# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
## 5 Discussion
### Challenges and Open Research Issues in Hardware-Based Methods  
Regarding the submodules of sensor nodes, as aforementioned, the use of low-power electronic components is necessitated [19; 21; 24]. However, the components having the highest energy efficiency do not necessarily achieve the best performance standards. That is why the specifications and selection of the hardware components should be meticulously studied before being adjusted to the sensor nodes. In addition, Power Optimization through the appropriate switch among active, idle, and sleep modes of operation cannot be used in applications in which continuous operation of nodes is necessitated [19; 23]. Moreover, Passive Sensors cannot be used in all types of applications [27]. The method of Dynamic Voltage Scaling that is applied in the processing unit of nodes, is effective only when sensing requests are less frequent [28; 29]. The usage of Cognitive Radio method in order to achieve the dynamic selection of the most suitable radio channel, inevitably requires the existence of multiple radio channels, thus increasing complexity and cost [21]. The increase of latency and the modification of routing paths are the main weaknesses of Adaptive Power Transmission [19; 24]. In many cases, the use of Directional Antennas is not feasible if localization methods are not applied in order to assist orientation procedures [21]. Likewise, the utilization of Short Communication Links requires the existence of many adjacently positioned nodes. For this reason, this methodology is not applicable in WSNs having sparse distribution of nodes [21]. Regarding Rechargeable Batteries, the recharge of all batteries of the nodes in a network may be a complicated or even impossible task to perform, depending on the deployment of the nodes. Additionally, besides their limited capacity, rechargeable batteries not only are unable to remain fully charged for long time but they also are characterized by a limited number of recharge cycles [19; 22].Of course, modern 3D-printed Lithium-Ion microbatteries have enhanced capabilities. Yet, further research regarding alternative materials, manufacture techniques and designs of microbatteries is needed [77; 78; 79; 80; 81]. Supercapacitors have certain advantages over batteries. Yet, not only they are costly, but they also have low energy density and high rate of self-discharge [82; 83; 84; 85]. Hybrid Ion Capacitors (HICs) (known also as supcapacuteries) that consist of one battery-type electrode and a capacitor-type electrode seem to be a very promising alternative that achieves higher power capacity, power density, energy density, and efficiency [86; 87; 88; 89; 90]. Energy Harvesting, as aforementioned, can substantially upgrade the energy sustainability in WSNs [33]. Thus, novel research is necessitated not only for the development of advanced equipment such as low energy harvesters, converters, and energy storage systems (ESS) [91; 92; 93; 94; 95; 96; 97; 98], but also in order to handle certain issues such as the high cost of implementation, the low power generated, and the presence of fluctuations and instabilities [99; 100]. Furthermore, health limitations associated with RF power obstruct the wide use of RF-based energy harvesting. Moreover, ambient RF is neither predictable nor controllable [35]. In light-based energy harvesting, the amount of the gathered energy is determined by the intensity of ambient light. Yet, although the availability of light is absolutely controllable in indoor applications, in outdoor applications is existing only during day time as long as the weather conditions are adequately good [38; 39]. Thermal-based energy harvesting not only is unpredictable and uncontrollable when caused by temperature variations but also has low efficiency [36]. Flow-based energy harvesting is not only unpredictable but also uncontrollable [35]. Biomass-based energy harvesting, due to its nature, is feasible only in a specific type of applications. Mechanical-based energy harvesting is unpredictable.