# Green IoT: A Review and Future Research Directions
### Related Works and Contributions  
There has been an increase in the number of survey papers on green IoT that have garnered attention in recent times [2; 10; 12; 13; 14; 15]. Reference [12] conducted an analysis of different approaches for achieving green IoT; however, they did not consider the use of specific green IoT models in their analysis. Reference [13] examined energy consumption under several cloud deployment scenarios, but their models did not take into account Quality of Service (QoS) metrics in some certain scenarios. Reference [14] conducted a thorough analysis of energy harvesting in wireless sensor networks (WSNs) using distinctive environmental sources. However, using a different storage medium for the harvested energy than the device itself may incur an energy loss, which would require additional effort to address. Reference [15] suggested that implementing energy-efficient measures in heating, air conditioning and ventilating systems would result in a significant amount of energy savings. Despite significant research in the field of GIoT, energy conservation schemes have yet to be thoroughly investigated. Reference [2] conducted a comprehensive analysis of green IoT techniques and proposed five principles for the GIoT. Additionally, the authors emphasized the use of case studies, specifically citing smart phones as an example, as a valuable tool for understanding IoT. However, the aforementioned studies were somewhat lacking in their depth of explanation. In the rapidly evolving field of green IoT, it is important to highlight new developments and provide clear insights to researchers in order to enable them to settle on the best solutions that promote eco-sustainability with attention to green IoT.  
This review paper differs from other reviews on green IoT in its goal to provide an extensive overview of the recent state-of-the-art energy-saving strategies and practices in the context of green IoT, specifically focusing on the energy efficiency of green hardware IoT devices. In order to achieve that goal, this study attempted to cover as many relevant topics as possible within the scope of the paper. However, due to size limitations, the focus was on in-depth analysis of particularly controversial or important sub-areas within the larger domain of green IoT in order to arrive at precise, concrete, and concise conclusions. This study covers the following sub-domains within the field of green IoT: (i) energy-efficient M2M communications, (ii) energy-efficient and eco-sustainable WSN, (iii) energy-efficient RFID, and (iv) energy-efficient integrated circuits and microcontroller units. M2M communication is a foundational aspect of IoT and sensor networking and serves as a crucial element in current systems, along with RFID technology. However, both the sensor networking and RFID technology will undergo thorough examination in this study, because they are considered key components in the field of green IoT. The main contributions of this paper are outlined as follows:  
* A review of current research on the green IoT ecosystem, including recent industry developments and embedded systems, key areas of application, deployment, challenges, and key players focusing on RFID, WSN, processors, MCUs, and ICs.
* A discussion of key design choices and features for RFID and WSNs that are considered to be among the highest priorities within green IoT networks.
* An exploration of the limitations and challenges that need to be overcome to fully realize the potential of a green IoT, including issues related to interoperability, security, and privacy, and recommendations for future research to support the efforts in developing hardware for green IoT that promotes eco-sustainability.