Download Offshore wind energy

TEMPUS ENERGY: OFFSHORE WIND ENERGY
1: Introduction
Worldwide, the importance of wind energy is growing rapidly. A distinction can be made between
onshore wind turbines (wind turbine farms) and offshore wind turbines (wind turbine farms). Using
offshore wind turbines instead of onshore wind turbines accounts for a number of advantages:
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The offshore wind speeds are higher than the onshore wind speeds. For instance when 10 km
offshore, the wind speed is typically up to 0.5 m/s higher. Since the power in the wind
increases with the cube of the wind speed, this higher wind speed is relevant.
The offshore wind speeds are less turbulent which also increases the generated power by the
wind turbines.
Onshore constraints due to spatial planning, noise effects and visual impact are reduced when
considering offshore wind turbine farms. This implies larger wind turbines can be used. Larger
turbines have higher towers which implies the wind speed reaching the rotor blades is higher.
The use of offshore wind turbines also accounts for a number of disadvantages:
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The offshore installation costs are considerably higher (which also stimulates the use larger
wind turbines).
Offshore wind turbines require more maintenance when compared with onshore wind
turbines. Wind turbines contain a lot of moving parts, they are complex mechanical and
electrical systems. To obtain a reliable power generation, sufficient maintenance is required
which is much more expensive since it asks a lot of effort to reach the offshore wind turbines.
Figure 1: Offshore wind energy
2: The evolution of offshore wind energy
Worldwide research on offshore wind power started in the 1970s. Mainly between 1977 and 1988,
national research has been performed concerning the potential of offshore wind energy and relevant
technologies. Roughly from 1990 to 1998, the potential of offshore wind energy has been evaluated
with some offshore wind turbine farms with medium-sized wind turbines (e.g. in 1992 a semi-offshore
wind turbine farm has been installed near the Northumbrian port of Blyth in the United Kingdom
which contained nine 300 kW turbines). From 1999 to 2005, large scale offshore wind power
demonstration projects have been realized. From 2005 to present, a large scale development of
offshore wind energy has been realized.
Figure 2: Overview of the Belgian offshore wind projects (source: Brabant)
At present, offshore wind turbine farms have been installed in a lot of countries and continents. When
restricting to Europe, the United Kingdom, Denmark, Sweden, the Netherlands and Belgium are
forerunners. Additional wind turbine farms are under construction and are proposed in several
countries. Due to the large coast line, the United Kingdom is an important forerunner (the United
Kingdom has a potential for offshore wind energy which exceeds the entire electricity demand of the
country). Figure 1 visualizes a number of locations where offshore wind turbine farms have been
installed.
Also in Belgium, a number of offshore (farshore) wind turbine farms are built or planned as visualized
in Figure 2 (http://www.mumm.ac.be/EN/Management/Atlas). Prior to installing a wind farm, the
project developer must obtain
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a domain concession in the zone which is reserved for wind energy development,
an environmental permit.
Once the domain concession and the environmental permit is available, the project developer is able to
built and exploit the wind turbine farm. As visualized in Figure 2, seven concessions are granted:
Norther, C-Power, Rentel, Northwind, Seastar, Belwind and Mermaid giving a total area of 238 km2.
Figure 3 shows a number of C-Power wind turbines and the offshore transformer station installed on
March 17th 2012.
Figure 3: Offshore transformer station of C-Power
3: The layout of an offshore wind turbine farm
The layout of an offshore wind turbine farm is given in Figure 4. The primary components are the
wind turbines which are located in the sea. Using electric cables, these wind turbines are connected to
an offshore transformer station which in turn is connected to an onshore transformer station using a
submarine cable. Due to the onshore transformer station, the wind turbine farm is able to inject its
power into the public high voltage grid.
The distances between the wind turbines are usually 4 to 8 times the rotor diameter. Placing the wind
turbines sufficiently far from each other, reduces turbulences and wake effects which increases the
power production. A wind turbine contains the foundation, the support structure, the transition piece,
the tower, the rotor blades and the nacelle (the nacelle contains the generator and possibly the
gearbox). The foundation system and the support structure is very demanding since it must keep the
turbine in its proper position while being exposed to forces coming from the wind and the sea waves.
Figure 4: Layout of an offshore wind turbine farm (source: Malhotra)
The electric voltage generated by the generator in the nacelle is converted to a higher voltage by a
transformer. This transformer can be mounted in the nacelle but the transformer can also be mounted
in the tower of the wind turbine.
