Those were the Douglas-Martin sunpower screens (which were on the roofs of the roadcities in "The Roads Must Roll" and used to extract power from the Great Big Reactor in "Blowups Happen" IIRC). Shipstones were (IIRC) described as energy storage rather than generation, and the descriptions of the development of them seemed very reminiscent of the discussion in "Snowball" about how patenting would be counterproductive.