## 9 Conclusions  
The energy-harvesting wireless sensor networks (EH-WSNs) belong to an important category in the scope of WSNs, and this paper extensively reviews a variety of aspects with respect to EH-WSNs, including energy harvesting (EH) techniques, energy management strategies, cognitive radio (CR) applications, physical layer security (PLS), and routing protocols. The identification of individual elements showed significant progress in the area which underlines the importance of employing EH systems other than conventional \begin{table}
\begin{tabular}{c c c} \hline \hline
**Aspects** & **Benefits** & **Challenges** \\ \hline Spectrum & Improved utilization of & Requires sophisticated \\ Efficiency [148] & available spectrum & spectrum management \\ \hline Dynamic & Ability to access unused & Complexity in spectrum \\ Access [149] & spectrum bands & sensing and adaptation \\ \hline Integration & Compatibility with existing & Integration with current \\ Issues [150] & systems and technologies & network infrastructure \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Cognitive radio benefits and challenges. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Techniques** & **Description** & **Challenges** \\ \hline Energy Harvesting & Security using & Effectiveness in various \\ Based Security [151] & harvested energy for & environmental conditions \\ \hline Encryption & Reduces need for & Requires robust physical \\ Alternatives [152] & traditional encryption & layer security \\  & methods & mechanisms \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Physical layer security techniques. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Protocol** & **Description** & **Key Considerations** \\ \hline Direct Routing [153] & Direct path from & Effectiveness in \\  & source to destination & various environmental \\  & & conditions \\ \hline Hierarchical Routing [154] & Data passed through & Efficient for large \\  & cluster heads in a & networks but adds \\  & tered structure & complexity \\ \hline Geographic Routing [155] & Routing based on & Requires accurate \\  & geographic location & location data and \\  & of nodes & may not scale well \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Routing Protocols Overview. energy resources towards sustainable and efficient WSNs, especially for low-power contexts. While the industry has come a long way, major obstacles remain. It is a research direction to, for example, optimize across EH coming from multiple sources such as solar, thermal, kinetic, and RF, maximizing a balance between energy consumption and storage efficiency. Moreover, the combination of CR and WSNs appears very appealing for performance-oriented spectrum access despite plenty of technical challenges such as spectrum management and energy efficiency waiting to be addressed. A future challenge in EH-WSNs is PLS, which provides a great opportunity to embed the security in the process of energy harvesting itself and therefore it replaces classical encryption techniques. Before PLS becomes a broad technology, more dangers and challenges are required to be addressed for the deployment of a solution. In this line, EH-WSN routing protocols need to be further developed such that they can provide proper security mechanisms against hostile attacks and cyber threats in addition to energy saving. Future work should focus on a number of key areas in order to advance EH-WSNs. Top-notch technologies such as artificial intelligence (AI) and machine learning (ML) can transform energy management, allowing it to change and react according to the levels of energy at every stage. Similarly, advancements in power storage technology such as hybrid powers storage systems and super capacitors might help reduce the existing constraints. Second, blockchain could be considered as a turning point in the security of EH-WSNs, realizing energy transactions and creating data integrity.