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MECH 4810 Energy Conversions
DRAFT REPORT
Nova Scotia Wind Turbine Project Team # 2
Maigoro Yunana
James Gough
Kyle Kearse
Submitted: March 8th, 2016
1
Table of Contents
1.0 Introduction .............................................................................................................. 4
2.0 Wind Energy ............................................................................................................ 5
3.0 Site Selection Analysis ............................................................................................ 6
4.0 Turbine Selection ................................................................................................... 12
4.1. Offshore wind turbine support structure ................................................................ 18
5.0 Wave conditions..................................................................................................... 20
6.0 Finances ................................................................................................................. 26
7.0 References .............................................................................................................. 28
2
List of Tables
Table 1 - Basic parameters for wind turbine classes......................................................... 12
Table 2: Yarmouth Histogram .......................................................................................... 16
3
List of Figures
Figure 1: Wind shear Profile (van Der Tempel, 2006) ....................................................... 5 Figure 2: Nova Scotia transmission and distribution map .................................................. 7
Figure 3: Sable Island bathymetric chart (National Oceanic and Atmospheric
Administration) ................................................................................................................... 8 Figure 4: Yarmouth and Shelburne bathymetric (National Oceanic and Atmospheric
Administration) ................................................................................................................... 8 Figure 5: Daily Average Speed Variation ........................................................................... 9
Figure 6: Site Monthly Average ....................................................................................... 10 Figure 7: Site Raleigh Distribution ................................................................................... 11 Figure 8: Annual Wind Energy Available ........................................................................ 11
Figure 9: Turbulence intensity for the normal turbulence model ..................................... 13 Figure 10: E-126 calculated power curve ......................................................................... 14 Figure 11: Wind farm layout............................................................................................. 17 Figure 12: Yarmouth Wind Rose ...................................................................................... 18
Figure 13: Monopile support structure ............................................................................. 19 Figure 14: Score protection of a monopile ....................................................................... 20
Figure 15 Wave Height Data for Yarmouth 2012-2014 ................................................... 21 Figure 16: Current speed data for Yarmouth .................................................................... 22 Figure 17: Experimentally determined inertia coefficients .............................................. 24
Figure 18: Experimentally determined drag coefficients .................................................. 24 Figure 19 Cost breakdown for construction...................................................................... 26
Figure 20 Total cost breakdown ....................................................................................... 27
4
1.0 Introduction
This project’s aim is to design a hypothetical wind turbine for Nova Scotia. The purpose
of this report is to comment on the current status of wind energy around the world as well
as Nova Scotia, provide design analysis based on wind maps and wind data, and provide
a cost report based on the turbine and site selection.
The ever increasing prices of oil and carbon emissions in the atmosphere provide a strong
pull for the installation of wind turbines around the world. Under the New Policy
Scenario that seeks to limit the average global temperature increase to less than 6 °C by
2099, the goal is to have wind turbines provide as much as 25 % of the global supply of
energy to meet the energy demands of tomorrow (Renewable Energy Outlook, 2013).
The 20 % Wind Energy by 2030 report published in the United States in 2008 shows the
USA’s pledge to push for more wind energy in the years to come. However, the general
consensus amongst wind energy industry leaders is that the rising prices due to smaller
fossil fuel reserves and increasing electricity prices are not enough to get investors more
energetic about putting their money in wind energy. More government grants and
programs and feed-in tariffs are needed to be able to reach the target goals for wind
energy, including Canada.
Canada currently only has one program to help pay for renewable energy projects called
ecoEnergy II (ecoEII) which pledges $268 million (Current Funding Programs, 2016)
where projects can earn $0.01 per kWh generated for 10 years. The program started in
2011 and will end in 2021. Here in Nova Scotia, the COMFIT Program was recently
discontinued in August 2015. The program’s incentives included $0.131 per kWh for
renewable projects >50 kW while projects <50 kW could make $0.499 per kWh by feed-
in tariffs. The current price of electricity in Nova Scotia is $0.149 per kWh (Report on the
Review of the Community Feed-in Tariff Program). Criticism for the program was
received because for large scale projects the feed-in tariff was less than the price of
electricity. Today, there is Enhanced Net metering for projects < 1 MW which seeks to
pay for energy put onto the grid. The Enhanced Net metering program is considered to be
the next step towards reaching agreement for carbon trading where countries can pay and
trade carbon emission in exchange for electricity from fossil fuels. Due to the small
population in Nova Scotia, the power demands are currently being met and additional
energy to the grid is not currently required. Instead of building a new turbine as the
project suggests a more likely scenario would be the replacement of a fossil fuel plant
with a windfarm.
