On a cool, overcast morning in Kaohsiung City, Taiwan, a chunky and colorfully painted ferry waits at a dock as a stream of passengers clambers aboard. It’s Chinese New Year 2017, and white-gloved officials are standing at the rails, beaming and waving. The boat, called Happiness, isn’t a new boat. Indeed, the years of wear and tear are quite obvious. But as a blue banner across the boat’s side proclaims, Happiness has been retrofitted with a direct-current hybrid-electric microgrid, and this is the reborn ship’s inaugural voyage. Most of the passengers have no idea what makes this boat so special, but all of them can appreciate the absence of diesel fumes, the whisper-quiet propulsion, and the lack of jarring vibrations from the deck.
Happiness now shuttles about 4,500 passengers a day—1.7 million per year—on a short hop across Kaohsiung Harbor. The plucky little ferry doesn’t look like a trendsetter, but it is. Since its introduction, dozens of other hybrid-electric and fully electric ferries have been unveiled around the world, and the number is growing year by year. According to the market research firm IDTechEx, the market for electric and hybrid boats will expand to US $12 billion by 2029. Car and passenger ferries represent more than half of that market, according to the quality-assurance and risk-management company DNV GL. Such upward trends show that the shipping industry is finally getting serious about finding alternatives to its highly polluting, fossil-fueled vessels.
While electrifying a large cargo ship or cruise ship is still impractical given the current state of batteries and electric motors, smaller ferries operating on shorter routes are ideal for electrification.
The main difference between Happiness and most of the other electrified ferries is the direct-current microgrid as its heart. Compared with more conventional shipboard systems that use alternating current exclusively, the Taiwanese boat’s DC microgrid offers lower power consumption, lower emissions, better reliability, and more seamless integration with other equipment like batteries and generators. Our experience with Happiness highlights why it makes sense for the designers of future short-range electric ships to incorporate a DC microgrid into their plans.
Using direct-current electricity on ships isn’t a new idea. Way back in 1880, Thomas Edison used one of his electric dynamos, which generated direct current, to power the lighting on the steamship SS Columbia. But just as land-based DC electrical systems gave way to those based on alternating current, shipboard DC systems failed to take off for much of the 20th century. It’s only recently that maritime power engineers have taken a renewed interest in direct current, to exploit DC’s advantages over AC.
One big problem with AC power on ships is maintaining the power quality. With AC, the current and voltage are slightly out of phase, but when the two become too far out of phase, it can cause the voltage to sag or spike, which can degrade the equipment over time and also cause the network to shut down if it goes too far. Because a shipboard electrical system is extremely small, compared with, say, a citywide grid, it can suffer from instabilities such as harmonic distortions, in which undesirable higher frequencies develop beyond the fundamental frequency—which in Taiwan’s case is 60 hertz. With DC, there are no waveforms to fall out of sync with each other, so the power quality is higher.
With a DC system, you don’t need a transformer to step voltages up and down, nor do you need rectifiers or inverters to convert between DC and AC. Instead, you can use solid-state power converters, which are much more compact and efficient and give you better control over the voltage and current. So if you’re operating equipment with variable-frequency drives, such as an electric motor driver, a power converter improves the control of the motor.
Best of all, a DC power system works very well with power sources like fuel cells, lithium-ion batteries, and supercapacitors, all of which are DC devices. So integrating them into a DC rather than an AC system saves on energy that would otherwise be lost in converting DC to AC and back again.
But to operate a ferry entirely on DC electric would require an enormous bank of batteries, which would be costly and tricky to manage. And DC versions of many types of electrical components and equipment, such as lights and air conditioning, are still not widely available.
For all these reasons, we decided to create a hybrid-electric AC/DC ferry. The main propulsion system and batteries would operate on DC, while loads such as lighting, air conditioning, and deck machinery would operate on AC. The entire system would form a microgrid, with power generation, distribution, storage, and loads comprising a self-contained network.
