TWO-STROKE ENGINES
BATTERY ECONOMICS LIMIT ESS SCOPE FOR DEEP-SEA SHIPPING
Credit: ForSea
MAN Energy Solutions has issued a white paper evaluating battery technology and how it could be applied on board large ocean-going vessels powered by two-stroke engines. The company concludes that, with the exception of short-sea ro-ro shipping, battery-electric propulsion does not present a feasible alternative to conventional 2-stroke powered shipping at present
Pure-electric solutions are considered and quickly dismissed as impractical, from both a weight and volume perspective, for the majority of ship types considered in the white paper. Techno-economic considerations around upfront battery cost differentials and the need for the replacement of battery cells after 10 years of operation are recognised. The white paper acts as a comprehensive introduction to current battery technology, and introduces the main lithiumion chemistries, and explains why Lithium nickel manganese cobalt oxide (LiNiMnCoO2) is currently most favoured for maritime applications. Some of the main considerations around battery sizing – including the temperature at which the battery system operates, and the effect of charging and discharging patterns on the operational life of the battery are reviewed. While space considerations mean that some phenomena are not addressed (such as the challenge posed by lithium dendrites), the impact of battery ageing and operational limits on ESS specification requirements is addressed. BATTERY MANAGEMENT SYSTEM The white paper introduces the concepts of the battery system, ranging from the individual cell level up to the entire battery level. The differences between power electronics control and the battery management system are outlined, identifying the dual role of the BMS in controlling the charging and discharging of individual cells as well as monitoring the state of charge and system voltage. The role of the BMS in balancing the system to minimise individual cell-level variations in cell voltage as a result of different self-discharge rates and temperatures is explained.
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This connects with a discussion of the role of the BMS in handling safety considerations, including the detection of faulty cells. The white paper provides an introduction to the causes and potential remedies to safety failures, including the rare but potentially serious thermal runaway issue. MARINE APPLICATIONS Having outlined the composition of battery systems and explained how the BMS system works in practice, the paper touches on a number of different configurations where battery systems are employed. The pure-electric configuration, such as that seen in the 4.1MWh pure-electric Tycho Brahe passenger ferry, operating between Helsingor in Denmark and Helsingborg in Sweden, is described. More relevantly from the perspective of operators in the deep-sea market, the paper discusses the potential applicability of battery systems for deep-sea vessels. The differences between a semi-hybrid diesel-mechanical set up and some of the different full-hybrid diesel mechanical set up are introduced. The white paper notes that the latter arrangement includes a battery in combination with a PTO/PTI attached to a 2-stroke engine. This allows for the battery and auxiliary engines to support the propulsion of the vessel and for the main engine to charge the battery while at sea. Appraising the different combinations, the white paper concludes that the higher system efficiency offered by a 2-stroke prime move directly coupled to the propeller shaft means diesel-mechanical arrangements remain the most efficient arrangement for large ocean-going vessels.
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