Article in Wikipedia states that specific energy of Superconducting magnetic energy storage is limited to 1 - 10 W·h/kg. http://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage Do I understand it correct that the larger will be radius of SMES ring the less problem with Laurence law we will have? And what would be energy density per kg if we have relatively thin ring such as couple of cm in diameter which circles entire Earth? And in addition to that, what if we would have a hypothetical superconductor in which absolutely all electrons contribute to conductivity? And how voltage of SMES is defined?