  Hydraulic energy is a key component of the global energy mix, yet there
exists no practical way of harvesting it at small scales, from flows at low
Reynolds number. This has triggered the search for alternative hydroelectric
conversion methodologies, leading to unconventional proposals based on droplet
triboelectricity, water evaporation, osmotic energy or flow-induced ionic
Coulomb drag. Yet, these approaches systematically rely on ions as intermediate
charge carriers, limiting the achievable power density. Here, we predict that
the kinetic energy of small-scale "waste" flows can be directly and efficiently
converted into electricity thanks to the hydro-electronic drag effect, by which
an ion-free liquid induces an electronic current in the solid wall along which
it flows. This effect originates in the fluctuation-induced coupling between
fluid motion and electron transport. We develop a non-equilibrium thermodynamic
formalism to assess the efficiency of such hydroelectric energy conversion,
dubbed hydronic energy. We find that hydronic energy conversion is analogous to
thermoelectricity, with the efficiency being controlled by a dimensionless
figure of merit. However, in contrast to its thermoelectric analogue, this
figure of merit combines independently tunable parameters of the solid and the
liquid, and can thus significantly exceed unity. Our findings suggest new
strategies for blue energy harvesting without electrochemistry, and for waste
flow mitigation in membrane-based filtration processes.
