I haven't looked into it, and while there is certainly "static electricity" generated in the atmosphere, I doubt that it would do all that was expected of it.
Near ground you have a voltage of 100V per meter. Quite impressive. But current strength is only in the order of pikoampere per square meter. Almost nothing.
When you have a high building or a long rod or, like in this case, a balloon, the potential gradient gets compressed, like this:
so at 100m height you get a voltage of 10,000V with a current of microamperes per square meter. I do not know why the current strength increases, but that seems to be the case. That should translate into a power in the order of 10 mW. Plauson claims that he produced a power output of thousands of Watt. That is an extraordinary claim. It seems though nobody has really repeated his experiments, except a certain Oleg Jefimenko, who made similar trials and reported much more realistic results, something like 70W.
At a height of 3000 meter you should get a current of 300,000V and, allegedly, 1 miliampere per square meter, which would give a power of 300W per square meter. That would compare well with solar panels. You need some long, conductive tethers though. It should actually be cheaper than a solar panel, because it is simple, and it is not intermittent. I guess you have to take it down during a thunderstorm.