# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction
### Energy-Efficient Routing and WSN Architecture  
Generally, designing of single-path routing protocol is easier than a multipath routing protocol. The drawback of a single-path protocol is that it swiftly drenches the energy when selected as the path. Additionally, in scenarios where a single-path protocol node is out of energy, a fresh route must be recomputed. Meanwhile, multipath routing creates a platform to equally re-distribute the energy among the sensor nodes by rotating the forwarding nodes. These have the capacity to increase network reliability by provisioning multiple routes, speeding up network recovering rate from a failure. For readers interested in the multipath routing protocols for WSN, a comprehensive survey is given in [40]. In terms of the energy efficiency of the multipath routing protocols for WSN, the Energy-Efficient Multipath Routing Protocol (EEMRP), discussed in [41], focuses on discovering multiple node-disjointed paths based on a cost function driven by the energy levels and hop distances of the nodes, and subsequently allocates the traffic rate to each selected path. Moreover, Energy-Efficient and Collision Aware (EECA), discussed in [90], is proposed as a dual node disjointed and collision-free route considering source and sink. The results showed that the efficiency of the multipath routing protocols in terms of the energy are better than single-path routing protocols. Moreover, it is capable of further improving both the EE as well as lifetime of the WSN, if the routing algorithms are not only the function of the shortest paths, but consider the residual energy before selecting the next hop, as reported in [91]. [PERSON] et al. [91] proposed dual novel energy-aware cost functions to improve the energy-balancing performance of the routing protocol by considering nodes in hotspots consumes more energy: (i) Exponential and Sine Cost Function-based Route (ESCFR) function, maps a miniscule variation in remaining nodal energy to a big variation in the cost function value. The idea of the ESCFR, operates by giving higher preference to sensor nodes having bigger remaining energy during route selection, thus creating energy equilibrium. (ii) The Double Cost Function-based Route (DCFR) protocol makes decisions by taking into consideration the energy consumption rate of nodes, as well as the residual energy, which enhances the energy-balancing performance of the routing protocol, even in networks facing obstructions. Unfortunately, the location of the sensor nodes may deplete energy in a given region or create energy holes. However, optimal sensor node placement via uniform distribution or by including a few sensor relay nodes with enhanced capabilities can be deployed to address the issue. Generally, this leads to energy balance improvement among the sensor nodes, avoiding hot-spots sensor nodes and guarantee RF coverage and link connectivity [92]. Plethora of research have focussed on locating the least number of sensor relay nodes or optimal sensor relay placement that will extend the network lifetime [42; 43; 44]. Meanwhile, other studies have proposed a cluster architecture approach, which organizes the sensor nodes into clusters. The motive of this approach is dependent on the cooperation among sensor nodes in the same cluster. Meanwhile, each cluster is managed by a selected node known as the cluster head, which is responsible for coordinating the members' activities and communicating with other cluster heads or the base station [38; 39]. Cluster architectures is one of the most desirable approaches to improving the EE of the WSN. Cluster architecture comes with many benefits, such as: improvement in WSN energy-efficiency and network scalability by maintaining a hierarchy in the network. To fully derive these benefits, these strategies must be considered:  
1. Reduction of transmitting distance of cluster members requiring lower transmission power. 2. Cluster heads limiting the transmissions frequency as a result of fusion. 3.