The FAA expects the next 20 years to transform aircraft propulsion, and hydrogen-electric aircraft will only arrive if their batteries and fuel cells are designed as one system.

In The Future of Aircraft Propulsion (8 October 2026), the FAA notes that electric motors are “usually 95% or more efficient”, but batteries store far less energy per kilogram than fuel. It calls hydrogen fuel cells the most promising hydrogen approach, while flagging the cryogenic handling, refuelling and boil-off that liquid hydrogen brings.

Why batteries and fuel cells belong in one model

A fuel cell is efficient at steady power but slow to follow sudden demand; a battery responds instantly but runs out quickly. A hydrogen-electric aircraft needs both, so sizing one without the other gives the wrong answer.

  • The fuel cell carries the cruise. It sets the hydrogen burn and the heat that has to be rejected.
  • The battery covers the peaks. Take-off, go-around and fuel-cell start-up all draw on it. The UK Aerospace Technology Institute puts the regional-aircraft need at about 50 kWh, 200 Wh/kg and more than 5C peak discharge.
  • The cooling ties them together. Both reject heat, and in liquid-hydrogen designs the fuel itself becomes the heat sink.

Two open twins, one evidence discipline

The battery twin designs a pack from the cell up: layout, mass, fast charging and a simulation of every cell, with real packs opened in 3D. The fuel-cell twin models PEM and solid-oxide cells, stacks and balance of plant, checked against Toyota Mirai production stacks and 13 public automotive stacks held out from fitting. Its latest Bayesian model predicts stacks it has never seen to 34 mV, against 71 mV for a fixed 0.7 V guess.

What coupling the twins shows

Per square centimetre is not per kilogram. Solid-oxide cells can approach PEM cells per area (about 0.5–1 W/cm² against 0.8–1.5), but today’s planar SOFC stacks deliver only 0.1–0.5 kW/kg against about 2 kW/kg for PEM.

Production hardware sets the benchmark. Toyota’s Mirai stacks give 114 kW from 56 kg (Gen1, 2.04 kW/kg) and 128 kW from 52 kg (Gen2, 2.46 kW/kg). At 600 kW gross that is six Gen1 stacks (336 kg) or five Gen2 stacks (260 kg), before any supporting equipment.

Where the heat goes decides the aircraft. Airbus’s ZEROe concept puts fuel cells and a heat exchanger in each of four 2 MW wing pods: distributed cooling, with liquid hydrogen piped along the wing. GKN Aerospace’s H2GEAR keeps all hydrogen and fuel cells in one rear-fuselage zone and uses liquid hydrogen to cool a central helium loop that cools the cables, inverters and motors: about 12 % lighter, with no hydrogen near the cabin.

Where HyFlux fits

HyFlux is building superconducting propulsion for hydrogen-electric aircraft, and the coupled twins show why cooling is the deciding question. Once liquid hydrogen is on board at 20 K, it can cool the motors, inverters and cables as well as fuel the cells. Superconducting machines take that idea further: the colder the conductor, the more power per kilogram.

Try the twins

  • Battery twin: design a pack from the cell up and open a real one in 3D.
  • Fuel-cell twin: PEM and SOFC models, the 3D system explorer and the evidence board.
  • PEM or SOFC for aviation: sizing along the H2FLY step ladder, the Mirai benchmark and a gallery of Mirai, Airbus and GKN layouts.

All model results are development results, not validated.

Sources: FAA, Toyota, Argonne National Laboratory (OSTI 1606353), Airbus 2025 Summit, FlightGlobal, Hales et al. (AIAA AVIATION 2023), Bauhaus Luftfahrt, Aerospace Technology Institute.