OFFSHORE WIND ENERGY

Group Pengmei Wu

Fan Zou Clément Bertrand

Loïc Delattre Aitor Colinas

OFFSHORE WIND ENERGY

Introduction

How does the energy use look like in future, so many current problems wake us up to the importance of the renewable energies supplying. Renewable energies offer the chance for a sufficient and secure energy supply, they will solve several problems caused by the burning of fossil fuel, and they will also create an opportunity for peaceful development and greater global security. It is obvious that harnessing this energy should be given a high political and economic priority in order to shift the world to a completely renewable energy supply. As one of these promising renewable energies, wind energy offers the potential to generate substantial amounts of electricity without the pollution problems of most conventional forms of electricity generation, no doubt wind power will be one cornerstone of the energy supply of the future. However, to build windmills is really difficult and expensive, and the environment would be influenced as well, but in the sea is where mills will do the least harm. People do not need to suffer from the noise and they will not ruin our landscapes, and also the strongest winds are at sea. Several countries demonstrate that wind energy use is technically and economically feasible. Denmark's offshore wind generation provides about 20% of total electricity demand in the country.

This paper first discusses the technology part of wind power, including the basic knowledge of offshore power turbine, how it produces the electricity, how to classify the offshore turbines and where to locate the windmills as well. Then it points out the economic factors of the offshore power generations. Thirdly, it makes some comparisons between the offshore wind energy and the other energy forms; it also analyzes the current situations and presents an overview of the wind power development and utilization of wind power.

I.) DESCRIPTIONS I.1) Conditions of the Offshore Wind Energy The wind turbine converts the kinetic energy of the wind to electrical energy. To do that, the wind generates a torque in the blades (rotor), which will move the turbine with a determinate force and it will generate an amount of energy in the generator (stator). This amount of energy depends on the density of the air, the swept area and the speed of the wind. Power generated depends on the area of the rotor, which is defined by the size of the blades and the speed of the wind. In the same way, the area depends on the diameter of the blades, so that the longer are the blades, the bigger is the power generated. We can represent de power with this equation: P �W �= 1/2�p�A�V 3

I.1.1) The most influential conditions I.1.1.1) The Density of the air The kinetic energy of an element is proportional to its mass. Therefore, the kinetic energy of the air depends on its density, on its mass per volume unit. So, if the air is heavier, then the turbine receives more energy. It is very useful when we are speaking about offshore wind energy because the wind or air in the sea has more density than in the land In addition, the air has more density when is colder and with more pressure (in higher positions there is less pressure and the air is less dense). It means that the winds are better in the sea because they have more density than the winds of the mountains. I.1.1.2) The swept area The area of the rotor determinates the quantity of the energy that the turbine is going to be able to take. The area of the rotor increases with the square of the blade's length, so if we have a turbine with the double of blade length, it will receive four times more energy.

Figure 1: Relation between blade lenght and power

It is very important for designers and investors to know the variations of the wind. They need this information to design more efficient turbines and to know how much they have to invest. If we do statistics of the wind speed and direction in the specific place where the turbine is going to be built, then we can know how much energy is going to be produced there. Rotor diameters may vary somewhat from the figures given above, because many manufacturers optimize their machines to local wind conditions: A larger generator, of course, requires more power (i.e. strong winds) to turn at all. So if you install a wind turbine in a low wind area you will actually maximize annual output by using a fairly small generator for a given rotor size (or a larger rotor size for a given generator) For a 600 kW machine rotor diameters may vary from 39 to 48 m (128 to 157 ft.) The reason why you may get more output from a relatively smaller generator in a low wind area is that the turbine will be running more hours during the year. The bigger turbines are well adapted to the offshore wind energy. The building costs don’t increase so much with the size of the tower and the maintenance cost is the same. Nowadays, the new technologies improvements are allowing cheaper building costs, so the offshore energy is becoming more efficient every time. I.1.1.3) The speed of the wind We can see in the formula of power that the speed of the wind is very important. The amount of energy of the wind varies with the cube of the speed. Double the wind speed, eight times the power. So, we have to look for the places were the speed of the wind is regularly strong. But there is a problem. If the wind is too strong, the blades and the rotor can get damaged. This is why if the wind speed is higher than 25 m/s approximately (it depends on the model), the rotor is stopped.

It means that all the wind power generators have a functionality rate, approximately between 5 m/s and 25 m/s (18 km/h – 90 km/h). We can divide this rate in different parts.

1) 0 – 5 m/s. We need a little wind speed to start the motion of the blade. It is supposed to be about 5 m/s approximately and below this speed, it is not efficient to turn on the turbine.

Figure 2: Relation between output power and wind speed

(Notes from University of Navarra www.tecnun.es (pdf archive))

2) 5 – 15 m/s. If the speed is higher, the power improves with the cube of the wind speed. This is the rate where we could use the function of the power (point I.1).

3) 15 – 25 m/s. This is the maximum power we can generate. We can regulate the rotational speed of the blades to maintain it in a constant speed.

