What is the source of energy that drives convection in the atmosphere?
The source of the energy that drives convection in the atmosphere is the same source that all energy on earth ultimately derives from: The Sun.
If you stand out in the sunlight, you will notice that it feels warmer than it does in the shade. That warmth you feel is heat energy directly from the sun. The ground, buildings and streets all absorb various energies from the sun (usually light) and then re-emit that energy as heat. That's why urban areas typically record higher temperatures than rural areas. Plants don't re-emit all the energy they take in, they store some of it in sugars. They still do emit heat from their own metabolism, but it's much less than the ground does.
The sun heats the air with a combination of its own heat, and heat emissions from light striking the ground. This of course, starts the convection process that ultimately generates wind.