Wireless Sensor Networks (WSNs) are crucial in supporting continuous environmental monitoring, where sensor nodes are deployed and must remain operational to collect and transfer data from the environment to a base-station. However, sensor nodes have limited energy in their primary power storage unit, and this energy may be quickly drained if the sensor node remains operational over long periods of time. Therefore, the idea of harvesting ambient energy from the immediate surroundings of the deployed sensors, to recharge the batteries and to directly power the sensor nodes, has recently been proposed. The deployment of energy harvesting in environmental field systems eliminates the dependency of sensor nodes on battery power, drastically reducing the maintenance costs required to replace batteries. In this article, we review the state-of-the-art in energy-harvesting WSNs for environmental monitoring applications, including Animal Tracking, Air Quality Monitoring, Water Quality Monitoring, and Disaster Monitoring to improve the ecosystem and human life. In addition to presenting the technologies for harvesting energy from ambient sources and the protocols that can take advantage of the harvested energy, we present challenges that must be addressed to further advance energy-harvesting-based WSNs, along with some future work directions to address these challenges.

In the last few years, the capability of Network Simulator 3 (ns-3) of simulating different aspects of wireless networks has increased rapidly, such that it now provides a wide range of models of real world objects, protocols and devices [1]. Simulation of communication systems and network protocols over realistic device operations is seen as a necessary task before implementation, because it allows for a flexible and fast, but still accurate, testing of the system evolution. Additionally, efficiently managing the energy consumptions of the different elements is a major requirement for an efficient design of wireless networks, since many wireless devices are battery operated.In this regard, the authors in [2] presented an ns-3 energy framework that allows users to simulate the energy consumption at a node as well as to determine the overall network lifetime under specific conditions. This framework adds sufficient support to ns-3 to devise simulations that include the energy consumption of the communication network. In order to do this, this framework defines the concept of an Energy Source, which represents an abstraction of the way in which the node is powered, a Device Energy Model, that defines models for the energy consumption of the different elements that compose the node, and several methods that provide different types of energy information (e.g., residual energy, current load, etc.) to other ns-3 objects external to the framework. The framework is developed with the objective of allowing the interoperability of different energy source and device energy consumption models, and it allows easy integration of new models. Moreover, some implementations of the energy source and device energy models are provided, so that an ns-3 user is able to incorporate them into their existing simulations.The increasing demand for battery operated devices with longer lifetimes has required researchers to explore energy availability from a different perspective, starting from the hardware itself. While the battery technology keeps improving, with the recent advancement in real life wireless devices, devices that are able to harvest energy from the environment, e.g., in the form of solar, thermal, vibrational or radio energy, are now commercially available.Given the above, the problem of designing optimal transmission protocols for energy harvesting wireless networks has recently received considerable attention [3][4][5]. When using an energy harPermission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. WNS3 May 7 2014, Atlanta, Georgia, USA Copyright 20XX ACM X-XXXXX-XX-X/XX/XX ...$15.00. vesting source, the objectives of these protocols are fundamentally different than those o...

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