Thursday, August 20, 2009

What is the relative cost of charging plug-in hybrid vs. non-plug-in hybrid electric vehicles?

The current crop of hybrid electric vehicles depends on the gasoline engine (and braking) to recharge the batteries. The coming generation of plug-in hybrid electric vehicles can also recharge the batteries by plugging into a normal electric receptacle. My question is what the cost differential is between paying for gas to do the charging on the go versus the hit to your electric bill to recharge at home overnight.

In the first case, you have the cost of gasoline and how much charging is accomplished per gallon of gasoline.

In the second case you have the cost per kilowatt-hour of your residential electricity as well as the charging efficiency. How much of each kilowatt-hour actually end up in the batteries of your vehicle?

I am not enough of an engineer to know the answer.

My hunch is that at least overnight, charging from "the grid" will be cheaper, as well as having no local carbon-based emmissions.

Any engineers out there?

-- Jack Krupansky


At 4:48 PM MDT , Blogger Lee Devlin said...

Hi Jack,

The charging efficiency from gasoline or from the grid is similar. About 70% of the thermal energy embodied in coal or gasoline is lost through the conversion. Natural gas can do a little better through 'combined cycle' where the fuel is combusted in a gas turbine and then the gas turbine exhaust is used to heat a steam boiler. In that case it's possible to get around 60% thermal to electrical conversion.

However, natural gas is expensive, at least historically, per BTU compared to coal so it's usually just used in 'peakers', i.e., gas turbines to respond to the peak load on the electrical system. It takes hours to stabilize a steam turbine, so peakers are not set up in combined cycle because they need to be brought on line in minutes, not hours.

The advantage of electricity vs. gasoline is that coal is amazingly cheap per BTU, about 1/10 the cost of gasoline per BTU. The energy gets more expensive as it makes its way through the extensive capital investment of the electrical company, but still comes out cheaper per kWh than using gasoline to charge your battery.

Then you can also make an argument that someone can use solar, wind, or hydro to power a car if it's a plug-in hybrid. I have a solar array that is over-producing by about 20% currently, and as I figure out ways to cut my domestic consumption of electricity, I am fairly confident that I can free up enough electricity to power a plug-in vehicle for a 40 mile per day commute, assuming a 4 mile per kWh car (i.e., similar to the Tesla). So I am pretty sure that my solar array can power my house and one car with no fuel cost.


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