ZEL+: Wearable net-zero-energy lifelogging using heterogeneous energy harvesters for sustainable context sensing

This paper presents ZEL+, a wearable lifelogging system designed to operate with net-zero energy consumption by leveraging multiple energy harvesting technologies for continuous context sensing. Self-powered wearable devices often encounter difficulties in environments with inconsistent or low-intensity energy sources, particularly indoors. To address this, ZEL+ incorporates three key design features:First, it employs a power-switching mechanism based on dual comparators and a capacitor to manage surplus energy and maintain operation under varying lighting conditions.Second, the system integrates heterogeneous energy harvesters not only for power supply but also for sensing ambient context. Specifically, a dye-sensitized solar cell is used to capture stable lighting patterns in low-light indoor settings, while an amorphous solar cell reacts dynamically to environmental light changes—both contributing to location and context recognition. Additionally, a piezoelectric element is included to complement motion-based features, providing supplementary cues for activity recognition.Third, a Spatial Consistency-Based Correction (SCC) algorithm is used to mitigate inconsistencies in location estimation caused by transient recognition errors, supporting more coherent lifelog construction.The system is implemented as a 192g nametag-shaped wearable device and evaluated in an office environment with 11 participants. The evaluation yielded F-measures of 94.7% for location recognition across eight office areas and 96.7% for activity classification. Energy performance analysis showed that the system maintained autonomous operation using harvested energy for approximately 94% of a standard 8-hour workday. These results suggest that ZEL+ can support practical, energy-sustainable lifelogging in indoor mobile computing scenarios.