Sunday, March 25, 2012

3 February 2011 – Braking wind in Texas

“If that is the way the wind is blowing, let it not be said that I do not also blow!”
- Mayor “Diamond Joe” Quimby, The Simpsons


Having spent the last few years looking at “renewable energy” issues, this story caught my eye yesterday:
Texas’ longest period of rolling blackouts ended around 1:30 this afternoon, eight hours after the power outages began across the state.
The Electric Reliability Council of Texas, which operates the state power grid, warned it may initiate more blackouts Wednesday evening or Thursday morning during the peak demand hours.
The planned outages, which lasted up to 45 minutes, were triggered when more than 50 power plants, including a few owned by Dallas-based Energy Future Holdings, stopped working Tuesday night because of the cold weather.
The Electric Reliability Council of Texas said 7,000 megawatts of generating capacity tripped Tuesday night, leaving the state without enough juice. That’s enough capacity to power about 1.4 million homes. By rotating outages, ERCOT said it prevented total blackouts. (Note A)
The reason it caught my attention is because Texas has, among other things, the single highest installed wind power capacity and the highest annual wind-powered electrical generation stats of any state in the Union.(Note B)  The installed wind power capacity in Texas at the end of 2009 was 7,427 MW, which accounted for 7.1% of the state’s total nameplate generating capacity of 140,900 MW (about 141 GW).  This, incidentally, is higher than Canada’s total installed generating capacity of about 126 GW.  The installed nameplate generating capacity of wind turbines in Texas is roughly equivalent to the installed nuclear generating capacity of Ontario.  And it’s only going up; according to a Reuters piece from a few weeks ago, wind power grew to more than 9500 MW by the end of 2010, accounting for 7.8% of the total electrical generating capacity of Texas. (Note E)  This isn’t surprising; state subsidies for renewable energy are quite generous in Texas, and taken together with federal subsidies, can make running a wind farm very lucrative.
Of course, the amount of electricity that all those wind turbines produce isn’t quite the same as their rated capacity.  In 2009, for example, Ontario’s nuclear reactors generated 81.7 TWh of electricity. (Note C)  More than half of this - 44.2 TWh - was generated by the 10 nuclear reactors at Pickering and Darlington.  Together, these sites have an installed nameplate capacity of 6600 MW. (Note D)  Since there are 8760 hours in a year, this means that the maximum generating capacity of both plants, operating at 100% of nameplate capacity for 24 hours a day, every day of the year, would be 57.8 TWh.  These 10 units, in other words, produced 76.5% of their nameplate capacity in 2009 (in reality, Darlington was high, at 90% of capacity, while Pickering was low due to planned outages for repair and refurbishment).
90% isn’t unusual for nuclear generating stations.  The 104 nuclear reactors currently operating in the US have an aggregate nameplate capacity of 106.6 GW, for a total potential annual generating capacity of 933.8 TWh.  In 2007, those stations generated 806.4 TWh for an aggregate capacity factor of 86.35%, an impressive figure when you consider that several of those reactors were offline for maintenance (more recent generation statistics have been lower due, as I noted in a post late last year, to lower demand for power as a consequence of the shrinkage of the US economy due to the recession).  The average capacity factor for an operational reactor in the US is above 92%.
As points of comparison, consider coal (the single largest source of electrical generation in the US) and natural gas.  The installed nameplate capacity of coal-fired stations in the US is 338.7 GW, more than three times the nuclear generating capacity, for a theoretical total annual capacity of 2,967 TWh.  In 2007, the US generated 1,764 TWh from coal, 59% of the rated capacity - a result, as I have noted elsewhere, of the regulations- and recession-inspired drop-off in coal-fired generation.  Natural Gas turbines, in sharp contrast, of which there are 4 times as many generating stations (5,470) as there are coal-fired stations (1,436), have a nameplate capacity of 459.8 GW (4,027 TWh), but in 2009 generated only 920,378 TWh - 22.8% of their nameplate capacity.  This is not because gas turbine generation is less efficient or more cost-effective; far from it.  Combined-cycle gas turbines are more expensive to operate than other generating plants, but they have the benefit of being very responsive, quick to start up and shut down according to shifts in demand.  As such, they make ideal back-up generators to provide power in the event that intermittent energy sources - like wind power - suddenly become unavailable.
Knowing all this, let’s go back to Texas.  Energy Futures Holdings, Inc. (EFH) owns and operates Luminant, the largest power producer in Texas, with 15,400 MW of generating capacity (1/5th of the State’s total), including 2,300 nuclear MW and 8,000 coal MW.  Neither EFH nor Luminant had offered any press releases on the blackouts by this morning, but local news media had some reports.  According to EFH officials cited in the stories, the problems were more related to the impact of the cold weather on new coal-fired generating stations, and on the combined-cycle gas turbines that normally kick in to deal with failures in generation and/or unforeseen surges in demand.
Energy Future Holdings’ plants accounted for less than half of the total missing capacity, said Allan Koenig, a spokesman for the Luminant power generation business. He said some equipment at the new coal plants is exposed to the elements and stopped working because of the cold.
