Classification des turbines à gaz

Brayton Cycle: The Ideal Cycle for Gas-Turbine Engines

In Relation to Power Plants

By Denise Lane

Preface:

Power generation is an important issue today, especially on the West Coast. Demand is outweighing supply because of lack of incentives for the utilities industry to build additional power plants over the past 10-20 years. Electrical innovations (such as the personal computer) were not accounted for in earlier predictions of power utilization and, now, the country is in dire need of streamlining the current power plants while pushing through as many applications as possible for new power plants. In response to this situation, power generation engineers will be in high demand. These engineers must have a thorough understanding of thermodynamics and, in particular, the Brayton cycle. It is the backbone of power generation. In order to deepen knowledge of how the Brayton cycle is applied at power generation plants, an interview was conducted via e-mail with Brian Lawson, who has obtained the P.E. designation and is the Senior Mechanical Engineer for Sierra Pacific Power Company’s Tracy Power Generating Station. This station provides a total electrical power output of 454 MW and supplies the majority of the population in northern Nevada. The italicized questions and answers asked and obtained are integrated throughout the various topics to provide further insight and understanding for the beginning engineer entering the power generation field. Further, bolded words are defined in detail at the end of each paragraph.

Brayton Cycle/Gas Turbine History:

The basic gas turbine cycle is named for the Boston engineer, George Brayton, who first proposed the Brayton cycle around 1870.1 Now, the Brayton cycle is used for gas turbines only where both the compression and expansion processes take place in rotating machinery.2 John Barber patented the basic gas turbine in 1791.3 The two major application areas of gas-turbine engines are aircraft propulsion and electric power generation. Gas turbines are used as stationary power plants to generate electricity as stand-alone units or in conjunction with steam power plants on the high-temperature side. In these plants, the exhaust gases serve as a heat source for the steam. Steam power plants are considered external-combustion engines, in which the combustion takes place outside the engine. The thermal energy released during this process is

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then transferred to the steam as heat.(2) The gas turbine first successfully ran in 1939 at the Swiss National Exhibition at Zurich. (3) The early gas turbines built in the 1940s and even 1950s had simple-cycle efficiencies of about 17 percent. This was because of low compressor and turbine efficiencies and low turbine inlet temperature due to metallurgical limitations at the time. The first gas turbine for an electric utility was installed in 1949 in Oklahoma as part of a combined-cycle power plant. It was built by General Electric and produced 3.5 MW of power. (2)

In the past, large coal and nuclear power plants dominated the base-load electric power generation. However, natural gas-fired turbines now dominate the field because of their black start capabilities, higher efficiencies, lower capital costs, shorter installation times, better emission characteristics, and abundance of natural gas supplies. The construction cost for gas-turbine power plants are roughly half that of comparable conventional fossil-fuel steam power plants, which were the primary base-load power plants until the early 1980s. More than half of all power plants to be installed in the foreseeable future are forecast to be gas-turbine or combined gas-steam turbine types.

In the early 1990s, General Electric offered a gas turbine that featured a pressure ratio of 13.5 and generated 135.7 MW of net power at a thermal efficiency of 33 percent in simple-cycle operation. A more recent gas turbine manufactured by General Electric uses a turbine inlet temperature of 1425°C (2600°F) and produces up to 282 MW while achieving a thermal efficiency of 39.5 percent in the simple-cycle mode. (2) Current low prices for crude oil make fuels such as diesel, kerosene, jet-engine fuel, and clean gaseous fuels (such as natural gas) the most desirable for gas turbines. However, these fuels will become much more expensive and will eventually run out. Provisions must therefore be made to burn alternative fuels.4

Q: Do I understand correctly that you use the gas turbine exhaust heat to serve as a heat source for steam turbines to produce even more power?

A: The Pinon Pine unit is a “combined cycle” which has a heat recovery boiler attached to the exhaust of the combustion turbine to produce about 22 MW of power from a steam turbine (Rankine cycle) that is located next door.

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Q: I understand that you are in the process of trying to use diesel instead of gas? Is the diesel providing enough expansion to turn the turbines as fast as needed to produce the same electric power produced from gas and/or proving to be economical in the trade-off?

A: There is only a small difference in the performance between firing a combustion turbine on natural gas and diesel. The cost of diesel, however, is typically much higher than natural gas on a $/MMBtu basis.

Brayton Cycle Components:

Gas turbines usually operate on an open cycle, as shown in Figure 1. Fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high-pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. (2) The resulting high-temperature gases then enter the turbine, where they expand to the atmospheric pressure through a row of nozzle vanes.5 This expansion causes the turbine blade to spin, which then turns a shaft inside a magnetic coil. When the shaft is rotating inside the magnetic coil, electrical current is produced. The exhaust gases leaving the turbine in the open cycle are not re-circulated.

Figure 1 – An Open Cycle Gas-Turbine Engine Figure 2 – A Closed Cycle Gas-Turbine Engine

The open gas-turbine cycle can be modeled as a closed cycle as shown in Figure 2 by utilizing the air-standard assumptions. Here the compression and expansion process remain the same, but a constant-pressure heat-rejection process to the ambient air replaces the combustion process. The ideal cycle that the working fluid undergoes in this closed loop is the Brayton cycle, which is made up of four internally reversible processes:

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