Twenty Hydrogen Myths

How do you figure it is about 75mpge?

The 60mpge is based upon the specifications released from Toyota. It holds 5kg of hydrogen; there is more left in the tank, but it may not be of high enough pressure to feed into the fuel cell. The announced target range is 300miles, but that maybe as low as 250miles. The 400+ mile range reported are under Japanese test cycles.

Toyota Mirai Fuel Cell Sedan Priced At $57,500 - Specs, Videos
2016 Toyota Mirai Hydrogen Fuel-Cell Car: A Few Things We Noticed

 

That is a convoluted way to determine an optimistic estimate.
We got hydrogen capacity, and an expected range, from Toyota. Why take the thermal efficiency of an unnamed, possibly generic example, FCEV, and develop a ratio for a mpge guess from the Prius? It is two different drive trains with different operational loses after the ICE and fuel cell.

Why is the FCEV 50% more efficient, and not 33%?

 

One way to look at it, 1 Kg of H2 is a GGE of (equal to 1 gal gasoline) (actually it's 0.997 Kg per GGE)
So 300 miles per 5 KG H2 so that's 60 MPG per GGE, assumimg as above you can get the 300 miles per 5 Kg in the tank.

I tried to see if Amory Lovins is getting into the anti-FCV-fray, and I don't see much, except he likes FCV, but he seems to be staying out of the politics of it.

 

Actually, we do know. The tanks weight 87.5kg, and have a storage density of 5.7%(which is good). So they can hold a few decigrams short of 5.289kg of hydrogen. Exactly filling a pressure vessel everytime is more difficult than a gasoline tank on a car though. So Toyota stated the 5kg tank size as approximate. Which is probably a reasonable number to use as the amount of hydrogen consumed for the target range of 300miles given that a full tank will still have a fifth of a kilo left after that. Which is probably when the low tank light comes on.

SAE J2799 is the communication standard between the car and pump according to the 10 questions link.
They might be referring to Japan adopting a new pressure standard, or stations that keep the in going hydrogen colder, so more a can get squeezed in before the max fill pressure is reached. There are simply more variables affecting how much pressurized gas can be filled into a tank than with liquids.

 

The 87.5kg is the weight of the empty tanks. The 5.7% storage density is based on the weight of filled tanks. It's 5.7% of @92kg for the hydrogen capacity.

 

Which is different from what the literature uses when discussing hydrogen tank storage capacity, which use the system weight for such percentages.

This paper uses 20bar as the empty pressure. http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/compressedtank_storage.pdf

If so, the Mirai's tank will contain roughly 0.14kg of hydrogen when empty based on that. Leaving 4.85kg for the 300 mile driving range. So 61.8mpge, maybe 61.9mpge.

The Prius gets near 70mpg on the JC08. 80mpge on the cycle may be 57mpge on the EPA.

 

Why the assumption that at 10 MPa the tank is actually empty? 1450psi is more than enough pressure to push hydrogen out of the tank and into the fuel cell. It appears that a PEM fuel cell only needs the hydrogen feed at pressures below 1 MPa.
https://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/44_4_NEW%20ORLEANS_08-99_0977.pdf
Horizon 5000W PEM Fuel Cell

The parameters of filling listed under Toyota's footnote for the range calculation, were just the one for that fill. The SAE standard covers a range of temps and pressures. "SAE TIR J2601 establishes safety limits and performance requirements for gaseous hydrogen fuel dispensers. The criteria include maximum fuel temperature at the dispenser nozzle, the maximum fuel flow rate, the maximum rate of pressure increase and other performance criteria based on the cooling capability of the station’s dispenser."- J2601: Fueling Protocols for Light Duty Gaseous Hydrogen Surface Vehicles - SAE International

Since the tank wasn't likely empty when the filled it, the range figure published probably includes the unused hydrogen in the tank.

 

wow that is high pressure for EMPTY rank

 

20% of tank capacity is a little more than conservative. Are they worried about the tank collapsing?

The pressure coming out of the tank is regulated down to the 100 to 200psi range, if not lower. That is all the fuel cell is going to need. The tank will be empty when it no longer has the pressure to feed the fuel cell. If the system goes to 'empty' at a higher pressure for some other reason, then that leftover gas shouldn't be advertized as part of the tank's capacity.

They are being built now, so the official JC08 results should be out soon.

 

I don't know anything about the fuel cell operation pressure, but maybe it's high?

 

The links I posted earlier on PEM fuel cells had a high of 0.55MPa. The Mirai could be higher, but not 10MPa/1450psi high.