Turbocharger development is being driven by a push for reduced energy consumption, environmental compliance and the development of alternative fuels
To meet the requirements of IMO’s Energy Efficiency for Existing Ships Index (EEXI) and Carbon Intensity Index (CII), ship operators should not overlook the role of the turbocharger in improving engine performance and energy efficiency, while reducing CO2 emissions.
Adopted at MEPC 76 in June, EEXI will enter into force on 1 January 2023, requiring ship owners to assess their fleets and choose the measures they need to take to comply. Some of these measures could involve adding energy-saving devices, engine power limitation (EPL), shaft power limitation, propulsion optimisation and engine derating. Most ships are expected to choose EPL, as this lowers fuel cost and can be carried out with less effort compared to other proposed measures.
However, engine derating, for example, can be an attractive fuel savings option if a ship operator can accept a reduction of 10-15% of the maximum speed at a specified maximum continuous rating (SMCR). The benefit will be a 10-12% fuel saving potential based on a new optimisation speed, according to MAN PrimeServ. Engine derating requires rematching of the turbochargers and swapping out the old propeller for a new one that has been optimised to achieve the highest efficiency at the reduced maximum speed.
ABB Turbocharging president Oliver Riemenschneider has noted that Norwegian tanker owner Odfjell has seen substantial opex and environmental benefits from derating the engine on the 45,000-dwt chemical tanker Bow Sun. Odfjell cut CO2 emissions by 3% and NOx by 20%, helping Bow Sun to maintain IMO compliance and meet IMO Tier I regulations.
“Zero-carbon fuels place different requirements on the turbocharger, demanding a fundamental technology development”
Among the three turbocharger-related solutions proposed by Mitsubishi Heavy Industries – Marine Machinery & Equipment (MHI-MME) for improving fuel efficiency is a turbocharger cut-out. This could be applied to an engine equipped with three turbochargers. A cut-out would stop one of the turbochargers during partial load operation, increasing the exhaust gas being supplied to the two remaining turbochargers. As a result, the operational turbochargers would achieve higher efficiency. MHI-MME anticipates this would yield a fuel efficiency improvement of about 2.5%.
A second solution involves a variable inlet turbine (VTI). MHI-MME was the first to introduce a hybrid turbocharger with electric-assist and VTI in 2014. With the electric-assist function, the ship can cut energy consumption during slow steaming operations by reducing the use of auxiliary blowers.
MHI-MME proposes that the turbocharger be modified on the turbine side to one with a variable turbine inlet. This makes it possible to optimise the turbine capacity for partial-load operation, achieving improved turbocharger efficiency at the operational point. This could yield about 3% improvement over a conventional turbocharger, says the Japanese turbocharger manufacturer.
A third solution involves retrofitting a standard MET turbocharger to an integrated exhaust gas bypass (EGB) turbocharger. Installation of an exhaust gas bypass line allows exhaust gas to be bypassed during high-load operation, making tuning suitable for low-load operation.
While these measures will help reduce GHG emissions from ships near-term, shipping won’t be able to achieve long-term decarbonisation goals without the use of low-carbon and zero-carbon fuels. Fuels such as ammonia and hydrogen have very different combustion rates. Mr Riemenschneider notes that this will put different requirements on the turbocharger regarding efficiency and pressure ratio, calling it “a real fundamental technology development.”
Turbocharger manufacturers like ABB will continue to co-operate closely with enginebuilders on the next generation of dual-fuel engines, keeping turbocharger development in the spotlight for years to come.