[smallscaleH2]

OBJECTIVE

I'm looking into the construction of a small-scale hydrogen tank refill station. The station would need to be able to fill up 1 SCUBA tank (80 cu ft per tank -11,25 liter-).

SCUBA tanks are able to store 206 bar worth of pressure (3000 psi). So filled with hydrogen at 206 bar, it would be able to contain 6,95 kWh or hence 78 % of the energy in a 1 liter gasoline tank.
Calculation: 11,25 l x 0,003 kWh/l x 206 bar = 6,95 kWh / 8,83 kWh = 0,78 x 100 = 78%

Since we'll need it able to fill one up, the electrolyser of the refill station would need to be able to generate 11,25 liter of hydrogen, and then compress this to 206 bar. I would want it to be able to do this in 7,5 minutes (1/8th of an hour), so have it produce 0,1875 liter per minute.

BENEFIT

A relatively small and cheap refill station like this could be useful for small enterprises to refill swappable hydrogen tanks and use it in a fleet of small vehicles (not in cars, as the tanks don't allow to store that much energy to drive any practical distance with it).

WORK OUT

To start designing this, I would first look at whatever is available and sold via hydrogengarage.com or trans-ss.com and meets these criteria.
I found that the largest HHO electrolyzers produce up to 10 to 15 liters per minute of HHO gas and have a cost of about 3000 usd. This amount of gas per minute is more than enough (see objectives above). The idea would be to use these electrolyzers, but just use distilled water in it, so it produces hydrogen (not HHO). I'm not sure whether anyone at ecomodder has ever done this and has experience with it ? Also, I'm not entirely sure it will produce the same amount of gas (hydrogen) this way as it was designed to produce that amount in HHO gas.

Next, I would want the tank to be mountable in a commercial CNG engine conversion kit so that such a kit can then be attached to an internal combustion engine (without too much fuss). Commercial CNG systems use 3600 psi (or 250 bar).

For the compressor: we'll need a 250 bar compressor, and the compressor would need to be powered by an electric engine (preferably a 230V AC engine, so I can just use grid power for this).

I found 1 cheap CNG compressor that allows to compress up (and even above) 250 bar: the Bauercomp Micro. I also found a cheap CNG compressor that allows to compress up to 207 bar (namely the BRCfuelmaker FMQ10). Besides this, I also found another CNG compressor that allows to compress upto 200 bar (the SMP Ltd Coltri MCH5). I also found a CORKEN reciprocating horizontal compressor that allows to compress up to 113 bar.

Inlet gas amounts to the compressor need to match with the outlet flow of the electrolyser (10 - 15 liters/minute). For the Bauercomp Micro, the inlet flow is 183 liter/hour (11 m³/h). For the BRCfuelmaker FMQ10, the inlet flow is 199 liter/hour (12m³/h). For the SMP Ltd Coltri MCH5, the inlet flow is 83 liter per hour (5 m³/h). For the CORKEN HG601FX 2,75" reciprocating horizontal compressor, the inlet flow is 214 to 929 liter/hour (12,9 to 56 m³/h). So, we'll need several compressors to match the electrolyser gas flow to the compressor gas flow (amount of compressors differs obviously depending on what compressor you take).

Between electrolyser and compressor, we'll place an automotive spark arrester (Chalwyn, ...). Between compressor and hydrogen tank, we'll place a bubbler. These restrict flow somewhat but are some essential safety features.

I have been thinking though that even 113 bar might already be suitable for my purposes (I only really need a little less than 1 liter of gasoline worth of energy anyway, so 2 SCUBA tanks @ 113 bar would do). The advantage of using a lower pressure (54% of what the SCUBA tank can actually withstand) is that the compressor will get less hot and will need to work a lot less hard (as heat and pressure builds up, far more energy is needed to be put into the compressor than the extra amount of energy you can actually store in the tank). In real life, this means that the compressor can use a much less powerful electric motor, and thus consume a lot less power. For the 113 bar compressor, CORKEN uses a 75 HP motor. By comparison, an 82 bar CORKEN compressor uses a 45 HP motor, and a 517 bar RIX Industries compressor uses a giant 500 HP motor.

With a 113 bar compressor, I'll thus also not need to worry about heat buildup (which might have been a problem with a 250 bar compressor, hence requiring cooling, and perhaps even a 2-stage compression system). If someone here at ecomodder decides to use a 250 bar compressor, perhaps you might want to consider just using one compressor, but simply adding an extra tank where the hydrogen can be left to cool in before routing it back to the same compressor to increase the pressure further (from 125 bar to 250 bar). You could avoid needing to buy 2 compressors then. Only thing to watch out for is to make sure the compressor is sturdy enough and thus won't fall apart when compressing it from 125 to 250 bar.

A last benefit is that the tanks themselves will be a lot safer (less pressure = safer).
Also since CNG car systems are also able to deal with lower pressures than 3600 psi, using lower pressure tanks won't give any problems (so the tanks don't need to be pressured to 3600 psi to be able to work in the system).

Notes
Commercial LPG systems use 100-200 psi (6,9 - 13,8 bar) but these probably aren't very useful as too little energy is stored at such pressures. If for some purposes they could be useful, they could make an even cheaper system as we'd only need to use a (simple reciprocating) 7 bar compressor then. FAS for instance sells them.