# Comparison Among Different Rainfall Energy Harvesting Structures
## 1 Introduction  
Research in the field of electrical engineering has been long engaged in the identification of new forms of energy production from common and everyday life sources. One of the latest techniques in this exploration phase is the conversion of mechanical energy, otherwise lost, into electricity by the employment of energy harvesters. This conversion creates micro-productions of electrical energy caused by vibrations of piezoelectric transducers, which is then used to feed sensors, or electronic devices with small energy consumption (e.g., wireless sensors used for monitoring applications, which still need chemical batteries with high costs of maintenance and disposal). Piezoelectric materials have a lack of symmetry in the crystal lattice, leading to dipole moment generation, which is also a function of the stress acting on the structure [1]. The lattice structure allows the fabrication of devices in both the macro and in the micro scales, overcoming the common limitations due to the number of windings to be used in the electromagnetic transducers employed in the same field [2; 3]. It can be stated that the kinetic energy of the ambient vibrations (which represents an energy normally wasted in the environment without a useful purpose) can be coupled to electromechanical transducers in order to feed electronic monitor apparatus. The vibration energy can be regarded as marginal energy due to its small entity, but it can be suitably adopted for utilities of reduced energy capacity, such as the transmission of information, as in the case of automotive tires [4]. Other studies have traced ways to recover energy from renewable sources [5; 6]. [PERSON] et al. faced the rainfall energy harvest as well [7; 8], with the idea of converting the kinetic energy possessed by the drops of rainwater into electrical energy by means of piezoelectric plates. A pioneering comparison of different piezoelectric materials finalized to investigate the possibility of energy generation by water droplet energy sources for low-power electronic devices was studied by [PERSON] et al. in [5]. [PERSON] [7; 8] has demonstrated that a single raindrop with diameters ranging between 1 and 5 mm can generate up to 12 mJ/s, whereas Italian studies have shown that the piezoelectric transducer-based conversion system produces 0.43 \(\upmu\)J of energy for each raindrop impact [9; 10; 11; 12; 13]. A recent work of [PERSON] [14] introduces different possibilities to harvest raindrop energy with piezoelectric and triboelectric transducers and evaluates the energy potential due to rain parameters. [PERSON] et al. [15] with a pioneering work suggested a recover energy of 30 nJ of electrical energy due to a droplet of 30 mL. These results leave some uncertainty on the actual capability to generate an adequate amount of electrical energy from rain. A complete overview of the possibility of harvesting energy in different scenarios is difficult to provide, since novel approaches and innovative ideas are continuously being developed. During recent years, particular attention has been given to the possibility of harvest energy from the raindrops by researchers from the region of Malaysia [16; 17; 18; 19; 20] and Bangladesh [21]. As reported in [21], in Bangladesh in June, July, August and September, there is maximum downpour; a comparison with the case of France is reported. In France, the available rain power is almost 1 Wh per square meter per year. In Bangladesh in the monsoon period, the amount of raindrops in mm varies between 1852 to 3082. If 15 raindrops 5 mm in size fall per second on a square meter of flat surface in Bangladesh (i.e., 900 raindrops per minute), the impact energy is equal to 0.18 watts. By considering four months, the potential energy can reach some kWhs each year. In [16] it is reported that for tropical countries such as Malaysia, Indonesia, Philippines, Myanmar, India, Panama, Brazil, and Nicaragua, the yearly rain rate is more than 2000 mm per year.