4) More than 25 m/s. Finally, if the wind speed is higher than 25 m/s, the components risk spoiling. This is why we stop the turbine when there is high speed.

It is very important for designers and investors to know the variations of the wind. They need this information to design more efficient turbines and to know how much they have to invest. If we do statistics of the wind speed and direction in the specific place where the turbine is going to be built, then we can know how much energy is going to be produced there.

This graphic represents the statistics of wind speed. We can see that most of the time the wind speed is near by 4-8 m/s, in this case. The black line represents the medium of the graphic. It means that there is the same probability to have a wind speed less or higher than the represented in the diagram by this black line. But the average wind speed is different. In this concrete graphic the average is bigger than medium speed. The average is got when we multiply each interval with the probability to have the highest value of the graphic. This is why the average is going to be higher and the power we can get may be higher. This statistics are necessary if we would be interested in building a general wind energy farm. These kinds of constructions are big and they require a lot of studies because of the responsibility they have. Offshore wind farm are more expensive to build, so the dates to know the viability of the projects are analyzed slowly. Therefore, there is a social responsibility. When the farms are built, they are thought to give an specific quantity of energy per year, and this predictions are made with this kind of statistics. I.1.1.4) Roughness The roughness is an important parameter. The land is not usually plain, so when the wind touches the surface it becomes slower. This difference between the speed at the top and below of the wind tower can produce some turbulences, and it affects on the durability of the wind turbine.

Figure 3: Wind speed percents (www.windpower.org/es/tour/wres/weibull.htm)

In a scale between 0 and 5, 5 represents lands with trees and small hills, and 0 is the surface of the sea. The parameter of the roughness in onshore wind turbines use to be 2 or 3. We can observe that the offshore wind turbines can have more duration than the onshore. With lower level of roughness, the shearing is also lower. Shearing is the effect of wind speed when it is lower in the surface (at least zero). If the roughness on the sea surface can be considered 0, the differences between both speeds are lower. It means that there is no vortex under the blades, so the distance between the blades and the surface can be smaller. This is why the use of smaller towers, 0,75 times the diameter of the rotor, is the most habitual choice. I.1.1.5) The number of blades The most of the wind turbine in the world have 3 blades per turbine. The reason is that a rotor with 3 blades can be considered as a disc when we have to calculate the mechanical properties of the turbine. Because of the instability, two bladed rotors is not a good idea. In addition, we would need more rotational speed to get the same amount of energy. I.1.1.6) Efficiency of the wind turbines To understand the efficient of the wind turbine, we have to understand the relation between input and output wind speed. When the turbine takes energy from the wind, this one slows down. It means that if we would be able to take the 100% of the energy, the output wind speed would be zero. But this is no possible, because in this case is not possible to enter any wind in the turbine. So, we would like to know how we can break wind speed with the most efficient energy conversion. The solution is less than 2/3. More precisely less than 59%. With the perfect turbine we would be able to have an efficient of the 59%. To understand this, we have to use the law of Betz. The mathematical description of the law:

Figure 4: Efficiency of the turbines http://www.windpower.org/es/tour/wres/powdensi.htm

The continuity equation says that the flow rate must be constant, so:

flow= ��A1�V1= ��A2�V2= ��A4�V4 �= density of theair V1�V2�V4 � A1� A2�A4 The force is the rate of change of momentum: F rotor= flow��V1− V4� The power is the rate of change of the kinetic energy:

W·

rotor= 1/2� flow��V12− V42�= 1/2� flow��V1− V4���V1�V4� The power also can be expressed as the force by speed in the point 2:

W·

= F�V2= flow��V1− V4�V2 Resulting: V2= 1 /2��V1�V4� The coefficient of the rotor efficiency, Cp, is the relation between the wind power in the point 2 and the the wind power without any turbine (point 1):

Cp=W

·

2

W·

1

= 1/ 2���A2�V2��V12− V42��1/2wp�A2�V1�V12�

We can define b = V4 / V1, so, Cp = 1/2*(1- b2)(1+b) The maximum value of Cp: Differentiating with respect to b and equaling to zero: b = 1/3 so,

Cp = 0,59 = 59% I.2) Functioning of a wind turbine

Figure 6: Different analysis points (Notes from University of Navarra www.tecnun.es (pdf archive)

I.2.1) The mechanism of wind turbines (WT)

Figure 6: Summary of the functioning of offshore wind turbines

(http://www.bwea.com/offshore/how.html)

Wind turbines convert the kinetic energy of the wind to mechanical power

First, a suitable place for the wind farm has to be found for the Piles building (1) into the seabed. In order to prevent damage to the sea floor, the piles are protected against erosion. The top of the foundation is painted with a bright colour to make it visible to ships and has an access platform for the maintenance.

Sensors on the turbine detect the wind direction and turn the nacelle, to face into the wind, in order to collect the maximum energy. A torque is created around a horizontal hub, when the wind interacts with the wind turbine blades (2), which are connected to a shaft inside the nacelle (3). Considering that the rotational frequency of the rotor is relatively low, this frequency has to be increased thanks to a gearbox. The gearbox output shaft turns a generator which converts the energy to electricity.

