Truck with ammonia fuel cell – more efficient than with hydrogen?

Liquid ammonia has three times the energy density by volume of compressed hydrogen. A relatively moderate pressure of 9 bar is sufficient for liquefaction at room temperature. Ammonia (NH₃) at first sight as fuel and energy source.

The US start-up amogy has now presented an articulated lorry with an ammonia fuel cell. According to the provider, the vehicle has an output of 300 kW and can bunker 900 kWh of energy in eight minutes – about as much as the batteries of the Tesla Semi. “Ammonia is the optimal fuel to enable rapid decarbonization of heavy transport because the necessary infrastructure is already in place around the world,” says Amogy in a press release.

Big Leverage with Ammonia in the Energy Transition?

In fact, ammonia is an often overlooked building block of the energy transition. It is already being produced in huge quantities worldwide, mainly for the production of artificial fertilizer. However, this requires hydrogen, and so far it has come almost exclusively from natural gas. “Green” ammonia produced from green electricity would therefore have great leverage here in replacing fossil fuels.

But does it make sense to use such clean ammonia in trucks? The advantage over batteries would be a longer range and faster refueling. The main advantage over hydrogen would be simpler handling. The tanks can be smaller and lighter, for example, because they don’t have to withstand the high pressures required for hydrogen (700 bar).

However, there are also significant disadvantages. Amogy does not give any details about the fuel cell used, but it is probably a polymer membrane cell (PEM). This cannot convert ammonia directly into electricity, but only pure hydrogen. So the truck still needs a “cracker” on board to break down the ammonia. This makes the entire powertrain more complex and less efficient than a pure hydrogen fuel cell. The online portal NewAtlas calculated – under very benevolent assumptions – that only around half of the stored energy reaches the wheels due to the conversion losses in crackers, cleaning and fuel cells.

That puts the increased energy density into perspective. And that doesn’t even include the energy to make the ammonia. The overall energy efficiency should therefore be even lower than with a hydrogen system that is already not particularly efficient (and lower than with battery-electric trucks anyway). Amogy itself does not provide any information on consumption and range.

Ammonia better for shipping

Still, it’s good news that companies like Amogy are continuing to develop ammonia propulsion systems. Because there are certainly many – more or less large – niches for them, although not necessarily in trucks. Rather in aviation and shipping. Actually has Amogy an ammonia tractor for this year announced with a megawatt output. By 2025, the drive is to be scaled up to ten megawatts. This brings you into the area of ​​container ships. Other manufacturers are also working on maritime ammonia drives.

Such ships would be particularly useful for a specific type of cargo: ammonia. Then they could tap into their own cargo and would not need a separate fuel bunker. In any case, the demand for transport is likely to increase sharply. Because in order to import the difficult-to-transport hydrogen from distant countries, several carriers are under discussion. Ammonia is one of them.

Here we have compared the strengths and weaknesses of the individual carriers: Ammonia scores above all in terms of energy density and costs, is in the midfield when it comes to conversion losses, but weakens when it comes to the purity of the hydrogen recovered and safety – it’s a question finally a flammable poison gas.

In the calculation, however, we assumed that Ammonia only as an energy carrier is used, so the hydrogen is recovered at the destination. However, this is not absolutely necessary, because there are enough possible uses for the ammonia itself. If you imported eco-ammonia and used it to replace the conventionally produced ammonia – for example in fertilizer factories – you could save a lot of natural gas or use it elsewhere. And compared to the other modes of transport, the disadvantages of ammonia in terms of efficiency and purity are no longer significant.

(grh)