Combined Cycles

Combined cycle are obtained by coupling a steam power plant (or sometimes a diesel engine plant) with a gas turbine installation. These systems are used to increase the overall efficiency of the gas turbine cycle, as the efficiency of the basic gas turbine plant is as low as 20 to 25%.

There are a number of combinations of combined cycles. However, we shall discuss the following two important combined cycle systems:

Combined cycle with reheat of exhaust:

The image above shows a gas turbine plant couple with a steam plant.

Gas Turbine :
Atmospheric air is drawn into the air compressor, when it is compressed to a high pressure. Fuel (Natural gas or Gasified Coal) is injected into the combustion chamber, CC and burns in the steam of compressed air. The products of combustion, comprising a mixture of gases at high temperature and pressure, are passed to the turbine. Products of combustion are expanded in the gas turbine and electric power is generated in the generator G1.

It should be remembered that the exhaust gases from the gas turbine have a temperature of 400 to 500’C and contain about 40 to 50% of the initial heat energy.

Steam Plant with Reheat Arrangement:

As seen in the image above, the exhaust gases from the gas turbine are heated in a reheater (RH). The hot flue gases then pass through the boiler to generate steam. This high pressure steam from the boiler is used to drive a steam turbine. Thus electric power is generated in the generator G2.

This combined cycle recovers much of the exhaust heat energy by reheating the high temperature exhaust gases from the gas turbine and passing to the heat recovery boiler for power generation.

Combined cycle with cogeneration arrangement:

The topping cycle system shown in the above image is another type of combined cycle with cogeneration arrangement. In this arrangement, the exhaust heat from the gas turbine is used for steam generation in a waste heat recovery boiler. Steam from the boiler is used for process plant.

Advantages of combined cycles

  1. The overall efficiency of the combined cycle plant is nearly 47 to 42% which is much higher than a simple gas turbine or stem plant.
  2. The capital cost of combined plant with supplementary firing is less than the combined cost of separate units of gas turbine and steam turbine plants for the same power capacity.
  3. The cooling water requirement of a combined cycle is much less than a pure stem plant having the same output. The reason is no cooling water is required for the gas turbine.
  4. The combined plant is more suitable for quick start and shut down than steam plant. Whether natural gas or gasified coal is the fuel, combined cycle greatly reduces CO2 emissions. Thus environmental pollution is minimized.
  5. By using combined cycle plants, energy resources could be used efficiently so that they last longer. Thus the concept of combined cycle systems will promote energy conservation.


alekhya bizconn said...
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adson stone said...

This is one of my favorite topics in engineering systems design (they don’t call me the “Energy Zarr” without reason). In fact, I often rant about waste in solving a problem with brute force. Now… with that said, sometimes a hammer is more effective when dealing with a nail, but in general, what goes in, must come out… and most of what comes out is heat. Take the quintessential LCD display like the 60” version sitting in your living room. That beauty has white LEDs for a back-light so it must be “green” right? Well, did you know that up to 80% of the light emitted by those LEDs is absorbed by the color filters on the LCD glass? It might be “thin” but it is definitely not efficient with the back-light energy. Technologies such as OLED or Sequential Frame LCD (SFLCD) do not use filters. OLEDs are self emitting and draw zero power when off. SFLCD technology still uses a back-light, but they are RGB LEDs. Each color frame (red, green, blue) is switched at such a high speed that the eye integrates the image into the proper colors. Each pixel is now larger and brighter with less power. How much less? Try 80 watts for an SFLCD TV versus 350 watts for the traditional LCD. Energy currently is a limited resource, so innovate where you can to save it.

adson stone said...

I remember talking with the late Bob Pease about the state of the art in digital techniques for solving complex problems. He politely let me babble for a few minutes and then laughed, “Yep, I solved that same problem 10 years ago with two op-amps”. I wanted to crawl under something, but his office was completely full of every magazine he had ever received… but that’s another story. He was correct – sometimes a straight forward analog solution can not only be the most elegant, but also the most efficient. Sometimes you need the power of a DSP processor when systems are non-linear or the signal processing is not realizable in the analog domain. However sometimes simple analog circuitry can solve the problem. Don’t forget your roots.

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Jay Joshi said...

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