Powering ahead: The big alternatives
It has taken a while, but cruise owners and operators are discovering their own reasons for investigating new power technologies, writes Stevie Knight.
Cruise vessels are big, but so are the drivers adding a push to the sector's alternative energy strategies. Which is why several current projects are examining the potential for both fuel cell and battery technology to help fulfil massive power requirements on modern cruiseships.
Fuel cells promise much: cleaner running than anything diesel engines can accomplish, and potentially even more efficiency. Not only could they soon be scaled for cruise ship installations, they even yield much needed flexibility.
Firstly, the efficiency is a matter of physics: fuel cells rely on a straightforward reaction rather than the ‘explosive’ mechanical conversion of a combustion engine. So there is, in principle, much more to be gained from fuel cell development than from trying to milk a few extra percent from diesel engines.
It has to be admitted the technology hasn’t hit the winning formula yet – but there’s industry recognition that it’s now getting close: for example, Royal Caribbean has said it’s looking at fuel cells for its new Icon class due out in 2022. More, fuel cells might be installed on passenger vessels earlier than you’d think: the company says it will begin testing the technology on an existing Oasis-class ship this year and will be running progressively larger fuel cell projects on Quantum class vessels being built in the next several years.
Of course there are “pros and cons” when it comes to applying the technology to cruise vessels, explains Joe Pratt of the US Sandia National Laboratory, and at the moment, Proton Exchange Membrane (PEM) cells are looking a better bet for immediate uptake than the much hotter solid oxide (SOFC) or molten carbonate (MOFC) cells because they are simply easier to deal with than stacks running at several hundred degrees.
Top of the ‘pro’ list is that fuel cells can be scaled up or down without any change in efficiency. “Many smaller units will give roughly the same efficiency as fewer large units,” he explains. It’s a very different scenario to traditional combustion engines which display markedly poorer efficiency at the bottom end of the scale.
This goes some way to mitigating one of the big ‘cons’: space. Cruise ships have a huge power draw in the multi-megawatt range but are tightly packed; crossing the tech over from commercial power plants hasn’t been easy partly because landside generators don’t usually have such a constrained footprint.
However, Dr Pratt explains since individual fuel cells have been packed down to “the size of a small suitcase… you could stash them all around the voids of a cruise ship - they don’t all have to sit together in one engine room”. And, as Gerhard Untiedt of Meyer Werft points out, “no exhaust gas means it’s not dependent on a funnel system”.
He adds footprint has been a focus: “We now have 5kW fuel cells in a box about 19 inches across... and the next generation coming in will be the same size, but 15kW or 20kW. Volume for volume we will output about the same as a medium-speed engine - although matching a high-speed engine will be a challenge.”
More, there’s a greater overall reliability: “If one small unit fails, the others keep working with little or no actual impact on operation: the failed unit can be replaced while the ship is still in service and even underway,” says Dr Pratt. It’s an obvious advantage when it comes to cruise vessels.
There are still a few snags. One is price: fuel cells look like remaining more expensive than combustion engines for the foreseeable future. However, this isn’t a show stopper according to Untiedt, if the design decentralises the power. Position the cells so they can feed into substations along the ship, and the layout can be configured for a lower energy density which makes for “a simpler grid, at less cost” he explains.
Interest in fuel cells is also being propelled by excitement about the development trajectory. In fact back in 2009, fuel cells performed just a little better than diesel engines at very low loads (up to around 25%), although they lost their advantage quite quickly at higher loading. As Untiedt explains, this has changed remarkably in the last seven years: while most fuel cells still lose out approaching diesel’s sweet spot at over the 70% load point, it's been predicted they could outperform diesel engines all the way across the range by 2022.
But the details are important and it’s worth noting PEM cells also come in two distinct flavours: low and high-temperature (though 'high' is relative and nothing like the heat generated by SOFC or MOFC cells). The former (which reach a moderate 80˚C) have shown themselves to be particularly attractive to the automotive industry, partly because they can pick up a load in milliseconds, much faster than a diesel engine. It’s this variety that’s being installed on the Aranda Arctic research vessel, a project under development by the MARANDA consortium.
The choice, says Per Ekdunge of project leader PowerCell, was partly governed “by the maturity of the technology”: these PEM cells have benefited from the commercial focus on reliability and are good value for money.
