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Hybridization in short-sea shipping
Emission-reduction options
Increased scrutiny is being put on shipping to reduce its environmentally damaging emissions. To address this, the IMO has adopted an initial strategy aimed at reducing total GHG emissions from international shipping by at least 50% by 2050. The question to ship operators however remains; is it possible to reduce GHG emissions whilst staying profitable? Hybrid power solutions, which use battery power to enable conventional diesel-electric combustion engines to operate at their most efficient load levels, are among the most promising measures to cut emissions.
Selecting the vessel type for the study
Hybrid power systems often achieve significant efficiency gains and emission reductions in situations in which a ship’s power demand fluctuates, for example when major energy consumers, such as cranes or thrusters, are powered up or down. This typically happens in port. The partners in the study therefore decided to investigate a ship type and size whose operating pattern included frequent port stays. A review of historical AIS data showed that container feeders meet this requirement.
Selected reference vessel
Searching for a suitable, representative container vessel design for the study, the consortium found the TOPAZ 1,700 TEU vessel by Neptun Ship Design GmbH to be a good match in terms of size and topology. With an overall length of about 170 metres and a 14.2-metre depth to main deck, the ship has a maximum speed of 19 knots. It is equipped with a bow thruster with a rated power of 950 kW, two cargo cranes with a rated power of 540 kW, and up to 250 reefer plugs.
Vessel configuration
In a conventional configuration, if built today, the design would include a diesel-mechanical system consisting of a MAN 6S60ME-C10.5 two-stroke, 11,280 kW engine and a fixed-pitch propeller, as well as four type 6L21/31 auxiliary generator sets with an output of 1,254 kWe each, and one 175 kWe emergency diesel generator set.
Operational modes
AIS data analytics was utilized to generate a representative vessel operational profile for a container feeder operating on a northern European trade route.
Operational modes
Further modelling was carried out to estimate both the propulsion and electric power demand for the given operation modes, and to create an artificial time-based load profile.
Cost analysis
The results of the study are presented in the form of two scenarios: 1) A new built hybrid vessel in 2020 using current battery technology, where a battery is applied for peak shaving and spinning reserve for the diesel gensets 2) A new built hybrid vessel in 2030 where batteries are applied for zero-emission
Parameters for calculations
To assess various hybridization strategies, the key figures investment cost (CAPEX), operational cost (OPEX), net present value (NPV) over ten years, and payback time were calculated and compared. The table lists the parameters used for calculating the OPEX and CAPEX of each configuration.
Calculation assumptions
A 10 per cent discount rate and a 2 per cent inflation rate were included in the NPV calculation. The baseline for the 2020 and 2030 scenarios was calculated assuming 500 US dollars per kWh of battery cost, 750 US dollars per tonne of MGO in fuel costs, and 0.07 US dollars per kWh for battery charging in port. Moreover, the residual values for the diesel generator sets (DGs), the shaft generator, the batteries and the power electronics after ten years were applied to the result.
Scenario for the year 2020
For a containership, the total number of reefers on board and the power they demand will have a significant impact on the electrical load and thereby on the potential benefits of hybridization. The “spinning reserve” capability achieved by installing batteries can provide substantial savings in cases where startup of additional gensets can be avoided or delayed. The study assumes that the battery system, using current battery technology will be used for peak shaving and as a ‘spinning reserve’ replacing one diesel generator. The figure above represents a simplified topology of the TOPAZ vessel with a hybrid power plant configuration.
Potential benefits of hybridization
This figure, with 150 reefers active, shows that the battery is also able to cover peaks when waiting and during port stay. It is then possible to run just one genset at a high optimum load point, instead of two.
Peak shaving
In the case where no reefers are active at least one genset is running during port stay in the conventional as well as in the hybrid case. Hence, most of the savings are attributed to the on-board cranes during cargo operation and the thruster during manoeuvering.
Result comparison
In both scenarios three diesel generators are sufficient to cover the maximum power demand and one generator set in the TOPAZ reference design can be fully replaced by batteries. To determine the absolute savings all cases were simulated and compared. Results for reefer power at 3.4 kW are shown in the table.
Scenario for the year 2030
The study went on to investigate battery-based zero-emission operation of a newbuild in 2030, assuming that many ports will require emission-free operation within a certain perimeter. For this application the size of the battery is exclusively determined by the anticipated ability to sustain zero-emission operation for three hours. This translates to an energy need of 7,000 kWh the battery system will have to cover. Since batteries lose some of their capacity over time and the study assumed a battery lifetime of ten years, the installed capacity was set at 11 MWh. During transit outside emission-free areas the battery system will be charged by a PTO/PTI shaft generator system. Diesel generator power will only be needed when the vessel is in “Waiting” mode.
Influencing factors
The zero-emission mode only looks economically viable if battery system costs are lower than today, fuel prices are higher, and incentive schemes or stricter emission regulations are in place, all of which may well be the case by 2030.
Conclusion
The joint study by MAN Energy Solutions, DNV GL and Corvus Energy concluded that based on typical load profiles and historical and AIS data, a TOPAZ-type hybrid feeder vessel equipped with properly-sized batteries as well as cranes and reefer plugs would require one diesel generator set less than the conventional configuration. The payback time for the additional investment in the battery system would be two to three years. A hybrid power plant enabling zero-emission operation as of 2030 will be feasible if battery prices drop, fuel prices increase, emission regulations become stricter and/or incentive programmes are in place.
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