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January 1, 2012

Getting Peak Returns out of Peaking Turbines

By Joe Zwers

Gas turbines are expensive. Although peaking units aren't as costly as baseload units, letting them sit idle is still a waste. Yet that is what peaking units spend nearly all their time doing. Some operators only fire up their units a few days a year. That is like putting money in the bank and collecting interest a few hours at a time, rather than all year round. That strategy only works when the payback is extremely high for those short periods.

The economic model is changing, however, both for good and bad. Peaking turbine owners used to be able to predictably rely on being called into action on summer afternoons. Now utilities are switching to demand curtailment strategies rather than purchases on the spot market. The good news is that increasing use of wind and solar power require peaking to be available at odd times, and that requires dispatchable generating resources to be standing by to make up for sudden shifts in the wind. In addition, the use of renewable power sources and the actions of shutting down generating stations near urban areas increases the need for grid stabilization.

According to the Operational Reliability Analysis Program (ORAP), which contains more than 20 years of data on turbine performance, gas turbines in North America are being started more frequently and being run for shorter periods of time. Strategic Power Systems Inc. (SPS) analyzed the ORAP data and found the average number of starts per unit increased from 77 per year from 1996 to 2000, to 100 per year from 2006 to 2010. During that same period, the service factor (the number of hours the unit was providing power divided by the number of hours the flame was on) dropped from 62% to 37%, so gas turbines are spending more time idling than generating.

"We are seeing market-driven cyclic loads tied to such factors as wind intermittency," said Tom Christiansen, senior vice president of SPS. "Fast start units are needed to keep the grid stable."

Meeting these needs, and increasing the economic return on peaking turbines, requires a different approach to turbogenerator sets. Instead of viewing them as a unit, using a clutch to separate the turbine and the generator in order to increase their flexibility allows them to provide a wider range of services.