7. Technical aspects of wind turbines

 

A typical wind turbine

 

Industry standard is now a 2.0 MW installed capacity machine, or often larger.

 

An example is the Danish manufactured Vestas V 80

 

Rotor Diameter: 80 m

Swept area: 5,027 m2

Speed revolution: 16.7 rpm

Operational interval: 9 - 19 rpm

Tower Hub height (optional approx.): 60 - 100 m

Total height (blade vertical) 100 - 140 m (depending on tower) i.e. 305 to 427 feet

Generator: Asynchronous

Nominal output: 2.0 MW at 50 Hz 690 V

Weight 

100 m Tower: 220 t

Nacelle: 61 t

Rotor: 34 t

Total: 315 t

 

 

Installed capacity and load factor (capacity factor)

 

The nominal maximum output is referred to as the "installed capacity". If the machine generated at maximum rate, continuously for a year, it would yield, per installed MW: - 1.0 MW x (365 x 24) hours = 8760 MWh. The actual yield is much less, mainly because there is insufficient wind to maintain full generation.

 

Onshore in the UK it is conventional to expect the achieved generation to be about quarter to one third of the maximum. The multiplication factor is called the "load factor" (synonym "capacity factor") - usually expressed as a percentage.

 

In 2003, Lord Sainsbury told the House of Lords that load factor was about 30% onshore and 35% offshore (Hansard 18 November 2003: Column 1851)

 

During the past two years of DTI records the average UK figures have been much less than this onshore: 24.1% in 2003 and 26.6% in 2004 (DUKES 2005).

 

Calculation of load factor –

 

 Example for a 1.0 MW turbine: -

 

 (Achieved generation/(Maximum possible generation)/ x 100 = Load factor

 

Maximum possible is 1.0 MW x 8760 h/y = 8760 MWh

 

Achieved generation is (say) 2190 MWh

 

Load factor thus = 2190 MWh / 8760 MWh = 0.25 i.e. 25%

 

The calculation should be based on yield over a stated time (the Ofgem period is January to December.

 

Windspeed

 

A wind turbine cannot generate until there is sufficient wind, usually about 4 m/s, called the 'cut-in' speed. The machine does not reach peak generation until about 15 m/s. It then maintains a constant output with increasing speed (see Physics of windpower, below) up to a safety 'cut-out' speed of 25 m/s.

 

A rotor can be allowed to idle (generator declutched) at wind speeds well below cut-in speed to take instant advantage of periods of stronger wind (a 30 tonne rotor otherwise takes time to come to speed).

 

Above cut-out wind speed the turbine is shut down for safety, with blades 'furled' (feathered), i.e. edge-on to the wind and with generator de-clutched and the wind-shaft locking brake on.

 

Some examples are given below, from manufacturers' specifications.

 

Vestas V 66 1.75 MW turbine. Rotor d. 66 m  cut-in 4  peak 16 cut-out 25 (metres/second)

 

Vestas V 80 2.0 MW turbine. Rotor d. 80 m  cut-in 4  peak 15 cut-out 25 (m/s)

 

General Electric 3.6 turbine.   3.6 MW Rotor d.104 m cut-in 3.5  peak 14 cut-out 25 (m/s)

 

Conversion of speed units: 4 m/s = 8 knots = 14 km/h = 9 mph = B3 : 15 m/s = 29 kt = 54 km/h = 34-mph = B7 :  25 m/s = 49 kt = 90 = km/h = 56 mph = B10.

Beaufort wind scale (B):  3 = Gentle Breeze; 7 = Moderate or Near Gale; 10 = Whole Gale or Storm

 

Prediction of the performance of a wind turbine may be obtained by previous anemometric recording of wind speed on the site but an approximate prediction of generating output may be made from maps of the distribution of wind speed in the UK. For example: - http://www.esru.strath.ac.uk/EandE/Web_sites/03-04/wind/content/ukwindspeedmap.html   

 

This map shows that average wind speeds in lowland Britain are 5-6 m/s, coastal and upland areas 6-7 m/s and exposed uplands 7-8 m/s. Only a few extreme sites in the uplands, west and north lie between 8-10 m/s average speeds. Note that the average wind speed, even in the windiest sites is below peak generating speed, suggesting that a wind turbine anywhere in the UK, exposed to a variable wind regime will spend much of its time well below maximum generation thus explaining the low load factor of about 26% (average for 2003 and 2004)

 

It is also this distribution of windspeed which makes high ground and coast the preferred target for wind developers.

