This article explore the wind turbines of the future. However, before looking into wind-turbines of tomorrow, it would be worthwhile to understand what really works for turbine design and what doesn’t. This would provide a fair idea of the challenges and opportunities that lie ahead for an industry that is the back bone of renewable energy.
During the centuries, there have been numerous Wind Turbines designs. Wind Mills of Netherlands had four blades, while those installed during 19th century in ranches in southern USA had multiple blades (up to 18). A study later-on found that although adding more blades does increase the energy extracted, but only up to a certain limit. Wind has to go past the blades of a turbine in the open and if the resistance along its passage is too much, it simply bypasses the turbine. In other words rather than going through the turbine it goes around it.
This limitation is commonly known as the Betz limit, which suggests that a maximum of 59.3% of the energy in the wind (confronted by the turbine) can be extracted by the blades. This limit however can be bypassed by creating a control volume around the wind turbine, which doesn’t allow wind to sidestep.
Three bladed wind turbines are the choice of the industry today because adding a fourth blade increases the efficiency slightly but the cost sees a much bigger jump. The blade profiles have also evolved over time. Turbine blades today are more aerodynamic. It should be noted that relative velocity at the root of the blade (wind velocity + blade velocity) can be much lower compared to the relative velocity at the tip of the blade. Aerofoils shapes that produce more lift at low speed resemble the shape of a sliced tear-drop. On the other hand Aerofoils that produce more lift and less drag at high speeds have flatter profile. Based on this information, the turbine blade are made such that their cross-section continuously changes from root to tip.
Another lesson that has been learned over time is the wind-boundary layer. Wind speed increases almost exponentially as one moves vertically upwards from the ground level. In simple terms, wind speed at 10 m is much slower than at 20 m. Therefore the higher the nacelle is, the faster wind it will confront. Today, nacelle height of more than 50 meters is a common place in wind turbine design. An added benefit of locating the nacelles high is that wind –turbulence decreases with respect to height. Turbulence can be induced by obstacles on the ground like trees, buildings, Poles, Pylons etc.
Off-shore wind turbine have an edge over their on-shore counterparts. They receive higher quality wind energy that is faster , less turbulent and more consistent. The biggest cost however of installing an off-shore wind turbine is the seabed platform.
In light of all the limitation mentioned above, the wind turbines of the future are taking the following shape:
Shrouded Wind Turbines
Shrouded/ Cowled or Ducted wind turbines have been tested and have been reported to have superior efficiency then open wind turbines.
The duct is shaped such that it creates a higher pressure differential across the blades. This allows the wind to further accelerates as it goes past the blades. Wind power has a cubic relationship with wind speed i.e. doubling the wind speed increases the power by 8 times. Thus in effect the shroud acts as a lens that converges energy to a smaller area where it can be easily captured.
There are concerns regarding the extremely high drag force during gale-force winds that can uproot the turbine. However these can be addressed by having a flexible or perforated cowling that can open gaps and allow high -speed wind to pass through.
Flying Wind Turbines
Flying or Buoyant Air turbines (BAT) have already been developed and are being deployed for test in remote places like Alaska. They have a unique advantage of meeting airstream at levels 350 meters above the sea level, where the wind is more powerful and consistent. They can also adjust height to the level where airspeed is the highest. In cold remote places there is also the advantage that air is more dense i.e 1.25 kg/m3 as opposed to 1.00 kg/m3 near equator. This means 25% more power for the same wind speed can be captured by the turbine.
Similarly Kite like wind turbines have also been designed. They will be able to capture abundant wind energy at higher altitude.
Floating Wind Turbines
Instead of Turbine installed on a concrete metal platform, which costs a fortune, future offshore turbines can be erected on a floating buoy-platform. If the centre of gravity, meta centre and buoyancy center are well designed, than turbines man maintain upright position. This is possible even if the turbines are swayed by extremely high winds or rough sea. The turbine are pushed back into normal position by a righting –torque, similar to certain life-boat designs that cannot capsize.
You can also look at the video below for a quick recap
Also worth reading