Making the decarbonisation of ships a reality: The story of a DC/DC converter for fuel cells

Here is a story that ranges from big decarbonisation strategies to small, concrete steps to achieve them. The European Union's goal, as set out in the Green Deal and other documents, is to achieve net zero greenhouse gas emissions by 2050, while ensuring that economic growth is decoupled from resource use. An interim target has also been revised. The new target of at least a 55% reduction in emissions by 2030 compared to 1990 levels was adopted as it became clear that the previous target of a 40% reduction by 2030 was insufficient.

In its hydrogen strategy, the EU states that hydrogen is essential to achieving these targets. Although renewable electricity is expected to decarbonise a significant proportion of the EU's energy consumption by 2050, it won't meet all energy needs. Hydrogen from renewable sources has significant potential to fill this gap, serving alongside batteries as a storage medium for renewable energy and facilitating energy transport. It provides a back-up for seasonal fluctuations and links production sites with distant centres of demand. The EU adds that the share of hydrogen in the European energy mix is expected to increase from less than 2% in 2020 to 13-14% in 2050.

This is the macro level. These are the global projections, for which the experts have to take into account many variables, and for each of the 27 countries that make up the EU. From there you have to go down to lower and lower levels until you start talking about the wind turbine, the battery or the fuel cell that will make such a reduction possible.

Hydrogen and fuell cells to decarbonise shipping:

For almost two years now I have been cooperating with KraftPowercon , a Swedish company that makes rectifiers for ballast water treatment systems (BWTS) and for reducing marine fouling on ships. I have enjoyed both topics, as they deal with what is perhaps the first environmental impact of shipping, and one that has only become more important since the middle of the 20th century. But when I was told that in November this year a product would be launched that would fit into this global decarbonisation architecture, even if it was only a small contribution to the massive task ahead, I was intrigued. I am talking about a small but concrete step to support decarbonisation: KraftPowercon will be launching a 200kW galvanically isolated DC/DC converter module for marine and land-based fuel cell systems.

Let's break it down and explain. Enjoy the electric journey and the many examples!

A DC/DC converter is an electronic device that converts direct current (DC) from one voltage level to another. These converters are essential in systems where different components require different voltages, ensuring that each part receives the correct voltage for optimum performance. For example, your laptop charger, the power brick you plug into the wall, converts high voltage AC (alternating current) from your home to low voltage DC (direct current) for your laptop. Once converted, your laptop uses a series of DC/DC converters to adjust the DC power to the correct level required by the various components, ensuring everything runs smoothly and efficiently. An average laptop typically has around 5 to 10 DC/DC converters.

Galvanic isolation is a technique used to separate electrical circuits, preventing current from flowing between them while still allowing data or power to be transferred. This separation is essential for electrical safety, voltage regulation, system protection and efficient power transfer. A good analogy is wireless charging, where a mobile phone is powered without a cable by transferring energy via a magnetic field. The term "galvanic" is named after the Italian scientist Luigi Galvani and has nothing to do with metals or chemical processes.

Galvanic isolation is critical in high power fuel cell systems because it separates the inherent ground impedance of the fuel cell from the main DC bus. This impedance affects the flow of current through the ground path and can affect the performance and safety of the entire electrical system. A simple comparison is the water system in your neighbourhood: if one house has a blockage (ground impedance), it affects the water pressure for the whole block. Disconnecting that house ensures proper flow for everyone else. Similarly, galvanic isolation prevents ground impedance problems, ensuring optimum power flow and system safety.

A DC bus is a vital component in electrical and electronic systems, acting as a central path for distributing DC power to different parts of the system. A busbar or bus is a solid metal bar or strip, usually made of copper or aluminium, used to conduct electricity within a power distribution system. It acts as a central connection point for multiple circuits and is designed to carry large currents and distribute power efficiently throughout the system. A simple analogy for a DC bus is to think of a tree, where the trunk is the DC bus and the branches are the subsystems. The trunk carries nutrients (electricity) from the roots to all the branches, ensuring that everything gets the power it needs.

Why is grounding necessary? Earthing is essential in electrical systems to safely dissipate excess electrical energy, prevent damage and ensure stability. It protects against electrical shocks, surges and interference, maintaining the safe and efficient operation of the system. Proper earthing also increases the reliability and longevity of electronic components. Imagine being safe in your house during a thunderstorm because the lightning rods are grounded. They safely conduct the electricity from the lightning into the ground, preventing it from harming you or your home.

How does grounding work on a ship where there's no ground? On ships with a metal hull, the hull itself is usually the primary ground reference. All electrical equipment and circuits are connected to the hull.

But how does grounding work on a ship where there's no ground? On ships with a metal hull, the hull itself is usually the primary ground reference. All electrical equipment and circuits are connected to the hull, providing a common reference point for the entire electrical system. On ships with non-metallic hulls, such as those made of fibreglass or composite materials, a special grounding plate or grid is used as the ground reference. This metal plate is immersed in seawater to ensure adequate grounding.

Fuel cells can produce higher voltages than the main DC bus, resulting in overlapping voltages. To manage this, the converter allows voltage regulation mechanisms to ensure that the fuel cell voltage matches the DC bus requirements. This isolation prevents overvoltage scenarios that could damage the DC bus and connected loads, and ensures that the entire power system operates within safe voltage ranges. A simple example is cooking on a cooker with two burners: very hot and moderately hot. If a delicate dish is placed on the hot burner, it could be burnt. The converter acts as a heat regulator, adjusting the temperature of the hot burner to match that of the warm burner, ensuring that your food cooks properly without burning.

If the converter fails, it creates an open circuit between the fuel cell and the DC bus, preventing reverse current from flowing back into the fuel cell. This prevents the fuel cell from being damaged by overheating, chemical imbalance or other causes. An open circuit failure mode is simple and reliable, eliminating the need for additional components such as chokes and filters that can complicate the system and introduce potential points of failure. Imagine a bridge with a missing section. Cars can't cross and stop before falling into the gap. An open circuit is like that missing section, stopping the flow of electricity to prevent problems.

The high frequency switching converter enables low ripple current, eliminating the need for chokes or filters. In electrical systems and power supplies, "low ripple current" means that variations or fluctuations in the supplied current are minimised. Think about riding your bike down the road. A perfectly paved road is smooth, but a bumpy road has little ups and downs. Ripple current is like those bumps, causing small fluctuations in what should be a smooth ride (current).

As you have seen, we have gone from the grand strategy of decarbonising Europe, to the vision of using hydrogen as one of the main tools, to using galvanically isolated DC/DC converters for a fuel cell, because fuel cells will be critical to using hydrogen to power everything that has been powered by fossils fuels.

We have gone from the grand strategy of decarbonising Europe, to the vision of using hydrogen as one of the main tools, to using galvanically isolated DC/DC converters for a fuel cell, because fuel cells will be critical to using hydrogen to power everything that has been powered by fossil fuels.

Finally, let's talk about the 200kW DC/DC converters mentioned earlier. What kind of machines, equipment or products would need such a powerful converter? Here are a few examples:

• Electric trucks and buses;
• Electric locomotives;
• High power industrial machinery;
• Mining equipment;
• And, of course, marine propulsion systems!

I hope you enjoyed the electric journey!

Pablo Rodas-Martini is a Contributor to KraftPowercon Marine. He holds a PhD and MSc from Queen Mary and Westfield College (now Queen Mary University), University of London.

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