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.
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
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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