  We present ZEL, the first net-zero-energy lifelogging system that allows
office workers to collect semi-permanent records of when, where, and what
activities they perform on company premises. ZEL achieves high accuracy
lifelogging by using heterogeneous energy harvesters with different
characteristics. The system is based on a 192-gram nametag-shaped wearable
device worn by each employee that is equipped with two comparators to enable
seamless switching between system states, thereby minimizing the battery usage
and enabling net-zero-energy, semi-permanent data collection. To demonstrate
the effectiveness of our system, we conducted data collection experiments with
11 participants in a practical environment and found that the person-dependent
(PD) model achieves an 8-place recognition accuracy level of 87.2% (weighted
F-measure) and a static/dynamic activities recognition accuracy level of 93.1%
(weighted F-measure). Additional testing confirmed the practical long-term
operability of the system and showed it could achieve a zero-energy operation
rate of 99.6% i.e., net-zero-energy operation.
