Combined cycle

A combined cycle is characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). The remaining heat from combustion is generally wasted. Combining two or more "cycles" such as the Brayton cycle and Rankine cycle results in improved overall efficiency.

In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the waste heat is used to make steam to generate additional electricity via a steam turbine; this last step enhances the efficiency of electricity generation. Most new gas power plants in North America and Europe are of this type. In a thermal power plant, high-temperature heat as input to the power plant, usually from burning of fuel, is converted to electricity as one of the outputs and low-temperature heat as another output. As a rule, in order to achieve high efficiency, the temperature difference between the input and output heat levels should be as high as possible (see Carnot efficiency). This is achieved by combining the Rankine (steam) and Brayton (gas) thermodynamic cycles. Such an arrangement used for marine propulsion is called Combined Gas (turbine) And Steam (turbine) (COGAS).

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Design principle

In a steam power plant water is the working medium. High pressure steam requires strong, bulky components. High temperatures require expensive alloys made from nickel or cobalt, rather than inexpensive steel. These alloys limit practical steam temperatures to 655 °C while the lower temperature of a steam plant is fixed by the boiling point of water. With these limits, a steam plant has a fixed upper efficiency of 35 to 40%.

For gas turbines these limitations do not apply. Gas cycle firing temperatures above 1,200 °C are practicable. So, a combined cycle plant has a thermodynamic cycle that operates between the gas-turbine's high firing temperature and the waste heat temperature near the boiling point of water.

A gas turbine has a compressor, burner and turbine. The input temperature to the turbine is relatively high (900 to 1,350 °C) but the output temperature of the flue gas is also high (450 to 650 °C).

The temperature of a gas turbine's flue gas is therefore high enough to make steam for a second steam cycle (a Rankine cycle), with a live steam temperature between 420 and 580 °C. The condenser is usually cooled by water from a lake, river, sea or cooling towers.

The output heat of the gas turbine's flue gas is utilized to generate steam by passing it through a heat recovery steam generator (HRSG).

By combining both processes, high input temperatures and low output temperatures can be achieved. The efficiency of the cycles add, because they are powered by the same fuel source.

Efficiency of CCGT plants

The thermal efficiency of a combined cycle power plant is the net power output of the plant divided by the heating value of the fuel. If the plant produces only electricity, efficiencies of up to 59% can be achieved. In the case of combined heat and power generation, the efficiency can increase to 85%.

Supplementary firing

The HRSG can be designed with supplementary firing of fuel after the gas turbine in order to increase the quantity or temperature of the steam generated. Without supplementary firing, the efficiency of the combined cycle power plant is higher, but supplementary firing lets the plant respond to fluctuations of electrical load. Supplementary burners are also called duct burners.

More fuel is sometimes added to the turbine's exhaust. This is possible because the turbine exhaust gas (flue gas) still contains some oxygen. Temperature limits at the gas turbine inlet force the turbine to use excess air, above the optimal stoichiometric ratio to burn the fuel. Often in gas turbine designs part of the compressed air flow bypasses the burner and is used to cool the turbine blades.

Fuel for combined cycle power plants

Combined cycle plants are usually powered by natural gas, although fuel oil, synthetic gas or other fuels can be used. The supplementary fuel may be natural gas, fuel oil, or coal. An integrated solar combined cycle power station is currently under construction at Hassi R'mel, Algeria.

Integrated Gasification Combined Cycle (IGCC)

An Integrated Gasification Combined Cycle, or IGCC, is a power plant using synthetic gas (syngas).

Automotive use

Combined cycles have traditionally only been used in large power plants. BMW, however, has proposed that automobiles use exhaust heat to drive steam turbines.^[1] It may be possible to use the pistons in a reciprocating engine for both combustion and steam expansion like in the crower 6 stroke.^[2]

Aeromotive use

Some versions of the Wright R-3350 were produced as "Turbo-compound" engines. Three turbines driven by exhaust gases, known as "Power recovery turbines", provided nearly 600 hp at takeoff. These turbines added power to the engine crankshaft through bevel gears and fluid couplings.^[3]

See also

• Heat recovery steam generator
• Mercury vapour turbine

References

1. ^ "BMW Turbosteamer gets hot and goes" by John Neff, AutoBlog, December 9, 2005
2. ^ "Inside Bruce Crower’s Six-Stroke Engine" By Pete Lyons, AutoWeek, February 23, 2006
3. ^ [1]