Steam Turbine Efficiency in Combined Cycle Plant

A1) Both. If there were no steam flow, the power output and efficiency would both be zero.

A2) It has very little to do with the stack temperature. It has much more to do with the turbine exhaust pressure, or rather, vacuum. The lower the temperature in the condenser, the stronger the vacuum, and the more work can be obtained from the same unit of steam, other things being equal.

A. Yes, a change in steam flow will change efficiency as well as power output. It could go up or down, depending on the turbine. An operating curve for the specific turbine would tell which. In the real world, efficiency is rarely a constant.

B. I think you are confusing real life with theory, and overall cycle efficiency with equipment efficiency. Carnot says we want the source temperature as high as possible and the sink temperature as low as possible. So we design a CCP with a gas turbine on the front end with as high a turbine inlet temperature as feasible. Then the goal in the back-end (HRSG and steam turbine) is to convert as much of the exhaust energy into electrical energy as possiblefeasible. This involves generating steam at a high temperature and pressure, so that the steam part of the cycle also has a relatively high source temperature. Then you have equipment design challenges to convert that heat to power as efficiently as possible at a design point. Back to your original question - operating your steam turbine with exhaust pressure above design will reduce cycle efficiency and power output, the thermodynamic efficiency may remain essentially the same. But operating it below the design exhaust pressure may reduce the power output and cause damage due to moisture formation in the low pressure stages of the turbine. While stack temperature is an indicator of boiler efficiency, it is not a good indicator of overall plant efficiency in converting fuel to electrical power. A boiler running at 5 bar can more readily be made efficient than a boiler running at 50 bar; however, the 50 bar plant will more likely produce more power because the cycle efficiency is higher, even if the stack temperature is somewhat higher. Sometimes design optimization leads to multi-level steam generation.

Overall, you have asked a lot of questions regarding power generation. It would help people here give better answers if we know what your background and responsibilities are? Engineer, technician, design, operation, optimization, etc.?

- Steam Flow affects mainly Power Output. Efficiency is another story.

- The steam turbine with the higher exhaust pressure is less efficient (at first sight).

Or it is less efficient than what it could be!

We are talking efficiency of the whole cycle: Energy input to energy derived at the shaft. This includes all the required energy to make the system function. Therefore, the answer is not 100% valid since the whole system is unknown. Therefore, it is not absolutely clear.

Many thanks for answers all. I understand what you guys write. In brief, HRSG and Steam turbine efficiency are different matters, aren't they?

As for Steam turbine efficiency, could anyone please let me know the equation? As far as I concern, enthalpy (kcal/kg) is engaged in the equation, but I hope to know how steam flow are affected. Please suppose two pressure system in condensing mode, only electricity production mode.

In above question, I mean the equation for steam turbine cycle (bottoming cycle) efficiency, not steam turbine itself.

And I hope to see how steam flow are related to cycle efficiency and if possible, how higher steam turbine exhaust temperature, for example, 50 and 60 degrees in Celsius, affects to the efficiency when stack end temperature is the same. Thanks for explaination in advance.

You can google " Bottoming Cycle" and select whichever topic might help you.

Try: http://www.uwasa.fi/materiaali/pdf/isbn_978-952-476-389-9.pdf

OR http://en.wikipedia.org/wiki/Rankine_cycle

It is up to you to analyse your overall system and make your equation. Every section of the circuit can be studied separately.

Refer to this link

http://zh.scribd.com/doc/46194431/Combined-cycle-Gas-and-Steam-Turbine-Power-Plants