A green rescue for reefers with PM shaft generators

There’s a lot of traffic on the high seas. Over the past two decades, there’s been a dramatic increase in the amount of sea freight being shipped – now over 80% of all cargo is transported overseas. And with billions of mouths to feed around the world, the amount of refrigerated sea transport vessels (reefers) has been steadily increasing since 2010. In increasing frequency, companies need to maintain major energy-consuming vessels to cool and ship meat, fruits and vegetables to global markets.

To meet demand, shipbuilders have been manufacturing larger and larger reefers, container vessels and bulk carriers. Yet shipowners also face the ongoing challenge to reduce fuel consumption and address increasingly restrictive IMO regulations for greenhouse gasses (GHGs). This has lately been seen with gas and oil carriers, where a permanent magnet (PM) shaft generator is becoming a standard solution to power the vessel.

A green rescue for reefers, PM machines can meet the increasing demand for more power while also saving on fuel consumption costs and meeting emission requirements. As an alternative to conventional electric machine types – asynchronous or synchronous – they can be applied as gensets, shaft generators or as propulsion motors to improve the energy efficiency of the vessel.

Let’s consider, for example, how a PM shaft generator can augment a scrubber solution when used as a shaft generator. To cut emissions, shipbuilders and owners often add scrubber systems to their emissions systems to “scrub” out harmful toxins in emissions. But if a ship is already required to power itself, plus a vast refrigeration system has extraordinary electricity demand, where are they going to get the extra energy from to power the scrubbers? Typically, this can be done by adding additional gensets or utilizing reserves from the existing ones.

Instead of adding gensets to cover the increased power demand, another more green and feasible solution is to upgrade a vessel that has a 2-stroke main engine with an in-line PM shaft generator. Thanks to the compact size of the PM generator, it can be easily installed on the propulsion shaft line – and no gearbox is needed. As the generator rotor is mostly hollow, the shaft line can run directly through the PM machine, connected to it with a flange connection. If the main engine has sufficient design margin, that can be utilized to power the shaft generator system. Or alternatively, the main engine can be upgraded, too. As a result, some – or even all – of the auxiliary gensets can be shut off during the voyage. It’s true that a PM machine is a bit more expensive than a conventional synchronous generator, but that difference will be typically paid back in a couple of years depending on the vessel’s operating profile, and the rest is pure saving. As an added benefit, the shaft generator system can be used as a propulsion motor for peak shaving and take-me-home functions.

Now, let’s consider the gains to be made from using PM machines while reducing fuel consumption by simply driving slower. It’s not rocket science that you literally save fuel with lower speeds. If you are not in a hurry, the main engines typically have the lowest specific fuel oil consumption at lower than the rated speed. This means when the vessel speed is lowered, fuel economy improves.

This is also where the permanent magnet machine is superior. The PM machine has higher efficiency in the entire operation range than a conventional machine, but especially at partial loads. If your onboard power consumption allows, you can reduce the power taken from the generators, operating the PM generators near the optimal point from an efficiency perspective. You save more by driving slower, as you operate both the main engine and shaft generator near their optimal points in terms of efficiency. It’s an unbeatable combination. It’s less fuel, more energy. It’s a win-win.

On top of the above benefits, the PM technology allows you to reduce your generator size and weight by even 50% compared to a conventional electric machine type.

It might sound like a miracle, but the future is here and the facts speak for themselves.


Interested in a PM shaft generator?