In thermodynamic gas turbine what the different between overall efficiency and thermal efficiency?

Question

what is the function of heat exchanger in gas turbine?

Answer 1

For my knowledge there are 3 efficiencies in thermal power generation system

thermal efficiency--- energy absorbed in relevant to the  energy given to the system through fuel.. less than 1 due to the heat loss to the environment and the heat in exhaust gas.....

mechanical efficiency-- is less than 1 due to the friction in rotationary parts ( turbine and generator)

electrical efficiency-- is less than 1  due to core and copper losses

overall efficiency is given by the multiplication of all three....

which is electrical power outcome to the given thermal power

heat exchanger transfer heat from on source to another (ex heat of the burning fuel to water)