RE: VRFBs7 Sep 2019 13:17
Lionel - much of the research presented at the recent UK Energy storage conference in Newcastle was modelling of ways to use Lithium-ion batteries in an economic way taking into account their inevitable degradation mechanisms.
If you simply ignore degradation then the optimal operation protocol is 100% hard on then 100% hard off any time there is a small difference between the current price of energy and the price that you predict to come up in the not too distant point in the future. Needless to say this wrecks a Lithium-ion battery pretty quickly. Why ? Because LiB degradation is higher at higher states of charge, and higher temperatures, and high currents produced high temperatures ...
A more intelligent algorithm goes a bit easier on the battery, requiring somewhat larger price differentials to be established before the Battery Management System says 'yes let's go for it'. This of course reduces the number of times you can charge-discharge the battery each particular day, which not unsurprisingly means it lasts somewhat longer.
All pretty obvious stuff that simply goes to show that if you give some people a hammer they think all problems are nails.
This analysis of course needs to be completely redone for a battery which does not degrade in such a way with repeated electrical and thermal cycling. I can tell you qualitatively what the results will be - VRFB's will be shown to be far superior as long term energy storage assets, despite their slightly lower DC-DC efficiency. Put simply you will need to pay for a little more electricity to charge up a VRFB to 1MWh energy, so costs a little more per charge, but this is simply put against the fact that each time you charge the LiB it costs you another 1/N th of your battery CAPEX cost, where N is the number of cycles that the LiB lasts. (after N cycles you have to completely replace the DC battery part).
Lazard quotes 3,500 cycles for Lithium-ion batteries in their LCOS analysis however I think this is an optimistic estimate taken from LiB manufacturers most optimal charge-discharge schedules. The data presented in Newcastle for real-world grid storage usage shows that the capacity will have fallen to 90% after around only 1300 cycles.
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