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.
Re: Equating "fuel" efficiency between electric, hybrid, and ...
The Wikipedia article also has a nice discussion of the difference between Tank-to-Wheel and Well-to-Wheel efficiencies. This is an important distinction. Aside from the fact that most of our electricity comes from burning fossil fuels, there are important differences in the efficiency of distributing energy (whether it is gasoline or electricity or hydrogen).
According to the Wiki article, gasoline has an efficiency of 0.83, or 83% of the energy makes it from the ground to the gas tank. The rest is lost in processing and transportation. Electricity has an efficiency of 0.303. Meaning only 30.3% of the energy coming out of the power plant makes it into the car battery. I didn't read the Department of Energy report that these numbers came from, but I hope that this 30% efficiency includes the efficiency of getting the coal to the electric plant. Being a DOE study, there is probably some weighting of electric sources (coal, nuclear, hydro, wind, etc) and it should indicate the overall average for a kilowatt hour of production nation-wide.
What this means for you and me is that when you look at the Tank-to-Wheel efficiency (the typical mpg rating for your car) you must remember that getting the gasoline into your tank is almost 3 times as efficient as getting the electricity into your battery. David came close to this point in his article. This is a big part of apples to apples. What does it really cost to move your (or my) butt around? Because electricity is cheap, it looks good right now. But how cheap will it be after the 'high efficiency grid' is implemented (probably by tax dollars if the current administration has anything to do with it) and after the demand on the electric supply is doubled or tripled by the use of 50 million electric cars plugging in every day.
We've gone down the road of ethanol without any consideration impact of the demand on our food supply when we squeeze all of our corn into our fuel tanks. And now we're going down the road to plug-in vehicles without considering the impact on the cost of heating and cooling our homes when we add the demand of American driving habits.
By the way, David, nice blog post. I really enjoyed this.
Differences between electric and fossil fuels
Between the electric fuels and fossil fuels which is more costly? Will the electric fuels not causes other problems or will the not demand lot of money then the fossil fuels in future
Re: Differences between electric and fossil fuels
The raw cost is relatively easy to calculate. Gasoline and electricity both are forms of energy, and you can calculate an equivalence between them .. $n of gasoline would perform a given amount of work, and $n of electricity would perform a given amount of work. I outlined all that above ...
But ponder that there are costs associated with them which aren't represented in the price of the fuel. Such as the requirement to send armies into the Middle East to maintain access to oil supplies (I am firmly of the belief the real reason for the Iraq war was to have a toehold in the Middle East for access to oil supplies). The cost for the war is not transferred to the pump price for gasoline. Nor are all the illnesses caused by poisoned atmosphere or water. And when it comes time that we run low on oil and oil supplies become tight, the resulting social upheaval and chaos (think "Mad Max the Road Warrior") will be very expensive indeed.
Re: disposal of batteries and cost of replacement.
Just what is the life of a hybrid battery, and what kind of environmental problems do we have to look at in the production of batteries and their disposal???
Re: disposal of batteries and cost of replacement.
"hybrid battery" is one of those mixed metaphors like "military intelligence"... The word "hybrid" means taking two or more elements and combining them together into one. Such as a car that has both a gas engine and gas tank and electric motor and battery pack, you mix all that together to make a hybrid car.
A battery pack is just a battery pack, it is inappropriate to call it "hybrid".
Okay, now that my Grammar Girl rant is out of me.. what was your question?
Oh, it depends on the chemistry of the battery. For example lead-acid batteries typically give 300 charge cycles and the primary constituent is lead which, uh, causes dementia and other diseases if it gets into the food supply. Fortunately lead batteries are highly recyclable. Unfortunately lead batteries are in pretty much every vehicle on the road. Fortunately that means there is a vibrant industry of making and recycling lead batteries.
Nickel-Cadmium batteries have another extremely toxic substance, cadmium, and hence have been banned in many countries.
Other chemistries are far more safe, very abundant, etc. For example Toyota has a Nickel-Metal-Hydride battery design that gets over 100,000 miles (essentially the life of the car) and NiMH is easily recyclable and nontoxic.
The lithium batteries are newer so the actual cycle life isn't as well known. The makers are often claiming 3000 charge cycles which is a pretty long life. That's nearly 10 years of one charge cycle per day, for example.
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