5
2.0 Wind Energy
Wind speed is intermittent and changes almost instantaneously, this a big factor which
affects the development of wind turbines. It also affects the assessment of a wind farm, to
produce the required amount of electricity. Wind power available to a wind turbine
placed in a steady airstream is given by the following fluid mechanics equation:
𝑃 = 0.5𝜌𝐴𝑈3 Where ρ is the density of air, A is the swept area of the turbine blades, and U is the speed
of the wind. The above equation shows a cubic relationship between speed and power,
therefore slight increase in wind speed will result in a greater increase in power available.
The annual energy output in kWh per m2 is calculated using the equation below:
𝐸
𝐴= 0.5 𝜌𝑈3 ∗ 8.76 𝑘𝑊ℎ/𝑚2
The wind speed increases with an increase in height due to wind shear as shown in Figure
1. As a result the taller the wind turbine the more energy available to convert to
electricity.
Figure 1: Wind shear Profile (van Der Tempel, 2006)
This can be approximated using the power law as shown in the equation below:
𝑈(𝑧) = 𝑈(𝐻) (𝑧
𝐻)
𝛼
(𝐿𝑦𝑛𝑛 𝑝. 36)
Where U(z) is the wind speed at height z, and U(H) is the speed at height H, and α is a
power law exponent that depends on surface roughness. For offshore wind, the power law
exponent is approximated to be 0.2 (p16, T.R.Camp). This value was used in the wind
analysis carried out for all the proposed locations.
6
The power available in the wind and wind shear profile change vary with locations.
Using wind data collected from a particular site, the above equations help analyze a site’s
viability for a productive wind farm. These are major factors used in site selection.
3.0 Site Selection Analysis
For an offshore wind turbine to capture as much wind power possible a suitable location
must be selected. The farm’s overall performance depends crucially on the conditions at
the particular site. Therefore the following features must be considered:
a. Proximity to the grid
b. Water Depth
c. Wind Energy Available (daily and seasonal)
These factors not only take into account the wind resource but also the projected
installation and maintenance costs of an offshore wind farm. Three sites, Sable Island,
Yarmouth and Shelburne were preliminarily chosen for analysis. This was based on the
availability of hourly wind data over the course of the year, proximity to the coast and
Nova Scotia wind atlas showing the average wind speeds.
Proximity to the Grid
One of the offsetting disadvantages of moving a wind farm site offshore is the expensive
nature of bringing the generated electricity by submarine cables. Laying submarine cables
for an offshore windfarm raise important technical and economic issues. Currently, Nova
Scotia is yet develop an offshore wind farm and therefore has no underwater grid
network. As a result, proximity to the on shore grid is very important to a site’s viability
as an offshore windfarm. Figure 2 below shows Nova Scotia Power’s grid map
7
Figure 2: Nova Scotia transmission and distribution map
Figure 2 shows the close proximity of the proposed Yarmouth and Shelburne offshore
locations to Nova Scotia’s grid.
Sable Island is a small island situated about 300 km south east of Halifax. It is also a
protected National Park Reserve. An offshore wind farm in close proximity to Sable
Island will require installation of at least 300 km of subsea high-voltage direct current
cables thereby increasing the construction costs. Laying such cable cost ~ $1M/km. This
makes Sable Island a less than ideal location for an offshore wind farm based on its
projected construction costs. In addition, a proposed wind farm would have undergo a
vigorous Federal Environmental Assessment process as it is part of a key migratory
flyway, with numerous bird species with over 350 recorded, such as the Roseate Tern and
Ipswich Sparrow. An offshore wind farm in proximity to Sable Island could pose a threat
to the migratory birds.
In terms of projected construction costs, Yarmouth and Shelburne sites are more ideal
location for an offshore wind farm because of their close proximity to Nova Scotia’s
electricity grid.
Water Depth
Analyzing a sites water depth is unique for an offshore wind farm. The water depth is
generally increases far from the shore as shown in Figure 3 and Figure 4. Offshore wind
turbines require a sturdy foundation to secure the wind turbine and handle the thrust
forces produced by the wind. A deeper water depth makes turbine installation more costly
and difficult. It is therefore recommend that offshore wind turbines should be ideally
located on shallow water. Sable Island has shallow water of 25 m, located just off its
coast as shown in Figure 3 below.
8
Figure 3: Sable Island bathymetric chart (National Oceanic and Atmospheric Administration)
Yarmouth and Shelburne both share shallow waters of 25 m depth, off their coasts as
shown in Figure 4. All locations are viable locations for an offshore wind farm.
