Alternative fuel sources
The first thing to say is that both gases are methane and the source can be both biogas based on organic waste or natural gas, to be used in parallel, making any switch from one to the other a straightforward and easy transition. The key point is that gas has different energy densities in different states. One unit of liquid gas energy takes up 3 times less volume than one unit of compressed gas energy. Simply, this means liquefied gas is denser and you can, therefore, get more energy on a vehicle than with compressed gas.