I think they're talking about something like this: https://www.theverge.com/2022/2/22/22945975/rondo-energy-decarbonize-steel-cement-making-renewable https://w

I think they're talking about something like this: https://www.theverge.com/2022/2/22/22945975/rondo-energy-decarbonize-steel-cement-making-renewable https://www.theverge.com/2022/2/22/22945975/rondo-energy-dec... Basically, in their words, "a large insulated shoebox full of brick". And I could be wrong, but I think you should be able to scale the amount of "brick" up to whatever size and keep the insulation the same thickness, so the storage capacity would increase by the cube of the scale and the amount of insulation would only increase by the square of the scale. That would allow you to minimize the fluctuation in temperature - i.e. if it takes 10 days to get up to temperature, because it's big, you don't have to cool it all the way back to room temperature when you take an afternoon worth of energy back out.