Sizing Generators to Run Greener and Cleaner

ALMOST ALL AUXILIARY GENERATORS IN YACHTS ARE OVER-SIZED AND CONSEQUENTLY, GROSSLY INEFFICIENT

May 24, 2023

It doesn't take a degree in electrical engineering to understand why, only a grasp of some fundamental principles concerning marine auxiliary power generating equipment — and some 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% of its running time. This is especially true in the case of diesel generators, which should operate at 65% to 75% of their rated continuous load capacity.

The single most important factor working against meeting the ideal generator loading in a yacht application is the wide variation, from minute to minute and hour to hour, of electrical load conditions on such vessels...

There 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. But 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 although it can be very effective in managing generator loading, it is also expensive.

I've actually sat in on dozens of meetings in which hours and hours have been spent developing a full load analysis for a yacht new-build, and more time spent matching the 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 generally insist on each genset in a pair or trio to be sized to carry the "full hotel load" on its own.

And when they do, I always recommend trashing plans for spending hundreds of thousands of dollars on load-sharing and paralleling and replacing that equipment with a simple giant double throw knife switch.

If you want each unit in a twin 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, understand that if you do, in most cases, you will end up with your (s) running almost 100% of the time badly under-loaded. Which some people might think increases their longevity. Wrong.

Running under-loaded generally causes a generating set, especially a diesel one, to run cool and/or fuel rich. This, in turn, causes smoking and coking. Smoking due to fuel being present in the exhaust. Coking (the rapid development of internal carbon deposits that degrade combustive efficiency and operating life) because the internal temperatures are generally cooler than they need to be for preventing excessive carbon build up.

So, why not increase the loading on the by heating the ocean?

Some years ago, the fad fix for this situation was to add a load-cell system that employed resistance coils to heat seawater. These systems would kick in when a was running under-loaded, to bring the loading up to where it needed to be for to run cleanly and efficiently. Neat solution. Wrong.

This solution is somewhat akin to opening your windows when your over-sized air conditioning system is short-cycling and running inefficiently. It not only wastes fuel and increases the yacht's "pollution footprint", it adds, however minutely, to global warming. For it generates additional load for an under-loaded generator by working to heat the ocean. Pretty dumb, if you ask me.

A better solution to the problem of under-loading generators is to find the primary determining factor(s) in the sizing calculations.

The single, most critical limiting factor is almost always the air conditioning system compressor starting loads.

It's important to understand that the starting load of most A/C compressors is 5 to 7 or even more times as much as their full running load. This means, for example, that six 1-ton compressors, running at full load all at the same time would draw about 12 kW of power. Whereas, the same compressors, all starting at the same time, could draw as much as 100 kW. That is a huge difference when it comes to sizing generator capacity.

If you size the generating system to handle the occasional situation where all or almost all of the A/C compressors are starting simultaneously, you will invariably end up with your (s) running badly under-loaded the rest of the time. Smoking and coking and generally requiring more frequent maintenance and repair than if they were running always at 65% to 75% of their rated continuous load. So what to do?

Enter "soft" and sequential starting management of compressor motors

There are a number of different approaches to soft starting, but suffice it to say that choosing soft-start equipped A/C compressors often reduces the starting loads by up to 65%. Which in the case of our example means that the maximum starting load for our 6 A/C compressors, starting all at once, is reduced to 35 kW. And which, given a generator's normal reserve capacity for reasonable transient surge loads, means a sized for this lower starting load will be hugely more likely to be properly loaded for most of its running time, when the A/C compressors are running (not starting).

The cost of soft-start compressors will generally not be more than about 15% that of conventional-start compressors. Soft-starts do not usually add significant weight. And they are not overly difficult to in terms of machinery volume. Even better is a manitoring/management system that allows only sequential starting of various compressors and block two or more compressors from starting at the same time.

Moreover, the small premium paid for soft and sequential management of A/C compressor starting represents money much better spent than that paid out to heat the ocean

In my experience in engineering yachts from 20 to 50 meters, once you get the A/C starting loads under control, and down to a reasonable level, you can often reduce the size of the quite significantly. Without sacrificing generator capacity for handling the rest of the "hotel load". Then, if you plan the electrical power distribution system with appropriate split-buss logic and sufficient switching capability, you will be able to assure that they run greener and cleaner all the time.

— Phil Friedman

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