Keep it cool

By Meghan Boyer

May 2011 - From electric vehicles to solar and wind energy installations, the markets for alternative energy are growing at different rates and represent a massive opportunity for companies that supply them with technology, says Memo Romero, president and CEO of MaxQ Technology LLC, Tempe, Ariz.

MaxQ develops aluminum liquid-cooled cold plates for use with high-power inverters in different applications including electric vehicles, wind turbines and high-speed rail. In the coming years, Romero expects to be shipping thousands of units annually as alternative-energy markets continue to expand.

His company has developed thermal-management products for power converters ranging from 60 kilowatts up to 2 megawatts capable of rejecting up to 50 kilowatts of heat. MaxQ's Q-Chill cold plate technology has been developed specifically for cooling high-power inverters in the renewable energy, industrial and military markets.

Liquid-cooled cold plates remove heat from high-current, high-voltage, power-switching semiconductor modules. For example, in an electric vehicle, an inverter takes power from the battery unit, which supplies DC voltage, and converts it to AC voltage needed for variable speed motor control, says Romero. "That power inverter has semiconductor modules that get hot as they operate," he says. The cold plates, mounted directly to the semiconductor modules, transfer heat away from them as coolant is pumped through.

Aluminum is a good material for the cold plates because it weighs less than some other metals, it can be welded or brazed and it has high thermal conductivity, says Romero. "There are other materials that are more conductive than aluminum, most notably copper, but they are too expensive and too heavy for high-volume use," he says. "Aluminum really does have the best combination of properties."

MaxQ is working with Sapa Extrusions Fabricated Components, Portland, Ore., a division of Sapa Group, Stockholm, to produce the aluminum cold-plate products. "Sapa is a leading aluminum extrusion company, and they are also one of the companies that helped develop the friction-stir-welding process," which is used to assemble the cold plates, says Romero. The collaborative relationship includes early-stage discussions on new designs, and Sapa provides production capabilities, supply chain infrastructure and quality assurance for MaxQ.

"We work together to make sure the design is going to work and that it's going to be manufactured," says Ray Goody, product manager of engineered products with Sapa Extrusions. The company's aluminum extrusion, machining and fabrication capabilities help lower production costs for MaxQ. Additionally, Sapa's global presence enables MaxQ to participate in business in the United States as well as Asian and European markets.

Friction stir welding
Friction stir welding is a beneficial process for creating the liquid-cooled cold plates because no additional materials are introduced to join the aluminum pieces together, says Romero. "If you are forming an aluminum-enclosed liquid cold plate that requires welding and if you use an arc or TIG welder, you will introduce other metals that can create corrosion or metal fatigue issues down the road, especially for long-life products such as wind turbines," he says. "With friction stir welding, you are just simply joining aluminum to aluminum with no other materials."

MaxQ uses the friction-stir-welding technology primarily with aluminum. "It's a process that can permanently join two adjacent parts. Through friction caused by a specially designed rotating tool, the aluminum is softened up in such a way that it can be mixed at the seam," says Romero. The cylindrical tool can then be moved along that boundary using standard computer numerical control software to complete the weld along an arbitrary path.

The friction-stir-welding process and manufacturing methods deliver a leak-free design, says Goody. Another option to attach the lid to the manifold cold plate is dip brazing, but this requires heating the complete unit up to very high temperatures, which can introduce distortion into the product, he says. A third option is gaskets, but they tend to leak.

Where each cold plate is fabricated depends on the size of the design and on specific customer requirements, says Romero. MaxQ has friction-stir-welding capabilities at its facility and can produce low to medium quantities of relatively small designs measuring roughly 12 inches to 18 inches long. The facility is not set up to produce high volumes or large cold plates. For such orders, Sapa fabricates the cold plates at its facility. Sapa recently made two units for MaxQ that measure 42 inches long by 17 inches wide capable of dissipating more than 12 kilowatts of heat.

Ongoing process
Very little standardization exists in the liquid-cooled cold plate market, which requires that most cold plates be custom ordered and designed to meet system-level specifications. "Every design is a little different. Every wind energy supplier has their own type of convertor," says Romero. MaxQ and Sapa must work together on each project to determine the design requirements and how to create the cold plate so it will meet each customer's size and reliability specifications.

"The first thing we examine is the layout and general topology of their power circuit," because it determines the size of the cold plate and the physical arrangement of the power semiconductor modules, says Romero. "Some customers have several modules all lined up in a row. We have one customer that has nine modules side-by-side, so it ends up being 40 inches long," he says.

MaxQ takes all of the customer information and creates a design that it believes will meet the requirements. Extensive computational fluid dynamic simulation is performed to ensure success in the application. Throughout this design and modeling process, the MaxQ team works with Sapa to optimize the design. "If you look at the whole process, it starts with working with a customer and also working with Sapa. There's extensive and necessary interaction to come up with the final design that satisfies not only the customer but results in a manufacturable product," says Romero.

For each cold plate, there is a multistep development process the companies must go through, says Goody. Sapa is working to develop best practices at its facility for creating the products. "There are multiple steps, so it does take a long time to develop the process," he says. "We are going to take that time to ensure we have an engineered solution that uses sound principles and sound practices so once we go into production, we have got a viable product that is going to work for us here in-house for manufacturability and is going to work for Memo and his customers for the intended use of the product."

The more cold plates Sapa and MaxQ create together, the more the two teams understand each other, says Romero. "There's a lot of give and take. I think as we've gone through this process a few times, we know which questions to ask," he says. "I think Sapa also has come to understand a lot more about the nature of our designs and as a result, has continued to play a key role in advancing the technology." MM


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