38 a curve of power to slowly replenish energy from one side of batteries to the other, increasing the lifespan of all the batteries,” Tyler notes. An automated system manages the charging and discharging of the batteries – AALTO refers to this as its power system, rather than a battery management system. That includes the balancing of cells for currents, voltages and temperature, monitoring individual cells and their strings for health problems, and isolating cells and strings to pre-empt thermal runaways if necessary. Most of the subsystems are powered by the main power bus, which can be supplied by the solar array or the batteries. The exceptions are the motors and ESCs, which are powered directly by the batteries at all times to ensure stable and consistent propulsion throughout flight. Briggs notes, “We don’t get a lot of transients, thanks to the slow-changing nature of daylight and our onboard batteries smoothing out the power coming from the cells. The batteries are most important for moments like when we turn and face the sun, because otherwise we could suddenly get a big voltage spike from the array.” Tyler adds, “Some careful programming and system architecture went into the ‘unlatching’ process by which we release the main bus from the batteries and attach it to the solar array, changing the flow of power. We’re able to see the sun through the clouds and the Earth’s curvature, so our ‘sunrise’ happens up to an hour before its expected time at ground level, giving the automated power system plenty of time to unlatch and start absorbing sunlight as soon as it’s available.” Batteries The batteries used in the Z8 are Amprius’ lithium-ion cells, based on the company’s proprietary silicon nanowire anode technology, to which it attributes its 400 Wh/kg specific energies. However, AALTO has announced plans to switch to the company’s 500 Wh/kg packs that were unveiled last year. “We bond the cells using rubber bands, so as they go through their thermal profiles from day to night, they can expand and contract,” Tyler says. “Compressing them that way is a key safety measure. Technically it also keeps them within a reasonable volume, but mass is far more of a concern for us than volume, which is why the high specific energy of the Amprius cells is important to us.” The cells are series-connected in strings, which are then connected in parallel, arranged as needed for each planned flight’s duration and the time of year. “During the 64-day flight for instance, we had 60 strings spread across 10 packs,” Tyler explains. “We can go anywhere from eight to 12 packs, and we could have done that flight with only eight, but 10 gave us a bit more capacity and hence redundancy. If we’d had a sudden degradation in one pack, we would’ve been fine.” Twelve packs means the highest weight penalty, so it’s a configuration reserved for missions with long nighttime missions, such as providing 5G in Europe in winter. Naturally, capturing more sunlight for longer night flights will also require bigger solar panels. Motor drives The electric motors resemble COTS products, but the rotors and stators are optimised for the Zephyr’s use case. Like the avionics, the ESCs are designed and assembled in-house. “Consider that the original Zephyr designs date back to the early 2000s, to a time when the industry around electric motors for UAVs was nowhere near as mature as it is now, so you’ll understand why we’ve made our own ESCs and e-motors,” Briggs says. “Even now though, it’s hard to imagine new suppliers who could make motors with the performance, efficiency and thermal stability we need to fly 200 days non-stop. The market for HAPS and other stratospheric platforms is not big enough for motor manufacturers to design and produce them in bulk.” Tyler adds however, “At peak climb, the motors produce just under 1 kW, and at cruise they’re normally at half that – not much power for a 75 kg aircraft. In the stratosphere, It’s nice and cold for the e-motors, especially since they’re not getting anywhere near the solar exposure of the wings, and there’s no moisture or dust ingress. That makes engineering them a bit easier.” December/January 2024 | Uncrewed Systems Technology The propellers and motors are optimised for efficient spinning in stratospheric air densities, running at around 2000 rpm at cruising height and 500-600 rpm near the ground
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