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Old 07-07-2010, 10:03 PM   #1 (permalink)
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Miles Per Gallon Equivalent by Four Different Methods

...and why EVs alone are of doubtful environmental benefit.

As more of us, myself included, are transitioning to plug-in vehicles of some type or another, the controversial topic of mpge is being discussed with increasing frequency. It's a complicated but important subject, as people will use mpge as a factor when deciding which car to buy and which mods to make.

We all have different goals and values. Some people want to reduce their carbon footprint, or reduce the rate at which we are depleting fossil energy resources, or rely less on petroleum, or just to save money. We won't all agree on a single KWh/gal factor because we have different values, but below I present four KWh/gal figures, and I recommend you use a figure somewhere in the middle. I will use 16KWh/gal.

24KWh/galCost Equivalencevaries with time and location
16.18KWh/galCO2 Equivalencefor US grid average
12.307KWh/galEnergy Equivalencefor US grid average
hundreds of KWh/galResource Depletion Potential for Renewable Electricityrenewable electricity is THE long-term solution to the transportation problem

The least contentious figure is cost equivalence. I pay $3.00 for a gallon of gas, and $0.12 for a KWh of electricity. 25KWh/gal.

The simplest KWh/gal figure is direct BTU equivalence: buring a gallon of gas releases 33.4KWh of heat. However, that's not a useful figure: it does not represent resource depletion, CO2 emissions, or any other item people care about.

If you wish to measure resource depletion or CO2 emissions, you must consider the entire life cycle of the fuel. The least efficient step in this life cycle is buring fossil fuels and converting heat to torque in a heat engine. For the EV, this happens in a power plant. For gassers, it happens under the hood. Direct BTU equivalence fails to capture this least efficient step for electrics but captures it for gas engines.

Using numbers from the DoE (see p3 column 2 which cites but does not include "Docket No. EE–RM–99–PEF"), the US grid average power plant is around 33% efficient, and transmission is 92% efficient, thus giving 30.3% efficiency for electric. Refining and distributing petroleum is 83% efficient per the same source. Thus the DoE finds a 12.307 KWh/gal figure by the life-cycle energy equivalence method. This is probably the right figure for academic comparisons of the overall efficiency of one architecture versus another. Unfortunately, it values a coal and petroleum equally, even though one is more versatile, rare, and troublesome than the other.

Take the above efficiency numbers and multiply by the values of 608g CO2/KWh and 8877g CO2/gal, and you have 16.18KWh/gal by CO2 equivalence. This varies with the carbon intensity of the grid. The number would be much higher in nuclear-powered France, and lower in coal-fired China.

If you're purchasing renewable electricity, then the CO2/KWh figure is very low (but certainly not zero; it takes diesel fuel to put up a windmill or to turn Si into a PV panel and ship it from Taiwan). Likewise, the resource depletion per KWh is very low. Unfortunately, we do not have a renewable-rich grid, and mass adoption of EVs at present would be a massive fleet of predominantly coal-fired vehicles with a massive CO2 footprint.

Consider a car that uses 250Wh/mi at the battery and 324Wh/mi at the wall. It gets:

38.0mpgeLife Cycle Energy Equivalence
49.9mpgeCO2 Equivalence
74.1mpgeCost Equivalence
107.1mpgeDirect BTU Equivalence

You can imagine which of these metrics EV advocates are pushing for, but in fact, they would underestimate the GHG emissions of a fleet of EVs by a factor of two.

If the X-Prize had used the more appropriate 16KWh/gal metric, only the X-Tracer electric Dalniks, the Li-ion EV, and the gas-fired Spira and Edison2 VLC's would still be standing. Aptera would have been eliminated with 67.8mpge before penalties.

Once more plug-in cars hit the market, we'll be able to make more direct comparisons between EV and HEV variants of the same car. Until then, study after study suggests a BEV with 100mi range (BEV-100) will be carrying enough weight in batteries to increase its CO2 footprint to larger than that of a HEV. A short-range plug in hybrid is a good compromise, with a recent Carnegie-Mellon study suggesting a PHEV-7 was optimal for minimum CO2 emissions.

There are other factors in the EV versus gasser debate that are beyond the scope of MPGe. These include the higher smog forming emissions of a gasser, the environmtental impact of oil spills, and mining for coal and rare metals to make batteries. There is also the matter of the cost of electric vehicles and windmills, and the rapid depreciation of current-generation high-density batteries.

It's wrong of the X-Prize Foundation to fail to look at the big picture when calculating MPGe. It's also incorrect to call EV's "zero emissions" (yes, that claim has been made) or even that they're low-carbon. EV's will allow us to burn diverse fuels and weather a petroleum crisis with less disruption (which is bad news for environmentalists, as fuel crises drive conservation).

We need renewable electric generation FIRST, and EV's afterward. I am voting with my dollars by selecting my utility's renewable electricity plan, and I'd encourage you to do the same. Your representatives in government also need to hear that it's an important issue to you, as I hope it is.

When confronted with an MPGe figure, I hope you'll be a little skeptical and dig for a Wh/mi figure. Then consider which KWh/gal figure makes sense to you, and come up with your own mpge figure. I recommend 16KWh/gal as a weighted average.

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