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It's only free in the sense that it's power that isn't being used for anything.
It's likely you would only have excess boost pressure to bleed off to produce electricity at very high engine speeds. If you're getting excess at moderate or low speeds, then you have a poorly sized turbo and you're throwing a lot of energy away, in the first place. Hybrid-electric turbochargers that generate electricity only make power at high engine speeds, and use it at low engine speeds. As a means to increase efficiency, it doesn't seem very efficient for ecomodders who tend not to drive flat out.
Also, looking at the diagrams and what goes into making power-generating turbos, seems like a lot of work for recovering what is potentially a few measly horsepower from the tiny turbine you would attach to a car engine. And that's before you go through the losses in converting it back to electricity. Would make more sense to simply have the turbine properly sized, in the first place, or to use a variable-geometry unit, instead.
This kind of thing makes sense on the industrial level or with ships, which have huge turbochargers, but on cars... I don't see the benefit.
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