Design concept for electric cars

(Originally entitled “Design concept for electric cars: short range car, extend range with rented towed trailer only on days when it’s necessary.”)

I was chatting on a mailing list, and we were mulling the basic problem of electric cars: short range. Batteries are very, very heavy compared to how much energy they can store, and they are expensive.

In principle, this shouldn’t matter, since on most days people don’t drive all that far before arriving at their destination—work, usually—and the car can be recharged there as they labor, then driven home. Given that the average commute in the USA is around sixteen miles and lasts 26 minutes, and even the average “bad day” commute is 46 minutes, it would seem that an auto with a range of sixty miles at sixty miles per hour before requiring a recharge would do just fine.

Things are not so simple. In practice, range matters a great deal; range-limited commuter vehicles have never fared well. An automobile is a major expense, and USAens are accustomed to being able to do anything that needs doing with a vehicle they own personally. The need for a long trip can come up with little warning… and ideally, it would be nice if we continued to be able to take long trips spontaneously.

On the list, Phil Gustafson came up with an elegant solution: when and only when it is needed, tow additional power equipment in a trailer.

The design concept is this: build the commuter electrics, but include the capability to draw power from a trailer being towed behind. (A short length of heavy wire and a plug will suffice to transfer the power.) Currently, the Tesla Roadster’s batteries mass about half a ton, can drive a sport’s car 221 miles, and cost about $30K. Assuming that much energy can drive itself and a commuter electric 120 miles, this extends the combination’s range that far. When the trailer’s batteries near exhaustion, pull into a service station, unhitch the spent trailer, and hitch up a new already-charged one. The station then hooks the old one up to recharging gear, beginning the process of readying it for the next customer.

Interestingly, this can be done with a relatively small total capital investment. Each vehicle that is undertaking a long trip on any given day will be making use of two trailers at any given moment on that day—the one dragged behind, discharging, plus the one ahead, which is recharging. $60K is a great deal of money… but relatively few vehicles need use of the two trailers on any given day, and for any given vehicle, odds are it won’t need them more than a few days a year. Even if we assume every vehicle spends a full 14 days a year—an entire two week vacation—on long road trips, that’s only 3.83% of the time. For each fleet of a thousand cars, there probably won’t need to be more than 80 of these trailers built and made available by rental companies.

Assuming a trailer price of $33K ($30K for the batteries, $3K for the chassis and wheels and whatnot), and a lifetime of three years, each day of use of two of them should go for around $60.23 wholesale, with a retail of perhaps $84.33 / day.

Obviously, the savings come from ensuring that highly expensive large banks of batteries aren’t spending most of their days sitting around doing nothing but twiddling their little electrochemical thumbs—they’ll be out working. The commuter electrics become perfectly affordable as well—60 miles worth of the Tesla Roadster’s 221 mile bank shouldn’t be more than $8145. Install that in a $10K body, and we have a reasonable pure electric for under $20K.

Better still, we have the option to make use of other battery technologies. Zinc-air comes to mind—very energy dense, but “recharging” them involves replacing spent zinc oxide paste with fresh zinc paste. A pain in the neck to do at home, but a reasonable light-industrial task for a service station—which would also serve as a collection point for the zinc oxide paste and dropoff point for zinc paste. And if a new battery technology comes along, it’s a lot easier to start building new trailers than it is to start building new cars; time-to-market drops significantly.

Note too that a trailer could instead carry a methanol-burning internal combustion engine, a generator, and a tank of methanol of essentially arbitrary size. Since unlike ethanol, methanol may be produced at a 100% yield rate from any mostly-cellulose material (including switchgrass, lawn clippings, kudzu, and so forth), such an approach would be carbon-neutral, with no need to burn food to fuel our cars.