Equating "fuel" efficiency between electric, hybrid, and gasoline vehicles

An earlier posting about Miles per gallon ratings with plug-in hybrid vehicles begs an interesting question. Just how does one measure "fuel efficiency" in a vehicle that runs on electricity and hence doesn't consume a fuel? The miles/gallon figure is what our society is accustomed to from over a hundred years of gasoline vehicle usage. An electric vehicle doesn't burn a fuel, it consumes electricity. Electricity has no weight and does not take up space in a container. Electricity is held in a battery but neither the size nor weight of the battery changes as the battery discharges. Yet a given battery can contain a given quantity of electricity, which determines the vehicle range and speed.

Electricity quantities are measured in kilowatt-hours. One watt is the electricity equal to one volt and one amp (watts = volts * amps). For example most handheld hair dryers use 750-1500 watts, and on a 120 volt circuit 750 watts requires 6.25 amps of current whereas 1500 watts requires 12.5 amps. Outside the U.S. where 220 volt circuits are more common the same 750 watt hair dryer requires 3.4 amps of current and at 1500 watts requires 6.8 amps. A kilowatt-hour is 1000 watts consumed over an hour of time. This is covered in more depth in: Overview of batteries and electric vehicles, Electrical Basics covering batteries in electric vehicles and Power Density in Batteries and Electric Vehicles

Why is this important? We are being asked to believe large miles/gallon efficiency claims for plug-in hybrid vehicles and we need to understand how to interpret those claims. If those claims are bogus then our society will have been fooled into a false solution to our transportation problems.

Miles/gallon to miles/kilowatt-hour are not directly comparable. My mind echo's with my high school science teachers yelling from across the room that you're comparing apples and oranges. What's going on is driving the vehicle down the road consumes energy. The different forms of energy are not directly comparable because one is gallons of gasoline, the other is kilowatt-hours of electricity, and the units are all different. In science class however the practice is to convert from one unit to another, leading my fellow classmates to convert all speeds to furlongs per fortnight. The U.S. Department of Energy has published a chart giving conversion factors and says that: Every fuel has different energy density. The most common way to measure how much petroleum is displaced through the use an alternative fuel is to convert the energy density of an alternative fuel unit to the energy density in a gasoline gallon. (Converting Alternative Fuel Units to Gasoline Gallon Equivalents (GGE))

Said chart shows one kilowatt-hour is convertible to .03 gallons of gasoline. Or 33.56 kilowatt-hours is convertible to 1 gallon of gasoline. Hence miles / 33.56 kilowatt-hours is equal to miles/gallon in this conversionary math system.

For example Tesla Motors publishes a chart showing Well-to-Wheel efficiency where they claim an energy efficiency of 177 W·h/mi. After a bit of calculation wikipedia converts this to 190 miles/gallon equivalent. Another interesting conversion is the cost per mile, assuming $.10 / kilowatt-hour to buy electricity, .177 kilowatt-hours consumed per mile is $.0177 per mile for the electricity.

I have a couple examples closer to home and more affordable than the Tesla Roadster (ahem). I talk of my electric motorcycle and electric bicycle. Last year I observed the electricity usage for my daily commute - 10 miles over very flat terrain in Silicon Valley. The motorcycle routinely consumed 3 kilowatt hours for the 10 mile trip while the bicycle routinely consumed 0.3 kilowatt hours for the same trip. Of course being a bicycle I pedaled to provide part of the power, meaning there is unmeasured energy input from my leg muscles, also meaning that it provided personal exercise benefits. In any case let's crunch these numbers a bit.

The motorcycle consumed 300 watt hours per mile (.3 kwh/mi), less efficient than the Tesla Roadster. However applying the same conversion derived by the wikipedia page, (1/300)*33705=112.35 meaning my motorcycle gives over 112 miles/gallon equivalent fuel efficiency.

The bicycle however consumed 30 watt hours per mile (.03 kwh/mi). Applying the same conversion, (1/30)*33705=1123.5, meaning my bicycle gives over 1120 miles per gallon fuel efficiency.

Getting back to my high school science teacher and apples and oranges. Saying my bicycle gets 1120 miles / gallon can be misleading, since it doesn't burn gasoline. Or putting it another way, to say a Tesla Roadster gets 190 miles/gallon fuel efficiency may keep the mind focused on improving miles/gallon and lead one to continue looking for high miles/gallon efficiency.

The problem here is not to achieve the most efficient burning of gasoline, instead the problem here is our need to transport our butts around town. The chosen popular transportation technology requires liquid fuels which are due to become in short supply soon, leaving us potentially unable to transport our butts around town. If we look at the problem purely through the lens of miles/gallon efficiency of burning gasoline, we remain stuck in the mindset of burning gasoline to solve the problem of transporting our butts around town. We need to focus on the energy efficiency of different forms of transportation, and we need to remain unattached to specific solutions of the need to transport our butts around town. This mathematical conversion technique simply helps us compare energy efficiency across different units of measurement.

Links: 

Converting Alternative Fuel Units to Gasoline Gallon Equivalents (GGE)

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Tables and Equivalents for Natural Gas and Propane for Motor Vehicle Fuels Tax Purposes

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Gasoline gallon equivalent

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Miles per gallon of gasoline equivalent

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Miles per gallon gasoline equivalent

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Well-to-Wheel efficiency (Tesla Motors)

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http://www.cngprices.com

Links: 

http://www.altfuelprices.com/

External Media

Stirling Engines and the return of Think Vehicles

Who says a hybrid vehicle has to burn gasoline? It's probably the oil companies pushing that idea. A hybrid vehicle is one driven with multiple power sources, such as electricity and gasoline. But that's only one model for hybrid vehicles, and today I have another model to discuss.

