While many older plants are now, in effect, peakers because they run so infrequently, there are new units being installed as peakers. For example, the proposed repowering of the E.F. Barrett plant on Long Island replaces two ancient steam units and some 1970-vintage combustion turbines and jets with a new 636 MW combined cycle unit and six simple cycle turbines (collectively 260 MW).
A combined cycle unit is, in effect, one to four simple cycle turbines that feed waste heat into a
heat-recovery steam generator (HRSG). The combined unit has higher efficiency than the simple cycle turbines: e.g. the CCGT replacing Barrett will have a 6,758 Btu/kWh while the CTs are at 10,122 Btu/kWh. But the extra complexity also increases the start time (notice and warm-up) and reduces the economic range of flexibility: CCGTs typically operate at no less than 40% loading. For this reason, I see the industry evolving to using more CTs (rather than CCGTs) when installed to support renewables. CCGTs are far better at replacing old baseload plants (coal, oil, or gas).