3.1: The mechanical loads of the wind turbine
Especially when considering offshore wind turbines, the support structures and the foundations are
very important. First of all, the support structures and the foundations must withstand permanent loads
due to the mass of the structure (e.g. a typical 3 MW wind turbine has a tower of 156 ton, a nacelle of
68 ton and a rotor of 40 ton). Moreover, as the blades of the wind turbine rotate, the blades travel past
the tower creating vibrations. For a typical variable speed wind turbine, the blade passing frequency is
between approximately 0.54 Hz and 0.78 Hz (which is three times the rotational frequency i.e. 0.18 Hz
to 0.26 Hz). The cyclic loading due to the sea waves typically occurs at a frequency between 0.04 Hz
and 0.34 Hz. The foundations and the entire construction must withstand these loads. The foundations
and the entire construction must also withstand loads from wind speed variations and moving sand
dunes.
A key assumption when designing offshore wind turbines is that the turbine must be able to withstand
extreme events which only occur once in a period of 50 years (the wind turbines have a design life of
20 years). Since wind turbines are generally unmanned during a storm, the key assumption is less
demanding than the key assumption when designing e.g. oil and gas industry structures which are also
manned during a storm (here, the structure must withstand extreme events which only occur once in a
period of 100 years). Possibly the structure must also withstand loads due to ice or seismic activity.
3.2: Support structures and foundations of a wind turbine
A number of support structures and foundations are quite common when considering offshore wind
turbines. An overview is given in Figure 5 in combination with their applicable water depth (gravity
structure, monopile tower, guyed monopile tower, tripod, braced lattice frame, floating tension leg).
Figure 5: Support structures and foundations of offshore wind turbines (source: Malhotra)
3.3: The electrical layout of a wind turbine farm
Figure 6: HVDC light transmission system for offshore wind power (source: Eriksson)
Each wind turbine contains a generator (asynchronous generator, doubly-fed induction generator,
synchronous generator,…) and the generated voltage is transformed to a higher voltage level (e.g. 33
kV) by a step-up transformer (this transformer can be mounted in the nacelle or in the tower of the
wind turbine) in order to reduce the 𝐼 2 𝑅 losses. The power cable (e.g. 33 kV) from each turbine is
inserted in a J-shaped tube which carries the cable to a cable trench in the seabed. The power is
transported to an offshore transformer station (Figure 3) where the voltage level is transformed to a
higher voltage level (e.g. 150 kV). A three phase submarine cable (protected with appropriate
armouring) transports the power to land as visualized in Figure 4 (in case of the Belgian Thornton
Bank of C-Power a distance of 38 km is crossed). Onshore, the power in injected in the public high
voltage grid.
Instead of using a three phase high voltage cable to transport the power to land, also a HVDC light
connection can be used as visualized in Figure 6. The AC voltage is rectified and a DC link is used to
transport the power. Onshore, the DC voltage is converted to an AC voltage and the power is injected
in the public AC high voltage grid.
4: Non-grid-connected wind power
Due to fluctuations in the wind speed, the power generated by wind turbines is also time-varying. In
case the contribution of wind energy exceeds 10% of the generated power (e.g. due to the installation
of large offshore wind turbine farms), it becomes more and more difficult to maintain the power
balance in the grid.
4.1: High energy consuming industries
In order to be able to realize a large number of large scale wind energy projects without disturbing the
power balance of the public electricity grid, the realization of non-grid-connected wind power is an
option. Instead of injecting the fluctuating generated power into a public electricity grid, the
fluctuating power is used by high energy-consuming industries which are located close to the wind
turbine farm. More precisely, a point-to-point private grid connection is made between the wind
turbine farm and the factory.
These high energy-consuming industries include aluminum-focused non-ferrous metallurgical
industry, salt chemical chlor-alkali industry, desalination industry, … For instance aluminum
companies are known for their large electrical power consumption and traditionally they are located
close to large hydroelectric power stations or nuclear power plants (or generally in countries where
electricity is cheap). Offshore and onshore wind turbine farms can also supply these aluminum
companies with electricity.
Aluminum is produced using the Hall-Héroult-process where aluminum oxide will be reduced to
aluminum using carbon i.e.
2𝐴𝑙2 03 + 3𝐶 → 4𝐴𝑙 + 3𝐶𝑂2 .
