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Author Topic: How does a hydrogen fuel cell-powered car work?  (Read 48170 times)

sooyeah

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How does a hydrogen fuel cell-powered car work?
« on: 04/12/2007 13:02:14 »
O.k how does a hydrogen fuel cell car work?



« Last Edit: 31/07/2008 00:36:58 by chris »


 

Offline Nobody's Confidant

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #1 on: 04/12/2007 17:29:58 »
Burns hydrogen instead of oil. So you can fill your car up with water, and only oxygen comes out the back.

Actually, I saw a commercial for this on TV, they said water vapor comes out which I find hard to beileve cause I thought we were years away from these things.
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #2 on: 04/12/2007 18:16:59 »
OK, firstly you cannot (except for some unsubstantiated claims) run fuel cells on water.  Water is the exhaust product, but hydrogen and oxygen (usually air, but sometimes pure oxygen) are the inputs.

Technically you might say that the hydrogen is burnt, insofar as it is oxidised, but since it is not a thermodynamic process, heat generation is not a desired (or even desirable, although it is inevitable) part of the process.

In any chemical reaction, the reaction requires some sort of exchange of electrons between the constituent parts of the reaction.  What a fuel cell (in its usual sense, although one might suggest it can in a literal sense have wider meanings) tries to do is divert this exchange of electrons and so create an electric current.

In essence, what a fuel cell is doing is in principle no different from an electric battery.  The main difference is that in an ordinary battery, the chemicals that are used to create the electric current are sealed within the battery, and when they are used up, the battery has finished its useful life.  With a fuel cell, you continuously feed more chemicals into the battery, so that it does not end its useful life simply because the chemicals already contained in there have been exhausted.

In the case of a zinc-carbon battery, the chemicals used are zinc and manganese, and they are contained within the battery, and when they run out, the battery is useless.  With a hydrogen fuel cell, the chemicals are hydrogen and oxygen, and they are continually fed to the fuel cell, so they do not run out (or, if your tanks run dry, you just fill up with more hydrogen, but you don't need to throw away the fuel cell).
 

Offline Mr Andrew

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #3 on: 04/12/2007 22:04:22 »
Anybody know how they are currently making the hydrogen transportable?  I thought you had to liquefy it and hold it under extreme pressures.  This is certainly not practical for the average car.  Is there some new method that I don't know about?
 

Offline kalayzor

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #4 on: 04/12/2007 22:17:41 »
Yeah -- some chemists at UCLA made a sort of hydrogen "sponge" out of an exotic alloy (a metal organic framework (of which there are MANY different possibilities at to composition, etc)) that can absorb up to 0.98-1.12% of its weight in hydrogen at 298K and...well...47.4atm (I also found 98.7atm...and both sources were accredited in some way, but neither had their methods shown.  I think that they were testing two different forms of the material).

That pressure is going to have to come down, but it should as the materials develop.
 

sooyeah

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #5 on: 05/12/2007 17:12:53 »
How much hydrogen is needed for an average fuel cell car, to run for say a week?

 
 

sooyeah

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #6 on: 06/12/2007 11:47:58 »
How much hydrogen is needed for an average fuel cell car, to run for say a week?

Do we not know the answer to that question? or is it too vague? lets say 100 miles, how much hydrogen do you need?
 

lyner

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #7 on: 06/12/2007 13:05:18 »
Quote
How much hydrogen is needed for an average fuel cell car, to run for say a week?
I understand that the Enthalpy of Hydrogen is  about three times that of petrol. You would need about one third of the mass of Hydrogen, compared with the equivalent mass of petrol. That only offsets, by a tiny amount, the overhead involved with storing hydrogen in a stable form. The best that has been achieved has been  about 6% by mass. That means you would still need to carry around about four times the extra mass of the (very expensive) storage medium.  This compares badly with the mass of a petrol tank. The medium can be easily contaminated by an impure hydrogen supply. Also, I should imagine that re-charging would take a lot longer than just pouring 40litres of petrol into your tank.
We have some way to go, yet but, when it can be made to work, it will be very clean - but not a free energy resource.
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #8 on: 06/12/2007 16:55:09 »
I understand that the Enthalpy of Hydrogen is  about three times that of petrol. You would need about one third of the mass of Hydrogen, compared with the equivalent mass of petrol. That only offsets, by a tiny amount, the overhead involved with storing hydrogen in a stable form. The best that has been achieved has been  about 6% by mass. That means you would still need to carry around about four times the extra mass of the (very expensive) storage medium.  This compares badly with the mass of a petrol tank. The medium can be easily contaminated by an impure hydrogen supply. Also, I should imagine that re-charging would take a lot longer than just pouring 40litres of petrol into your tank.
We have some way to go, yet but, when it can be made to work, it will be very clean - but not a free energy resource.