Sub sea cables (4) deliver the electricity to an offshore transformer (5) which converts the electricity to a high voltage.

The grid system transmits the electricity to the transformer substations on land (6) which reduce the voltage to domestic or industrial values.

The local low voltage networks transmit the electricity to homes, offices and factories.

I.2.2) Components of turbines

Figure 7: Description of a wind turbine (http://www.capewind.org/article19.htm)

The components shown in figure 1-2-2 are mainly:

0) A rotor composed of three blades;

1) A hub through which the blades attach to the low speed drive shaft;

2) A nacelle including a gearbox and generator, shafts and couplings, a mechanical disk brake and a yaw system;

3) A tower and foundation that supports the rotor and the drive train;

4) Electrical controls and cabling, and instrumentation for monitoring and control.

I.2.2.1) The rotor

The rotor is composed of the hub, three blades and a pitch regulation system. Rotor blade design has advanced with knowledge from wing technology, and utilises the aerodynamic lift force that an air foil experiences in a moving stream of air. The shape if the blade and its angle in relation to the relative wind direction both affect its aerodynamic performance.

What makes the rotor turn?

It is of course obvious that the wind but not only. It is a bit more complicated than just the air molecules hitting the front of the rotor blades. Modern wind turbines borrow technologies known

from air planes and helicopters, plus a few advanced tricks of their own, because wind turbines actually work in a very different environment with changing wind speeds and changing wind directions.

The wind which hits the rotor blades of a wind turbine will not come from the direction in which the wind is blowing in the landscape, (lets say from the front of the turbine). This is because the rotor blades themselves are moving.

Since most wind turbines have constant rotational speed, the speed with which the tip of the rotor blade moves through the air (the tip speed) is typically some 64 m/s, while at the centre of the hub it is zero. 1/4 out from the length of the blade, the speed will be about 16 m/s.

Rotor blades for large wind turbines are always twisted.

Seen from the rotor blade, the wind will be coming from a much steeper angle (more from the general wind direction in the landscape), as we move towards the root of the blade, and the centre of the rotor.

A rotor blade will stop giving lift, if the blade is hit at an angle of attack which is too steep. Therefore, the rotor blade has to be twisted, so as to achieve an optimal angle of attack throughout the length of the blade.

Rotor blade materials The materials used in modern wind turbine blade construction may be grouped into three main classes:

1 wood (included laminated wood composites);

2 synthetic composites (polyester or epoxy reinforced by glass fibres);

3 Metals (steel or aluminium alloys).

Lifetime The lifetime of a rotor is related to the variable loads and environmental conditions that it experiences during its operation for instance the frost, the corrosion, the metal fatigue etc. Therefore, the rotor’s inherent mechanical properties and design will affect its useful service life.

I.2.2.2) The nacelle

The nacelle houses the turbine’s drive train and generator assemblies, plus the yaw mechanism and any control components.

I.2.2.3) Gear box

Gears connect the low-speed shaft to the high-speed shaft and raise the rotational speeds from about 30 to 60 rotations per minute to about 1200 to 1500 rpm. It’s the rotational speed required by most generators to produce electricity.

I.2.2.4) The generator

Wind turbine generators are very unusual, in comparison to other generating units which are attached to the electrical grid. One reason is that the generator has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power but at the output with the utility’s power distribution or to local power requirements.

Most of the wind turbines in the world use a so-called three phase asynchronous generator also called induction generator, whose magnetising current is drawn from the grid, ensuring that the generator’s output frequency is locked to that of the utility, and controlling the rotor speed within limits. Synchronous generators produce electricity in synchronisation with the generator’s rotating shaft frequency, and the rotor speed must exactly match the utility supply frequency.

The current is sent through a transformer next to the wind turbine or inside the tower to raise the voltage depending on the standard in the local electrical grid.

Large manufacturers usually supply both 50 Hz wind turbine models (for the electrical grids in most of the world) and 60 Hz models (for the electrical grid in America). The rotor’s frequency is typically about 0.5 Hz and so the increase in frequency is obtained by a combination of a gearbox and a multi-pole generator.

The cooling system Generators need cooling while they work. Some manufacturers use generator placed in the duct and some of them use large fan for air cooling, and some use water cooled generators. Water cooled generators may be built more compactly, which also gives some electrical efficiency advantages, but they require a radiator to get rid of the heat from the liquid cooling system.

I.2.2.5) Yaw

In order to extract as much of the wind’s kinetic energy as possible, the rotor axis should be aligned with the wind direction. Larger WT’s with upwind rotors require active yaw control to align the machine with the wind. When a change in wind direction occurs, sensors activate the yaw control motor, which rotates the nacelle and rotor assembly until the turbine is properly aligned.

Downwind machines of all sizes may posses passive yaw control, which means that they can self-aligned with the wind direction without the need for a tail vane or yaw drive.

I.2.2.6) Tower

The tower of a WT supports the nacelle assembly and elevates the rotor to a height at which the wind velocity is significantly greater and less perturbed than at sea level, due to the wind shear effect. Therefore the tower’s structure must withstand significant loads, originating from gravitational, rotational and wind thrust loads. In addition, the tower must be able to withstand environmental attack for the entire design life of the turbine, which may be 20 years or more.

