In a lab at the Dominion Power Innovation Center in Virginia, Dr. Weston Johnson is preparing a 100W prototype generator for trials. If his vision pans out, conventional wind farm architectures will be considerably simplified so that current obstacles to deployment disappear.
Unlike conventional designs built around magnetic field principles, this generator was built to use electro-static charges. The payoff, says Johnson, will be a direct drive, multi-megawatt generator that is significantly lighter and less costly than conventional drive-trains. “Its advantages will radiate throughout a wind farm especially those offshore,” says Johnson. “For instance, these ultra light weight generators can be installed in turbines mounted on significantly reduced foundations, even those that float, and their high-voltage dc output will make costly substations unnecessary. What’s more, the design handles vibrations better than legacy cylindrical magnetic generators, so much so that two bladed turbine designs are now feasible.”
The technology involved uses an electric field, which is an inherent property of a charged particle. “While the forces generated by static charges have been known since Faraday’s time, the materials did not exist until recently that would allow generating and holding an electric field of sufficient intensity to meet industry requirements for power density,” says Johnson. While the high insulation properties of the advanced materials and coatings in the new design are proprietary, their manufacturing methods come out of the aerospace industry. And there’s more. “The techniques allow for a fully automated process, thus our motors and generators can be produced in almost any locale without a price penalty.”
A little background
The new generator design is based on static-charge principles that Ben Franklin toyed with: Opposite charges attract and like charges repel. Although Franklin invented a spark-gap motor, he considered it a novelty and not worth further development. By the 1890’s Nicola Tesla had invented the induction machine. Motors and generators based on his designs were broadly adopted and the industry never seriously looked back.
But the large generators needed by today’s wind industry are pushing their limits and are so heavy, the static-charge design is getting a closer second look. For instance, one drive train for a conventional 5 MW unit in an offshore turbine weighs some 83 tons while a direct drive, permanent magnet generator with a similar rating weighs about 130 tons. Johnson says his direct drive Brisa generator –Spanish for breeze – will weigh less than 40 tons. This makes a floating wind turbine platform possible, a useful capability for greater water depths.
Johnson says the generator will eventually produce up to 100,000 Vdc and even higher. Hence, substations that typically house step-up transformers and frequency-synchronizing equipment at the wind farm will all be unnecessary. Using high voltage dc also eliminates the need to synchronize ac phases from different turbines and then again with the grid frequency. Transforming dc to the ac grid will happen closer to a point of use, providing greater efficiency in transmission. By some estimates, the U.S. ac grid loses 8% of produced power in transmission.
Eliminating the offshore substation also eliminates the heavy marine equipment needed to erect them. Johnson says the developments are applicable to onshore wind farms as well, but the more costly offshore work will see the biggest installation costs cuts, 25% or more by calculations from others in Johnson’s company, Electric Force Motors LLC.
The company also anticipates much shorter lead times for controls and support equipment, to about four months which improves wind farm development schedules. “However, that four month estimate is for once we move to production,” says Johnson.
For the designs in development, high voltage means 35,000 Vdc. “But we will be able to get up to 100,000 Vdc or more once we’ve better defined our manufacturing requirements and can test on a larger scale. About 132,000 Vac is a standard offshore transmission voltage,” he says.
Johnson adds that efficiency at low speed also improves over conventional designs. “The reason for that is the high “static” charge. Once the generator is charged, it can be removed from the rest of the system and the charge stays on the capacitor. It does not go anywhere. The charges are not in motion so there is no current. There are no I2R –power or heat losses – those common to induction machines in the electric field generator, even at stand still, because the charge is static or stationary. For this reason induction machines produce losses, especially at low speeds, because they are always running current.”
The way an induction generator is built today is much the same as Tesla built them in the early 1900s, says Johnson. It’s a cylindrical format with the stator and rotor made of laminated electrical steel. Each lamination is stamped and coated with insulation. These are pressed together into a component around which copper wire is wound to form a pole that will generate a magnetic field.
Two common ways increase power density in a generator: run more current or add more poles. Both have inherent limitations. “More current means producing what electrical engineers refer to as I2R losses. In essence, more current means more heat and as current increases to achieve greater power density, at some point it’s necessary to pull heat out of the generator with air or water cooling which lowers system efficiency.” To avoid using higher current in an individual coil, designers can also add more coils or more poles, but at some point they run into physical or space limitations. A more expensive alternative to higher currents or number of coils, is to use permanent magnets because they are a source of a static magnetic field and don’t need a copper coil with current to maintain their field. However, magnets don’t eliminate copper coils, they only lower their number. Magnets can also fail (crack or degauss) if they get too hot or cold. Commodity prices for some of their constituent rare-earth materials have fluctuated more than 300% in the last few years when China (which controls 97% of global supply) shifted market prices to drive out competition and to meet its own national interests. Because Johnson’s design uses the electric field it eliminates the use of anything magnetic, including permanent magnets.
Rather than generating forces from north and south magnetic fields with copper coils, Johnson’s design uses positive and negative charge on individual plates to attract the rotor. “Our system uses a thin conductive trace to store the charge. With this Electric Field Technology (EFT), the generator is no longer limited by the heat capacity or size of the copper wire. Now the limit is the manufacturability of a conductive circuit trace which can be orders of magnitude smaller than legacy copper coils. For this reason our generators and motors will easily achieve 10 or more poles per phase.”
Because the technology can put a significantly higher number of poles than a magnetic generator in an equivalent frame size, it can operate with high efficiency at significantly lower speeds than traditional units. “That will let us produce power at low wind speeds and do so without high maintenance gearboxes which is a huge cost and weight savings for wind turbine designers.”
Testing is set to begin in late September, about the time of this publication. The initial test unit is on a 100W motor and generator intended to prove that performance meets predictive models.
With the process defined, the next step will be to produce a 1 kW unit before the year is out. Johnson says field-worthy prototypes will include a 100 kW generator-motor in Q2 next year before building multi-MW field test units. Advanced product information is currently available for 2.5 MW and 5 MW models.
“The science is known, so we are not trying to prove anything new. The objective is to validate the material properties in a working generator. We’re also looking for vendors to grow and move to a larger volume. So it’s more about validating manufacturing techniques than anything else.” With regard to the control side and overall operation, says Johnson, “users will see no difference other than higher efficiency and lower installation and operating costs”. WPE
Filed Under: Generators, News