Analysis of information sources in references of the Wikipedia article "Hydrogen-cooled turbo generator" in English language version.
hydrogen's low density, high specific heat and thermal conductivity make it a superior coolant for this application ... Hydrogen has one of the best heat transfer properties of any gas, with a specific heat of 3.4 Btu/lb-F at standard conditions. On a mass basis, hydrogen is 14 times more efficient than dry air for removing heat ... Hydrogen, as the lightest gas, has the lowest density of any stable gas. Wind resistance losses are kept to a minimum because the rotor's wind resistance in a hydrogen-cooled generator is far less than in a similarly sized air-cooled generator
