economic use of energy has emerged as another requirement. The concern lies in estimating the periodicity and magnitude of the exploited energy source, deciding which parameters to tune, and simultaneously avoiding premature energy depletion before the next recharge cycle [@sudevalayam2011energy]. Hence, the goal is to characterize a tradeoff between the performance levels and lifespan of built-in energy units. This new perspective compels the need to understand energy storage technologies and the associated performance levels.

Related Work
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Since the natural energy sources are stochastic, the power and data management policies in EH wireless communication systems are different from their counterparts that depend on the grid energy or non-rechargeable batteries. Moreover, considering a system that has a data buffer as well as a battery, the optimal control of an EH wireless communication system requires managing the transmission rate by monitoring both the traffic load and the stored energy, and respecting the causality constraints in both the data and energy arrivals. The authors in [@ozel2011transmission] and [@6202352] controlled the transmit power levels subject to energy storage capacity and causality constraints, and introduced a directional offline water-filling algorithm that optimizes the delay-constrained throughput. Separately, the authors in [@ozel2011transmission] considered a transmitter model with 