General Windfarm
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General
What is a wind farm?
What is a wind turbine generator?
How do wind generators work?
Blade speed and materials
Where are wind farms sited?
How much energy does a wind turbine produce?
How much does electricity from wind turbines cost?
How does New Zealand currently generate its electricity?
How fast is demand for electricity growing?
Are wind turbines reliable?
Are wind turbines efficient?
How reliable is wind power?
What happens when the wind stops blowing?
How much wind power can we have in our generation mix?
The following material has been prepared by the New Zealand Wind Energy Association, with assistance from the Australian Wind Energy Association, and gives an excellent background to wind farms generally:
What is a wind farm?
'Wind farm' is the name used for one or more adjacent wind turbine generators that are connected electrically. This includes vehicle access tracks, underground cabling for electrical interconnection and communications and the switchyard at the point of connection to the grid. In New Zealand wind farms have been built with between one and 103 turbines. Each wind turbine acts independently, generating from the available wind resource. The electricity flows through common cabling out onto the local or national electricity network. The turbines are usually arranged to maximise use of the wind available and placed sufficiently far apart to receive a reasonable share of the available wind. Wind turbines produce electricity directly from a natural, clean and sustainable energy resource - the wind.
In terms of growth rates, wind energy technology is now the world's fastest growing form of electricity generation. New Zealand is also now experiencing a rapid growth in the number of wind farms due to the precipitous decline in the Maui gas field, rising electricity prices and the falling cost of electricity generation from wind turbines.
Every development is different and site specific issues and local planning controls ultimately determine the design and layout of a wind farm.
What is a wind turbine generator?
A wind turbine generator consists of a foundation, tower, nacelle and a rotor. The nacelle contains the main drive shaft, the gearbox and the generator. Together these three components are referred to as the 'drive-train'. The rotor usually consists of three blades on a central hub although some innovative two bladed designs are currently being developed and tested. The foundation is typically a single block of concrete the dimensions of which will depend on the turbine. It may be up to 12 metres wide and 3 metres deep and, whatever size it is, the foundation is always buried below ground level. What you can see at ground level is generally no wider than the turbine tower itself. Whatever type of foundation is used, stock can graze right up to the base of the tower.
The nacelle rests on a large bearing and is driven by a motor such that it and the rotor constantly face into the wind.
Towers are typically coloured white or light grey as these colours have been found to be best at blending in with the background. Internal ladders provide access to the nacelle, which contains the drive train, gearbox, generator and controlling equipment. The price of a wind turbine obviously depends on its size and generating capacity. Nonetheless a good 'rule of thumb' to use is about NZ$2 million for every Megawatt (MW) of installed capacity.
How do wind generators work?
Some of the energy in the wind, as it passes over the blades, is converted into kinetic energy (energy of motion) in the blades and consequently the main shaft of the turbine. The main shaft turns inside the nacelle and this motion is passed, through the gearbox to the main generator where it is converted to electrical energy. As the wind speed increases more energy is delivered to the wind turbine's rotor. The energy delivered to the rotor is extremely sensitive to the wind speed - a doubling of the wind speed gives, in theory at least, eight times the amount of energy. Wind turbine generators deal with these huge variations in power using several aerodynamic strategies that regulate the power captured by the rotor.
| Wind speed metres/sec | Wind speed km/hr | Turbine operating mode |
| < 4 | < 14 | Machine Shut Down |
| 4-12 | 14-45 | Output increases steadily with increasing wind speed |
| 12-25 | 45-90 | Output remains steady with excess energy 'spilled' from rotor |
| >25 | >90 | Machine shut down for self protection |
Turbines in New Zealand are typically operating (i.e. not shut down because of too low or too high, wind speeds) for more than 90 per cent of the time.
Blade speed and materials
The blades of electricity-network connected wind generators typically range from 25-50 metres in length and usually extend from the nacelle to about half way down the tower. Depending on the size and design of the machine, the rotor will turn at between 10 and 25 revolutions every minute. From a distance this rotation seems quite slow and stately. The blades are made from advanced composite materials that have a high strength and are lightweight and flexible. The maximum blade tip speed is about 250km/h and it is quite normal to see the blades fl ex backward several metres under the enormous pressure of the wind. Integrated lightning protection systems ensure that the blades can withstand a direct strike without serious damage.
Where are wind farms sited?
Wind farms are usually sited where there is a good wind resource, access to transmission lines, local community support and plenty of open land available. Typically the best wind resources are in coastal regions or inland at higher altitudes. Wind farms are unlikely to be built offshore around New Zealand in the foreseeable future due to the higher costs of constructing offshore, the deep waters around New Zealand, as well as the fact that the onshore wind resource here is already extremely good.
How much energy does a wind turbine produce?
This depends on the size of the machine and the wind resource. A single 1.65 MW turbine operating at a 45% capacity utilisation factor (which is common in New Zealand) throughout the year will generate enough power to meet the needs of about 800 average New Zealand households.
How much does electricity from wind turbines cost?
There are currently no taxpayer subsidies for wind turbines in New Zealand. The cost of generating electricity from wind turbines has fallen to about one-tenth of what it was 20 years ago. Because of this, and the fact that New Zealand has some of the best wind resource in the world, it is now possible to generate electricity, from wind turbines, at prices of between six and seven cents per unit of electricity. This is directly competitive with coal and gas fired generation. In addition to which it is also worth noting that the cost of coal and gas fired generation is rising due to declining supply, rising demand for coal particularly in China, Asia and India, as well as the need for these industries to meet increasingly stringent environmental standards.
How does New Zealand currently generate its electricity?
