Living without CO2 in a Prius

Sometimes you are wandering happily into the local bookstore, when a blog post just punches you in the face. For me it was the personalized license plate I saw on a Toyota Prius:

The Zero CO2 Prius

The Zero CO2 Prius

For people unfamiliar with bizarre American practices, a personalized plate is one for which a person pays an extra fee for a special design or a customized message (instead of the ordinary sequential number). It gives people an opportunity to express their individuality [sic] and it gives the state a chance to profit from such silliness. Our Prius owner has done both, choosing the whale tail along with the personalized message, “SANS CO2”, meaning without CO2 in French.

This is so painfully arrogant… and so factually wrong. For starters, the only people living without CO2 emissions are living quite sedentary existences, like those in the photo below.

Living without CO2 (image from Wikipedia commons)

One may question the extent to which these people are living, but we shan’t bias our judgment solely on account of their death. And we won’t get into the environmental and social impacts involved with mining the metals necessary to make the battery packs for the Prius. Let’s just take a stab at a rough calculation of CO2 emissions. There has been much said on this topic (e.g., the Hummer v Prius wars), for people love to either love or hate the Prius. We’ll try to step above the prattle by being fair-minded and clear in our assumptions. We’ll make just four:

  1. A car takes 76000 kWh of energy to produce (call this Ee for embodied energy),
  2. A typical car takes 40 kWh/day to run (call this amount Fo),
  3. A Prius gets twice the fuel economy (at 20 kWh/day, which we call Fp), and
  4. Energy is a reasonable proxy for CO2 since most of the CO2 emissions are from energy production.

Remember, this is a rough calculation. The first two numbers are from a fabulous reference that anyone interested in sustainability should read: Sustainable Energy without the Hot Air. The last two are just reasonable assumptions. Now let’s picture a scenario: you have owned a car for (forgive me as I get more mathy) No years. You are considering replacing it with a Prius that you plan to own for Np years. A second (do-nothing) option is to continue operating the old car for the whole NNp years. We’re interested in the annualized energy, so we divide the embodied energy by the total number of years the car will be in operation and then add in the yearly operational energy. This gives us for the following annualized energies for the do-nothing option:

Eao = FoEe/(No+Np)

and for buying the Prius instead:

Eap = Fp + Ee/Np.

After being careful to multiply Fo and Fp by 365 to convert days to years, we compare the Prius with the do-nothing option (percent difference = (EapEao)/Eao), looking at a range of values: No = 5, 10 or 20 years and Np = 5 or 10 years.

No (years)
5 10 20
Np (years) 5 1% 14% 28%
10 -24% -19% -13%

What we see is that if we plan on keeping the Prius for only five years, we break even at best. At worst we incur a moderate penalty in energy (and consequently CO2 emissions). If we plan on keeping the Prius for 10 years, we can realize some benefits up to a quarter less energy (and consequently CO2 emissions). This isn’t bad, but it’s hardly “SANS CO2”.

There are obviously plenty more comparisons we could make, but this is enough for now. Making decisions about these matters is hard and complicated. Purchasing a Prius may be justifiable, but let’s try to avoid walking around acting like Mother Teresa.

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