Cycle Modifications Enhancements

In addition to improved high-temperature materials, there are several variations to the gas turbine cycle that can be used to increase efficiency and unit capacity:

Gas turbine combined cycle: In this cycle, superheated steam generated from turbine exhaust in a heat recovery steam generator (HRSG) is supplied to a steam turbine to produce additional power. Typically, power output and overall thermal efficiency can be increased by up to 50%. Combined-cycles are discussed in detail in Chapter 12.

Steam (or water) injected gas turbine cycle: As discussed, the largest share of energy collected by the turbine section is used by the primary air compressor, leaving a relatively small fraction of the total available for export work. Because of this, relatively small changes in the ratio of compressor work to export work result in significant changes in the useful turbine output. One way to change this balance is to make the mass flow passing through the turbine greater than that passing through the compressor. For example, if the compressor takes two-thirds of the work done by the turbine and an increase in turbine mass flow increases overall power production by 10%, the effective increase in export work is 30%. This effect can be seen when steam is injected into the turbine combustor for emissions control. The small increase in exhaust mass flow increases power output of the turbine, usually by 3 to 10%.

Some gas turbine designs lend themselves to modifications that allow injection of significantly greater quantities of steam (or water) specifically for power augmentation. The result is that the gas turbine functions in a somewhat similar fashion to a combined cycle, which uses both gas and steam turbines to produce power. This process is frequently referred to by trade names as the Cheng™ or STIG™ cycle. Examples of these turbines and their relative power increases with steam injection include the Kawasaki GPCC15 (1.3-2.3 MW), Allison 501 KH (3.6-5.4 MW), the Mitsubishi MF-11AB (6-11 MW), and the General Electric LM2500 (22-28 MW). Depending on the turbine design, power augmentation can increase capacity by up to 50% while also increasing thermal efficiency. Steam-injection cycles are discussed in detail in Chapter 12.

Air bottoming cycle: In this cycle, hot gas turbine exhaust (or other process exhaust) is passed through an air-to-air heat exchanger. The heated air is then used to drive an air power turbine. Combined, the power output and efficiency of this type of combined cycle is about 30% greater than the Brayton cycle. While efficiency and total power output are less than with a conventional combined cycle, the use of a bottoming-cycle air turbine is simpler and less costly.

Regenerative cycle: A regenerator or recuperator is a heat exchanger that transfers heat from turbine exhaust to compressor discharge air prior to combustion. Recovered heat displaces a portion of fuel that would otherwise be required. Regeneration usually decreases unit capacity (as a result of heat exchanger pressure drops), but enhances cycle efficiency.

Regeneration causes the exhaust gases ultimately leaving the turbine to be much cooler than they would otherwise be. The reduced energy and temperature of the exhaust stream results in decreased heat recovery potential. Whereas the typical gas turbine exhaust temperature is about 1,000°F (538°C), the ultimate exhaust from a recuperative turbine is about 600°F (316°C). Both the quantity of recoverable heat and the maximum achievable temperature and pressure of heat recovery generated steam are greatly reduced. The simple open-cycle gas turbine cycle with regenerator is shown schematically in Figure 10-57 and in T-s diagram in Figure 10-58.

For the ideal cycle with regeneration, thermal efficiency y

Regenerator

Combustion Chamber

Compressor

Turbine

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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