Super Yachts (The Aqua)
Sinot Yacht Architecture & Design made headlines in February when they released a design for the €500M superyacht “Aqua”. While the (false) rumor that Aqua had already been purchased by Bill Gates captured the public’s attention, it was the Aqua’s use of a hydrogen fuel cell electric powertrain that caught mine.
I started working with hydrogen fuel cell technology over 20 years ago. And the first applied project I did was examining their feasibility to power boats. From 2014 until today, I have been focused on the maritime application of hydrogen fuel cells first with research at the US Department of Energy’s Sandia National Laboratories where I established the nation’s only Zero Emission Maritime program, and now with Golden Gate Zero Emission Marine, a company I co-founded in 2017 to commercialize packaged hydrogen fuel cell marine powertrains.
In the transportation world, if you want to achieve zero emissions there are really only two choices: batteries or hydrogen fuel cells. On the maritime side, because of the high duty cycles, power requirements, and – what’s really key – high energy consumption, batteries are not technically possible to support the vast majority of vessel types and voyages. It takes a lot of weight and volume to store the enormous amount of energy required, using even the best projections for known battery technologies. This leaves hydrogen fuel cells as the only solution. However, even hydrogen fuel cells have a difficult time meeting the demands of some types of vessels.
To understand why, it is important to take a quick look at the fundamentals behind the technology. Hydrogen is a flammable gas and has properties remarkably similar to natural gas. This makes sense because the main constituent of natural gas is methane, or CH4, which means 80% of the atoms in methane are hydrogen. But there are a few differences. While natural gas liquefies at -162 C (making LNG), hydrogen will not liquefy until -253 C – just 20 degrees above absolute zero. Handling liquid hydrogen (LH2) is industry standard today, but the colder temperature means more energy is needed to make LH2 and there are some different handling procedures.
Made simply of a single proton and an electron, hydrogen is also the lightest gas in the universe. This is good and bad. The good side of this is that if hydrogen leaks it immediately goes straight up into the air at about 70 km/hr – that’s enough to clear an 8 story building in 5 seconds – and is so light that it escapes the earth’s atmosphere and leaks into space. What this means is that while flammable, in open air it is extremely difficult to build up to the concentrations needed to ignite. However, this benefit comes with a price. Hydrogen is so diffuse that storing it takes up a lot of space. In the automotive world, where Toyota, Honda, and Hyundai have retail hydrogen fuel cell vehicles, the hydrogen is stored in thick carbon fiber tanks at 70 MPa pressure. Even then, the volume of the tank is about 8-times larger than a tank of diesel containing the same amount of energy. And pressurized gas tanks do not scale well – to contain hydrogen for a vessel, a pressurized tank array would have a volume 12-15 times that of the equivalent diesel tank.
Liquefying the hydrogen reduces the volume far more than pressurization. A tank of LH2 is still about twice the volume of LNG and 4-times the volume of diesel, but much more manageable than pressurized gas. (Interestingly, hydrogen is much lighter than diesel on an energy basis, which means that even though the tank will be larger, it won’t add as much weight to the vessel as the diesel tank. Displacement-limited vessels can benefit from this characteristic.)
Today’s commercially-available fuel cells – which are needed to convert the hydrogen to electricity to power the on-board electric powertrain – are larger and heavier than their diesel engine counterparts. Throw in the electronics needed to manage the high power flows for the powertrain, and this part of the system adds even more volume.
The key takeaway here is that hydrogen fuel cell systems are larger than their diesel engine counterparts. Small, high-powered catamarans have a difficult time squeezing all of that volume into their slender hulls. Conversely, large monohulls typically have plenty of space available belowdecks. In the commercial sector vessels typically get larger as range increases, and are able to more easily accommodate the larger equipment.
Large luxury yachts are in a different class though, because their specifications often call for moderate speeds and long ranges that are similar to a commercial cargo vessel, but in a smaller package. Two conventionally-powered yachts of similar size to the 112 m Aqua, the 115 m Luna and the 107 m Andromeda can be compared to as off-the-cuff examples. The Luna has about 11 MW installed power for a 22.5 knot top speed, with a 1 million liter fuel tank allowing a stated range of 9,000 nm. The Andromeda has 4.4 MW for a 16 knot top speed with enough fuel for a 8,500 nm range. Commercial cargo vessels with these power and range numbers would typically be in the 150 m – 200 m size class, and some of that added volume could be used to accommodate the larger hydrogen fuel cell powertrain. In a yacht there is simply less space to work with, making packaging a challenge for today’s hydrogen fuel cell systems.
