EVs and Energy Density

If the new vehicles on display at this year's Detroit Auto Show have you wondering whether 2010 might be the Year of the Electric Car, you're not alone. GM's Volt plug-in hybrid is due out this fall, and purely-electric options like Nissan's Leaf aren't far behind. The global auto industry is investing billions of dollars in developing this technology, and the US government is putting up additional billions in loan guarantees for EV manufacturers and consumer purchase subsidies. No one should dismiss the seriousness of these efforts or their potential to reshape the vehicle and transportation energy markets over the next couple of decades. At the same time, their ultimate success depends on whether a combination of improved technology and significant changes in consumer expectations concerning vehicle performance and characteristics can overcome the core challenge of vehicle electrification: either matching the effective energy density of liquid fuels or giving up the flexibility they provide.

Understanding the practical consequences of energy density, which refers to the amount of energy that can be stored in a given volume or mass of fuel or battery, requires putting electricity and fuels onto a common basis of comparison. Although I've generally tended to do this in terms of gallons, barrels or BTUs, for a change I'd like to consider the fuels we commonly use in terms of their equivalent electrical energy. The units may be less familiar at first, but this should make a side-by-side comparison with the battery capacities of new electric vehicles (EVs) easier.

According to the Department of Energy a typical gallon of gasoline delivers 116,000 BTUs of energy, and a gallon of diesel fuel 128,000 BTUs, based on their lower heating values. Converting to electricity units gives us 34 kilowatt-hours (kWh) per gallon and 37.5 kWh/gal., respectively. Using typical volumetric densities for these fuels, I come up with figures of 5.5 kWh/lb. for gasoline and 5.3 kWh/lb. for diesel. By comparison, the battery for the extended-range GM Volt hybrid, which is rated at 16 kWh, appears to weigh 400 lb., yielding an energy density of just 0.04 kWh/lb., or less than 1% of the energy density of hydrocarbon fuels. If this were the entire story, EVs would look like a hopeless proposition, and we could dismiss them for another generation.

The factor that helps to bridge the enormous gap in energy density between the best batteries and liquid fuels is efficiency. While neither electric motors nor internal combustion engines (ICEs) can turn 100% of that stored energy into motion, the EV motor has an efficiency advantage of roughly 4:1 over ICEs. Even after taking that into account, we're still left with a requirement for roughly 25 lb. of batteries to deliver the same range as a pound of gasoline, with the effective useful capacity of the Volt's entire battery pack storing the equivalent of no more than one gallon of unleaded regular. Plug-in hybrids like the Volt cleverly finesse this limitation by using on-board generators running on liquid fuels to extend their range. Of course this entails big trade-offs of cost and weight, but the designers of such vehicles hope to come up with a mix that will satisfy consumers who are accustomed to cars that can go 300 miles without provoking "range anxiety".

In some respects the bigger concern related to energy density might be the one that proved to be the Achilles' heel of GM's first effort to produce a consumer-friendly electric car, the EV-1. To understand why recharging EVs is such a tough problem, let's take a look at your last visit to the gas pump in terms that would never occur to most people. Gas pumps in the US are limited by EPA regulations to deliver a maximum of 10 gallons per minute. Half that is probably more typical. But even at 5 gallons per minute, the gas pump is "recharging" your car at the power equivalent of 10 megawatts (MW), effectively delivering the entire daily power consumption of the average US household every 12 seconds. Even if you discount that figure by the lower conversion efficiency of an internal combustion engine, it's still the equivalent of a couple of megawatts. Matching that for an EV would require either stupendous voltages or currents well above most designers' comfort level. For example, a car recharger drawing 100 amps would have to operate at 25,000 Volts--more than ten time the voltage of the electric chair--to deliver a comparable charge in the same interval. At the 240 V of your home's appliance circuit, you'd need about 10,000 amps--similar to what a transit train draws from the "third rail." Almost inevitably, the safe recharging of EV batteries must take longer--hours longer--than refueling your gasoline vehicle, or entail clever-but-costly workarounds such as the battery-swapping scheme of Better Place and other firms.

From the above it's hard to avoid the conclusion that EVs and plug-ins might not be quite ready for prime time. However, I was struck by a comment from a GM official cited in a New York Times article on the Detroit Auto Show, concerning the need for first-generation EVs to pave the way for an eventual mass market. There's every indication that these cars will shortly be ready for "innovators" and "early adopters." The Volt, Leaf, and cars like them will prove out not just the technology of vehicle electrification--a trend that began with the original Honda Insight and Toyota Prius and still looks like the strongest competitor to the ICE in the long run--but also the response of real drivers who aren't engineers or environmentalists. My own experience with energy density in the more modest realm of battery-powered lawnmowers suggests that this will require adapting our expectations and usage patterns to this new vehicle type, rather than treating it as plug-and-play in our current lifestyles. In the meantime, the automotive mainstream has some very attractive non-plug-in options for getting the most out of the energy density of our current fuels, based on the steadily-growing variety of conventional hybrids, advanced diesels and downsized gasoline cars with direct injection and other innovations.