Technology of Wind Energy
This section gives an overview of the different types of wind technologies and how they work:
Wind Energy Converters
A wind energy converter is another term used for the device which converts the kinetic energy in wind to mechanical energy and in turn to electrical energy i.e. a wind turbine. Since the beginning of the 20th century numerous types of wind turbine have been tested to identify the most effective designs to capture the energy in the wind. Present day wind turbines may vary in their appearance particularly in relation to size.
Development trends are towards larger turbines that use higher towers to avail of the higher wind speeds further from the ground. They also use longer blades to maximise the energy capture from the wind.
These turbines use proportionately less raw materials than smaller turbines and require a smaller ground area per unit of power output than their smaller predecessors. Larger turbines have the advantage of a slower blade rotation than smaller turbines. Economies of scale exist in wind energy as in many other businesses. For reasons of visual harmony, projects should incorporate the same turbine types and ensure that the blades of each turbine rotate in the same direction.
How a wind turbine works
- Most wind turbines operating commercially today in Ireland consist of 3 rotor blades that rotate around a horizontal hub. The blades face into the wind and rotate as the wind passes through them. The rotor is connected to a nacelle (a housing for the generator and other electrical equipment) that is located at the top of a tower to ensure a higher and less interrupted wind flow.
- Wind turbines start operating at approximately 4 - 5 metres per second (approximately 16-18 kmph) reach a maximum output at 12 - 14 m/s and automatically shut down for safety at wind speeds greater than 25 m/s (approximately 80 km.p.h.).
- The rotating motion is accelerated through the turbine transmission (which usually includes a gearbox although they are becoming less common) into the generator that converts the motion to electricity. When more air passes through the blades, more electricity can be produced.
- The low voltage electricity from the generator is 'stepped up' through a transformer to match the national grid voltage. The electricity is transported from the wind turbine to the grid along electric cables which may be buried underground within the wind farm site.
- The electricity from the wind farm joins the national grid at a sub-station.
- For a more detailed explanation of how wind turbines work please have a look at the guided tour of wind energy.
Choosing a Wind Turbine
Wind turbines are available in various sizes from a number of wind turbine manufacturers, agents and developers. Size however is not the only aspect of the wind turbine that should be thoroughly investigated by developers when deciding which turbine to use with their project.
The wind profile and wind speeds at each specific site need to be evaluated to identify which turbine is suitable for the particular site conditions. As the wind turbine itself may be as much as 70% of the total project cost it is vital that it produces optimal electricity for the given site. To assist the decision, manufacturers are required to classify their turbines in accordance with International standards (IEC 61 400-1). There are different classifications for operation of a turbine with respect to maximum wind speeds and average wind speeds.
The most common commercial wind turbines installed in the last number of years are between 1.5 and 2 MW. One of the most common wind turbines in Ireland is a 0.85 MW turbines. This scale or turbine has a rotor diameter of 52m and the nacelle is typical perched on a tower at a height of about 50 metres above ground level.
For a 2 MW wind turbine the rotor diameter may be up to 70 or 80 metres with the nacelle typically located 80 metres above the ground and a maximum blade tip height of almost 120 metres. Improvements in turbine design and efficiency means that we can expect wind turbines to continue to increase in size for some time yet. Where larger turbines are used, less are required to produce the equivalent power output achieved by a greater number of the their smaller predecessors. That is to say less wind farms are needed and less land is needed to produce the same amount of energy than is smaller ones are used.
Wind Turbine Selection Should Reflect:
- Wind profile at site and turbulence
- Technology availability
- Electricity production and investment cost
- Experience of similar models
- Experience of similar climatic conditions
- Size of site
- Capacity availability at grid access point
- Noise level certification
- Warranty and maintenance costs
Popular modern horizontal axis wind turbines (HAWT) employ 3 distinct technology types:
Turbines that run at one rated speed regardless of wind speeds. A constant speed turbine will stall when the wind becomes too strong. Constant speed turbines are robust and relatively cheap but are liable to mechanical stress, aerodynamic efficiency and noise.
Variable Speed Double Feed Induction Generator (DFIG)
Because the DFIG turbine runs at variable speeds it offers greater aerodynamic efficiency than the constant speed turbines and lower noise levels. The stress on the gearbox is lowered using variable speed generators. The improved performance naturally comes at an increased initial investment cost.
Variable Speed Direct Drive (DD)
Direct drive generators eliminate the gearbox (and therefore, any subsequent gearbox problems) completely. They offer improved performance through there choice of components utilised but are the most expensive initial investment cost option because of this.
Experienced professionals are the only people in a position to make informed choices when selecting which wind turbine is most suitable for each specific wind farm. In any event, a bank will not loan to a development which has not demonstrated due diligence and satisfied their conditions precedent.
For a more detailed explanation of wind turbine technology and concepts please have a look at the excellent guided tour of wind energy.