Inmodern wind turbine systems, large and complex power electronics are deployedto correct for frequency and voltage of the power produced. These powerelectronics, or more specifically full-power converters, go through significantthermal stresses and generate heat under load. 

Typically the components in these converters are air-cooledor have a water-glycol fluid loop. While both these technologies work well intheory, the limited efficiency of heat transfer system requires large volume orsurface areas and smaller delta T between the electronics and the coolingmedium. This in turn leads to using a higher than optimal number of modulesthus increasing complexity and cost of the system while reducing reliabilityand efficiency. 

Using precision cooling technology from Parker Hannifin'sClimate and Industrial Controls Group on these power electronics, thermalcycling of the components can be greatly reduced and complete system can bepackaged into a much more compact and modular space. This type of cooling alsoincreases the life of the IGBT modules and helps increase efficiency andreliability.

"There is a dynamic shift in powercontrol and conversion coming to the Wind Industry. As renewable energy becomesa larger part of the power-grid, regulations will require a much higher qualityof power be delivered from wind turbines to the grid. To comply with theseregulations, turbines will shift from using doubly-fed induction generators topermanent magnet synchronous generators with full power converters," says DheerajChoudhary, business unit manager for Global Renewable Energy at ParkerHannifin.

The doubly fed machines only useapproximately 30 percent of rated power as a rule of thumb for sizing of powerconverters if it is a variable speed turbine. So for a 3MW plate capacity, apower converter with a rating of 1MW is typically deployed. And with a 97percent average efficiency, the cooling system needs to remove only 30KW ofheat from the system at the most.

As the industry moves to permanent magnet synchronousgenerators and larger medium-speed generators, instead of six or 8 poles thegenerators will now have 192 poles and the armatures and rotors will be muchbigger. The turbines will require full power converters as well.  For a five MW generator at a 3 percent typicalpower loss, and then a 3 percent power loss at the power converter, we arelooking at 300KW of dissipated heat that needs to be rejected.

"With a 10X higher efficiency of the Parker's patentedsystem when compared to a water-glycol loop, we can dramatically reduce thefootprint of the cooling system because we are not flowing as much fluidthrough the loop or requiring a comparable surface area for heat transfer,"says Dale Thompson, business development manager for Parker. "The system is also hermeticallysealed, so there is never a need to de-ionize or filter the water, and there isno need for a secondary water loop. No consumables to replace and no service tobe performed, all valuable attributes especially for turbines at sea.  We can exhaust the heat into the air at theocean or convert into useful heat for heating components or space whererequired, so the system is flexible, movable, modular and there are fewerchances of breakdowns."

Parker started the development of this technology in 2004 tocool high power electronics, primarily IGBTs and SCRs used to do switching andconnecting in and out of the grid.

The primary method used on thesesystems was to pump water through the electronic circuit and connect to thebase of the IGBT. The system works well for small drives and converters but itfails to address the needs of scalability into the larger power systems. Whenyou look at three megawatt turbines and above, these large systems dissipate atremendous amount of heat.

"How we are doing it differently isby using a vaporizable, dielectric fluid and flow boiling it across the base ofthe silicon," says

Thompson.

Thompson says that with the current method of cooling, userstake the hot fluid and run it through a radiator to dissipate some of the heatin a continuous loop. But the problem is that the system can't have multipleIGBTs in that loop because each one is adding heat to the fluid.

"What we are doing is taking overwhen the fluid starts to boil.  If a potof water is starting to boil, you can turn the heat up all of the way and itjust comes out in the steam," says Thompson. "It's the change of state of thewater from liquid to gas that we're working on, or the latent heat ofvaporization. It's a more effective approach, and we can increase the powerdensity about 40 percent on the inverters."

The key is size, weight and powerespecially in the cell. The process can shrink the footprint, increase thepower density and create a hermetic system (basically a refrigerator) that isflow-boiling a liquid to vapor and never needs maintenance. A long life pumpoffers over 50,000 hours of continuous duty or over 10 years of life, andtesting over even higher hours shows no signs of wear, so the lifetime may bemuch longer.

"We are the only company in theworld doing this, and our sister division in Charlotte has created a "cooldrive" which offers a tremendous advantage," says Thompson. "The system, whenwe started, managed 800 kilowatts of power and had 27 IGBTs in three cabinets.We are down to 18 IGBTs with a little over a megawatt of power that it managesin one cabinet."

"The power converters are also beingused to store energy for the grid, so instead of a blade on the turbine idling,when the grid-demand for power isn't there, we are using the turbine to storeenergy into a megawatt of lithium ion batteries in containers that arepositioned at the renewable energy field," Thompson adds.  "That is the basis of our technology and, inessence, it is a very efficient and reliable heat pump. We're moving heat fromwhere it can do damage to where we can safely dissipate it or use it in a veryefficient, cost effective manner, while preventing any damage to theelectronics, either from heat or from the cooling system itself, which happensoccasionally in a water-glycol loop."