Oversized Generators Don't Run Better or Last Longer

UNDER-LOADING IS THE ROOT OF ALL GENSET EVIL

The only prerequisite to properly sizing your genset is to grasp some fundamental principles concerning marine auxiliary power — coupled with a bit of sensible load analysis.

Notwithstanding relatively recent improvements in fuel delivery systems on power plants, the single most effective step in maximizing generator life and minimizing fuel waste and pollution is to assure that a genset is sufficiently loaded during (nearly) 100 percent of its running time.

This is especially true in the case of diesel generators, which should operate at 65 to 75 percent of their rated continuous load capacity. This is because a genset that is oversized and, consequently, under-loaded most of the time generally runs too cool internally. And that means a greater internal build-up of carbon and other byproducts of combustion, leading to what we in the yachting industry call “coking and smoking.”

There are all manner of sophisticated electrical regulation and distribution equipment intended to enable naval architects and engineers, as well as yacht builders, to match generator capacities to anticipated loads. This includes sophisticated paralleling and load-sharing devices that make it possible for twin and multiple generators to work cooperatively in carrying total power loads at any given moment.

However, although such equipment has been around for nearly three decades, its proper employment continues, in my experience, to be poorly understood at the consumer end, and even by many yachting-related professionals. And while it can be very effective in managing generator loading, it is also quite expensive.

The single most important factor that works against meeting ideal generator loading in a yacht application is the wide variation in transitory load conditions, from minute to minute and hour to hour.

I've sat in dozens of meetings during which hours have been spent developing a full load analysis for a yacht new-build, with additional time spent matching the genset capacities with the anticipated loads, then still more time invested specifying load-sharing and load-paralleling equipment to enable the auxiliary power generating system to adjust capacity to meet differing load conditions efficiently, only to have ...

The yacht's prospective owner or captain or chief engineer insist that each genset in the pair be sized to carry the full hotel-load on its own.

At which point, I’ve generally recommended trashing plans for spending hundreds of thousands of dollars on load-sharing and paralleling equipment, and instead, simply replacing that equipment with a giant double throw knife switch.

The “hotel load” is the load created when everything, repeat every electrical light and appliance, is turned on at the same time.

Understand that, if you want each unit in a twin genset arrangement to be able to handle the full “hotel load," save a pile of dough by specifying a giant knife switch to control which unit powers the house electrical system at any given time. However, also understand that, if you do, in most cases, you will end up with your genset(s) running almost 100 percent of the time badly under-loaded.

Which some people think increases their longevity. And which it doesn’t.

Running under-loaded causes a generating set, especially a diesel one, to run cool and/or fuel rich. This, in turn, results in smoking and coking — smoking due to un-burned fuel being present in the exhaust and coking because the genset’s internal temperatures are cooler than they need to be in order to prevent excessive and rapid build-up of carbon. Consequently, matching generator size properly to load is the right way to go, not simply installing the largest genset that will fit. 

— Phil Friedman

Copyright © 2024 by Phil Friedman — All Rights Reserved

Note: An earlier version of this article originally appeared on the author’s LinkedIn blog, then later, in Passagemaker magazine.