## 1 Introduction
Switching off BSs in a HetNet may not be sufficient to reduce energy consumption; therefore, a more energy-efficient solution needs to be sought urgently because increasing energy consumption due to the massive deployment of HetNets has contributed significantly to greenhouse gases emission (GHGE) as well as pollutants related to global warming. According to the studies performed by the authors in [9], GHGE contribution by information and communication technology (ICT) would surpass half of the current transportation sector contribution by 2040 and about 14% of the total global carbon footprint. Thus, substituting non-renewable energy with renewable energy (energy harvesting) in HetNets is seen as one of the possible solutions to reduce brown energy consumption by HetNets, leading to lower carbon emission; therefore, energy sustainability is important in order to maintain the equilibrium between the degrading environmental factor and increasing HetNet user demand. In addition, energy harvesting also improves the energy efficiency (EE) of a HetNet by reducing the grid power consumption (GPC) as suggested by the studies conducted in [10]. Undoubtedly, GPC is also regarded as one of the factors contributing to increasing carbon footprints. Apart from energy efficiency, hybrid power source also greatly contributes to communication during emergency situations. Wireless communication by itself is considered an emergency communication strategy [11]. Nevertheless, depending solely on grid power source can backfire during grid power outages caused by natural or man-made disasters, i.e., earthquake, landslide, war and so on; therefore, it is very crucial to have a back-up plan, such as renewable energy sources during emergency situations. Nevertheless, irregular patterns of energy harvesting should be taken into consideration while evaluating their contribution to the EE of a HetNet. In spite of improving just a single-cell EE, sharing the excess harvested energy (HE) among the coordinating BSs would further enhance the EE of the overall HetNet [12]. This can be explained by comparing the available energy per BS and its user strength and /or distribution, which rarely compensates one another. Several technologies, such as power lines and smart grids, are being studied for energy cooperation among the connected BSs. The research conducted in [13] discusses safe and reliable architectures and current implementations of the smart grid; therefore, it is clear that utilising this technology in this work has the potential for significant and reliable contribution. In recent works [14], less complex and simplified smart grid applications are designed specifically for communication systems. In this work, a smart grid is considered to coordinate the transfer of the excess HE among different BSs and also as an energy storage medium. As the case studies carried out in [15] suggest, energy storage management for smart grids is a vast separate topic and was not taken into consideration in this work. The remainder of this paper is structured as follows. Section 2 describes some existing work related to this research and the contributions of this work. Section 3 illustrates the CoMP-based hybrid-powered HetNet model together with an explanation of the problem. In Section 4, the proposed algorithm is described in detail and the principal algorithm is explained in parts for better insight. Next, Section 5 presents the performance evaluation results and the discussions. Finally, the conclusions and potential future research directions are drawn in Section 6.