According to the designers ”The system uses an external water tank, hence the Plantbook continuously absorbs water when soaking it in water and generates electrolysis using power stored in a solar heat plate installed on the top. In this process, it is operated using hydrogen as energy source and discharges oxygen. If you put it into a water bottle while you don’t use the laptop, it automatically charges a battery and discharges oxygen. A leaf-shaped strap hanging on the top is made with silicon. It plays a role of a hand ring and a green LED indicates when the battery is charged. Using this LED, users can check how much spare capacity the batter has”.
Source : yankodesign