## 2 Background
### Energy Harvesting  
Energy harvesting techniques are different sets of processes of converting the ambient energy present in the environment into electrical energy to be used in powering wireless sensors or embedded devices [14]. These technologies often supplement batteries in situations where their integration is feasible. Energy harvesting become popular in wireless sensor technology and autonomous devices due to the nature of the environment these devices operate in. For example, devices that are deployed in drones, underwater, and so on, cannot have access to any physical power source; as such, harvesting energy from the environment becomes necessary. This technique provides energy ranging from microwatts to megawatts. When selecting energy-harvesting techniques, factors such as the nature of the environment, the source of available energy nearby, and the energy consumption of the devices have to be put into consideration before a particular method is selected. According to [PERSON] et al. [15], energy-harvesting techniques according to the source of energy can be classified into seven (7) categories, namely, solar energy, wind energy, vibration energy, thermal energy, ocean wave energy, acoustic noise energy, and radiofrequency energy. However, in this paper, we only discussed the four most popularly used methods:  
1. Solar Source: This is one of the most common and easiest methodologies of harvesting energy from sunlight and converting it into electric energy. The major component here are photovoltaics, which can be arranged in different forms, depending on the design. These photovoltaics are solely responsible for collecting the energy from sunlight and converting it into electric energy. The solar energy harvester can be used with batteries or directly to the equipment it needs to power, with the help of inverters which regulate the power into an appropriate type, suitable for the available devices [16]. 2. Wind Source: The wind available in our surroundings can also be tapped and produced energy from it using electromechanical conversion methods. The system works with turbines popularly known as turbine generators which are mechanical devices that are installed in the direction of the wind. These turbines have a rotor and spines, which rotate with the power of wind speed. The movement generates mechanical energy, which can be collected and converted into electrical energy [17]. The wind turbine generator converts mechanical energy to electrical energy. The generator has to work with a power source (the wind turbine rotor), which supplies fluctuating mechanical power (torque). Inverters have to be integrated into the wind energy harvester system to convert electricity generated by the system into compatible power needed by the devices. 3. Vibration Source: Energy harvesting from a vibrating source is the art of converting vibrational energy to electrical energy. The technique works with different types of converters and technologies such as electromagnetic, electrostatic, piezoelectric, and triboelectric, as listed by [18]. Many vibration sources exist, such as ocean movement or the motion of people, which can be utilized and harvested for an effective energy source. The majority of the techniques used electromechanical converters, while others used piezoelectric generators which convert mechanical energy generated from the vibration source into electricity. 4. Thermal Source: Thermal energy harvesting is another method of cultivating energy from the environment by capturing free heat and converting it to electric energy using special technologies such as thermoelectrics or pyroelectrics. This heat exists in two forms, either naturally occurring heat, for example, heat from volcanoes refuse dumps, or artificially generated heat, usually as waste from industries [19]. With the above-mentioned methods for harvesting energy from ambient sources, the question is, which method is simpler to achieve and which one suits our deployment environment?