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Engineer’s Angle: Energy efficiency may damage generator; load banks help

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Engineer’s Angle: by JD Anson

As climate change has a growing effect on our planet, we try to do the right thing by becoming more energy conscious. The most popular move is to change incandescent halogen and xenon lighting to LED.  Along with the benefit of much longer bulb life, power consumption drops significantly. Now each bulb uses about a 10th of the power for the same light output. This may seem small, but if you multiply that decrease by the hundreds of bulbs on board a typical yacht, it adds up quickly. An average tri-deck has about  400 fixtures on board. At 50W each, they would consume 20kW.

The way that LEDs are more efficient is that they lose less power to generated heat. This reduction of heat generation will also require less air conditioning to keep the interior cool. Newer motors and other equipment are also now more efficient than ever. This is great, right?  Not so fast.

Now there is less power required, so the generators originally specified to accommodate less efficient equipment are now oversized for their typical loads.  Some builders may also install oversized generators as a marketing ploy, thus compounding the problem. While they are burning less fuel by running at lighter than designed loads, they now have lower exhaust temperatures. This leads to several problems for the prime mover.

The most common issue is wet stacking. This is a black, oily liquid that can leak from exhaust joints or pass through and produce an oily sheen on the water’s surface, leading to black scum lines on the hull, a hefty fine or even banning from more stringent ports. Long periods of light load can also cause deposits on turbocharger blades, behind piston rings and exhaust valves, as well as cylinder polishing. All these lead to worse economy and costly repair.

For best efficiency and longest life, a generator should run at a load near its designed rating. Though some debate exists as to how much is enough, most recommendations fall in the 75-80 percent range. With most generators sized for full guest load using hot tubs, showers, zero speed stabilizers and a full bore galley, this load represents a very small fraction of the overall hours put on a generator. Most hours are during repositionings and alongside with only crew on board. These hours usually are running at well under 50 percent load, and in cool weather, under 20 percent. This is when the damage is done.

So what to do? The addition of a load bank can alleviate much of the damage being done to the generator’s engine. Though there are several designs, at their most basic they are water-cooled heating elements. A tank filled with coolant has several water heating elements installed in it. This tank is cooled by a heat exchanger,  much like the one on a generator that is cooled with raw water pumped through it. The elements are activated at the level required by a control system that monitors the load on the main bus bar and, comparing that with the programmed set points, decides how many elements to turn on to create the needed load. These resistive loads are the least efficient use of electricity, thus the best for creating a load. They also are the smoothest form of consumption, thus keeping stress on the electrical system to a minimum.

Installation of a load bank is a straightforward process. After an onboard survey of the requirements, a properly sized load bank is installed.  Newer ones have become much smaller, thus easier to fit. A raw water supply from the seachest or dedicated thru hull is connected to the bank, as well as an overboard discharge. Once the proper electrical connections are made, commissioning is performed and the programming of parameters is done.

LED lights are initially more expensive to purchase, but over their lifetimes much cheaper to own and operate. Similarly, the initial cost of purchase and installation of a load bank will be offset by reduced engine wear,  thus paying for itself by extended periods between rebuilding.

Best of all, not having to listen to the deck crew gripe about having to scrub the waterline again or having the port police knock on the door to kick the boat out of the harbor is well worth the price of admission.

JD Anson has more than 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric (finelinemarineelectric.com) in Fort Lauderdale. Comments are welcome below.

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