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Wind power farms need diversity planning.
Large commercial wind generators consist of a giant 3 blade propeller (mounted horizontally) on a high tower which drives an electrical generator. Many of them are grouped together as a 'wind farm' and sizes have grown to 2, 5 and in future 8 or10 MW so you would need 260 of 5 MW units to produce the same power output of a 1300 MW coal or nuclear power station.
As wind varies from place to place, the full installed capacity will not always be useable but extra units across the country will provide diversity. To make use of this diversity, extra transmission may be necessary, as load factors could be as low as 33%.
The blades on a large machine may rotate at 20 times a minute (rpm) depending on size and drive through gears an electrical generator at 1500 rpm. The output cables are grouped on platforms and fed onshore to the 'grid.'
The disadvantages of onshore wind turbines are noise and appearance but the land under them can be farmed. Wind farms offshore tend to be larger but there is more wind and they are more acceptable in crowded lands such as UK.
A future plan is to locate wind farms in the Irish Sea, on the continental shelf in the Atlantic coast off Ireland, in the North Sea, in the English Channel, in the Baltic and the Western Mediterranean connected by an undersea European supergrid (HVDC) for diversity (ref 118).
33 GW of this wind power - which could supply half the UK demand when the wind is suitable - is planned for the UK around the British Isles. An amount of 'spinning reserve' from coal fired power stations as standby, for times of low wind strength, may be necessary.
Part of this plan, the Burbo Bank (mouth of Mersey UK) wind farm looks like this from the coast:-
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The total installed world capacity of wind generators in 2007 is 75,000 MW, of which 20,000 MW is in Germany (meeting 7 % of Germany's needs), 28,000 MW rest of Europe, 11,000 MW in the US.
BBC video of windfarm off Skegness UK
Floating offshore wind generators
Wind generators may be floated on a structure to enable use in deeper seas. StatoilHydro is to build a 2.3 MW experimental wind turbine by 2009 attached to the top of a 'spar-buoy' with 100 m draught off the coast of Norway. Rotor blades 80m dia, the 'nacelle' is 65 m above sea level and the structure is suitable for depths between 120 and 700m.
Vertical wind turbines.
Smaller size vertical wind turbines are suitable for homes and small businesses. Advantages claimed are that they are quieter, non directional, less vibration, and can have gust tracking measures to increase efficiency. The Quiet Revolution QR5 is rated up to 7 kw but would have 24% load factor on 3.5kw, operates at 4.5 to 16 m/s wind speed.
Electrical aspects - large wind generators must not destabilize the system.
To generate ac at 50 cycles per second (hertz) (or 60 in US or Canada) electrical generators run in synchronism - at a constant speed of 3000 rpm in large power stations 3600 rpm in us - and contribute to the grids frequency stability. They are 'synchronous generators.'
Early wind generators ran 'asynchronously' ( ie at slightly lower speed out of step with the grid frequency). The disadvantages are that this provides no help with frequency stability and under a fault conditions (in the one generator system) they would draw large reactive power which lowers voltage over a wide range and could cause healthy circuits to disconnect. Also they are less able to make full use of the full range of wind speeds.
Large wind farms will run synchronous generators and /or include ac to dc to ac conversion. The maintenance of the brushes on a wind generator, being difficult to access, are a problem. A development is the Brushless Double Fed Generator (BDFG) with two 3 phase stator windings of different pole numbers, one connected to the grid and the other supplied with variable voltage and frequency from a converter. The first is a 20KW BDFG at the Cambridge (UK) Engineering Division.
The power output of a wind turbine varies at lower wind speeds up to the maximum capacity. At higher wind speeds the blade pitch is varied to maintain the maximum power and above 25 m/s wind speed the system shuts down:-
Maglev Vertically mounted Wind Turbine
Maglev Wind Turbine Technologies (Arizona) claim that (in conjunction with Guangzhou energy China) the cost of generation can be reduced to under 5 cents per kwhr using very large vertically mounted 1000 MW wind turbines. These would require 100 acres per machine. The bearing supporting the rotating part of the machine would include permanent magnets, to reduce friction losses and energy transferred to linear generators. Advantages are low maintenance, long life span and use of wind speeds down to 1.5 m/s resulting in 1000 hrs extra generation per year. Nothing like this size of vertical wind turbine has so far been constructed. The shape could look like this:-
