Energy efficiency audits on ships. Impact of a variable speed shaft generation system on ship fuel consumption and propeller hydrodynamics

This article outlines the impact of a variable speed generation system on a ship’s energy performance figures, analyzing electrical and hydrodynamic issues.

Energy efficiency audit is the first step for evaluating the energy efficiency of a ship; it is fundamental for identifying the key areas where more fuel can be saved with the best return on investment. The audit must include a thoughtful analysis of the ship’s operational profile and machinery together with an evaluation of the different choices for fuel saving.

One of the most interesting solutions for vessels with different operating profiles and fitted with controllable pitch propeller with shaft generator, is the use of a variable speed generation system, so the synchronous generator can be run at variable speeds over a wide range of engine rpm.

This approach offers benefits by improving the energy efficiency of the propulsion train. On the one hand, it improves the performance when running at partial load; on the other hand, hydrodynamics of the propeller are also improved, increasing the open water efficiency by running on a higher pitch and advance ratio, and at the same time reducing the problems associated with cavitation and noise.

The main technical consideration on this type of installation has to do with the new operational envelope of the diesel engine together with the hydrodynamic performance and possible redesign of the propeller blades. Redesigning the propeller blades for the new condition can be advantageous since an additional gain can be achieved.

When it comes to the electrical installation of the ship, the main work is to evaluate the electrical loads and space required, especially for retrofitting projects, which are becoming very common for existing vessels due to the increase in fuel prices.