Less is more in radical turbine design

TALK ABOUT BREAKING WITH TRADITION. The engineers with MegaWindForce have designed a turbine that has no hub, no nacelle, no gearbox, may need no crane for construction, and if the design can be tweaked just a bit, it will run with less noise than modern conventional turbines.

MegaWindForce’s 250 kW prototype will have a 5-m diameter stator and 5-meter blades for a 15-m diameter rotor. Blade cord will be about 70 cm, and the tower height, 25m.

MegaWindForce CEO Ton Bos says the invention of the generator is as important as the rest of the turbine. “It is makes the other innovations possible and is based on two significant developments. First, a magnetic flotation system results in nearly zero friction, and the direct-drive generator allows getting rid of the gearbox. The design also allows an air-gap speed of over 25 m/s as opposed to existing direct-drive designs that are limited to about 5 m/s. In addition, the generator works with adaptive cooling and at high frequency resulting in a significant weight reduction. It’s easily scaled for large units, because there are no restrictions that are limiting construction of a conventional 10 MW wind turbine. We could not imagine we were the only ones looking at the generator design this way. No one had picked up the principle of the stator and rotor we defined, so we patented it,” says Bos.

Magnet levitation in the generator allows eliminating some rolling element bearings for a nearly friction free rotation.

The second generator development is that it is made of more than 100 switchable units or sections. The generator rotor is kept at a constant linear speed of about 100 km/hr (27.7 m/s). “We regulate this speed by switching generator units on or off depending on wind speed all the way up to 30 m/s. This way, it is unnecessary to transfer energy from a low-speed axis through a gearbox to a generator and it is far more reliable,” says Bos.

A problem with conventional generators is that they must function in all wind speeds. In low winds, the generator is far from efficient. A generator design suitable for higher wind speeds makes the situation worse, says Bos. He says the turbine will begin producing power in 2 m/s wind and continue up to 30 m/s. “Most turbines reach their rated power output at about 12.5 m/s and must shut down at 25 m/s. Our design continues working beyond that which accounts for the greater power production.”

What’s more, MegaWindForce blades are kept at an optimum pitch for best aerodynamic profile for all wind speeds and without turbulence. Current wind turbines must slow the rotational rate in wind over 12 m/s by twisting the blades off the ideal pitch (stalling actually) which slows the rotor speed by creating a lot of turbulence and additional forces. In addition to omitting the gearbox, other components will contribute to a weight loss of 30 to 40% over conventional turbine of the same capacity. “We will make as many parts as possible out of carbon fiber. The manufacturing techniques are already at use in the marine industry. For larger turbines, the weight reduction is greater.” Also, the design is sufficiently modular to ship in standard 40-ft containers so shipping costs will be lower.

Construction will also get a boost because a built in hydraulic mechanism will carry the generator and rotor up the tower and into position, eliminating need for a costly crane.
The turbine opens additional possibilities. For example, in Norway the design makes sense when it is mounted offshore because it can be mounted on towers half the height of conventional offshore units.

When will the prototype fly? “We have a two-stage approach. One is to work on a small model with about a 5-m diameter rotor to prove out the principles. The second step is to build a 250-kW prototype for licensing to communities of about 550 households. That will cost about €2.7 million and we just signed a contract with another organization to make that happen. All the permits and licenses that allow us to move forward are in hand.”

Bos says the second prototype will mount to a 25-m tall tower and with a 15-m rotor will produce 1,314 MWh/year. “This comparable to a Vestas V44 – 600 kW with a 44-m diameter rotor. The MWF figure is almost three times as high and with less noise. The annual production capacity and cost are key. It is evident that we are able to reduce the price by more than 50%. For a fair comparison between turbines, examine their annual outputs. We will be able to produce electricity at the same price of natural gas and without subsidies. Even without further improvements to the current design, we can cut 50% off of today’s price for electric power, which means we don’t need subsides to move forward,” he said.

Balancing high frequency with switching losses 

The key benefit of operating at high frequency is that it allows use of smaller external components, such as inductors and capacitors, because the inductor size is primarily determined by the amount of ripple current allowed in a given switching regulator’s specification. For a given inductance, the ripple current decreases as the switching frequency increases. Consequently, a progressively smaller inductor can be used to maintain the same amount of ripple current as a switching regulator’s frequency increases – reducing the size and cost of the power supply.

Traditionally, a higher switching frequency also means greater power loss, demanding more board space or a heatsink to dissipate the heat. Switching losses increase with greater frequency due to the larger number of constant-energy-switching events per time. Some of these losses are due to the switching regulator’s MOSFET which takes a finite time to turn on or off. This generates voltage and current overlaps during the switching transients. Bos says his company has solved the problem and is able to use high frequency without the switching losses.