Air- or Water-Cooling Resistors for EVs: The Case for H2O

REO water-cooled resistor. (REO)

The choice between water cooling and air cooling for internal-combustion engines was for decades a very real debate in the auto industry, with Volkswagen and Porsche for years successfully campaigning for air-cooling—against the majority of OEMs—before eventually bowing to the pluses of H2O.

Now the air-versus-water issue reemerging—this time in the context of cooling the electronics of electric vehicles (EVs).

REO U.k> chief executive Steve Hughes: "The benefit of using a water-cooling resistor in an EV is that it can be easily integrated into existing cooling systems of the vehicle and is able to deliver cooling on the areas where fans often fall short.” (REO)

Efficiency and reliability of battery performance is crucial and overheating needs to be avoided both to deliver required performance and, particularly, driving range. Totally effective EV thermal management is a must—and that includes cooling resistors used to dissipate heat and slow down a mechanical system to avoid overheating.

“Brake resistors have relatively low ohmic values, a high power rating and are typically encased in a frame to create a safe distance between surrounding components. To increase its dissipation capability, the resistor’s frames feature cooling fans or liquid coolants,” explained Steve Hughes, UK Managing Director of German electrical-components specialist and manufacturer, REO, which has developed a water-cooled brake resistor.

Most manufacturers understand the importance of effectively addressing and understanding heat-transfer considerations early in the design process, saving time and minimizing prototyping costs. “But despite this, many OEMs are unsure how to choose between the two main ways of regulating device temperature: air-cooling or water-cooling,” said Hughes.

Water: lower cost, reduced energy consumption

Air cooling has been most commonly used because it is relatively inexpensive and requires little maintenance, other than filter replacement. Also, the fan is invariably a robust component able to endure some damage (such as losing a fin) without significantly impacting performance, while also remaining functional and safe.

But Hughes cites two minuses of air-cooling: noise (anathema to EV designers) and packaging, with typical air-cooling enclosures creating space challenges.

REO’s water-cooled D350 braking resistor converts excess electricity that is not needed for recharging an EVs battery into heat that can be dissipated safely or as energy to be regenerated into the vehicle’s power supply.

“This is an effective use of energy that would otherwise simply be lost. It also reduces the need for (cabin) resistance heating using the battery’s energy, which can reduce the vehicle’s driving range,” stated Hughes. “The benefit of using a water-cooling resistor in an EV is that it can be easily integrated into existing cooling systems of the vehicle and is able to deliver cooling of the areas where fans often fall short.”

Water-based systems operate on a simple principle of heat exchange, with pipes containing a liquid coolant circulating around an enclosure to keep components cool. Any excess heat generated during use is thermally conducted by the water in the pipes; that transferred heat energy then can be regenerated for other processes.

EVs must be as efficient as possible and Hughes said that not only are fans in air-cooling systems limited by their reliance on ambient temperatures to cool overheating components, but depending on the size of the applications, multiple fans may be necessary.

His arguments for use of water cooling also include its high heat conductivity and requirement for only a single pump to cool infrastructure, “significantly” lowering the overall energy consumption and cost of the cooling system.

Water cooling is the only method that permits cooling at temperatures less than the ambient air temperature and coolant can be stored in the pipes below the application’s ambient level for even faster cooling, Hughes added.