The structure of a mobile device or a sensor node in WSNs is usually system-dependent but generally comprises the following four elements: a power supply unit, a set of sensors and/or actuators, a processing unit (usually realized with a microcontroller) and a wireless communication system.
Thus, in the end, the design of such systems must be carried out with the aim of power consumption minimization. Of course, a possibility is to use higher capacity batteries to ensure the required energy constraints are met, but this results in increased size for the device that is often not acceptable. In many of these systems, such as WSNs deployed in inaccessible areas or medical devices implanted in the human body, the replacement or recharge of depleted batteries is not feasible, so the device must be designed to ensure that the energy provided by battery is enough for the system’s entire anticipated lifecycle. In general, mobile devices are powered by battery. Moreover, the large diffusion of smart mobile phones (smartphones) that integrate powerful microprocessors with high processing capability, wireless communication protocols and a rich sensor set, makes the smartphone an optimal platform for sensing applications. In recent years a strong increase in the adoption of mobile sensor systems was reported in different fields of application, such as: quality assessment of olive oil, electronic noses for food quality determination, characterization of tomato paste, determination of solid fat content in vegetable oils and fats, characterization and production control of ice-creams, environmental monitoring, particulate concentration measurement, toxic volatile organic compound detection, microbial analysis, corrosion detection in metal structures, control of industrial processes, characterization of saline solutions and forensic analysis. In this paper, the main strategies to design a wireless mobile sensor system powered by energy harvesting are reviewed and different sensor systems powered by such energy sources are presented. Since the presence of these energy sources is discontinuous in nature, electronic systems powered by energy harvesting must include a power management system and a storage device to store the scavenged energy. A very interesting alternative to power mobile devices is energy harvesting where energy sources naturally present in the environment (such as sunlight, thermal gradients and vibrations) are scavenged to provide the power supply for sensor nodes and mobile systems. Moreover, the depleted battery must be properly disposed of in accordance with national and international regulations to prevent environmental pollution. This poses important problems, in particular for sensor nodes that are placed in inaccessible areas or biomedical sensors implanted in the human body where the battery replacement is very impractical. Wireless sensor network nodes and mobile devices are normally powered by batteries that, when depleted, must be recharged or replaced.