Rather than designing a new vessel from scratch for this experiment, we opted to retrofit an existing diesel boat. This decision was driven mainly by cost considerations: Because our focus was on the electrical and propulsion systems, we figured that starting with a boat that was already proven to be seaworthy was the more practical and economical option. The cost of retrofitting Happiness came to $400,000. By comparison, a later hybrid-electric ferry that we built from scratch cost about $2 million.
Happiness originally entered service in 2009, operating on the short 650-meter hop between the Gushan Ferry Pier and Cijin Island, a popular tourist destination in Kaohsiung City. The ferry was equipped with two diesel engines, each with a rated power of 225 kilowatts, to drive the main shaft of the propeller. There were also two 88-kW diesel generators to power the engine room pumps, air conditioning, lighting, and deck machinery.
The diesel engines were seriously oversized for the type of duty they were performing. Although its theoretical top speed was 10 knots, or 18.5 kilometers per hour, the ferry rarely moved that fast. On its short route, the vessel left the dock, briefly reached about 5 knots, and then decelerated before arriving at the opposite dock. It spent about two-thirds of its time idling at Gushan or Cijin, as passengers got on and off.
The frequent stopping, starting, and idling were causing the diesel engines to wear out prematurely and lose efficiency. And local officials and businesses weren’t happy about the clouds of diesel fumes hanging over Kaohsiung Harbor, thanks to Happiness and the three other ferries that plied the waterway.
As it turns out, short, frequent hops are an excellent match for electric propulsion. The project to retrofit Happiness with a hybrid AC/DC electric system began in June 2016. With funding from Taiwan’s Ministry of Economic Affairs, the new system was developed by the Ship and Ocean Industries R&D Center (SOIC), in collaboration with maritime propulsion researchers at National Kaohsiung University of Science and Technology and Aalborg University, in Denmark.
Although the electrification project could have been outsourced to a large electrical systems integrator, like ABB or Siemens, managers at SOIC decided instead to oversee the retrofit themselves. SOIC’s mission is to support the technological development of Taiwan’s ship industry and nurture homegrown manufacturing, so it made sense to keep the project in Taiwan.
Our team started by creating two interconnecting power systems: one operating on AC, the other on DC. On the AC side, the existing diesel generators were connected to a 440-volt AC panel. In addition to powering the lighting, air conditioning, the engine room pump, and deck machinery as they had been doing, the generators would now feed the new DC propulsion system on occasion. The AC system has a total load of 35 kW.
On the DC side, we replaced the diesel engines with two synchronous permanent-magnet motors from Danfoss Editron of Denmark, each with a rated power of 130 kW. The new motors were integrated with the original gearbox, main shaft, propeller, and throttle system. We also installed lithium-ion batteries from the Dutch company Super B and power converters from Danfoss. The DC and AC systems are connected by a bidirectional power converter, which allows power to flow between the two systems, converting AC to DC and vice versa.
The new propulsion system is more suitable for lower-speed travel. Before, the engine room was a noisy, smelly space below deck. Now, the propulsion room is clean and quiet, and all you hear when the motors are activated is a faint, high-frequency buzz.
The battery system’s capacity is 100 kilowatt-hours, which provides enough power for cruising. The batteries can be charged by the diesel generators, and they can also be charged using grid power, when the boat is docked for the night. The dockside 380-V charging station is similar to a fast-charging station for an electric car. The batteries and propulsion motors are connected in parallel to power converters, which in turn connect to a 750-V DC bus. The result is a shipboard microgrid with a total power of 1 megawatt.
The new equipment takes up much less space than what it replaced. The old 225-kW diesel engines were each as big as a household refrigerator, whereas the new electric motors are each the size of a host computer. The old electric control lines were bulky and complicated. They’ve all been replaced with a streamlined communication system, based on the controller-area network (or CAN) protocol used in automobiles. The control room dashboard now sports a touch screen, which lets technicians easily view the system status and troubleshoot problems by looking through historic data.
When the retrofitted Happiness took its inaugural voyage in January 2017, it became the first hybrid-electric passenger ferry in Asia.
