High altitude platform

An apparatus for generating electrical energy at altitude, comprising a tether connecting a substantially ground-level location to a platform at an elevated location, the tether comprising a conduit coupled to an electrical generator at the platform, the conduit arranged to allow the flow of a fuel fluid from the substantially ground-level location to the elevated location, and the electrical generator being operable to convert energy in the fuel fluid to electrical energy at the elevated location, wherein the elevated location is at an altitude of from 5,000 m to 30,000 m.