They call it an electric turbocharger

I'm no engineer, but I do know there is no such thing as free energy. Any forced induction system requires energy to do it's thing whether it's an engine driven supercharger, and electric supercharger or a turbocharger. Efficiency can be gained by capturing energy that otherwise be wasted as heat, which I suppose a turbocharger does indirectly by using exhaust gas to drive the turbine. An electric supercharger could do the same if coupled with a hybrid drive system by using electricity generated in braking. Of course this adds complexity and expense, but if coupled to an existing hybrid this would be minimal. A small electric-supercharged hybrid diesel car (like a Prius with a 1.2L diesel) could possibly approach 80-90 mpg. Of course like most hybrids it would be about as exciting to drive as stirring a bucket of paint! :)