bolosky

I’ve made a rough estimate of the relative carbon footprint of driving a Tesla Roadster and riding a bicycle. You might think that a bike has no carbon footprint, since it doesn’t have an engine. Really, though, it’s powered by a person and the person is in turn powered by burning hydrocarbons. These hydrocarbons come in the form of food, which has a very high carbon footprint per unit energy compared to other sources. Furthermore, batteries and electric motors are way more efficient than human bodies at converting input energy into kinetic energy. The bottom line is that they’re about the same.

Some caveats about my calculations. They only consider the cost of a trip, not the production and end-of-life costs of the bike and car, which would doubtless greatly favor the bike. They’re at best crude approximations and depend on assumptions (speed of biking, diet and size of biker, fuel mix of electricity) that vary greatly from person-to-person and region to region. I got a number of facts from websites, and I didn’t try to verify that the websites are accurate so there might be large errors there, too.

Let’s start with the car. 250 Wh/mile seems to be roughly the efficiency that I get when I drive, although different sources for this number vary considerably: the Tesla website (Tesla Motors - well-to-wheel) claims 110 Wh/km = 177 Wh/mi, but that seems too good to me if you’re driving at speed; Tom Saxton said that he measured over 330 Wh/mile at the plug for about 900 miles of driving (only a little of which was drag racing). While I’m more inclined to believe Tom than the numbers from the Tesla website, I’ll just stick with my guesstimate from the display in my car. The Roadster’s charging efficiency is about 86%, so it takes about 291 Wh from the plug to put the 250 Wh in my battery.

Here in the Pacific Northwest, our local energy company is Puget Sound Energy (PSE), which reports a fuel mix of 36% coal, 41% hyrdo, 20% natural gas, 1% nuclear, and 2% “other” (mostly wind) (Power Supply Fuel Mix). This is actually overrepresenting PSE’s carbon production, because they have much more wind power than that mix represents, but they sell the credits for it to other utilities. So, I’ll stick with what they report.

Wikipedia (Carbon footprint - Wikipedia, the free encyclopedia) says that that the EPA estimate of various forms of electricity generation in grams of CO2/kWh is 950 for coal, 600 for natural gas thermal, 11 for hydro, 6 for nuclear and 5.5 for wind (the nuclear and wind numbers are from Vattenfall, since EPA didn’t have estimates for them). I’m not sure if these numbers include transmission losses, but I’ll assume that they do; even if not, it would only be a modest percentage loss and this is a very rough estimate, so it probably wouldn’t change the conclusion. Prorating by the PSE mix gives about 467 g CO2/kWh electricity at the plug. Multiplying by the Roadster efficiency of 291 Wh at the plug/mile gives about 140 g CO2/mile driven.

Now let’s move on to the bike. I tend to ride at about 20 mph on the level, but I’m in better shape than many people, so let’s assume that someone who rides every day for their commute goes 15. Calories Burned During Exercise (selected because it came up first on a web search, not because I have any reason to believe it’s accurate) says that a 190 lb person (just under the US male average of 191 lb) riding 14-15.9 miles/hr burns 863 calories/hour. Dividing that by the 15 miles/hour speed gives 58 cal/mile.

Food sources (again selected by being high on a web search) says that the average (British) person releases 2.2 tons (I’m assuming they mean metric tons, since it’s a British website) of CO2 equivalent per year for food. Assuming a 2500 cal/day diet, that’s 2.4 g CO2 equivalent/cal. Multiplying by the 58 cal/mile on a bike gives about 140 g CO2/mile ridden, the same as the Roadster (actually a hair more if you don't round off, but I put no faith at all in those low order digits).

I listed two digits of accuracy for the final results, but the truth is that they depend so heavily on the assumptions that even if the facts I pulled from the web are perfect, the bottom line will vary over a large range. Maybe you’re a thin, slow-biking vegetarian who lives in a place that gets all of its electricity from coal and knows a bikes-only shortcut to work. On the other hand, you could be a fat person living in Chelan, WA where essentially all of the power is hydroelectric and your trip to work is on 35mph roads. The point that I’m trying to make is that driving a Roadster is roughly comparable in CO2 production to riding a bike given a somewhat reasonable set of assumptions. That is, I'm not telling you to get off of your bike to save the atmosphere, just pointing out that the EV versus bike tradeoff is pretty even.

