Thank you @sacmoens. To answer your question I think that solar energy is the only energy than can be harnessed using photoelectric effect. This is because the phenomenon, photoelectric effect is dependent on incident light and the only source of light that can produce a significant amount of energy is the sun.

Remember that light do display a dual nature, sometime acting as a wave and in cases like photoelectric effect it acts like a particle. I bet that when you teach them this, they will be quite amazed by this dual ability of light.