# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
###### Abstract  
Wireless Sensor Networks (WSNs) are considered to be among the most important scientific domains. Yet, the exploitation of WSNs suffers from the severe energy restrictions of their electronic components. For this reason there are numerous scientific methods that have been proposed aiming to achieve the extension of the lifetime of WSNs, either by energy saving or energy harvesting or through energy transfer. This study aims to analytically examine all of the existing hardware-based and algorithm-based mechanisms of this kind. The operating principles of 48 approaches are studied, their relative advantages and weaknesses are highlighted, open research  
a constantly increasing range of human activities [4]. Specifically, although the first WSNs ever used served only military purposes [5], nowadays WSNs are not only still used in the military sector [6] but they also are utilized in an extensive variety of civil applications. Environmental and habitat monitoring, energy management, object tracking, fire detection, machine failure diagnosis and various other industrial applications, health and biomedical applications, agriculture, livestock farming, inventory control, traffic control, smart homes, smart cities, surveillance and reconnaissance are some of the domains in which WSNs are utilized [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17].  
Yet, despite the evident advantages that WSNs offer, their use is obstructed due to the inborn constraints of wireless communications and the limited resources of the nodes. Actually, the main imperfection of WSNs is the particularly constrained energy adequacy of their nodes [18]. It is initiated by the fact that nodes are powered by batteries of restricted capacity that are difficult or even impossible to recharge or replace. Thus, the operational time of WSN nodes is limited and consequently the overall network lifespan is restricted.  
So, the attainment of energy conservation is an issue of critical importance for WSNs and that is why numerous research methods have been proposed by researchers that pursue energy sustainability [19; 20; 21; 22; 23; 24; 25]. In this research work, as illustrated in Figure 2, energy sustainability methods are classified into two main categories, i.e., the hardware-based methods and the algorithm-based methods.  
In accordance with the above, the remainder of this article identifies the causes of energy consumption in WSNs and analyzes all (i.e., 48) mechanisms that have been proposed in order to reduce this energy consumption and consequently extend the lifetime of nodes in WSNs. Specifically, in Section 2 the causes of energy consumption and waste in WSNs are analyzed. Section 3 describes the architecture of sensor nodes and analyzes the existing hardware-based mechanisms for energy sustainability in WSNs, via energy saving, energy harvesting, and wireless energy transfer. Section 4 describes the protocol stack of sensor nodes, presents the communication technologies used in WSNs and analyzes all algorithm-based mechanisms that have been proposed for energy sustainability in WSNs. In Section 5, the main findings of this survey along with open research issues are discussed. Finally, in Section 6, the corresponding concluding remarks that are extracted from this research work are resumed.