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There are many well-known challenges associated with reducing the charging time for electric vehicles, including the amount of power available at the charging station, the charging station power-conversion electronics, the power cable carrying the power from the charging station to the vehicle and the charging subsystem within the vehicle itself.
You can add another one to the list: the heat developed via I2R dissipation as hundreds of amps flow through the power cable, even with its low resistance on the order of an ohm and less. Reducing that current to keep the heat within allowed bounds has an immediate impact on charge time.
Note that power cable self-heating is not unique to charging EVs: The National Electrical Code in the U.S. (and similar codes elsewhere) has detailed tables for derating AC power cables, showing maximum allowed ambient temperature at different current levels. There are also new restrictions on packing these self-heating power cables in bundles, as well as restrictions on laying low-voltage power cables, such as Power over Ethernet IEEE 802.3bt (Type 3/60 W and Type 4/100 W), alongside these higher-power AC cables.
Recognizing this barrier as an opportunity, an experienced team led by Issam Mudawar (Betty Ruth and Milton B. Hollander Family Professor of Mechanical Engineering at Purdue University) analyzed, devised and tested a way to increase the current-carrying capacity from its present maximum of 520 A to over 2,400 A. Mudawar has been working for 37 years on ways to more efficiently cool electronics by leveraging how liquid captures heat when boiled into a vapor. The work was done in conjunction with the Gwangju Institute of Science and Technology in South Korea.
That 520-A figure was the highest by far when they did their work, as many chargers are well below that (Figure 2).
To achieve this significant increase in capacity, the team employed fluid-based cooling for the power cable. The team accomplished this via rigorous fluid and thermal modeling, detailed equations and additional insight into the dynamics of cooling.
The project began with a basic thermal model of the power cable (Figure 3), but that does not begin to reveal the subtleties and real-world considerations they took into account as the fluid-dynamics modeling progressed. They added complex equations characterizing the system dynamics from multiple perspectives, including advanced modeling and a subsequent simulation tool kit. By capturing heat in both liquid and vapor forms, a liquid-to-vapor cooling system can remove at least 10× more heat than pure liquid cooling.
While it’s one thing to model and simulate with a high degree of sophistication, it is another to build and test what these indicate. The team’s arrangement involved pumping highly subcooled dielectric liquid, HFE-7100 (an engineered non-flammable fluid used in various critical industrial applications). The high boiling point and low surface tension through a concentric circular annulus mimicked a segment of an actual power cable, with its uniformly heated 6.35-mm–diameter inner surface representing the electrical conductor and adiabatic 23.62-mm–diameter outer surface for the external conduit.
At these levels of power, heat and fluid flow, the basic plumbing itself is complicated, as are the various transducers needed to control and measure the parameters of interest (Figures 4 and 5).
Their prototype mimics all the traits of a real-world charging station: It includes a pump, a tube with the same diameter as an actual charging cable, the same controls and instrumentation, and it has the same flow rates and temperatures (Figure 6).
Their work shows just how complicated the EV charging problem is as it reaches higher currents, as well as the advanced efforts needed to understand and solve these problems. It is described in extreme detail in two somewhat overlapping and very lengthy papers in the International Journal of Heat and Mass Transfer. Each is packed with copious analysis, modeling data, detailed tables, insights and results. The papers also thoughtfully include a full page defining all the nomenclature, Greek symbols, subscripts, acronyms and abbreviations used.
The two papers, with intensely lengthy titles and just three authors, are accessible at Purdue’s site here and here, respectively.
Have you ever been involved in a high-current power-rail design in which I2R dissipation created a thermal problem, separate from IR voltage drop? How about even a lower-current (under 10 A) situation, where the power-cabling gauge was inadequate and offered excessive resistance, leading to dissipation?
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