Biggest key problem imho is how they expect to store this heat energy. It looks like this cell will, like a PV cell, constantly be absorbing photons. If those

Biggest key problem imho is how they expect to store this heat energy. It looks like this cell will, like a PV cell, constantly be absorbing photons. If those photons aren't creating electricity/voltage across a gap then they are being converted into heat. So to keep the medium at temperature you would need to insulate it, to wrap it in a mirror, only exposing the flux to the energy-collecting cell as needed. That means moving parts. As for available temperatures from fission reactors: https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA "When the reactor was operating at full power, about 1,140 MW, the chamber temperature was 2,272 K (2,000 °C)"