Current battery technology unsuitable for emissions-reduction on cargo vessels

Different propulsion options can have a significant influence on fuel consumption and emissions when manoeuvring close to port, according to a new report titled Impact of Battery-Hybrid Cargo Ship Propulsion on Fuel Consumption and Emissions during Port Approaches. The report was written by Delft University of Technology’s Dr Congbiao Sui, Professor Douwe Stapersma, Klaas Visser and Peter de Vos, and Harbin Engineering University’s Dr Yu Ding. Dr Sui presented the study in a paper at the CIMAC Congress in Vancouver, Canada, in June this year.

Using a benchmark simulated chemical tanker, researchers investigated CO², NOx and hydrocarbon emissions. Close-to-port operations were broken down into three smaller areas: sailing in coastal areas; sailing in restricted areas; and sailing in the open seas. The effectiveness of various propulsion options in these modes was measured through assessing competing power-take-in and battery configurations.

Results showed that during harbour approach, electric propulsion modes can reduce local emissions and even result in zero emission operations, but that the total fuel consumption, including operating at sea, was higher in such situations compared to conventional mechanical propulsion.

Why cargo ships?

Similar studies to date have tended to focus on ship types, such as offshore support vessels and ferries, while cargo ships have received less attention.

Cargo ships sail at lower speeds and require more manoeuvring when sailing in coastal areas and approaching harbours than when operating in the open sea, which increased their specific energy consumption.

The authors noted the increasing pressure shipping is under to reduce emissions, and that air pollution concerns and greenhouse gas reduction policies mean coastal and port areas often have stringent emissions control regulations.

Given these contributing factors, methods of lowering emissions by using battery hybrid propulsion when operating near ports could be attractive as a means of safe, energy-efficient and environmentally friendly propulsion and power for cargo ships.

How emissions and efficiency were assessed

The researchers simulated a coastal chemical tanker with a propulsion package comprising a two-stroke 4,170 kW main engine, three four-stroke 750 kW auxiliary engines, a 546V Li-Ion battery pack and shaft motor/generator operating in various configurations across three cases.

• Case 1: In the tanker’s benchmark operating mode, the main engine provided all power for ship propulsion and onboard electrical loads via the shaft generator operating in power take-off (PTO) mode, with auxiliary engines and battery disconnected, across all three sailing areas.
• Case 2: In restricted and coastal areas, auxiliary engines were used to drive auxiliary generators and provide ship propulsion power via the shaft motor operating in power take-in (PTI) mode, with both main engine and battery disconnected. When sailing in open seas, the main engine provided all power needed for propulsion and electrical loads via the shaft generator in PTO mode.
• Case 3: Battery power is used for propulsion and onboard electrical modes via the shaft motor in PTI mode when in coastal and restricted areas, with main and auxiliary engines disconnected. When sailing in open seas, the battery is charged via generators driven by the auxiliary engines, and the main engine powers propulsion and onboard electrical modes through the shaft generator in PTO mode.

Case 1 is the current operating practice of the simulated vessel, while Case 2 and Case 3 are conceptual options that could form part of future solutions.

The vessel itself was chosen as a “typical ocean-going chemical tanker,” according to the authors. The benchmark propulsion system is a slow-speed two-stroke diesel engine driving a controllable pitch propeller, and the electrical system comprises three auxiliary diesel gensets and a shaft generator operating in PTO mode. It had a length of 113.8 m, a moulded breadth of 22 m, a moulded depth of 11.4 m and a design speed of 13.27 knots.

This vessel was chosen as being representative of ocean-going cargo ships, with a lot of data available, explains Dr Sui. It is suitable for hybrid propulsion as it has a controllable pitch propeller enabling engine, shaft and propeller to run at constant revolution, and it has a shaft generator installed that can, in principle, work in PTI mode as well as PTO mode, he adds.

The methodology used is generic and so can be transferred to other vessel types or sizes, provided requisite data is available, Dr Sui notes.

Mission profiles were defined for each of the three sailing areas. In coastal areas, this was a speed of 7 knots, a distance of 40 nautical miles and a sailing time of 5.71 hours. For restricted areas, this was a ship speed of 5 knots, a sailing distance of 10 nautical miles and a sailing time of 2 hours. For open sea sailing, this was a speed of 13 knots, a sailing distance of 18.43 nautical miles and a sailing time of 6.26 knots.

Performance indicators used in the study were based on equations used to evaluate energy conversion effectiveness, fuel and emissions indices. In order to express the results of these in a single number, an operational average value of energy effectiveness was used, with the same averaging method used to calculate the mean value of energy effectiveness, hub distribution factors, fuel index and emissions index across the whole voyage.

The study focused on emissions of CO₂, NOx and HC. For CO₂, the authors assumed that fuel injected into the engines would be completely combusted, so CO₂ emissions would be directly determined by fuel consumption and composition.

Fuel consumptions and emissions: specific areas and overall

Case 1 consumes less fuel and produces less CO₂ emissions than Case 2 when operating in coastal and restricted areas, due to having a higher specific fuel consumption for the auxiliary engines and experiencing extra electric losses in the hybrid drive line, the report’s authors found. However, due to the superior emissions-reduction qualities of the auxiliary engines, Case 2 results in less NOx and HC emissions than Case 1 in these areas. While Case 3 does not produce any emissions or consume any fuel whatsoever when sailing in coastal and restricted areas, when sailing in open seas there is much higher fuel consumption and emissions of both types than for Case 1 or Case 2, due to the extra power draw from the auxiliary engines necessitated by battery charging.

Case 1 had the lowest total fuel consumption and CO₂ emissions overall, but had the highest NOx and HC emissions across the whole manoeuvre. Case 3 consumed the largest amount of fuel and produced the most CO₂, but generated the least amount of hydrocarbon emissions, though Case 2 came very close behind (a mean of 0.0076 grammes per ton-mile versus 0.0077, respectively). Case 2 generally ranked between Cases 1 and 3, but had the lowest total mass of NOx emissions.