## 7 Conclusions  
Data collection and transmission operations are often marred due to the limited lifetime of the energy-constrained sensor-nodes in an IoT network. A battery-powered sensor-node has the huge limitation of finite node-lifetime. This is a fundamental bottleneck that limits the WSN's operational performance and efficiency. In this paper, we considered a recent breakthrough in the application of WPT technologies and provided a comprehensive review of WPT technologies that are applicable for IoT-SC application-scenarios. We presented the different application models of WPT and closely re-examined each model with a view to determining the most energy-efficient and cost-effective technique. We pro  
Figure 7: Effect of number of sensor-nodes on average energy charged for MCBR-Flooding and IEEABR routing protocols.  
posed a model that combines both data collection and wireless energy transfer schemes and also investigated the WPT efficiency of a power-hungry opportunistic IoT sensor network whereby mobile collectors (data mules) are opportunistically exploited as wireless energy transmitters, which traverse the network and recharge the energy-constrained IoT sensor-nodes placed within their charging vicinity or coverage area. We conducted an empirical study of WPT model to determine the effect of varying the distance on the power transfer efficiency.  
Given the different trade-off factors, the energy provisioning of the sensor-nodes follows the inter-meeting time between the energy transmitters and the IoT-nodes. Essentially, a slow-moving wireless energy transmitter within the charging vicinity of the node transmits more energy than a fast-moving energy transmitter within the same coverage area. The former has a higher power transfer efficiency than the latter. This finding gives some useful insight into the design of energy-efficient opportunistic IoT sensor networks. The proposed model combines both data collection and wireless energy transfer techniques. The scheme is focused on exploiting the recent breakthrough in WPT technology and the advantages offered by opportunistic networks. This is the first of such schemes that leverage on the advantages inherent in the separate schemes for an energy efficient opportunistic IoT network. After a careful study of the problem description and formulation, we provided some simulation analysis to further backup our proposed technique. The simulation results further justified the result obtained through empirical model. From the results, we showed that a network system operating under our proposed scheme can indeed remain perpetually operational. The results of the simulation also give some useful insights into future research directions.  
Conceptualization, G.K.I.; Data curation, K.L.-M.A.; Investigation, J.K.P.S.; Software, G.K.I.; Validation, G.K.I.; Formal analysis, G.K.I.; Visualization, K.L.-M.A.; Supervision, K.L.-M.A. and J.K.P.S.; Writing--original draft preparation, G.K.I.; Writing--review and editing, K.L.-M.A. and J.K.P.S. All authors have read and agreed to the published version of the manuscript. This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.