MAIL: Electrical Energy Equivalency

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Electricity Sources Pie Graph
Even with the best engine technology, the second law of thermodynamics puts a limit on the percent of heat energy that can be converted.
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Electricity Sources Pie Graph
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In the article “Plug-In Vehicles” (HP151), Andy Kerr points out that 1 gallon of gas has the “energy equivalent of 33.7 kWh” and that an electric vehicle that gets 3 miles per kWh would be equivalent to 100 miles per gallon [of gasoline].

This is a bit misleading because not all the energy in 1 gallon of gas can be turned into electricity. Even with the best engine technology, the second law of thermodynamics puts a limit on the percent of heat energy that can be converted.

Typically, utilities turn about one-third of the heat generated into electricity. The best they can do is close to 50%. This puts the equivalent energy consumption of 3 miles per kWh between 33 miles per gallon and 51 miles per gallon.

Larry Schlussler • Arcata, California

The U.S. EPA developed “miles per gallon-equivalent” (MPG-e) because consumers are used to mpg for their gasoline- (or even diesel-) powered cars and needed a way to compare. The EPA says “Electric vehicles convert about 59% to 62% of the electrical energy from the grid to power at the wheels—conventional gasoline vehicles only convert about 17% to 21% of the energy stored in gasoline to power at the wheels.” (See bit.ly/EPA_EV.)

The methodology they developed (described here: bit.ly/MPGequivalent) came up with 1 gallon of gasoline equaling 33.7 kilowatt-hours. In other words (and yes, I would agree it is approximate, if not misleading), a car can travel the same distance on 1 gallon of gas or 33.7 kWh of electricity.

Andy Kerr

[Editor’s note: If you scroll to the bottom of the Wiki page that Andy mentions above, it details “well-to-wheel” comparisons, “accounting for the upstream efficiency of electricity generation and transmission, etc. In the example provided by the U.S. DOE in its final rule, an electric car with an energy consumption of 265 Wh per mile in urban driving and 220 Wh per mile in highway driving resulted in a petroleum-equivalent fuel economy of 335.24 miles per gallon.”]

Comments (1)

jerryd's picture

I'm sorry but way too much misiformation here.

Coal is about 1/3 of US electric production now and dropping as more coal plants get shut down as too polluting and not cost effective. The data shown is seriously out of date. Typically they are now getting about 45% of the heat to electricity because both coal is about 40% eff as the less eff ones have shut down and NG is about 58-60% eff now.

Hydro, wind, solar and nuke of course have almost no CO2 emissions.

While they are right that EV's get about 65% eff of electric supplied, ICE's only get about 7% of it's fuel to the wheels because they never except when accelerating operate in their eff zone. Most of the time they are either idling, coasting or only using 5-10% of the engines power when at steady speeds. Thus they only get about 7% of their fuel to the road. Many use as much power as it takes an EV to do 50 mph just idling the engine. It called part throttle eff.

Gasoline requires 3kwhrs just to refine it amoung many other losses.

Most EVers either make there own RE or buy it from utilities thus little CO2 used. It only takes 1kw of PV now costing $1k for 20-25 yrs of fuel. sunelec among other suppliers.

So from these real facts I can hardly see how EV's get the same CO2 as a compact car. But far less in real life today and even less by the time serious numbers of EV's are on the road so many more coal plants are going the shut down over the nex 2 yrs and many more hiugh eff NG and more RE takes a larger share..

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