http://en.wikipedia.org/wiki/Energy_density#Energy_density_in_energy_storage_and_in_fuel
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In energy storage applications, the energy density relates the mass of an energy store to its stored energy. The higher the energy density, the more energy may be stored or transported for the same amount of mass. In the context of fuel selection, that energy density of a fuel is also called the specific energy of that fuel, though in general an engine using that fuel will yield less energy due to inefficiencies and thermodynamic considerations—hence the specific fuel consumption of an engine will be greater than the reciprocal of the specific energy of the fuel. And in general, specific energy and energy density are at odds due to charge screening.

Gravimetric and volumetric energy density of some fuels and storage technologies (modified from the Gasoline article):

    (Notes: Some values may not be precise because of isomers or other irregularities. See Heating value for a comprehensive table of specific energies of important fuels. The symbol ** indicates the item is an energy carrier, not an energy source.)
   
storage type
energy
density
recovery
efficiency
by mass
by volume
peak
practical
MJ/kg
MJ/L
%
%
**mass-energy equivalence89,876,000,000
**binding energy of helium nucleus675,000,0008.57x1024
nuclear fusion of hydrogen (energy from the sun)300,000,000423,000,000
nuclear fission (of U-235) (Used in Nuclear Power Plants)77,000,0001,500,000,00030% 50%
**liquid hydrogen14310.1
**compressed gaseous hydrogen at 700 bar1434.7
**gaseous hydrogen at room temperature1430.01079
beryllium (toxic) (burned in air)67.6125.1
lithium borohydride (burned in air)65.243.4
boron (burned in air)58.9137.8
compressed natural gas at 200 bar53.610
gasoline46.934.6
diesel fuel/residential heating oil45.838.7
polyethylene plastic46.342.6
polypropylene plastic46.341.7
gasohol (10% ethanol 90% gasoline)43.5428.06
lithium (burned in air)43.123.0
Jet A aviation fuel42.833
biodiesel oil (vegetable oil)42.2030.53
DMF (2,5-dimethylfuran)4237.8
crude oil (according to the definition of ton of oil equivalent)41.8737
polystyrene plastic41.443.5
body fat metabolism383522-26%
butanol36.629.2
LPG34.3922.16
**specific orbital energy of Low Earth orbit33 (approx.)
graphite (burned in air)32.772.9
anthracite coal32.572.436%
silicon (burned in air)32.275.1
aluminum (burned in air)31.083.8
ethanol3024
polyester plastic26.035.6
magnesium (burned in air)24.743.0
bituminous coal2420
PET pop bottle plastic?23.5 impure?
methanol19.715.6
**hydrazine (toxic) combusted to N2+H2O19.519.3
**liquid ammonia (combusted to N2+H2O)18.611.5
PVC plastic (improper combustion toxic)18.025.2
sugars, carbohydrates & proteins metabolism1726.2(dextrose)22-26%
Cl2O7 + CH4 - computed17.4
lignite coal14-19
calcium (burned in air)15.924.6
dry cowdung and cameldung15.5
wood6–171.8–3.2
**liquid hydrogen + oxygen (as oxidizer) (1:8 (w/w), 14.1:7.0 (v/v))13.3335.7
sodium (burned to wet sodium hydroxide)13.312.8
Cl2O7 decomposition - computed12.2
nitromethane11.312.9
household waste8-11
sodium (burned to dry sodium oxide)9.18.8
iron (burned to iron(III) oxide)7.457.9
Octanitrocubane explosive - computed7.4
ammonal (Al+NH4NO3 oxidizer)6.912.7
Tetranitromethane + hydrazine explosive - computed6.6
Hexanitrobenzene explosive - computed6.5
zinc (burned in air)5.338.0
Teflon plastic (combustion toxic, but flame retardant)5.111.2
iron (burned to iron(II) oxide)4.938.2
**TNT4.1846.92
Copper Thermite (Al + CuO as oxidizer)4.1320.9
Thermite (powder Al + Fe2O3 as oxidizer)4.0018.4
**compressed air at 300 bar40.14?
ANFO3.88
hydrogen peroxide decomposition (as monopropellant)2.73.8
Lithium Thionyl Chloride Battery2.5
Regenerative Fuel Cell1.62
**hydrazine(toxic) decomposition (as monopropellant)1.61.6
**ammonium nitrate decomposition (as monopropellant)1.42.5
Molecular spring~1
**sodium-sulfur battery?1.23?85%
**liquid nitrogen0.77   0.62
**lithium ion battery0.54–0.720.9–1.995%
**lithium sulphur battery0.54-1.44?
kinetic energy penetrator1.9-3.430-54
5.56 × 45 mm NATO bullet0.4-0.83.2-6.4
**Zn-air batteries0.40 to 0.72???
**flywheel0.5??81-94%
melting ice0.3350.335
**zinc-bromine flow battery0.27–0.306
**compressed air at 20 bar0.27?64%
**NiMH Battery0.220.36?60%
**NiCd Battery0.14-0.22??80%
**lead acid battery0.09–0.110.14–0.17?75-85%
**commercial lead acid battery pack0.072-0.079???
**vanadium redox battery.09.1188?70-75%
**vanadium bromide redox battery.18.252?81%
**ultracapacitor0.0206???
**ultracapacitor by EEStor (claimed capacity)1.0???
**supercapacitor0.01?98.5%90%
**capacitor0.002???
water at 100 m dam height0.0010.001?85-90%
**spring power (clock spring), torsion spring0.00030.0006?
zero point energy00

