Simple-Cycle Gas Turbine
Prerequisites
A combined-cycle plant takes an hour to start. When 30,000 people turn on their air conditioners at 4 PM, the grid cannot wait an hour. Simple-cycle gas turbines start in 10-15 minutes, trading thermal efficiency for the ability to respond when the grid needs power now.
A simple-cycle gas turbine runs only the Brayton cycle: compress air, burn gas, expand through the turbine. No heat recovery, no steam turbine. Exhaust heat (500-600 degrees C) goes straight up the stack. Thermal efficiency: 35-42%, versus 55-63% for a combined-cycle plant. Capital cost: roughly $700-1,300/kW, about two-thirds the cost of combined-cycle. These plants are called "peakers" because they run only during peak demand hours.
Calculate why low efficiency is acceptable. A peaker running 500 hours per year at $3/MMBtu gas and 9,500 BTU/kWh heat rate: fuel cost = $28.50/MWh. Annual fuel spend: modest. A CCGT costing roughly $400/kW more in capital would need years to recover the difference at only 500 hours of operation.
At what capacity factor does a CCGT's efficiency advantage pay for its higher capital cost?
The crossover. Below roughly 15-20% capacity factor (about 1,300-1,750 hours/year), the simple cycle's lower capital cost wins despite higher fuel cost per kWh. Above that, the CCGT's efficiency savings accumulate enough to justify its capital premium.
Peakers exist because electricity demand is spiky. Building an efficient plant that sits idle 85% of the time wastes capital.
Simple-cycle gas turbines achieve only 35-42% efficiency versus 55-63% for combined-cycle plants. Utilities build them anyway because:
At low capacity factors, capital cost dominates total cost. A cheap plant running a few hundred hours costs less overall than an expensive efficient plant sitting idle most of the year. Fast startup is essential for responding to sudden demand spikes.
The answer is DLesson complete
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