 

 

Physics of wind power

 

i) Theoretical output is proportional to the square of the blade-length (radius).

 

A wind turbine converts the kinetic energy of moving air into mechanical work. The theoretical electrical output is thus related to the mass of air passing through the rotor. Doubling the area of the rotor doubles the amount of power available and, because the area of the swept circle is pi x radius squared, the output is proportional to the blade-length squared.

 

ii) Theoretical output is proportional to wind speed cubed so even a small increase in average wind speed should give substantially more electricity over the course of time.

 

Real wind turbines follow the first rule closely hence any increase in height allowing increase in rotor radius gives substantially more power. The practical consequence is that machines originally designed for offshore installation (both V80 and GE 3.6) have quickly migrated onshore.

 

The second rule is not followed closely by real wind turbines. At first as wind rises above cut-in speed the power output increases dramatically with speed (because of the cubic relationship a doubling gives 2 x 2 x 2 increase in power). However the output then becomes more or less proportional to wind speed up to peak generation (i.e. x 2 increment doubles power) and then between peak and cut-out wind speed the output remains almost constant (because the generator is running at maximum output).

 

This lack of conformity to the cubic relationship is a result of aerodynamic (stall) regulation, or pitch regulation of power conversion by the blades, of 'electrical-braking' and of the alternator reaching its peak capacity. In the first case the shape of the blades allows wind-flow to become turbulent over an increasing part of the blade as the speed rises, reducing theoretical power conversion. In the second case the whole blade pitch is varied, or control surfaces (ailerons) are moved to 'spill' wind with the same effect. The load imposed by the generator also controls rotor speed (just as an idling car engine slows if the headlights are switched on) - this loading, like pitch regulation, is under operator or computer control. Such modification of the aerodynamic and electrical-braking characteristics allows a modern wind turbine to harvest maximum power from fairly low wind speeds but also safely to continue operation in high winds up to gusts of almost 60 mph.

 

Rotor speed (and see section 9. The effect on birds)

 

Wind turbines are so gigantic that the rotor appears to be travelling quite slowly but this is illusory. A big turbine like a Vestas V 80 2.0 MW machine rotates at 16 rpm and so, with a blade radius of 40 m, the blade tip velocity is 241 km/h (149 mph), over twice the motorway speed limit. The GE 3.6 turbine at its maximum 15.3 rpm has a blade tip velocity of 300 km/h (186mph), approaching the average speed of a Formula 1 racing car and its blade-swept area is substantially larger than that of the V80, at 8,495 m² [larger than a football pitch which is 7392 m²]

 

A bird which just avoids a GE 3.6 blade tip has only 1.3 seconds to dodge the next blade, approaching from about 93 yards away on a strongly curved path! Further discussion of this in section 9. The effect on birds.

 

 

Spacing of turbine:  area of land needed

 

To avoid “taking the wind out of each others sails”, wind turbines require spacing at 8 to 10 rotor diameters (downwind) and across-wind at c. 5 diameters (Manwell et al; 2002). Some authors suggest even greater spacing.

 

An example is Horns Rev off the Danish coast where 80 turbines (2.0 MW) are in a square array of 20 km², thus 0.25 km² per 2 MW turbine (or 0.125 km² per MW installed). This is rather more closely packed than the counsel of perfection above.

 

The biggest onshore windfarm in the UK has 39 turbines (1.5 MW) on a land area of 7.5 km² giving 0.2 km square per turbine (or 0.13 km² per MW installed).

 

For comparison a 1500 MW fossil fuel station with a load factor of 80% would occupy no more than about 2 km² and generates 1500 x 0.8 = 1200 MW. With wind load factor of 25% a 2MW turbine yields 0.5 MW - so we need 2400 turbines to equal this electricity and occupying 2400 x 0.25 = 600 km² of land.

