**3. Far-field wireless power transfer**

In the far-field range, the power is transmitted through microwaves and in practice has been developed for low-power applications, because of its low efficiency. However, despite the low intensity the light rays are able to transmit the power. For instance, Sun rays can generate large amounts of energy in spite of travelling enormous distances. Similar to the other far field sources, the power generation occurs in specific conditions and in large amounts. A great use of this technique will be the solar farms in large areas of Saudi Arabia, which are able to generate nearly the same level of electricity per year compared to the traditional power generation stations [15].

The Radio Frequency (RF) signals have powered very low power application and is more considered as a harvesting energy solution. The ultrasound waves and vibrations are also utilised in similar applications. The waves are converted through the piezoelectric effect as transducer for electrical signal and are considered as energy harvesting from the environment.
