  Electric energy generation from falling droplets has seen a hundred-fold rise
in efficiency over the past year. However, even these newest devices can only
extract a small portion of the droplet energy. In this paper, we theoretically
investigate the contributions of hydrodynamic and electric losses in limiting
the efficiency of droplet electricity generators (DEG). Noting that the
electro-mechanical energy conversion occurs during the recoil that immediately
follows droplet impact, we identify three limits on existing droplet electric
generators: (i) the impingement velocity is limited in order to maintain the
droplet integrity; (ii) much of droplet mechanical energy is squandered in
overcoming viscous shear force with the substrate; (iii) insufficient
electrical charge of the substrate. Of all these effects, we found that up to
83% of the total energy available was lost by viscous dissipation during
spreading. Minimizing this loss by using cascaded DEG devices to reduce the
droplet kinetic energy may increase future devices efficiency beyond 10%.
