While there is no perfect fuel, says Mr Valland, the most applicable and with the least greenhouse gas emissions are methanol, ethanol, butane, LNG, LPG and propane.

The problem, is that many of these fuels have lower energy densities than heavy fuel oil and marine diesel oil, requiring more space to provide the same energy, he told London's Vessel Performance Optimisation (VPO) during a tour of his lab in Trondheim, Norway.

"It is more challenging for these fuels to bring the required energy for the operation of larger ships, and those that contribute the highest emissions," he said.

Liquid hydrogen from renewable energy is one of the cleanest fuels, but it has a very low energy density and a large storage volume.

"If you have low energy density fuel, you will probably only be able to carry enough for one or maybe two legs of your journey. This means coming into port with low fuel supply," he said.

Mr Valland went onto say that these large ships will fuel wherever the fuel is cheapest. The supplier knows these ships are looking for the cheapest fuel.

"If you change this and say that any vessel coming into port is going to have to refuel, you put him into a seller's market rather than a buyer's one. This has huge implications," he said.

"There are also consequences if ships have to refuel much more often than they do today. We already have congested harbours that will become even more congested. Fuel stations will pile up because of this.

An alternative is LNG, a fuel that contains no sulphur and emits very low levels of CO2. However, its release of unburned methane has led to widespread debate around LNG as a long-term sustainable fuel suited to achieving the IMO's 2050 target.

The release of methane, a greenhouse gas, is up to 30 times more than standard bunker and low-sulphur blends. But LNG is twice the volume compared to the same energy stored in the form of heavy fuel oil.

While alternative low-carbon fuels like hydrogen offer the potential to make shipping cleaner, emissions during their production before onboard use, should also be considered. "Today, 80 per cent of hydrogen production is from natural gas. It means there really is a lot of carbon emissions," Mr Valland said.

"The overall emissions of fuels depends on how you produce it. At the moment there are no zero greenhouse gas fuels on a well to wake basis."

LNG is produced in much higher quantity than what is currently used by shipping. In contrast, the production of methanol and hydrogen would need to increase to meet shipping's energy needs.

Some 70,000 ships of the 90,000 in the global fleet could adopt alternative propulsion solutions such as batteries, fuel cells, hydrogen or methanol fuel to reduce greenhouse gas emissions.

But the challenge, he says, is that the other 20,000 ships, 22.5 per cent of the 90,000, are the biggest emitters and are least suited to alternative fuels and propulsion systems, he said.

Shipping today consumes 250 million tons of fuel annually, with 75 per cent of fuel consumption from heavy fuel oil, 23 per cent being distillates and two per cent being liquefied natural gas (LNG) and other alternative fuels.

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