Albert Einstein once remarked that there is no difference between the past and the future in the four-dimensional space-time-world. That perspective might be useful for theoretical physicists, but the rest of us have to live on a one-dimensional time line, where the past seems much clearer than the future.
In the mid-1990s, most industry pundits believed that the replacement of aging coal plants by high-efficiency combined-cycle plants was inevitable. For some developers of combined-cycle plants, deregulation created a perfect storm that required fast construction of large, low-emission, quick-starting baseload plants. Many developers made their fortunes in those early years. Ten years later, the best project sites had been taken and regional overcapacity of gas-fired plants had reached a tipping point, so the less well-financed developers began a slow slide into bankruptcy.
With 20/20 hindsight, it now seems obvious that many projects were based on overoptimistic predictions of stable natural gas prices and relied on an infrastructure not designed for dispersed delivery of large quantities of gas. But the price of natural gas, which was less than $2/mmBtu throughout the 1990s, skyrocketed to $5 by 2003 and to $8 by 2005. Merchant plants built a decade ago cannot meet their pro formas when gas prices quadruple. Today, on most grids run by independent system operators, there are many combined-cycle plants competing head-to-head for sales of intermediate and peaking power (Figure 1).
1. No more baseload. This duty-cycle analysis of the SGT6-6000G gas turbine takes its data from 20 units between July 2005 and June 2006. Classifications are based on ISO definitions. Source: Siemens Power Generation
Shelter from the storm
Siemens Power Generation (SPG) designed the 270-MW-class SGT6-6000G gas turbine (Figure 2), formerly known as the W501G (the "6" suffix stands for 60 Hz), to meet the demand criteria of the 1990s. Its 58% efficiency in a combined-cycle configuration was a step above the 54% to 55% efficiency of that era's state-of-the-art units. The table lists the key performance specs of the SGT6-6000G in two different SCC6-6000G (combined-cycle) configurations.
Typical SCC6-6000G combined-cycle plant performance. Conditions are at sea level and 59F ambient temperature. Figures include inlet and exhaust losses. Source: Siemens Power Generation
Because they are more efficient than other combined-cycle plants, many SCC6-6000G plants are dispatched more often than competing units in today's overcapacity markets.
The prototype SGT6-6000G engine first saw service in April 1999 in simple-cycle mode at Unit 5 of the Macintosh Power Plant, owned and operated by the City of Lakeland, Florida. This plant configuration used an auxiliary once-through steam generator to produce the necessary turbine cooling steam (more on the turbine design details later). Later, a bottoming cycle was added, enabling the plant to run in combined-cycle mode. It went commercial again in this configuration at the end of 2001.
Today, 22 Siemens G-Class turbines are operating worldwide and two units are under construction. The fleet recently passed the 250,000 operating hours milestone and should surpass 325,000 operating hours by the end of this year.
Peaking service remains the standard by which turbines are judged. One SGT6-6000G operated at a 10% capacity factor during its second year of service, but during that year it racked up 123 successful starts with a 95% starting reliability. At last report, the fleet's rolling reliability and availability had reached 96.7% and 93.1%, respectively. By these measures, the SGT6-6000G certainly fits the classical definition of a mature turbine.
Email
Comments
Print
Reuse
