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What’s wrong with electric cars?

Recently, we’ve heard that Volvo are abandoning the internal combustion engine, and that both the United Kingdom and France will ban petrol and diesel cars from 2040. Other countries like China are said to be considering similar mandates.

All cars use stored energy to overcome air resistance at higher speeds and rolling resistance at lower speeds. Energy is also needed for acceleration. The advantage of the electrical car is the efficiency with which it uses the energy stored in the battery with minimal losses. By contrast the petrol or diesel powered car only uses about 20% of the chemical energy stored in the fuel. The disadvantage is that the energy density, the energy that can be stored for a given weight, in even the best batteries is 50 times worse than gasoline. The weight of gasoline in a car is less than 5% of the total weight of a car, while the weight of the battery is about 25% the weight of the Tesla electric car. However the extra weight of the battery is more than offset by the much higher efficiency of the electric motor compared to a petrol or diesel engine. The problem is filling them up.

Nobody likes spending time in petrol stations, but due to the high energy density of petrol or diesel fuel, it doesn’t take very long. I filled up my car with 12 gallons of petrol in 75 seconds. In power terms, that’s 20 MW, equivalent to the electrical consumption of 10,000 average homes consuming 2 kW. Taking account of the greater efficiency of the electric car would lower the electrical consumption equivalent to about 4.5 MW, or 2,250 homes.

Image credit: Tile by MabelAmber. Public domain via Pixabay.

Or we could look at it in terms of battery sizes. The new Tesla S has a 100 kWh battery, the much more modest Chevy Bolt 60 kWh. Filling them up from a home charging point limited to a few kW will take overnight. Charging from a fast charging point will be limited by the ability of the battery to accept charge. It takes an hour for a Li battery to accept a full charge if it is to last a few years and be recharged many times. Do you really want to hang around a filling station that long?

Charging at home will mean that cars have to be parked in garages. That’s fine for the wealthy with space to spare in their three-car garage. I don’t see this happening in most of England. My brother is the only person in his neighborhood of a few hundred houses who parks his car in the garage. It’s a small car by today’s standards and he has only centimeters to spare on either side!

The extra electrical energy needed if all vehicles were converted from gasoline or diesel can be estimated from the energy content of oil consumed, divided by about four to account for the increased efficiency of the electric car compared to the petrol or diesel car. In most developed countries this would imply a need to generate 25% more electrical energy. However the electrical companies will have to distribute substantially more power to the home. To fully charge a car overnight requires up to 10kW, a major increase on average consumption. Fast charging stations also present a challenge for the electrical company. The equivalent of the local petrol filling station will require 1-2 MW or electrical power, about the same as a small shopping mall.

If the cars are charged at home don’t expect to use solar to charge your car, unless you only drive at night and leave the car in the garage during the day! Given that most people will want to charge their cars at night (like their cell phones!) this will smooth out the electrical demand during the day. Constant electrical power demand is best met by nuclear, coal, hydro electricity, or combined cycle natural gas. In fact if coal were used to generate the extra electricity required by electric cars there would be no reduction of CO2 emissions compared to petrol or diesel cars.

So even if the cost and the range problem are solved, the time taken to fill up an electric car still makes it unattractive compared to a hybrid or a conventional vehicle.

Featured image: Electric car by geralt. Public domain via Pixabay

 

Recent Comments

  1. Daniel

    This is a ridiculously outdated article. I thought it was written in 2015. Read up. Do some research.

  2. Batterygeek

    I thought this argument became a bit lame toward the end of the article. In the U.S. the average daily commute is 40 miles roundtrip. Since the Chevy Bolt gets 238 miles per battery charge, this leaves plenty of room for errands. As for the annoyance of charging overnight, the Bolt does take 8 hrs at 120v but at 220 it can charge in four hours. By leaving out these facts, the author has tried to shore up his ify conclusion.

  3. Peter Rez

    Daniel:

    I used to do research on the physics of battery materials and am a co-author of 5 papers on this subject. On attending the Spring 2017 Materials Research Society conference I found that nothing fundamental had changed. All that’s happened is that the cost has been dramatically lowered by Musk and others adopting more efficient manufacturing techniques. There is a possibility of marginally improving energy density and eliminating the risk of the batteries catching fire by changing to solid-state electrolytes. The limitations in energy density are mainly from the cathode, where lithium cobalt oxides and the lithium iron phosphates are still being used. The conductivity of the cathode, anode and electrolyte limits charging rates.

    Batterygeek:

    You are correct it’s quite feasible to use electric cars, even with their battery limitations, for short commutes. Given the long charging times it actually makes more sense to use electric cars for an on-demand autonomous vehicle taxi (think Uber or Lyft). The vehicles could remain at charging points until summoned for a journey that could be matched to their state of charge.

    When you say that the car took half the time to charge at 220 V than 110 V what’s happened is that you are supplying the same current at twice the voltage. Power is the product of current and voltage, so the power you are supplying also doubled. That’s why I express times in terms of the power available.

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