 

 

Foundations

 

Onshore wind turbines, according to size and site conditions may require a wide range of different foundation types and sizes. The commonest is the gravity base comprising a ferro-concrete slab loaded with aggregate. Other options might be rock-anchors on a hard rock site, piled foundations or an embedded concrete cylinder in soft conditions (Civil Engineering, November 2005).The hole excavated for a turbine's foundation has a volume of 200 - 800 m3 depending on site conditions. This would need a maximum of about 1700 tonnes of concrete and aggregate for a gravity base. Only a quarter or less of the concrete will be cement - the energy intensive component which emits CO₂ in manufacture.

 

An average gravity base for a 2.5 MW turbine requires about 40 truckloads of concrete - up to about 250 m³ compared with only 40 m³ for the smaller 250 kW turbines, common a few years ago (Civil Engineering, November 2005).

 

Myths of our own making

 

Olympic swimming pool. Opponents of wind power have created a myth of their own, by suggesting that foundations are of "Olympic swimming pool size". That would be 50 x 25 x 2 or 3 m = 2500 to 3750 m³. This is an average 12-fold exaggeration!

 

Failure to payback energy and CO2. It is often said that wind turbines fail to pay back the energy and CO₂ cost of their manufacture and erection, or even that the CO₂ emission from cement manufacture alone is enough to offset the lifetime saving of CO₂ by a turbine. All of these assertions are untrue.  Don't repeat them - there is enough to complain about in wind power without resorting to easily exposed misinformation but for more detail see Roads (below) and Payback time for energy and CO₂ (section 5).   

 

Wind turbines only operate 30% of the time. In fact the industry is quite correct in saying that wind turbines generate for near 80% of the time – but what they fail to say is that for a large proportion of that 80% the amount of generation is very small.

 

Wind turbines need back up so they don’t save any CO₂. It is certainly true that the more wind power we install, the more backup will be necessary when wind speeds are low but there is a high demand for electricity. That backup will cost some of the saved CO₂ emission but it will certainly not negate all of it. Thus wind power undeniably displaces some fossil fuel burning and saves some CO₂ emission.

 

Roads and site clearance

 

Importing the turbine components requires access for very large low-loader trucks and a large mobile crane able to move 50 tonne or larger components. This is achieved by construction of a network of access toads which themselves require excavation of overburden and infill with large quantities of crushed rock aggregate. This work and borrow-pit sourcing of aggregate can do an enormous amount of ecological damage in vulnerable habitats of semi-natural vegetation especially on deep peat soils. The photo gallery accessible on the Cefn Croes website is a remarkable illustration of this literal holocaust: - http://www.users.globalnet.co.uk/~hills/cc/gallery/index.htm

Further discussion is posted at http://www.users.globalnet.co.uk/~hills/cc/scoutmoor.pdf

 

Transmission lines

 

One gigawatt of generation by a large power station is a very different matter from a gigawatt’s worth electricity from 1666 two-megawatt turbines spread over perhaps 500 km² of countryside! Yes it needs that number, given a load factor of 30%.

 

The large network of low voltage transmission lines results in substantial line losses compared with that of the single high voltage super-grid line linking a power station to often nearby industry. The wind ‘farmers’ say little about line losses  but it is a matter of some importance if their electricity is supposed to be displacing carbon emission from fossil fuel stations. Attempts to suggest that ‘local consumption’ can mitigate this are patently daft – where do we find 600 megawatts’ worth of consumers at peak wind generation on the Isle of Lewis? At present, even without the additional lines needed by dispersed wind generation, the grid and network system has a total delivery loss of  over 30 TWh/y which is about 16 times as much as  UK wind power (date from AAS, 2005 and DUKES, 2005).

 

Construction of the power lines raises another problem. – There is as much opposition to power lines in open country as there is to wind turbines – maybe more! The two are of course interdependent and numerous low power wind generators will inevitably create many miles more power line. The current proposal for a major super-grid power line from Beauly to Denny, in Scotland, had by February 2006 attracted some 18,000 objections and almost no letters of support (source Scottish Executive). The Highland Council, Stirling, Clackmannanshire and Perth and Kinross councils also objected in April 2006. These objections raise questions of landscape amenity, risk to birds which is proven, health-risks which are unproven but a matter of great public concern, and of course the vexed matter of transmission-loss.