Saturday, 13 December 2014
Parasitic Consumption of Wind Turbines
The below post is intended for debate purposes only and conclusions are based on evidence made publicly available. I do not have access to any commercial sensitive data or other data not made publicly available that can confirm the following conclusions.
An interesting element of the current energy policy is that, with all the new wind farms scattered around the country, the demand for power has risen among our generating fleet, whilst in the other sectors of our economy it has decreased. The power used from the grid by generating units is known as house load or parasitic power.
We can show how much parasitic power is now potentially been used by our wind turbine fleet by looking at data from Eirgrid on a windless day(s). During these days, wind turbines will often used grid power to continuously turn their naccelle to face the direction of any small gusts that may arise in the area. This is in contrast to a strong windy day, where the prevailing wind will be coming from one direction, so the nacelle will remain fixed in that direction for as long as the weather system lasts. They also use grid power in calm cold conditions to ensure that the blades don't freeze.
To demonstrate this, first of all lets look at the demand profile for two periods of 4 days (Friday to Sunday) in October 2013 and October 2014. Each trough and crest represents one day.
As expected, the demand is identical for both years. The first two weekdays in both years reach nearly 4,000MW at peak, and at the weekends reach about 3,500MW. There are no large industries that have relocated here in the meantime, nor any serious economic upturns, or booms, that could result in larger demand for this year. Ireland's economy is still sailing along in post-recessional / bailout mode. By the way, we wouldn't be expecting any large energy intensive industry to have relocated here because of our soaring electricity prices but that's another story. Here is another period with the same type of profile:
Wind generation was average to good during the above periods. Now, let's look at the wind generation profile from the previous period :
The blue line shows that this year, the wind output was particularly poor, even reaching minus 3MW on occasions. The part we need to focus on is the second half of that graph, which covers the Saturday and Sunday when the wind was crawling along between -3MW and 10MW in 2014. We would expect that the wind fleet would have been using grid power during this time to try to pick up any gusts and that this power consumption would be much larger than in 2013. So lets look at the demand profile for these days:
On the first and last days (Thursday and Sunday), we can see the demand profiles are very similar but in the case of Thursday, the wind output is much the same for both years. On Friday, the wind begins to deviate towards lower outputs and we see a spike in demand. But this could also be attributable to a larger load from industry on Friday morning. Then on the Saturday, we see two spikes in demand - one of about 400MW and the other about 200MW. We are now back at weekday levels of demand, which is odd because the factories and offices are closed down just like any other weekend. These spikes on Saturday fit in nicely with the wind graph above, where by Saturday afternoon, the wind was non existent, and the wind turbines were struggling to pick up on any gusts. As they did this, they consumed quite large amounts of grid power. The amounts are actually quite shocking and unexpected. On Sunday, it appears that they gave up on trying to use grid power (which indicates that they are charged for it) to start the turbines.
Let's look at that Saturday demand profile in more detail :
There is slightly less demand from about 3pm to 7pm but I can't see how wind turbines could have used less power in 2014, since there were more of them, so it is most likely attributable to something else, perhaps load shedding. When we extract the data where the demand is higher - from 9am to 3pm and 7pm to 10pm - we arrive at a figure of 1,906MWh for total additional power consumed from the grid by the wind farms during this calm day (assuming all of it is due to wind turbine fleet). This doesn't include parasitic power used on average wind days which is not possible to see with this data. So the 1,906 MWhrs refers to grid power consumed only on very calm days when the wind fleet are trying to catch gusts and maintain standby power (i.e keep oil and parts warm etc). There are around 1,124 wind turbines in Ireland so it works out at 1.7MWh per turbine.
To put this in context, the average 4 bed house requires approx 6.8MWh per year. So the power consumed over 9 hours by our total wind fleet in Republic of Ireland was the equivalent of the power required for 280 four bed houses for a whole year or 102,000 four bed houses for one day.
A week later the IWEA were telling us how much power their wind farms had generated (they hit a record) but we never get told how much power they consume, particularly on very calm days.
What are the associated emissions from this extra demand for grid power and how is it possible to say that wind energy is "zero carbon" ?
It is important to state that I accept that I cant prove with certainty that all or some of this additional demand is directly related to parasitic consumption of wind energy but given that 2013 and 2014 demand profiles are generally a perfect fit (as above) and that this variation occurred on a weekend day when there is less scope for variation (factories closed etc), it is difficult to attribute the variation to any other factor. But of course, comments are welcome.