Fluid-cooled cable quadruples EV charging current capability

What you will learn:

  • New cooling technology has been proposed for the high current charging cables of EVs.
  • The system relies on the use of a sub-cooled boiling stream to remove heat from the cable.
  • Experiments were performed to acquire data for subcooled boiling in rings using HFE-7100 as refrigerant.

Many challenges remain when it comes to reducing the charging time of electric vehicles (EVs). These include the amount of energy available at the charging station, the charging station power conversion electronics, the cable carrying electricity from the charging station to the vehicle, and the charging subsystem in the charging station. the vehicle itself. (Fig. 1).

Each of these obstacles requires new approaches and solutions, which can be tackled by different industry specialists. Of course, when it comes to charging currents of several hundred amps and above, each solution requires extensive analysis, testing and verification to ensure that it performs well, reliably and safely. security.

Obviously, the amount of current delivered to the VE is an important factor in determining the required charging time. A team at Purdue University is focusing on this charging cable and has analyzed, designed and tested a way to increase the current carrying capacity from its current maximum of 520A to over 2400A. They note that the figure 520-A is by far the highest, as many chargers fall well below this (Fig. 2).

The cooling benefits allow using a smaller wire diameter inside the charging cable and dissipating a higher current, while removing just over 24kW of heat. “My lab specializes in finding solutions for situations where the amounts of heat produced far exceed the capabilities of today’s technologies to remove,” said Issam Mudawar, Professor of Mechanical Engineering at Purdue Betty Ruth and Milton B. Hollander.

Fluid based cooling

To achieve this significant increase in capacity, the team used fluid-based cooling of the cable, as even a tiny amount of inevitable resistance in the cable and its connectors results in a large amount of I2R heating. The team achieved this through rigorous fluid and thermal modeling, detailed equations, and additional information on cooling dynamics,

The project started with a basic thermal model of the cable (Fig. 3), but that doesn’t begin to reveal the intricacies and real-world considerations that they took into account as the modeling of fluid dynamics progressed. They added complex equations characterizing system dynamics from multiple perspectives to the advanced modeling and subsequent simulation toolkit.

Their objective was to develop a consolidated theoretical / empirical method to predict the heat transfer and pressure drop characteristics of laminar and turbulent flows through concentric circular rings with a uniformly heated inner wall and an adiabatic outer wall. By capturing heat in liquid and vapor form, a liquid-vapor cooling system can remove at least 10 times more heat than pure liquid cooling.

While maintaining a sub-cooled boil along the entire cable is a key practical goal, this consolidated method has been found to be able to tackle multiple flow regimes (single-phase liquid, sub-boiling). cooling, saturated boiling and single-phase steam). Their model is very effective in predicting local surface and fluid temperatures.

Build and test the model

If it’s one thing to model and simulate regardless of sophistication, it’s another to build and test what they indicate. The team’s arrangement involved pumping highly subcooled HFE-7100 dielectric liquid through a concentric circular ring mimicking a segment of an actual cable. A uniformly heated interior surface of 6.35 mm in diameter represented the electrical conductor and an adiabatic exterior surface of 23.62 mm in diameter for the outer conduit.

At such levels of power, heat, and fluid flow, the basic plumbing itself is complicated, as are the various transducers needed to control and measure and the parameters of interest. (Fig. 4 and 5).

Although the prototype has not yet been tested on electric vehicles, Professor Mudawar and his students have demonstrated in the laboratory that their prototype can withstand a current of more than 2.4 kA. (Note that he has been working for 37 years on ways to cool electronics more efficiently by taking advantage of the way liquid picks up heat when boiled into vapor; by capturing heat in liquid and vapor form, a liquid-vapor cooling system can remove about an order of magnitude more heat than liquid cooling alone.)

Their prototype mimics all the characteristics of a real charging station: it includes a pump, a tube the same diameter as a real charging cable, the same controls and instruments, and maintains the same flow rates and temperatures. (Fig. 6).

For more details…

The work is described in tremendous detail in two somewhat overlapping articles, each one of the longest I have ever seen in an academic journal. The articles include extensive analysis, modeling data, detailed tables, information and results. They also carefully include a full page defining all nomenclature, Greek symbols, indices, acronyms and abbreviations used.

The two articles with long titles, but only three authors, are “Experimental study of a subcooled flow boiling in rings with reference to the thermal management of ultra-fast charging cables of electric vehicles“(20 pages) and”Consolidated theoretical / empirical predictive method for subcooled flow boiling in rings with reference to thermal management of ultra-fast charging cables of electric vehicles”(22 pages). Both are behind pay walls, but accessible copies are posted on the Two-Phase Thermal Management and Flow Management Laboratory (TPFTML) webpage at Gwangju Institute of Science and Technology, South Korea. ). here.

The project was funded by a research and development organization alliance between Ford Motor Co. and Purdue. The researchers applied for a patent for their charging cable invention.

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