## 1 Introduction  
Presently, smart electronic systems and wireless sensor network technologies are widespread in home and industry environments [1; 2; 3]. Their influence over people's daily activities such as health and well-being, security, education, entertainment has increased substantially [4]. Moreover, there is taking place a change in automation and control process as smart sensors for communication, control and interoperability are being integrated within enterprise business systems [3]. Furthermore, efficiency and interconnectivity of the Industry [5] are improving with the aid of the aforementioned technologies and systems. Nowadays, smart electronic systems and wireless sensor networks must be whole in order to achieve low cost and energy efficiency, which is one of the main features [6], especially when aiming for green electronics [7]. Therefore, energy harvesting becomes the most appropriate sources of energy for these electronic systems. Furthermore, energy harvesting systems have become a key technology to gather energy from the environment and give answer to some of the technical challenges that face the deployment of wireless smart sensor networks [8; 9], namely their independence from the mains power grid [10]. An energy harvester is a transducer devoted to obtaining the maximum amount of energy from a given physical phenomenon [11]. Usually, an energy harvesting system has three main elements [12]; a harvester, a low power management system, and a low power storage system. The harvester's characteristics determine the type of power conversion required: Direct Current/Direct Current (DC/DC) or Alternate Current/Direct Current (AC/DC) [13], which sets the specifications of its architecture and conditions the power device selection. Summarizing, there are harvesters that collect energy in DC form and others that provide AC signals, such as wind or liquid flow harvesters. Consequently, the power converter of the system must be suited to the waveforms and energy that the harvesting technology targets. Hence, designing a whole energy harvester system [14; 15] requires specific research of the available converter. The core idea of dynamic fluid harvesters is to use large-scale generator technology principles in small-scale generators [16]. Furthermore, currently three types of generators are used to harvest energy from the wind [17; 18]: electrostatic, piezoelectric, and electromagnetic. The electrostatic harvesters base their operational principles on the friction between electrocharged materials, which induces variations of capacitance [19]. This charge is converted into electricity thanks to electrodes [20]. However, these generators are not very efficient, and the power output levels they provide are low compared with other technologies listed in [18]. The piezoelectric (PZT) harvesters base their operation principles on a vibration and deformation of piezoelectric materials [16]. Early PZT harvesters used a mechanical kinetic source to generate energy, but the state of the art PTZ harvesters also use them as the energy source [21]. These harvesters include mechanical parts into the PZT material to optimize and increase its efficiency [22]. The electromagnetic (EM) harvesters base their operational principles on the Faraday and Lenz laws. The rotation of magnets produces an electromagnetic field that is harvested and converted to electrical energy with inductive coils [18]. On the one hand, PZT harvesters produce more energy when the resonance frequency is achieved, but on the other hand, EM harvesters increase the power generation when the rotational or linear velocity is increased on the harvesting device [23; 24]. One of the challenges with small electromagnetic generators is that, due to the reduced size, the output voltage level only reaches a few hundreds of millivolts in AC, but the operational electronics need a few volts (i.e., 3.3 V) in DC to work.