by Gary Flomenhoft
4-26-09
Internal combustion is so inefficient that it's doomed. Why? Because only 15% of the energy in fuel drives the wheels, and only about .3% moves the driver (Grabianowski). The reason we use gasoline is because it stores a lot of energy, not because it's efficient. Electric drive systems are far more efficient.
Autonomous transportation like automobiles and trucks need to store energy on board, unlike electric railway trains, electric trams, cable cars, or municipal subway or light rail systems, which have access to utility power through wires or metal rails. Note that these have all gone to electric drive already. Autonomous vehicles like cars have two key factors: how much energy can you store, and how efficiently can you convert it to motion?
Let's take a look at gasoline, our favorite transportation fuel: Gasoline stores a tremendous amount of energy, around 36,000 Watt-hours (Wh) per gallon. What does that mean in non-geek terms? 36,000Wh is like running a 100 Watt light bulb for 360 hours. That's a lot of energy! Sunlight at noon on a clear equatorial day shines at about 1000 Wh per square meter. So a gallon of gas is equal to 1 sq meter (approximately10 square feet) of sunshine for 36 hrs. Plants convert sunlight to energy at about 2% efficiency, and fossil fuels took millions of years to form. Well, you do the math...
Putting it in human terms, an average person can maintain .1 horsepower, which is about 74.6 Watts. Dividing 36,000 watt-hours by 74.6 Watts we get 482 hours of human labor equivalent to 1 gallon of gas. At a minimal salary of $6/hr, that would cost you around $2900 for the human labor equivalent of one gallon of gas. In the fall of 2008 gasoline cost around $3.75/gallon, which everyone was whining about. $3.75 vs. $2900, pretty good bargain. To get 482 hours of labor for $3.75 you would have to pay the person $.00778 or .778 cents per hour, less than 1 cent per hour. But the situation gets even more interesting.
Now let’s put that gasoline in your car's engine. Internal combustion engines run on the Carnot cycle, which is about 18% efficient at turning energy into work in a gas engine, although only 15% makes it to the wheels or runs accessories (fueleconomy.gov). The rest turns into heat. So if you hired someone to work for you, it means that they would actually work only 15% of the time, or 9 minutes an hour, and take the other 51 minutes off. This slacker would be FIRED! But if you were only paying them 1 cent an hour, maybe you wouldn't care? So it is with gasoline. Gasoline is so cheap, we don't care that our engines are so inefficient. Because of the current peak oil plateau period, gasoline is about to get a whole lot more expensive! So in summary, gasoline stores a tremendous amount of energy per unit of weight or volume, but the conversion of energy to work in a petrol engine is very inefficient.
So now let's take a look at electric propulsion, the way of the future, and the situation is exactly the opposite. Electric drive systems convert energy very efficiently, but energy storage is a problem (It's the batteries, stupid!). Electric motors are very efficient, converting about 80-90% of the electrical energy into motion. Let's say 80% gets to the wheels. In human terms, this person is working about 48 minutes every hour, not bad, maybe even about average for a typical worker. The problem with electric propulsion comes with energy storage. Traditional lead-acid batteries store about 30 Wh per kilogram or about 13.6Wh per pound. So a 1000 pound battery pack stores about 13,600 Wh hours of energy, or the same as .377 gallons of gas which weighs 2.36 pounds. Dividing 1000 by 2.36 we see that gasoline stores about 440 times as much per unit of weight as lead-acid batteries. The bigger problem is that you can refill your gas tank in 5 minutes, but it takes hours to charge your batteries.
So now let's talk about useful energy. Let's say the average car carries 20 gallons of gas. 20 times 36,000Wh/gallon is 720,000 Wh. At efficiency of 15% that's 108,000 Wh of useful energy. A 1000 lb pound battery pack carries 13,600 Wh. At 80% efficiency that's 10,880 Wh of useful energy, or about 1/10 as much as a tank of gas, which explains why electric cars have 1/10 the range of gas cars or about 30-50 miles.
New lithium batteries have around 100Wh/kg or 3.3 times as much energy storage per unit weight as lead-acid, which extends the range, although at much higher costs. But there is still the problem of recharge time. Meanwhile, the interim solution to the problem of energy storage in electric drive systems is to use hybrid vehicles, which carry a liquid fuel source to store energy and extend driving range with an auxiliary combustion engine. New plug-in hybrids will allow electric-only driving for a limited range, but beyond that they will still use liquid fuel and combustion engines.
The real limiting factor of electric drive is not power, range, or acceleration. All these problems have been solved. The real problem is recharge time of batteries or energy storage in general. Even if you could drive your lithium powered electric hypercar 300 miles on a charge, you would still have to wait hours for a recharge. This will not work for trucks or for long distance personal driving. People will not accept a new technology which is worse than the old one. Some new lithium batteries in the lab at MIT are claimed to recharge in 5 minutes, but we'll see if they make it to the real world (Kang & Ceder). Another possibility is mechanical recharge systems like metal-air batteries. In these types of batteries the electrolyte fluid can be changed quickly and regenerated offsite, but I haven't heard much about them lately. How about hydrogen fuel cells? They are currently too expensive and overall system efficiency including producing the hydrogen is less than combustion engines, and far less than batteries. This is a little known fact about fuel cells, and a limiting factor. Another method is standardized battery packs that can be changed out in minutes like replacing the battery on your cordless drill. This method was tried in the 1990s but didn't advance very far. I am skeptical of the handling problems and standardization necessary for this system to work. So energy storage and/or recharge time for batteries remains the holy grail of electric drive systems.
When energy storage is solved, the superior efficiency of electric drive will immediately make electric vehicles the overwhelming choice for cars and trucks, as they already are for vehicles which don't need to store energy onboard. In this light, does it makes sense to put a lot of effort into replacing fossil fuels with biofuels, destroying rainforests for oil palms and using agricultural land to make fuel for an inherently inefficient engine? Perhaps effort would be better spent on electrical energy storage and renewable electricity production.
Sources:
How Hypercars Work, by Ed Grabianowski
downloaded 4/26/09
http://www.fueleconomy.gov/FEG/atv.shtml
downloaded 4/26/09
Kang, Byoungwoo & Ceder, Gerbrand.
Battery Materials For Ultrafast Charging And Discharging. Nature, March 12, 2009, p190.