Gas storage tools: tank capacity calculator and fuel energy comparison

What the tools are for

Most questions about gas storage end in arithmetic: how many kilograms fit in a cylinder, how far a vehicle goes on them, or how a tank of hydrogen compares with a tank of diesel. The calculator answers the first two for a specific vessel, and the comparison answers the third for every common fuel at once. Both use the same density data as the guides on natural gas storage tanks and hydrogen storage.

Two typical questions

A depot manager comparing a CNG van with a fuel-cell van wants to know how far each goes on a full tank. In the calculator an 80-liter CNG cylinder at 200 bar gives about 13 kg of methane, of which nearly all can be used, and at 6.5 kg per 100 kilometers that is close to 200 kilometers. A 120-liter hydrogen tank at 700 bar holds about 5 kg, and at 1.2 kg per 100 kilometers for a light commercial vehicle it covers a little under 400 kilometers. The calculator shows the arithmetic, so the assumptions can be changed to match a real vehicle.

An engineer sizing a storage system for a remote site wants to know how much space each fuel needs for the same energy. The comparison shows that a liter of LNG holds about 21 megajoules, a liter of CNG at 250 bar about 10, and a liter of hydrogen at 700 bar under 5. Diesel, at about 36 megajoules per liter, remains the reference every gaseous fuel is measured against.

Reading the results

The calculator reports two masses. Stored gas is everything inside the cylinder at the fill pressure; usable gas is what remains after subtracting the gas left behind when the pressure falls to the minimum an engine's regulator can work with. For a compressed tank run down to 10 bar the difference is a few percent. For an adsorbed tank it is much larger: an 80-liter ANG vessel rated at 150 V/V holds about 8 kg of methane at 35 bar, but at 5 bar the carbon still keeps about a quarter of it, so roughly 6 kg reaches the engine. That gap is the main reason adsorbed natural gas has struggled to match CNG on range.

Gasoline gallon equivalents put gaseous fuels on the scale used at American pumps: 2.567 kg of CNG or 1 kg of hydrogen carries the energy of one gallon of gasoline. The 13 kg in the CNG example is therefore about 5 GGE. Range is the usable mass divided by a consumption figure, which the calculator lets you change, because a delivery van, a city bus and a scooter differ by a factor of ten or more.

Where the numbers come from

Densities of methane and hydrogen at 15 °C are taken from the NIST reference equations of state and interpolated between tabulated pressures; other temperatures are scaled with the Peng–Robinson equation. Energy content uses lower heating values. The fuel comparison draws on the same tables for the compressed gases, fixed boiling-point densities for LNG and liquid hydrogen, and the IPCC 2006 default factors for CO₂. Every figure is listed with its unit on the page, so the results can be checked by hand or carried into a spreadsheet.

What the tools leave out

Both tools describe the fuel inside the vessel. The mass and volume of the tank itself, valves, insulation and the space between cylinders on a rack all come on top, and for compressed hydrogen the tank can weigh more than ten times its contents. Fill losses, the heating of gas during a fast fill and the energy spent on compression or liquefaction are also outside the calculation. The tank guide covers vessel weights, and the CNG vs LNG comparison covers the energy cost of each route.