How much energy is required to compress the O2 or H2 for storage/transport or even use? Pure O2 or H2 at atmospheric pressure isn't that useful. For example,

How much energy is required to compress the O2 or H2 for storage/transport or even use? Pure O2 or H2 at atmospheric pressure isn't that useful. For example, it is very hard to use atmospheric pressure H2 to run an internal combustion engine. It's easier to run an external combustion engine with atmospheric pressure H2, but said engine will have to be right on top of the H2 generator and since the engine needs a cooling source and the H2 generator requires sunlight, you may have enough of a temperature differential to run a heat engine directly, without the H2 intermediary.