[Wonder Warthog]

"So, how large would the ‘stack’ of fuel cells have to be to generate the electricity needs of New York City, for instance?"

As I recall, a unit to run a single-family home is about the size of a refrigerator. But there would not be a "single stack to run NYC". The power generators would be installed on a distributed basis (which is the whole idea behind a "microgrid"). Distribution would more likely be determined by existing pipeline infrastructure distributing natural gas.

[Parmy]

I think it is still 'pie-in-the-sky' concept. Until it is brought into reality it is nothing.

Still, until it is actually producing a quantifiable amount of electricity, more than a household, that energizes a large area it is not worth anything.