RE: Tony Abbott18 May 2019 13:28
However the question of whether the battery efficiency is the issue comes down to whether you are using a fossil fuel power station or a PV system to charge the battery. In the first case you have to pay for the fuel to charge the battery in the second case you would only be charging the battery with excess power from the PV system - and then the alternative would simply to discharge the electrical energy into a resistor bank, effectively just burning it off. In the second case the input energy should be considered as zero cost.
So a battery that is, say only 50% efficient might take twice as long to charge zero to 100% but when driven by PV, or wind, it doesn't actually cost twice as much to charge as 2xzero is still zero. What it would mean is that perhaps you only get to use the battery less often - but the benefit is not half that of a battery that is perfect 100% efficient.
How can you see this ? Well imagine that there is a potential for battery energy storage from the daily demand/supply imbalance eg there is 1 hour of excess supply due to the sun shining, and then a couple of hours later when everyone comes home from work there is excess demand and you would like to use energy stored from earlier. Now compare a 100% efficient battery with a 50% round trip efficient one.
Let's think about exactly this scenario and talk specific numbers - a 1MW battery with 1MWh energy capacity, battery A is 100% efficient whilst battery B is only 50% efficient.
You might think 'ohhh you'll always get only 50% of the energy back out of battery B as compared with battery A' but no it depends on the amount of excess and demand you have.
i) if it is less than 1MW then neither battery charges fully in the 1 hour charging opportunity you have. B gives you only half of what you ultimately get from A
ii) if it is more than 1MW but less than 2MW then battery A charges fully whilst battery B is perhaps still increasing its charge. Eg for exactly 2MW battery A is fully charged after only half an hour then it spends the next 30 minutes sitting there like a greased bodybuilder waiting to strut its stuff, but a bit of a lump nonetheless. In the second half hour battery B is catching up and by the time the first hour is up it too is fully charged. Tortoise and hare.
iii) Above 2MW both batteries enter the discharge phase fully charged A gives you 1MWh hour back, battery B maybe only 71% (0.71x0.71 = 0.50) single trip efficiency.
So only when the energy excess is less than 1MW does the perfectly 100% efficient battery do twice as well as the 50% efficient one. At the other end of the spectrum it ends up doing significantly better than 50%.
I appreciate that this may seem counterintuitive to some but it is a fact and it is one of those that we will have to get used to explaining over and over again in the next 20 years
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