Are there any systems in the brain that are responsible for monitoring energy For example, the amount of energy that is available for the brain to use, or the amount that the brain consumes with respect to time?
This is a very broad question. I'll simplify greatly.

Like most systems, the brain regulates it's moment-to-moment energy needs through control of it's vascular supply. While the vascular supply (to the heart and brain) is regulated (by both baroreceptors and other) to protect against decreased blood flow (that is, the heart and the brain will be preferentially protected against decreased blood flow), the day-to-day energy need fluctuations are still met by brain parenchymal vasodilation/vasoconstriction, often mediated by ion concentration in response to CO2 levels, etc. There are a host of local and peripheral vasodilators. The most instantaneous of these are a result of Ca++ and K+.


  The matching of blood flow to regional brain function, called functional hyperemia or neurovascular coupling, involves the coordinated activity of neurons, astrocytes, and parenchymal arterioles. Under physiological conditions, localized neuronal activation leads to elevated astrocyte endfoot Ca(2+) and vasodilation, resulting in an increase in cerebral blood flow. (see also Proc Natl Acad Sci U S A. 2012 May 22;109(21):E1387-95. for Ca2+-activated K+ (BK) channels.).


The simplest answer to your question of how the energy needs are met by the brain is through vascular control of the supply of nutrients.