60 January/February 2024 | E-Mobility Engineering battery cells. “What we have developed is manipulating the boiling process at very low heat densities so we manage to have great heat dissipation capabilities. In normal operation this leads to low heat generation, but in a pack it is large amounts of heat so we reduce the temperature difference between the boiling point and the cell.” While many cooling systems have a thermal gradient of 10 to 15 C, the dual phase immersive cooling has a gradient of just 1.5 to 2 C, which allows lighter enclosures so the system is more efficient. For an operating temperature of 25 to 26 C for the pack the target boiling point of the liquid will be 24 or 23 C within atmospheric pressure, although Carrar can work in a slight vacuum and positive pressure meaning that can be a temperature of anywhere from 15 to 35 C as Friedman explains. “It’s easier to design for positive pressure, harder for vacuum and because the pressure range is low it is not for a very deep vacuum,” Friedman said. “This type of fluid handling is an engineering challenge we are solving all the time. It’s easier to work in positive pressure for the other components,” he said. Carrar controls the pressure inside the pack, typically from 0.7 bar to 1.1 bar, so that the boiling point of the refrigerant will be at the required temperature. The fluid then goes as vapour to a compressor and to a condenser to bring it back to liquid and then goes back to the modules. Keeping the temperature at 25 C allows fast charging and discharging at rates of 3C or even higher. “When you are talking about phase change you are talking about the specific heat to latent heat transfer and with latent heat there is almost no limit to the amount of heat you can remove in theory,” Friedman said. The limit is the condenser. A rule of thumb is that a condenser is four times more efficient than a radiator. “We have shown that we can extend the time of the high C rate; eg for 2C or 3C we can do it continuously. For a 60 x 40 mm condenser that is 30 mm thick we can dissipate 25 kW while keeping the batteries at 25 C.” The biggest reservoir is the battery modules, but the gas phase from the outlet of the modules travels to the condenser and there is a small volume to pump some of the refrigerant back to the modules. A 100kW battery pack typically uses 20 litres of dielectric in the pack with 2 litres circulating in the rest of the system. Compressor Another part of the system is a compressor, which is only needed for specific applications such as high ambient temperatures or for fast charging. “It depends on the ambient temperature if we need a compressor; it’s usually only needed for high ambient, say at 45 C, or for fast charging,” Friedman said. “We are developing a specific compressor for our system with a coefficient of performance (COP) of 5, so with 5 kW input we can dissipate 25 kW of heat with a typical 2 litre volume. The compressor sees only the vapour and this is one of the challenges; to compress the vapour to 3 or 5 bar at 55 C at the inlet to the condenser. “We need to keep the refrigerant as close to the boiling point as possible so we design the system to the extreme points; 45 C or -30 C, and usually these are the challenges,” remarks Friedman. “In the medium range the system is much more efficient as the compressor can control the speed of converting the vapour to fluid and in this way manage the pressure differences.” Cooling pumps Using immersive cooling also fundamentally changes the design of the e-mobility platform. With today’s IC engine systems there is usually a large pump that works with the cabin heating and air conditioning system (HVAC) and those hoses are relatively short. The move to EV designs means the need for cooling is more distributed throughout the vehicle, requiring longer hoses. “We want to replace the radiator system with the condenser so the only additional component is the compressor”, said Friedman. “We Deep insight | Immersive cooling The IMMERSIO Cell-to-Pack (CTP) system supports packs with an energy density of up to 200 Wh/kg and a fast charge capability of less than 15 minutes from 20-80% SoC (State of Charge) using high nickel NMC cells and a mineral oil single phase coolant. The pack includes a Cell Management Unit (CMU) with an Intelligent Active Safety Module that monitors and controls the temperature, voltage, and current of each cell (Image courtesy of XING Mobility)
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