Ofcourse, all of the above only reflects the energy density of the fuel itself, and as you have mentioned, this is ultimately meaningless without taking into account the volume and mass of the container, and other peripheral systems required for the storage and production of energy (for instance - nuclear fuel has a very high energy density, but building a whole nuclear powerstation around it does rather reduce the overall system wide energy density; and ofcourse, with hydrogen too, you have high containment costs).

Also interesting to note how much less efficient petrol/ethanol mix is - so if you put the same amount of petrol/ethanol mix in your car as you would have previously put petrol in your car, it will not take you as far.

The other thing you have to bear in mind is that energy density by volume is at least as important as energy density by mass (and it is only when we look at energy density by mass that hydrogen looks good at all).  The problem is that if you require a bigger fuel tank (even if the mass of the fuel may be lighter), then the amount of material you need to build into the car increases (and in the worst case, so might the aerodynamic efficiency of the car).  Also, an increase in required volume for the same energy requires an increase in engine size to produce the same power.
« Last Edit: 06/12/2007 17:04:05 by another_someone »
 

lyner

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #9 on: 06/12/2007 18:35:25 »
I agree. What price the 'wonder fuel'?
Even batteries seem as attractive.
 

Offline Dick1038

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #10 on: 06/12/2007 19:37:45 »
There has been a new fuel cell technology developed. It uses a hydrogen compound that is liquid at room temperature and doesn't require the expensive platinum catalyst.  I don't recall the details.
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #11 on: 06/12/2007 21:03:32 »
There has been a new fuel cell technology developed. It uses a hydrogen compound that is liquid at room temperature and doesn't require the expensive platinum catalyst.  I don't recall the details.

Since petrol, and other oil and alcohol products, are all compounds of hydrogen, so I am not sure what exactly you are thinking of her, because we already have liquid compounds of hydrogen that we can use as a fuel.
 

lyner

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #12 on: 08/12/2007 11:05:11 »
well said!
 

sooyeah

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #13 on: 08/12/2007 15:08:39 »
With a hydrogen fuel cell, the chemicals are hydrogen and oxygen, and they are continually fed to the fuel cell, so they do not run out (or, if your tanks run dry, you just fill up with more hydrogen, but you don't need to throw away the fuel cell).

So oxygen is used as well? It's a hydrogen oxygen liquid fuel mix? Could you not also used liquid helium, or nitrogen? With transfer of electons, surely any Gas atom could be used. Could a combination liquid work better?

If oxygen is also used in the current fuel cell, could it not run on just liquid oxygen? If you could use liquid helium, it is not a far more stable liquid compared to hydrogen?

So could you not have a high proportion of helium and lower levels of hydrogen and oxygen in order to make it a safer liquid? Or, extending that idea, could you develop an helium fuel cell?
« Last Edit: 08/12/2007 15:11:16 by JOLLY »
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #14 on: 08/12/2007 17:08:30 »
So oxygen is used as well?
Yes, it is crucial, but oxygen is readily available from the air, so you don't need to tank up on it.