Figure 4: Yarmouth and Shelburne bathymetric (National Oceanic and Atmospheric Administration)
Wind Energy Available
Hourly wind data over a year was provided by the Canadian Weather Energy and
Engineering Datasets (CWEEDS) for all three locations. An analysis was carried out to
compare the availability of viable wind energy for generation in each proposed location.
The data was measured at a height of 10m. Using the power law equation and an
exponent of 0.2, wind speed at a proposed height of 120 m was calculated and used for
the wind resource analysis. Figure 5 below shows the hourly average wind speed variation
for each site. As expected it shows the variability of the wind over the course of the day,
this has a big impact on the annual energy intercepted by the wind turbine rotor. Higher
9
average wind speeds are experienced during the daylight hours and lower at night. Small
changes in the average wind speed has a cubic effect on the energy available for
production as was shown in the wind power equation. The Sable Island site has the
highest annual arithmetic average wind speed of 11.5 m/s while the Shelburne site has the
lowest with 7.1 m/s. The Yarmouth site has annual average speed of 8.1 m/s.
Figure 5: Daily Average Speed Variation
The seasonal changes in average wind speed shown in Figure 6, were as expected. Higher
wind speed averages are witnessed during the winter months and lower averages during
the summer months. Sable Island has distinctly higher wind speeds through the whole
year as compared to the Yarmouth and Shelburne sites. Seasonal variations in wind speed
are important in assessing wind generation with regards to meeting up seasonal changes
in electricity demand.
0
2
4
6
8
10
12
14
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Ave
rage
Win
d S
pee
d (
m/s
)
Hour of the Day
Hourly Average Wind Speed Variation
Yarmouth Site
Shelburne Site
Sable Island Site
10
Figure 6: Site Monthly Average
Predicting the future availability of wind in a particular location is difficult. The trends
generally remain the same, however there are variations in wind speeds year to year.
Given the data acquired from CWEEDS, we can perform statistical analysis to predict the
probability of witnessing a particular wind speed. Rayleigh distribution was used to
model the annual wind speed distribution and the total wind energy available at each of
the proposed sites. It is worth noting that these measurements were taken onshore,
however the coastal location of all proposed sites makes the measurements provide more
confidence in its accuracy.
The distribution is calculated as follows:
𝑝(𝑈) = (𝜋
2) (
𝑈
𝑈𝑎2
) exp {(−𝜋
4) (
𝑈2
𝑈𝑎2
)}
p is the probability of the actual wind speed being wind speed, U and Ua is the average
wind speed of the location. Figure 7 shows the Rayleigh distribution of the sites in
comparison to each other. The modal peak of each distribution occurs at a lower speed
than its corresponding average speed. The modal peak of Sable Island occurs at a higher
wind speed compared to the Yarmouth and Shelburne sites. It also tails off at higher wind
speed compared to the other sites. The probability of lower wind speeds occurring in the
Sable Island site is less than the other sites, however it has higher probability of
witnessing larger wind speeds. Given the cubic relationship between the wind speed and
power, this results in a larger annual wind energy available as shown in Figure 8.
0
2
4
6
8
10
12
14
16
Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec
Ave
rage
Win
d S
pee
d (
m/s
)
Month
Monthly Average
Yarmouth Site
Shelburne Site
Sable Island Site
11
Figure 7: Site Raleigh Distribution
The annual energy available per m2 of the wind turbines, swept area. In correlation with
the Rayleigh distribution comparison shown in Figure 7, the Yarmouth site produces less
energy than the Sable Island site but more than the Shelburne site. Sable Island has an
energy availability of 18332 kWh/m2, Yarmouth with 5399 and Shelburne with 3702.
Given such results, Sable Island is the most ideal wind farm site.
Figure 8: Annual Wind Energy Available
0
0.02
0.04
0.06
0.08
0.1
0.12
00
.51
.52
.53
.54
.55
.56
.57
.58
.59
.51
0.5
11
.51
2.5
13
.51
4.5
15
.51
6.5
17
.51
8.5
19
.52
0.5
21
.52
2.5
23
.52
4.5
p(u
)
Bin Avg Speed (m/s)
Site Rayleigh Distribution
Yarmouth Site
Shelburne Site
Sable Island Site
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
Yarmouth Shelburne Sable Island
Win
d E
ner
gy A
vaila
ble
(kW
h/m
^2)
Site
Annual Energy Available (kWh/m^2)
12
The analysis of each site based on its proximity to the grid, water depth and availability
of wind energy has made Yarmouth an ideal location for a wind farm. All sites are ideal
based on water depth. Yarmouth is ideal primarily because it does not run into the
complexity of being a large distance away from the grid, thereby incurring large
construction and grid integration costs. This overrides Sable Island’s significantly larger
value of wind energy available. In addition, Yarmouth, has a higher amount of energy
available compared to the Shelburne site.
