Why not use use the cow's body heat?

Animals don't give off much waste heat. They evolved not to. And it's very hard to extract energy from small temperature gradients. The thermoelectric chargers that convert body heat to energy are only ~1% efficient. Compare to sunlight, which gives ~20x the watts per square inch, and can be converted at 20% efficiency. So a body heat charger needs to be roughly 400 times as big as a solar panel does.

What is the Matrix?