72 They did so because microturbines exhibit significant fuel inefficiency relative to reciprocating engines as their power outputs drop past 2000 hp, and their thermal inefficiency becomes pronounced at power levels below 300 hp. Recuperating exhaust heat and using it to preheat intake air reduces the fuel quantity needed to achieve target power outputs; hence, boosting fuel efficiency to levels desirable to fleet operators. Mohseni does not dispute this. “Below 30 kW, you get around 10-11% thermal efficiency – that’s compared with around 20% for a similarly scaled piston engine. The advantage of the microturbine compared with the piston engine is lower weight, and if you look at the overall mission, having a lighter engine that consumes more fuel and makes the aircraft lighter over time becomes more advantageous than a heavier engine consuming less fuel, in certain aspects.” Furthermore, NEX Power has accumulated copious experience in building and integrating heat recuperators onto its microturbines, per the company’s CHP days. However, it has also found that heat recuperators are expensive, heavy and challenging to operate. For example, when a microturbine with a heat recuperator turns off, its electric motor must keep running so as to pull cooling air through the engine, owing to the recuperator forming a giant slug of super-hot metal around the combustor and exhaust ports. “If you don’t put a load of extra time and energy into cooling that recuperator completely, the heat will spread through to the electric motor and destroy it,” Mohseni explains. “So, for customers that really need long range, we can customise and install a recuperator suited exactly to their platform, especially with advances in additive manufacturing making it easy to reduce the weight of a recuperator, and optimise its thermal and mechanical pathways. “But it’s important that customers understand just how severely adding a recuperator will change their UAV’s mission envelope and price.” Hence, the GenSet’s standard issue 15 kW design does not feature a heat recuperator; instead, the system is a simple cycle gas turbine with four main sections, and all active parts run on a single shaft, with (technically) no power turbine because, semantically, a power turbine drives a propeller. Starting at the front, air is pulled in through an air filter and an alternator by a compressor, enabling cooling for the alternator – a critical requirement given the lack of a propeller and potentially of any forward motion to otherwise forcibly air-cool the alternator – as well as providing compressed air for the combustor. That compressed air flows into the combustor chamber, where fuel is injected for mixing with the air, and conventional igniters provide ignition for gasoline. If heavy fuels, diesels or other compression ignition fuels are used, ignition aiding in the form of glow plugs are integrated and used. The ignited charge then runs into and drives the turbine that, as indicated, links to the compressor and alternator via the common shaft. “The alternator serves as both starter and generator, initially motoring the compressor to about 40,000 rpm, at which point there’s enough compressed air for the combustor to ‘turn on’ with the incoming fuel, and thus enough power coming from the combustor for the turbine to start helping rotate the shaft,” Mohseni explains. Najafi continues, “So, below 40,000 rpm, 100% of rotation is provided by the alternator via a battery, but at 60,000 rpm the turbine has picked up enough power and speed to provide 100% of the rotation and maintain August/September 2025 | Uncrewed Systems Technology The standard 15 kW GenSet does not integrate a heat recuperator, owing to their cost, weight and operating challenges; however, NEX can engineer one upon request, optimised for end-user specs Recuperating exhaust heat to preheat intake air [boosts] fuel efficiency to levels desirable to fleet operators
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