Polymer Electrolyte Membrane Fuel Cells for Unmanned Aerial Vehicles: Characterisation and Modelling under Varying Ambient Conditions

Promovendus/a: Jorben Mus

Promotor(en): Prof. dr. Frank Buysschaert, Prof. dr. ir. Maarten Vanierschot, Prof. dr. Veerle Vandeginste

Hydrogen (H) is the simplest and most abundant element in the universe, which explains its widespread presence, including in water and many compounds on Earth. Conversely, hydrogen gas (H2, commonly called hydrogen) is rare in nature on Earth because it is highly reactive, quickly bonding with other atoms to achieve a lower energy state. This reactivity makes hydrogen useful as a fuel, as it easily converts to a lower-energy form, releasing energy in the process. This principle is used in combustion engines, which burn fuel with air to produce mechanical power. When hydrogen is used instead of fossil fuels, the exhaust contains mainly water vapour rather than CO2, although nitrogen oxides (NOx) can still form.

As hydrogen gas does not occur naturally on Earth and must be produced, it is considered an energy carrier rather than an energy source. It can be generated by splitting water using renewable electricity (green hydrogen), but most hydrogen today is grey, coming from hydrocarbons and releasing CO2. Hydrogen is colourless and odourless and often perceived as unsafe, but it can be handled safely with appropriate measures. It also has a very high gravimetric energy density (33.3 kWh/kg), almost three times that of gasoline, making it attractive for mass-restricted applications such as aerospace. However, its low volumetric energy density means it occupies a large volume, requiring storage under high pressure or as a cryogenic liquid.

Fuel cells (FCs) generate electricity through a reaction between hydrogen and oxygen, typically from air, forming only water without combustion. The core of a fuel cell consists of a membrane sandwiched between two electrodes, where hydrogen is supplied to one side and ambient air to the other. Hydrogen splits into protons and electrons at the anode. In polymer electrolyte membrane fuel cells (PEMFCs), the protons pass through a polymer-based membrane, while the electrons travel via a different path through an external circuit, creating a direct current (DC). At the cathode, they recombine with oxygen to form water, achieving a lower state of energy. Fuel cells are highly efficient, converting roughly 40–60% of the hydrogen’s energy into electricity, compared to about 20–35% in combustion engines. Multiple cells form a stack to increase power, with supporting systems such as cooling, gas conditioning, and water removal grouped under the balance of plant (BoP). Today, fuel cells are used in some vehicles and in stationary backup power systems, but overall deployment remains limited due to high costs and lack of infrastructure.

Focusing specifically on flying drones or unmanned aerial vehicles (UAVs), hydrogen PEMFCs show significant promise because they combine high gravimetric energy density with direct electricity generation. Compared to combustion engines, electric powertrains are easier to control, quieter, and produce no direct greenhouse-gas emissions. This is why most UAVs currently rely on batteries. However, the relatively low energy density of batteries restricts endurance: adding more batteries increases weight and demands more power to stay airborne, which can even reduce flight time. Fuel cells do not have this trade-off: the high gravimetric energy density of hydrogen results in a relatively low weight penalty when taking extra hydrogen, enabling longer flight durations. However, PEMFCs have limited power density and slow response, so hybridisation is needed. UAV takeoff demands high power, and supplying this with a PEMFC alone would lead to a heavy powertrain. Combining the fuel cell with a battery allows high power during takeoff and climb, while battery recharging can be performed during cruise. While demonstrators have shown impressive endurance, with flight times up to around 90 minutes for rotary-wing drones and several hours for fixed-wing UAVs, commercial adoption remains limited. This thesis investigates the gap between optimistic projections in academic literature and the challenges faced in real-world applications. It presents a stepwise approach for integrating PEMFCs into UAVs for commercial use.

A key finding is that PEMFC systems for UAVs differ substantially from those in other sectors. To reduce mass, which is critical for UAVs, the BoP is simplified. For instance, no liquid cooling is used. Instead, a fan blows ambient air over the stack, and inlet air conditioning systems are omitted. These simplifications make the systems lighter, but may reduce performance and robustness, especially under varying weather conditions. Variations in ambient humidity and temperature, for example between colder and warmer days, can influence both performance and degradation.

In this context, a dedicated testbench with a 250 W PEMFC was built to assess performance under different ambient conditions representative of real-world operation. The measurements reveal up to 6% variation in performance, with air humidity having the greatest impact. Based on these results, a model was developed to predict fuel cell behaviour as a function of ambient conditions. This helps system designers estimate performance more accurately, refine stack design, and size the fuel cell and hybrid system, resulting in light and reliable powertrains. In this way, the thesis contributes to making hydrogen-powered UAVs more practical and predictable, and helps increase readiness for commercial deployment.