I.2.3) The control system

Some wind machines require a control system to start up when the wind speed is sufficient or to turn off the machine at about high speeds to prevent overheating of the generators.

An anemometer and wind vane measures the wind speed and direction then transmits the data to the controller.

Wind turbines are therefore generally designed so that they yield maximum output at wind speeds around 16 meters per second because stronger winds are rare.

In case of stronger winds it is essential to waste part of the excess energy of the wind to avoid damaging the wind turbine. All wind turbines are designed with power controls which are:

The turbine's electronic controller which checks the power output of the turbine several times per second. When the power output is too high, it sends an order to the blade pitch mechanism which immediately turns the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again.

Stall controlled wind turbines have the rotor blades closing onto the hub at a fixed angle. it is the inherent aerodynamics properties of the blade that determine the power output; there are no moving parts to adjust. The geometry of the rotor blade profile (twist, thickness), has been designed to ensure the creation of turbulence on the side of the blade which is not facing the wind when the wind speed becomes too high. This stall prevents the lifting force of the rotor blade from acting on the rotor. As the actual wind speed in the area increases, the angle of attack of the rotor blade will increase, until at some point it starts to stall.

I.2.4) Safety system

Most of wind turbines are equipped with safety devices to ensure safe operation during their lifetime.

The most simple safety device is the vibration. It consists of a ball resting on a ring. The ball is connected to a switch through a chain. If the turbine starts shaking, the ball will fall off the ring and switch the turbine off.

There are also electronic thermometers located in the nacelle, which check the oil temperature in the gearbox and the temperature of the generator.

It is important that wind turbines stop automatically in case of dysfunction of a critical component. For instance, if the generator overheats or is disconnected from the electrical grid, it will stop breaking the rotation of the rotor, and the rotor will start accelerating rapidly in a second. So the wind turbine is fitted with an over speed protection system.

The normal ways of stopping a turbine without any stress, tear on the tower and the machinery, are to use the aerodynamic braking systems. They have to work in case of electrical power failure. They are also activated if the hydraulic system in the turbine loses pressure. They can stop the turbine in a matter of a couple of rotations.

Currently, the wind turbines are supported by tubular steel towers. The first advantage is that it makes it safer and far more comfortable for service personnel to access the wind turbine for repair and maintenance but it’s very expensive.

The primary danger in working with wind turbines is the height above ground during installation work and when doing maintenance work.

So, the straps are connected with a steel wire to an anchoring system that follows the person while climbing or going down in the turbine.

The wire system has to include a shock absorber, so that persons are reasonably safe in case of a fall.

Fire protection and electrical insulation protection are governed by a number of national and international standards.

During a maintenance work, it’s important to stop the machinery. The rotor can be locked in situ with a pin in order to prevent any movement of the nacelle.

I.2.5) Size of the wind turbines

A typical land-based turbine has a tower height of about 30 to 50 meters, and blades between 12 and 25 meters long. Offshore wind turbines are predicted to be on 60-80 meters towers, with 35-60 meter long blades, capable of generating at least 2 megawatts, although it is very likely with time that the size of installed turbines will increase.

Turbine manufacturers are already designed machines for the offshore market, mostly in the range of 1.5 to 2.5MW.

I.2.6) Turbines can be upward or downward machines

The rotor can be adjusted upwind of the tower and nacelle. In this way, the tower doesn’t perturb the wind flow.

It also can be placed downwind of the tower. It enables self alignment of the rotor with the wind direction. The problem is that the wind arriving at the rotor is deflected and turbulences are created by the tower. (See Figure 1-2-3)

Figure 8: Upwind and downwind horizontal axis wind machines.

I.3) Location I.3.1) Where? Although it is possible to build structures in water deeper than 30m (for example the oil platforms), it is very expensive and would not be economically viable for offshore wind turbines. Nevertheless, some studies and applications are currently developing in order to find mean to put offshore wind turbines in water deeper and deeper, this part will be more developed in the part (I.3.3)

The more you get far from the shore, the more the wind velocity tends to increase. This means that turbines built further offshore should capture more wind energy. Unfortunately, as the distance to land increases, the cost of building and maintaining the turbines and bringing the electricity back to shore also increase, limiting the distance out to sea at which offshore wind projects will be built.

The Department of Marine has indicated that a minimum distance of 5km offshore is appropriate. Shorter distances would need to be justified. In some parts of Europe, there are offshore projects closer than 5km to the shore.

Figure 9: repartition of the offshore turbines in northern Europe

I.3.2) what materials? Steel is more competitive than concrete for larger offshore wind farms. It appears that all of the new technologies will be economic until at least 15 meters water depth. In any case, the marginal cost of moving into deeper waters is far smaller than what was estimated.