It's a hydrogen oxygen liquid fuel mix?

The hydrogen and oxygen are stored separately.  To premix them would have a dangerously explosive mix, and would anyway have to be separated when entering the fuel cell.  In any case, as I said, for most applications, you don't need to carry your own oxygen (it is normally available from the air around you - unless you are looking for applications in space, or submarine applications).

As for whether the hydrogen is liquid - cryogenically stored liquid hydrogen is one choice, so is high pressure gas storage, and so is gas absorption in some solid matrix.  Each solution has its own problems.

Could you not also used liquid helium, or nitrogen? With transfer of electons, surely any Gas atom could be used.

No, it depends on the structure of the electrons around an atom.  All atoms are naturally electrically neutral (i.e. the number of positively charged protons in an atom normally balances the number of negatively charged electrons), so there is good reason not to give away or accept another electron, since that would make them no longer electrically neutral.  On the other hand, atoms also like to have certain specific configurations of electrons around them in preference to other configurations, and if they have too few or too many electrons for their ideal configuration, then they may give up or accept another electron in order to obtain an ideal configuration.  When you get an atom that has too many electrons for its ideal configuration, and one that has too few electrons for its ideal configuration, then the one that has too many electrons will give away its electron, and the one that has too few will accept an electron, and between them you will get a stable chemical compound (that, in a nutshell, is the basis of all chemistry).

The first rule is that electrons like to go around in pairs.  Normal helium has exactly two electrons, and so for it, that is an ideal number of electrons, and so it will have great difficulty either accepting another electron or giving up an electron.

For oxygen, it naturally has 8 electrons, but the ideal number of electrons for an atom of about oxygen's size is 10 electrons.  Neon is slightly heavier than oxygen, and has 10 electrons, and so, like helium, neon is very difficult to make chemical compounds with.

Hydrogen would really like to have 2 electrons (like helium, its heavier cousin), but it will just as readily give up its one electron, so it has no electrons.

If you then have two hydrogen atoms, each willing to give up one electron, and one oxygen desperately seeking to get two more electrons, you can see they are in a strong position to make a swap - and between them make a chemical compound of two hydrogen atoms and one oxygen atom - this compound is water.

« Last Edit: 08/12/2007 17:10:02 by another_someone »
 

lyner

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #15 on: 09/12/2007 18:12:15 »
Is this the silly season for nakedscientist forums?
Doesn't anyone read books before starting to post here?
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #16 on: 09/12/2007 19:13:53 »
Is this the silly season for nakedscientist forums?
Doesn't anyone read books before starting to post here?

Does it matter?  Maybe they do not know which book to read.  Some people are more comfortable gaining information through human contact than through inert books.  Whatever the reason, why does it matter?
 

sooyeah

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #17 on: 10/12/2007 10:48:43 »
So oxygen is used as well?
Yes, it is crucial, but oxygen is readily available from the air, so you don't need to tank up on it.

It's a hydrogen oxygen liquid fuel mix?

The hydrogen and oxygen are stored separately.  To premix them would have a dangerously explosive mix, and would anyway have to be separated when entering the fuel cell.  In any case, as I said, for most applications, you don't need to carry your own oxygen (it is normally available from the air around you - unless you are looking for applications in space, or submarine applications).

As for whether the hydrogen is liquid - cryogenically stored liquid hydrogen is one choice, so is high pressure gas storage, and so is gas absorption in some solid matrix.  Each solution has its own problems.

Could you not also used liquid helium, or nitrogen? With transfer of electons, surely any Gas atom could be used.

No, it depends on the structure of the electrons around an atom.  All atoms are naturally electrically neutral (i.e. the number of positively charged protons in an atom normally balances the number of negatively charged electrons), so there is good reason not to give away or accept another electron, since that would make them no longer electrically neutral.  On the other hand, atoms also like to have certain specific configurations of electrons around them in preference to other configurations, and if they have too few or too many electrons for their ideal configuration, then they may give up or accept another electron in order to obtain an ideal configuration.  When you get an atom that has too many electrons for its ideal configuration, and one that has too few electrons for its ideal configuration, then the one that has too many electrons will give away its electron, and the one that has too few will accept an electron, and between them you will get a stable chemical compound (that, in a nutshell, is the basis of all chemistry).

