Feasibility of Bipolar DC Grids on Ships

Abstract

Global average temperatures are setting new records compared to the previous decade, primarily due to the accumulation of greenhouse gases such as CO2. The growth potential in emerging economies worsens the current global situation. These CO2 emissions are needed to be curtained and efforts are underway. The emissions target set by the International Maritime Organization seems far-fetched given the forecast for the growth of the shipping sector. Although electrification has proven to reduce emissions, the energy efficiency metrics (EEDI, EEXI) are incapable of providing the quantitative benefits that can be realized by switching to a bipolar DC (BiDC) grid when compared with the AC grid.
This
thesis aims to evaluate the feasibility of BiDC grids over the complete
operating profile of the ship by comparing ten arrangements, five for AC and
five for DC, with the Key Performance Indicators (KPIs). The arrangements are
two diesel generators (AC1 & DC1), two diesel generators and battery (AC2
& DC2), two diesel generators, battery, and shore power (AC3 & DC3),
one diesel generator and battery (AC4 & DC4), one diesel generator,
battery, and shore power (AC5 & DC5). The comparison is made for key
performance indicators like CO2 emissions, fuel consumption, electrical power
requirement, propulsion power cable losses, capital costs, operating costs,
propulsion system weight, and load carrying capacity of the ship. In this
study, a ferry is chosen to compare the two grid architectures.
To
establish the required background knowledge for the reader of this thesis, the
literature on the current emission regulations, the different propulsion
systems, and the differences in AC and DC grids are presented. The ship’s
propulsion and auxiliary power demand are not freely available and are in the
possession of the ship operator. Therefore, first, a preliminary methodology
was developed for estimating the ship’s mechanical power requirements. Second,
the ship’s electrical power requirements are estimated from the understanding
of the literature. The cable power losses are also calculated for the AC and
BiDC grid topology. Third, the diesel generator model was developed from the
data points of a marine diesel generator for the AC and BiDC grid operation,
i.e., the fixed speed and variable speed operation. Finally, an optimization
problem was formulated for all the ten arrangements for minimising the CO2 emissions over the complete operational profile of the ship.
Overall, it was
observed that the minimum emissions achieved by BiDC grids, when compared with
AC grids for all the arrangements, the emissions from the former were lower. Furthermore,
these results depend on the operating profile of the ship, the shore
infrastructure, the diesel generator and battery configuration and their
technical parameters. However, these findings demonstrate the possible
potential of BiDC grids in emission reduction from the shipping sector.