## 1 Introduction  
Recent years have witnessed the emergence of a significant promising technology, Wireless Sensor Networks (WSNs), containing numerous inexpensive sensor nodes randomly scattered over the area of interest to collect information on entities of interest. WSNs have been used for wide-ranging applications, such as in the military [1], environmental [2], health [3] and smart home fields [4]. In WSNs, sensor nodes are generally equipped with limited energy battery and limited power storage with low power, small size and low cost [5]. Sensor nodes are usually deployed in remote or dangerous areas. They can sense ambient conditions and generate data to reflect the real-world scenario [6]. Sensors can communicate with each other over a short distance and relay information on a wireless transmission device. Through multiple hop transmission, this information can be transmitted to a remote terminal and processed by other devices. In WSNs, since researchers have not made breakthrough progress in battery technology, a fundamental problem is the limited lifetime of sensor nodes due to limited and fixed energy supplement [7, 8]. Note, it is generally accepted that the usefulness of a wireless sensor expires when its battery runs out. A significant amount of work has been carried out across the protocol stack to prolong the sensor's lifetime by saving energy [9, 10], including duty-cycling strategies [11, 12] and Medium Access Control (MAC) protocols [13, 14]. However, it is inevitable that the energy will be exhausted and sensors have to be discarded because it is generally impractical to render servicing for sensors. Meanwhile, it is inevitable that the accumulation of discarded batteries will contaminate the environment. Currently, Energy Harvesting or Rechargeable Wireless Sensor Networks (EH-WSNs or R-WSNs) have attracted more and more attention. They benefit from extending the lifetime of sensor nodes by equipping them with rechargeable technologies [15, 16], which convert sources, such as body heat [17], foot strike [18], finger strokes [19], and solar [20] into electricity. A sensor can operate perpetually by using supercapacitors (with virtually unlimited recharge cycles) to store the harvested energy [21]. Note, a harvesting node is said to achieve energy-neutral operation if the energy used is always to a lesser degree than the energy harvested [22]. The energy rechargeable device and energy storage device as the energy supplement components of R-WSNs replace the common accumulator battery that is used in traditional WSNs (T-WSNs), as is shown in Figure 1, in T-WSNs, the the system architecture contains four components: battery, low-power sensor node, micro-controller and wireless transceiver device. In the R-WSNs, the system architecture contains five parts: rechargeable system, battery, low-power sensor node, Micro-controller and Wireless Transceiver device. The rechargeable system is utilized to extract the energy from ambient conditions, which is the key difference between the new WSNs and T-WSNs. The system architecture of a rechargeable sensor node is illustrated in Figure 2. Although their lifetime is less of an issue, a sensor node has to confront two issues. In R-WSNs, the energy available is usually very dynamic, which is influenced by the environment and can vary significantly over time. First and foremost, owing to the restriction of development of conversion technology, the sensor has to stay in a dormant state most of the time in order to recharge the battery and use the energy prudently. Moreover, due to the limited energy storage capacity, a node cannot always be beneficial at conserving energy when a network can harvest excessive energy from the environment [23, 24]. Since more energy can be extracted from the ambient environment in R-WSNs, the harvested energy should be consumed as soon as possible [25, 26]. Therefore, the surplus energy of a node can be utilized for strengthening packet delivery efficiency and improving network data collection rate.