally stratified boundary layers are described. Mean wind speed, mean temperature and turbulence statistics measured in this laboratory facility are found to be similar to corresponding data obtained from measurements in the atmosphere. Examples of simulated dispersion over a variety of surface features including urban areas and complex topography are described. (Author)
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THE AIM OF THIS REVIEW IS TO POINT OUT MAJOR FIELDS OF KNOWLEDGE AND PLACES WHERE MORE RESEARCH IS NEEDED IN BUILDING AERODYNAMICS.METHODS FOR OBTAINING WIND EFFECT DATA ARE DESCRIBED, AS WELL AS A SURMISE OF NATURAL WIND CHARACTERISTICS.EMPHASIS IS PLACED ON THE FLUCTUATING PRESSURE DISTRIBUTIONS ON BUILDINGS, WIND-INDUCED BUILDING MOTIONS, AND AIR MOVEMENT AROUND BUILDINGS.SUBJECT MATTER IS SELECTED WHICH BEST SUPPORT AERODYNAMIC APPLICATIONS IN REGARDS TO WIND INDUCED ENGINEERING PROBLEMS.(FROM PAPER)
Strong winds in the atmospheric boundary layer, Part 2: discrete gust speeds
ESDU (1993b) Strong winds in the atmospheric boundary layer, Part 2: discrete gust speeds, ESDU Report 83045, ESDU International.
Wind Tunnel Model Studies of Buildings and Structures
ASCE, American Society of Civil Engineers (1999), Wind Tunnel Model Studies of Buildings and Structures (ASCE Manual of Practice Number 67).