The first rule is that electrons like to go around in pairs.  Normal helium has exactly two electrons, and so for it, that is an ideal number of electrons, and so it will have great difficulty either accepting another electron or giving up an electron.

Could you not give helium an extra electron? Or is that what helium 3 is?

For oxygen, it naturally has 8 electrons, but the ideal number of electrons for an atom of about oxygen's size is 10 electrons.  Neon is slightly heavier than oxygen, and has 10 electrons, and so, like helium, neon is very difficult to make chemical compounds with.

Hydrogen would really like to have 2 electrons (like helium, its heavier cousin), but it will just as readily give up its one electron, so it has no electrons.

If you then have two hydrogen atoms, each willing to give up one electron, and one oxygen desperately seeking to get two more electrons, you can see they are in a strong position to make a swap - and between them make a chemical compound of two hydrogen atoms and one oxygen atom - this compound is water.

So the trick really is to make a electron full store of hydrogen to put in your car. Could you not, return the hydrogen to the fuel cell after use, by running it through a coil where it could pick up more electrons? 
 
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #18 on: 10/12/2007 14:11:16 »
Could you not give helium an extra electron? Or is that what helium 3 is?

That is nothing to do with what 3He is.

When you see a number with an atom, like helium 3, or polonium 210, it refers to the number of nucleons (the number of protons + the number of neutrons) in the atom, and is not directly related to the number of electrons.  I say it is not directly related to the number of electrons, because indirectly, in an electrically neutral atom, the number of protons must be the same as the number of electrons.  But then, the number of protons for any given element is always fixed (i.e. for helium, the number of protons is always 2, otherwise it would not be helium; for polonium, then number of protons is always 84, otherwise it would not be polonium), so what the number really tells you is how many neutons are in the atom.  In the case of 3He, since there must be 2 protons, so it tells you there is one neutron; whereas the more common form of helium is 4, there are still 2 protons, but now with 2 neutrons.

Yes, you can add another electron to helium; but since the objective of the exercise is to extract energy from the system, so one wants to use a situation that naturally exists in order to obtain energy from that natural condition.  If you have to add an electron to helium in order to obtain the energy from helium later giving away that same electron; then the energy you have to add to the helium will exactly match the energy you will obtain by letting the helium give away that electron, so you have no nett energy gain.

Ofcourse, the same problem occurs with hydrogen and water.  Free hydrogen does not exist (except in trace quantities that are of no use to anybody) on this planet, so the common way to obtain hydrogen is by splitting water (which is plentiful on this planet) into hydrogen and oxygen.  But there you come to the Achilles heal of the arguments for using hydrogen as a fuel - you need as to use as much energy to obtain the hydrogen from water in the first place as you would obtain from the hydrogen when it is converted back to water.  So the only way you can use hydrogen is not as a primary fuel source, but just as a means of carrying energy from one place to another (e.g. you take water to a power station, such as a nuclear power station; convert that water into hydrogen (thus using the energy of the power station to extract the hydrogen); then take the hydrogen to the hydrogen powered car, and let the car use that energy from the hydrogen to obtain the power to move the car).

Aside from that, there would be particular problems with simply having helium (or any other atom) with an extra electron added.  In water, you have hydrogen that has given away an electron (and so is electrically positively charged, because it is missing the electron), and you have the oxygen which has accepted the extra electrons (and so is electrically negatively charged), but because then two are combined in a single molecule, so the overall molecule remains electrically neutral.  If you have atoms that are not electrically neutral (either because they have accepted an extra electron, to become negatively charged; or have given away an electron, to become positively charged), if they are not bound up in a molecule that is electrically neutral in total, then those atoms can become very dangerously reactive (and typically are also poisonous because of their reactivity).

So the trick really is to make a electron full store of hydrogen to put in your car. Could you not, return the hydrogen to the fuel cell after use, by running it through a coil where it could pick up more electrons?

I hope the explanation above will tell you why that is not possible within the car.

In any case, you could not pick up electrons from a coil, but you could pick up electrons from an electrode (a metal plate through which you run an electric current), but you need energy to do that, and so you still need to take the water (which is how the hydrogen exists when it has given its electron to an oxygen atom) to a different energy source where it can obtain the energy with which it can now be given back its electron (which is now taken back from the oxygen).
 

Offline Sjoeberg

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #19 on: 11/12/2007 18:48:25 »
The interesting thing about fuel cell cars in my opinion is not hydrogen as fuel, at least not at these times. The problem with using hydrogen is, like previously posted, that we have to produce it first... using quite a lot of energy (actually more, than we get when using it in a fuel cell due). But there is another way.

