RF-Powered Wearable Energy Harvesting and Storage Module Based on E-Textile Coplanar Waveguide Rectenna and Supercapacitor

Abstract

This paper presents a high-efficiency compact ( $0.016\lambda _{0}^{2}$ ) textile-integrated energy harvesting and storage module for RF power transfer. A flexible 50 $\mu \text{m}$ -thick coplanar waveguide rectenna filament is integrated with a spray-coated supercapacitor to realize an “e-textile” energy supply module. The meandered antenna maintains an $S_{11}< -6$ dB inside and outside the fabric and in human proximity with a 2.3 dBi gain. The rectifier achieves a peak RF-DC efficiency of 80%, across a 4.5 $\text{k}\Omega $ load, and a 1.8 V open-circuit voltage from −7 dBm. The supercapacitor is directly spray-coated on a cotton substrate using carbon and an aqueous electrolyte. When connected to the supercapacitor, the rectifier achieves over an octave half-power bandwidth. The textile-integrated rectenna is demonstrated charging the supercapacitor to 1.5 V (8.4 mJ) in 4 minutes, at 4.2 m from a license-free source, demonstrating a significant improvement over previous rectennas while eliminating power management circuitry. The integrated module has an end-to-end efficiency of 38% at 1.8 m from the transmitter. On-body, the rectenna’s efficiency is 4.8%, inclusive of in-body losses and transient shadowing, harvesting 4 mJ in 32 seconds from 16.6 $\mu \text{W}$ /cm2. It is concluded that e-textile rectennas are the most efficient method for powering wearables from $\mu \text{W}$ /cm2 power densities.

Keywords

• Antennas
• energy harvesting
• flexible printed circuits
• impedance matching
• microstrip antennas
• rectennas