Energy Harvesting
Harvesting energy from environment is not a new technology; it has been used since decades to scavenge energy from wind, sun and moving water. The new thing is to implement this idea in modern systems. It is a process of conversion of environmental energy into electrical energy. Energy is around us in many different forms such as sound, vibration, mechanical, thermal, chemical, electrical, solar, wind, RFetc. Figure1(Hi-globe,n.d.)shows the ambient energy sources in the environment. There are many types of energy harvesters each offering differing degrees of usefulness depending on the application (Alippi C. et al.,2009).Energy harvesting or energy scavenging has an important application in areas where wiring is not practical or it is difficult to supply energy, hence it has great scope in wireless sensor networks to keep them operational for almost infinite time.
Figure 1. Different sources of ambient energy
Wireless sensor networks are suitable to deploy in areas with harsh weather condition and flexible enough to embed with any system of interest.However, constant power supply is a big challenge to keep sensor node functional. To address power problem in WSN, we broadly have two solutions:
Currently, low power consumption in WSNs is a hot research area to increase the life cycle of sensor networks. Research has been conducted to improve/investigatelow-power MAC protocols (Lu C. et al., 2010) (Filip N. et al., 2011), routing protocols (Hua W.S. et al., 2014)(Raghunathan V. et al., 2005)transport protocols (Nacef A. B. et al., 2011), and even operating systems (Dash. S et al., 2012).
Another feasible option is to extract energy from ambient environment as the total power cost of wireless sensor nodes has been reduced to low level by now (Zhou H.-Y. et al.,2008) with the development of microelectronics, low-power design, and network technology (Vullers R. J. M. et al., 2009), and this give rise to adapt energy harvesting techniques for the provision of energy in WSNs.
Traditionally, conventional batteries used for power supply in WSN. Although batteries are common and reliable source for energy supply but they have limitation of size and aging even they are not used at the fullest. Function of the wireless node is to sense/compute energy from the environment in the form of heat, sound, pressure and to communicate neighboring nodes. It is good to capture operational energy from the ambient environment. Wireless sensors are useless when their battery dies, also batteries can cause a serious environmental hazard because batteries contain heavy metals and toxic chemicals for example mercury and lead. Moreover, it is expensive to maintain and replace batteries especially for the places difficult to reach such as environmental monitoring, biomedical fields and military surveillance. Life of the network depends on the life of the sensor nodes, which requires continuous power supply. Using batteries as a power is not suitable in long run, because average battery life is 1-18 months and they discharge 0.1 to 5% a month, the discharge rate will be increased if they exposed to high temperature (>40C) constantly and consistently. Batteries can be a good choice for the applications of wireless sensor networks if they require less energy conversion overhead. Due to short life span of batteries and other limitations, it is important to find an alternative energy source. Energy harvesting is a promising technique to power sensor nodes continuously at lower cost. However, it is important to consider environment, size and life of energy harvesting system. Different type of energy harvesters are available, their usefulness depends on the type of application. Some of the potential applications are reviewed below: