The Stirling engine is not a photovoltaic system, in which

I guess the trap is that we see ", but" and expect an independent clause to follow. However, if you take a close look, ", but" does not precede a complete independent clause. In fact, the "," is used as an end to the modifier. Thus, this is a sentence where only one independent clause is separated by a modifying clause, which causes confusing.


The Stirling engine is not a photovoltaic system[highlight], in which sunlight excites electrons in a semiconductor to generate electricity,[/highlight] but rather a parabolic dish that concentrates solar power, generating heat that is converted to grid-quality electricity.

If we get rid of the modifiers:
The Stirling engine is not a photovoltaic system but rather a parabolic dish that concentrates solar power.

Now, we just have one independent clause. This is how we should consider our problem statement.