Diverse solutions satisfy electrical power demands

Generating electrical power at sea imposes challenges on prime mover, alternator and system designers.

Electrical power demands on ships were traditionally met by auxiliary diesel gensets running on marine diesel oil. Higher fuel prices and advances in fuel treatment and injection systems undermined the viability of such sets and stimulated the wider adoption of generating systems based on heavy fuel-burning engines.

One or more of the following options are now typically specified to secure economical auxiliary power supplies:

Independent heavy fuel-burning diesel-alternator sets may be arranged to operate on the same bunkers as the main engine in a so-called uni-fuel installation. In-line engines rather than vee-cylinder models are normally favoured, with running speeds from 720/750 rev/min to 900/1000 rev/min specified for 50Hz and 60Hz power supplies.

Steam for powering a turbo-alternator set can be raised by a waste heat recovery system tapping exhaust gas, scavenge air and cooling water sources of heat. Installations range from the simple to complex integrated energy-saving generating plants which have been applied to large container ships with a substantial refrigerated cargo capacity.

The latter systems may secure auxiliary power directly from the main diesel engine, diesel gensets, a steam turbo-alternator and a gas turbine-alternator; the optimum combination of sources is automatically selected for a given ship operating mode.

Peter Brotherhood of the UK recently supplied 1500kW geared steam turbines to replace the existing diesel prime movers of gensets installed on seven Shell shuttle tankers; the turbines were connected to the original alternators.

Main diesel engine-driven (shaft) alternators whose economic

attraction is that they exploit the high thermal efficiency, low specific fuel consumption and low grade fuel-burning capability of the ship?s propulsion plant to generate electricity at sea. The auxiliary diesel genset(s) can be shut down at sea, their reduced running hours giving benefits in fuel, maintenance and spares costs.

Shaft alternators are commonly driven by a speed-increasing power take-off (PTO) gear from the gearbox of a high/medium-speed diesel-controllable pitch propeller installation.

Alternatively, shaft alternators can be incorporated in the shaftline between a low-speed main engine and the propeller; this configuration – with a large air gap of 7.5mm between stator and rotor, and without additional bearings – has proved very successful, reports German specialist STN Atlas Elektronik. It is simple and sturdy and requires little maintenance; and, in contrast to some other arrangements, torsional vibration problems are not anticipated.

Other solutions include shaft alternators directly driven from the end of the main engine crankshaft or via a reduction gear from the front end of the engine.

Shaft alternators became even more popular thanks to the development of reliable constant frequency generator drives to serve low-speed diesel engines driving FP propellers, the most common propulsion plant for mainstream cargo tonnage. Such a system maintains the shaft alternator speed (and hence output frequency) constant over a wide range (typically 70-110 per cent) of the nominal main engine speed.

A shaft alternator/motor may also be specified to function as a generator in meeting auxiliary power demands, or as a motor (when there is surplus electrical power available) to boost propulsion effort. This facility is appreciated at a time when propulsion redundancy is valued.

All these options effectively result in single-fuel installations, with one grade of cheaper heavy bunkers meeting propulsion and auxiliary power demands. The choice between them is determined by the specific ship and the intended service profile. In practice, a main engine-driven alternator is commonly specified for at-sea generating duties on deepsea tonnage, supplemented by a number of diesel gensets dictated by classification society requirements.