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Second installment of BAM’s Motor Boats dossier to try to shed some light: what does sustainable boating mean? After an initial introductory article, let’s get to the heart of the battle: how to “store” electricity? How to recharge when at the dock?
On the one hand, there are “traditional” batteries, lead-acid or gel batteries. On the other, lithium ones (in turn distinguished into between iron-lithium or lithium-ion ones). The former find decidedly little use on electric and hybrid boats, and are somewhat more common for powering small sailboats with low power ratings, up to about 1.5 kW. They are still used because of their lower costs and especially because they are less problematic from the perspective of digestion systems, have simpler controllers, and do not need exaggerated attention to installation. Now, however, lithium ones are used almost exclusively. With a view to more sustainable boating they have strong advantages, but also with some problems.
The advantage in using lithium-ion batteries is not only in their characteristic higher energy density (in a nutshell, the ratio of the amount of energy stored to the volume of the battery) but especially in their speed in charging or discharging, thus in recovering and supplying energy to the system. Traditional batteries can charge at “1C“or slightly more, where C indicates the ratio of battery capacity to one hour of use. So if you have a 1kWh battery to charge it you can power it with a maximum power of 1kW.In discharge on you can get as low as 2 or even 3C, depending on the models.
In lithium, we are getting, in this respect, to very high levels: we get as much as 10 C charging. The advantage is obvious: with a conventional battery to recharge an 80 kWh battery (reference power for many full-electric powerboat models that are seeing the light of day right now) would take more than 80 hours. Making a four-wheel comparison, and all electric boating technology comes from there, to recharge a Polestar, precisely 80 kWh, with a 150kW supercharger takes it from 20 to 80 percent charge in less than 20 minutes. This is a charge at just over 3 C. But as we know, and we will discuss this in more detail later, superchargers in the marine world are very rare. It should be noted that then to get to the final 100 percent and stabilization of the cells the final part of the process becomes significantly more complex. Very quickly, however, you still have a ready-to-use battery.
In this regard, it should be pointed out that lithium batteries can be air-cooled or water-cooled. In the first case they have cooling fins and are allocated in rooms that are forcibly ventilated at the inlet and outlet, in the second from case, used when you need large charge and discharge rates on board (typically very powerful motors working with small capacity battery packs), there is a dedicated cooling circuit that often also goes to cool the controllers. In any case, the problem of overheating, especially in charging, should definitely be kept under control. Manufacturers now make batteries that are watertight, which are created to avert the risk of inadvertent self-combustion triggers. But still it exists.
Premise, as we said before, all the know-how comes from the car world, which has boundless funds at its disposal compared to the boating world, and safety levels and construction standards have become very high. The risk of fire comes when either directly (e.g., due to a violent mechanical impact with a spike) or indirectly (due to overheating due to mishandling by the controller during charging or discharging) anodes and cathodes come into contact. In addition, automobile manufacturers, and consequently also marine manufacturers, with their suppliers have since gone down the road of cell separation, a solution created to prevent any damage from causing more serious damage.
There is a widespread belief that it is impossible to put out lithium battery fires–the reality is. Traditional methods of putting out fires do not work because lithium batteries contain both fuel and oxidizer, so removing oxygen as is normally done to extinguish traditional fires is useless. When a lithium battery ignites, either you can lower its temperature enough to stop the chemical reaction (but that is difficult) or you have to wait for the combustible and comburent material to be completely consumed.
Hence the idea of separating them into many cells: an 80 kWh battery goes on burning for a day, but if divided into 100 properly thermally insulated cells obviously the time is reduced and the fire cannot spread. As we were saying now on cars, safety levels have become very high, and it is no coincidence that marine manufacturers are teaming up with automotive manufacturers to adopt similar solutions: Candela-Polestar, or Torqeedo and Tyde with BMW, are just a few examples.
Unlike regular internal combustion engines, with their diesel fuel tanks, electric propulsion has an amount of energy to carry that is more “bulky.” Batteries weigh a lot and take up a lot of space. This is obviously an issue, from a sustainable boating perspective, central to the sailing world. The way forward now is research toward batteries of different types, some interesting developments seem to have solutions involving nanotechnology, such as graphene batteries.
They are still in the prototype state, but they promise much higher energy density, the ability to break free from dependence on rare earths, and allowing far less environmentally impactful end-of-life disposal. So much smaller weights and footprint. As is easy to imagine graphene batteries are also very expensive, but even here great strides are being made with early applications on cars, for example in the Aion range for Chinese GAC electric cars. But even for smartphones there are the first mass-produced applications.
Also very interesting are the new salt batteries. These, while they do not have a much lower energy density than lithium ones have the valuable distinction of not having to use rare earths in their construction. So less environmental impact both at the source and at the end of life when it comes to recycling.
Given that before Graphene and Salt batteries catch on, and it will take quite a while, it is worth noting that as far as lithium batteries are concerned, the path to efficient disposal is making great strides. It is still definitely expensive now, but some estimates say that within about three to five years it will be cheaper to produce new batteries using recycled material from spent batteries than to go digging for rare raw materials. Fundamental step to becoming economically sustainable as well.
Of course, the ports already have charging stations. Along the highways, no. Everything easy? Not really. In fact, the reference standards are always those of cars. All electric cars can also be recharged with a three-pronged household plug; power management systems can figure out what kind of power is being supplied to them. But in one night you recharge 10% of an 80 kWh battery.
Supechargers reduce charging times, but there are very few of them along the marina network. They also work on direct current while in the nautical world alternating current is used for certification reasons and because along the docks it is not supplied except in very rare cases.
In alternating current, at “normal” boat columns you can have alternating current at 220 rarely a or 380 volts, the possibility of recharging more or less quickly much depends on the choices, and the possibilities of individual ports and marinas. In many places the maximum charging limit is at 2 kW, although the standard is definitely rising toward 3.5. But even then, charging times remain very long.
The situation is very different in super yacht moorings, here you have higher powers, you “work” at 380 volts and powers as high as 50/60 kW. In this case if you have an 80 kWh battery on board (again using the latest generation of electric boats as a reference) in less than an hour and a half you go from 20 to 80 percent charge. But obviously it is not very convenient to moor in place of a 40-meter, nor is it easy for ports to convert to more efficient systems. In short, with a full electric planing boat, you either have a supercharger nearby for your commute, or you have to factor in very, very long charging times.
Fortunately, there are, or are coming, other solutions for replenishing electrons. For smaller motors, e.g., dinghies or sailboats, the batteries are either removable (and thus “refillable” at home) or they are AC rechargeable, so even sa board. Virtuous examples come from Mercury and Torqeedo. But also very interesting, for larger boats, are developments on increasingly efficient solar panels, from state-of-the-art range extenders (small gensets) or even from hydrogen. But that will be discussed in another installment.
by Luca Sordelli
Technical advice Paolo Portinari and Mario Bonelli
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