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Need Help : Hydrogen Tank, Molar Quantity.

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Patrick

Wed Apr 30 2014, 02:26AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I need some others to check my work.

Question : how much hydrogen is in my tank?

Given : (I will force these values)
Volume = 819 cm cubed.
Pressure = 3.2 Mpa
Temp = 297 kelvin

I want to know how many moles of H2 are present in that tank, under these conditions.

So re arranging PV=nRT, to n =PV/RT I get :
(3,200,000 Ã— 0.000819) / (8.314 Ã— 297 ) = 1.06 mol of particles in my tank.

So if H's heat of combustion = - 286kj/mol, then do I divide by 2, multiply by 2 or do nothing due to it being diatomic?

286kj/mol X 1.06 mol = 303.2kj of H Available.
303.2 X 0.4eff = 121kj usable after conversion losses.

if it takes a battery 6.6Ah at 12 v average, under load to hover for 8 minutes, then....

I think I need a greater H mass.

Sulaiman

Wed Apr 30 2014, 01:00PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

From this reference

you get 237 kJ/mol
and since in PV = nRT
n is in moles, I don't think there is a times or divide by 2
so only 96 kJ electrical output

for your battery I get 6.6 x 3600 x 12 = 285 kJ
2.7 times more than your fuel cell
... just under 3 minutes

Patrick

Wed Apr 30 2014, 02:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Sulaiman wrote ...

From this reference

Yeah, I got less than 3 minutes too. I'm shocked at how poor the PEM fuel cells measure up.
It looks like doubling the battery mass is all I really have left. maybe for 20 minutes of flight.

Ash Small

Wed Apr 30 2014, 03:18PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

It looks like doubling the battery mass is all I really have left. maybe for 20 minutes of flight.

That 'is' the way the maths works. For the same payload, increasing battery mass, and if necessary, increasing prop size= longer range. Plot a graph depicting these parameters.

Hint: As well as increased lift, larger props are also more efficient, all other factors being equal.

Sulaiman

Wed Apr 30 2014, 05:18PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

After I wrote my previous post it occured to me that maybe you should be using 297 kJ/mol
because the thermal loss is probably accounted for in the quoted 40% efficiency.
Helps, but not massively.

BigBad

Wed Apr 30 2014, 05:26PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I'm a bit surprised you're still investigating hydrogen fuel cells; they're practically guaranteed to be a loss with pressurised tanks.

The hydrogen plane stuff mostly wants to use liquid hydrogen. You're really up against it with small vehicles though; the square cube law is not your friend for LH2 tank.

You might get away with insulating with styrofoam though; but cryopumping may be a problem, and the tendency of LOX to form condensing from the air makes it rather 'interesting', but might be OK for short periods.

A lot of hydrogen tanks use thermos flasks for these kinds of reasons.

Patrick

Wed Apr 30 2014, 10:44PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im a desperate, desperate man, im seeing as Dr. Slack and others are pointing out that a fixed wing multirotor may solve the range, vertical and hover requirements.

some are already doing this.

let me ask, im using 10 and 11 inch diameter props, and relaize effciency goes up as the prop-disc loading goes down. But why is this? im thinking accelerating high mass with a tiny change in velocity, is more effcient than a tiny mass accelerated by a huge delta V. (holding force constant in both cases, due to KE=mv^2) that squared part gets to be problem.

2Spoons

Wed Apr 30 2014, 11:58PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

You are exactly right: the force is due to the momentum change M.dV , while the energy used is M.dV^2. So for the same force a big M and small dV needs less energy.

Have you looked at methanol fuel cells?

Ash Small

Thu May 01 2014, 12:06AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The way I look at it, the losses with props are all at the circumferance. As you increase the diameter, area increases faster than circumferance, hence more area = less losses. The other factors I mentioned in my earlier post also hold true.

I'm no mathematician, I don't convert everything into 'digital' and then back into 'analogue'. I think in 'graphs', if you like.

I know you need to 'input' some figures at some point, but if you want to increase range, it's not a 'linear function'. Doubling battery weight won't increase the prop size by any 'linear function'. I think this is where your '^2' function comes into play.

As I said, I'm no mathematician.

Patrick

Thu May 01 2014, 02:11AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

What about the pitch? in reguards to M.d and M.d^2 ? do i go larger disc with shallower pitch? i presume id want thrust at lower RPM instead of higher RPM.

also, i cant keep going larger though i clearly need to, so what bout a three blade prop? does that entrain more mass and lower the delta V over the same diamter as a 2-blade prop?

as for the methanol fuel cell, no but are they functionally differnt or better ? butam i trading poor tankage for poor molar concentration?

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