In solar thermal electric generation, the sun's energy is concentrated by ___ to heat the fluid in a pipe that is used to generate electricity.

Mirrors

Explanation:

In Solar thermal power generation system, mirrors are used to collect and focus sunlight onto a fluid pipe to produce high temperature required to generate electricity.

There are two main components: mirrors capture and focus light onto a receiver. The heat transfer fluid is circulated in the receiver. Steam is produced which is converted to mechanical energy in a turbine. It powers a generator and electricity is produced.