Tin-can turnaround
Aluminium is a bewilderingly useful material. While most probably know that it is used to store Pepsi and Stella Artois, it is less well known that a single one of these cans contains enough energy to fully charge ten iPhones – or, that cramming aluminium filings together with ammonium perchlorate was what once powered NASA’s space shuttle off the launchpad.
In fact, aluminium contains around 24kWh per litre – more than twice the 10kWh stored in the same volume of diesel, and many times as much as LNG (6kWh). Surprisingly, it is also competitive on a weight level as well. Though heavier than diesel and LNG, it contains more energy per kilogram than coal, as well as outperforming the new generation of proposed e-fuels, ammonia and methanol. In both measures, it also completely trounces what is possible using commercially-available lithium-ion batteries.
The problem, however, is in figuring out how to access the energy. A proposal called an aluminium-air battery, under consideration for cars, would involve using aluminium in an arrangement very similar to a fuel cell. Oxygen atoms are separated into hydroxide ions (OH-) which react with an aluminium anode to create aluminium hydroxide – Al(OH)3 – and generate power through electron flows.
Hot-swappable aluminium-air batteries could be useful in the automotive sector, but a technology based on similar principles could be even more useful in the maritime realm – where weight may not matter as much as it does on the road.
MIT-spin off Found Energy is promising to power vessels using recycled aluminium. The reaction would oxidise aluminium with water, thereby generating hydrogen as well as an output of usable heat. The hydrogen could then be fed through an onboard fuel cell to generate electricity and a further supply of heat.
Storing hydrogen this way, Found Energy claims, could be responsible for a power output of around 23MWh per m3. It is not unusual for a 14,000teu container vessel to have a fuel storage tank around 13,000m3, and halving this does not seem like such a drastic proposition. But to store a ULCV-sized portion of hydrogen fuel would be a different matter, requiring at least 100,000m3.
Another advantage being touted by Found Energy is its system would be able to power a ship using aluminium scrap. While this would be sub-optimal from a storage point of view, it would be an effective way of utilising as-yet hard to recycle aluminium, Gadi Ruschin, Found Energy co-founder, tells Marine Professional.
“We shred it and compact [the scrap] with a briquetter. It takes around 2% of the output energy to do this. Aluminum is one of the most recycled metals but there are still millions of tons which are being landfilled annually around the world. There is no good technology to separate different types of aluminum waste from unwanted or unidentified alloying elements.”
However, Ruschin claims that the process only oxidises aluminium, thereby effectively filtering out other components. “Only the aluminum is oxidized in the process, so everything else -- plastics, steel and other contaminants -- are just left and physically filtered after the reaction. This is why we can handle aluminium waste which is not so desired by the recycling industry.”
Around 75% of all aluminium ever produced is still in use today, circular-economy logic of aluminium stands to be improved even further if it ends up being used as ship fuel.
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