FAQ

Q: Why do vertical wind turbines look so different while horizontal turbines are quite similar in appearance?

A: Because the lack of design theories and design basis, many companies apply the experiences of horizontal turbines design to vertical turbines, and some companies design a vertical turbine based on their preference. Most of the vertical turbines designed in this way are drag-type.

Q: Why do so many vertical wind turbines have difficulty generating electricity?

A: Vertical wind turbines could be categorized into drag-type and lift-type. Many drag-type vertical turbines or drag lift hybrid type turbines could easily rotate because of aerodynamic design, but generate very little electricity or couldn’t generate electricity at all. Although some vertical turbines are lift-type, the designers don’t master the characteristics of vertical turbines; they have chosen the wrong airfoil and other important parameters. Consequently the aerodynamic performance is very low. For some lift-type turbines, the RPM of the turbine windmill and generator don’t match, which could cause less electricity generated as well. These are some major reasons that many vertical turbines don’t generate electricity

Q: How to choose a good vertical turbine?

A: A good turbine must have good performance in power generation, particularly in low wind speed, and must be able to work in a wide range of wind speed similar to large horizontal wind turbines. In addition, good methods of over-speed control and automatic mechanic brake are necessary features of a good vertical turbine as well. Generator short circuit is quite common for braking, but it is not a good solution. It causes the degaussing of the permanent magnet generator, and eventually shortens the lifespan of the generator.

Q: What is over-speed control?

A: Over-speed control functions when the wind speed exceeds the rated wind speed, so the turbine won’t increase its RPM and output power too much. Turbines may disintegrate in strong winds, since the RPM is proportional to the wind speed, while the centrifugal force is proportional to the square of the RPM. The horizontal axis wind turbine can yaw to reduce the windward area, but the vertical axis wind turbine can not. Therefore the vertical axis wind turbine must have an over-speed control device.

Q: How many over-speed control methods are available?

A: There are many kinds of over-speed control methods, such as pitch angle control, reducing the windward area, dump load etc. However, if dump load is used for over-speed control, the effective dump load power is related to the cut-out wind speed and rated wind speed of the vertical axis wind turbine

The formula is:
Effective dump load power ≥ rated power * {(cut-out speed/rated speed)3-1}times
For example: a 3 kW turbine has its rated wind speed at 12 m/s, and cut-out wind speed 26 m/s, the dump load power ≥ (26/12) 3-1 = 9.2 times, so the dump load power shall be at least 3 *9.2 = 27.6 kilowatts.

Q: How to determine the effectiveness of the brake?

A: Vertical axis wind turbines have lower rotation speed and higher torque, and the ratio of the torque at survival wind speed and at the rated wind speed is the square of the ratio of the respective wind speeds: Torque(survival wind speed) / Torque(rated wind speed) = (survival wind speed/ rated wind speed)2 The braking torque of magnet valve brake is restricted by the size of the magnet valve, so it does not have enough braking torque to stop the wind mill at survival wind speed. Therefore, magnet valve brake systems need two-stage brakes. The first stage brake works when the wind speed reaches cut-out speed or slightly higher than cut-out speed, and the second stage is used to lock the turbine when the survival wind speed is reached. For hydraulic brakes, the braking torque is large enough for small vertical axis wind turbines, so one-stage brake is enough.

Q: Is short-cut of the generator able to replace the brake of the vertical wind turbine?

A: No. Short-cut of the generator can only be used in comparatively low wind speeds for a short period (a few minutes). It can not be used in very high wind speeds for long time. So it can not replace the brake.

The reasons are:

First, the generator does not allow for long time operation under short cut condition. But usually the typhoon lasts for hours or even longer. Long time short-cut will cause demagnetization of the generator. Demagnetized generators can not generate any electricity, and short-cut of demagnetized generator can produce hardly any reverse braking torque, so the turbine may disintegrate in strong winds.

Second, using short-cut of the generator as a brake is based on the principle that permanent magnet generators could produce reverse braking torque in short cut condition. Usually a well-designed permanent magnet generator short-cut current is 5-6 times of its rated current, and the ratio of the torque at survival wind speed and the rated wind speed is the square of the ratio of the respective wind speeds:

Torque(survival wind speed) / torque(rated wind speed) = (survival wind speed / rated wind speed)2 It means that the torque from wind mill in high winds is far bigger than the reverse braking torque of a short-cut generator. For example, a wind turbine with rated wind speed at 12 m/s, survival wind speed at 60 m/s, in the survival wind speed, the torque from the wind mill is (60/12) 2 = 25 times, so short-cut of the generator can not replace brakes in vertical axis wind turbines.

 

Q: Why do many vertical turbines fall apart and collapse when the wind speed is not very high?

A: As we know that wind power is proportional to the cube of wind speed, a doubling of wind speed results in 8 times of wind energy. Because vertical turbines can’t yaw; if a vertical turbine does not have pitch angle control for over-speed regulation and automatic mechanical brake, it could break up and collapse very easily even when the wind is not very strong.

 

Q: What are the necessary features for vertical wind turbines to have commercial value?

A: A vertical turbine must have the electricity generation performance comparable to large horizontal turbines; have pitch control, over-speed regulation and automatic mechanical brake. Otherwise it has no commercial value.