In the past, WSN have not been the focus of much EH but now they are coming center stage because battery problems are preventing most of the potential rollouts. Wireless Sensor Networks are, by definition, mesh networks by which any tag ie node can signal to any other node provided it is in range. Such a sensor network is self-organizing. Drop another node in (and you may literally do this from a helicopter) and it automatically joins the network without human intervention. It is self-organising. It operates rather like the internet so it is self-healing: even if some nodes are inoperative or out of range, the message eventually gets through somehow by multipath hopping of the signals in and out of the network. In principle WSN are massively scalable: it should be possible to deploy millions of nodes in one such system one day at very low cost. Extremely small, locally powered, wireless sensors are a fundamental part of this vision. Indeed, the military envision dust scattered over the enemy that monitors what they are doing. A high proportion of WSN concepts for industry and healthcare involve nodes that need to last for decades without maintenance.
WSN is therefore a multibillion dollar opportunity but, as yet it is only a business of a few hundred million dollars partly because affordable, reliable provision of power to inaccessible nodes, often over decades, is usually impossible with current technology. For example, at a recent IET conference on energy harvesting in the UK, one presenter reported results of a survey that showed that 70% of users did not want to roll out wireless sensors after successful trials because the battery life in the sensor is inadequate. Even though the battery may be cheap, the cost to manually access the sensor and replace the battery can be prohibitive. Size constraints are a problem too. That is why WSN is the focus of a huge amount of work developing next generation energy harvesting. The opportunity is huge and the batteries and energy harvesting of today are not good enough for more than a small number of the envisioned deployments. They include automatically anticipating and monitoring forest fires, avalanches, hurricanes, failure of country wide utility equipment, traffic and much more over wide areas. Meanwhile, WSN are so power hungry that they typically employ several AA or AAA cylindrical batteries and lifetimes of nodes are sometimes only weeks or months. Read "Wireless Sensor networks" www.idtechex.com 
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