Hydrogen fuel cells?

Question

ok so how much energy do you need to extract hydrogen from water

how much energy do you get frim combining hydrogen and oxygen

how much energy do you need to extract hydrogen from carbon based fuels

plz post a link i will appericiate it

Answer 1

The enthalpy change for water to hydrogen and oxygen is 68 kcal/mol.

2 H2O_liq + 136 kcal -> 2 H2_gas + O2_gas

This means the reaction needs 136 kcal of energy to produce 4 grams of hydrogen gas.  However, the electrolysis process is only about 60% electric energy efficient, so it actually takes 227 kcal of electric energy to produce 4 grams of hydrogen gas.

The combustion of hydrogen is just the above equation reversed, so burning 4 grams of hydrogen with oxygen will release 136 kcal of thermal energy.  However, that thermal energy cannot be converted into work perfectly.  Fuel cells are at about 50% efficient, while combustion is about 15% efficient.  So the usable energy output from burning 4 grams of hydrogen gas is 68 kcal for a fuel cell, and 20 kcal for a combustion engine.

If hydrogen is extracted from natural gas, the reaction looks like this:

CH4_gas + 2 H2O_liq + 60 kcal -> CO2_gas + 4 H2_gas

So steam reformation of methane consumes 60 kcal to produce 8 grams of hydrogen gas, or 30 kcal to produce the 4 grams used above.  This process is actually very efficient, and the actual input energy may be close to 43 kcal for 4 grams of hydrogen gas.

So for electrolysis:

227 kcal in -> 68 kcal (fuel cell) or 20 kcal (combustion) out

For steam reformation:

43 kcal in (+ 106 kcal in NG)-> 68 kcal or 20 kcal out.

Obviously, steam reformation + fuel cell is the way to go, but it still uses fossil fuels.  Also energy is not actually being gained in this chain, even though it looks like 43 kcal became 68 kcal.  Remember, that hydrogen comes from using energy to partially burn the natural gas.  The 25 kcal "gained" is what comes from completing that combustion.  Simply burning the natural gas would have produced 106 kcal (16 kcal in an engine) with no energy input.

The 106 kcal of natural gas used directly instead of making hydrogen;

In a combustion engine:

0 kcal in (+ 106 kcal of NG) -> 16 kcal out.

In a combined-cycle turbine (to make electricity)

0 kcal in (+ 106 kcal of NG) -> 74 kcal out.

Answer 2

I don't have any links, but I can tell you a few things:

- Hydrogen extraction from water is very costly; typically, industrial hydrogen is made from natural gas.  Which is also costly -- just a bit less so.

- You get as much energy from combining hydrogen with oxygen as it took to strip them apart in the first place -- except that there are losses in the process, so you actually lose.

- The energy content of hydrogen, assuming a standard welding-gas cylinder of the stuff at 2200 psi, is equivalent to about a half gallon of gasoline.  The density of gaseous hydrogen is very low, so tanking it in a vehicle is a pain.