The overall effect of this challenge in the case of the Aqua can be seen in her resulting performance specifications: 4 MW installed power for a 17 knot top speed and a 3,750 nm range. In other words, she has a similarly-sized powerplant to the Andromeda but less than half the range. The volume issue appears to have affected more than the range, however. A look at the deck plans and capacities also reveals compromises, with fewer decks, accommodations, and excursion toys than the two comparison vessels. Indeed, the entire “lower” deck of the Aqua appears dedicated to the propulsion and hotel load system, while in conventional yachts this is able to be confined to just a portion of a single deck. Adding this all up, the specifications of the Aqua on paper are more similar to a class of vessels in the 80 m range. Indeed, it is perhaps unfair to compare the Aqua to her 110 m class cousins and instead to think of her as an 80 m yacht that has been masterfully “stretched” to accommodate the larger powerplant.
In general, the Aqua’s hydrogen fuel cell powertrain design is a great example of how this technology would be typically implemented on a vessel. She hosts a full-electric system where the power is primarily generated by 4 MW of fuel cells. There are two Voith Schneider Propellers (VSPs) powered by electric motors each rated for 550 kW, and two conventional propellers turned by electric motors, 1 MW each. This gives a total 3.1 MW of propulsive power. A diesel-electric system would not look much different, with just diesel generators producing the power instead of fuel cells.
The proton exchange membrane (PEM) fuel cells are arranged in four 1 MW blocks, and from the publicly-released 3D image of the Aqua each block appears to be about 16.8 m3. (This exactly matches the predicted volume for this amount of power in a report I co-authored from my time at Sandia, so this is either a happy coincidence or a result of that work. Either way, the Aqua designers appear to have accurately estimated the volume required for this system.) The selection of a PEM-type fuel cell, which is the kind used all over the world in vehicles, trucks, backup power systems, equipment, and more, means the designers understand that these are the lightest, quietest, most responsive, and most mature of all types of fuel cells today.
Power electronics are an important part of the system, because this equipment converts the fluctuating DC voltage output of the fuel cells into the regulated AC and DC power needed by the propulsion equipment and house loads. They are located in the stern near the fuel cells, likely intended to handle the full fuel cell output, and are shown as four blocks each about 8.4 m3. In other words, the designers assumed the power electronics are about half the size of a fuel cell on a per-MW basis. With so much variety of power electronics from different vendors today, this is a good approximation. In both cases (the fuel cell and the power electronics) the volume estimates are conservative. By the time this vessel is commissioned, it is likely that both technologies will be smaller than as shown today and this savings can either free up volume for other uses, or pack more power into the same envelope.
The cryogenic hydrogen fuel is stored in cylindrical tanks located amidships. To keep the fuel as a cold liquid, LH2 tanks have an inner tank surrounded by a vacuum space, superinsulation, and an outer tank. The inner tank, which holds the LH2, is typically only 50-70% of the overall volume of the assembly. I estimate the outer volume of the tanks on the Aqua is about 350 m3 each, and for this size tank I suppose the inner tank to be around 200 m3. This gives a rough hydrogen-carrying capacity of 14 tons per tank, or about 28 tons total. In terms of stored energy, this is equivalent to about 930 MWh. The stated range of the Aqua is 3,750 nm and the cruise speed is stated as 10-12 knots. Using very rough approximations, at 12 knots the Aqua might consume about 22 tons of fuel to reach maximum range. This would leave about 20% margin in the tanks, which is reasonable. Unlike diesel or fuel oils, contamination build-up is not an issue for LH2 tanks so while 20% is a conventional and comfortable margin a case could be made to decrease this margin in order to reduce tank size or allow a longer range.
