Why a hybrid approach makes sense for small tankers

Ole Vidar Nilsen* (DNV GL) makes the case for auxiliary engines on all future small tanker new building projects

ole-vidar-nilsen-dnv-glDNV GL has initiated a joint industry project (JIP) to better understand how specific battery configurations can reduce operational costs and improve emissions profiles. The JIP includes Total, ship designer FKAB Marine Design, battery manufacturer SAFT Batteries, shipowner Viken Shipping, and ship operator Wallem Shipmanagement.

The JIP is focused on the long-term benefits of lithium-ion battery systems for auxiliary power management on board smaller-sized oil or product tankers in the 17,000 DWT range. Reference data, such as trading patterns, typical modes of operation, and power loads were aggregated from conventional sister vessels.

On one of these ships, live power-load recording equipment was installed to obtain realistic data. The deployment area considered in the JIP included typical routes in northern and western European waters.

This information served as the input for the development of a new ship design concept. Four possible hybrid auxiliary power system configurations were identified, while the main engine and the shaft generator configurations were identical.


The project looked at the potential benefits of batteries when used for various purposes:

  • to boost the available supply of power at times of maximum demand, or in case of machinery failure (spinning reserve)
  • to avoid loading down the generator sets excessively, which causes high fuel consumption (peak shaving)
  • to achieve a more even loading of the generators (load leveling)
  • to allow the generators to run at the most economical load levels and reduce the number of generator sets required on board
  • to feed on-demand power to equipment as needed (such as discharge pumps, tank cleaning equipment, and thrusters during port navigation)
  • and to provide propulsion power at very low transit speeds (in port) as well as in high-sea emergency operation

Based on the power needs of the reference ships, the required battery size when eliminating one generator set was calculated to be 400 kWh.


Building the business case

These areas of battery used were subjected to comprehensive cost-versus-benefit and return-on-investment analyses and environmental evaluations. Safety considerations, operational requirements, dependence on third parties and specific technical requirements were likewise considered.

For the capital expenditure calculations, the study assumed a battery lifecycle of 10 years and a ship lifetime of 20 years. The operating expenditure consisted of fuel and maintenance costs and was dependent on the engine running hours. A battery system should reduce both the installed engine power and the hours of operation to lower the total cost of ownership. The fuel bill for the auxiliary engines was cut by around 11% compared to the base case. The payback period was less than two years, and net savings were in the range of US$800,000.

The auxiliary engine running hours were reduced by 50% to 62%. While the effects on the EEDI remained below the tolerance threshold, CO2, SOx, and particulate matter emissions were reduced by about 10%, and NOx emissions by roughly 5% for all hybrid configurations reviewed for the study.

Besides the economic and environmental benefits, the project demonstrated that batteries offer other means of energy saving, such as storing surplus and regenerative energy on board for peak demand situations, providing instant backup power and acting as load optimizer. This makes them a favorable solution for small tankers. Further, the generator sets can run at their most energy-efficient levels, while providing enough reserve capacity for peak loads and emergencies.

Over a 20-year operating period, all hybrid configurations investigated performed better than the conventional base scenario.

These findings lead me to say that hybridization of the auxiliary machinery is beneficial, at least for smaller tankers, and should be considered in all new building projects. The required battery package size should be determined based on the ship’s power needs.



*These comments are an edited version of a longer article penned by DNV GL business development manager, Ole Vidar Nilsen, which originally appeared in the May issue of DNV GL’s Tanker Update publication