China Ship Survey

‘Permanent magnet’ makes propulsion technology different

An interview with Mr. Jonas Nyberg, Managing Director West, Berg Propulsion, and Dr. Jussi Puranen, Head of Product Line, Electric Machines, Yaskawa Environmental Energy / The Switch.

English translation of the full published text. For the original Chinese version, see here.

By Zhao Bo, Journalist with China Ship Survey

Although permanent magnet (PM) machines were invented more than 100 years ago, they haven’t been used in megawatt-class applications until the past few decades. Today, nearly all the top ten wind turbines use the PM generator, with power up to 16 MW. The increasingly stringent environmental regulations in the shipping industry will make permanent magnet machines more and more popular in ships.

On 10 November 2021, Berg Propulsion signed a key supplier and cooperation agreement to provide systems integration capability for Yaskawa Environmental Energy / The Switch permanent magnet and power electronics products. The cooperation creates a new and combined force offering the robustness and efficiency of propulsion systems where electric motors directly drive the propeller shaft, consolidating Berg’s growing presence in electric propulsion.

This represents the latest strategic step forward for Berg Propulsion, a mature electric propulsion system integrator. Since its establishment in 1912, the company, which is specializing in propulsion and system integration, has been committed to customizing efficient operation solutions for various applications. In the past few years, it has set up efforts to provide wisdom for the sustainable development of the marine industry. Yaskawa Environmental Energy / The Switch devotes itself to advancing the world with electrical drive trains. They’re on a mission to enable more profitable power generation, energy storage and use, while lowering emissions and the cost of ships. In the eyes of people in Marine, the cooperation shows the confidence of the two companies in electric propulsion, which they believe could be the mainstream technology in the next few years.

To provide insights into the solution and the cooperation, we interviewed Mr. Jonas Nyberg and Dr. Jussi Puranen.

Simple and efficient solution

At present, Berg and Yaskawa Environmental Energy have already collaborated on an initial integrated project, delivering a gearless system to a large bulk carrier, to make the carrier less vulnerable to ice damage while maximizing efficiency. Berg will be responsible for systems design, automation and integration. Shafts will be powered by The Switch PM electric motors. What is the design concept of the solution? What benefits can this solution bring to ships?

Zhao Bo: How does permanent magnet shaft generator work in the electric propulsion solution?

Jussi Puranen: It’s actually very simple: permanent magnets are mounted on a rotor, which is rotated by a propeller shaft. This creates a rotating magnetic field, which induces voltage and electric power on the stationary stator side. It’s fully analogous to those small dynamos used on a bicycle’s front wheels to provide light before the LED era. The only difference compared to conventional generators is the fact that the magnetic field is produced with permanent magnets instead of conventional electromagnets, making the machine far simpler and more efficient. From the generator stator, electric power is fed via a frequency converter to the grid. With the frequency converter, the constant frequency can be kept on the grid side, even though the propulsion shaft speed changes, depending on how much propulsion power is needed by the vessel at any given point in time.

In PM machines, strong Neodymium magnets are used that need no external energy for magnetization. This results in higher generator efficiency. When used as a shaft generator, the PM machine is installed inline onto the propulsion shaft – between the main engine and propeller. So, it takes the power from the propulsion shaft and converts it into electric power via a frequency converter, which allows a freely adjustable propulsion shaft speed while keeping the frequency constant on the grid side.

Zhao Bo: Why can this technology save costs compared to conventional generator?

Jussi Puranen: Since the vessel’s 2-stroke engine has a better specific fuel oil consumption (SFOC), this means significant fuel savings for a vessel operator compared to having electric power produced with 4-stroke gensets that have lower efficiency with higher SFOC. Also, when using a shaft generator, the gensets can stay turned off, which also leads to significant savings in their maintenance costs. The shaft generator can also act as a propulsion motor, taking electric power from the gensets in that case and allowing boost and take-me-home functions in special circumstances, such as harsh operating conditions or a main engine failure. When peak propulsion power is produced with the assistance of a shaft generator running in the motor mode (boost mode), a 10–15% smaller main engine can be selected, which then lowers CapEx costs.