Every major component on the Aqua’s powertrain consists of multiple redundant blocks. This not only allows flexibility in placement, it also provides inherent resiliency. If there’s an issue with any part of the fuel cell or hydrogen supply system the vessel will continue to have power to propel itself and support the house loads. The fuel cells themselves provide another layer of redundancy, because each 1 MW block is itself comprised of smaller units anywhere from 30 kW to 100 kW each depending on the manufacturer. If a single fuel cell “fails” the power output of the block is barely affected. Crew can isolate the failed unit and replace it with a backup from the ship stores in a matter of hours with no noticeable effect on operations. Back in port, the failed unit can be sent back to the manufacturer for refurbishment and returned in like-new condition to maintain replacement stock. Over time, as fuel cells naturally degrade, all units will eventually be replaced in this manner. There is never a need to take the vessel out of service for an extended overhaul or repower. This is just one of the ways maintenance is drastically reduced with the use of a fuel cell-based, solid-state power system.
A curious aspect of the Aqua’s power system design is the inclusion of a 1.5 MWh Li-ion battery pack. Battery packs are typically required with diesel electric powertrains to allow the diesel engines to operate at peak efficiency and lowest emissions. With fuel cells, the need for batteries is different. PEM fuel cells provide electrical power faster than diesel engines, so there is no need for batteries to provide power in case of demand spikes. Fuel cells are also instantly load following, which means that they only produce the power that is needed and there is no need to absorb extra power during demand changes. Being zero emission themselves, and having similar efficiency across the load range removes the need for batteries to optimize emissions or efficiency. As noted above, the multiple redundancy nature of a fuel cell powertrain also precludes the need for a battery pack for backup power, although the battery could be thought of as a second redundancy measure.
For some fuel cell vessels, batteries make sense to provide additional power for intermittent peak demands such as high-speed sprints (ferries), high-power maneuvering (tugs), or specialized on-board equipment requiring large house loads (work boats). Although Li-ion batteries are the lightest and smallest large-scale batteries on the market, their ability to hold energy pales in comparison to LH2, which is just 1/10th the size and over 100-times lighter for the same amount of energy. On-board Li-ion batteries also bring additional safety considerations because of the potential for thermal runaway. Unless needed for one of the special cases mentioned above, minimizing the amount of on-board batteries to reduce added weight and volume and to enable the use of conventional technologies (e.g. lead acid) is often desirable. I can only assume that Aqua’s designers included this large pack for either added redundancy or a special use-case of which I am unaware.
The mechanics of fueling Aqua would be based on well-established practices for handling and transferring LH2 on land, with modifications to meet maritime regulations. Because of the similarity between LNG and LH2 many of the existing regulations can be used with only minor modifications. However, the logistics of obtaining 28 tons – about 8-10 truckloads – of LH2 would be challenging outside of North America, which has nearly 90% of the world’s LH2 generation capacity today at close to 300 tons per day. Today’s total generation capacity in the EU is just 26 tons per day, Japan at about 35 tons per day, and a smattering of small plants in a few other locations around the globe. This is changing rapidly though, with multiple projects in the EU, Japan, and other places focused on increasing LH2 production and import/export terminals to support the coming global hydrogen economy. A vessel of this magnitude will single-handedly encourage this development, in some cases directly, for the benefit of the entire globe.
The Sinot design team appears to have thoroughly and accurately depicted the hydrogen fuel cell powertrain on the Aqua, and to have incorporated it in a deliberate and thoughtful way to take advantage of its benefits. The design theme of water is appropriate, for the only emission from the fuel cells is ultra-pure water. Passengers will enjoy the silent, vibration free powerplant with no possibility of smoke, diesel fumes, or diesel fuel spills whether moored offshore or cruising. Combined with the on-board design elements, passengers will feel an uninterrupted deep connection to the sea that is unprecedented.
Aqua’s eventual impact cannot be overstated. Far beyond decreasing the pollution by a single vessel, or increasing the cachet of an owner looking to showcase the latest trend, the Aqua will impact an entire industry. It will support awareness, be responsible for building infrastructure, and chart a new path that others can more easily follow. Although a magnificent vessel, the action of commissioning the Aqua will not be a self-indulgent one. The owner who does this will be responsible for enabling the elimination of all maritime pollution.