4.0 Turbine Selection
There are variety of wind turbine manufacturers in the world, choosing the appropriate
turbine is paramount to a wind farm’s ability to generate the required amount of
electricity, safely. A variety of parameters included in selecting the appropriate turbine,
however this is based mainly on the site conditions, and turbine performance. All this is
driven in large parts by the wind resource analysis performed in the previous section.
Selection based on Site Conditions
The turbine selected must be able to handle all the conditions of the Yarmouth site, whilst
generating a significant amount of electricity, based on the electricity demand. The
International Electromechanical Commission published an international standard, IEC
61400 regarding the engineering integrity of wind turbines based on site conditions. This
is to ensure that the wind turbine is protected from all hazards during its 25 year life cycle
(Wind Turbines). These site conditions include the maximum wind speed and turbulence
intensity. (Manuel et al, 41). The turbines are classified in 3 different categories for each
site condition as shown in Table 1.
Table 1 - Basic parameters for wind turbine classes
Wind Turbine Classes I II III S
Vref m/s 50 42.5 37.5 Values
specified by
the Designer A Iref(-) 0.16
B Iref(-) 0.14
C Iref(-) 0.12
Vref is the reference wind speed average over 10 min, Iref is the expected value of the
turbulence intensity at 15 m/s
Turbulence is random short term wind speed variations imposed on the mean wind speed.
It is more serious at onshore sites than at offshore ones where the wind blows more
steadily. Such random fluctuations over time intervals of ten minutes or less affect the
turbine design considerations. This includes maximum load and fatigue prediction,
structural excitations, control, system operation and power quality. Therefore quantifying
the turbulence allows a more appropriate selection of a wind turbine using a calculated
value know as turbulence intensity. This is the most basic measure of turbulence and it is
as follows:
13
𝑇𝐼 =𝜎𝑢
𝑈
σu is the standard deviation. The turbulence intensity ranges from 0.1 to 0.4, with the
highest occurring at the lowest wind speeds (Manuel et al, 41) as shown in Figure 9. The
main consequence of increased turbulence is are increased thrust loads and fatigue on the
turbine blades.
Figure 9: Turbulence intensity for the normal turbulence model
The Yarmouth site conditions can be described based on the above parameters. The
maximum gust wind measured at the site was 49 m/s, therefore the wind turbine has to be
rated under Class I with a reference speed of 50 m/s. There is no data to accurately
quantify the turbulence intensity at this location. As a result a safe turbulence intensity of
0.16 is chosen, placing it under Class A. Given that the turbulence intensity is low on
offshore locations than onshore, a wind turbine rated for Class A is more than appropriate
for the site’s conditions. Therefore the wind turbine to be selected must have an IEC
Class of IA.
Turbine output
A turbine’s performance is usually characterized by its power coefficient, Cp:
𝐶𝑝 =𝑃
0.5𝜌𝑈3𝐴=
𝑃𝑜𝑤𝑒𝑟 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛 𝑡ℎ𝑒 𝑊𝑖𝑛𝑑
This non-dimensional value represents the fractional value of power in the wind that is
converted to electricity. It changes depending on the wind speed, U at the hub height. The
theoretical limit of rotor efficiency, named the Betz Limit was discovered to be 0.59. This
states that a rotor in a steady airstream cannot convert more than 59%, of the wind’s
14
kinetic energy into mechanical energy. Manufacturers test their wind turbines in ideal
conditions to find the Cp curve detailing the Cp value of wind speed, U at hub height as
shown in Figure 10. Using the Cp values we can calculated the power generated by a
particular wind as a specific speed. This correlates with the turbine’s power curve shown
in Figure 10 which shows the manufactures estimate of power output at each value of wind
speed, and its rated power. The power curve also shows important specifications of the
wind turbine, which includes the following:
Cut in speed: lowest wind speed at which the turbine starts to generate power
Rated Speed: lowest wind speed at which the turbine reaches its rated output
power
Cut out speed: speed at which the turbine stops to prevent any damage, usually at
25 m/s for offshore wind turbines.
Turbine Selection
The Enercon E-126 wind turbine was selected. It has a rated power of 7,580 kW and a
127 m rotor diameter. It was designed for an IEC wind class IA. Other specifications of
the E-126 are as follows:
Cut in speed: 3 m/s
Rated speed: 15 m/s
Cut out speed: 25 m/s
The E-126 is a gearless, variable speed, variable pitch wind turbine. The rotor shaft is
attached directly to the generator, which spins at the same speed as the blades. The power
curve and Cp values are shown in Figure 10.
