I looked at that, the energy density seems too low. 10-20Wh/kg vs 260 for lithium ion batteries. For as the internet told me cargo ships carry 30 days worth of fuel. I think I figured flow batteries could power a cargo ship for a couple of days. Which isn't enough.
Interesting idea though in that you could pump the two chemical solutions into the ship. Sail it to it's destination and then pump the used fluids out and replace them with fresh. And it's not a bomb like lithium ion batteries.
I bet we are nowhere near the engineering limit of flow battery energy density. It is fairly limited in research compared to the various conventional battery technologies. I think a 5x increase in energy density over Vanadium redox (to 100-200 Wh/kg, 75 Wh/kg is achievable by some chemistries already) would put it in striking distance.
Interesting idea though in that you could pump the two chemical solutions into the ship. Sail it to it's destination and then pump the used fluids out and replace them with fresh. And it's not a bomb like lithium ion batteries.