I read the following about thermogenin:
"When thermogenin is inserted into the inner mitochondrial membrane, it accentuates mitochondrial proton leak and dissipates the proton motive force. Since oxidation is no longer coupled to phosphorylation, thermogenin is said to cause uncoupling. In the presence of thermogenin, oxidation and proton pumping continue at high rates but with low rates of ATP synthesis. So Thermogenin enhances mitochondrial proton leak so there would be heat."
Why does proton pumping without ATP synthesis generate heat?