An Exhaustive Analysis of Energy Harvesting Absorbers and Battery Charging Systems for the Internet of Things

Introduction

In the last few years, the demand for the power has increased enormously. Researchers are looking for alternate energy sources such as solar, wind, thermal, vibration and radio frequency (RF) for energy harvesting. Inexhaustible RF energy sources with zero harmful emissions can pave way for harvesting energy unlimitedly for powering low power sensors and microcontrollers. Internet of Things (IoT) enables to connect enormous sensor nodes for remote monitoring applications to the internet. RF Energy harvesting reduces the complexity of IoT nodes by avoiding power circuits, reduces the overall cost and improves the overall efficiency of the system. RF Energy harvesting or Green Energy harvesting is an excellent technique which allows size reduction in comparison with other harvesting systems such as photonic cells or wind turbines, where miniaturization really matters a lot in portable devices which uses battery (Almoneef, T. S. 2014). RF energy harvesting plays a key role in providing a sustainable energy source towards the future of wearable electronics too. Further, RF energy harvesting is reliable, portable environmental friendly and also cost effective. These advantages have attracted the researchers to unveil novel research in this field. A typical EHS consists of Antenna/absorber, matching circuit, rectifier circuit, charging circuit, battery and low power electronic devices such as sensors, microcontrollers etc. as shown in Figure.1.

Figure 1.

RF energy harvesting System (RFEH)

Generally materials are classified on the basis of permittivity (Ɛ) and permeability (μ) in the four quadrants as shown in Figure.2.

Figure 2.

Material classification

Double positive materials (DPS) with (Ɛ > 0; μ > 0) i.e. naturally existing dielectric materials lie in quadrant-I. In such materials, the direction of poynting vector and wave number is same. These are known as right handed materials (Almutairi, A. F. 2019). The refractive index is positive in these materials. Epsilon negative materials (ENG) with (Ɛ < 0; μ > 0) i.e. plasmas and metals at optical frequencies belong to quadrant-II. The wave travels in evanescent mode in these materials. Double negative materials (DNG) with (Ɛ < 0; μ < 0) are referred as metamaterials and lie in quadrant-III. The refractive index is negative in these materials and are also known as Left handed materials (LHM). The effective (Ɛ & μ) of a material can be tailored to design an artificial structure for any specific application. The real part of the permittivity Ɛʹ(ω) defines the dielectric constant of the material and the imaginary part Ɛʹʹ(ω) defines the attenuation provided by the material (Aminov, P. 2018). The real part of permeability μʹ(ω) defines the energy stored in the magnetic field and the imaginary part μʹʹ(ω) gives the measure of energy dissipation in the material for the incident magnetic field. by combining the structural elements of electric and magnetic response together in a unit cell a material with effective metamaterial properties can be designed. The electric and magnetic responses can be generated by coupling between metamaterial elements. Moreover, the flexibility of engineering ‘μ’ and ‘Ɛ’ values match the impedance of a designed metamaterial absorber with free space to reduce the reflection (Amiri, M. 2018). Subsequently, it leads to the perfect absorption at the desired frequency. To synthesize negative permeability was demonstrated as shown in Figure.3(a). The transmission response for SRR is represented by bold line and the response for the combined structure (thin wire and SRR) is shown by dashed curve in Figure.3(b). For the bulk structure, if SRR is alone present, there is no transmission around 5 GHz. But when SRR is combined with thin wire, energy is transmitted. The effect of the combined negative permeability and negative permittivity derives the energy transmission.