Figure 10: E-126 calculated power curve
Total power output was calculated using a histogram of compiled hourly data collected
from 1953 – 2005. The wind speed were put in bins, frequency of each being was
calculated as shown in table. The total energy available per m^2 for each bin was
calculated using the energy equation, previously shown. The total swept area, A of the E-
126 was calculated to be 12667.6 m^2. The total energy out output for each bin is
calculated using the equation:
15
𝐸 = ∑ 𝐶𝑝 𝑏𝑖𝑛 ∗ (𝐸/𝐴)𝑏𝑖𝑛 ∗ 𝐴𝑡𝑢𝑟𝑏𝑖𝑛𝑒
The power coefficient of the bin, Cp bin corresponding to the bin average speed is shown
in Figure 10. The energy density for each bin, (E/A)bin is shown in Table 2.
16
Table 2: Yarmouth Histogram
m/s W/m^2 kWh/m^2
Raleigh
Probability
Bin# Umin Umax hrs/yr Frequency
Bin avg
speed,
Uavg
Power
Density
From
bin
From
Raleigh p(u)
1 0 0 448 5.11% 0 0.00 0.0 0.00 0.0%
2 0 1 0 0.00% 0.5 0.08 0.0 0.01 1.2%
3 1 2 192 2.19% 1.5 2.07 0.4 0.63 3.5%
4 2 3 535 6.11% 2.5 9.57 5.1 4.64 5.5%
5 3 4 472 5.38% 3.5 26.26 12.4 16.60 7.2%
6 4 5 667 7.62% 4.5 55.81 37.2 41.23 8.4%
7 5 6 470 5.37% 5.5 101.90 47.9 81.65 9.1%
8 6 7 769 8.78% 6.5 168.21 129.3 138.01 9.4%
9 7 8 976 11.14% 7.5 258.40 252.2 206.95 9.1%
10 8 9 892 10.18% 8.5 376.15 335.4 282.01 8.6%
11 9 10 228 2.60% 9.5 525.14 119.7 354.89 7.7%
12 10 11 649 7.41% 10.5 709.05 460.2 417.05 6.7%
13 11 12 439 5.02% 11.5 931.54 409.3 461.40 5.7%
14 12 13 488 5.57% 12.5 1196.29 583.4 483.51 4.6%
15 13 14 630 7.19% 13.5 1506.98 949.0 482.17 3.7%
16 14 15 278 3.17% 14.5 1867.28 518.6 459.26 2.8%
17 15 16 138 1.58% 15.5 2280.87 314.9 419.05 2.1%
18 16 17 91 1.04% 16.5 2751.43 250.5 367.15 1.5%
19 17 18 110 1.25% 17.5 3282.62 360.0 309.49 1.1%
20 18 19 90 1.03% 18.5 3878.12 349.1 251.42 0.7%
21 19 20 49 0.56% 19.5 4541.61 223.8 197.1003 0.5%
22 20 21 36 0.41% 20.5 5276.76 188.0 149.288 0.3%
23 21 22 40 0.46% 21.5 6087.25 245.5 109.3571 0.2%
24 22 23 31 0.35% 22.5 6976.76 215.0 77.54073 0.1%
25 23 24 11 0.13% 23.5 7948.95 87.2 53.25993 0.1%
26 24 25 8 0.09% 24.5 9007.50 68.3 35.46057 0.0%
27 25
No
upper 24 0.28%
Total 8760 6162.5 5399.135
Air density
(kg/m^3)
1.225 Annual power on cubic average 5444.5
Arithmetic Avg speed 8.1 Annual power on arithmetic avg wind speed 2851.45
Cubic Average Speed 10.05
17
The annual energy output for an E-126 turbine was calculated to be 23.6 GWh. The
average value of Cp is 0.30.
Site Specification
• Location: Yarmouth
• Distance off the coast: 20 km
• Average wind speed: 8.1 m/s
• Annual Energy Output per turbine = 23.6 GWh
• No of units: 14
• Nameplate capacity: 101.92 MW
• Total Energy Output: 330.4 GWh
• CO2 offset: 323931 g of CO2 (coal)
• Number of households: 32797
Site Spacing and Orientation
Spacing the wind turbines is important to the productivity of each individual wind
turbine. The extraction of energy by those wind turbines that are upwind of other turbines
results in lower wind speeds at the downwind turbines and increased turbulence (Manuel
et al 422)
It is therefore recommended that the wind turbines be accurately spaced, in order to
negate those effects. Experts say that the turbines should not be placed closer than 5-9
rotor diameters in the prevailing wind direction and 3-5 rotor diameters in the
perpendicular direction (McKay, 2009).
