Hi Carl
The 0.5ohm resistor is used to represent the TEC. TEC will change its resistance under different delta T.
You are right, it's not a linear circuit. I previously wanted the BJT to operate in linear region, but it's very difficult. But this circuit still owns unipolarity, and I have thermistor work as the feedback.
For the DAC and hbridge, I am considering to put an inverter before one side of the input. And adjust the dac output by summing Opamp. By doing that, I can adjust the input to TEC, for example, sth like -2.5V~2.5V.