# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
## 5 Discussion
### Challenges and Open Research Issues in Algorithm-Based Methods
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Method** & **Basic Operation** & **Advantages** & **Disadvantages** \\ \hline \multirow{2}{*}{**Low power electronic units**} & Use of low-power sensors, processors, and transceivers & Energy efficiency and low power consumption. & Increased cost of application \\ \cline{2-4}  & Use of active, idle and sleep operation modes of hardware. & Energy saving when nonstop nodes’ operation is not needed & Not applicable where continuous measurements are required. \\ \hline \multirow{2}{*}{**Use of Passive Sensors**} & Sensors containing no active circuits are used. & Practically no energy dissipation takes place. & They cannot be used in all kinds of applications. \\ \hline \multirow{2}{*}{**Dynamic Voltage Scaling**} & Frequency and voltage in line with the processing tasks. & This technique increases energy efficiency of the processing unit. & It is effective only when sensing requests are less frequent. \\ \hline \multirow{2}{*}{**Cognitive Radio**} & Communication needs define radio channel selection. & High power channels are not used for wakeup-call communication. & The existence of multiple radio channels adds complexity and cost. \\ \hline \multirow{2}{*}{**Adaptive Transmission Power Control**} & Power in line with the distance and energy residues of nodes. & Energy spent for transmission is in line with existing conditions. & Delay is increased. Routing paths are modified. \\ \cline{2-4}  & Signals are received and sent in one direction at a time. & Increase of throughput, decrease of power needed and overhearing & Localization methods may be needed for orientation purposes. \\ \hline \multirow{2}{*}{**Short Communication Links**} & Communication is made by using many transmissions over short distances. & Less energy consumption during transmission. & More nearby allocated nodes are needed to be deployed. Not applicable in sparse networks \\ \hline \multirow{2}{*}{**Rechargeable Batteries**} & Batteries that can be recharged many times are used. & High energy density. Low cost. Low rate of self-discharge. & Long charging time. Short recharge cycle life. Limited lifetime. \\ \cline{2-4}  & Capacacity of high capacitance are used. & Short charging time, long recharge life cycle and lifetime. & Expensive. High rate of self-discharge. Low energy density. \\ \hline \multirow{2}{*}{**Supercapacitors**} & DC electricity is made from Ambient/dedicated wireless signals carrying RF waves. & Dedicated RF is at least partially predictable and partially controllable. & There are health limitations for RF power. Ambient RF is neither predictable nor controllable. \\ \hline \multirow{2}{*}{**Light-based Energy Harvesting**} & Electricity created by photons emitted by light (solar/indoor) & Solar-based is predictable. Indoor is predictable and controllable. & Solar is uncontrollable, available only in daytime of weather is good. \\ \hline \multirow{2}{*}{**Thermal-based Energy Harvesting**} & Energy is generated due to the existence of either heat or variations in temperature & This method is controllable when caused by heat. & It is unpredictable and has low efficiency. It is uncontrollable when caused by temperature variations. \\ \hline \multirow{2}{*}{**Flow-based Energy Harvesting**} & Energy produced by wind and water is scavenged. & This type of energy harvesting is environmentally friendly. & It is neither predictable nor controllable. \\ \hline \multirow{2}{*}{**Biomass-based Energy Harvesting**} & Energy is made from various types of biological material & It is an inexpensive method with high efficiency. & It can be used in specific types of applications. \\ \hline \multirow{2}{*}{**Mechanical-based Energy Harvesting**} & Energy scavenged from strain, vibrations, and pressure. & This type of energy harvesting is controllable. & It is unpredictable. \\ \hline \multirow{2}{*}{**Human-based Energy Harvesting**} & Energy harvested from human activity or physiological tasks. & Human activity-based energy harvesting is controllable. & Physiological: unpredictable, un-controllable. Activity: unpredictable \\ \hline \multirow{2}{*}{**WET: Inductive Coupling**} & Energy transferred from a primary to a secondary coil. & Simple and safe to apply. High efficiency in small distances. & Loss of power. Inefficient for long distances. Non-directionality. \\ \hline \multirow{2}{*}{**WET: Magnetic Resonant Coupling**} & Energy transferred between coupled resonant coils & Non-radiative. No need of line of sight. Long distances covered and resonant frequency tuning. \\ \cline{2-4}  & Energy transferred via electromagnetic waves. & Energy transfer over long distances is achievable. & Life of sight is needed. Radiation emitted is harmful. \\ \hline \hline \end{tabular}
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