Figure 11: Wind farm layout
Turbines are spread 635 m apart in prevailing wind direction and 381 m apart in the
perpendicular direction in two rows of 7. Figure 12 below shows the wind rose for
Yarmouth. Majority of the wind comes from the west, so therefore the wind turbines will
be placed facing that direction.
18
Figure 12: Yarmouth Wind Rose
4.1. Offshore wind turbine support structure
The support structure of the offshore wind turbine is responsible for fixing the turbine in
place and keeping it upright. This type of structure is subjected to highly dynamic loads
due to the combined wind and hydrodynamic loading and the complex dynamic loading
from the wind turbine. When designing a support structure for an offshore wind turbine
it is crucial to accurately capture the effects of wind loads, wave loads, and dynamic
loading from the turbine. The reason for this is due to total loading likely being much
smaller than the sum of each consecutive load, since the loads are not coincident and
aerodynamic damping from the rotor greatly damps motion from wave loading. Several
types of support structures were looked into for designing the wind turbine in Yarmouth,
it was decided that a monopile design would be the best choice. The monopile foundation
was chosen due to its simplicity in design, ease of manufacture, and from it being the
cheapest foundation option.
The monopile foundation
The monopile foundation is the most common type of support structure installed with
offshore wind turbines, the European Offshore Statistics Association reported that 97%
foundations installed in 2015 were monopiles. The design of the monopile foundation is
relatively simple; it is a cylindrical foundation pile with a transition piece that connects
the turbine tower and monopile (Figure 13). It is important to note that monopiles are
N
NE
E
SE
S
SW
W
NW
Yarmouth Wind Rose
19
more of a shallow water type of foundation, and can be employed in water levels up to 30
meters.
Figure 13: Monopile support structure
The monopile foundation is implanted deep into the seabed, typically 10-20 meters in
order to provide a stable support structure for the turbine. There are two methods
employed when implanting monopile foundations into the seabed. One method involves
driving the monopile into the seabed with the use of a vibrating hammer, and is method
only viable when the seabed is not rocky. The other method employed is required when
the seabed is rocky and it involves drilling into the seabed then implanting the monopile.
The diameter of the monopile is determined through force analysis and is typically 4.5-9
meters in diameter. One problem that must be addressed in the design of the monopile is
erosion. The sea currents passing by the monopile tend to erode to soil around it,
exposing more of the foundation and increasing the hydrodynamic loads felt by the
support structure. In order to prevent this from happening a method called score
protection is employed (Figure 14), this method involves placing two layers of stones
around the monopile in order to prevent the soil from being eroded.
20
Figure 14: Score protection of a monopile
Once the monopile foundation has been imbedded in the seabed, a transition piece
(Figure 13) is attached to the top of it using a grouted joint. The purpose of the grouting
is to get rid of tolerances between the monopile and transition piece, and it helps deliver
loads from the turbine down into the seabed. One flaw with this design is that the
grouting will start to wear away with time, because of the forces involved with the
turbine, which will increase maintenance costs as employees will have to go refill these
worn away areas.
5.0 Wave conditions
When conducting a load analysis on a site where an offshore wind turbine is going to be
installed, it is crucial that the wave conditions surrounding the area be accurately
measured. These conditions not only affect how the support structure of the turbine must
be designed, it also affects its installation. Proper analysis of the wave conditions will
give information on how high the waves are, their velocity, and the force exerted by the
waves. This information is very useful in while constructing offshore wind turbines, since
it gives the contractors knowledge on times when the wave conditions will be minimal,
which will make the turbine installation much easier and less expensive. For this turbine
design project, the team is expecting that Yarmouth will be the location where the turbine
will be installed. All wave condition data presented in this section is for Yarmouth, Nova
Scotia.
The Figure 15 shows measured heights of the waves in Yarmouth; this data was taken
from the Government of Canada. This graph represents the various wave heights
measured in Yarmouth for 2012 to 2014. The data measurements were taken four times a
day, approximately every six hours for everyday for the three year span. There has also
been current data included in Figure 16 to give an idea of the wave conditions in
Yarmouth. As can be seen from the graph below, the maximum wave height is 5.1
meters. The maximum wave height is important since this will be the point where the
waves exert the largest bending moment on the pile foundation. The wave force analysis
will not be completed in this report, due to the lack of information available for waves.
21
The process on how to calculate these forces will be explained in the next section in order
to give the reader knowledge on how these calculations are carried out.
