Can cloud charge imbalance be used as an energy source?
First off quite some energy must be present in clouds: a lightning path is quite long, and electrical breakdown of air requires about 1MV/m. Most (many, smaller) electric discharges are not seen because they occur within clouds. The electric energy built up can also be lost in other ways (i.e. Lorentz force).
Some of it comes to ground and when it happens it has such large peak power (on average about a trillion watts) that it can not efficiently be harvested.
I assume that charges build up quite continuously during the lifetime of a cloud: if we knew what cloud to select for discharge we could control (by selecting the right cloud or part of a cloud) the power flux, though not easy its hard to rule out as impossible.
Now how exactly do I propose to do this? Air can be made conducting through bruteforce electrical breakdown (not efficient), or by heating it (not efficient) or by an energy form encouraging with higher specificity the ionization of specific molecules present in air (be it oxygen or nitrogen or ...).
Apparently non-lethal weapons were designed which do not use projectiles: a UV laser with 2 optical, parallel paths, each connected to an electrode at the weapon side can tetanize (stimulate at such a pulse rate as to keep the muscle contracted) with specificity any of the 3 broad muscle types (voluntary skeletal muscle, involuntary smooth muscle, and cardiac muscle all respond to different pulse patterns/frequencies). A weapon on same principle was designed to stop cars up to 2km by frying the control chip in the car.
According to hstvi (not sure how credible this source is) ionizing wavelengths with lasers can make a path of air conductive to 2km:
"With an ultraviolet wavelength of 193 nanometers, the maximum effective range is about 100 meters. The effective range increases to two kilometers when a wavelength of 248 nanometers and a more intense beam are used."
Now for our clouds we would not need 2 optical paths to the laser.
Problems I envision: clouds heights are on kilometer scales, so possibly stages of such lasers would need to be mounted on weatherballoons, preferably locked by cable to a certain height/location.
Each laser would could be surrounded by a thick piece of transparent insulator (glass?) so as to allow UV to pass but not current to enter the setup. On the glass electrodes are placed which are connected to a similar electrode on say a retroreflector of this balloon (a lower one targets this balloon).
If forming a conductor with the lasers consumes more energy than would be got from the lesser charged clouds (or parts of them), we may need to allow electrical breakdown to occur at which point switching of the laser beam would not stop the lightning.