“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Monday, September 06, 2010

Unit ambiguity in the New York Times?

I've made several posts (here for example) decrying people's cluelessness with respect to scientific concepts and, in particular, with respect to units. But do we have an example in the New York Times? I follow the New York Times Twitter feed (nytimesscience) and read the following: When It Comes to Car Batteries, Moore's Law Does Not Compute http://bit.ly/bSuX3e. Naturally, I clicked on the link and it took me to an an article on the rate of advance in battery technologies, both with respect to energy density and charging rates. All very interesting, and the interview subjects were from IBM and Better Place.

But about midway through the article, I read the following:
He illustrated the challenge of building a battery with the energy density of gasoline by recounting that it took 47 seconds to put 13.6 gallons of gas in his car when he stopped to fill up on the way to San Francisco. That’s the equivalent of 36,000 kilowatts of electricity. An electric car would need to pump 6,000 kilowatts to charge its battery.
Does this make sense? Let's parse it.13.6 gallons of gasoline contains (at 125*10^6 joules/gallon) 1.7*10^9 joules. Putting this in the vehicle in 47 seconds yields a "rate of energy transfer" of 1.7*10^9 joules/47 seconds, or 36.2*10^6 joules/second or just over 36,000 kilowatts. OK, so far so good.

But where does the 6,000 kilowatts come in? Well, the internal combustion engine (ICE) in the gas powered car might use energy at something like 22% efficiency, so the 1.7*10^9 joules might translate to 3.74*10^8 joules of useful work. OK, an electric motor with the power to drive a car will typically have a minimum efficiency of 92%, so that means that we'll need 3.74*10^8/.92 = 4.07^10^8 joules of electrical energy put into the battery. Doing this in 47 seconds yields a charging rate of 8.6*10^6 watts or 8,600 kilowatts. Not really so far off, and I guess that that's where the figure came from.

Stepping back, it's a very good illustration of why we love fossil fuels. Suppose IBM and Better Place succeed in creating a lithium air battery with the required energy density and ability to accept a charge at the rate described above. Now, imagine a "charging station" with, say, 4 cars charging their batteries at the rate of 6 megawatts, and imagine that you're at a corner with another station across the street doing the same. As I drive around, this is not at all an unusual circumstance at some times of day. So, at this corner, we'll need to be able to supply at least 48 megawatts of electrical power. That's about 5% of the power from a gigawatt generating station (this is a big facility). Getting energy from a power plant to a charging station at this rate, with the ubiquity of today's gas stations, is an incredibly difficult transmission problem

As I've demonstrated in a previous post, I believe it will be possible to install sufficient capacity to supply electrical energy to a fleet of electric cars. But the ability to deliver it at an acceptable rate to batteries that can store it is definitely the sine qua non of the widespread adoption of electric vehicles.


Michael Tobis said...

Yow. I suppose that there's no way to store the energy locally?

You wouldn't have a weight limit on the battery which presumably would make things easier.

I understood a major point of electric vehicles was that they would charge at night, using the otherwise useless wind power. That is, electric vehicles and windmills are symbiotic.

I had also heard that instead of filling up there would be a battery swap station for long trips.

GRLCowan said...

Stepping back, it's a very good illustration of why we love fossil fuels.

And why I now, and everyone by and by, will love nuclear-generated boron more.

(How fire can be domesticated)

King of the Road said...

I agree that wind energy and cars are symbiotic. Charging at night is definitely the way to go and charging over eight hours instead of 47 seconds would only require about 10 kilowatts. With a 220 volt circuit, this is about 45 amps.

I have a machine shop in my garage wired for 220 volts to serve my mill, lathe, and welder and the highest amperage circuit I have is 20 amps. A typical modern house is wired for 200 amp service in total. On an average summer day with my pool pump on and the air conditioner running, I use a little under 8 kilowatts and my average continuous use is about 2 kilowatts taken year round.

10 kilowatts over 8 hours is (drum roll) 80 kilowatt hours. As a point of reference, the Nissan Leaf battery has a capacity of 24 kilowatt hours, but is good for about 100 miles.

The battery trading idea is a good one but the station would either need a lot of storage or the ability to charge very quickly and, of course significant standardization would be required.

Certainly, distributed generation would be a boon to adoption as well.

I have no illusion that stations capable of delivering energy to electric vehicles at a rate equal to that of today's gas stations is what we'll see. But it does bring home the point.

King of the Road said...

Forgot to say, storing electrical energy is, to put it mildly, not in a very advanced state with respect to distribution scale quantities. Pumped hydro, compressed air, ultracapacitors, batteries, flywheels, hydrolysis and fuel cells, etc. are all under active investigation but all have severe issues. Pumped hydro and compressed air suffer high losses; batteries, flywheels, ultracapacitors suffer problems of scale, and fuel cells suffer from a variety of problems, not the least of which are spectacularly high cost, capacity, and loss. Still, I'd say though that the fuel cell might be where I'd place my bet for the local charging station.