Ballonets are typically filled with air, not helium, to compensate for the expansion/contraction of helium as the ship climbs/descends, and to keep the ship trim fore/aft.
A fan(s) is used to maintain constant pressure within the envelope plus or minus a couple of psi difference with ambient pressure. This maintains the ships aerodynamic shape. Ordinarily, the large volume of the ship contains the helium.
I don't know if anyone has experimented with Hydrogen as the primary lifting gas, and then used helium as a trimming medium along a fore/aft containment system. Would, IMO, still require a free-air ballonet to obviate any complex over-pressure design requirements.
Since the ship is solar, I don't know what design innovations may have been applied to reduce or eliminate the need for electric ballonet fans. From what I could see, I do not believe the design is sophisticated enough to accomplish complex maneuver via ballast changes (nor is there a reason to IMO). Manuever is typically (and most easily) accomplished via airflow across the steering finst.
Hope this was info was useful.
Sky
