Dredging on liquid hydrogen: feasible or not?

The emissions of dredging vessels maintaining the Dutch coastline are responsible for 20% of the annual CO₂ emissions of the Dutch Ministry of Infrastructure and Waterways (Rijkswaterstaat) and for a much larger percentage if you look at NO_x and particle matter. Back in 2019, Rijkswaterstaat made a budget available to stimulate innovations from the market, with so-called Innovation Partnerships (IPS).

In such an IPS, the government co-finances R&D projects of the private sector and has the possibility to act as launching customer for these innovations to prove them in practice, without the normal rules for public contracting.

One such innovation submitted was the LEAF-Hopper, from shipbuilder Royal IHC. The LEAF-Hopper in this case is not the critter eating away at the leaves in your garden, but an acronym which stands for a trailing suction hopper dredger powered by Low Energy Adaptive Fuel (LEAF).

After exploring the particular needs of Dutch coastal maintenance, and comparing a variety of energy carriers and prime movers, Royal IHC made a concept design for a dredging vessel with the following main characteristics: shallow draft for more dumping and less rainbowing, relatively low sailing speed and pumping speed, liquid green hydrogen as energy carrier, fuel cells as energy converter and one week autonomy. This with a hopper size of 4.300 m³ hopper size and about 5 million m³/year production capacity.

Energy saving
It was obvious from the start that any Renewable Fuel of Non-Biological Origin (RFNBO) would take more space onboard and would be significantly more costly than diesel. Therefore, IHC’s designers focused on energy efficiency from the start. Up until now, dredger design maximised the production capacity rather than minimising the energy use, but as the energy costs become a larger percentage of the total cost of ownership, low energy consumption is a must. IHC achieved this by creating a wider vessel with a shallower draught, and a design with recessed bottom doors. With a fully loaded draught of only 5.5 m, the vessel can use the bottom doors much more often in foreshore replenishment, which is quicker and more energy-efficient than rainbowing. Another design choice was to use a submersed dredge pump. This allows for more dense sand/water mixtures than an inboard pump, and hence less water needs to be pumped around to transport the sand.

Hydrogen
When the project was started in 2019, one of the requirements was that the technology had to be available on the market for a start of construction in 2024. As neither dual-fuel methanol or ammonia engines were available at that time, the choice for hydrogen was an obvious one. Later on in the project, this choice was recalibrated against new technologies but stood the test. A life-cycle analysis was carried out comparing various energy converters and energy carriers, and the choice was made to go for a combination of green, liquid hydrogen in combination with fuel cells.

Fuel cells
It’s often overlooked in comparisons, where the internal combustion engine is considered a given, but fuel cells have some very attractive properties for energy conversion. A fuel cell is basically the opposite of an electrolyser. While an electrolyser uses electricity to transform water into hydrogen and oxygen, a fuel cell transforms hydrogen into electrical energy, heat and water.

Bruno Bouckaert, Technical Manager at Rijkswaterstaat: “Before I joined the project in early 2024, I was never a big fan of hydrogen. Like many, I considered hydrogen to be the eternal “technology of the future”. That is because hydrogen is often compared to battery technology or even direct electrical power. I am convinced that hydrogen will never win the battle from batteries for cars (and perhaps even trucks and buses). Neither will it win from electric heat pumps for heating of houses. The windturbine-to-work efficiency is so much higher if you don’t have to convert the electricity to molecules and back to electricity but keep it as electrons. Yet those are the hydrogen applications we are most familiar with. When batteries are not a feasible option, such as on a coastal dredger with 24/7 operations and a high-power demand, the equation becomes different.“

Fuel of the future
It’s now clear that all long-range ships in the EU will mostly run on some form of hydrogen by 2050. The question is rather how that hydrogen will be brought onboard: either in its pure form as compressed or liquid hydrogen or bonded in other molecules such as e-methanol, e-diesel, e-LNG or e-Ammonia. This choice will depend on each ship’s characteristics: range, power requirement, fuel cost, whether the design is weight-driven or volume-driven, availability of the fuel, etc. It’s clear that there will not be a single synthetic “fuel of the future”. There is not a single fuel of the present (HFO, LSFO, VLSFO, MGO, LNG, HVO, etc.), and we shouldn’t assume that it will be different when we switch to synthetic fuels. In general, we can say that the easier the molecule is to implement – take for example e-diesel which is a drop-in fuel – the harder and more energy-intensive it is to produce this molecule. This leads to higher fuel costs for lower returns. So it depends of the balance between capital costs (the ship) and fuel costs which fuel may be more suitable.

What about e-methanol?
What we know now, is that fuel costs will be a larger part of overall operating costs of dredging vessels in the future. The project team also looked at e-methanol. Dredging contractors seem to prefer e-methanol, as it can be bunkered like diesel (at normal temperatures and pressure), requires a relatively modest upfront investment and a dual-fuel motor always has (bio)diesel as an alternative, when RFNBO’s are not demanded by the client.

E-methanol is a synthetic fuel, in which hydrogen is bonded to CO₂. This CO₂ is emitted by the engines onboard, but that’s not a problem if the CO₂ is short-cyclic and therefore does not come from a fossil source. It can either be captured from biomass or directly from the air (DAC). Currently e-methanol is only scarcely available, but it will be widely available in a few years, and it’s certainly a valid option, which will be used on dredging vessels.