The ferry now has two operating modes: pure-electric mode and range-extended hybrid mode. In pure-electric mode, the ferry operates solely on energy from the battery pack, much like the pure-electric mode in a hybrid-electric car. When the battery system reaches its minimum state of charge, the ferry automatically switches to range-extended mode, with the generators driving the propulsion system and charging the batteries. Although a pure-electric boat would have zero local emissions, it would also require a much larger battery system. The hybrid configuration permits a considerably smaller and less costly battery pack.
The microgrid on Happiness operates in what’s known as islanded mode. That is, the microgrid’s generation, electric load, and energy storage form a self-contained system that isn’t synchronized to the larger power grid. The only time the ferry is connected to the Kaohsiung City power grid is when it’s plugged into the dockside charging station.
Operating an islanded microgrid poses some challenges in terms of balancing voltages and frequencies as well as generation and load. The ferry’s new power-management system acts as the brains of the microgrid, continuously calculating the boat’s loads and power flows to determine which combination of generators and batteries to use and to keep the voltages and frequencies within acceptable ranges.
The power management system can also maintain power output in the event of a sudden outage—when a generator or motor suddenly stops working due to a short circuit, for instance. The safe and uninterrupted operation of the electrical and propulsion system is of course a fundamental requirement for a ship that transports passengers. Traditional AC power systems are well protected from short circuits because they typically rely on large, rotating generators, which can sustain a very high fault current for several hundreds of milliseconds without suffering catastrophic damage. That’s enough time for a relatively slow mechanical circuit breaker to activate and isolate the fault, leaving the rest of the power system unaffected.
In a shipboard DC microgrid, a fault caused by a short circuit is handled differently. The high fault current will typically cause the discharging of the capacitors inside the power converters, which are connected to the main DC bus. The power converters then shut down to protect themselves, typically within a few tens of microseconds. In such a situation, mechanical circuit breakers can’t react quickly enough, which can lead to a system malfunction or blackout.
Instead of mechanical circuit breakers, you could use solid-state circuit breakers, such as ones based on IGBTs (insulated-gate bipolar transistors) or integrated gate-commutated thyristors. These power semiconductor devices can be configured to switch off within several milliseconds when a fault is detected.
However, there are still some challenges with using solid-state circuit breakers for fault protection in DC microgrids. As a relatively new technology, they’re more expensive than more mature alternatives. And the communication between components when using these devices can be tricky to automate. For our project, we also had to consider the ferry operators: If a solid-state circuit breaker malfunctioned, would the crew be able to quickly figure out what to do? In the end, we decided it wasn’t practical to implement this cutting-edge technology.
Instead we used a time-honored low-tech solution: fuses. We put them between the main DC bus and the power converter. Like any other fuses, they’re simply pieces of conductor that melt when too much current passes through them, thereby interrupting the flow.
In addition to the quiet, clean, smooth ride that passengers now enjoy, Happiness has reduced its diesel fuel consumption by more than 30 percent. We expect the electric propulsion motors to last much longer than the diesel engines would have. The batteries will probably have to be swapped out after five years, but the savings in diesel fuel will more than pay for the new batteries.
Based on the results from Happiness, two other hybrid-electric AC/DC ferries have been built and launched in Kaohsiung Harbor. Engineers at SOIC are also designing hybrid-electric propulsion systems for other types of vessels, including yachts, tugboats, and tenders.
At present, the use of such “green” ships around the world is just beginning, with Norway and a few other Western European countries leading the way. We believe a growing share of electric and hybrid-electric ships will be equipped with DC microgrids because they offer clear advantages in fuel efficiency and stable operation. That’s the lasting lesson of Happiness.
About the Authors
Chun-Lien Su is a professor of marine engineering with the National Kaohsiung University of Science and Technology, in Taiwan. Josep M. Guerrero is an IEEE Fellow and a professor of energy technology at Denmark’s Aalborg University. Sheng Hua Chen is an electrical engineer with the Ship and Ocean Industries R&D Center in New Taipei City, Taiwan. Their work on Happiness was supported by a grant from the Ministry of Science and Technology of Taiwan.