Small-scale H2 tank refill station

[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.

[seifrob]

I don't want to sound like a spoilfun here, but:

- energy stored in one scuba diving tank full of hydrogen sounds about right (back of envelope calculation though)

- iirc, you cannot electrolyse distilled water. You need to have some electrolyte (sulphic acid etc) to electrolyse. So there will always be both hydrogen and oxygene, but you can cope with it.

- it sounds like you are trying to make carbon free bussiness. Now, where are you going to take power to electrolyse and compress that hydrogen? Solar array? If that, you than need a real gas balloon, as you do not want few clouds to stop your bussiness. It adds to the bill

- if you are going to buy your electricity to power these electrolysers and compressors, than be ready to invest MUCH more kilowatt-hours in electricity to produce few kilowatt-hours in storage tanks. Its my wild guess, but I would say the ratio can be from around 6 to 1 to 12 to 1 (one kilowatt-hour in scuba tank will cost you six kilowatt-hours you pay in electricity). From financial point of view, this is disastrous. It would be better to buy loooong extension cords.

- If you want offshore emission free system with limited range, compressed air will serve you as well, with less hassle. You can compress your air using wind power. you can rebuild engines to run with compressed air. You can store compressed air in scuba tanks. you can refill it as easily as possible with compressors in case the wind stops blowing. They tested this in France in eighties, no more info though, but someone may chime in.

[oil pan 4]

Do you even realize how much proper hydrogen and natural gas compressor equipment costs?

[smallscaleH2]

cRiPpLe_rOoStEr notified me on some safety issues.
So, due to the very narrow safety margin of SCUBA tanks, the idea is changed to use a 3 gallon, type 1, CNG tank (as type 1 tanks are the cheapest CNG tanks, and are also the easiest to recycle). These tanks can handle up to 900 bar. We'll use it at 250 bar of pressure (so as to keep a large enough safety margin).

So filled with hydrogen at 250 bar, the 3 gallon CNG tank would be able to contain 11,35 kWh or hence 96 % of the energy in a 1 liter gasoline tank.
Calculation: 11,35 l x 0,003 kWh/l x 250 bar = 8,51 kWh / 8,83 kWh = 0,96 x 100 = 96%

[JockoT]

A litre of petrol will take me 11 miles, and most people even less. So a 118 lb tank, costing $795 (https://www.cngunited.com/shop-cng-t...g-e-4-gallons/) will take me 11 miles. That's not including H2 cost. A litre of petrol, even at UK prices is only $1.45. And it fits in my petrol tank.

[smallscaleH2]

Originally Posted by seifrob

I don't want to sound like a spoilfun here, but:
you cannot electrolyse distilled water. You need to have some electrolyte (sulpheric acid etc) to electrolyse. So there will always be both hydrogen and oxygene, but you can cope with it.

Yes, I made a mistake there. I think I'll probably change this to make plain HHO (not hydrogen). I'll either use baking soda, sodium or potassium hydroxide (probably sodium since this seems to be the cleanest agent to use). I heard HHO has about 4x the energy of H2, and it changes to H2 rapidly. So even if it isn't used fast enough so that the full 4x more energy can be used, it will at least still be as potent as H2. The only problem I see here is that storage of HHO in tanks isn't well described anywhere, so if anyone here has experience with it, let me know. I'm mainly concerned of increased safety issues compared to regular H2. I would be using it at just 250 bar though (not 700 bar as what most H2 pumps supply) so I would assume my safety issues would still be lower (even with HHO) than that of the H2 pumps at 700 bar. But as said, the more info I can get on this, the better.

Originally Posted by seifrob

it sounds like you are trying to make carbon free bussiness. Now, where are you going to take power to electrolyse and compress that hydrogen? Solar array? It adds to the bill

No, the idea is to use grid power (via either a subscription to renewable power only, or nuclear power or renewable power + nuclear power, no fossil fuel power !)

Originally Posted by seifrob

if you are going to buy your electricity to power these electrolysers and compressors, than be ready to invest MUCH more kilowatt-hours in electricity to produce few kilowatt-hours in storage tanks. Its my wild guess, but I would say the ratio can be from around 6 to 1 to 12 to 1 (one kilowatt-hour in scuba tank will cost you six kilowatt-hours you pay in electricity). From financial point of view, this is disastrous.

You are comparing the price the owner of the refill station would pay to the price of the electricity from the grid ? If so, that's a wrong comparison; you should be comparing it to the price of fuel (say gasoline).
When you compare this, it should be way cheaper.

Originally Posted by seifrob

If you want offshore emission free system with limited range, compressed air will serve you as well, with less hassle. You can compress your air using wind power. you can rebuild engines to run with compressed air. You can store compressed air in scuba tanks. you can refill it as easily as possible with compressors in case the wind stops blowing. They tested this in France in eighties, no more info though, but someone may chime in.

I thought about that as well, but most people already own an internal combustion engine-powered vehicle (so you'd then be making compressed air, which no one could use as they don't have any machinery or vehicles that can run on that). Besides this, I think compressed air has a much lower energy content than air. So you need even more compression to store a same amount of power, and thus also require more energy to be put into it (well for the compression-part at least), and you'll hence also need more expensive compression equipment.

[smallscaleH2]

Originally Posted by JockoT

A litre of petrol will take me 11 miles, and most people even less. So a 118 lb tank, costing $795 (https://www.cngunited.com/shop-cng-t...g-e-4-gallons/) will take me 11 miles. That's not including H2 cost. A litre of petrol, even at UK prices is only $1.45. And it fits in my petrol tank.

You're comparing the cost of the H2 (or well HHO) tank to the cost of the gasoline you buy. Not a correct comparison. You should be comparing your $1.45 then with the cost of 8,83 kWh worth of HHO, produced by this particular refill station (which hasn't been made yet). Or, compare the price of a 3 gallon -type 1- CNG tank to the price of a 1 liter gasoline tank.

[JockoT]

Amazon sell a one gallon gas can for $7.17.

[cRiPpLe_rOoStEr]

HHO is quite challenging, since it usually doesn't recover all the energy spent to break the water molecules. Well, some folks have claimed that they could make an engine run on HHO only, but I haven't seen any real evidence of its supposed efficiency.

[oil pan 4]

You are not considering compressing HHO I hope?