In practice, alumina 𝐴𝑙2 𝑂3 (aluminum oxide) is dissolved in a carbon-lined vat of molten cryolite
𝑁𝑎3 𝐴𝑙𝐹6 (sodium hexafluoraluminate) where some aluminum fluoride 𝐴𝑙𝐹3 is added giving a “cell”.
The molten mixture of alumina, cryolite and aluminum fluoride is electrolyzed by passing a direct
current through it (the carbon-lined vat is the cathode and the anode placed in the molten mixture is
also made of carbon). Hundreds of “cells” are connected in series and a direct current is passing
through it. Approximately, 8𝑘𝑊ℎ is needed to produce 1 𝑘𝑔 of aluminum. Moreover, the cells are
electrically heated to reach the operating temperature (approximately 960 °C).
The salt chemical chlor-alkali industry is also a high energy consuming industry. The Chlorine-Caustic
process produces caustic soda (𝑁𝑎𝑂𝐻) and chloring (𝐶𝑙2 ) using salt (𝑁𝑎𝐶𝑙 = sodium chloride) based
on the chemical reaction
2 𝑁𝑎𝐶𝑙 + 2 𝐻2 0 → 2 𝑁𝑎𝑂𝐻 + 𝐻2 + 𝐶𝑙2 .
A direct current is needed to produce this 𝑁𝑎𝑂𝐻 (sodium = 𝑁𝑎 is an alkali metal) and 𝐶𝑙2 which are
used for the production of pulp and paper, soaps, detergents, plastics, fertilizers, disinfection
chemicals…
When considering the applications of these high energy consuming industries, it is important the
efficiency of the production process does not depend on the current density since the generated power
is fluctuating. Indeed, the power fluctuations influence the size of the production but they do not
influence the efficiency.
The development of renewable energy production to provide the high energy consuming industries
sufficient energy is not only important for the development of these so called upstream industries. It
also stimulates the development of the so called downstream industries which use the produced
aluminum, hydrogen, caustic soda or chlorine as their inputs.
4.2: Energy storage
In order to be able to realize a large number of large scale wind energy projects without disturbing the
power balance of the public electricity grid, the fluctuating power can be stored. For instance, the
power is used to produce hydrogen (𝐻2 ) and oxygen (𝑂2 ) from water (𝐻2 𝑂). More precisely,
2 𝐻2 0 → 2𝐻2 + 𝑂2 .
The produced hydrogen can be stored, transported and distributed which is the main philosophy of the
so called hydrogen economy. The hydrogen
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can be burned producing heat,
can be fed to a fuel cell producing electricity which allows e.g. to drive a car using an
electrical motor,
can be fed to a fuel cell producing electricity which is injected into the public grid (giving a
constant power instead of a fluctuating power),
can be fed to a combustion motor to drive a car,
can be used in the chemical industry,
…
Figure 7: Options of electricity storage (source: Maegaard)
As visualized in Figure 7, the power generated by the wind turbine farm can also be used
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to heat water which can be stored and used,
to produce compressed air which can be stored (CAES) and can feed a turbine which drives a
generator injecting power into the grid (giving a constant or controllable power instead of a
fluctuating and non controllable power),
to pump water to a high reservoir (pumped storage) and when needed, the water is flowing
down driving a hydraulic turbine and a generator injecting a controllable power into the grid,
to load batteries (the generated voltage is rectified and used to load the batteries) and using an
inverter the stored energy is released.
5: Condition monitoring and condition based maintenance
The total cost of installing an offshore wind turbine farm exceeds the costs of installing an onshore
wind turbine farm. Also repairing broken parts of an offshore wind turbine is more expensive than
repairing broken parts of an onshore wind turbine. Moreover, when performing repairs (and other
tasks) on offshore wind turbines, the impact of the weather is very large which requires an adequate
planning including reliable weather forecasts (which is highly non trivial).
In order to limit the repair costs, adequate technology is used by combining existing experience from
onshore wind turbine farms and offshore oil platforms. In practice, different approaches are used but
some possibilities are:
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using a direct-drive generator avoiding the use of a gearbox,
using a fixed-pitch rotor eliminating complex systems,
using a generator with a simple structure,
using cathode anti-corrosive protection (rust protection),
using appropriate surface protection (painting pigments),
…
In general, corrective maintenance (i.e. repairing broken parts) must be avoided as much as possible.
When a wind turbine comes to a standstill due to a broken part, production losses occur. Repairing the
broken part is expensive and can take a long time due to bad weather conditions (implying production
losses during a longer period of time).
