A SELF-POWERD BACKPACK FOR WEARABLE SYSTEMS

Abstract

The solar-powered backpack described in this paper provides a portable charging solution for people who are frequently outdoors, such as hikers, students and travelers. Four prototypes were built and experimentally evaluated. Prototype-1 carries the photovoltaic cell alone. Prototype-2 places a rubber water bladder filled with tap water behind the cell, prototype-3 fills the same bladder with iced water, and prototype-4 fills it with hot water. Varying the bladder temperature allowed the effect of rear-surface temperature on cell output to be measured while keeping the water available for drinking. Each prototype was characterized outdoors in direct sunlight, outdoors in shade, and indoors, with the load resistance swept from 47 Ω to 10 MΩ. The highest output, 1223.3 mW at a load of 73 Ω, was obtained from prototype-1 and prototype-2 outdoors in direct sunlight. Cooling the rear surface did not improve performance: prototype-3 delivered 905.4 mW and prototype-4 634.0 mW under the same conditions, indicating that the cell was already operating near its optimal temperature and that further cooling moved it away from that point. Prototype-1 was then connected to BQ25570 and LM2596 power management circuits and used to charge a 2400 mAh battery, the LM2596 reaching 2.4 V in 5 min against 11 min for the BQ25570.