The answer is extremely hot metal, Amy explained in a Skype call. Molten silicon heated to 2,400°C emits very bright light. “At these higher temperatures, you get enough radiation that is strong enough to use a photovoltaic heat engine,” he said.[While an “engine”. “This would have had to be an external combustion turbine otherwise, and have a heat exchanger and other components that don't exist yet,” Henry noted. The temperatures are. This solar heat engine would allow instantaneous response to grid needs, because each unit inside the thermal storage could be. “This is the technological step that we made that preceded this,” said Henry. At this scale, you would need to able to pump a very large volume of very hot silicon through the enormous network of carbon graphite pipes. Pumping was the breakthrough that.