Electric Vehicles — How clean are they? [Part 1]
19 November 2009 @ 6:46 am
Electric vehicles are cleaner than fossil fuel driven ones. True or False? Did I hear, “Of course, it’s true!” ? Or was it “False, you’ve only moved the pollution from the cars to the power plants!”? The right answer, as in most cases, is “Well, it depends…” — the answer to life, the universe and everything else we have no clue about.
The latest lecture in my class on Sustainable Energy Systems and an article by a Senior Lecturer at my university (Gas vs Electric Mini Cooper) provided fodder for this post.
First of all, what is an electric vehicle? It could be one of four types :
- Battery electric vehicle -> Propelled by an electric motor which is powered by a rechargeable battery.
- Hybrid Electric Vehicle -> Propelled by a combination of an electric motor and an internal combustion engine. The electricity is generated from regenerative braking or from the internal combustion engine.
- Plug-in Hybrid Electric Vehicle -> Propelled by an electric motor powered by a rechargeable battery. Backup electricity is generated by an internal combustion engine
- Fuel Cell -> Propelled by an electric motor. The electricity is generated from a fuel cell that combines hydrogen gas and oxygen.
A hybrid electric vehicle uses only petrol (or gasoline) as a fuel. A smaller engine, automatic switching off of the engine when the vehicle comes to a stop and electric motor drive at low speeds (where ICEs are inefficient) results in a cleaner vehicle than one running using fossil fuels in Internal Combustion Engines. So the answer here would be True.
The answer for the other three would depend on how the electricity is produced (or how hydrogen is produced). In the US, about half the electricity is produced using coal. Which means that generating and using 1 MJ of electricity (1 MJ Life Cycle Energy which included the energy needed to mine the coal, refine uranium etc. ) in the US would release 1.7 times more CO2-equivalent than producing and using gasoline containing 1 MJ of Life Cycle Energy (including the energy required to extract the crude, refine it, transport it etc.)
So that makes electricity dirty right? NO!
An electric motor is far more efficient than an internal combustion engine. From John DeCicco’s article, where he compares a gasoline driven Mini Cooper with the Mini Cooper Electric, I gather that the Electric version needs
33 kWh of electrical energy to travel 100 miles on city roads and 36 kWh of electrical energy to travel 100 miles on a highway
The combined average (55% city driving and 45% highway driving) of these two mileages is 34.4 kWh per 100 miles.
A gallon of gasoline (or about 4 L of petrol) has an energy content of 33.7 kWh. (More on conversion from electricity to gasoline here)
To find the mileage,
33.7 kWh / 1 gallon * 100 miles / 34.4 kWh = 97.7 miles per gallon
A Mini Cooper Hardtop has a mileage rating of :
City : 28 miles per gallon
Highway : 37 miles per gallon
The average mileage is 32.1 miles per gallon.
This means that an electric car is around 3 times (97.7/32.1) more energy efficient than an internal combustion engine propelled car. Given that electricity is 1.7 times more polluting to generate than gasoline, an electric car is roughly 2 times cleaner than its petrol or gasoline alternative. The assumption here being that around 50% of electricity is produced from coal. In India, where the proportion of coal generated electricity is higher, electric cars would be far less cleaner than their US counterparts, but still cleaner than a petrol driven car. DeCicco’s article says that the gasoline powered Mini Cooper emits 357 g of CO2 per mile driven. If the electricity is generated completely from coal, the electric Mini Cooper would emit 355g of CO2 per mile.
So, finally, is an electric car cleaner than a fossil fuel car?
A battery electric car definitely is. Even if you assume that all the electricity is generated from the “diritiest” options — coal fired plants (which is never the case) you still emit marginally less Green House Gases (GHGs) than in using a petrol fuelled car. With the increase in penetration of solar and wind energy generation, it only gets cleaner.
But what about fuel cells and hybrids or Compressed Natural Gas fuelled cars? More analyses and more posts.