You have a heat reservoir, i.e. a well-insulated and very hot object, that stores energy as heat. If you insulate it with mirrors, that can look like bouncing p

You have a heat reservoir, i.e. a well-insulated and very hot object, that stores energy as heat. If you insulate it with mirrors, that can look like bouncing photons back into the reservoir. When you want to generate energy, you open the insulation and let heat out to hit this chip. It’s like opening an oven door to let some hot air out.