I think this whole reasoning about willpower as a energy (whether renewable or not) is horribly confused. It is just a surface analogy taken too far. For physical work we have muscles that burn calories -- that is a scientific fact. Mental work can also make us kind of "tired", but that is not enough to conclude that we have some analogical mental muscles that burn mana points or whatever.

A big difference is that unlike physical work, mental work does not make us literally tired, but more like frustrated. One can think about pleasant things all day long, one can do their hobby, even when it is difficult, all day long, without getting tired. But a work that does not make sense, makes one burn out quickly. When one solves a problem, the mere information that the problem is easy to solve, gives one a lot of "energy"; on the other hand, a suspicion that there is a mistake in a problem which makes it unsolvable, takes the "energy" away. I guess in a similar way, belief in one's "unlimited willpower" makes the task easier, while belief in a "limited willpower, already largely spent" makes the task more difficult.

I propose a completely different point of view that includes what happens inside the human mind. Human brain is basicly a problem-solving machine, by nature focused on problems of survival and reproduction, and all related stuff (like status, human relations, etc.). All other goals must be somehow connected to this basic goals, even if the connection is only imaginary. This is what the word motivation means: you connect solving of problem X with something that given person already cares about; for example you say "solving problem X will make you rich" or "solving problem X will make people like / admire you". Thus the brain includes solving problem X in its collection of problems worth solving, and that makes a human willing to do it.

The opposite of this process should be called demotivation; it is usually not done explicitly, but it just happens under some circumstances. If motivation is seeing a link between X and one's natural goals, demotivation is weakening of this link; a person becoming suspicious that maybe X does not really contribute to one's goals, or at least that X is so difficult that a completely different way to achieve one's goals would be more efficient. Simply said, my model is something like "the brain chooses a rational solution, but it may have completely wrong data on its inputs". For example a little problem on the way towards X may create an exaggerated thought "X is impossible", which is followed by a rational conclusion "I don't want to do X anymore" -- and the person reports being too tired to do X. Next day the little problem is forgotten or put in a correct perspective, and the person is ready to work on X again.

If this model is good, the the "unlimited willpower" is simply a strong conviction that X is possible and efficient to achieve one's goals; a convinction so strong that it can't be even temporarily unbalanced by little problems.

Where exactly is the glucose in this picture? Perhaps consuming glucose is a chemical reinforcement which gives the organism a signal "whatever you were doing, it is good for your survival". Simply said, you are doing X because you believe "doing X is good for me". Then you have some problems and switch to "doing X does not work". Then you eat glucose and your body says "you are on the right path". And in some situations the brain interprets it that perhaps X is actually good (as if X somehow made you eat the glucose), so it is worth doing X again.