If the stored heat is on the level of getting the emitter into the white-hot, how is the "battery" turned off, to hold the "charge"?

The article mentions a mirror layer as part of the cell, for retaining the energy of out of band photons. Would that be the "off" solution, just bounce them back when they are not needed? Cell moved out of the path? Somehow that triggers "too simple to be true" heuristics in me, but on the other hand... yeah, mirrors (or just very white surfaces, precise direction is not needed) can be quite capable of not getting heated by incoming photons, and that must mean bouncing them back.