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Advanced Cooling for Wind Turbine Generators

In modern wind turbine systems, large and complex power electronics are deployed to correct for frequency and voltage of the power produced. These power electronics, or more specifically full-power converters, go through significant thermal stresses and generate heat under load. 

Typically the components in these converters are air-cooled or have a water-glycol fluid loop. While both these technologies work well in theory, the limited efficiency of heat transfer system requires large volume or surface areas and smaller delta T between the electronics and the cooling medium. This in turn leads to using a higher than optimal number of modules thus increasing complexity and cost of the system while reducing reliability and efficiency. 

Advanced Cooling for Wind Turbine Generators
Using precision cooling technology from Parker Hannifin's Climate and Industrial Controls Group on these power electronics, thermal cycling of the components can be greatly reduced and complete system can be packaged into a much more compact and modular space. This type of cooling also increases the life of the IGBT modules and helps increase efficiency and reliability.

"There is a dynamic shift in power control and conversion coming to the Wind Industry. As renewable energy becomes a larger part of the power-grid, regulations will require a much higher quality of power be delivered from wind turbines to the grid. To comply with these regulations, turbines will shift from using doubly-fed induction generators to permanent magnet synchronous generators with full power converters," says Dheeraj Choudhary, business unit manager for Global Renewable Energy at Parker Hannifin.

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

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

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

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

The primary method used on these systems was to pump water through the electronic circuit and connect to the base of the IGBT. The system works well for small drives and converters but it fails to address the needs of scalability into the larger power systems. When you look at three megawatt turbines and above, these large systems dissipate a tremendous amount of heat.

"How we are doing it differently is by using a vaporizable, dielectric fluid and flow boiling it across the base of the silicon," says
Advanced Cooling for Wind Turbine Generators
Thompson.

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

"What we are doing is taking over when the fluid starts to boil.  If a pot of water is starting to boil, you can turn the heat up all of the way and it just comes out in the steam," says Thompson. "It's the change of state of the water from liquid to gas that we're working on, or the latent heat of vaporization. It's a more effective approach, and we can increase the power density about 40 percent on the inverters."

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

"We are the only company in the world doing this, and our sister division in Charlotte has created a "cool drive" which offers a tremendous advantage," says Thompson. "The system, when we 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 manages in one cabinet."

"The power converters are also being used 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 store energy into a megawatt of lithium ion batteries in containers that are positioned at the renewable energy field," Thompson adds.  "That is the basis of our technology and, in essence, it is a very efficient and reliable heat pump. We're moving heat from where it can do damage to where we can safely dissipate it or use it in a very efficient, cost effective manner, while preventing any damage to the electronics, either from heat or from the cooling system itself, which happens occasionally in a water-glycol loop."
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