{"id":51,"date":"2020-12-06T17:38:18","date_gmt":"2020-12-07T00:38:18","guid":{"rendered":"https:\/\/web.siei.org\/?page_id=51"},"modified":"2020-12-06T17:38:18","modified_gmt":"2020-12-07T00:38:18","slug":"storage-tank","status":"publish","type":"page","link":"https:\/\/www.siei.org\/?page_id=51","title":{"rendered":"Storage Tank"},"content":{"rendered":"\n

Storage is possibly the most pernicious bottleneck for hydrogen use, because of hydrogen’s very low energy density (see last paragraph below). We use a 500 gallon propane tank for storage, at 200 psi maximum pressure. A project at Humboldt University<\/a> , CA uses this same method.<\/p>\n\n\n\n

We were told the tank must be new, because a used tank might have residual gases which could poison the fuel cell (but why would thorough purging with nitrogen not eliminate this?). The tank may have a limited lifespan, due to the reactive nature of hydrogen and consequent risk of corrosion (see Hydrogen Safety<\/a>). Presumably the reactivity of hydrogen is increased by moisture in the tank (since this would lead to loose H+ ions in aqueous solution, ie an acid). Most electrolyzers have a built-in dryer to ensure dry hydrogen.<\/p>\n\n\n\n

While higher pressures (eg 2500 psi, as in commercial K-cylinders) would have vastly increased our storage capacity, the cost and complications of adding a pressure pump kept us to 200 psi. Also, an off-the-shelf propane tank is rated to only 215 psi (we do not know whether it can be rated to a higher pressure). Conveniently for our system, our electrolyzers put out hydrogen at 200 psi.<\/p>\n\n\n\n

We chose a 500 gallon tank size for three reasons. First, our current electrolyzers are small and therefore cannot fill a larger tank in any reasonable time. Second, we have only a 1 kW fuel cell. Third, we figure we can add another 500 gallons of capacity easily. In short, we built our system on a relatively small scale in all parts, to control price and because we were experimenting.<\/p>\n\n\n\n

That said, bigger is better when it comes to storing hydrogen (See Useful Equations<\/a> to follow this calculation): In a tank filled to 200 psi, you have 160 psi (11 atmospheres) of usable gas, since the fuel cell has a minimum 40 psi feed. Thus our 500 gallon tank at 200 psi stores 5500 gallons \u224821,000 liters (STP) of usable H2. At 3.5 Wh\/liter, this is about 73 kilowatt-hours of power. Given a fuel cell electrical efficiency of 40%, the effective stored electric power (measured “at the plug”) of our 500 gallon tank is about 29 kWh. This is equivalent to 3\u00bd commercial K-cylinders of hydrogen, or a bank of 30 L-16 batteries. If your electrolyzer is big enough, a 1000 gallon tank is desirable. Its footprint is not much greater and it doubles your energy reserve.<\/p>\n\n\n\n

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Electric code required an explosion-proof box at the tank (box is for the pressure transducer wiring)<\/figcaption><\/figure>\n\n\n\n
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The pressure relief valve must be at least 7′ above the tank. Since the 1″ pipe is stainless steel, at $175 per 20′, this was an expensive valve. A relief valve threaded at both ends would eliminate the need for the stainless pipe. Ordinary pipe would do, since it would not be exposed to H2.<\/figcaption><\/figure>\n\n\n\n
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Most of the fixtures are ganged at the tank.<\/figcaption><\/figure>\n","protected":false},"excerpt":{"rendered":"

Storage is possibly the most pernicious bottleneck for hydrogen use, because of hydrogen’s very low energy density (see last paragraph below). We use a 500 gallon propane tank for storage, at 200 psi maximum pressure. A project at Humboldt University , CA uses this same method. We were told the tank must be new, because … <\/p>\n

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