Abstract
Liquefied natural gas (LNG) is recognized as a preferable alternative fuel for ship owners, since it can substantially reduce harmful emissions to comply with stricter environmental regulations. The increasing number of LNG-fueled vessels has driven up the number of LNG bunkering vessels (LNGBVs) as well. A key issue of LNGBVs is boil-off gas (BOG) generation, especially the huge amount of BOG that is generated during loading and unloading (bunkering) processes. This study proposes a hybrid system that combines conventional onboard LNG-fueled generators with an energy storage system (ESS) to solve the BOG issue of LNGBVs. This hybrid system is targeted at an LNGBV with the cargo capacity of 5000 m3. The amount of BOG generation is calculated based on assumed operation modes, and the economic study and the environmental analysis are performed based on the results. By comparing the conventional system to the proposed ones, some benefits can be verified: about 46.2% BOG reduction, 66.0% fuel saving, a 7.6-year payback period, and 4.8 tons of greenhouse gas (GHG) reduction for one voyage in the best case, with some assumptions. This proposed hybrid system using the ESS could be an attractive green solution to LNGBV owners.
Highlights
The air pollutant emissions from ships have increased over the last 50 years, and these have had negative impacts on the marine environment and human health [1]
In order to reduce a considerable amount of boil-off gas (BOG) for the LNG bunkering vessels (LNGBVs), two energy storage system (ESS)–hybrid power systems are suggested compared to the conventional system (S1)
The total mass of the BOG generated is depending on the volume of the liquefied natural gas (LNG) cargo capacity (V [m3]), LNG density (ρ [kg/m3]), boil-off rate (BOR) [%/day], time spent (t [hour]), and calculated as below: BOGgenerated [kg]
Summary
The air pollutant emissions from ships have increased over the last 50 years, and these have had negative impacts on the marine environment and human health [1]. Some methods have been continuously studied for LNG carriers; the partial re-liquefaction system [14], the high-efficient small-scaled re-liquefaction system [15], etc It might be impractical for the LNGBV, because the design capacity of the re-liquefaction system for the short BOG peak period would be unnecessarily large [16]. The LNG composition is very crucial for meeting the required methane number to reduce knocking and misfiring risks in an LNG-fueled engine In this vessel, there are three gensets with 1.5-MW power each to supply electric power for propulsion as well as ship service loads; one or two gensets are sufficient to supply the load demands for all the operation modes, and the last genset is just for redundancy in accordance with the rule requirements [29]. It is assumed that the ship has 17 tons as a weight margin and 48 m3 as a volume margin as per the average value of the manufacturers’ specifications [30,31]
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