[Myrddin]

You figured out the value. If you have water, waste CO2 and the catalyst, you can transform back to methane (storable) with electrical input. Photo voltaic cells can capture solar energy and provide the input to do the conversion. Methane is much easier to store than hydrogen. In a closed system such as a space craft, that would be a useful way to manage the carbon cycle. It doesn't make much sense on earth. We can burn plentiful methane and let green plants do the carbon/nitrogen fixation for us using sunlight. My lawn is a spectacular tool for capturing CO2 all summer.

[Ramius]

Roger that. I was also thinking of long distance transportation. One of the difficulties of electrical generation is that it needs to be fairly close to the consumer. Long distance transmission is terribly inefficient, and means losing non-trivial amounts of power the farther it goes. This sounds like it might be a good way of storing up energy in one place and moving it over long distances.