Conventional directly on-line gensets must run at full speed all the time to keep the frequency constant. However, adding a frequency converter in the system allows our machines to be run in variable-speed mode. This results in significant fuel savings when lower than full power is needed.

Joining forces to collaborate and innovate

Berg and Yaskawa Environmental Energy provide shipowners with a more efficient propulsion system solution – a solution to convert vessel projects from geared installations to direct-drive applications with fixed-pitch and controllable-pitch propellers. However, behind this seemingly simple solution is the joining forces of professional knowledge and technology from both sides.

Zhao Bo: It was reported that Canada Steamship Lines (CSL) with permanent magnet-powered electric motors has successfully completed sea trial in China. What are the roles of Berg and Yaskawa Environmental Energy in this project?

Jonas Nyberg: Berg Propulsion acts as the system architect and a consultant to the yard, shipowner and designer of the vessel. Our role is to apply our experience and expertise toward how these systems are optimized. During the project, we take on the role of the project manager and engineering partner for the solution, working primarily with the automation, energy flow and installation aspects. Berg is also delivering fully feathering controllable-pitch propellers and its patented Twin Fin solution.

The direct-drive electric propulsion market for large ships based on PM technology is right now in its infancy. Strengthening our collaboration will enable us to influence the trajectory of the market by electrification and to provide energy-efficient, flexible and future-proof electric propulsion and power generation solutions. This will allow shipping companies to meet demanding emission regulations. Besides the bulk carrier project, we are currently in discussions for a few new applications and segments for the direct-drive electric system. We believe this to be the best-in-class solution for any vessel with a high requirement for both efficiency and durability. The solution offers an unparalleled simplicity amounting to the highest possible efficiency and robustness. To mention a few, we believe this solution will make its way into ferries, offshore, ice breakers and cargo among others.

Jussi Puranen: We provided two 3 MW direct-drive PM propulsion motors, four PM variable-speed gensets with a total power of around 10 MW and all the power electronics to control these. This included The Switch DC-Hub for DC distribution. The DC-Hub contains several applications to take care of our motors and generators but also the vessel grid. The DC-Hubs are connected with an electronic bus link (EBL) to reach high efficiency, flexibility and redundancy. Our proprietary electronic DC breakers (EDCB) ensure high selectivity and ride-through in each DC-Hub.

Zhao Bo: How to examine the usefulness of PM machines in shaft generator systems?

Jussi Puranen: We compared an induction machine and a PM machine in a 1.8 MW converter output system. This study used a 174,000-cbm LNG carrier that has two of these machines per vessel – one on each of two propulsion lines. The reference was our PMM 1500M PM marine generator. To make a 1:1 comparison, the same frame size (1500) was used for the induction machine, but its axial length was increased. Induction machines must draw a large magnetization current from the grid/frequency converter, causing significant ohmic losses in machine winding, and therefore must have lower power density to avoid severe overheating issues. Such magnetization losses do not exist in PM machines, since the magnetic field is produced with permanent magnets, and their power density can be pushed significantly higher. This makes the machine more compact. Electromagnetic analysis was performed with FEM software (Flux 2D), and CFD (Fluent/Ansys) was used for thermal analysis. The electromagnetic and thermal behavior of the PM reference machine had been already verified in full-load tests before delivery of the first machines.

Prerequisites to become one of the mainstream technologies in the marine industry

It is essential to reduce greenhouse gas emissions through new technologies and low-carbon/zero-carbon energy to improve the energy efficiency of ships, as well as to meet International Maritime Organization (IMO) regulations. One of the most effective ways to achieve this target is to use variable-speed shaft generators for power generation, rather than conventional generators.

Zhao Bo: Why do you say that electric solutions will become one of the mainstream propulsion options in the shipping industry?

