Dual fuel diesel electric propulsion systems LNG carriers-Electric transmission losses & fuel cost
One of the strongest arguments against electric drive of LNG carriers has always the losses in the electric
transmission between the prime mover and the propeller.
Indeed there are electric transmission losses of less than 8%, but even with these losses a dual fuel
electric propulsion system is far more efficient than a steam turbine drive system. The thermal
efficiency of the dual fuel diesel engine in gas mode is approximately 47%. Then the thermal efficiency
at the electric motor flange to the gearbox would be over 43.5%.
Currently, all electric propulsion (EP)
systems for LNGC’s have medium speed electric motors driving the propeller via a reduction gearbox,
because medium speed electric motors are cheaper and smaller than low speed electric motors.
Factoring in the losses in the gearbox (1.5%) and the shafting losses (1%), the total thermal efficiency
of the DFDE propulsion system is about 42.5%. This compares very favorably with the steam turbine
drive system’s thermal efficiency of less than 30%. Reducing fuel consumption by 30 – 40% has a
significant impact on the total cost price of the LNG that is being transported to the consumer market.
Fig :Electric transmission losses by LNG ships
Maintenance
Medium speed diesel engines, which include the dual fuel diesel engines used in the DFDE LNGCs,
need more maintenance and more manpower to carry out the maintenance than steam turbine
propulsion systems. Steam turbine propulsion systems are much lower in maintenance
requirements.
Apart from the maintenance on the dual fuel diesel
engines, there is also maintenance required on the electric drive system. Initially, there was a great
deal of uncertainty about these costs, but careful examination of the major cost drivers has shown that
these cost are a mere fraction of the maintenance cost of the dual fuel diesel engines.
Fig: Duel fuel electrical propulsion system
Lube Oil
Medium speed diesel engines have higher lube oil consumption than steam turbines. The main reason
is that the lube oil in the diesel engine is exposed to the combustion process in the cylinders. The
specific lube oil consumption for the dual fuel diesel engines is quoted at 0.5 g/kWh when running in
gas mode. On an annual basis this means up to 100 tons of lube oil.
When running in HFO mode the
lube oil consumption might be higher as it has to protect the engine against the corrosive attack of the
sulfur contained in the residual fuel. The difference in properties between gas fuel and HFO might
make it necessary to change between lube oils with higher and lower TBN number to adequately
protect the engines. Most recently a preliminary advice came to use a TBN 30 lube oil when
alternating between HFO and gas fuels. At this stage experience with HFO on dual fuel engines is too
limited to draw any firm conclusions.
Gas combustion unit (GCU)
Electric driven vessels need additional equipment in the form of a gas combustion unit (GCU) to
handle excess boil-off gas (BOG), while steam turbine vessels burn the excess BOG in the boilers and
dump the steam in the condenser. The additional CAPEX for procurement and installation of the GCU
are included in the 2 – 4% higher initial price for a DFDE LNGC.
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