Does a Nissan Leaf or a GM Volt really use considerably less energy than a comparable conventional compact or midsize sedan? Could the same reasoning given for seductive ”100+ mpg” and “Zero Emissions” stickers be used for more insidious purposes? With the first shipment of 2011 model electric passenger vehicles ready to hit the streets and plug-in hype at an all-time high, it’s worth considering the implications of the EV methodology from the EPA (no, not the Electric Plug-in Association) and all its faults for another measure used to compare energy options: the energy return on investment (EROI) of primary fuels. In fact, the same rationale used to justify head-scratching mileage ratings is used in a misleading context to make sources of renewable energy, such as the sun, appear less attractive in terms of energy return than conventional dirty coal and oil.
The biggest problem with 100+ mpg ratings for vehicles of the plug-in type is that primary fuels, or slightly refined derivatives thereof, are compared directly to units of electricity. It should be noted that electricity does not simply come out of the ground as do coal or oil; it is a highly refined and nearly universal energy carrier whose versatility gives it far greater utility to society than raw fuel with its limited direct uses. But according to the methodology of EPA, “a BTU is a BTU,” and units of electrical energy can be compared directly to the energy content of primary fuels such as gasoline and diesel fuel. This is rather like comparing oranges to orange juice. The high apparent efficiency of electric vehicles derives from the actual inefficiency in converting thermal energy to usable electric or motive energy (and in fact the conversion between these two forms is also close to 100% efficient). If you want to know how many “gasoline gallon equivalents” any vehicle is actually using, simply multiply the given mileage using the EPA method by the conversion efficiency of the power plant used to generate its motive power. In the case of a conventional vehicle, the power plant sits under the hood. In the case of the typical U.S. grid mix, the power plant sits on somebody else’s backyard and is usually around 33% efficient. So your 120 “mpg” EV actually gets, all told, around 40 miles per “gallon-equivalent” of its primary fuel, assuming you charge it from the grid. Not too shabby, but 40 is not the same as 120.
Similarly, when considering the EROI of primary energy sources used to generate electricity as a secondary carrier, one often encounters analyses that limit the study of coal, for instance, to the EROI at the mine-mouth. But an EROI of 80:1 for simply getting rocks out of the ground does not give us much useful information to compare to potential alternatives. Even if all electric power were generated directly at the mine-mouth (and we know that is not the case), we would still need to know the energy losses associated with conversion to electricity. Converting thermal energy from a coal fire in turn to steam, to the mechanical energy of a rotating turbine, and finally to the energy carried by electrical currents induced in a power plant’s connection to the grid, introduces losses, mostly in the form of waste heat, at each step. So if the EROI for the thermal energy content of mine-mouth coal is 80, then the EROI for mine-mouth-coal-generated electricity is roughly a third of that figure, or around 27:1, based on the conversion efficiency of typical power plants.
Comparable figures to primary fuel EROI for most renewable energy sources makes little sense – the primary energy converted into electricity, e.g. sunlight or wind, is free, in both monetary and energy terms. Instead, the ultimate EROI of wind- or PV-generated electricity is used as the basis for comparison, usually yielding relatively lower figures such as 8:1 for photovoltaics or 19:1 for wind power. While we may also charge certain support infrastructure, such as long-distance transmission or energy storage, as additional energy costs unique to renewable energy in determining EROI, these considerations are no different from assessing the energy costs of networks of pipelines and railroads needed to support fossil fuel consumption, and such costs are (in the case of renewables) of far lower magnitude than the 60-70% losses of primary energy associated with thermal power plants. And while 8:1 or 19:1 may still be lower than 27:1 (which should be an upper bound for the EROI of coal-fired electricity, given that any transportation beyond the mine-mouth will lower the ultimate EROI), these numbers are also a far cry from the 80:1 figures that make renewable technologies appear ridiculous by comparison.
Hopefully it is clear that my goal here is not to disparage the EV but to promote a clearer understanding of how to compare apples to apples. And heck, the EV is better than nothing – we’ve known petroleum was unsustainable for about as long as we’ve been using it to carry around ourselves and our stuff, and what could be better evidence than the daily transfer of over $1 billion in wealth every day from Americans to petroleum exporting nations thanks to our gas-guzzling habits? It is undoubtedly a good thing that automakers have had the foresight to put into production advanced models that can convert different forms of energy into propulsion power and conserve energy that would otherwise be wasted in transit. But we cannot act as if the EV, should it prove a viable replacement for a major share of transportation, gives us a blank check to continue our happily motoring culture and all its trappings of luxury without considering the physical limitations of the biosphere and humanity’s existence within the larger ecosystems on which it depends.
Case in point: what we call primary fuels are, in fact, secondary products of the bounty of sunlight harvested over eons by the Earth’s organisms. Solar energy remains the only primary energy source directly driving the vast majority of life processes, and as a consequence the replenishment of fuels on which we depend is fixed at a certain rate given the biophysical conditions under which minable fossil fuels deposits can form. Fossil fuels may have given us the temporary illusion of freedom from Earth’s primary productivity, but the overall energy balance equation hasn’t changed much over billions of years. When one considers ancient photosynthetic and geologic conditions as input requirements, and the impossibility of recreating those conditions on timescales meaningful to humans, the long-term EROI of fossil fuels really doesn’t look so good.
“No it doesn’t!”