Predictions of fuel costs in the future are always based on the cost of renewable energy, which is the main cost driver for RFNBO’s. Research institutes estimate that e-methanol will have a production cost per GJ which is 40-80% higher than liquid green hydrogen in the period 2030-2050. E-methanol is made of hydrogen and its cost is closely related to the cost of green hydrogen, onto which the cost of the captured CO₂ and the synthesis to methanol has to be added. The availability and the cost of captured CO₂ are the biggest unknowns.

Apart from the higher cost per unit of energy, also more energy is needed when using e-methanol. Fuel cells can achieve an efficiency of about 50%, a number which can’t be reached with dual-fuel engines on methanol. So the choice between hydrogen and e-methanol is one between the more expensive ship (hydrogen) or the more expensive fuel (e-methanol), along with a number of other considerations, such as required autonomy and availability of the fuel.

And ammonia?
E-Ammonia is currently not at a Technological Readiness Level for application on a dredging vessel. Safety concerns about toxic clouds will likely limit the application of e-ammonia to the largest seagoing ships. For dredging vessels which frequently operate near densely populated areas, it’s not likely to become the fuel of choice in the short term.

Technical feasibility
Is it technically and economically feasible to dredge on liquid green hydrogen?

The Innovation Partnership provided the budget to find out. Royal IHC completed the basic engineering of the vessel, in which all principal design choices are made, leading to an integrated design, approved by class (Bureau Veritas). To dredge about 6 Million cubic metres of sand per year – in a mix of fairway maintenance and coastal dredging – the H₂-Hopper will have to bunker about 20 tons of liquid hydrogen, once per week. Liquid green hydrogen is available on the market right now, and production capacity will increase in the coming years.

To obtain maximum efficiency, the fuel cells feed into a battery bank at a constant pace. Sudden shifts in load – a characteristic of hopper dredgers – are taken up by a super capacitor, which lengthens the battery life by smoothing out the power draw. The whole system is governed by an Energy Management System, which – contrary to a more common Power Management System – looks into the future to ensure that enough energy is available at any moment during the dredging cycle. The power for the main consumers is distributed onboard through a 1kV DC network. The liquid hydrogen is stored in two double-walled vacuum-insulated tanks on deck.

Cost
But certainly, it’s a lot more expensive than dredging on diesel? That, unfortunately, is indeed the case. Not only is the liquid green hydrogen significantly more expensive than diesel per GJ of energy, but the capital costs for the vessel are far superior, due in large part to the liquid hydrogen tanks. Then there’s also the fact that the frugal H₂-Hopper has a lower production capacity than a similar diesel-powered vessel, which means operational costs and capital costs are spread over less cubic meters of production. Averaged out over 30 years, dredging on liquid hydrogen with the H₂-Hopper is expected to be about 60% more expensive per cubic metre than dredging on diesel on a representative (newbuild) reference vessel. This includes capital costs, maintenance and fuel costs and is based on a distribution of production over fairway maintenance (50%), foreshore replenishment (40%) and beach nourishment (10%).

This extra production cost however, is offset by lower environmental costs. The Environmental Cost Index is a shadow cost which is added to the contracting cost, when comparing bids. Taking into account a multiplier of six, which is now typically used in public tenders for frontrunner projects, the difference in environmental costs will bridge the cap between the costs of dredging on diesel or on green hydrogen. This means that a contractor will not only win that contract with a H₂-Hopper, but he’ll do so with a 60% bigger turnover. Other ways to bridge the cost gap are a subsidy on the initial investment, and international regulations such as ETS and Fuel EU Maritime. It’s already known that dredging vessels above 5000 GT will be subject to ETS from 2027. For smaller vessels and particularly Fuel EU Maritime, there is still uncertainty.

Cryogenic
How about bunkering liquid hydrogen? At minus 253 degrees Celsius (yes, that’s only 20 Kelvin above absolute zero), and with a risk of explosions, precautions are necessary. In terms of bunkering, liquid hydrogen is very similar to bunkering liquid natural gas (LNG). At first it will be with trucks, but as demand increases, bunker barges with liquid hydrogen will become available, reducing the supply cost.

What’s next?
A major hesitation for dredging contractors to invest in a H₂-Hopper is that the vessel may be very well suited and optimised for Dutch coastal works, but big contractors tend to use their vessels worldwide, and there’s currently no market for such a vessel elsewhere. This is a valid argument, but at the same time, one must recognise that there are many hopper dredgers designed, built and operating in dedicated areas, to maintain a specific river, canal or harbour. What if optimisation and specialisation are necessary for the energy transition to succeed? Maybe a dedicated Dutch vessel in the short term will spread its wings as EU regulations and later IMO regulations shift the scales in favor of RFNBO’s.

Rijkswaterstaat acknowledged the concerns but is now confronted with the fact that there are still no means to dredge at large scale in a more climate-neutral way or in a locally zero-emission mode. Other ways are being explored to accelerate the transition, and to not let the dredging industry lag behind the energy, industry, construction and mobility sectors. After all, The Netherlands has always been – and should stay – the Silicon Valley of dredging technology.   

Contact

Royal IHC
Hans Hesen – [email protected]

Rijkswaterstaat (IKZ)
Rutger Rebel – [email protected]

Note: The opinions, beliefs, and viewpoints expressed in this article do not necessarily reflect the opinions of Offshore-Energy.biz