Your next tow vehicle?

The hype is strong with Musk (see the Coyote: Elon Musk Is The Master of Yelling “Squirrel” ). The latest is his tractor. See Tesla Semi: 500-Mile Range, Cheaper Than Diesel, Quick to Charge By Bill Howard. The key item of interest is energy storage and both the Coyote and Howard comment on this.

The Coyote says Here Are the Two Problem With EV’s — energy density and recharge time.

15 gallons of gasoline weighs 90 pounds and takes up 2 cubic feet.  This will carry a 40 mpg car 600 miles. … the Tesla gets  0.22 miles per pound of fuel/battery while the regular car can get 6.7.  That is a difference in energy density of 30x.
 The problem is that it is MUCH faster to refill a tank of gas than it is to refill a battery with a full charge.

and here is Howard on the tractor:

As for battery capacity, Tesla gave us a hint: Tesla says the Semi uses “less than 2 kilowatt-hours” of energy per mile. Based on vehicle battery packs in use now, we know lithium-ion battery packs of at least 50 kWh weigh about 15 pounds per 1 kilowatt-hour of stored energy. So if the Tesla Semi uses 1.5 kWh per mile and travels 500 miles, that means the battery is 750 kWh and weighs 11,250 pounds. If consumption is closer to 2.0 kWh per mile, the battery at is as much as 1,000 kWh — 1 megawatt-hour — and 15,000 pounds.

Now, if you want to recharge via solar, a typical cargo container has 320 square feet of roof. At 15 watts per square foot, this could accommodate about 4800 watts of solar panels. At 2 kWh/mile, bright sun might get enough energy from the roof of that typical semi-trailer cargo container in an hour to run the vehicle 2 miles.

Another way to look at this is that, at a 60 mph highway speed, the tractor would need a 30 kW power delivery. (60 miles, 2 kWh/mile, 1 hour). A gallon of diesel has about 38 kWh of energy storage. Since the battery and electric motor efficiency is about three times that of a heat engine like the diesel, these calculations indicate the electric tractor running at the equivalent of about 5 miles per gallon. That sounds a bit low but isn’t that far off.

I wonder what an RV park would do if you pulled in for the night to recharge your tow vehicle from a day’s driving. They aren’t geared up for covering a night time use of 400 kWh energy draw (2 kWh/mi and 200 miles). At ten cents per kWh that would be $40 of electricity and most places have more expensive electricity. Compare that to the current headache, A/C on a hot day, where a large RV might pull 30 – 50 kWh. That’s an order of magnitude increase in energy needs.

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