In and of itself, it certainly "is" usable heat. But keep in mind that the technology is at the stage of the first one cylinder Diesel engine. Interestingly enough, one of the materials that could possibly be used in place of nickel is tungsten, which has a MUCH higher melting point. I think much higher temperatures will become available as the tech advances.
one of the materials that could possibly be used in place of nickel is tungsten
***I hadn’t heard that. Got any links? I had heard that Rossi used tungsten to separate H2 gas into H1 monoatomic gas and that was his initial secret “catalyst”...