Figure 15 Wave Height Data for Yarmouth 2012-2014
0
1
2
3
4
5
6
2012 2012.5 2013 2013.5 2014 2014.5 2015
Wav
e H
eig
ht
(m)
Date in years
Wave Height Data for Yarmouth 2012-2014
Series1
23
Wave Force Analysis
When designing and offshore wind turbine it is important to understand the forces that
the turbine will be subjected to. As waves pass the tower they create both an inertial and
drag force opon it. The inertial force acting on the tower is created from the acceleration
of the waves as they make contact, the equation for the inertial force is given below as:
F(t) = π
4ρCMD2 ∙ u̇(t)
Where:
𝜌 = The density of sea water (kg/m3)
𝐶𝑀= Dimensionless inertia coefficient (-)
𝐷= outer tower diameter (m)
�̇�(𝑡) = time dependent undisturbed flow acceleration (m/s2)
The second component of the force acting on the tower comes from drag, and is created
from the the velocity of the wave as it passes the tower. Here the velocity is multiplied by
its absolute value instead of being squared, this is to maintain the orientation of the
velocity vector.
F(t) = 1
2𝜌𝐶𝐷𝐷 ∙ 𝑢(𝑡)|𝑢(𝑡)|
Where:
𝐶𝐷= Dimensionless drag coefficient
𝑢(𝑡)= wave velocity
A very useful equation for predicting the wave forces on an exposed vertical pile goes by
the name of Morrison Equation. This equation is the result of superimposing the linear
inertia force and the drag force of the waves. The equation is shown below and gives the
force per unit length of the pile up to sealevel. One difference in this equation comes
from the u(t)tot variable. The velocity of the wave isn’t the only thing we need to
consider, the velocity of the current needs to be accounted for as well. This is done
simply by vectorially superimposing the current velocity onto the wave velocity. The
acceleration of the waves is the only thing considered in the inertia force since the current
is not accelerating.
F(t)tot = π
4ρCMD2 ∙ u̇(t) +
1
2ρCDD ∙ u(t)tot|u(t)tot|
The dimensionless inertia and drag coefficients can be acquired from graphs published by
Det Norske Veritas (DNV). The DNV suggestions for drag and inertia coefficients is one
24
of the most accepted for design purposes. In order to properly obtain values from the
graphs, the Keulegan Carpenter number (KC) is obtained through the equation given
below.
KC = uaT
D
Where:
𝑢𝑎= flow velocity amplitude (m/s)
𝑇= oscillating flow period (s)
Using the KC value along with the surface roughness values in the graphs below will
yield the drag/inertia coefficients.
Figure 17: Experimentally determined inertia coefficients
Figure 18: Experimentally determined drag coefficients
25
With all of the variables needed to calculate the wave forces having been obtained, they
are then inserted into the Morrison equation. After the forces have been determined, the
moments of these forces are found and then used to calculate the bending stess (σ). The
equation given below gives the bending stress of a hollow cylinder. The perpindicular
distance from the neutral axis (y) will be taken as Do in the equation for bending stress, as
this is where the maximum stress will occur.
σ = My
I
I =𝜋
4(𝐷𝑜
4 − 𝐷𝑖4)
M= ∑ Mo = D1𝐹𝑤𝑎𝑣𝑒 + 𝐷2𝐹𝑤𝑖𝑛𝑑
Where:
I= Area moment of intertia for the tower (m4)
Do= Outer diameter of tower (m)
Di= Inner diameter of tower (m)
D1= Vertical distance from the base of the foundation to the transition piece (m)
D2= Vertical distance from turbine rotors to the base of the foundation (m)
Fwave= Force acquired from the Morrison equation (N)
Fwind= Force exerted on the turbine by wind (N)
From here, the required inner and outer diameter of the tower can be determined by
setting the bending stress of the tower equal to the yield strength of its material, with a
proper factor of safety. Once this has been done, the inner diameter minus the outer
diameter value will be known, these values can then be played with to offer the best
possible load transfer to the seabed.
26
6.0 Finances
The mass of the wind turbine is just a small part of the cost of installing and maintaining
such a large piece of mechanical/electrical equipment. Shipping, machining,
transportation, all influence the total cost of each project. Furthermore, each project is
inherently different than other projects due to primarily, geographical location. Strictly
speaking, the geology of the ocean floor of the chosen location is not known. A steep,
rocky, location would make construction much more difficult than a flat bottom. The
geographical location, as well as the limited resources available on the Internet and the
limited number of offshore wind farms, an estimate of the cost can only be made.