The use of preventive maintenance, i.e. maintenance of the system before faults occur, is a good
practice. Scheduled or planned maintenance i.e. performing routine maintenance actions in order to
keep the system working is useful. However, sometimes the scheduled maintenance is too late
implying still faults occur or sometimes the scheduled maintenance is performed before it is really
needed (implying unnecessary repair costs).
Preventive maintenance also includes the option of condition based maintenance. Condition based
maintenance relies on measurements of the system components. These measurement results are logged
and evaluated which allow
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to identify changes and to know the causes of these changes,
to make statements of the condition of the installation which allows early fault detection and
repair.
Condition monitoring and condition based maintenance is useful in many branches of industry. Also
when considering onshore wind turbine farms and especially when considering offshore wind turbine
farms, condition based maintenance is important.
When considering offshore wind turbine farms, approximately 25% to 30% of the energy generating
costs are due to maintenance and repair. By using appropriate condition monitoring techniques, the
need for scheduled and corrective maintenance can be reduced which reduces the maintenance costs.
Indeed, wind turbines contain complex electrical and mechanical systems with a large number of
moving parts. To obtain a reliable power generation, appropriate maintenance is crucial. An offshore
wind turbine will not survive long when there is a lack of maintenance.
First of all, sensors are needed to measure a number of relevant parameters. The measurement data is
sent to a monitoring center (e.g. using an optic fiber which is integrated in the high voltage power
cable connecting the wind turbine farm and the mainland). The monitoring center logs the data and
detects trends over a period of weeks, months and years. A distinction can be made between warning
and alarm thresholds.
When considering wind turbines, vibration analysis is very important. Using accelerometers,
vibrations of the gearbox, the bearings of the gearbox, the bearings of the power generator,… are
measured. The logged data allows to perform an analysis in the time domain or an analysis in the
frequency domain (e.g. power spectrum density analysis). Not only accelerometers are important to
measure vibrations, also noise measurements and even electrical parameter measurments are
important.
An imbalance in the rotor of the wind turbine affects directly all drivetrain components as well as the
structural health of the wind turbine. This rotor imbalance could be due to mass imbalance arising
from inhomogeneous mass distributions, or aerodynamic imbalances arising from errors in the pitch
angle or blade damage. This imbalance causes the presence of the rotating frequency and its multiples
in the Fourier spectrum.
Oil analysis is also important. More precisely, the quality of the oil is inspected and reveals the
presence of scrap iron in the oil. Measuring the oil filter pressure drop and the oil temperature is also
relevant.
Instead of monitoring vibrations, pressures, temperatures… of individual parts, a more holistic
approach is possible by monitoring the actual power output of the wind turbine and compare this
power with the normal power output at the available wind speed. In case the difference between the
actual power production and the normal power production exceeds a threshold value, the presence of a
fault is revealed.
References
Becker E., Telemonitoring of wind turbines, magazine ‘antriebtechnik’,Prüftechnik, May 2008.
Boyle G., Renewable Energy: Power for a Sustainable Future, Oxford University Press, 2012.
Brabant R., Degraer S. and Rumes B., Chapter 2: Offshore wind energy development in the Belgian
part of the North Sea and anticipated impacts: an update.
Eriksson K., Liljegren C. and Sobrink K., HVDC Light Experiences applicable for power transmission
from offshore wind power parks, AIAA, 2004
Grainger B. and Thorogood T., Beyond the harbour wall, IEE Review, March 2001, pp. 13-17.
Gu L. and Jin C., Performance Evaluation of Non-Grid-Connected Wind Power Industry Cluster,
Jin C. and Zhou Z. and Bai K., Construction of Non-Grid-Connected Wind Power Industrial Bases in
Jiangsu Coastal Areas, IEEE, 2010.
Liu Y., Zhang P. and Fang M., Research on Construction System of Large Scale Non-Grid-Connected
Wind Power Offshore Wind Farms,
Maegaard P., Wind Energy Development and Application Prospects of Non-Grid-Connected Wind
Power, IEEE, 2009.
Migueláñez E. and Lane D., Predictive Diagnosis for Offshore Wind Turbines using Holistic
Condition Monitoring,
Malhotra S., Selection, Design and Construction of Offshore Wind Turbine Foundations
(http://www.intechopen.com)
U.S. Energy Requirements for Aluminum Production: Historical Perspective, Theoretical Limits and
Current Practices, Prepared for Industrial Technologies Program Energy Efficiency and Renewable
Energy, U.S. Department of Energy, February 2007.