A hybrid turbocharger (e.g. the MAN B&W TCS-PTG or the ABB/ composite "electric turbo-compound" versions) couples an electric machine (a motor/generator) to the turbocharger rotor shaft. In principle, the compressor and turbine are on the same shaft as a high-speed electric machine (PM machine or induction). Because the electric machine can be driven as a motor or run as a generator, power can be either drawn from or fed to the turbocharger shaft:

• In "motor" mode, electric power is supplied to assist the turbocharger when the exhaust energy is insufficient (e.g. at low load or for the load pick-up of a large two-stroke engine), providing extra boost.
• In "generator" mode, when the exhaust has surplus energy at high load, the electric machine extracts power (an electric turbo-compound / power turbine) which is recovered into the ship's electrical system - improving overall efficiency;

The same machine can also start the turbocharger to reduce the motoring oil pump requirement.

Advantages: (1) improves part-load air delivery and reduces smoke/NOx, giving better acceleration and load acceptance; (2) allows the engine to be optimised/scrubbed or run on a higher efficient point; (3) recovers waste exhaust energy at high load, improving total plant efficiency and reducing specific fuel consumption; (4) reduces auxiliary energy demand (replacing the electric blower at low load).

Challenges: (1) high-speed electric machine and power electronics must be reliable and robust; (2) the rotor/bearings must withstand the extra axial/radial loads and the added mass; (3) control system complexity and integrating the power electronics with the ship's electric system; (4) cost and maintenance of the rotating machine in the hot turbocharger environment; (5) space/weight and the efficiency of the machine itself.

The statement is broadly correct but must be nuanced. Overall plant efficiency (SFC and electrical energy) improves because:

1. Exhaust energy recovered: at high load the turbocharger produces more exhaust gas energy than needed for the required boost; the extra energy is converted by the generator into electrical power which would otherwise be produced by burning extra fuel in an auxiliary engine - so the total fuel used per vessel is lower, improving the overall (propulsion + electrical) plant efficiency.
2. It reduces the auxiliary electrical load: it can supply power rather than the turbocharger being a pure consumer; conversely at low load it replaces an electric blower (which would otherwise spend electrical power), so net electricity demand falls.
3. Part-load optimisation: boosting at low load improves the main engine's combustion, reducing smoke and improving its own efficiency; good air/fuel ratio lowers fuel consumption at off-design.
4. Exhaust gas boilers/waste heat recovery interact: if an exhaust gas boiler/economiser already recovers heat, hybrid recovery of the "pressure energy" via the turbo-compound does not waste the downstream heat - the two can be combined for best efficiency.

However, challenges limit the gain: the electrical machine and electronics have losses, so the net gain depends on matching; the recovered power is only significant at higher loads; the total saving must be weighed against auxiliary load and the engine's own turbocharging needs. When the ship is generating electricity partly by the hybrid unit and partly by auxiliary engines/EG, the integrated plant efficiency (not just the main engine) is what matters; properly designed and matched, a hybrid turbocharger raises overall plant efficiency, so the statement is valid as long as the system is optimised and operated in the right load range.