# Current Trends on Green Wireless Sensor Networks
## 3 Energy Harvesting and Energy-Neutral Operation  
Sensor nodes are usually powered by a battery with limited capacity. The use of conventional batteries does not always permit designing long-lasting WSNs. Moreover, replacing batteries can be too difficult when severe environmental conditions exist. Therefore, to avoid having to replace batteries, one possibility is to recharge the battery of the nodes by means of an energy harvesting system. Energy-harvesting techniques allow the sensor nodes to obtain energy from the external environment such as from sun, wind, vibrations, radio frequency, thermal, etc. These techniques transform the energy of the environment into electrical energy that can be used in the nodes. This energy can be used to avoid depletion of the node batteries. Energy harvesting does not guarantee that the nodes can operate continuously and indefinitely because the energy sources are uncontrollable, making them unpredictable and difficult to model. An example is sunlight, which is not available at night, or wind whose speed is variable and difficult to predict. Although sensor nodes could operate by adapting energy consumption to the energy harvested at any given time, it will often be desirable to store the harvested energy for later use. For this purpose, sensor nodes should be equipped  
Figure 5: Classification of clustering-based routing protocols reproduced from [PERSON] et al. [156]. with storage devices such as rechargeable batteries, although the use of supercapacitors has become more popular due to their high energy storage density and their smaller size, which is more suitable for WSN nodes. A fundamental problem in EH-WSNs is to properly manage both the energy consumption in the WSN and its energy-harvesting system. In a battery-powered node, the goal of a power-management scheme is to minimize the energy consumption or to maximize the lifetime achieved while required performance constraints are satisfied. In an energy-harvesting node, one possibility is to consider the harvested energy as a supplement to the battery energy and try again to maximize the lifetime of the network. However, a more interesting possibility in this case is to use the energy at an appropriate rate so that the WSN keeps on operating continuously. This mode was called energy-neutral operation (ENO) [178]. Therefore, a node is said to obtain energy-neutral operation if the desired level of performance can be sustained indefinitely. In this section, we are interested in the power-management mechanisms and ENO in EH-WSNs. We will refer to them as neutral power management. Moreover, we will focus on energy prediction strategies. The research area related to power management and ENO in EH-WSNs is moderately active (one hundred papers a year indexed by Scopus), but there has been a slight increase in the last few years. However, only a couple of interesting surveys have been published in last years about power-management mechanisms and ENO in EH-WSNs. In [179], [PERSON] et al. present the technologies for harvesting energy from ambient sources (RF, solar, mechanical, thermal and hybrid). Moreover, it dedicates one section to the architecture of a energy-harvesting node (storage, energy prediction and power management). Finally, they study the protocols that can take advantage of the harvested energy: physical layer (power adjustment based on link quality parameters, multi antenna-based protocols or joint source-channel coding-based protocols), MAC layer (adaptive duty-cycle-based protocols, CSMA/CA-based protocols), routing (offline routing protocols, route cost-based protocols, geographic routing-based protocols and clustered network-based protocols).