"Flow venders (sic) have to convince customers that they will still be around for the multi-decade lifespan of the devices. That's a hard sell to financiers as well. But having a brand like Mitsubishi Hitachi stand behind the technology would be a major vote of confidence."
No offence taken Paludina, its just ships effectively have ready made electrolyte tanks and some of the ballast or cargo weight could be substituted for electrolyte.
Now with ships, you can have sails! https://www.theguardian.com/environment/2017/mar/14/spinning-sail-reboot-cut-fuel-make-ocean-tankers-greener Can save 10% of the energy. Now these large ships have an awful lot of room for ballast, up to 200,000m3 for large tankers. If you could put aside a chunk of ballast for electrolyte you could be storing 15-25kWh/m3 1/2 the ballast would be That ferry was ~60m and could do 40km on a 4.3MWh battery, big tankers are about 10x larger, but increasing size does not need a proportional increase in power. Ferry uses 0.1075 MWh/km, say a large tanker uses 0.86MWh/km. !/2 the ballast as electrolyte and getting say 20kWh/m3 gives you a 2000MWh storage gives you a range of 2300km