Wednesday, August 02, 2006
Why Use Electricity rather than Oil
Recently the Economist had an article about the electric car that include some inaccuracies. Here is the text of the letter I sent to the editor:
"SIR - Your article on Electric Cars (July 29) repeats a common misconception that because the electricity used by such a car is produced by burning fossil fuels the "claims of greenery might sound a bit fishy." However, the thermodynamic efficiency of car engines is in the 20-30% range while the corresponding efficiency of the steam turbines of the electrical power plants is about double these numbers. Thus only half the fuel need to be burned for an electric car as compared to a gasoline power car. In addition, power plants can afford better emission filters, so the overall decrease in greenhouse gasses is even bigger."
While most of my US career has been in Computer Science, I was trained as a mechanical and electrical power engineer and I spent two years (1959-1961) working as an engineer in a power plant in Athens, Greece. So let me elaborate a bit.
When a heat engine (be it the gasoline of your car or your lawnmower, a truck diesel, or a stem turbine at a power plant) it cannot convert all heat to mechanical energy. The fraction converted to mechanical energy is the efficiency of the engine. Because of the laws of thermodynamics, the higher the temperature reached inside the engine the higher the efficiency. (If you to know why see, for example, http://ecen.com/content/eee7/motoref.htm but be prepared to deal with thermodynamic equations.) This is the basic reason why diesel engines are more efficient than ordinary gasoline engines and why large steam turbines are more efficient than internal combustion engines. It also happens that electricity is converted to mechanical energy with very high efficiency, usually well over 90%. Compare now a gasoline engine with 30% efficiency (on the high side) with an electric motor. The generating plant may have a thermal efficiency of 50% (on the low side) and the motor itself 90% (also on the low side). The total efficiency from fuel to mechanical energy is 45%, or 50% higher than that provide by the gasoline engine. Simple arithmetic shows that using an electric motor will burn only two thirds of fuel than using a gasoline engine.
The effects on pollution are even bigger. A lot of the pollution occurs when a gasoline engine starts or idles because combustion is imperfect. In a power plant starts and stops are far less frequent and, of course, they can have better emission filters. Power plants can also burn coal that decreases our dependence on oil. Coal produces more pollution but we need burn much less coal than oil.
Keep also in mind that electricity can be generated by hydroelectric power or wind power, so the effects on emissions or dependence on oil are even bigger than those suggested by the above analysis.
Because electric car batteries are going to be recharged mostly during the night when demand of electricity is low, it is unlikely we will need more power plants, we will only keep busier the ones we currently have. (I know from my days of working in such a place that demand between midnight and 6am was quite low.)
Why do we hear so often the argument that electric cars are not a solution because we must ultimately burn fuel to power them? Why is not important to reduce oil consumption by a significant fraction (30% or more)? Or do the oil and auto industries not like you to know the truth?
While we wait for the development of the electric car, try to help by using as much as possible electric powered tools rather than gasoline powered ones. I use an electric lawnmower, snow blower, and power saw. They are lighter, quieter, cleaner, and cheaper than the gasoline versions and the long power cord is less of a nuisance than people think. (The tools are also much easier to start.)
After I first posted this a friend pointed a particularly relevant site: