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If we can use hydrogen to power electric motors, why can’t we use water to run a car?

Can’t you just break down water, use the hydrogen to power the electric motor, and I don’t think O2 as a byproduct is bad, now this is of course an ideal condition, but why hasn’t this been looked into more?

37 comments
  • I'm not a chemist/physicist, but if I had to guess it's because it takes more energy to break apart the water into Hydrogen than what you will get by burning the Hydrogen to power the motor.

    • Your guess is accurate.

      The other side of the issue is that "water" is rarely only water. There are tons of other shit floating in the water that cause problems with the splitting process, so you usually have to clean the water first which takes even more energy/resources.

    • Correct. Splitting hydrogen from water is quite energy intensive. Burning hydrogen into oxygen to make water releases energy, but not as much energy as it takes to split the hydrogen off in the first place. The reason to use hydrogen fuel cells is that the extra energy needed to generate the hydrogen is still far better than the carbon output and costly materials needed for making and charging a battery. Batteries need rare earth metals, and they lose their charging ability over time. Splitting water into hydrogen creates "potential energy" from the later creation of water again, making it a useful, clean way to store electricity.

      Same as the plans for using cranes stacking concrete bricks to store electricity. It takes more electric to stack them than is produced by unstacking them. But it's a clean way to store potential energy, and far more efficient and sustainable than a battery.

      • The reason to use hydrogen fuel cells is that the extra energy needed to generate the hydrogen is still far better than the carbon output and costly materials needed for making and charging a battery.

        This is just absolutely a false statement. Hydrogen is a carbon fuel, because all of it for practical purposes comes from natural gas. Although it is possible to get hydrogen through electrical hydrolysis, this simply is not where hydrogen as a fuel source comes from today.

        If you see or hear hydrogen being discussed, translate the word hydrogen to mean 'natural gas' or fossil fuels, because that's what you are actually talking about. We do not currently get hydrogen as a fuel by splitting. We currently get hydrogen as a fuel by splitting hydrogen from natural gas. You would likely be better off just driving a gas car than a hydrogen powered if your goal is overall emissions reductions. Batteries represent an actual renewable technology because right now (not hypothetically) we can and do power the electrical networks that charge them with renewables. In as far as renewable hydrogen is concerned, there basically is none, because it costs so much more so to produce in this manner than it does to get hydrogen from fossil fuels.

  • As a lot of people have already said, breaking water down into hydrogen and O2 requires more energy than that produced by running a car on hydrogen.

    Luckily, there's some promising research underway, on solar panels that converts water vapour in the air into hydrogen. Last I read, they're approaching kilowatt scale, but it takes a big system to produce just 500 grams of hydrogen in a day. Which will only produce around 2kW of output power.

    Assuming they can somehow make all of that much smaller, and produce much, much more hydrogen from that smaller system, there's the secondary problem of storing and pressurizing the hydrogen produced, for use in a vehicle. That will take more energy again.

  • Because it will always take more energy to break the water than you will get burning the Hydrogen in Oxygen back into water - it's basic thermodynamics.

    You will lose some energy as heat that you cannot get back.

    You can't power a car from a process that loses energy. Even if you use a battery to donate the lost energy, then you might as well just cut out the lossy middleman and just run off the battery or generate the Hydrogen elsewhere - which is what we currently do.

    It is better to think of Hydrogen as an energy transporter than as a fuel, as you'd need to generate the Hydrogen somewhere that has abundant energy (ideally renewable), then transport I where needed, such as a Hydrogen powered generator.

    Interestingly the fact that all processes generate waste heat means the only theoretically 100% energy efficient process is heat generation itself, as all forms of energy eventually degrade to heat (as it is essentially the universe's waste energy).

    • Electric heaters aren't 100% efficient, because some of the energy is wasted altering the resistive material chemistry and emitting other electromagnetic radiation, and even sound, that doesn't heat the air inside your house (right away). Still at a perfect 100% efficiency, it takes 1 joule to raise your house temperature by 1 joule, ideally. Heat pumps, which are just an AC unit running in reverse, are more efficient than electric heaters. Some heat pumps have a coefficient of 3. Meaning they take 1 joule to heat the air in your house by 3 joules. Because they don't try to heat the air, they move heat to the air from outside, and they can achieve this even if outside is freezing.

      • That's the theoretically part - there are processes that will capture the energy generated that would've otherwise become heat, but that only affects the timeliness. Given enough time, all workable energy generated by a heater would become heat, even if you had to wait for the matter itself to decay trillions of years from now when all the stars have long since breathed their last breath.

        Also has somebody watched Technology Connections by any chance?
        Heat pumps are so cool - if you showed onw to someone even a hundred years ago, even knowing what electricity was, they'd think it was magic.

  • What you're asking for is, as far as my basic college understanding of physics goes, pretty much impossible. You're asking for a perpetual engine, looping electrolysis and catalyzing hydrogen and oxygen, creating energy at a net positive. This is impossible in terms of the laws of energy conservation. Energy can neither be created nor destroyed. It can only be converted from one form into another. Basically, you will only ever get what you put into it. Moreover, in real world applications, you will lose energy to things like heat runoff.

    The reason hydrogen works as a fuel source is because of its potential energy. Hydrogen really wants to bond with things. Same thing with oxygen and so they have a high potential energy. However, combine hydrogen and oxygen into water and you've got a basically inert molecule that then takes a lot of energy to break back apart. That energy is then converted back into potential energy. The problem again is that this relies on your engine being a perfect system, which can't exist in practical, real world applications. You WILL lose energy in the reaction in any real world scenario. Meaning you will alway need to put more energy into the system. Thus, always returning a net loss.

    That's kinda why we haven't gotten fusion reactors working quite yet. Solve that problem and you've solved the world's energy crisis.

37 comments