To me, at least, it’s a pretty surprising result. After all, you’re lugging around a whole car (+ 900 pounds of battery) with you when you drive. What happened to make the numbers come out so far from intuition? First is that PSE is way more efficient in getting energy in exchange for releasing carbon than farmers are. We saw above that food releases 2.4g of CO2 equivalent per calorie. A (dietary) calorie is about 1.2 Wh, so that’s 2000 g/kWh, or about 4x worse than PSE. Second, the human+bike system is way less efficient at turning food energy into miles traveled than the Roadster when you normalize for the mass. Say the (191lb average size) person + bike (+ clothes, water, etc.) is 100kg. It takes 59 cal = 71 Wh to go a mile for an efficiency of 0.71 Wh/kg-mile (sorry for the ugly mixed metric/English units). The Roadster + driver + stuff in the trunk is roughly 1350 kg and uses 250 Wh/mile for an efficiency of 0.19 Wh/kg-mile, almost 4x better as well. These two factors of roughly 4 (together a factor of nearly 16) make up for the factor of 14 difference in mass that favors the bike.

Now, I’m going to drive my Roadster to the gym and ride a stationary bike, completely killing any carbon advantage I might have had by driving electric…

vfx

Is the Roadster sentient?

Have the bike charge the car.

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The world loves to be deceived.

Squint

58 kcal/mi is way too high. I've yet to see a website, heart rate monitor, GPS, or gym machine that doesn't overestimate calorie expenditure for cycling.

From my experience using SRM Powermeters and Velotron ergometers, calories per mile can be estimated at 35 kcal/mi. It's not affected as much as you might think by rider weight, intensity of the ride, and fitness.

Someone else's calculations and assumptions:

How Much Do Bicycles Pollute? Looking at the Carbon Dioxide Produced by Bicycles

bolosky

I just pulled the numbers from websites; it's entirely plausible that they're all wrong and all wrong in the same direction since they have some incentive to err in that way. I've certainly never tried to bike inside a calorimeter to see my heat output. If we use 35 kcal/mile rather than 58 and the rest of my assumptions are correct, then the bike goes down to about 60% of the CO2 generation of the Roadster, which is still closer than I would have guessed.

I'm not all that impressed by the numbers from the bike guy's website, though, because he didn't really attempt to take into account the carbon released by the production process of the food and gasoline. In the case of food, presumably ALL of the net CO2 release is in production, because as he points out plants get their carbon from the air, and animals get it from eating plants (or other animals that eat plants, etc.) Although I suspect that his conclusion ("the argument that a bicycle pollutes more than a[n ICE] car is entirely without merit") is correct.

One number that's intersting from his site is the CO2 release for cars. He certainly lowballs the real cost, because he's ignoring production, but even with that the average car comes out to 590 g CO2/mile (my conversion from his report of 1300 lbs/1000 miles), 4x the Roadster.

edo

If the food is "renewably" grown, biking would always be carbon neutral, assuming the bike was "renewably" made.

There are two ways this breaks down:
1.)Tractors, Fertilizers, etc. are not carbon neutral.
2.) If a forest was torn down to plant the field, then the carbon consumption of the earth goes down, meaning a net increase in carbon production.

An interesting spin off would be whether it is better to burn the food to make electricity or eat it and ride your bike.

doug

I never find these arguments particularly meaningful. What almost always seems to be missing is a consideration for the number of calories you're burning anyway while at "rest". Just because you're not biking or walking, doesn't mean you're not breathing, metabolizing, generating heat.

Yes, ultimately energy in equals energy out. But some of that energy gets stored as fat and some of it ends up in the septic tank. The body is a complicated system with various feedback mechanisms. Eating less often slows the metabolism making it more "efficient" causing the body to store more energy as fat. People who are in shape with more muscle mass often burn more rest calories.

For me, I don't feel I noticeably eat more on days I exercise. If anything, I crave healthier foods. So I figure it's all a wash, and as common sense tells you, you and the environment are probably better off if you ride your bike.


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Tdave

Interesting. And the bike carries one passenger and the Tesla two. Per passenger, the Tesla wins.

BlackbirdHighway

Unless you have a bicycle built for two!

Brent

Yeah, what Doug said.

For what it's worth, bike racers eat huge quantities of food,* with their evening race meals containing on the order of 10,000 calories . They need so much food that eating becomes a bit of a chore; food isn't calorie dense enough for more "leisurely dining."

I remember reading one article that made the argument that bicycles contribute to global warming** more than cars (or some stalking horse) ... by increasing the average lifespan. Dying earlier was just about the most effective way of reducing your carbon footprint. It was tongue-in-cheek, but only a little.

* http://sports.espn.go.com/espn/page2...e=stein/090701
** http://www.npr.org/templates/story/s...toryId=5595169

doug

Thanks. I considered mentioning the case of athletes where the energy output can be quite high, but didn't want to convolute things too much. (Talk about Michel Phelps's diet during the last Olympics sticks out in recent memory.) Again, ultimately energy out equals energy in.

I figure the average commuter, who's casually walking or biking, is using energy that would otherwise be stored on his gut or backside.

I suppose at some point someone should calculate the carbon footprint of being a burden on the health care system and add that into the calculation.


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