The metal parts of the turbine structures will be specially coated to protect them from corrosion using (electrical) corrosion protection. About the cabling which is used to transport the electricity to the shore, undersea cables are used and must be buried in order to avoid damages caused by the fishing equipment and anchors of boats. The voltage within these cables is up to 150kV. I.3.3) What kind of foundation? Currently, there are no technical barriers to installing offshore turbines but the construction, delivery to site and assembly of such large machines will need specialist equipment, facilities at ports and careful timetabling to make sure that the possibilities of using calm weather windows are maximised. Now there are four main fixed bottom substructures: Traditional Concrete, Gravitation + Steel, Mono Pile, and Tripod. But some studies and projects are carrying out in order to develop new fixations which consist in using big floaters. Traditional Concrete This kind of foundation is the more widespread. The process uses the gravity in order to hold the tower in an upright position. The concrete caissons are previously built in a dry dock next to offshore sites, after they are towed to their final place and finally sand and gravel are put into the caisson for increasing the weight. This application is very similar to those used for the bridge construction. The problem of these concrete caissons is the price, because the more the water is deep, the more the price increases, so this device is not economical at all when the wind turbine is built at more than 10 meters of depth. Gravitation + Steel This is a new technology which use a method very similar than the concrete caisson but the main difference come from the material. The concrete is replaced by a cylindrical steel tube which is fixed on a slab of steel placed at the bottom of the sea. The interest of such device comes from the fact that steel foundations are lighter than those in concrete, because the tubes are hollow. It’s a very significant point which makes easier the installation; Of course once the steel concretes are fixed they are filled up with Olivine (a very dense ore) in order to increase the weight. It’s necessary to do that for resisting against the waves and the ice pressure because the sea can get frozen in winter. Mono Pile This type of foundation is pretty simple because it consist of just one steel pile (from 3,5 to 4,5m of diameter). And this steel pile is just the extension of the tower itself through the water to the bottom of the sea. So this steel pile is pushed very deep into the flour, but the depth is up to the quality of the basement, it can reach 10 to 20 meters.

Tripod The tripod foundation is very similar to the geometry used by the oil industry. A frame made of three steel tubes leaving from the basis of the tower permit to share out the forces from the pile to these three slim steel tubes which are pushed into the floor (10 to 20 meters), one more time the depth is up to the conditions of the floor and the eventual ice pressure. This technology is very adapted when the water is very deep, it’s interesting because thanks to this device it’s possible now to get farter and farther from the shore. The main raison why tripod foundations are not appropriated for depth less than 6 or 7 meters is that service vessels at low water depths will face problems approaching the foundation due to the steel frame.

Figure 10: The different foundations currently used (http://www.energy.gatech.edu/presentations/BBell.pdf)

Floating concept This technology is not really used yet, but it’s going to be more and more widespread in the years to come. The concept consist in putting the tower on a big floater, assembling the turbine on the shore, then it is placed on a raft in order to be towed to the offshore site. The last step of the installation is the fixation of the turbine at the floor. For that, the floater will be attached to long steel tethers reaching down to a concrete block at the bottom of the sea hundreds meters below. The tethers must be tight enough to allow only side-to-side movement, but not open down.

These kinds of floating-mounted turbines could be installed in very larger water depths ranging from 50 to 200 meters. The interest is that unlike the traditional offshore turbines, they could be until 150km from the shore. The only problem will be the way to transmit the electricity back to the cost.

Figure 11: Different types of floating substructures (http://www.scienceagogo.com/news/20060819023800data_trunc_sys.shtml)

Park effect It’s necessary to keep a minimal distance between turbines because of the wake effect. In fact, the wind leaving the turbine has lower energy content than when it arrives to the turbine. The wind velocity is decreased when it goes through the rotor (because of the energy exchange) and also turbulences are generated. For these reasons turbines must be placed at least three rotor diameters from one another, the aim is to have a wind velocity profile as straight as possible in front of the rotor. I.4) Environmental Impacts There are some considerations to take into account concerning the environment because such devices can have a direct impact on the nature, especially on the marine life.

The environmental impact of offshore wind farms is considerably reduced compared with those onshore; both noise and visual impact are unlikely to be issues, but there are still some considerations. For example, there could be an environmental impact from carrying out work offshore, such as localised disturbance of the seabed.

Nevertheless, the word “consequence” does not only mean a negative point because in some cases, the implantation of OFF SHORE wind turbine has a positive impact but regarding a global result.

Let’s find here a listing with of the good aspects, followed by the negative points which result from the installation.

I.4.1) Interesting aspects for the environment and human activities

I.4.1.1) Marine life

Foundations can act as artificial reefs with a resultant increase in fish populations from the new food supply (like mussels for instance) for marine birds.

