Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: colarris on 11/08/2019 15:30:46
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My school science lessons taught me that water can be split and I have read of some high profile attempts at producing a water fuelled engine. Wikipedia lists quite a few but they all seem to eventually be shot down by science. What is the problem and is it actually posibble?
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What is the problem
It takes at least as much effort to split the water as you could get by using the hydrogen produced as a fuel.
It would be better to use that effort to drive the car directly.
is it actually posibble?
No.
Well, strictly, it's not impossible, but it would be stupid.
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You can get net energy by reacting chemicals like lithium or fluorine with water, but it wouldn't be worth the trouble. Unlike an air-breathing engine, you'd have to carry those extra chemicals in the car and you wouldn't get nearly as much range as you would burning gasoline. Then there is the hazardous nature of lithium and fluorine to consider...
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Appreciate the replies.
That explains why the most high profile attempt was discredited. They said that his numbers didn't add up and if they did then he had essentially invented a perpetual motion engine. I didn't quite understand what they were saying but you made it quite clear. Thanks again.
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It is entirely possible and has some potential value. The key parameter of transport fuel is energy density. Assuming a free and unlimited supply of oxygen (from the air), the energy density of hydrogen is about 140 megajoule/kilogram, about 3 times the value for any other chemical fuel and 30 times that of any battery.
The joy of hydrogen is that it can be "made from" surplus electricity generated by solar and wind farms, stored and transmitted by the existing gas grid (which originally handled "town gas", 50% hydrogen), and burned in conventional vehicle engines with some modification to the intake manifold. The problem is carrying the high-density liquid at - 253 deg C, or a useful volume of low-density gas. However the gap between theory and practicality is not all that great: at 200 atm pressure the energy density of hydrogen gas approaches that of the best batteries and recharge time is comparable with liquid fuel.
An alternative use would be to synthesise conventional liquid fuels by reducing atmospheric carbon dioxide or organic waste to methane or propane (easily transportable as liquids and requiring very little engine modification) or higher alkanes that can be substituted directly for petrol and diesel. The ability to synthesise a narrow spectrum of alkanes instead of distilling it from crude oil would lead to cheap, clean aviation fuels as well as solving all sorts of problems with pollution and excess atmospheric CO2.
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You can get net energy by reacting chemicals like lithium or fluorine with water, but it wouldn't be worth the trouble.
Depends on your definition of "net". Neither is present as element in nature, only as compounds which have to be dissociated at energy cost to provide your fuel. It just happens that of all the available compounds that can be readily dissociated with freely available electricity, water is cheap, plentiful, and easy to handle.
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In the early days of the underground railways locomotives were fueled by hot water (the fires shut down to prevent smoke) , of course the amount of energy that can be stored in hot water is very low but a car could be run on water for a short distance.
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freely available electricity,
Freely, but not free.
Other storage media (like batteries) are available.
Fuel cells might just be competitive, but the big problem with hydrogen is the poor energy density.
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the big problem with hydrogen is the poor energy density.
It is close to feasible. 1000 liters of hydrogen at 200 atm pressure has the same energy content as 63 liters of diesel fuel - a standard tankful for a car. Now whilst a 1000 liter tank is certainly bigger than a 63 liter one, the contents weigh less than 20 kg.
A further compromise is worth considering in view of the limited range of the competitive battery vehicles. The standard 60 kg diesel load will take a car about 600 miles at 70 mph and refuel in less than 5 minutes for the return journey. To cover 1200 miles in the best battery car will require about 4 x 30 minutes = 2 hours' recharging en route with best present technology at 50 mph, more if you do the whole trip at 70 mph. If we limit our hydrogen car to 200 miles range, with a 300 liter tank, the five refuelling stops at 70 mph will only take about 30 minutes total, and we have saved about 200 kg of battery weight so acceleration and stopping should be better.
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freely available electricity,
I was recently in Norway, and they quoted some very high statistics in the percentage of electric vehicles being sold today.
Electric cars are being actively promoted by the government
- They have concessions like permission to drive in bus lanes (this might work until the majority of cars on the road are electric!).
- Unlike most countries, Norway has a lot of "green" hydro power.
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the big problem with hydrogen is the poor energy density.
It is close to feasible. 1000 liters of hydrogen at 200 atm pressure has the same energy content as 63 liters of diesel fuel - a standard tankful for a car. Now whilst a 1000 liter tank is certainly bigger than a 63 liter one, the contents weigh less than 20 kg.
A further compromise is worth considering in view of the limited range of the competitive battery vehicles. The standard 60 kg diesel load will take a car about 600 miles at 70 mph and refuel in less than 5 minutes for the return journey. To cover 1200 miles in the best battery car will require about 4 x 30 minutes = 2 hours' recharging en route with best present technology at 50 mph, more if you do the whole trip at 70 mph. If we limit our hydrogen car to 200 miles range, with a 300 liter tank, the five refuelling stops at 70 mph will only take about 30 minutes total, and we have saved about 200 kg of battery weight so acceleration and stopping should be better.
It's certainly an interesting technology.
I gather they are talking about 700 bar tanks, which makes matters "easier" from some points of view.
However, all this stuff is off- topic.
A hydrogen powered car (possible but tricky) is not a water powered car (a breach of the laws of thermodynamics).
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Pedantically true (we expect nothing less!) but the OP was talking about dissociation and recombination of water, which to my mind is pretty similar to the oxygenation of hydrogen.
There are a few wartime film clips of buses and taxis with huge gas bags, and some with coal braziers actually making producer gas en route. I've not heard of any elfinsafety concerns with these contraptions, but compared with the attentions of the RAF and Luftwaffe, I guess any form of urban transport was better than standing still.
The other gas that intrigues me is "Brown's Gas" - the stoichiometric mixture of H and O that arises from the AC electrolysis of water. It turns out to be quite stable and wellbehaved, and has the advantage over all other combustion fuels that it doesn't deplete the atmosphere - it burns all by itself to produce pure steam and can be used indoors. AC electrolysis has all sorts of advantages: no electrolytic corrosion of the electrodes, and the possibility of "tuning" the cell to optimum yield with minimal ohmic loss. Its widespread introduction would require a through revamping of the gas grid, but its advantages as a domestic, industrial or transport fuel are significant, and it is potentially the most efficient dump for surplus electrical energy.
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It turns out to be quite stable and wellbehaved
I think that's best described as " a matter of definition".
https://deepblue.lib.umich.edu/bitstream/handle/2027.42/37313/690060330_ftp.pdf?sequence=1
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Just like petrol!