The Drive System Options
Roughly speaking, when we double the diameter of a wind turbine the swept area and power output increases by a factor of 4. The rotational speed is cut in half and the torque on the output shaft is 8 times greater. Therefore, as wind turbines increase in size, the speed increaser becomes disproportionately more expensive. The main factor when selecting a gearbox is the torque required for the application.
The rotational speed of the output shaft on the NREL 5 MW reference wind turbine is 12.1 rpm. When it is producing 5 MW the torque is about 4,000,000 newton meters or 3,000,000 foot pounds. The combination of this slow speed and high torque is rarely seen, if at all, in any other machine built today.
Speed increasing gearboxes convert this slow speed, at high torque, to the desired generator speed but can cost 25% or more of the overall cost of the wind turbine.
Direct drive generators rotate at the same speed as the output shaft but require a large number of poles and a massive structure that is equally as expensive as the gearbox. Both the direct drive generator and the gearbox need a frequency converter to connect a variable frequency current to a constant frequency electrical grid. The cost of the converters can also be a high percentage of the overall cost of the wind turbine. Here are two more options.
Hydraulic systems on some wind turbines use a pump, connected to the slow rotating output shaft, to convert the wind energy to a high pressure fluid flow, which connects to one or more hydraulic motors that drives one or more generators at a constant speed. This system does not require a frequency converter; however, the pump is also massive and therefore, expensive.
This picture shows the pump as being multiple times bigger than the motors. For this reason, the pump is the most expensive part of the system. One way to lower this cost is to use the reciprocating motion of hydraulic cylinders to create a high pressure flow. The piston rod of each cylinder is connected to a crank driven by the output shaft and the Cap-end is mounted to the frame of the turbine.
As the piston in the cylinder moves back and forth, one side of the piston pressurizes the fluid while the other side is filled with fluid. Each end of the cylinder has two ports with one way valves, at each port, guiding the fluid through the cylinder.
The table below shows the cylinder dimensions for random scenarios.
The cylinders are small in comparison to the largest cylinders built today and almost any machine shop anywhere in the world has the capability of building or repairing them. Since the flow rates are larger than most hydraulic applications none standard motors could be considered for driving the generators. Impact turbines such as the Pelton wheel can produce power at an 82% efficiency when the pressure is 2,500 psi. Efficiency usually increases as the pressure increases so the Pelton wheel may be beneficial especially if the flow from more than one turbine travels through the wheel.
The cost to build the prototype hydraulic pump for the Lux wind turbine, not including labor for installation, was 1/5 the cost of a positive displacement pump with an equivalent flow rate. Some of the cost savings was attributed to the outer frame, containing the cylinders, which was already in place and only required minor changes. Also, commercial grade cylinders with a maximum pressure of 3,000 psi were used in the demonstration project, which would not be adequate for long term operation. High quality components are necessary to ensure long time periods between maintenance schedules. Also, the material and machining costs were at retail prices, which did not include any discounts for volume purchases.
Items to consider when designing this pump:
- Number of cylinders per row
- Number of rows
- System maximum pressure
- Size of bore
- Length of stroke
- Materials for seals and other components
- Hose and Tube connections
- Ease of replacing a cylinder
- Environmental control
- Sensors for leakage detection
The Lux wind turbine showing the Traction Drive at the bottom
A magnified view to the Traction Drive.
The Traction Drive is used to increase the rotational speed of the output shaft. A large diameter wheel is attached to the output shaft and one or more small diameter wheels are forced against the large wheel, creating friction, forcing the small wheel to rotate at a higher speed. The Traction Drive is similar to a locomotive transferring 5 MW of power from the steel wheels to the steel rail. This drive system has been used by the transportation industry for over a century.
The system is easy to build on a VAWT because there are no space limitations at ground level and the ground or foundation can be utilized to support the drives. Recyclable bushings mounted on the outer diameter of all the wheels can be replaced after excessive wear rather than replacing the entire wheels. The large diameter wheel could have segmented bushings simplifying the process even further.
The contact area of a locomotive wheel and rail is the size of a dime. This small area is due to the shape of the wheels and rail that is needed to keep the wheels centered and to prevent the wheel flange from contacting the rail. The contact area on the Traction Drive, however, can be adjusted to maximize adhesion between the surfaces, thus reducing surface wear. Also, the rail system has to contend with vegetation and other adverse environmental conditions, which reduces the coefficient of friction. The environment around the Traction Drive can be controlled and the coefficient of friction can be maximized at all times.
The coefficient of rolling resistance of the steel wheel is low, which provides a power loss similar or lower than a multistage gearbox. A Traction Drive with several small wheels, each driving a generator can be engaged or disengaged as the wind speed changes. The drive system can also be orientated to help diminish or remove the overturning moment at the bottom of the rotor of a VAWT.
A Traction Drive system was installed on several of our prototypes and has proven to be a viable alternative to the gearbox system. Future models will combine the mounting brackets for the blades with the large diameter wheel. The hub and wheel diameter will be approximately 1/5 the rotor diameter. The small wheel diameters will be 1/10 the large wheel diameter yielding output shaft speeds, 10 times the rotor speed.