(2) 19% of Denmark’s total energy production is energy converted from wind energy by its wind turbines. This category looks into:
a) how electricity is harnessed using wind power, i.e. the methods etc in Denmark
b) where is this 19% energy converted from wind energy used, e.g. domestically, in industries, recreational, just to increase/take over some part of Denmark’s energy production.
Denmark has relatively modest average wind speeds in the range of 4,9 to 5,6 m/s measured at 10 m height. Onshore wind resources are highest in the Western part of the country, and on the Eastern islands with coastlines facing South or West. The country has very large offshore wind resources, and large areas of sea territory with a shallow water depth of 5 to 15 m, where generating wind power is most feasible. These sites offer higher wind speeds, in the range of roughly 8,5 to 9 m/s at 50 m height.
Using of Wind Energy in Denmark: Past, Current, Future
In the 1980s, most of the electricity production in Denmark was based on coal and the acidification of forests and lakes by acid rain was predominant n the environmental debate. Early Danish wind turbine development was thus a far cry from simultaneous government sponsored research programmes on very large machines in Germany, USA, Sweden, the UK, or Canada. In the end, improved versions of the classical, three-bladed upwind design from the Gedser wind turbine appeared as the commercial winner of this wild competition, but admittedly not without a number of wreckages, mechanical, and financial.
Denmark has, within the last 20 years, invested more in wind energy than any other European country. This is consistent with Denmark’s long tradition of using wind as an energy source. The main objective of investing in wind energy in 1976 was to make Denmark less dependent on imported energy supply. One of the main drawbacks of wind energy is that electricity can only be produced where there is wind. In Denmark this problem was avoided by connecting the private wind turbines to the national grid, allowing fluctuations to average out and so provide a constant supply.
Today wind power provides 20% of Danish electricity consumption. Within a few years, the wind power industry has grown to become a significant industrial sector providing huge benefits for exports and employment. From single turbines at the beginning, wind power generation plants are now used and the Danish wind power industry is at the leading edge in an ever more competitive global market.
As Denmark’s future energy supply faces numerous challenges and has become subject to unstable international conditions, offshore wind now has a key role to play. Offshore wind power can contribute significantly to achieving the EU goals of a 21 percent share of renewable electricity by 2010, halting global warming and reducing Denmark’s dependence on coal, oil and gas.
In Denmark’s energy strategy for 2025, their government expects to see a significant increase in the use of renewable energy in the years to come. The market-based expansion of this sector will be brought about through incentive schemes and investment in physical infrastructure as well as research, development and demonstration. With higher oil prices and high CO2 allowance prices, a significant proportion of the renewable energy expansion would be expected to be delivered by large, offshore wind farms. At sea, wind resources are better and suitable sites are more readily available to enable these large projects to operate in harmony with the surrounding environment.
In Denmark, as offshore Wind farms impact on their natural surroundings, it is essential to ensure that conditions in unique marine areas are not detrimentally affected. Spatial planning when identifying potential locations for offshore wind farms, taking into account grid connection routes and other areas of interests, must ensure that future offshore wind farms are established in suitable areas in such a way that substantial adverse environmental impacts can be avoided or diminished.
One of the challenges Denmark face is to assess the cumulative effects from multiple offshore wind farms to arrive at optimal site selection. Thus a committee on future offshore wind farms is currently updating the Danish action plan from 1997 to use the experience and learning gained to date in order to identify appropriate locations and at the same time to minimize visual disturbances and the effects on animal species such as marine birds and mammals.
Wind Generation in Denmark: The methods and machines
1) Modern Wind Turbines
Denmark has (in 2003) around 3,000 MW wind power, which is supplied by approximately 5,500 wind turbines. Individuals and cooperatives own around 80% of the capacity.
