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Thanks all. Gotcha Cowboy.
I’d be worried about that level of loss if we were a lithium battery co. mind you. We know what happens when you don’t fully discharge and charge those up.
Good news this week hopefully (fingers crossed emoji).
My Lithium car has 1-2% 'Vampire drain' every night when left idle. Doubt those figures Drifter mentioned will occur in real life but the upper end of possibilities.
the end result of this discussion is that only in the worst case scenario or an undrained stack is there any self-discharge in a VRFB - even in that case it is only the electrolyte that is stuck in the stack that would be affected and this is a tiny fraction of the overall electrolyte. So BE were simply quoting the very worst case figure they could find so as they were being conservative or didn't want to get involved in complex caveating whether the VRFB stack is drained or not.
Having said that as I've explained it is a non issue because you're unlikely to be keeping a VRFB fully charged for more than a couple of days - you're certainly not going to be keeping it charged for months at a time waiting for an overcast day.
In the grand scheme of things, I don't suppose it matters one bit. It just shouldn't happen, unless the power stack is left flooded. The original question was, how long could a VRFB hold it's charge? My answer was, effectively for ever, then Drifter said that the rate of self-discharge was 2.5% per day. I queried this. As Alfacomp then pointed out, the rate of self-discharge will be effectively zero if the power stack is drained. So I stand by my original answer.
Indeed 1210 - the typical way in which a grid scale or commercial and industrial (C&I) battery would operate would be as a grid-tied asset which can charge from the grid once or twice a day or as an isolated system connected to a PV system when 1 charge per day would be the best you could get. The VRFB's may be long-duration storage in that they may have capacities of 4-10 hours which is long in that it is longer than the 1 hour typical of Lithium-ion cells.
This is not to be confused with long-term energy storage - which is the kind of thing that you would want to be able to deal with interseasonal rather than intra-day peak shifting. Here the interseasonal energy storage system is charged and discharged once per year. Which such a low rate of usage it would take any electrochemical battery a few millenia to pay for itself, even if you could charge it up in the summer and retain all the charge until the winter.
A battery is not the right sort of technology for that application - instead a relatively expensive machine that can convert cheap raw materials from one low energy form into another easily storable higher energy form is the sort of thing that would be required. Better still if the higher energy form can be used in transportation systems as a movable energy store that you can easily ferry around - so perhaps ethanol or ammonia before methane and hydrogen.
1210. My angle was this; If there was a week of windy, sunny days, if all of the excess energy could be stored it could then be used in the future during a week that is cloudy and still. Obviously not practice if the energy previously stored has dissipated before you want to use it.
Sorry if I am missing something - why would it matter if a vrfb lost circa 2.5% of charge when doing nothing? Surely that is the point of the solar array or similar that it is attached to... to keep it topped up / full. And importantly, charging even when it is in use which cannot be done with lithium. Under what circumstance(s) would a vrfb be left sat idle without being connected to an array or similar?
Indeed those numbers from Drifter88 seem very high to me.
As reteps says there should be essentially zero loss in energy when the electrolyte in the tanks is kept separate and away from one another - this self-discharge figure will only be for the electrolyte that is sitting inside the power stacks. Some VRFB systems automatically drain the power stacks back to the storage tanks when they are not running - this then naturally eliminates any self-discharge of the electrolyte that is in the power stacks. The only downside to doing this is that it then takes some time to fill the stacks (10 seconds ?) before being able to switch the battery back on - it depends then on whether your battery is intended to provide fast frequency response services (sub 10s) to the grid or not, as to whether you can operated with the self-draining stack design or not.
There is of course no option with Lithium-ion to optimise anything like this to reduce self-discharge.
Where did you find those figures, Drifter? They both seem high to me. VRFB self-discharge should be effectively 0% over 24 hrs . How can electron movement take place when the electrolyte is in two separate tanks?
That is why VRFB are great for black start batteries
Short answer, it will keep for ever. Actually, it will keep for as long as the electrolytes stay in the system unmixed. You're more likely to spring a leak in the tanks or pipework than anything else.
Hi. I was thinking and wondered if anyone might know the answer to this;
If I charge my vrfb via my renewable energy source. How long can the vrfb hold its charge for? Would it still have a full charge or will it reduce over time? Will the electrolyte sit quite happily as V2/V5?
What I was thinking was, if you had enough batteries, could you store wind energy and keep it for cloudy/still days that might occur in a weeks/months time?