DC hubs are often seen as a modern, future‑proof choice for electric and hybrid vessels. But whether they are the right choice depends on how your ship produces, distributes and uses electrical power.
For ship designers and technical decision‑makers planning a newbuild or a retrofit, electrical distribution is not a one‑size‑fits‑all choice. DC, AC and hybrid AC/DC arrangements each have strengths and limitations, depending on factors such as power sources, consumer types and operating profiles.
When you want to make a cost‑effective and energy‑efficient design decision, you need to think about where DC hubs add value – and where they will introduce unnecessary losses.
There are plenty of electrical distribution concepts available to ship owners who choose diesel-electric propulsion for their vessels.
These range from low voltage systems for smaller power ranges – like those seen on ferries or bulk carriers – to medium voltage systems for larger power ranges like those on a large container or cruise vessel. These concepts all have one thing in common: the power producer is an engine connected to a generator producing electricity at alternating current (AC).
Electricity on ships has typically been distributed to consumers in AC and converted to direct current (DC) for consumers that need it. These consumers are usually smaller onboard electrical systems.
A DC hub is a combined DC switchboard with converter modules. In DC distribution, everything is connected to a common DC link through power electronics, and power is transferred in DC.
DC hubs have become a hot topic as fully electric and hybrid vessels have become increasingly popular. However, DC hubs can present challenges in terms of electrical losses.
In a newbuild, electrical distribution can be designed holistically from the outset, which makes it easier to combine AC and DC where each delivers the most value.
In a retrofit, the existing power generation and distribution architecture often sets clear boundaries, which means that introducing DC hubs may require additional conversion steps. This usually makes the whole system more complex and, even more importantly, can quickly increase electrical losses.
DC power distribution clearly makes sense when the main power source on your ship is a battery producing DC because you can eliminate a transformation step.
If your ship uses variable-speed gensets, DC distribution has one potential advantage over traditional AC arrangements. Many small power consumers rely on fixed frequency electricity and may not work optimally with variable frequency. These small consumers can be anything from pumps and fans in the electrical system to coffee machines and photocopiers used by crews.
DC hubs deliver electricity to these small consumers at a fixed frequency, so they can be optimised independently. You then need to weigh up whether this added efficiency offsets the energy losses inherent in the electricity conversions.
DC power distribution with DC hubs is not the best choice if the main power source onboard your ship is a diesel generator that produces AC power and your main consumer is an electric motor or the ship’s hotel load. Both the electric motor and the hotel load need AC power. Pure physics tells you that converting the power to DC and then back to AC will produce heat and create additional electric losses.
“DC hubs are often assumed to be more efficient by default — this is not always true.
The electrical conversion losses when using a DC hub depend on what the power is being used for.
For propulsion power, the conversion losses with a DC hub are 3.5% to 4%. With a classic direct front-end AC distribution system, the losses measured between the generator and the motor are just 1.5%.
For auxiliary power and hotel load, a DC hub suffers losses of between 3.5% and 5% between the generator and the AC grid. With an AC system the losses are just 1%.
It can be argued that the additional electrical losses seen with a DC hub are offset by the efficiency gain offered by variable-speed gensets. The argument goes like this:
To back up the argument for DC hubs, OEMs will typically use a comparison curve from a high-speed engine, where the gap between fuel consumption at variable speed and constant speed is greatest. However, it is typically medium-speed engines that are used for DC hubs, so in reality this gap is smaller.
DC hubs are not always the optimal choice for hybrid propulsion systems where both engines and batteries are part of the powertrain. In these configurations, the potential efficiency benefits of variable‑speed gensets must be weighed against the additional electrical conversion losses that DC distribution introduces.
When these losses are taken into account, the efficiency advantage of variable-speed gensets typically appears only at higher loads of around 60-70%. At medium-to-low loads, constant‑speed gensets can be even more efficient than variable‑speed alternatives, especially when one includes the auxiliary and hotel load losses from DC/AC conversion.
In diesel-electric systems the separate propulsion chains – let’s say two propellers for a RoPax ferry or six thrusters for a wind turbine installation vessel – are connected via a circuit breaker. In a well-designed AC system there are no losses, but DC systems have circuit breakers that cause additional losses of 0.6% to 3%. If you run your wind turbine installation vessel’s six thrusters with two of the six gensets in dynamic positioning mode, these losses can be as high as 12%.
DC hubs can make sense, and they are a good choice for:
However, they are not an optimal solution for every vessel. A DC hub is not the best solution for:
For these types of applications, the optimal solution is to combine the best of both worlds by connecting the gensets to an AC grid and connecting the batteries and selected consumers to small DC hubs that are part of the total diesel-electric hybrid power train.
Wärtsilä has helped many ship owners to optimise the electrical distribution systems for their newbuild vessels. Contact Wärtsilä to discuss your next newbuild project and find out how we can help you to make smart decisions about electrical distribution.
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