# Green IoT: A Review and Future Research Directions
###### Abstract  
The internet of things (IoT) has a significant economic and environmental impact owing to the billions or trillions of interconnected devices that use various types of sensors to communicate through the internet. It is well recognized that each sensor requires a small amount of energy to function; but, with billions of sensors, energy consumption can be significant. Therefore, it is crucial to focus on developing energy-efficient IoT technology and sustainable solutions. The contribution of this article is to support the implementation of eco-friendly IoT solutions by presenting a thorough examination of energy-efficient practices and strategies for IoT to assist in the advancement of sustainable and energy-efficient IoT technologies in the future. Four framework principles for achieving this are discussed, including (i) energy-efficient machine-to-machine (M2M) communications, (ii) energy-efficient and eco-sustainable wireless sensor networks (WSN), (iii) energy-efficient radio-frequency identification (RFID), and (iv) energy-efficient microcontroller units and integrated circuits (IC). This review aims to contribute to the next-generation implementation of eco-sustainable and energy-efficient IoT technologies.  
internet of things; green IoT; energy-efficient; energy harvesting; wireless charging; green 6G; green communications; eco-sustainable WSN; RFID; M2M +
Footnote †: journal: Journal of Applied Soft Matter  
technology [1]. The IoT enables the network objects to perform tasks by coordinating decisions and sharing information through these sensors.  
According to [6], each active RF identification device requires a small quantity of electrical power to function. However, when billions of such network devices are connected that are consuming energy on a daily basis, and a significant number of data points transmitted by sensors need to be processed by large data centers, it becomes necessary to have significant processing and analytics capabilities [7, 8], which can be energy-intensive. Therefore, the energy consumption in large-scale IoT systems should be carefully considered. To address these energy concerns, a new field of research known as \"green IoT\" has emerged. This initiative aims to elevate energy efficiency and reduce CO\({}_{2}\) emissions for IoT technology [9]. It is clear now that the IoT will have a considerable economic and environmental impact in the forthcoming years. As a result, there is a pressing need to research and develop advanced techniques and strategies to address the energy needs of billions of devices. As traditional energy sources become scarce and energy consumption continues to increase, the idea of green IoT (GIoT) has gained significance for researchers and vendors alike. The concept of green IoT is crucial for finding new ways to downsize energy consumption and increase eco-sustainability.  
The objective of the GIoT initiative is to enhance energy efficiency throughout IoT systems. It aims to make every aspect of the IoT more energy-efficient, from the design phase to implementation [10]. This has sparked research in both academia and industry to come up with various methods to escalate energy efficiency in IoT. As a result, the research in the context of green IoT encompasses a wide range of challenges and mitigation techniques. However, research on GIoT techniques can be divided into two main categories [11]: (i) software, which aims to reduce energy waste through more efficient resource utilization, and (ii) hardware, which envisions improving energy efficiency in IoT devices. This latter category is the focus of our study.  
Figure 1: Top 10 IoT application areas in 2020.