Tuesday, February 19, 2013

CNBC Tries To Recreate Tesla Test Drive





Today (February 19, 2013) CNBC the business news arm of NBC had its reporter Phil LeBeau reenact the fateful test drive of the Tesla Model S that John Broder of the New York Times performed three weeks ago.   The problem with the reenactment is that the weather today is 20 degrees (F) warmer than the chilly day Broder set out on his eventful drive (50 F versus 30 F). 

The press has simply missed the point of Broder’s report.  Broder did not set out to out the Model S on its range he simply set out to report on the fast charging stations Tesla is now promoting as part of their service offering.  Fortunately for us thermodynamicists the day Broder chose for the drive was a very cold day with daytime highs of only 30 degrees F and nighttime lows of 10 degrees F.  This is the kind of winter weather a Bostonian expects, a Chicagoan thinks is spring, and a Californian pays millions of dollars for a shack to avoid.

For me the weather Broder encountered was a realistic test of how much energy is sapped out of the batteries in the EV simply to keep the cabin warm.  I blogged in late 2011 that EVs were not suited for highway driving on cold North American winter’s days or nights.  

Highway driving at speeds of approximately 65 MPH implies a significant amount of heat loss from the vehicle especially through the windows (non-insulated surface of the vehicle).  The heat convection away from the vehicle’s surface is forced due to the high velocity of the air rubbing the surface.   In my 2011 blog I described that the heat transfer from the vehicle to the surroundings can be modeled and should approximate 40 watts per degree Kelvin of heat loss per meter squared of glass surface on the vehicle.  The degrees Kelvin of temperature difference between the interior of the vehicle and the outside air determine how many joules of energy transfer happen each square meter each second.

The CNBC test was performed with an outside temperature that is 20 degrees F warmer than when Broder drove.  This is about 11 degrees Kelvin warmer.  Hence the heat loss per second per meter square of glass was 440 joules greater when Broder drove.  The model S Broder used had the added option ($1,500) of an all glass panoramic roof.  LeBeau also had this option on his test car.  With the panoramic roof the car has a window surface area in excess of 6 square meters (63 square feet) and Broder’s drive required an extra 2,640 watts of heating to attain the same interior temperature as LeBeau.   This is like running two hair dryers at full blast full time while driving.

If one uses 2,640 watts for an hour one uses 2.64 kilowatt hours of energy sucked from the battery for the purpose of heating.  LeBeau reported his total time between DC and Boston was 8 hours.  With 1.5 hours stopped for charging LeBeau was on the move for 6.5 hours.  If the trip required 6.5 hours of driving a total of 17.16 extra kilowatt hours of battery power is needed simply for the sole purpose of heating the vehicle when the outside air temperature is 20 degrees F colder than it was today. 

The EPA rates the Tesla at 90 MPGE on Highway which equals 0.374 kilowatt hours per mile.  If LeBeau actually could have driven 245 miles on a full charge as the car reported to him he averaged 0.347 kilowatt hours per mile.  Phil LeBeau is no lead foot.    Per Tesla’s logs, Broder averaged the efficiency that the EPA predicted but had he driven on a 20 degree warmer day he too would have matched LeBeau’s performance.  Broder is also no lead foot.

Note I simply performed the calculation using the glass surface area.  The car’s cabin has significant metal surface area that also will lose heat to the surrounding air.  Since the car has carpets and door panels the loss through the metal surface is lower but could equal another 5 to 10 miles of range in the 540 mile trip in 20 degree F colder weather.

The results of this real comparison show the following:

Broder did not fake the problems with his test drive.  It was simply a case of cold weather and poor communications between Tesla and him.

Electric cars are not well suited for long distance highway driving in extremely cold weather. 

Don’t buy the panoramic roof option if you live in Boston.

CNBC should have performed the same drive in the same winter weather so as to not compare apples to oranges.

The Tesla cannot achieve 265 miles of range on a single 85 kilowatt hour charge in 30 degree F weather, the range is approximately only 225 miles.

In 50 degree F weather the range with an 85 kwh battery is approximately 245 miles as Phil LeBeau proved today

Tesla claimed 300 miles range (55 mph and no climate control) is not achievable under real driving conditions.

The EPA should investigate how they rate the Model S as 88 MPG City, 90 MPG highway that would result in a maximum range of 245 miles, yet Tesla claims a 265 mile rating from the EPA??
Phil LeBeau was dressed in a warm winter jacket inside the car while he test drove, I guess he was not toasty warm in the cabin of the Model S?

Here is a post on Tesla’s website dated November 29, 2012.  The temperature of that date was much like today’s temperature.  The Tesla owner pretty much achieved similar mileage as LeBeau (0.351 kilowatt hours per mile) and also drove very conservatively and used the heater sparingly.

Terry and Kevin | NOVEMBER 29, 2012
I live in NY and travel to VT frequently. Just completed my first drive on one charge of 220 miles each way. Range is definitely an issue here, as the 265 mile max range on full charge with the 85 KW battery will just get you to southern Vermont with no real battery life to spare. This is driving at 50-55 MPH, with mostly cruise control on where ever feasible. As for heat in the cabin, it works well, but if you increase the temp setting on the dash, it'll definitely decrease your range further. I averaged 351 wh/mile on each leg of my trip, driving VERY conservatively. Heating the interior is a real drain on the battery range, although using seat heaters (Front only available) is more efficient. Good luck to those sitting in back.

In summary Broder wrote an accurate and honest article about the Model S in cold weather conditions and Tesla does not attain the 265 mile range in winter in New England.  Broder achieved 225 mile range in 30 degree weather, Terry and Kevin as well as LeBeau achieved 245 mile range in 50 degree weather.  None of these drivers were toasty warm. 

My 1999 C280 keeps me toasty warm and I could do the 445 mile trip in 6 hours that LeBeau took 8 hours to accomplish.  My two hours of saved time and being toasty warm are worth more than the 70 bucks of gasoline I would have spent.  I am not sure 8 hours of Phil LeBeau’s time in worth the publicity Tesla will claim with this test drive that in no way replicated Broder’s ordeal.  

Broder was bloody lucky he did not report for the Winnipeg Free Press and did not try the test drive in Manitoba.  

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