I.4.1.2) Emissions Each unit of electricity generated from the wind that saves a unit generated from fossil fuels, which will help reduce greenhouses gases, pollutants, and waste products that result from fossil fuel use. I.4.1.3) Visual impacts. The way to put the wind turbines far from the shore permit to decrease the visual pollution. I.4.2) Negative aspects of this device on the environment: I.4.2.1) Migrating birds Some studies have shown that offshore wind turbines have no significant effects on birds But after several years they realized that the population of water birds has been decreased a lot. Besides potential collisions (bird strikes), it is possible that the birds would need to consume more energy to avoid collisions and maintain their orientation when navigating around the turbines. Tower illumination may also cause navigational disorientation for birds. Also, the increases in fish population may also have stimulating effects on bird populations in the area, which could cause collisions between birds and towers or rotors. For these reasons, it’s necessary to find places where the population of marine bird is not too developed in order to have the less cases of death and by the same time the less damages on rotor blades. I.4.2.2) Interference with navigation for endangered and threatened species. Electromagnetic fields created by the electric cables running from the turbines and underwater noises and vibrations could affect orientation and navigational ability I.4.2.3) Potential alteration of natural environments and diminution of habitats. Underwater support pilings, anchoring devices, scour-protection materials, and electromagnetic fields could cause a decrease in benthic communities, alter natural environments, and possibly affect migration patterns. I.4.2.4) Marine traffic, recreation, and other sea space uses. It is possible that wind turbine energy plants may disrupt air traffic control and maritime radar systems. The turbines would be spaced such that small vessels, assuming the depth of water is adequate, would be able to sail through the array and any developer wishing to build a wind farm offshore would identify whether their proposed site would interfere with shipping routes or pleasure craft. I.4.2.5) Noise impacts. The noise from rotating turbine blades is higher because the devices are bigger than onshore turbines. It has been suggested that the noise from the turbine travel underwater and disturb sea life. This is not apparently the case according to a Sea Mammal Research Unit report which says that as long as there has been some previous exposure to boats, engines etc. it doesn't bother seals at all.

II.) THE ECONOMICAL ANALYSIS OF OFFSHORE WIND FARMS Faced with the possibility of an energy crisis, the fight for energy will be the global conflict in future. So the development of new energy sources and the active use of renewable energy are more and more important. Offshore wind power as clean, free and renewable energy, most countries in world pay attention to it. Moreover, its capacity is huge.

Table 1: Offshore Potential relative to national electricity consumption (Matties.H.G.et.al:Study of Offshore Wind Energy in the EC.Final Report of Joule contract commissioned by the

Commission of the European Communities CEC,Verlag Naturliche Energien Germany,1995.)

As it can be seen from this table, a huge amount of energy could be gained from offshore windpower. We can find large potential of the offshore windpower farms. II.1) Expenses of offshore wind farms. Quantitative research and analysis for the technical and economic benefits of offshore wind power is an important topic. It is helpful to the rational use and development of offshore wind power.

But the cost of initial investment is high; we must pay for its collection, conversion and use. Moreover, offshore wind power has its intermittent and stability characteristics. So we should take the scientific method to analyze the feasibility of offshore wind power projects. The rational use of the limited financial, material resources and manpower maximize the economic benefit of investment.

A Initial Investment a The costs of technical and economic feasibility study and design expenses b The costs of offshore wind power turbine and transportation c Foundation cost and installation cost d The costs of grid connection e Other costs

B Operating Costs a The costs of materials (parts, lubricants, etc.) b The operation and maintenance costs c The management cost d Power generation costs

C Total Revenue

Table 2: The classification of the costs

The total costs for the system are composed of A, B and C is the revenue. Initial investment are generally all the cash flow from design to put into operation .And operating costs are the cash flow of input into the project and output from the project year after year.

Operating Costs: It is the costs of the normal operation. Operation costs become low after the completion of offshore wind power turbines. The main costs are the preliminary costs. Management cost: Experience shows this kind of cost increase with the use of turbines. Most of maintenance cost is a fixed amount per year for the regular service of the turbines. Studies shows the wind turbines (25-150 kW) have annual maintenance costs with an average of around 3 per cent of the original turbine investment. For newer machines, it estimates a little lower, around 1.5 to 2 per cent per year of the original turbine investment. The costs of materials: it including costs of refurbishment and major overhauls. Some wind turbine components are more easily tear and wear. In order to prolong the life-span of turbines, it should do major overhaul termly. There are some factors affect the magnitude of these costs. Distance to shore and water depth are one of the most important influencing factor on the cost of offshore wind power farms. It will affect foundation cost. From Figure 12, we can see the relationship between the water depths and the foundation cost. We can see the rise of the foundation cost as the water depth increase.

Figure 12: Foundation cost in relation to water depth (turbine types 1MW to 1,5 Mw)

(www.windpower.dk.)

Other costs including foundation cost also depend largely on the distance of the offshore site from the coastline and thus on the water depth. We can see some data of Germany. As the Table 2 shows, the costs are calculated on the basis of different distances from shore.

Table 3: The cost as a percentage of the WT price (869 Euro/kw) in relation to the distance from shore

(www.windpower.dk.)

On the other hand, the estimated power costs of offshore wind farms also relative to distance from the shore. The common distances are 30km, 50km and 70km. The expenses are expected to vary in relation to the different distance. But it is impossible to give very detailed data of power generation costs. Therefore, the following figures only give a range.