Avedøre Holme, Denmark: The picture shows the Avedøre Wind Farm, just 5 kilometres from the city centre of Copenhagen, Denmark. The 12 Bonus 300 kW wind turbines, (and one 1,000 kW power company test wind turbine) are located next to a 250 MW coal-fired power plant. (Photograph Søren Krohn, © 1997 DWIA)
2) Large Onshore Wind Farms
Rejsby Hede wind farm consist of 40 turbines from Bonus Energy each of 600 kW. The turbines were erected in 1995 near Tønder in southern Jutland. Totalling 24 MW it was the largest wind farm in Denmark at the time.
Today the largest onshore wind farm in Denmark is Syltholm on the southern island Lolland. The farm consist of 35 NEG Micon 750 kW turbines i.e. a total capacity of 26,25 MW.
3) Multi-Megawatt Wind Turbines
NEG Micon 2 MW: The prototype of the NEG Micon 2 MW turbine was commissioned in August 1999. It has a 72 m (236 ft.) rotor diameter. In this case (Hagesholm, Denmark) it is mounted on a 68 m tower. In the background you see the foundations for two sister machines. The turbine is intended for offshore applications.
From the outside it resembles the 1500 kW NEG Micon machine so much, that you’d have to see the turbine in its stopped state (with the blades pitched out of the wind) in order to notice the difference: The rotor blades are pitchable, since the machine has active stall power control, whereas its 1500 kW cousin has passive stall power control.
4) Megawatt-Sized Wind Turbines
On a pitch controlled wind turbine the turbine’s electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again.
The rotor blades thus have to be able to turn around their longitudinal axis (to pitch) as shown in the picture. During normal operation the blades will pitch a fraction of a degree at a time – and the rotor will be turning at the same time.
Designing a pitch controlled wind turbine requires some clever engineering to make sure that the rotor blades pitch exactly the amount required. On a pitch controlled wind turbine, the computer will generally pitch the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximise output for all wind speeds. The pitch mechanism is usually operated using hydraulics.
The Future for Megawatt-Sized Turbines: The megawatt market really took off in 1998. Since then, it has been clear that the market trend is towards bigger projects with bigger wind turbines. Megawatt-sized machines will be ideal for offshore applications, and for areas where space for siting is scarce, so that a megawatt machine will exploit the local wind resources better.
5) Offshore Wind Turbines: Offshore wind energy is a promising application of wind power, particularly in countries with high population density, and difficulties in finding suitable sites on land. Construction costs are higher at sea, but energy production is also higher. The largest offshore wind farms in Denmark are Horns Rev by the west coast of Jutland and Nysted close to Lolland – 160 and 158 MW respectively. The Danish energy plan, Energi21, from 1996 set up a target for 4,000 MW offshore wind power in 2030. These 4,000 MW are expected to produce 13.5 TWh per year equivalent to 40% of the Danish electricity consumption.
Nysted Offshore Wind Farm: The most recent large offshore farm is Nysted Offshore Wind Farm at Rødsand built in 2003. The wind farm is located app. 10 km south of the town of Nysted on Lolland and consists of 8 rows with 9 turbines each. The total power of the 72 wind turbines each of 2.3 MW thus reaches 165,5 MW. The annual electricity production of the wind farm is 600GWh, enough to supply 145,000 (Danish) households. The wind turbine towers are about 70 m tall, and the rotor blades 41 m long. The picture is from the initial phase of the installation.
Horns Rev – Denmark’s Largest Wind Farm: The largest wind farm in Denmark is the offshore wind farm of Horns Rev, which was completed in 2002. It is situated in the North Sea, 14-20 km off the coast of Jutland. With its 80 Vestas 2MW turbine, the wind farm has a total capacity of 160 MW. That makes it the largest offshore wind farm in the world today (2003). The farm supplies the equivalent of 150,000 (Danish) households. The larger production compared to Nysted is due to better wind conditions.
Aerodynamic Improvement Devices
A number of technologies known from the aircraft industry are increasingly being applied to improve the performance of wind turbine rotors.
