Selecting generators for new build yachts
by Roger Marshall
When a utility company generates power, it produces just enough to meet demand. That demand is monitored carefully, but is usually so large that the utility can easily predict how much power is needed at any given time.
A yacht has a different problem. With one, two or three generators, the load may be too much for one generator, but under capacity for two, which can lead to a common problem: that of generators running under their load capacity much of the time. This is not good for the generator and can shorten its life considerably.
Ideally, the generator size will be matched to the electrical load capacity of the vessel, but this load can vary considerably in the course of one day. For example, overnight there might only be air-conditioning and lighting loads running, but by early morning the galley has come alive, the engineer might need power to run pumps and charge batteries, and if the vessel is to get under way, the captain might need anchor handling power, thruster power, and power for bridge electronics.
This load fluctuation can give designers and engineers nightmares. A designer can make many load calculations to get the generator size just right, but all those calculations can be thrown out of whack because the owner wants a larger refrigerator or larger galley stove than the boat was originally designed for.
Consequently, weeks of calculations can be tossed out the window in an instant.
‘Selecting the right generator size to suit the load range is as difficult as picking tyres for a car you haven’t yet purchased,’ says David Laska of L & L Electronics in the US.
The range of options is enormous. For example, one 45m superyacht might carry two 125kW gensets and a 7kW set for backup use, but an identical yacht might have two 150kW powerplants. A third option might be three 92kW units for a similar-size yacht by a different builder. According to Laska, two 150kW units, or around 300kW, are about average for the house loads on a 45m yacht.
As the yacht gets larger, the range of options increases dramatically. An 85m yacht recently constructed in the US had seven generators putting out a total of 2,300kW, although not all the power was for the house load. One drove the tier B mechanical systems and a second drove the steering and tier A mechanical systems. A 90m yacht built in Europe has five generators: two at 350kVA, two at 560kVA, and one 190kVA, for a total of almost 2,000kVA.
But just as the Emission Control Areas in North America, where nitrous oxide (NOx) and sulphur oxides (SOx) are regulated, and Tier III emissions standards are affecting main engine emissions, environmental controls are also leading to more stringent exhaust management systems for generators. Thus power generation, like propulsion, is changing as new electrical monitoring and control programs and more efficient generators come into the marketplace.
One of the new generators comes from Northern Lights where the challenges outlined above have been met by the company’s Hybrid Marine system.
‘In this system, the generator runs at a speed sufficient to generate enough power to meet demand, and if desired, to run on batteries as well,’ says Mike Maynard, vice president at Northern Lights. ‘With Tier III arriving in 2013, the regulations get even tighter.
‘At Northern Lights, our Hybrid Marine system is designed to work with our Tier III engines and increase efficiency, eliminate wet stacking and provide longer generator life. Plus, it’s smaller and lighter than a conventional generator.’
Power generation, like propulsion, is changing as new control programs and more efficient generators come into the marketplace
Northern Lights has also introduced new generators with it’s DECS exhaust gas cleaner as an interim step toward meeting Tier III. Other manufacturers have similar, but slightly different methodology, and as propane and natural gas (the logical next steps to cut emissions) are not available in this size yet, options are limited. Manufacturers are waiting to see how Tier III shakes out before developing new machines.
In conclusion, it would seem that generator options are varied and selecting the right one to meet future regulations is going to be difficult in the short term, and extremely difficult in the longer term until Tier III controls come into force and indicate the best direction to go.