Storing The Wind

Would storing wind power sort out the intermittency problem ?

One of the solutions offered for wind intermittency is storage. Let's take the recent calm spell, which was interspersed with very high winds, to see if storing wind would balance out the variances and lead to a more stable predictable electricity supply.

Let's remind ourselves of what the recent wind output looked like (Figure 1) :

Figure 1: Raw wind power which is what we have at present

We can see from the 23rd to 30th September, there was some very high wind output, and again at 4th till 7th October. This was interspersed with a very calm spell lasting a couple of days. After the 4th, there was a long period of poor winds lasting about 12 days.

So what would happen if we balanced out this wind variation to provide more stable constant output ?

Assumptions

There are a number of storage options - pumped storage, battery or compressed air. For this study, I assume a 500MW storage device is built and 1MW of stored power produces 1MW output when needed at all times. As far as I know, only pumped storage can provide this type of reliable storage. My understanding is that a battery produces less output when it's stored power is running low than it does at full charge i.e. the 1MW you put in does not produce 1MW 20 days later.

The other assumption made is that once wind power goes above 500MW, the surplus power is stored. When wind goes below 500MW, the storage device makes up the difference to bring output back to 500MW. Only wind power is stored.


Results

In a perfect world, this would result in a constant wind plus storage output of 500MW. However, as you can see below (Figure 2), there is a blip in the storage output at around 3 days in on the 25th September. But output really hits a brick wall on the 1st October, at 9 days in.

Figure 2: Output from a storage unit powered solely from wind

At this point, there is a sudden drop in power from 500MW to 335MW and then to 11MW that lasts a full day as stored power runs out. Then we get reliable output for the next four days but the battery runs out again on the 6th October (two weeks in) and we are once again at the mercy of the wind. At one point it reaches 5MW. This lull in wind and storage power lasts for 12 days.

On the 18th we get good winds again and the battery begins re-charging but the problem is you can't use this power when it was most needed i.e. a couple of weeks prior. 

Figure 3 shows the contribution of the wind plus storage unit towards demand. The storage unit actually comprises 2,500MW of wind and 500MW storage, so 3,000MW in total. For 3,000MW of capacity, as you can see, you only get very small amounts of power out - about one seventh or 15% of peak demand - when the unit is running at full output i.e. 500MW. This compares very badly with gas powered stations (or indeed coal, biomass or any dispatchable plant) which could power 85% of peak demand for the same amounts of capacity.

Figure 3: Contribution from the storage unit towards demand

To add further insult to injury, you still get unreliable and intermittent amounts of power for large portions of the month - 43% of the time in this scenario. So we are back to square 1. 

Conclusions

Storage doesn't work. There are simply too many calm days to make it a reliable worthwhile solution. It's no wonder the Spirit of Ireland proposal, now dropped, intended to use grid power to move water uphill, rather than raw wind power.