July 6, 2011 § 4 Comments

The most obvious benefit of electric vehicles (EV’s) is the replacement of imported oil with electricity.  The zero emissions at the tailpipe are another plus, but as is often pointed out, the problem is merely shifted to the power generating plant.  Carbon sequestration is more tractable at such locations than at the vehicle.

A relatively less known fact is that electric vehicles use less energy than conventional cars.  To be specific, EV’s expend fewer units of energy to travel the same distance.  This is important because simply using less energy to receive the same gratification is a powerful arrow in our carbon mitigation quiver.  So, how much less energy do they in fact use?

In a departure from blogs of the past, we will calculate this right here.  We will use the following facts and assumptions:

  • A gallon of gasoline has 116,100 BTU which equals 34 kWh
  • The average car being replaced delivers 35 miles per gallon
  • For years the dogma has been that EV’s use 0.2 kWh per mile.  Nissan claims that the Leaf averages 0.25 kWh per mile.  As in all electric and hybrid cars, stop and go gives better mileage than continuous operation.  So, that number could be higher in some cases.  We will use the 0.25 number for this exercise.
  • Refining oil to give gasoline consumes 20% of the energy in the oil
  • Coal fired plants have efficiency of 40% (by using coal not gas we are being conservative, but this figure is that of newer supercritical combustors)
  • Electricity lost in transmission is 8%
  • Energy to get the oil out of the ground washes with coal mining.  Had we used the less conservative gas source for electricity, the offset would have been precisely correct

So, energy losses for gasoline prior to being consumed in the vehicle are 20%.  Energy used after combustion is:  34 kWh in a gallon divided by 35miles to the gallon, further divided by 0.8, equals 1.25 kWh per mile

Energy losses for EV’s are 60% at the generating plant, minus 8% in transmission, equals 32%.  Energy used by EV’s equals 0.25/0.32 equals 0.78 kWh per mile

Ratio of these two puts it at 1.6.  In other words, a conventional vehicle uses 60% more energy as an EV for the same purpose.  Is this exactly right, probably not, but it is not off by much.  The key take away remains that the EV advantage has a facet that is not commonly recognized in quantitative terms.

Implications to other oil replacement means:  The other principal avenues to replacing oil for transportation are:  natural gas fired vehicles, biofuels and gas-to-liquids derived fuel.  For the latter two, the energy used to produce is certainly worse than for conventional gasoline or diesel.  So, the EV advantage holds in this regard.  Natural gas powered vehicles offer some interesting possibilities.  The energy to produce should be somewhat less than for gasoline.  But the intriguing possibility for additional efficiency is in taking advantage of the high octane rating of around 125.  Diesel type compression ratios (14 to 22) would certainly provide more work per unit volume of gas, although I know of no move to capitalize on this opportunity.  But the sheer inefficiency of the Carnot Cycle will doom it to always compare unfavorably with EV’s.

The foregoing notwithstanding, oil replacement is too important an objective to not pursue all the alternatives.  For one, the alternatives discussed can be retrofitted to the current fleet.  EV entry will necessarily be slow and conventional vehicles will continue to be built.  We need to provide alternatives to imported oil to power them.

Implications to federal targets on vehicle mileage:  A recent story reports on negotiations between the White House and the auto industry on mileage standards.  The proposed target of 56.2 mpg by 2025 is almost double of the average today.  On the face of it, the only real impact of EV’s is that of an effectively very high mileage to offset the guzzlers.  I am not clear on how they calculate the mpg for EV’s given that no liquid fuel is involved except for Plug-In Hybrids when not in electric drive mode.  One way to do it would be to use the data shown above.

The Nissan Leaf drives 100 miles using 0.24 X 100 = 24 kWh.  Gasoline has 34 kWh per gallon.  So the gasoline equivalent of 24 kWh is 24/34 = 0.705 gallons.  So, the equivalent mpg for the Leaf is 100/0.705 = 141.8 miles per gallon.

The mileage target has two purposes:  reduce import of oil and reduce emissions.  The latter is recognized in that EV’s have zero tailpipe emissions, but we need also to take into account that the overall energy used is less.  Since the majority of electricity is produced using fossil fuels, using less per mile driven is a direct reduction in fossil fuel based emissions.   This somehow needs to be recognized in the mileage target debate.


Gallons Per Mile: Its Time May Have Come

July 6, 2011 § 1 Comment

Three years ago Richard Larrick and co-workers at Duke University proposed the logic of using gallons per mile instead of miles per gallon (mpg).  In their paper they pointed out that mpg was not linear and hence not intuitive in making decisions.  Such a decision may be for example when considering replacing one of your two vehicles with more fuel efficient ones.  Vehicle X is a land cruiser with 10 mpg and Vehicle Y is a compact with 25 mpg.  One would assume that replacing Y with a 40 mpg vehicle would save more than replacing X with a 20 mpg vehicle.  Not so.  Not even close.

Assume that you drive 200 miles a week.  Vehicle X would consume 20 gallons of fuel and the replacement would consume 10 gallons.  At $4 a gallon, the saving would be $40.

Vehicle Y would consume 8 gallons and the replacement would consume 5 gallons.  The saving would be $12.

This underlines the character of mpg: it is not intuitive for decision making.  Gallons-per-100-miles is completely linear.  Using that metric it is abundantly clear that the Vehicle A replacement results in more savings.

Three years ago that paper did not catch people’s fancy.  One possible reason is that we have difficulty with a smaller number being more desirable.  Also, gallons per mile would be a fraction; not wanting to deal in fractions forced the use of gallons per 100 miles or some such. The biggest reason was likely inertia and not wanting to replace the familiar.

But now it’s time may have come.  Electric vehicles (EV’s) could force the issue.  The easiest way to compute the “fuel” cost in EV’s is to determine the electricity used per mile and multiply by the cost of electricity.  EV’s are likely to be very similar to each other in the charge used per mile.  This is a huge departure from conventional vehicles, where the fuel efficiency can be dramatically different.  EV differences will primarily be premised upon body weight.  Other than that the variability will largely be in the type of driving: stop and go city driving will be more economical than distance driving, largely due to the regenerative braking systems.  This is of course the opposite of conventional cars and the EPA will have to consider this in their testing.

The capacity of an EV will be measured in kilowatt hours (kWh).  The Nissan Leaf has a capacity of 24 kWh and uses 0.25 kWh per mile, so it has a range of 100 miles for average driving on a full charge.  There you have it.  The kWh per mile is your gallons per mile analog.  For EV’s nothing else makes more sense as a metric because the consumer knows what she pays for electricity at home, and easily computes fuel cost.  In fact the ease of computation is going to be a key to driving the right battery charging behavior.  We expect utilities to incent night time charging by heavily discounting rates at night.  Absent that, significant day time charging could create serious problems for the grid.

Now, one could argue for EV’s to most usefully employ kWh per mile but for conventional cars to remain as before.  Sure, but when being responsive to federal fuel efficiency mandates, the mixed fleet will need to be assessed.  In so doing, the gallons per 100 miles, taken together with kWh per 100 miles for EV’s will allow direct comparison.  This is because we know that a gallon of gasoline has 34 kWh of energy.  So gallons per 100 miles could convert to kWh per 100.  Somehow I doubt the populace will go for this wholesale switch to kWh.  But changing from mpg to gallons per 100 miles will allow an easy computation of fuel efficiency of a mixed fleet and allow direct comparison with EV’s for consumer choice.

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