Segway Inventor Focusing on Green Cars

Have you driven a Fjord lately?

In the 1990's a Norwegian company, Pivco, designed and built a small electric vehicle, the Think Citi. It was attuned to being used in a city and its limited range and speed made it unsuited to America where we've installed superhighways with speeds above 70 miles/hr. But at the same time California had enacted a requirement that some percentage of all vehicles sold in the state must be zero emissions, which sent the car companies scrambling to find an electrically driven alternative. Ford's solution was to buy Pivco and import the Citi's, and within a couple years they drove it into bankruptcy. Fortunately some investors in Norway saw an opportunity and bought the remains from Ford, resurrecting it into a new company.

The reborn company is now ready to unveil their new car, and is it a doozy.

The design is simple enough they make parts from around the world, and can have simple assembly plants that are built near to where the vehicles will be sold. Rather than build a large centralized factory from which vehicles are shipped worldwide, it appears they plan to have a distributed production stream.

The base vehicle should cost $15,000 (or so) but that's without the battery pack. Part of their plan is to radically change how vehicles are made and sold. The battery pack they are using, derived from the Lithium-ION pack designed by Tesla Motors, will be very expensive. The cost for the battery pack would make this simple little car out of reach for all but the most well heeled of buyers, and the well heeled buyers wouldn't be interested in a simple vehicle like this. To enable a mass market for the Think vehicles, they are leasing the battery pack and bundling into the lease maintenance and replacement services.

The hybrid portion of the car is a Stirling Engine designed by Dean Kamen. Dean Kamen is a super rich inventor who made a splash with the Segway, and has his sights set on redesigning how vehicles are powered. Stirling engines take heat and create rotary motion, and is a design dating back to the 1800's. Stirling engines, however, have never been harnessed in a commercially viable way to produce useful work. But according to these articles Dean Kamen has worked out a way to harness a simple Stirling engine that can take any fuel and produce electricity. The vision is for a Stirling engine to be mounted in these Think vehicles, and used to charge the onboard battery pack.

The CNNMoney article discusses how they intend to market the car through carsharing programs. First, the car would fit well with a carsharing program because of its leased nature. If you encounter their car through a carsharing program, and like the car, you can then order your own if you wish. Hmm, interesting possibility except I don't think there's many people using carsharing programs. This may be a questionable part of their plan, one I think most Americans will have difficulty overcoming.

In any case the overall vision is that the car company will be offering a range of services related to the vehicle, and charging a monthly fee. This includes car insurance, maintenance, wireless internet access, and more. The CEO of Sun Microsystems, Jonathan Schwartz, has described a meeting he once had with car company executives. The exchange started with the question, what's the monthly lease or car loan payment for the typical car. Then he asked how much in monthly fees would be required for a car company to give the car away for free. His point was that if a gadget maker could offer enough services bundled with the gadget, the gadget could be given away for free, and the customer is obligated to continue paying for the services.

The Think is coming close to this model. They're offering to sell the base vehicle inexpensively, and bundle with that vehicle a range of services.

A thought in my mind is, who owns the vehicle itself. There have been instances of computer companies selling a cheap or zero-cost computer, expecting to make a mint by in monthly service fees. But instead the geek crowd bought the cheap computer, didn't buy the service package, and installed Linux on the cheap computer, driving the computer maker out of business because they lost their shirt giving away hardware.

If the customer owns the Think vehicle who says they have to buy battery service from Think?

In any case this all looks to be very exciting.

Vehicle-to-Grid is such a strange idea, why are people pursuing it?

"Vehicle-to-Grid" is a kind of reverse plug-in hybrid electric. The idea is some kind of poppycock wet dream that I don't understand the purpose its development. This idea comes around occasionally and involves some overly hyped breathless descriptions of the possibilities inherent in a car that has an onboard electricity generating plant. I've seen this idea applied to fuel cell cars and now to hybrid-electric cars.

PG&E Demonstrates Vehicle-to-Grid Technology and PG&E demonstrates first ever vehicle-to-grid charging and PG&E sees plug-in hybrids as potential profit centers discuss a recent showing by Pacific Gas and Electric of a modified Prius that has vehicle-to-grid capabilities. "The basic principle here is that electric cars charged at night while electricity is cheap can actually give some of that power back during the day when electricity costs more, and the owners of the vehicle that is giving that power back can get a credit towards the purchase of electricity when the car charges back up."

That's the idea .. that a hybrid electric car, or for that matter a fuel cell car, contains on board the car equipment capable of generating electricity. So why not, these people suggest, connect the car to the grid and use the onboard generator to generate electricity and sell it to the power company. And then the article suggests that time-of-use metering will allow the car owner to buy that electricity back in the evening when rates are cheaper.

Since the articles linked above are doing such a good job of portraying this as a positive thing, I want to list out some alternative points of view.

First is the cost and effort to load power into a car. For a hybrid car that means making a stop at a gasoline station, or for a fuel cell car it's a stop at a different kind of gas station. The use of the onboard generator means depleting that fuel, which then decreases the range of the car requiring the driver to make a stop at the gas station (of either kind) more frequently.

Second, time of use metering is rarely available to consumers. Instead consumers most often see flat electricity rates. The advantage accrues only when time of use metering is available.

Third, it's especially egregious to tie vehicle-to-grid to a gasoline burning vehicle. The goal is to reduce gasoline usage to zero, not to continue gasoline usage.

Fourth, a comprehensively installed vehicle-to-grid system means electrical intertie points at most or every parking spot in a parking lot. This would drastically increase the cost of building parking lots ... think, do you see power outlets installed in parking lots today? No. And we're not talking normal power outlets, but one capable of accepting power as well as sending power. This sounds like complexity, and would we trust average consumers to properly operate a power intertie system connected to their cars?