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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 will be necessary, as load factors (annual output / installed capacity) could be as low as 33%. Wind can provide 2 watts/sqm of land space onshore to 3 w/sqm offshore, which if significant power is provided would take up a large area (100MW needing 50,000 sq km.)
The blades on a large machine may rotate at 20 times a minute (rpm) depending on size and drive an electrical generator through gears at 1500 rpm. The output cables are grouped on platforms with transformer and fed onshore (eg at 33kv) to the 'grid.'
The disadvantages of onshore wind turbines are noise and appearance but the land under them can be farmed, offshore there is more wind.
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 or other storage schemes..
Support systems for Offshore wind turbines
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The photo of Burbo Bank above shows a wind farm close to the coast. For turbines in shallow water up to 20m a steel support 6 foot in diameter (monopole) hammered into the sea bed may give sufficient support. These are installed using a giant 'Jack up' boat, a platform 290 ft long (90m) sketched below.
The 'boat' carries three turbines and is towed to the second and third sites, completes installation before being towed back to shore to reload.
For a turbine over 3MW in depths greater than 30m alternative support systems are being considered for 'Round 3' wind farms in the UK. The Gifford team design of a concrete base on concrete foundations is one alternative as in the sketch of a 7.5 MW turbine above. Their system of installation uses submersible barges which avoids a jack up boat.(ref 210)
Three other alternatives (ref 211) are being considered, sketched below:-
BBC video of wind farm off Skegness UK
Floating offshore wind generators
Floating wind generators can be moored further out to sea in water up to 700 m deep. the advantages are a greater amount of steady wind say 8 m/s and invisibility from coast. Use is made of oil exploration and drilling platform techniques.
Statoil Hydro is to build a 2.3 MW experimental wind turbine 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. See StatoilHydro web site
Local wind generators for home and business.
Smaller size vertical wind turbines are suitable for homes and small businesses. Advantages claimed are that they are quieter, non directional, less vibration, and include 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.
Two vertical wind generators above Sainsbury's store Dartmouth, UK.
Electrical aspects - large wind generators must not destabilize the power system.
To generate ac at 50 cycles per second (hertz) (or 60 in US or Canada) electrical generators run in 'synchronism' (with the grid frequency)- at a constant speed of 3000 rpm in large power stations (3600 rpm in the 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 they would draw a 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 large horizontal wind turbine varies at lower wind speeds up to the maximum capacity (at 14 m/s wind speed shown in sketch below). At higher wind speeds (14 to 25 m/s below) the blade pitch is varied to maintain the maximum power and above 25 m/s wind speed the system is shut down:-
Maglev Vertically mounted Wind Turbine Idea.
Maglev Wind Turbine Technologies (Arizona) claim that (in conjunction with Guangzhou energy China) the cost of generation can be reduced using very large vertically mounted 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:-
More details of proposed large vertical wind generator and comment.
Wind turbines incorporated in high tower buildings (Bahrain).
The buildings are shaped to funnel and change wind direction and even out the wind flow between the different heights of the three turbines. Steel rods are incorporated in the blades to prevent them flying off in the rare case of a blade failure. The bridges are an aerofoil shape strengthened to prevent resonance with the blades which run at 38 rpm geared down to 1500 rpm for generation. The blade tips can be separately varied to shut the turbine down.
Video of Bahrain World Trade Centre building incorporating 3 wind turbines.
Wind at high altitudes
Winds are far more powerful and more reliable at high altitudes and a number of ideas have been put forward to harness such power:-
Sky Windpower, based in Oroville N Carolina propose a flying electric generator at 6000 to 24,000 ft. The experimental model has four helicopter 35 ft blades that lift and then spin at height. Power is carried to the ground via an aluminum cable. Larger devices could generate 20 MW.
A kite that rises to upper atmosphere, tethered on a drum which rotates and drives a generator. Having reached maximum height it glides down to statrt again.
A floating ball tethered at 1000 ft that rolls in the wind.
A ladder of Kites driving a turbine at the top.
There are obvious problems with spacing, radar interference , storm damage.
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