Capacitor energy storage, dielectric materials, capacitor banks for grid applications.
W = ½·C·V². Power-factor correction (kVAR = P·(tan θ_old - tan θ_new)), tuned LC harmonic filters, DC-link capacitors in inverters, supercapacitors (5-10 Wh/kg, 5-10 kW/kg, >1M cycles).