Using the right catalyst, you can easily convert for instance methanol directly to hydrogen and carbondioxide, and sending it directly into a proper fuel cell, thereby producing a current. The trick here is, that you don't have to transport the hydrogen... you use methanol for that. Yes yes, in these times, where people panic about CO2 rates, i know it's almost taboo to mention carbondioxide. But please remember that a fuel cell has the potentiel to reach an effiency of 90%, whereas a standard diesel engine at best reaches 25%.

Other fuel cell uses methanol directly as the fuel - these are called direct methanol fuel cell, as opposed to the reformate methanol fuel cells described above. Using methanol directly naturally also produces carbondioxide. But if we produce this methanol from say wheat or biological waste (you know, stuff we flush out the toilet), it's carbondioxide neutral: The carbondioxide our fuel cell exausts comes from a plant, which has grown by consuming the same amount of carbondioxide from the air. Or the exaust comes from our waste, which comes from plants and animals grown on the exact same principle. The circle is complete. Plants consume CO2, we consume the plants sending back into the atmosphere, where it is consumed by other plants. Beautiful, ain't it?

Another idea (for the future) is to produce the hydrogen using only renewable energy. I imagine hydrogen plants, where solar cells produce hydrogen. Perhaps we'll all have a solar power plant (imagine roofs consisting of solar panels), where the excess electricity produced is sent directly to a electrolyser producing hydrogen, which is then stored in a tank. Your home power plant (a fuel cell in the basement) produces electricity from the stored hydrogen on a cloudy day, and you fill up your car also using the stored hydrogen.

Obviously this is a bit utopian... we need far more effective solar panels and there are numerous other problems, but the idea is still there.
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #20 on: 11/12/2007 19:42:05 »
Fuel cells using methanol, or even methane, are a totally different matter from using hydrogen.

As you say, both of these feedstocks contain carbon, and so will produce carbon dioxide, but since both can (and often are) created from plant matter, thus the carbon dioxide created is from carbon dioxide taken from the atmosphere (true, the same can be argued for mineral oil combustion, but in the case of mineral oil, the counter argument is that it was taken from the atmosphere millions of years ago, whereas boifuel sources of methenol or methane could be take from the atmosphere with the preceding 12 months).

The arguments about efficiency are far more complex that that stated.

Firstly, the efficiency of a diesel engine can be raised if the temperature of the combustion is raised (the theoretical limit to the efficiency of the engine is totally determined by that one factor).

Secondly, looking at the efficiency of just the engine alone is meaningless.  What matters is the efficiency of the use of the original energy source in providing the desired result (the motion of the vehicle).  Thus what matters is looking at the cost of production of the fuel, the efficiency of power generation from the fuel (the figures you have quoted), and the efficiency with which that power is translated into the movement of the vehicle.  While I am not arguing that the figures are necessarily any better for the diesel engine when looking at the entire process, I am merely saying that looking at only one small part of that process is meaningless if one is looking at that component part when the entire of the rest of the process is incomparable.
 

Offline Sjoeberg

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #21 on: 12/12/2007 10:42:38 »
Quote
As you say, both of these feedstocks contain carbon, and so will produce carbon dioxide, but since both can (and often are) created from plant matter, thus the carbon dioxide created is from carbon dioxide taken from the atmosphere (true, the same can be argued for mineral oil combustion, but in the case of mineral oil, the counter argument is that it was taken from the atmosphere millions of years ago, whereas boifuel sources of methenol or methane could be take from the atmosphere with the preceding 12 months).

On this, i agree with you totally. Yes, mineral oil is also produced, to some degree, from plants and other organic matter, but the point is, like stated, that using methanol and ethanol produced from surface plants and organic matter, which has just ended their life-cycle will have no net effect on the environment. So yes, I do indeed agree with this.

