A comprehensive thermodynamic model reveals how hydrogen-powered, heavy-duty trucks could drive for the longest possible distances before needing to refuel.

Samuel Jarman, SciencePOD

Hydrogen is an incredibly powerful fuel: when burned in controlled conditions, it can release far more energy weight for weight than more traditional carbon-based fuels. The only by-product released is water, so it creates a minimal impact on Earth’s climate.

Because of these properties, hydrogen fuel is now fast becoming an important part of global strategies for slashing the carbon emissions of the transport sector. Before this can happen, however, researchers will need to develop better approaches to storing hydrogen safely within compact, space-efficient fuel tanks.

In a new study published in Cryogenics, Laura Stops and colleagues at the Technical University of Munich (TUM), Germany, present a new thermodynamic model for describing how hydrogen behaves when stored in the tanks of heavy-duty trucks.

Through simulations based on this model, the team shows that these vehicles can reach their longest possible driving ranges when hydrogen is stored at a combination of high pressures and ultra-cold temperatures.

The results could offer important guidance for future designs of hydrogen-powered vehicles. “By developing a model which facilitates sustainable transportation, this research is an important step towards a cleaner future,” says Stops.

Today, heavy-duty trucks and buses account for some 28% of all the CO₂ emitted by the EU’s transport sector – which itself accounts for around 26% of its total emissions. For hydrogen-powered vehicles to make a dent in these statistics, it will be crucial to maximise their driving ranges: this will involve ensuring high energy storage density, quick refuelling times, and minimal energy losses in their fuel tanks.

In their study, the TUM team addresses this challenge by developing a fully comprehensive thermodynamic model: incorporating a combination of differential and algebraic equations to account for several important aspects of a fuel tank’s everyday operation.

“In our work, we define three operating scenarios: discharge, dormancy, and refueling,” Stops explains. “The model also defines three operational modes: namely standard, minimum pressure, and maximum pressure mode.” In addition, the model’s equations can account for whether hydrogen exists in a liquid or gaseous phase, or in a combination of both phases simultaneously.

When using a model to simulate any fixed-volume hydrogen fuel tank, different equations are needed to describe each operational mode. Using this method, the team’s model could do this automatically.

"All but three equations can be used independently of the operational mode, and those three equations are switched by a smart decision logic whenever a change in the operational mode is detected,” Stops explains. “The model can then represent the tank in the single- and two-phase region, as well as simulate the transition between both.”

This enabled the team to compare a simulated fuel tank’s performance when it stores hydrogen in three different ways. The first of these was cryo-compressed hydrogen (CcH₂), which combines ultra-cold temperatures with ultra-high pressures. Here, hydrogen is stored in an ‘overcritical’ state, where the distinction between its liquid and gas phases ceases to exist.

Secondly, subcooled liquid hydrogen (sLH₂) stores hydrogen as a liquid, but without additional compression. Finally compressed gaseous hydrogen (CH₂), instead stores hydrogen as a high-pressure gas, without need for ultra-cold temperatures.

Through their simulations, “we discovered that CcH₂ allows for the highest driving ranges due to the highest storage density,” Stops describes. Based on these results, the researchers now hope that their thermodynamic model could soon lead to more feasible designs for hydrogen-powered heavy-duty trucks. “This publication is timely due to the urgent need to reduce greenhouse gas, combined with recent advancements in hydrogen technology and strong policy support for clean energy,” says Stops.