Permanent magnet (PM) shaft generators are being touted as a way to cut power demand and emissions on large containerships, particularly those resulting from plugging in reefer boxes to ship refrigerated goods.

Yaskawa Environmental Energy — a Finnish builder of PM shaft generators that is also branded as The Switch — has sold 100 PM generators for ships since 2014. They have been mainly used in large LNG and LPG carrier newbuildings, but have also been installed on product tankers and car carriers.

The head of the company's electric machines product line, Jussi Puranen, said the systems provide "low-hanging fruit" to cut fuel use and emissions on any vessels with considerable power demands.

PM machines are hollow steel cylinders with magnets fixed on their surface built around a propulsion shaft that leverage its rotational power to transform kinetic energy into electricity.

The mechanically simple machines can be used as generator sets and as propulsion motors to improve overall vessel energy efficiency, Puranen said.

The Switch was the first company to move into the marine market for PM machines in 2014 after developing them for wind turbines where they have become a leading technology due to their greater efficiency and lower maintenance requirements than conventional generators.

Puranen said none of the top 10 wind turbines used PM generators in 2001, but all the biggest ones do now.

A Yaskawa Environmental Energy shaft generator is seen at the company's Lappeenranta factory in Finland. Photo: Yaskawa Environmental Energy/The Switch

Coming of age

PM generator technology was first invented a century ago, but has only more recently come of age, being used in other modern applications such as elevators and hybrid cars.

Other companies are now entering the market for large PM generators, including ABB, General Electric, VEM Group and Ingenieria.

Puranen said the generators are suitable for large containerships, and that retrofitting where old diesel engines needed to be replaced to cut emissions is technically possible — particularly if there is space where a variable-speed shaft generator was previously fitted.

In the past, mechanically complex electrically excited synchronous generators (EESG) have been used in ships that are equipped with variable-speed shaft generators. They are larger, require extra equipment to supply rotor current and suffer energy losses.

By contrast, PM machines have "direct positive effects on the dynamic performance of the whole shaft line, including torsional vibrations and design flexibility for the ship", Puranen said.

Annual fuel savings of about $75,000 over an EESG have been measured with LNG carriers, he said.

Ships' main engines consume substantially less fuel than auxiliary diesel generators per kWh produced, so using energy generated through the propeller shaft makes sense with reefer containers.

Demand for reefer boxes has been predicted to rise by up to 8% on a compound annual growth rate in the decade up to 2030, and a 40-foot reefer container will average a daily power consumption of 180 kWh.

"People are starting to see what I call the magic of permanent magnets and realising how it can accelerate the shift to enhanced performance and sustainability," Puranen said.

Used with batteries, the technology can also be employed as a motor to enable peak shaving or load levelling of main diesel engines, or deliver the primary energy source for entering and exiting harbours, Puranen added.