RegGuheert wrote:... and can achieve a 70% round-trip efficiency[/url]. That's the kind of performance we need to achieve to make long-term storage a viable possibility.
Long term storage is more driven by cost than efficiency. The opposite of short term storage.
Let me try out an example to illustrate:
$0.10 primary generation from solar cells. 10% ROI + expenses.
Li-ion batteries, $100 per kWh, 90% round trip efficient.
Flow battery, $1 per kWh, 25% efficient.
Cost for a short term storage: assumed to be once per day.
Battery total cost = energy cost + capital cost = 0.10/0.9 + 100*.1/365 = $0.14/kWh
Flow total cost = energy cost + capital cost = 0.10/0.25 + 1*.1/365 = $0.40/kWh
Cost for long term storage: assumed to be once per year.
Battery total cost = energy cost + capital cost = 0.10/0.9 + 100*.1/1 = $10.11/kWh
Flow total cost = energy cost + capital cost = 0.10/0.25 + 1*.1/1 = $0.50/kWh
To build a system with a minimum cost, some fraction of the storage should be low cost (and perhaps low efficiency) and some should be high efficiency and higher cost. Lowest cost will come with some amount of excess generation, enough short term storage to cover daily to a week or so, and some lower efficiency longer term storage.
This is the point to something like hydrogen fuel cells, which is a type of flow battery. Flow batteries are generally not potentially useful for cars, as the potential low cost technologies have low specific energy densities (kWh/mass).