drive peltier element
OK so your max output on the op-amp is about 9V and the MOSFET needs 4V to turn on.... 9-4 = 5V. That sounds about right to me for what you are seeing.

Ditch the MOSFET and use a regular NPN transistor or Darlington. Since you are running it in linear mode anyway, the MOSFET does not buy you much other than low gate current.

Or better, change your circuit to use a P channel MOSFET like so.







simulate this circuit &ndash; Schematic created using CircuitLab