# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction
Accordingly, research on the green IoTs encompasses wide spectrum of topics, research issues and challenges. Green IoT techniques can be classified into two main categories [11]: (i) Software, which focuses on energy wastage as a result of inefficient resource  
Figure 1: The most popular IoT applications (2018). utilization, i.e., algorithms; and (ii) Hardware, which focuses on improving the EE in IoT components, and it is our target in this study. In the recent times, green IoT survey papers have attracted wide attention [1; 10; 12; 13; 14; 15]. The authors in [12] analysed various strategies for achieving green IoT; however, explicit green IoT models were never considered. [PERSON] et al. [13] discussed various cloud energy consumption scenarios. However, their models never discussed Quality of Service (QoS) metrics capable of increasing the energy consumption further in some scenarios. Reference [14] provided an in-depth discussions on energy harvesting in wireless sensor network (WSN) by exploiting various environmental resources. However, storing the harvested energy in a different medium other than the device will result in energy loss and hence, more work is needed in this regard. Reference [15] proposed that by implementing EE in heating, ventilating and air conditioning, could result in high energy saving design. Though extensive work has been done in the area of green IoT, energy conservation models are yet to be analysed. Reference [1] provided an extensive analysis of green IoT strategies, leading to the proposition of five green IoT principles. Moreover, the authors considered the case study approach to be a vital tool of IoT (in smart phones). However, the above studies lack depth in their explanation. Since then, both sensor networking and RFID have been considered the two main pillars of IoT applications. In a hot research topic in ICT like green WSN and green RFID, there are many developments that quickly come into the spotlight and need to be highlighted in order to provide clear insights for researchers to choose the best solutions that provide green IoT and eco-sustainability. Thus, this review paper is different from the other review papers related to green IoT, in that this study gives full consideration to discussing green WSN and green RFID, which is considered one of the two main pillars of IoT applications, and this study aims to provide an overview of current state-of-the- art energy saving practices and strategies for green WSN and green RFID. To this end, this study tries to incorporate as many directions into this article as possible. Restricted by size constraints, the hot research topics have been investigated deeply based on their respective sub-domains to achieve a precise, concrete and concise conclusion at the end of this article. The key contributions of this study are summarized as follows:  
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including: _Long Range_ (LoRa), Bluetooth, LR-WPAN, Mobile communication, WiMAX, and WiFi is summarized in [17; 18; 19]. Table 1 provides a summarised comparison of wireless systems on the basis of notable attributes, such as standard, energy consumption, frequency band, data rate, transmission range and cost. Table 2 shows the suitability of these wireless technologies for IoT applications. Wireless sensors are designed to continuously and autonomously transmit data for a long period. WSNs that continuously send data are sensitive applications of weather forecast, traffic report, water quality, healthcare and embedded system. Wireless sensors are powered by batteries. However, the replacement or maintenance cost of exhausted batteries is prohibitive, and these batteries are difficult to replace, especially in remote locations due to geographical limitations (e.g., challenging terrain) [20]. Accordingly, developing energy-aware solutions to increase battery life and reduce replacement costs has become indispensable for WSN sustainability.