Tuesday, 2 September 2014

Aghada (AD2) Combined Cycle Power Station

Aghada (AD2) in County Cork is a 431MW Combined Cycle power station operated by ESB. The diagram below (Fig 1) shows how central it now is to power generation in the South West region since Ireland’s rush for wind energy. In 2009, 920MW of wind was installed. By the end of 2013, installed wind capacity was at around 1,772MW. This has resulted in more frequent ramping of this generator.

The second half of the month is the most suitable for comparative purposes. Back in 2009, the plant was ramped up to meet peak demand and then ramped down to exactly 50% load for extended periods - the threshold below which significant inefficiencies begin. It is well known in the industry that running a gas generator at below 50% load results in more fuel consumption and consequently, more CO2 emissions being emitted than if the plant were running at full load. But in 2013, the plant was used to accommodate even the smallest variances in wind generation, hence the sawtooth edge. De-ramping was to between 48% and 50%, teetering over the inefficiencies cliff. The rugged irregular edge of 2013 compared with the smooth fine edges of 2009 proves that wind is many more times variable than demand, contrary to what SEAI claimed in their recent report “Quantifying Ireland’s Fuel and CO2 emissions savings” (Wind generation variability in 2012 was less than electricity demand variability - Page 9 of SEAI Report )

Interestingly, this inefficiency threshold of 50% is the same as the threshold for non-synchronous wind penetration and interconnector imports, meaning that wind generation and imports can be accommodated in the system up until the point where they reach 50% of demand, without causing large scale thermal inefficiencies. So the question is, what will happen when Eirgrid try to integrate 75% wind penetration into the system as is being investigated under their DS3 Programme - a requirement that will be necessary to meet renewable targets. This could result in de-ramping of gas plants below even 30% load where fuel consumption and CO2 emissions are circa three times what they are at full load, thereby completely negating any fuel / emission savings due to wind energy. This situation would also be bad financially for gas plant operators as their plants would be consuming more fuel but receiving less from the market. Its doubtful that they would be allowed to close down though, which means more subsidies in the form of capacity and/or constraint payments.

The graphs below show Aghada (AD2) and two large Cork wind farms - Coomacheo and Boggeragh. Together these wind farms total 116MW. Fig 2 shows actual output while Fig 3 shows Aghada’s output adjusted to make the comparison easier. It can be seen that in the early and latter parts of the month gas generation was used to fill gaps in supply due to calm winds. According to Eirgrid, Aghada (AD2) had a 93% availabilty in December, so its shut down “cold” status during the middle parts of the month indicate displacement by wind and possibly coal. Fig 4 shows Moneypoint (MP3) and Aghada (AD2) lined up. The erratic profile of both generators is notable in the second half of the month, due to increased wind penetration in the system. Indeed, on 17th December, a new wind output record was set of 1,769 MW. Aghada’s “cold” status may also have been due to operational constraints in the Cork region, where the maximum thermal generation at any one time is restricted to 1,100 MW. (Eirgrid Operational Constraints, March 2014)

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