Figure 13: WT power generation cost in the offshore area at a distance of 30 km from the shore

(Matthies H.G.et.al:Study of offshore Wind Energy in the EC.Herausg.Germanischer Lloyd.Garrad Hassan and

Partners.Windtest KWK.Verlag Naturliche Energien.Brekendorf ，1995.)

Figure 14: WT power generation cost in the offshore area at a distance of 50 km from the shore

(Matthies H.G.et.al:Study of offshore Wind Energy in the EC.Herausg.Germanischer Lloyd.Garrad Hassan and Partners.Windtest KWK.Verlag Naturliche Energien.Brekendorf 1995.)

Figure 15: WT power generation cost in the offshore area at a distance of 70 km from the shore

(Matthies H.G.et.al:Study of offshore Wind Energy in the EC.Herausg.Germanischer Lloyd.Garrad Hassan and

Partners.Windtest KWK.Verlag Naturliche Energien.Brekendorf 1995.)

From the above analysis, we can find a large cost reduction potential. Particularly, there will be a larger cost reduction on the aspects of grid connection and foundation cost.

II.2) The calculation of economic analysis At first, we should refer to the definition of NPV. NPV(Net Present Value): It refers to the cost difference between the total output value and the current value of total value, in their effective use of n-year period. Obviously, the NPV is below zero, and its economy is poor; NPV is zero, the inputs and outputs are the same; NPV is above zero, its economy is good. The greater of its value is the better of its economy. After the completion of offshore windpower projects, the annual net income is:

Net annual output value = Annual Production Value - Depreciation - Operating Expenses - Other Expenses Depreciation: It is the loss of capital asserts, their value will be gradually transferred to the products. It is currency performance of the labor loss. For offshore wind power farms, the calculation of depreciation is taking the initial investment as the depreciation factor and using straight-line depreciation method. It can be simply expressed as:

Dj = m*(P0 / n)

Dj——Depreciation of j year, j=1,2,3,…n , Depreciation Year. P0——Costs per kilowatt m——Total installed capacity

We take an offshore windpower farm in southern of China as our example. The conditions of this offshore wind power farm are as follows:

Single capacity

(kw)

Numbers Height(m)

300 32 30

500 40 35

600 78 40

750 10 50

Table 4: The capacity of offshore windpower farm (http://www.dtzzfd.cn/fdxx.asp)

Total installed capacity: m = 8.39×104 kw Costs per kilowatt: P0 = 9300 RMB/kw Life-span: n = 20 year Then: Depreciation Dj = 9300×8.39×104/20 = 3.9×109 RMB/Year

Costs Depreciation The expenses of materials

The operation and maintenance expenses

The management expenses

Total

percentage 82.6 6 4.8 6.6 100 Table 5: Statistics of costs

(http://www.dtzzfd.cn/fdxx.asp)

Therefore, the annual costs: 3.9×109/82.6% = 4721.5×108 RMB

Power Output of Last Year: 1.8×108 kwh The Costs per kwh: 4721.5×104/1.8×108 = 0.262 RMB Some possible ways to reduce the costs of wind power:

1. On the one hand, to increase single capacity and the turbine number of a offshore wind farm. On the other hand, to reduce the cost per kilowatts. The power generation costs of offshore wind farms have a relationship with economies of scale, including the capacity of offshore wind turbine, Mass production can reduce unit cost. In example above, if other costs unchanged, the cost per kW reduces to 7000RMB. Then the cost of electricity falls to 0.209 RMB.

2. To increase generating capacity and reduce the development cost of electricity.

3. Reasonable protection to increase life span. According to a research of Danish electric power company: with respect to the IEA (International Energy Authority) standards, on the current technical level and life-span of 20-year, the estimated cost is 0.36 kroner per kilowatt-hour (0.05 U.S. dollars). If life-span is 25 years, there will be a decrease of 9%. With wind power generation capacity increased by 100%, the cost will fell 15%. In example above, if life-span increase to 25 years, other costs unchanged. Then the cost of electricity falls to 0.219 RMB.

III) COMPARATION III. 1) Wind power capacity of different countries

(http://china.lbl.gov/china_renewable-wind.html)

Europe has always taken the lead in generalizing renewable energy to replace fossil fuels in order to decrease the effect of green house. European community made a project of development of renewable energy in 2001, which aim to make electricity produced by renewable energy take 12% of the whole European electricity system. In 2002, total electricity productions by wind is 32000MG, while the countries belong to European Community take 74% of the whole world. Therefore, in this part, we will give some information about those countries performing important roles in the world in wind energy utilization, including Germany, Denmark, Spain and USA.

III.1.1) Wind power capacity of the world

(According to the certain statistics)

Obviously, wind power capacity of the world increases dramatically, in the last 10 years, the number of units installed for wind power increases almost 10 times. Wind power plays a more important role in the renewable energy. III.1.2) Development of the wind energy over the world

(According to the certain statistics)

From the figure, we can see that wind generating capacity increased a lot in the recent 25 years, in 1980,it’s almost zero, and now it turns to 70000 MH, It will keep increasing in the future since it’s advantage getting more and more obvious and the decrease of the total setting cost. III.1.3) Social attitude to different power plants (surveys by Thayer)

(www.ilr.tu-berlin.de/WKA/design.html) Opinion surveys show that wind has high public support, but this support erodes once specific projects are proposed. Because support is fragile and can be squandered by ill-conceived projects, the industry must do everything it can to insure that wind turbines and wind power plants become good neighbours. One means for maximizing acceptance is to incorporate aesthetic guidelines into the design of wind turbines and wind power plants.

