We know that the power coefficients of small vertical axis wind turbines (VAWTs) are low. However, it is just because they are small. In the s, Sandia National Laboratories conducted field tests of their 30m curved Darrieus turbine (egg beater or phi shaped). The measured power coefficient is 0.40. Because the high latitude part of the classic curved Darrieus rotor does not contribute to the power well, it indicates that modern straight blade VAWTs (rectangular shaped swept area) will show the power coefficient over 0.45. It is competitive with large HAWTs.
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Unfortunately, however, there are many journal papers which report the low efficiency of VAWT using small scale wind tunnel measurements without knowing that the size (or Reynolds number) matters.
The above discussion is on the lift type VAWT. Although the drag type VAWT is useful in small applications, its material heavy design is not economical in large applications.
The largest VAWT ever built is the Éole turbine (Canada). It was 4MW rated power and 100m height. Although it is smaller than the present large HAWTs, it was built in ! At that time, the mainstream of large turbines was VAWT and the biggest HAWT was only around 30kW. The advancement of material science and numerical analysis so far has made the present large HAWTs possible. With the same technologies, VAWT can be as large as the present HAWTs, if you want.
The generator and other heavy electromechanics of VAWT can be installed below the turbine. Therefore, the turbine part is light-weight and low center-of-gravity position. It reduces the size and cost of the floating foundation significantly. Also, large VAWTs do not require the yaw and blade-pitch controls. Rotor speed controll is by the power electronics of the generator. This simplicity reduces the operation and maintenance (O&M) cost.
Because of the vertical axis position, you can mount the VAWT rotor on a “rotating” circular cylindrical float. In this configuration, sea water is the main bearing support. Loss of energy by the sea water friction on the floater surface is not significant and its loss ratio decreases with the turbine size.
Because an HAWT blade is a long and thin cantilever structure, the scale-up of HAWT is getting more and more difficult. On the other hand, a VAWT blade is a series of both-ends-supported beams. For the convenience of production/transport, the blade can be divided into shorter parts. Because of the constant section shape of the VAWT blade, we can use the FRP pultrusion process for low-cost blade production. In the assembly of VAWT, you don't require any onshore cranes or crane ships because heavy parts are installed at low altitude. These features and the previous item 4 will make super large floating VAWTs possible.
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I’m not saying VAWT is better than HAWT in onshore and offshore-bottom-fixed applications. It is in the floating offshore wind application where VAWT can be more economical and easy to scale-up than HAWT.
Vertical axis wind turbines often have less rotation efficiency. This is part of the reason why vertical axis wind turbines have lower efficiency.
Due to the rotor design, not all the blades on the vertical axis rotor receive incoming wind at the same time. In fact, only the wind-facing blades are driven by the wind to turn while the others are simply following along. During rotation, vertical axis rotors are also faced with more drag— or aerodynamic resistance— on the blades.
This is especially shown on Savonius wind turbines because they have wider blade surfaces.
Since vertical axis wind turbines are typically installed on ground level, they do not harness higher wind speeds often found at higher levels. Consequently, less wind energy is available for ground-level vertical axis wind turbines. A common solution to this is to install the turbine on the rooftop of a building.
To address this problem, we have improved the rotor design so that it can be mounted on top of a mast. The rotor and the mast combined tower our vertical axis wind turbine to 10 meter high, with the generator and power electronics located at the 4-meter height.
Often placed on ground level and populated environments, vertical axis wind turbines face more turbulence and issues of vibrations. When in operation, not only do the blades need to withstand more force, the bearing between the rotor and the mast also needs to endure higher pressure. In earlier models, it was more likely for the blades to bend or crack. For others, this can result in more maintenance and therefore more cost.
To strengthen our own turbines, we have taken wear-down calculations into consideration for design and manufacture. The LuvSide vertical axis wind turbines are now robust enough to stand wind speed equivalent to a strong tropical storm.
Vertical axis wind turbines are known to have less efficiency compared to horizontal axis wind turbines. This is mainly due to the nature of their design and operational characteristics.
On average, the efficiency of a horizontal axis wind turbine lays between 40 to 50 %, meaning the turbine is able to convert 40% to 50 % of the kinetic energy it receives into actual electrical power. On the other hand, a Savonius vertical axis wind turbine has an average efficiency of 10 to 17 %, while the Darrieus vertical axis wind turbine reaches 30 to 40 %.
Even so, given the suitable environment, a Savonius wind turbine can still produce enough power to support annual consumption of a normal two-person household.
While horizontal axis wind turbines and Savonius wind turbines start turning automatically once receiving wind, Darrieus wind turbines often rely on a starting mechanism. This is because the wing design of Darrieus wind turbines does not always guide the wind to form enough torque for rotation. At the moment, this is still a challenge that many, including us, are trying to tackle.