Quote
The arguments about efficiency are far more complex that that stated.
Naturally it is. I'm not saying that you can achieve 90% net efficiency and comparing this to 20% efficiency in a diesel engine. I'm just saying, that the potential of a fuel cell powered vehicle exceeds that of a diesel powered vehicle. But both methanol and diesel synthesis/refining requires energy, so you'd have to do an extensive energy calculation to find the pro's and con's. My point in the efficiency is therefore only referred to the engine (as i do not have the results from that calculation) And one thing, that you can not get around is, that the efficiency of a combustion engine is limited by the fact that it is a Carnot engine (and there limited by its operating temperature and surroundings temperatures), whereas the fuel cell is only limited by the free energy of the fuel (which is about 90% of the total enthalpy in the hydrogen). Naturally you have to account for the electric motor as well, which i do not have confirmed figures for, but a quick google search tells me, that a 100 hp electrical engine is able to do +90%. Mechanical resistance can be assumed to be the same for the two vehicles (though I'd expect a combution powered vehicle to be even more inefficient due to shafts etc etc.)

So to conclude: My point is, that an electrochemical powered engine has a greater _potential_ in efficency, than a combustion engine will ever have, because of the Carnot factor.

Though I think highly of fuel cell technologies, I'm also reasonable enough to know that even if fuel cell cars will be a reality, it will be sometime in the future. Everyone has been saying "just five more years, then we'll all have fuel cell powered cars"... yeah, and they've been saying that since 1990. :o)

« Last Edit: 12/12/2007 10:47:49 by Sjoeberg »
 

sooyeah

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #22 on: 12/12/2007 13:59:26 »
I don't really get the it takes energy to produce it arguement, it takes a huge amount of energy to produce petrol surely.

And helium as a fuel, could be, or would be, highly volitile?
 

Offline Sjoeberg

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #23 on: 12/12/2007 14:46:05 »
Ok to make things clear:
- Petrol definetely requires energy to "produce" (bring up from the underground, transport, refine etc.)
- Hydrogen also requires energy to produce, which would typically be through electrolysis (sending a current through water, thereby releasing hydrogen and oxygen: 2 H2O -> 2 H2 + O2)
- Methanol/ethanol producing also requires energy, this is typically done in bio-plant, where specially "constucted" organisms (bacteria) "eats" dead organic material such as plants and waste and releases (m)ethanol. Exactly as when you brew beer og produce wine... microorganisms eats the sugar and releases ethanol. This proces obviously requires heat (and also transport etc.etc.)

So yes, all proceses consume energy... but which consumes the most per unit energy "produced" in the form we need it in, is not an area i know much about.

I have no knowlegde of using helium as fuel, but i imagine, that would be hard, as helium in my knowlegde does not take part in chemical reactions (unless we're talking about nuclear reactions ;o) ). Using _hydrogen_ involves some problems:

1. It's highly explosive when oxygen is around. (the explosion actually gives you an idea of how much energy hydrogen/oxygen-blends contain - it is (some of) this energy that are used in a fuel cell producing electricity instead of heat and light.)

2. It's actually quite difficult to transport hydrogen in large quantities. The H2-molecule is so small, that it diffuses slowly through a steel tank! Naturally you can keep hydrogen in a tank for some time, but long streches of hydrogen pipes is a bad idea. We just can't transport hydrogen lossless like we do with for instance crude oil.
 

another_someone

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Re: How does a hydrogen fuel cell-powered car work?
« Reply #24 on: 12/12/2007 16:13:37 »
I don't really get the it takes energy to produce it arguement, it takes a huge amount of energy to produce petrol surely.

As SJoeberg has mentioned, it is all about how much you spend for how much you get.

Think of energy as money (very much, energy is the currency of science).

If you spend 1 million, and get 2 millions back, you are in profit, and it is a good deal.  If you spend 1 millions pounds, and get only 1 million back, you have achieved nothing, and by the time you have paid your brokers commission (because all real life processes have inefficiencies that lose energy beyond the theoretical ideal), you will actually have made a small loss - certainly not the kind of deal you want to make.

Yes, it does take a huge amount of energy you have to put in when processing petrol; but in that particular case, the amount you get back exceeds what you put in, so you are in profit.

And helium as a fuel, could be, or would be, highly volitile?

Technically, the term 'volatile' merely means that it is easily turned into a gas - and both hydrogen and helium are normally gasses at room temperature, so they would both be regarded as volatile by nature.

There is the more colloquial use of the word 'volatile' to mean something that is unstable, but the problem with helium (if we are talking about electrically neutral helium, without any electrons added or taken away), is that it is too stable.  If you want to get energy out of a system, you needs to have some instability in that system - but not so much as to make the process unmanageable.
 

The Naked Scientists Forum

Re: How does a hydrogen fuel cell-powered car work?
« Reply #24 on: 12/12/2007 16:13:37 »

 

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