If energy from the grid costs 13 cents/kW-hr and a 1000 cycle battery costs 27 cents/kW-hr, and diesel is 40 cents/kW-hr, then a 33% efficient energy storage device that is capable of thousands of cycles over its lifetime suddenly starts to look interesting.
Maybe it's possible to trade a little efficiency for higher energy density or lower initial costs.
For example, the flywheel below is 86% efficient but has an energy density of only 5.5 W-hr/kg.
It would be cost effective to find something that was only 50% or 40% efficient but with 10X the energy density.
Bret Cahill
System Description Continuous rated power 50 kW Peak power 120 kW Charge or discharge time 60 seconds at 50 kW Mass 135 kg Volume 110 litres Diameter 24 in (610 mm) Height 18 in (460 mm) Specific power 370 W/kg (continuous)
890 W/kg (peak) Specific energy 5.5 Wh/kg Typical DC interface 600 - 750V Self-discharge rate 0.5 - 1.0 kW Typical Efficiencies at Rated Power Motor/generator 98% Inverter 95% Net one-way 93% Net round-trip 86% Flywheel Rotor General description Composite flywheel rim with aluminum hub Operating speed range 15,500 rpm - 31,000 rpm Polar inertia 0.720 kg=B7m2 Energy storage 1,000 Wh @ 31,000 rpm 250 Wh @ 15,500 rpm Net energy storage 750 Wh Motor/Generator General description Permanent magnet, 3-phase, synchronous, liquid cooled Frequency range 515 Hz - 1,030 Hz Line-line voltage 230 Vrms at 515 Hz 450 Vrms at 1,030 Hz Total inductance 100 =B5H Line-