It is hoped that solar cell arrays can be made cheap enough to be a source of utility power. Quartz and certain ceramics generate voltage when mechanically strained. Very little current can be drawn, since they are insulators, but they are very useful transducers of mechanical stress and strain. These are storage batteries in which a charge is frozen into an insulator. It produces an electrostatic voltage but can provide no current.
Lightning is the discharging of enormous voltages and energy storage in clouds.
No means of using it have been developed, but enormous efforts are made to prevent it from destroying buildings and utility systems. Two pair of junctions between two dissimilar conductors, if held at different temperatures and connected in series, produce a voltage. Stacks of such junctions comprise a thermopile. Some thermopiles have been heated in satellites by nuclear decay heat. Thermocouple temperature transducers are thermo-electrics.