Abstract: The common high power wind turbine erected, generally has a heavy gearbox attached to the turbine rotor to a synchronous generator. It also contains a cabinet full of electronics which convert the generator output to grid-compatible power. The normal nacelle itself has to be elevated some 50 stories off the ground, so major maintenance issues are involved while dealing with the removal and replacement of components. At the least, technicians must make the 50-story climb periodically to handle ordinary upkeep.
It is fair to say that there are a number of interrelated mechanical and electronic systems in the nacelle of the characteristic utility-scale wind turbine.
With this in mind, our paper addresses the possible solutions like why can't we use a hydraulic drive? Why not just place a hydraulic pump there and let the hydraulic pressure spin a generator on the ground?
Analysis about ordinary wind turbines:
Wind turbines convert the kinetic energy in the wind into mechanical power. So how do wind turbines make electricity? In an intelligible way, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which are associated to a generator which sets up electricity.
Sizes of Wind Turbines
Efficacy-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are clustered together into wind farms, which provide bulk power to the electrical grid.
Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping. Small turbines are sometimes used in association with diesel generators, batteries, and photovoltaic systems. These systems are called hybrid wind systems and are normally used in remote, off-grid locations, where a connection to the utility grid is not available.
Internal composition of Wind Turbine
Anemometer:
Measures the wind speed and transmits wind speed data to the controller.
Brake:
A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
Pitch:
Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity
Gear box:
Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine
Generator:
Usually an off-the-shelf induction generator that produces 60-cycle AC electricity
High-speed shaft:
Drives the generator
Low-speed shaft:
The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
Rotor: The blades and the hub together are called the rotor
Nacelle:
The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake
Controller:
The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
Working:
A radial piston type pump is mounted in the nacelle and an in-line or bent-axis motor on the ground to drive the generator. A radial piston-type pump provides the best performance at the low input speeds typical of wind turbines, and an in-line or bent-axis generator motor provides the greatest efficiency. This arrangement simplifies the task of regulating generator speed under varying wind conditions. It also reduces the nacelle's overall weight and isolates the generator from the low-frequency torsional vibrations that characterize wind turbines. A hydraulic control system controls the pitch angle of the turbine blades hence controlling the speed and power production. A multiton planetary gearbox hooks the turbine rotor to a synchronous generator. A cabinet full of electronics converts the generator output to grid-compatible power.
Possible changes with Hydraulics:
Mounting a radial piston-type pump in the nacelle: One way would be to mount a radial piston-type pump in the nacelle and an in-line or bent-axis motor on the ground to power the generator. A radial piston-type pump yields the best performance at the low input speeds typical of wind turbines, and an in-line or bent-axis generator motor bestows the greatest efficiency.
Effect: This arrangement simplifies the task of controlling generator speed under varying wind conditions. It also reduces the nacelle's overall weight and the generator will be free from the low-frequency torsional vibrations that characterize wind turbines.
Motor and generator can mount at ground level (Ground level Implementation): As a realistic matter in most, but not all, cases it makes sense to split the system so the motor and generator can be installed at ground level. But no matter which approach is used, a hydraulic drive of this nature could result in smaller, lighter nacelles. Nacelle-mounted hydraulics for a 100-kW system would typically weigh 700 to 1,000 lb. In the same vein, the towers and bases to support the smaller, lighter nacelles would themselves be less expensive. Moreover, most maintenance activities could take place at ground level.
A hydraulic drivetrain for a megawatt-scale wind turbine might replace a mechanical gearbox in the nacelle with a hydraulic pump, lightening the load on the tower and decoupling rotor vibrations from the generator.
Effect: Such hydraulic systems would eliminate the need for mechanical gearboxes. Consequently, the overall wind-turbine mechanics would be less complex and the uptime of the apparatus would likely improve. And a setup of this nature would decouple torsional vibrations generated in the rotor hub from the generator. Finally, a hydraulic pump would have less inertia than existing wind-turbine mechanics and thus would let the turbine begin generating power in lighter winds. The turbine's operational envelope would expand as a result. All in all, such systems would have lower operating costs over their lives than those we see today.
Higher Input speeds: One factor often ignored is the relatively low rotational speed of a wind turbine. Most hydraulic pumps are designed for input speeds ranging between 500 and a few thousand revs/min, while wind turbines normally max out at 150 rpm or less.
The issue is that pump losses are not strongly related to input speeds. Operating a piston-type pump at 150 rpm when it is designed for 600 rpm, for example, reduces the volumetric output linearly by a factor of four.
But it does not reduce losses by a factor of four. Pumps are less efficient below their designed speed, and the extreme variability of the input speed with changing wind conditions makes matters worse.
Adoption of Hydraulic drives: Early adopters of hydraulic drives will have to depend on off-the-shelf components, which lead directly to a second set of challenges. In general terms, today's off-the-shelf hydraulic components are practical for wind turbines with outputs up to about 500 kW.
For megawatt-scale systems, however, designers must resort to suboptimal solutions like multiple pumps and motors to get enough capacity. When they do, efficiency suffers further. These drawbacks are a reflection of the industrial and mobile applications that historically have been the primary focus of hydraulic manufacturers.
Effect: Pressure drops, for example, typically are a square function of port size. We certainly have the design tools to overcome these nonlinear effects, but the market dynamics present challenges to any company developing the necessary technology.
A Further insight into the implementation:
There is no doubt that a hydraulic solution can offer solid economic benefits compared with today's electromechanical systems. Hydraulics offer power density unmatched by any other technology and power density is precisely what is needed in a wind-turbine nacelle. The efficiency gap may eventually succumb to new technologies like more-efficient fluids to reduce line losses and electrohydraulic-control systems to optimize performance. But even without such developments, hydraulics look attractive on a life-cycle cost basis. Hydraulic wind turbines could potentially make it feasible to field less-expensive nacelles, towers, and tower bases. But though there are benefits to a hydraulic approach, there is no free lunch. The hydraulic solution is currently less efficient than an electromechanical system, and it is not as easily scaled up to handle the loads of multimegawatt turbines.
The simple fact is that even operating at maximum efficiency in the 5,000-psi range, the hydraulic system would be from 10 to 30% less efficient than a mechanical system in moving energy from the turbine blades to the generator input shaft.
It's also likely that ground-level maintenance would be more frequent because it would take less planning. Yes, the pump in the nacelle would still need maintenance, along with the hub and other equipment. But the overall cost would still be much lower.
Futuristic approach:
One must consider the fact that hydraulics are much more reliable than mechanical systems in extreme environments. It should also be said that the specialized gearboxes used in today's wind turbines are proving to have a much shorter service life than what their manufacturers have predicted. This reality perhaps more than any other may provide the opportunity to launch hydraulic drivetrain alternatives.
All in all, the benefits of large-scale hydraulic drives can only be inferred from other hydraulic applications because nobody has successfully demonstrated them on a large wind turbine - yet. But the benefits would seem to be compelling. It's just a matter of time until hydraulic drives are put to work harnessing the wind to produce clean, renewable energy.