About 50 to 60 per cent of our electricity is provided by hydro power stations. A further 10 per cent comes from geothermal power stations and the remainder has traditionally been from natural gas although this is changing fast as output from the Maui gas field drops away rapidly.
How fast is demand for electricity growing?
The demand for electricity is growing by two to three per cent annually. If we are to meet this demand growth we will need to install between 150 and 300 Megawatts (MW) of new electricity generation capacity every year. Our ability to increase the capacity of both our hydro and geothermal stations is currently limited which means that this increasing demand will probably have to be met by wind turbines, coal or natural gas.
Are wind turbines reliable?
In terms of mechanical operation and maintenance, wind turbines have a reliability that is slightly in excess of that typically enjoyed by the steam turbines used by coal plants - about 95 to 97 percent. Because turbines in New Zealand are located to take advantage of strong and consistent winds, their utilisation rates (the amount of time they are in use) are generally in excess of 90 per cent. This compares very favourably with conventional power plants.
Are wind turbines efficient?
To a certain extent the concept of efficiency is irrelevant when the raw material (the wind) is free. Wind turbines are nonetheless very efficient at converting the energy in the primary fuel (the wind) to electrical energy. Today's large-scale machines typically operate at efficiencies in excess of 45 per cent. This compares with coal fired electricity generation where only 30 to 40 per cent of the energy in coal is actually converted into electrical energy. Most of the remaining energy is lost as heat in the combustion exhaust. For gas fired power stations the efficiency of energy conversion may be as high as 60 per cent.
How reliable is wind power?
The amount of electricity produced by a wind turbine can be very different from one day to another or one week to another. For this reason it is often said that the wind is intermittent and cannot be relied upon, as you never know when it is going to be there. In fact this is not true for three main reasons:
- Wind turbines and hydro power stations work well together. When the wind is blowing, less water needs to be run through hydro turbines and it can therefore be stored behind dams instead. When the wind is not blowing this stored water can be released from behind the dams and used to generate electricity. As a result intermittent wind generation ceases to be intermittent when wind turbines and hydro power stations are working together as they can do so well here in New Zealand.
- The output of wind farms can be usefully predicted as much as 24 to 48 hours in advance. With increasingly effective data collection and interpretation from around the country, forecasts are likely to improve with significant benefit to electricity network managers.
- The total output of wind turbines spread around the country (or even those across a single wind farm) is much less volatile than that of a single turbine. This is because when the wind is not blowing in one place it almost certainly is somewhere else.
While the wind speed may fluctuate in the short term, the energy in the wind is remarkably consistent in the long term. For instance the data from the first four years of operation of Stage 1 of the Tararua wind farm shows that the annual power output has averaged 132.4 Gigawatt hours (GWh) (enough to supply 15,000 households) and that it has never deviated more than 7.5 per cent above or below this level. Operational data from the Brooklyn wind turbine, just outside Wellington, shows that the average annual electricity production for the 10 years through to the end of 2003, has been 938.5 Megawatt hours (MWh) (enough to meet the needs of 120 average households). During this 10-year period the annual electricity production from the Brooklyn turbine has never deviated more than eight per cent above or below this average amount. Compare this with an increase in crude oil prices of more than 50 per cent in the first 10 months of 2004, the rapid decline in domestic natural gas reserves and significant increases in the price of coking coal over the same period.
What happens when the wind stops ?
The short-term variability in the output of wind turbines currently has no impact on the overall efficiency with which the New Zealand electricity network is operated. This is because wind turbines still generate a relatively small proportion (less than two per cent) of the total amount of electricity generated. Today when the output of wind farms increases, fossil fuel generators and/or hydro stations, simply reduce their output and vice-versa when wind farm output decreases. The electricity network already has to be able to accommodate such changes at all times as it has to have the ability to cope with constantly fluctuating demands from users of electricity as well as the risk of existing generators 'tripping off' the system. In the future more wind farms will be built and the percentage of electricity that we generate from the wind will increase. How well the network copes with this fluctuating level of electricity supply depends on the nature of the other types of generation connected to the system as well as whether the lessons learned in other parts of the world can be successfully applied here.
How much wind power can we have in our generation mix?
In some parts of Germany wind energy can contribute up to 70 per cent of a region's needs. In Denmark 15 to 20 per cent of annual electricity needs are supplied by wind turbines. Nonetheless we have to recognise that Germany and Denmark are part of a much larger electricity network and hence they are better able to cope with fluctuating electrical output from wind turbines.
It is not yet clear what the maximum level of wind energy penetration can be here in New Zealand and further studies are clearly required. However New Zealand's great strength, as far as wind turbines are concerned, is that hydro power stations and wind turbines are ideally suited to each other. When the wind is blowing more electricity is generated by wind turbines and consequently less electricity is needed from hydro stations. Water can therefore be stored behind hydro dams when it is windy. When it is not windy this stored water can be released and used to generate electricity in the normal way. A quick calculation indicates that the dry years of 2001 and 2003, when we were short of electricity as a result of which prices rose sharply, would not have happened if there had been only a modest number of operational wind turbines connected to the electricity network. Even the skeptics estimate that 700 MW of wind turbines could be installed on our electricity network without affecting overall system reliability or performance. In fact, the wind industry believes that 700 MW is overly conservative and that it would be eminently possible to install 1,000 MW of wind energy around New Zealand by 2010. Looking out beyond 2010, the industry and others such as Transpower, the electricity grid owner and operator, believe that the maximum level for wind energy penetration could ultimately be significantly higher than 1,000 MW. Whatever the absolute limit may be it is clear that with only 170 MW of wind energy currently installed around the country, we are still far short of even the lower estimates of current forecasts of the theoretical limit for wind energy penetration.
Last updated: Tuesday, 24 October 2006
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