1. How much of the fuel's energy is released as heat? They have a heat recapture device, but that's only used to preheat air/fuel, and not used to generate electricity. Is the energy in the heat simply discarded?
2. Can this be made to work without the process of burning? i.e. can it function purely from heat? If it can, it might be able to replace steam turbines in, for example, nuclear plants or CSP plants. That could be hugely beneficial.
2. Thermophotovoltaics in general can operate with any heat source, though this device is clearly optimized for combustion. However, the efficiency is far too low to compete in the large-scale power generation segment. This is almost certainly aimed at light aviation, heavy drones, military applications, etc., where there are not a lot of alternatives that combine small size, high power density and good efficiency.
The heat is being used to generate electricity.
But in any case, I believe that the more you heat sodium, the more light it emits, probably there is a practical limit on an incoming heat power after which the thing will go boom, but before that it will follow some roughly linear law: the more heat energy in, the more light comes out. Though I'm not a physicist, so I make be wrong, even if I do not see how I can be wrong.