70 Dossier | NEX Power 15 kW GenSet gas micro-generator Turbo development As Mohseni explored in his PhD, the costs associated with both developing and manufacturing microturbines are a major reason why such units are not used more widely in UAVs. “Capstone Turbine [now Capstone Green Energy], for instance, invested hundreds of millions to develop their stationary power microturbines and, respectfully, they’re now struggling financially because of how much they need to recoup, which drives up their product prices, and that makes it harder to scale-up production to bring that price down to market levels,” he says. “But going into UAVs first, with a technical strategy of modularising our microturbine design, enables us to optimise for different use-cases with much lower non-recurring engineering costs.” In pursuing other means of reducing development costs, Mohseni and his colleagues also looked into COTS solutions such as automotive turbochargers for inspiration, given analogous architectural features such as their very small compressors, turbines and high-speed shafts. That led to the 3 kW microturbine design featuring a turbine with a screw, a very different compressor design with a screw, and an alternator mounted between the two that acted as both starter and generator for the system. “I later found out that other companies used similar architectures, including Ferrari in their Formula One engines of the time, and Garrett in one of their hybrid-electric turbocharger designs,” Mohseni recounts. However, the design had a couple of problems revealed through lifetime testing. For one, its shaft ran in an oillubricated bearing and, over time, the oil would leak through to the hot side of the engine. The heat would cook the oil and that would gradually impair the turbine’s rotor dynamics, reducing its lifespan significantly. “For Formula One turbochargers, that’s fine – they don’t need much runtime – but micro-CHP units need a mean runtime of 20,000 hours between failures. So, we switched to an air bearing technology, also called foil bearings, to get rid of the oil,” Mohseni says. Additionally, as lifetime testing also revealed that the alternator’s magnets would become demagnetised owing to the heat of the turbine, the alternator was moved to the front of the engine (in front of the cool compressor section) as part of the overall redesign. That redesign came in useful when the microturbine design was being conceived and rescaled for applicability in the UAV space, with the air bearing technology and frontal alternator configuration both being incorporated into what became the 15 kW GenSet. As Dr Esmail Najafi, director and product lead at NEX Power recounts to us, “Such technologies were not invented, but customised and scaled-up for this engine, with the concept of the 15 kW turbogenerator coming around 2019. “As well as the good commercial fit being a general motivator, demand for a power unit of this size came from our sister company ARC Aero Systems, which develops crewed and uncrewed aircraft – and it took about five years of engineering, with prototype tests starting in mid-2024.” Several prototype iterations were created over the course of NEX’s development (each round of modifications coming as a response to bench tests running to a failure of some sort). Two units of the presentday version are built and running on test benches to validate and characterise their capabilities, with an estimated 250 hours of test data recorded on those units at the time of writing. “On the previous iteration, we’d exceeded 500 hours of testing before deciding to modify it further; of the two modern units built, one drives its alternator via a coupling, and the other is direct-drive – the latter being the approach we’re taking commercially,” Najafi adds. “We’ve also tested the engine on hydrogen, including static tests on one of ARC’s cargo UAVs; we integrated the 15 kW turbogenerator and a bottle of hydrogen and it ran smoothly.” Operating principles The 15 kW GenSet differs notably from the other gas microturbines we have featured in this publication – specifically, UAV Turbines’ (Issue 14), Turbotech’s (Issue 31) and Aant Farm’s (Issue 51) – in that those microturbines all integrated some form of exhaust heat recuperator. August/September 2025 | Uncrewed Systems Technology The filter and alternator both integrate at the front of the engine, keeping the compressor’s air coming through the alternator, and the turbine’s heat far away
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