Jonas Nyberg: As the fuels and energy sources for ships become more diverse in combination with an ever so strong drive for energy efficiency, this inevitably means solutions becoming more electric. The reason being some of the new energy sources such as batteries and fuel cells are inherently electric, as well as future/alternative fuels are easier to run in a generator engine rather than a propulsion engine. Integration of a direct-drive electric system further simplifies the installation and provides a minimalistic approach to the number of components needed to create an efficient propulsion plant.

With the increasing use of hybrid powering and electronic applications, many traditional components of modern ships have been replaced by new technologies and components. An electric drive system as an advanced solution could decrease energy loss, improve environmental performance and reduce operating costs. Electric solutions have already become mainstream in some segments such as offshore and workboats. And as long as the drive for increased efficiency and alternate fuels becomes more prominent, their application will be growing for the merchant fleet as well. 

We are currently engaging with a few different vessel designers and shipowners to benchmark the best solution concept for their vessels. What we’re finding is that the direct-drive electric solution comes out as the best solution for most of the cases where the vessel propulsion solution is based on a diesel, gas or battery electric power plant. The reason for this is the simplicity of the concept and that there are almost no losses whatsoever in the way we have integrated this.

We see that future segments for this are really any high demanding application that puts a premium on reliability and efficiency.

Jussi Puranen: We have two decades of experience using PM machines and full-power converters in megawatt-class wind turbines. Since 2014, we have delivered these products also for marine applications – shaft generators and propulsion motors. We have now delivered over 130 megawatt-class machines and more than 1,300 dedicated marine drives. We see a huge demand for these highly efficient machines and drives that support both AC and DC distribution on a vessel in the near future, since they help vessel owners/operators meet the IMO regulations for greenhouse gas emissions (EEXI and CII, which go into effect already in 2023).

Furthermore, a PM machine is extremely simple mechanically, leading to increased reliability and less need for maintenance (lower OpEx). There are two reasons why a PM shaft generator system saves fuel. First, electric power is produced with a vessel’s 2-stroke main engine, which is more efficient than 4-stroke gensets. Second, a PM machine is more efficient than conventional machines. Often, it is also common to use a controllable-pitch propeller (CPP). This means both the speed and propeller pitch angle are freely and independently controlled in a so-called combinator mode. This results in the highest possible efficiency with any given propulsion load.

PM shaft generators for the large container vessel market are emerging too. These vessels can carry up to 24,000 TEU (twenty-foot equivalent unit) containers on board and are often powered by a single very large 2-stroke engine, with power ratings up to 80 MW. As these vessels often have a single propulsion shaft due to economy reasons, the shaft generator power rating is often 4–5 MW for these vessels.

Furthermore, such large main engines typically rotate at somewhat lower speeds than smaller ones. This means a container vessel shaft generator needs relatively large torque from the shaft to produce the required output power. To meet these increasing power requirements, we started the development of our biggest frame size shaft generator, the PMM2000M, around a year ago. The design of these machines combines extensive feedback from over 100 shaft generators delivered with state-of-the-art engineering to achieve three main goals – maximum efficiency, high reliability and compactness. With extensive type-test data from over 20 different shaft generator types so far, the electromagnetic performance of the PMM2000 machine has been maximized, resulting in extremely good efficiency and high power density.

The comparison between a permanent magnet machine and an induction machine also reveals that a PM machine often has peak efficiency at partial load operation. This means PM machines are especially suitable when slow steaming is a desired function. There were significant differences – varying between 3–8% in efficiency at partial load operation. An induction machine needs constant external magnetization power from the grid, regardless of the load. This severely affects the partial load efficiency values. The efficiency difference between the two machines means that the 174,000-cbm LNG carrier with two propulsion lines used in the analysis will save approximately USD 70,000 annually through lower fuel consumption when a PM machine is used, taking a 174,000 cubic meter LNG carrier as an example. Assuming that the ship has a lifecycle of 25 years, the total fuel savings will be close to USD 2 million.