What is Seebeck Effect? How can we produce current using heat?

Seebeck effect is a thermoelectric effect in which heat converts into electricity. In this effect potential difference across the circuit is generated by creating temperature difference across the junction of hot and cool side.

When one side of semiconductors is heated the electrons in their valence band absorbs energy and get excited. Excited electrons move in conduction band and become free to move. As holes are majority charge carriers in p-type semiconductor, all the excited electrons(minority charge carriers) in p-type participate in the flow of current through conductor. Now, due to higher energy at hot side, all the electrons in n-type semiconductor and holes in p-type semiconductor moves towards junction of cool side and this will create a potential difference across the circuit and current starts flowing though it. 

The output of this effect is very small, i.e., in millivolts. So, to make the effective flow of current, thermoelectric substance should have high electrical conductivity and low thermal conductivity.

Calculation for obtaining amount of Potential Difference and Current:-
Open Circuit Voltage, V= S*dT
where S is Seebeck coefficient, dT is temperature difference
s=dV/dT
dT=Th-Tc
Current drives through the circuit, I=S*dT/R