The truth about “zero emissions”

The truth about “zero emissions”

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The truth about "zero emissions" vehicles
The truth about "zero emissions" vehicles

Everyone wants to go “green”, in theory anyhow. And it has been asked many times why the electric car is not more commonly used, as it is said to give off “zero emissions”. And recently with Tesla motor company joining forces with Toyota to create the Model S electric sedan, it seems it’s time to really evaluate just what “zero emissions” means in the USA.

First, we should examine where our electricity comes from. It is estimated that in the USA about 50% of electricity is derived from coal.  After coal, the second most common energy source is natural gas, which produces about 20% of the USA’s electricity.

The final 30% of the USA’s electricity comes from non-combustible resources (gives off no CO2), what one could consider “green”; Nuclear, Hydro, and finally about 2% of electricity is derived from “renewable resources” Solar and / or wind.

The 2010 Tesla Roadster boasts a 0 – 60 mph time of under 4 seconds, a fine time for any automobile; but even more impressive when one considers that the Roadster is an electric car. Acceleration like that must need a lot of power. According to the Tesla website the Roadster’s motor puts out about 288 hp, or 215 kilowatt’s. So we can assume that if we were driving the Roadster for one hour, at full throttle, we would use at least 215 kilowatt/hours of electricity.  Or closer to 86 kilowatt/hours at 40% throttle.

As was stated earlier coal power is the most common form of electricity in the USA (and Canada). So just how much coal would need to be burned to power a Tesla Roadster at full throttle for one hour?

One Kilogram of coal can produce 6.67 KWh. Unfortunately a coal power plant is not 100% efficient, in fact it is only about 30% efficient, the rest of the energy being lost to heat and friction. A kilogram of coal actually only produces about 2 kilowatt hours of electricity. Meaning, to drive a Tesla Roadster for one hour (215 KWh) at full throttle we would need to burn over 100 kg’s of coal, or 30-40% of that in actual daily driving conditions.

What about gasoline?

Gasoline at full efficiency produces 13 kilowatts per kg. Of course a car’s engine isn’t 100% efficient, though it is much more efficient than a coal power plant. It can be estimated that the average car engine is about 40% efficient, of course power is also lost through the transmission, so in the end we can estimate that about 20% of the energy given off by the gasoline is actually reaching the wheels. So to produce 215 KWh’s of power a car’s engine would have to create 288 HP (1HP is 746Watts) which is unrealistic.  But 86 HP is more attainable and is 30% of the power; more in line with daily driving conditions.  That would require approximately 5.1 L, or 6.6kg of gasoline.

What does this mean for “zero emissions”?

With over half of the USA’s energy coming from coal, driving any electric vehicle more often than not would be creating more CO2 than if one drove a standard gasoline based vehicle:

1 Kg of Coal produces 1.83 Kg’s of CO2, 1 Kg of gasoline produces 3.2 Kg’s of CO2. If we drove our Tesla Roadster at 30% throttle for one hour, using 65 KWh’s of electricity, the electric car using coal power would create about 55 Kg’s of CO2, the gasoline based car would clearly need much less.

Now of course these numbers are rough, as no one drives a car the same or necessarily at 30% throttle all the time. But as you can see per kilowatt hour coal actually produces far more CO2 than gasoline.

So why do we call it “zero emissions”?

The truth is an electric car does give off zero emissions, at the tail pipe. But to create that electricity pollution is given off a few hundred miles away at the nearest power plant.

Does “zero emissions” exist?

“Zero emissions” only exists in cases where energy is not derived from combustible sources. As was stated earlier, 30% of the USA’s energy comes from non-combustible sources, such as Nuclear and Hydro. Technically, both these forms of electricity give off zero emissions, but both still create other forms of pollution. So in a short answer for all practical purposes right now “zero emissions” is a myth.

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