The driving range of CNG vehicles remains one of the major concerns of fleet managers when evaluating CNG as a motor fuel. Fleet managers have relied on the published rated capacity of CNG cylinder manufacturers when selecting size and quantity of cylinders to provide a minimum driving range. Experience over the last five years shows that most fleet managers have incorrectly sized tanks by basing their calculations on CNG cylinders manufacturers published “full” capacity. The error has been perpetuated by the dispenser industry claims to provide a temperature compensated fill. The actual lack of driving range for CNG vehicles is not due to poor CNG fuel mileage but rather in most cases is a miscalculation of fuel capacity of a CNG cylinder in fast fill operations.
Pinnacle CNG Systems has recorded the beginning and ending temperatures of more than 100,000 cylinder fills. This data indicated that some CNG fuel cylinders were more efficient in terms of storage, (standard cubic feet of gas stored per cubic foot of water tank volume) than others. This data clearly identified aluminum CNG cylinders as having consistently lower ending fill temperatures than composite cylinders of roughly the same capacity and filled under similar conditions. (fig. 1)
The “Ideal Gas Law” defines that pressure and temperature determine the volume of gas contained in a vessel (Eq. 1). If two vessels of the same internal water volume are filled with gas to the same indicated pressure, and one cylinder has a lower temperature, then that cylinder will contain more gas. The greater the difference one temperature is relative to the other, the greater the difference in gas volume contained.
This paper establishes that aluminum CNG cylinders have a ten percent greater storage efficiency than composite tanks (Table 1). Cylinders constructed from steel have slightly lower storage efficiencies than aluminum, but a greater storage efficiency than tanks constructed from composite materials.
Comparing cost per therm (100 SCF) of storage capacity, type 2 steel tanks emerge as the most economical at 70% the cost of type 4 composite cylinders. (Table 2) Type 2 aluminum cylinders compare at 80 % the cost of type 4 composite cylinders.
The differences in gas storage efficiency of CNG cylinders are primarily a function of the specific heat, thermal conductance, (Eq. 2) and mass of the tank construction materials cylinders. The greater these three properties are the more efficient the cylinders are for gas storage.