He couldn’t predict when the Central Texas plants will be working again, and he declined to say which other plants were down.
When large coal plants go down, ERCOT calls on natural gas plants to fire up quickly to meet demand. However, the state’s natural gas network was also grappling with the cold, and the pipelines had lost pressure. So some natural gas plants including at least one Luminant plant couldn’t get fuel.(Note F)
Another news article notes that the drop in pipeline pressure was due to greater demand upstream, again due to the unexpected cold.
It’s also interesting that Luminant has 900 MW of wind generating capacity, and touts itself as “the largest wind purchaser in Texas and the fifth largest in the United States.”(Note G) 900 MW is 12.1% of the wind power capacity of the entire state.    The whole question of wind power capacity raises eyebrows, because the amount of capacity lost in Texas yesterday - 7,000 MW, according to the news reports - is close to the amount of wind power generating capacity presently installed in Texas (about 9000 MW).  So the question becomes, what was happening with wind power in Texas when the lights went out?  Believe it or not, the actual data is available online at the website of ERCOT (Electrical Reliability Council of Texas), which regulates about 85% of the state’s electrical generation and transmission [].  A quick overview of the data shows that an awful lot of windfarms were down for maintenance, derating, or were on “forced” withdrawal from generation, although quantifying the outages is difficult as not all wind farms identify as such.  ERCOT hasn’t yet published its load data for this month (the latest spreadsheets available are for 2010), but it will be interesting to see what those figures show.  According to METARS data, the past 48 hours haven’t seen prolonged, unusually low wind activity in Texas []; windspeed lows have occasionally been in the single digits, but while this is generally too low for a wind turbine to produce power at the nameplate rating, these lows did not persist throughout the period in question.  So while the Texas wind farms were probably (almost certainly) operating at far below their rated capacity, it seems unlikely that the whole state was entirely becalmed.
That said, with such a high proportion of its power coming from wind turbines, a sudden drop-off in windspeed at exactly the wrong time - say, during the daily spikes in demand in early morning and early evening - could prove exceptionally problematic, especially if coal and gas-fired plants were also producing at below their rated capacity.  The underlying problem with wind power is that because it is intermittent, wind farms put a disproportionate amount of strain on power grids that, by virtue of their design, tend to operate close to maximum transmission capacity, and that as such have only a limited capability to absorb shocks (or “step changes”) in generation.  As one study into the problem of transmission step changes resulting from the increasing adoption of wind power notes,
When more wind power plants are connected to the system, the diverse wind resources of various locations will make the aggregate output less volatile. The statistics of power fluctuations (step changes and ramping rates), when expressed in terms of total wind power capacity, will be smaller than those of an individual wind power plant. However, the actual magnitudes of step changes in MW or kW will be larger than those of an individual wind power plant. In this situation, the system control areas will face higher regulation duty. Although the probability of calm wind at every wind power plant becomes smaller and smaller as more plants are scattered over wider areas, such an event is still possible. The maximum step changes will also be higher in magnitude.(Note H)
In other words, the more wind power plants that a grid must support, the greater the step changes in electrical production that the grid must be able to absorb, and the greater the aggregate impact of “a calm wind at every power plant”.  This is fairly obvious when you think about it, and it appears to be one of the problems that contributed to the failure in Texas this week.  It’s not immediately clear what “critical mass” of wind power a given grid can support, but yesterday’s events provide us with at least one data point.  Wind power accounts for only 7.8% of the total generating capacity of Texas, but at a time of high demand, low wind power production, and failures at conventional power plants - all due to unusually low temperatures and foul weather - the combined impact was enough to force the state’s electrical power producers to implement rolling blackouts in order to prevent a more widespread collapse of the grid. 
It’s worth noting that none of the nuclear power plants in Texas went down; only the new-fangled “clean” coal plants, the emergency gas plants with their exposed and overtaxed pipelines, and the unpredictable wind turbines were affected by the record cold and snows in the southwest.  If nothing else, it’s a cautionary tale for those who advocate replacing reliable, continuous electrical generating technologies with intermittent generating options like wind turbines. 
Oh, and did I mention that Texans pay $0.1099 per kWh, 12.1% more than the national average?  That must be especially annoying when it’s -14F outside and your baseboard heaters aren’t working. 
By the way, in case you missed it, Ontario Premier Dalton McGuinty recently announced a $7B deal with Samsung to build 2500 MW worth of wind and solar power plants in Ontario.(Note I)  According to government statistics, in an entirely unrelated development, Ontario consumer electricity rates have risen 48% since McGuinty took office.(Note J)
B) Unless otherwise noted, all of the energy statistics are derived from the US Energy Information Administration at