Science Forums 2

[CraigD]

Originally Posted by C1ay

First consider the fact that there are power grids now that suffer rolling blackouts during peak usage times as a result of overwhelming demand. These grids are already short of capacity and loading them with the local transportation loads would simply add to that problem.
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These numbers get better of course if you allow an hour for a fill up or you build in enough capacity that cars only charge at night but I don't currently think the grids are even sufficient for that.

As you note, C1ay, the inconstant demand that results in the need for rolling blackouts is a point in favor of electric cars. It’s often cited by electric car proponents.

Inconstant demand for electricity is what leads to the need for rolling blackouts. Mostly because of building cooling (air conditioning), demand for electricity is greatest during the day, and increases on hotter days. As a result, electric generator plants must be designed to produce much more power than their daily and yearly averages at peak hot day times, and much less at night and on cold days.

Because most electric cars are recharged between the time their users return home in the evening and leave for work in the morning, their demand occurs during off-demand hours, “leveling” power generation demand. Also, as an uninterrupted supply of constant voltage isn’t needed to recharge a battery, car charging could be supplied by separate supply lines, or managed by in-house systems, giving power managers more flexibility in supplying electricity.

Some have gone even further, utilizing the collective storage capacity of electric cars as part of the power grid itself. Because most cars are parked during periods of peak daytime cooling demand, their users having commuted to work earlier, if their batteries were connected to charging stations, they could actually be drawn down to make up for shortages from the power plants. After peak demand had passed, they would be charged back up.

All of these “power leveling” schemes are financially advantageous to power supply companies, savings that they could pass on to EV owners as reduced electric bills.

I couldn’t find a good reference on daily power demand, but based on the graph in this wikipedia article, typical daytime peak power demand appear to be about 40% greater than nighttime lows.

Originally Posted by C1ay

Secondly lets play with a little math to get us in the ball park. For the sake of easy computing let's assume the average fill up of gasoline is around 10 gallons. Let's also assume that by converting to electric we can gain an increase in efficiency that would allows us to fill up with only 40% of that figure since combustion engines are considerable less efficient than electrical propulsion systems. That brings our fill up equivalent to 4 gallons of gas.

Various sources on the internet give an equivalency of 120,000,000 joules to 130,000,000 joules of energy per gallon of gasoline, an average of 125,000,000 joules. That 500,000,000 joules per fill up.

I agree with the calculation so far. It agrees with the reference I gave to this wikipedia reference table in post #24.

Originally Posted by C1ay

Now suppose 10 minutes is a reasonable amount of time for someone to pull into the station and pump 500,000,000 joules of power into their vehicle. At 600 seconds that's around 833,333 watts per second or 3,000,000 kwh per vehicle to fill up. Multiply that by the number of cars per day filling up at a station and you'll get an idea of the power requirements.

: You’ve made a unit converting mistake here. “Watts per second” (which would be a unit of change in power) is not a normally used unit. 1 watt (a unit of power) is defined as 1 Joule per second.



so , not 3,000,000.

Practically speaking, there’s no current market-ready technology (at least none I know of) that will allow a car-size battery, such as the lithium-ion batteries in the Tesla roadster, to be recharged in 10 minutes. The Tesla, for example, requires at least 3.5 hours (210 minutes) to recharge its 53 kWh battery system, and is capable of this rate only because it has a motor driven liquid cooling system. It’s wise, I think, to assume that electric car recharging will remain an overnight processes for some time to come.

I’m not disputing that replacing all or a large fraction of oil-powered vehicles with electric ones would not be an engineering challenge. I am disputing that it’s an overwhelming one. As I noted in post #24, according to US energy use data, it’s less of a change than the routine electric system changes that were made from 1979 to 2007.