As Ekdunge explains, Aranda will use two fuel cell stacks to provide the energy for the analysis equipment and other hotel loads, giving it silent, vibration and emission free working. Most importantly although it’s only 150kW in size “it will have an 18 month trial in the marine environment, returning some real figures”.
It’s a smart, simple solution - spills are biodegradable in the marine environment - and it has an amazing 50% energy conversion rate which puts even the best diesel engines in the shade – but while all these virtues make low-temperature fuel cells appealing particularly to cruise vessels, they have one or two quirks.
Firstly, while they can ramp up or down fast, Ekdunge explains that just like many living organisms they need oxygen “so the limiting factor is how fast you can supply them with a clean, salt-free air flow”. Secondly, they are sensitive beasts. The stack requires a pure feed as these fuel cells can be poisoned by carbon monoxide or other impurities. Therefore Aranda’s being run from compressed hydrogen tanks, and unfortunately, when you scale it up these are probably too large to feature in a larger, commercial cruise vessel design.
But don’t write low-temperature PEM cells out of the cruise picture just yet. The coming availability of liquid - not just compressed - hydrogen may make a difference. Although both the energy density and the temperature are lower than liquefied natural gas, they are “still in the same order of magnitude”. Further, Ekdunge points out that the basics of cryogenic design have been broached by LNG installations... so pure hydrogen feed fuel cells may yet find a foothold in the market.
On the other hand, the alternative could be high temperature (HT) PEM cells. These run at around 160 degrees C and aren’t as quick to respond, taking five or so minutes to ramp up the power. But they have one major advantage: they are more forgiving of impurities than their lower temperature cousins. More, the heat demand on a cruise ship is high, so recovering the thermal energy by using direct, liquid cooling of these HT PEMS will play an important part.
Therefore, this is where the Pa-X-ell project, led by Meyer Werft is focusing: it already has a 90kW HT PEM demonstrator planted on Viking Line´s cruise ferry MS Mariella which has been in operation since last summer. At the heart of it is an environmentally friendly, simple but effective fuel: methanol. This hits the spot for two reasons: it’s liquid at room temperature and non-toxic if spilt into the marine environment.
Part of the energy is fed into an internal reformer to convert the methanol into hydrogen, a fairly simple business. It looks like being a winner: the reforming process itself absorbs just a few percent of the power and the more relaxed nature of these HT PEM cells allows them to cope with the small stream of carbon monoxide by-product.
However, both Gerhard Untiedt and Per Ekdunge agree that fuel flexibility will play a much greater part in future of vessel designs: “I see a big fuel mix coming,” predicts Untiedt.
It’s also worth noting that it’s also possible to run hydrocarbons through an HT-PEM cell if you use an external reformer, therefore a ship’s existing LNG engine tanks could be used to feed the onboard fuel cells. While less ‘green’, there are certainly benefits to this idea, not least of which is being embraced by the IGF code rules already carved out for LNG-fuelled ships.
Further, although presently limited to landside generation, PowerCell Sweden has pulled off a nice trick in that it’s able to run a stack from hydrocarbon fuels, normally anathema to low temperature PEM cells. It can even take diesel, extract the gas via a heat and catalytic Auto Thermal Reactor (ATR) and mop up the sulphur and carbon monoxide afterward. This leaves a flow pure enough for the sensitive PEM cells although Prof Ekdunge admits “you do lose about 15% of the power to the reformer and cleaning process”.
However, Joe Pratt makes the point: “If you are using a hydrocarbon, then one way or another, you are going to be splitting off the carbon and dumping it into the atmosphere.” But, he adds, “The overall worth is in the detail, how much pollution, and how much more efficient is it than running LNG or diesel through a combustion engine?”
The ‘clean energy’ quest is something that’s close to the heart of both Per Ekdunge and Gerhard Untiedt who adds that “methanol and all sorts of alcohols” can be produced from a number of different streams, including by using the electricity generated by solar and wind energy. This last is particularly interesting he says “because although you are releasing a couple of carbon atoms from the methanol, it’s the same amount as you are picking out of the air to create it in the first place – so it is, in effect, carbon neutral”.