The average cost of a wind turbine is approximately $1.3 to 2.2 million per MW (How
Much Do Wind Turbines Cost, 2016). The cost of preventative maintenance and
corrective maintenance can be as high as $10.00/MWh (Engels et al, 2009). In the same
report, listed is the maintenance cost for the Barrow offshore windfarm as $20.00/MWh.
The location of the Barrow wind farm relative to shore is similar to the chosen Yarmouth
location. At a depth of 18-22 m and a total energy output of 330 GWh it provides a good
measure of the maintenance costs for the proposed wind farm. According to another
report, construction costs can reach $4500 US/kW (RENEWABLE ENERGY
TECHNOLOGIES: COST ANALYSIS SERIES, 1992). Without going into too much
specific detail about the cost of individual components, you can see in the figure below,
the general breakdown of the construction cost per component of the wind turbine as
created by the International Renewable Energy Agency.
Figure 19 Cost breakdown for construction
Figure 19 below shows a cost breakdown of an offshore wind turbine provided in the
same document as the above Figure 20.
27
Figure 20 Total cost breakdown
The numbers in Figure 20, about 4500 US/kW provides a much higher cost estimate of
the values given in reference (Engels et al, 2009). Simple calculation based on the
numbers in Figure 20, shows us that construction costs total to about $450 million US for
the construction of the Yarmouth project. From the Barrow Wind Farm project,
preventative and corrective maintenance costs are about $6 million US yearly. With a
total energy output of about 330 GWh per year sold at a price of $0.10 US ($0.131
CAD)/kWh over a project lifespan of 25 years, the payback period is 14 years.
28
7.0 References
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Agarwal, Puneet, and Lance Manuel. "Wave Models for Offshore Wind Turbines."
American Institute of Aeronautics and Astronautics 1336 (2008): n. pag. Web.
"Current Funding Programs." Natural Resources Canada. Natural Resources Canada, 25
Feb. 2016. Web. 07 Mar. 2016.
Engels, Wouter, Tom Obdam, and Feike Savenije. Current Developments in Wind - 2009.
Tech. Energy Research Center in Netherlands, n.d. Web. 7 Mar. 2016.
"How Much Do Wind Turbines Cost?" Http://www.windustry.org/. N.p., n.d. Web. 7
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Lynn, Paul A. Onshore and Offshore Wind Energy: An Introduction. Chichester, West
Sussex: Wiley, 2012. Print.
Manuel, J. F., J. G. McGowan, and A. L. Rogers. Wind Energy Explained. 2nd ed. N.p.:
John Wiley & Sons, 2009. Print.
McKay, David. "Wind II." (n.d.): n. pag. Www.withouthotair.com. 2009. Web. 8 Mar.
2016.
"Renewable Energy Outlook." World Energy Outlook 2013 World Energy Outlook
(2013): 197-229. International Energy Agency, 2013. Web. 7 Mar. 2016.
"RENEWABLE ENERGY TECHNOLOGIES: COST ANALYSIS SERIES." Science
Scope 16.3 (1992): 32-34. International Renewable Energy Agency. Web. 8 Mar.
2016.
Report on the Review of the Community Feed-in Tariff Program. Halifax, N.S.: Dept. of
Energy, 2014. Mar. 2014. Web. 7 Mar. 2016.
29
"Sable Island National Park Reserve." Parks Canada. Parks Canada, Mar. 2012. Web. 8
Mar. 2016.
Wind Turbines – Part 1: Design Requirements. Tech. no. 61400-1. N.p.: International
Electrotechnical Commission, n.d. Print.
Miñambres, Oscar Yanguas. "Assessment of Current Offshore Wind Support Structures Concepts
– Challenges and Technological Requirements by 2020." Http://e-archivo.uc3m.es.
Karlshochschule International University. Web.
Espinosa, Julio García. "Design and Calculus of the Foundation Structure of an Offshore
Monopile Wind Turbine." Https://upcommons.upc.edu. Facultat De Nautica De
Barcelona, 1012. Web.
Marino, Enzo. "An Integrated Nonlinear Wind-Waves Model for Offshore Wind
Turbines." Google Books. Web. 08 Mar. 2016.
"Foundation Design Loa." Http://www.fema.gov. Web.
Techet, A. H. "Morrison's Equation." Http://web.mit.edu. MIT, 2004. Web.
"2015 Tide Tables." Government of Canada, Fisheries and Oceans Canada, Science.
Web. 08 Mar. 2016.
"Ocean Surface Currents (OSCAR)." Ocean Motion : Data Resources : Ocean Surface
Current Visualizer. Web. 08 Mar. 2016.
"The European Offshore Wind Industry - Key Trends and Statistics 2015."
Http://www.ewea.org. The European Wind Energy Association, Feb. 2016. Web.