Originally Posted by silentm
but when we are talking about fuel in a car we have to look at how much energy it can produce per unit of volume and here hydrogen puts out a measily 2.8 GJ/m³ (that's 1000 litres) at 35 MPa in its gaseous state. (i don't have the exact number for petrol here as it's not written in the text, but when i look at the graph i have it's about 1/10th of petrol)
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There is that issue, but it's not as big as it seems. A modern midsize car has at least 150 of total volume. Each cubic foot being 7.5 gallons, a 20-gallon fuel tank is just about 3 cubic feet, 2% of the car.
1/10 is about right. So if you had the car using compressed H2, the tanks would take 12% to 20% depending on the pressure (there are 70-MPa storage tanks on the market).
But then a 20-gal tank is for a 600-mile range, which you don't really need. These 20-gal tanks are there only because it costs very little to enlarge the tank. With half that capacity, the tank will take just 6%-10% of the car. It's not a big compromise.
Of course, tankage weight needs to be worked on, so that this tank doesn't weigh 200kg.
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when hydrogen is a liquid things look a bit better, where we have about 8.5 GJ/m³ but that means cooling the hydrogen to 20 K (that's -423 °F and -253 °C) which takes in itself a huge amount of energy.
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Yes, LH2 is pretty much out of the question for civilian non-aerospace applications. Energy loss is just the smaller of the problems. A bigger one is when you spill some LH2 on your car and it cracks apart there, due to a thermal shock. Or when you're about to drive after spending a week on an air trip, and find out the tank is empty, because LH2 has all evaporated... so you have to call a special emergency refueling vehicle.
OTOH, the military is already interested in liquid hydrogen. Fuel cells are not necessary. And cost is less of an issue there.
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now the biggest problem probably is obtaining the hydrogen... the easiest way to obtain it is splitting water into Oxygen and Hydrogen.
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Actually the easiest way is steam reformation of natural gas. Electrolysis only sounds easy.
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so the energy we invested in the first place will be produced again when burning the hydrogen so we end up neither gaining nor loosing energy. which is not very efficient right?
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It's the same way with batteries. Of course there's no perpetuum mobile. We're running out of cheap portable fuel (oil), it will have to be central power plants (at first coal, later probably nuclear, by 22nd century possibly fusion), and simple power carrier.