Combined cycle power plants (CCPP) utilize two different heat engines and a single heat input (the combustor) to produce electricity. In a combined cycle, waste heat from the exhaust of the system supplies the heat for a second system. The heat engines include gas turbines, internal combustion engines, steam turbines, fuel cells, and micro-turbines. The most commonly used combination is a gas turbine and a steam turbine. In a simple cycle natural gas power plant, the exhaust from a gas turbine (usually 900–1200°F), is rejected into the atmosphere, wasting approximately 2/3 of the energy input. A combined cycle, however, utilizes a Heat Recovery Steam Generator (HRSG) to harness this energy. A HRSG is a heat exchanger that transfers energy from the exhaust to a water system which then becomes steam. This steam is then used to drive a steam turbine, producing more electricity than just a simple cycle natural gas power plant alone.
Two Heat Engines Running in Series
In a combined cycle gas turbine plant (CCGT), a gas turbine and a steam turbine are operated simultaneously. The gas turbine runs the same way it would in a natural gas power plant, except that in a combined cycle the exhaust from the gas turbine is diverted through a HRSG before it goes out the stack. In the HRSG, much of the thermal energy that would otherwise have been lost is used to heat water into steam in order to run a steam turbine and generate additional electrical output.
In a typical vapor power plant, a boiler is used to create steam that is sent through a steam turbine to generate electricity. The steam is then condensed into a liquid so that it can be pumped back to the boiler where the whole cycle starts again. The steam cycle in a CCPP operates similarly to a vapor power plant except an HRSG is used in place of a boiler. Unlike a boiler, the HRSG creates steam without the need for a fuel input because the gas turbine process has already created the heat. The gas turbine exhaust provides enough energy to superheat the steam that is needed to avoid damaging the steam turbine. If the exhaust from the gas turbine does not contain sufficient energy to superheat the steam, duct burners can be installed in the system to add the energy required or increase the quality of the process.
Waste Heat Recovery
Waste heat results from equipment and system inefficiencies as well as the thermodynamic limitations of the processes. Improving the industrial efficiency typically involves adding new equipment or reducing the amount of energy consumed by existing equipment..
A HRSG is the basis for the recuperation of waste heat in a combined cycle. A HRSG reduces the temperature of the gas turbine exhaust from 900-1200°F to around 150-300°F. By recovering the heat that would have been lost in a simple cycle plant, more electricity can be generated from the same fuel input.
Duct burners, supplementary burners that increase the quality or quantity of the steam being produced within the HRSG, enable the steam turbine to create more electricity. After the temperature of the exhaust from the gas turbine decreases, it is raised again by lighting the duct burners in order to continue heating the water. However, because burning the extra fuel incurs additional costs, the duct burners are turned on only when electric companies are buying electricity at a high price. In other words, although more electricity is being produced, the cost of producing each MW also increases.
The efficiency of power plants is normally expressed by heat rates (Btu/kWh), similar to the way that cars are rated by mpg. The heat rate compares how much energy is put into the system per kWh of electricity produced. Unlike a car, however, the lower the heat rate the more efficient the power plant is. According to the U.S. Energy Information Administration (EIA), the heat rate is averaged to be 10,330 Btu/kWh for steam cycle power plants, 12,560 Btu/kWh for gas turbine power plants, 10,200 Btu/kWh for internal combustion plants, and 8900 Btu/kWh for combined cycle plants (based off of 2012 data) . This data demonstrates that combined cycles are much more efficient than any other type of power plant in the U.S..
Recovering and using waste heat raises the efficiency of a power plant significantly. The efficiency of a CCGT can approach 60% . The newer gas turbines are close to 40% efficient, and the steam cycle operating under combined cycle conditions is around 30% efficient. A comparison of the two simple cycle systems reveals that the combined cycle power plant is much more efficient than either of the two systems alone, because the waste heat in the combined system is used to create more power. The steam turbine creates 1/3 of the total power output—which would otherwise be lost as heat through the power plant stack without the use of a combined cycle.
Industry uses the equation shown in the sidebar to calculate the efficiency of a combined power plant: add the efficiency of the first cycle to the efficiency of the second cycle minus the product of the two cycles’ efficiencies. For example, if the efficiency of a simple cycle gas turbine system is 35% and the simple cycle steam turbine cycle is 30% efficient, the combined efficiency would be 0.35 + 0.30 - 0.35 * 0.30 = 0.545, or 54.5%, which is significantly higher than for either of the simple cycles because more power is being created for the same fuel input.
Physical sites that include Combined Cycle Systems:
Faribault Energy Park
|How has the gas turbine advanced? ASME discusses this.|
|How does a combined cycle power plant work? This video explains.|
|Want to know more about HRSG designs and benefits? COSPP discusses this here.|
|The second law of thermo states that, without external work, heat will never travel from a cold body to a hot body.|
|Combined Cycle Efficiency Equation
ηC1=efficiency of cycle 1
ηC2=efficiency of cycle 2
|Want to know more about the efficiencies and emissions of combined cycle plants? Siemens discusses this here.|