“There are a lot of low hanging fruit for fuel-saving and environmental friendliness that we have not yet reached, so there’s still room to be clever,” says John Roger Nesje of Rolls-Royce.
Take the latest Hurtigruten expedition cruise ships which demonstrate a tour-de-force of nice, solidly proven technology. The vessels, Roald Amundsen and Fridtjof Nansen, will each have 2.4MW of installed power, supplied by four of the updated Bergen B33:45 gensets while the main azipull thrusters will be using Rolls-Royce’s permanent magnet technology. Perhaps more surprisingly, both of them are hybrids.
Batteries have found a way into the high-tech spec of other smaller vessel types, but cruise, so far, has been left out of the picture. This is partly down to the sheer scale of the ships, and partly because the propulsion chunters on at a steady rate and doesn’t really suffer the kind of peaks and troughs that recommends itself for hybrid configuration.
However, there are a couple of drivers that are starting to push batteries into the limelight, even if the installation is limited to providing a small portion of the ship’s total energy draw. Most obviously, cruise operators are susceptible to the stains that air quality issues leave on public perception and would like to slip into harbour with a clean stack. Further, cruise ships typically want to head for ‘unspoiled’ regions, so the advantage of silent, emission-free running is highly prized adds Nesje. These can be reefs, fjords, inland waterways or wildlife territories – and of course the polar zones have lately been put on the agenda for ‘explorer experiences’.
But perhaps most surprisingly, peak shaving can be added to the list. In fact, while both Hurtigruten vessels are eventually aiming at emission-free propulsion through sensitive areas, the Roald Amundsen (due to hit the water in 2018) will initially be using a smaller battery array for peak shaving alone.
As Nesje explains, although there isn’t usually a sharp spike in demand from large winches or cranes such as you might see on a commercial boat “you still find you have wide fluctuations in the hotel loads”.
While these variations may not be fast, they can still be deep. “A cruise ship has a number of big pumps onboard to deal with the sanitation and galleys; there’s also the air-conditioning, night time heating, and even things like warming the swimming pool,” he says.
There’s also a peak in demand when approaching the berth, at this point a number of systems suddenly spring into life, including those large bow thrusters. It’s therefore sensible “to keep a certain margin” around the power draw which can be nicely managed by a battery installation, “even avoiding starting up another engine”, says Nesje.
For eco-sensitive cruise ships there are additional benefits. “Ramping up and ramping down... that’s the time when it’s harder to control emissions,” he explains. This applies to relatively clean-running LNG engines which can be prone to methane slip when faced with large load steps. Therefore, he says it makes sense to support them with the batteries “for just a few minutes” till the engines hit their stride.
Probably the battery array’s most valuable role is that of a marriage broker, tying the knot around several different green energy power sources. “Solar, wind, fuel cells – all these produce electricity,” he points out. Installing a battery array can absorb the different energy spikes, either in supply or demand: in fact Nesje goes as far as saying “once you have batteries onboard, you have no particular challenge with integration”.
So, despite issues around size, weight and overall energy density compared with traditional fuel, John Roger Nesje is confident batteries will find “more and more of a place onboard cruise ships”.
“There is currently no limit on size,” says Grant Brown of PBES. “We have engineered systems large enough to power small cities.” He adds: “In my estimation, the companies generally like to start small to test the water, so to speak. They then realize the potential and usability and scale up on subsequent systems.”
But what about sizing the batteries? It’s still a tricky business, even for a 3,000 gt ferry and when it comes to a cruise vessel installation five times as large, the whole issue becomes a lot bigger. While Brown adds that dealing with size and complexity is “just math”, there’s a caveat. “As long as our applications engineers get correct data about all loads and when they need to be served, they can make sure the battery meets the mission.”
Hurtigruten, as noted, is taking on the challenge in bite-size chunks. Roald Amundsen is being installed with a smaller set of batteries, while the second vessel – Fridtjof Nansen – is to have a larger bank scaled for periods of fully electric sailing. Only after the latter has been operating for a while will the first vessel be recalled for an update.
Although Hurtigruten hasn't been explicit about their reasoning, this approach could be a way to mitigate the difficulties in hitting battery installations on the nail. Further, the operator may well be hoping the extra year or two will mature a couple of significant trends: rising power, dropping prices and the blossoming of further innovation.
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