Wind and coal-fired generation's share of the Electric Reliability Council of Texas' fuel mix rose month on month in September, while natural gas-fired units' cut slipped below 50%, a mark held for two months, as less cooling load and strong winds pushed more peaking units out of the stack, according to the grid operator.
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The largest monthly increase in percentage terms came from wind generation, which rose about 5% to make up nearly 14% of the total generation, after hitting a two-year low of about 9% in August, the data show. The year-ago level was 11%.
ERCOT data showed wind farms produced 4.4 TWh in September, jumping about 33% from August output of 3.3 TWh.
Meanwhile, coal's share rose 2.7% month on month to 33.5% to account for more than a third of total generation in September, its highest mark since April, after ranging between 30% and 33% from May through August.
However, coal-fired plants supplied 10.7 TWh in September, dropping from the year-to-date high of 11.2 TWh in August, as total energy demand fell from 36 TWh in August to 32 TWh in September and was expected to dip further following the end of the peaking demand months.
Total energy demand in September was slightly lower than the year-ago level of about 33 TWh, as US National Weather Service data showed cooling degree days in Texas totaled 389 in September, 73 below the same month last year but 17 CDD above average.
More seasonal temperatures in September trimmed load as well as output at gas-fired plants. Gas-fired units supplied about 41% of total demand in the most recent month, down from 50% in August and 50.6% in July. The year-ago level was more than 44%.
Spot gas prices at Houston Ship Channel averaged about $2.935/MMBtu in September, up about 2.5% from August's average of $2.863/MMBtu and almost flat to the year-ago level.
Despite the decline in gas-fired plants' share of the fuel mix heading into fall, when less demand was expected, gas topped in the fuel stack for seven months through September this year, second to coal only in January and April, according to ERCOT data.
Nuclear output ticked up 2% to account for about 11% of total supply in September, as Luminant's 1,124-MW Comanche Peak-2 nuclear unit stayed offline for the first half of August and had only a four-day outage in September.
Looking ahead, fuel mix in the footprint seems likely to be more uncertain as retirements of both gas and coal-fired units have been announced heading into the shoulder season.
In late September, Talen Energy told ERCOT of plans to retire the 300-MW gas-fired unit at its Barney Davis plant near Corpus Christi, Texas, on December 31. About a week later, the City of Garland filed with ERCOT to indefinitely shut two gas-fired units with a combined capacity of 118 MW at its Spencer plant on January 3.
On the coal side, Luminant filed plans Friday to retire its 1,208-MW Big Brown plant and 1,200-MW Sandow plant in early 2018 and announced October 6 plans to retire the 1,800-MW Monticello plant in January.
--Jeff Zhou, email@example.com
--Edited by Keiron Greenhalgh, firstname.lastname@example.org