Combine Cycle Power Plant(Reciprocating Engine and Steam Turbine)( Flexicycle)

Combine Cycle Power Plant-Engine + Steam Turbine

This combination combine Reciprocating Engine's(Otto cycle/Diesel cycle) and Steam Turbine's ( Rankine Cycle).
The reciprocating engine's exhaust gases are made to pass through exhaust gas boiler which extract their energy to make steam which in turn use in steam turbine to convert it into electrical energy by electric generator.

    

These power plants have combine cycle efficiency of 45-48%.

This vapor cycle is also called Flexicycle. Wartsila is the most efficient engine manufacturer internationally with model 31 SG Engine to be world's most efficient Engine.  For Flexicycle Overview
https://www.youtube.com/watch?v=NPI1P6ex9_Y   

These power plant are of small capacity with power generation capacity ranging from a few MW to 400MW and these power plant are supportive plant for emergency power source or for any maintenance work taken as stand by for bigger power plant such CCPP(gas turbine+steam turbine) and Steam turbine of Capacity from 1200MW to 4800MW and also for Hydel Power Plant of capacity from 150MW to 22000MW. 

Engines' efficiency depend upon the category like Slow, Medium and High speed engine with slow speed engine to be more efficient (45-50%)as the thermal process efficiently followed in them as compared to that in medium/high speed.

And Diesel Engines are less efficient than petrol engine but their power producing range are much higher than that of petrol engine as their compression ratio (15 to 23) is higher as compared to that of  petrol engine (8 to 12).

Diesel engines are often run on furnace oil which is cheap but having less calorific value and render engines to higher maintenance outages. And these engine produce more environmental problem as this fuel is most dirty fuel from refineries.

With the readily available less fuel cost of Gas fuel( LNG, CNG), with lower maintenance routines gas engines run on Otto cycle are becoming more famous in today's energy mix.

Gas engines CC power plant are of less power producing capacity (5MW to 100MW ) as compared to Diesel Engine CC power plants.

Gas engines power plants are cleaner and required less auxiliary equipment because of cleaner fuel as compared to that diesel engines. So installation cost and auxiliary load are lesser for gas engine CC power plants.

Single Cycle Steam Turbine Cycle Efficiency

Single Cycle Steam Turbine 

When it comes to thermal engines and vapor power cycles, pressure and temperature or Enthalpy  or simply the energy level  which of the fluid ( working fluid ) entering into the engine is the one of most important factor on which engines are designed and efficiency with which energy is extracted. 

Steam turbine cycle is called Rankine Cycle which is the combination of thermal processes which made a thermal cycle completed and effective practically. Rankine Cycle Ideally contain four processes two of them add energy into a fluid and other two extract energy from other fluid.

The Processes are as follows:

• Isentropic expansion of steam in turbine( energy extraction for Electricity from Turbine ).
• Constant Pressure heat removal in condenser.
• Isentropic compression in a pump( famously known as Feedwater Pump)
• Constant Pressure heat addition in boiler.

This Cycle wasn't efficient enough to produce electricity, efforts after designing of blades, were done to increase its efficiency which include:

• Reheating 
• Regeneration 
• Super Critical cycle

Reheating 

The Idea is to use the energy of the steam (at around 50 bar, 550 C)  first at higher enthalpy , at first portion (High pressure turbine) and then this steam expands and give energy to turbine and this steam reaches at lower pressure( around 30 bar, 380 C ) and then here only option to increase the temperature which is done by reheating the steam in Super-heater of boiler and increase its temperature. Then this steam at IP portion(intermediate pressure turbine) same pressure but higher temperature ( with higher enthalpy as compared to at the end of HP turbine) enter into IP turbine.

Usually in higher capacity power plants, steam turbine with double reheating is being used, more than two reheat is theoretically won't be effective.

Double reheating is done by reheat the steam at the end of IP turbine in the super-heater and then the same process is done with now at lower pressure and temperature. Steam then enter into the last portion of turbine which called LP turbine( Low Pressure Turbine).

Regeneration

There is regeneration cycle which is called Stirling cycle, follows in Stirling Engine, whose ideal efficiency is same as Carnot Cycle and Ericsson Cycle, which are ideal cycle.
In power plant, regeneration cycle or regeneration is used to achieve the positive side of regeneration cycle along with reheating, co generation and super-critical cycle to increase overall efficiency.

This regeneration cycle is used by extracting high pressure and temperature steam from turbine intermediate stages to different other equipment(Auxiliaries of steam turbine cycle steam heater,ejector, deareator etc.). Air used for combustion of fuel is preheated and feed water is heated using steam etc.

Steam Turbine Thermal  Efficiency is maximum to the date is 42 % .