原创 Supercaps to power electric vehicles?

Supercapacitors (also called ultracapacitors) are a relatively recent fundamental technology innovation for passive devices, with the first ones coming to market in the 1970s with widespread use by the early 1990's. Prior to their development, the "conventional wisdom" and textbook view that even a one-farad capacitor was impractical for real designs, as it would be the size of a desk. Yet today, the supercap is a standard component in the engineer's bill of materials (BOM) kit.

These capacitors have both advantages and disadvantages compared to rechargeable batteries. They typically can store 10 to 100 times more energy per unit volume or unit mass than standard electrolytic capacitors but have only about 1/10 the energy density of batteries (and thus are physically larger for a given amount of energy); can be charged and discharged more quickly than batteries; and tolerate many more charge/discharge cycles than rechargeable batteries. In many designs they replace or complement batteries for short- or long-term backup and operation.

So what about using them in electric vehicles (EVs) and hybrid electric vehicles (HEVs), instead of the battery packs? So far, none of the commercially available EVs and HEVs use them, as far as I can tell. I'm not a battery expert, but I suspect it is a combination of factors: size, cost, perhaps power-management issues, difficulties in using them in series and parallel combinations, failure-mode issues, and other factors. I am sure the technical experts at EV/HEV vendors have considered them and decided they don’t make sense, at least at this time.

But that hasn't stopped people from speculating, and this speculation can make it all sound so easy. I recently saw that NASA Tech Briefs gave an honorable mention to an idea – and I emphasize the word "idea" – of using an array of one type of supercap for energy storage in an EV with a 3000-mile (approximately 4800 km) range, see here .

Wow, that's impressive…until you realize that this idea is entirely speculative. The detailed contest entry freely uses words like "revolutionary," "easy," and "standard" in the discussion of a large array of multi-layer ceramic capacitors (MLCCs) with dielectric constant (the ratio of the permittivity of a substance to the permittivity of free space) of 300 million.

I'm not saying that such a design isn't possible; we know that when it comes to technology advances, you should "never say never". Nonetheless, the issues associated with such a dense power pack in an EV go beyond the storage component itself. The author proposes an array of 12,000 MLCCs of 5.5 F each, for a total of 66,000 F.

That's an amazingly high energy density and capacity, which bring in major issues of safety and actual system design. How do you reliably connect all these MLCCs? What happens when one or more MLCC fails open, or shorts internally? How do you deal with the large current flows and high voltages into and out of such a dense package?

Talk to any engineer who has worked with high-energy battery packs ranging from relatively smaller ones used in laptops to larger ones in EVs, and you'll hear that the reality is that the battery/supercap itself is only part of the design and manufacturing challenge. There are many other issues such as internal interconnects; external connections; charge/discharge management; current, voltage, and thermal monitoring; and overall safety monitoring and protection. While it is easy to say that those are all manageable issues that can be easily overcome (a project manager I worked for casually declared these peripheral functions to be "mere details"), these are actually all very difficult problems, especially in a high-volume, manufacturing-oriented product.

We often hear reports about the next big thing in batteries (or supercaps) promising five or even ten times the density of today's best units. Yet most batter progress over the past decades has been through modest, incremental improvements adding onto each other, rather than the one big breakthrough. A balanced and perceptive article "Tech World Vexed by Slow Progress on Batteries " in The Wall Street Journal (of all places) pointed out that the time and effort to get a new battery enhancement to the mass market is about ten years, and many promising advances in the lab don’t make it to market adoption due to manufacturing, material, and functional issues, even if the underlying technology is sound.

Will an array of MLCCs be the next big thing for EVs? I'll admit it: I don’t know. I do know that when someone says it will be easy, and yet has never actually built and tested an actual unit, it's a good idea to be skeptical.

What's your experience with high-energy/high-density batteries and supercaps? Have you even been involved with projects that seemed so easy in concept, until you had to bring them into a moderate/high-volume manufacturing situation?