This is entirely theoretical, but within the realm of possibility. Inside all mammalian and plant cells exist organelles which produce energy and create what is essentially a zero point energy reaction inside of the cell wall during the Krebs cycle.
I am not clear on how one would go about the process of harvesting this energy except through biotech science, and I think as a thought experiment or even a novel suggestion for “green energy” that it is less interesting from a purely engineering perspective and far more intriguing from the perspective of ethics.
After all, in order to actually harvest the energy created by these animals – and organelles are after all a type of animal inside a cell wall inside the matrix that is a human or a tree or a cat… we would have to imprison the cell and use its product of labour for our purposes without giving consideration either to its suffering or how our intent might interfere with what it is motivated to do by biological programming, whether or not it has a “mind”. And because as of yet we have not developed a means for a human mind to communicate with an organelle, this enslavement would necessarily be done involuntary on its part, and possibly with great glee and smug assuredness we are doing what is best for society by those carrying out this experiment.
Is the possibility of a source of almost “free” perpetual energy from a living being of any kind that cannot consent therefore ethical? If this experiment failed massively or did major harm to the scientists performing it, who would be responsible for the blame?
And if this thought experiment makes you wonder, then one must also wonder if the cost of so called green technology that uses heavy metals found in conflict zones is worth it. We love the idea of free energy and green tech but almost never ask ourselves where things originate.