# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction  
The Internet of Things (IoT) is a paradigm which aims to advance telecommunications in all spheres of human life. In the near future, symmetry technologies for the Internet of Things (IoT), along with symmetry and asymmetry applications for IoT security and privacy, will re-design the socio-ecological human terrain morphology. This advancement leads to substantial improvement in quality of human life and the world's economic growth at large. IoT is considered the backbone of emerging applications (Figure 1) [1], as innovation plays a key role in the massive evolution of machine communications. Machine-to-machine (M2M) traffic is estimated to account for approximately 45% of the total Internet traffic by 2022 [2]. IoT creates a platform in which physical objects can mimic certain human sensory capabilities, such as perception, vision, hearing, smell and thinking. Buoyed by these human sensory capabilities and the emerging tactile Internet, machines can communicate with one another, share relevant information and make real-time decisions with less human input, especially now that we are migrating to the 5G era, in which the expected wireless network delay is 1 ms. In this scenario, divergent sensors, such as radio-frequency identification (RFID) and sensing technology, can undertake the collaborative function of sensing, collecting and transmitting sensor informationthrough the Internet [2]. Experts report that IoT holds incredible potential for smart homes, smart cities and healthcare applications [3]. Healthcare sectors can generate an annual revenue of $1.1-2.5 trillion by 2025 [4]. Moreover, the worldwide economic effect of IoT is approximated at between $2.7 and $6.2 trillion by 2025 [2]. With the rise in ubiquitous IoT innovations, it has dawned on the telecom operators that innovative, and creative solutions are the essential tools needed to tackle the challenges and the potential of IoT. Today, the number of objects with Internet connectivity capability is greater than that of the Earth's population. As the capabilities of the IoT expand, it is expected that close to 50 billion devices will have Internet connectivity by 2020 [5]. Generally, the IoT will be capable of interconnecting billions/trillions of heterogeneous objects through the Internet by using different types of sensors like RFID and sensing technology. Both technologies will undertake collaborative functions of sensing, collecting and transmitting sensor information via the Internet. Though active RFID tags are low energy consuming wireless devices [6], when these RFID tags are connected in an industrial scale involving billions of them, they generate millions of payload transmitted to the data centres for processing. To process these enormous sensor payload, huge processing and analytics capabilities are needed, which, in turn, consumes huge energy resources [7; 8]. Herein, it is necessary to consider the issue of large-scale consumption of energy resources by IoTs. Motivated by this, a relatively novel research interest has evolved known as \"green IoTs\". This initiative not only tend to improve EE but also reduce the emission of CO\({}_{2}\) from ICT products [9]. It is generally accepted that IoT will have great economic and ecological significance in years to come. Driven by these realities, it has become imperative to analyse the state-of-the art techniques and approaches capable of tackling the energy needs of these billions of energy hungry devices. Thus, the green IoT has become an important topic for researchers and vendors now more than ever as the conventional energy resources are dwindling and the energy consumption increasing exponentially. Green IoT initiatives concentrate on the IoT EE. Thus, green IoT is defined as the process of achieving EE in IoT technology. This entails making every design process in IoT green until the implementation phase [10]. This has motivated researchers in both academia and industry to develop different techniques to improve IoT EE.