Answer is: fusion of hydrogen atoms.
For example, this nuclear reaction is in the inner core of the Sun and hydrogen is conveted into helium. The inner core is
the central region of the Sun and all solar energy is produced in the core
by nuclear fusion.
Cesium has low ionization energy
and lose one electron very easy, but for that energy is needed.
Distillation of ethanol also reqiured energy and does not produce it.