(www.ilr.tu-berlin.de/WKA/design.html)

Though offshore wind plants create some environmental impacts, there are no containment buildings around wind plants to shield their inner workings from view. It found that a negative view of wind turbines on the landscape is the major factor determining opposition to wind energy. Other, though much less significant, factors are the disbelief that wind turbines will make a difference in improving air quality [usefulness], and the fear that the wind turbines will harm residents.2 Opposition is primarily determined by a negative reaction to seeing wind turbines on the landscape. III.1.4) The cost of different renewable energies

(http://news.bbc.co.uk/2/shared/spl/hi/pop_ups/04/sci_nat_global_energy_crisis/html/4.stm)

Renewable energy use is predicted to increase by nearly 60%, but this will do little more than keep pace with rising overall demand. By 2030, renewable sources (not including biomass and nuclear) will have increased only 1% from 3% to 4% of total world use. However, costs for most types of renewable energy are expected to decrease. III.1.5) Annual Wind Power Development

(http://jxj.base10.ws/magsandj/rew/2002_04/wind_energy.html)

Europe continues to account for two thirds of total worldwide wind development, and Germany - seemingly the perennial pacesetter - continues to outdistance all other markets. It again led the world in wind development, accounting for 40% of the total market; in 2001, Germans installed an amazing 2600 MW, more than 60% greater than the year before, and itself a record.

III.1.6) Installed wind power capacity per person III.1.6.1) Installed wind power capacity per person in Europe

(http://thewatt.com/modules.php?name=News&file=article&sid=758&mode=nested&order=0&thol

d=0) Europe is clearly a leader in wind turbine installations, and although Denmark only accounts for 6.6% of the world’s total installation capacity, they are the clear world leader, dominating the installed capacity per person by more than 380 watts per person compared to second place Germany. III.1.6.2) Installed wind power capacity per person in the rest of the world

(http://thewatt.com/modules.php?name=News&file=article&sid=758&mode=nested&order=0&thol

d=0) In terms of installations per person, the rest of the world doesn’t even come close to Europe. Denmark also uses their land area very efficiently, managing to squeeze in 66kW/km2 compared to 0.5kW/km2 in the US. Denmark has the cheapest installations of wind power as well, they install the most wind capacity per gross national product in the world. Compared to Spain, Denmark spends 2 times more on wind power installations but they have 3.8 times more installed capacity per person. III.1.7) Figure to show the primary energy contribution of Sweden

(http://www.renewable-energy-policy.info/relec/sweden/index.html)

[MW] 1990 1995 1999

Total installed capacity thermal, nuclear, hydro, wind and geothermal

34187 33623 33588

Total thermal supply power stations 7880 7349 6870Nuclear power stations 9970 10055 10076Total hydro-electrical production capacity 16330 16152 16432

Hydro plants < 1 MW - 209 168Hydro plants ≥ 1 MW and ≤ 10 MW - 762 775Hydro plants > 10 MW - 15181 15489

Wind energy 7 67 210Geothermal energy - - -Photovoltaics 0 0 3Solar thermal - - -Municipal solid wastes 30 76 77Wood/wood wastes 1200 1200 1490Biogas 0 0 18Industrial wastes 0 0 0

References: [1] Matties.H.G.et.al:Study of Offshore Wind Energy in the EC.Final Report of Joule contract

commissioned by the Commission of the European Communities CEC,Verlag Naturliche Energien Germany,1995.

[2] Verband der danischen Windkraftindustrie,www.windpower.dk. [3] Schwenk.B.Rehfeldt.K:Studie zur aktuellen Kostensituation der Windenergienutzung in

Deutschland Herausg.Bundesverband Windenergie.Osnabruck 1999. [4] Madsen.Peter Stenvald:Tuno Knob Offshore wind farm,European Union Wind Energy

Conference Goteborg Schwedn 1996. [5] Matthies H.G.et.al:Study of offshore Wind Energy in the EC.Herausg.Germanischer

Lloyd.Garrad Hassan and Partners.Windtest KWK.Verlag Naturliche Energien.Brekendorf 1995.

[6] Gerdes.G.Strack.M:Long-term Corrleation of Wind Measurement Data in DEWI-Magazine(1999),No-15.

[7] Rehfeldt.Knud:Wind Energy Use in Germany-Status 31.12.1999.DEWI-Magazine No.16. [8]Lemming.Drang Trong Danisch Investitgations and Plan of Action for Offshore Wind Power

European Seminar-Offshore Wind Energy in Mediterranean and other European Seas.OWEMES’97 La Maddalena,Italien.

[9] www.windpower.org [10] Notes from the subject “Energetic Technologies” of the University of Navarra (Spain)