## 3 Transfer and Harvesting Energy Metamaterials
### Energy Harvesting Metamaterials  
Energy harvesting is the process of capturing and storing energy from the environment in order to use it later. Metamaterials, due to their specific electromagnetic properties, have shown great potential in the field of energy harvesting. They can be used to create devices that can more efficiently capture and convert energy from sources, such as solar [50], wind [51], kinetic [52], or thermal [53; 51]. Metamaterials in such devices control the way that energy is absorbed and scattered. In this paper are listed a few applications of MM in the energy harvesting field, such as solar cells, wind turbines, or thermal management, but these are just a few of many more.  
Most energy harvesting technologies are now focused on the areas of RF signals and microwaves. Metamaterials can be used to create devices that convert EM signals into electricity. These devices can be used to harvest specific RF signals or microwaves, but they can also take lost signals from sources, such as wireless networks or television signals. What makes these technologies sustainable is that taking the lost radiation from the environment makes the economy circular.  
Microwave energy harvesting devices usually use antennas and other components to capture and convert energy from microwaves to electricity [54; 55]. Microwave energy harvesting devices typically use rectifying antenna (rectenna). These rectennas consist of a patch antenna and a diode. The patch antenna captures the microwave energy, and the diode rectifies it into DC electricity [56]. The way to harvest energy from microwaves through the use of metamaterials is to apply them to the system, where MM can control the way that energy is absorbed and scattered. Let us note that the amount of energy that can be harvested from microwaves is relatively small, and the efficiency of the energy harvesting devices is still relatively low. Therefore, it is mainly used as a supplementary source of energy, rather than a primary one.  
[PERSON] et al. [57] demonstrated a RF energy harvesting rectenna design based on a metamaterial perfect absorber. With the embedded Schottky diodes, which are often used in wireless communication systems, the rectenna converts captured RF waves to DCpower. He pointed out that optimal RF energy harvesting occurs when the metamaterial is constructed, such that a high Q-factor resonance occurs at the same frequency as the external radiation source. [PERSON] confirmed that later, in [58], and [PERSON] pointed out that a perfect metamaterial absorber makes a promising solution for collecting ambient RF energy in low-power density environments because it is tunable, highly efficient, and electrically small [58].  
[PERSON] et al. [56] conducted research, which proved that energy high-frequency electromagnetic field absorbers, based on resonance MM, constitute a competitive alternative to conventional antenna systems. They achieve efficiency exceeding 90%, which is more than five times higher than the conventional dipole-type antenna arrays. He also pointed out that practical application of MS structures is easier than classic antenna systems due to greater flexibility in the geometry. Resonant MM can also be used as high-efficiency couplers and receivers in the microwave wireless power transfer [59].  
The amount of energy that can be harvested from RF or microwaves depends on several factors, such as the intensity of the signal, the energy efficiency of the device, and the size of the energy storage device. In general, it is possible to harvest enough energy from RF or microwaves to power small electronic devices, such as wireless sensors, but it would not be sufficient to power larger devices or to charge a battery. The harvested energy is usually used as a supplementary source of energy, rather than primary one.