There seems to be some considerable confusion over the "amount of
potential energy stored in a tank of compressed air". Let's discuss
this!
First, note an important quotation from the Chairman: "There are no
blocks" (Richard Feynman "Lectures on Physics", Ch. 4). I note that
no one has ever observed potential energy in its naked state,
independent of the system to which it pertains.
In this case there is no unique answer to the question. We speak of
certain kinds of potential energy as being stored in, for example,
springs. I'm going to assume that everyone is familiar with that
concept. We can agree that the capability of performing work is
intrinsic to a system consisting of a tank of compressed air. One
could use it to turn a turbine and extract shaft work therefrom, for
example.
The quantity of potential energy, it may be said, is the capacity to
do an equivalent amount of work. The constraints on doing that work
will affect the quantity, however. That is why, for thermodynamic
systems, more than one sort of potential energy can be defined.
Values will usually differ for these quantities, but all have the
dimension of energy. One can calculate the order of magnitude of the
"energy storage" by dimensional analysis alone, and when evaluating
an explosive output that is probably about as much as one needs. More
precision would require more detailed specification of the details of
the explosion.