One example is vortex generators, which are small fins, often only about 0.01 metre (0.4 inch) tall, which are fitted to the surface of aircraft wings. The fins are alternately slightly skewed a few degrees to the right and the left. The fins create a thin current of turbulent air on the surface of the wings. The spacing of the fins is very accurate to ensure that the turbulent layer automatically dissolves at the back edge of the wing.
Curiously, this creation of minute turbulence prevents the aircraft wing from stalling at low wind speeds. Wind turbine blades are prone to stalling even at low wind speeds close to the root of the blade where the profiles are thick. Consequently, on some of the newest rotor blades you may find a stretch of one metre or so along the backside of the blade (near the root) equipped with a number of vortex generators. (Picture © LM Glasfiber A/S).
How things work?
1) Wind Turbine Generators
The wind turbine generator converts mechanical energy to electrical energy. Wind turbine generators are a bit unusual, compared to other generating units 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 (torque).
Generating Voltage (tension): On large wind turbines (above 100-150 kW) the voltage (tension) generated by the turbine is usually 690 V three-phase alternating current (AC). The current is subsequently sent through a transformer next to the wind turbine (or inside the tower) to raise the voltage to somewhere between 10,000 and 30,000 volts, depending on the standard in the local electrical grid. Large manufacturers will 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).
Cooling System: Generators need cooling while they work. On most turbines this is accomplished by encapsulating the generator in a duct, using a large fan for air cooling, but a few manufacturers use water cooled generators. Water cooled generators may be built more compactly, which also gives some electrical efficiency advantages, but they require a radiator in the nacelle to get rid of the heat from the liquid cooling system.
Starting and Stopping the Generator: If you connected (or disconnected) a large wind turbine generator to the grid by flicking an ordinary switch, you would be quite likely to damage both the generator, the gearbox and the current in the grid in the neighbourhood.
Design Choices in Generators and Grid Connection: Wind turbines may be designed with either synchronous or asynchronous generators, and with various forms of direct or indirect grid connection of the generator. Direct grid connection mean that the generator is connected directly to the (usually 3-phase) alternating current grid.
Indirect grid connection means that the current from the turbine passes through a series of electric devices which adjust the current to match that of the grid. With an asynchronous generator this occurs automatically.
To find out more about how the size of wind turbines could affect certain areas and the reasons for choosing different sizes of turbines, you could visit: http://www.windpower.org/en/tour/wtrb/size.htm
2) Rotor Aerodynamics
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 you 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 that 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. However, in the case of stall controlled wind turbines in particular, it is important that the blade is built so that it will stall gradually from the blade root and outwards at high wind speeds.
3) The Wind Turbine Yaw Mechanism
The wind turbine yaw mechanism is used to turn the wind turbine rotor against the wind. The wind turbine is said to have a yaw error, if the rotor is not perpendicular to the wind. A yaw error implies that a lower share of the energy in the wind will be running through the rotor area.
4) Aerodynamics of Wind Turbines: Lift
The rotor consisting of the rotor blades and the hub are placed upwind of the tower and the nacelle on most modern wind turbines. This is primarily done because the air current behind the tower is very irregular (turbulent). Modern wind turbines borrow technologies known from aeroplanes 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 reason why an aeroplane can fly is that the air sliding along the upper surface of the wing will move faster than on the lower surface.
This means that the pressure will be lowest on the upper surface. This creates the lift, i.e. the force pulling upwards that enables the plane to fly.
The lift is perpendicular to the direction of the wind. The lift phenomenon has been well known for centuries to people who do roofing work: They know from experience that roof material on the lee side of the roof (the side not facing the wind) is torn off quickly, if the roofing material is not properly attached to its substructure.
Sources:
http://www.energyquest.ca.gov/story/chapter16.html
http://www.renewableenergyworld.com/rea/news/infocus/story?id=46749
http://www.managenergy.net/download/opet_gp/summaries/_s023p_gpr_ntua-on-wind-denmark.htm
http://www.windpower.org/en/pictures/modern.htm
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