Originally Posted by TxThundr
Understanding that Fuel Cells are a net negative energy source, it seems they come into play if you have a Wind/Solar setup that produces more energy than you can store in your current battery bank.
If stored Hydrogen has a greater density of Energy storage (or potential energy storage) than a battery, then I would think it would make sense to store your excess Solar/Wind energy in stored Hydrogen then just throwing it away.
From a Tesla forum discussing fuel cells:
Compressed hydrogen has an energy density of 142 MJ/kg.
Lithium ion batteries have an energy density of 0.6 MJ/kg.
That's 236 times as much energy per kg for hydrogen.
So if I have a choice between throwing out the excess generated electricity from Solar/Wind, or use the electricity to generate Hydrogen to store for later use in a Fuel Cell, then I think it's worth considering.
First you are quoting values for energy density that are actually specific energy, energy density is energy per volume, MJ/l or similar units.
Second, your argument fails to to take into account 3 things:
1) The total weight of the installation
. The specific energy of the fuel (hydrogen) looks great, that that doesn't take into account the rest of the system. The total weight of the fuel cell system would include the weight of the hydrogen, the fuel cells, the storage containers for the hydrogen, the hydrogen generator
and the hydrogen compressor
. The weight of the battery system would just be the batteries. The weight of wiring
, solar panels
and controller would be the same either way. The specific energy of the system as a whole will be somewhat capacity dependent, but for a sailboat scale system I doubt the fuels cells will have much of an advantage if any.
Let's say that 1kg of H can be stored at 200bar in a 13l aluminum
tank which weighs about 17kg (See link below). Let's assume the generator
weighs the same, and the compressor
and let's assume the fuel cell weighs half that. So 3.5 x 17kg +1kg = 60.5kg plus misc for the system. Let's call it an even 60kg. Tesla forum folks say 142MJ/kg fuel, Sandia Labs says 120 MJ/kg (see pg 8 of link below). Let's call it 142 MJ from 1kg of H but 60kg of total system so about 2.4 MJ/kg.
2) The complexity of the system. The fuel cell system has a lot more subsystems which is a disadvantage. Some of them have moving parts
which is a very serious disadvantage compared to the battery option.
3) The energy density of hydrogen in compressed gas form at 200bar (scuba tank pressures) is 1.5MJ/l (see pg 13 of Sandia link below) vs Lithium ion which ranges from .9 to 2.4 MJ/l (see other link below). This would be a wash if you were just considering the fuels but you also need to consider the volume demands of the other parts
of the system, the hydrogen generator, the fuel cell, the compressor and the plumbing