Vapor ,Gas and Combined Power Cycles

Types of Cycles Heat Engine Rankine Gas Power Systems Brayton Internal Combustion Engines Otto, Diesel,Stirling, Atckison Refrigeration Heat Pump Air Conditioning

Carnot Cycle Feed pump Boiler Condenser Energy reservoir at high temperature, TH low temperature, TL Turbine W 2 3 4 1

1. Operation (4-1). 1 kg of boiling water at temperature T1 is heated to form wet steam of dryness fraction x1. Thus heat is absorbed at constant temperature T1 and pressure p1 during this operation. 2. Operation (1-2). During this operation steam is expanded isentropically to temperature T2 and pressure p2. The point ‘2’ represents the condition of steam after expansion. 3. Operation (2-3). During this operation heat is rejected at constant pressure p2 and temperature T2. As the steam is exhausted it becomes wetter and cooled from 2 to 3. 4. Operation (3-4). In this operation the wet steam at ‘3’ is compressed isentropically till the steam regains its original state of temperature T1 and pressure p1.

Trained as a civil engineer, William Rankine (1820-1872)was appointed to the chairman of civil engineering and mechanics at Glasgow in 1855. He worked on heat, and attempted to derive Sadi Carnot's law from his own hypothesis. He was elected a Fellow of the Royal Society in 1853. Among his most important works are Manual of Applied Mechanics (1858), Manual of the Steam Engine and Other Prime Movers (1859) .

Vapor Power System Model

Rankine Cycle, Ideal

Process 1-2: Isentropic expansion of the fluid through the turbine from saturated vapor to the condenser pressure. Process 3-4: Isentropic compression of fluid to compressed liquid. Process 4-1: Heat transfer at constant pressure through the boiler to saturated vapor Process 2-3: Heat transfer at constant pressure through the condenser to saturated liquid.

Rankine Cycle Energy Analysis Energy balance, each process For pump

Rankine Cycle Energy Analysis - For boiler -For turbine -For condenser

Rankine Cycle Energy Analysis Thermal efficiency

Usually, The properties: p1, t1 and p2 are available for a power plant,then: From p1, t1 , get h1 , s1 h2: From p2 , get s2f , s2fg h2f , h2fg So, x can be known h4 : From p1 , s1= s4 get h4 h3: From p2 , get h2f , s2f h3= h3f s3= s3f’

Real vs. Ideal Cycle

Real vs. Ideal Cycle Major difference is irreversibilities in pump and turbine

Increase Efficiency? - Lower condenser pressure - Increase superheat temperature

- Increase Efficiency? - Increase boiler pressure

Reheat

- Reheat - Equations become: - Purposes of reheat: keep turbine inlet temps within limits, increase quality of steam in last stages of turbine

Ideal Regenerative Rankine Cycle Regeneration: effective use of energy Open (direct contact) feedwater heaters (mixing chambers) Closed feedwater heaters (heat exchangers)

Ideal Regenerative Rankine Cycle

Open feedwater analysis: #2 Mass Balance: #1 #3 Often will use the term “blend fraction” consequently, Energy Balance: Thus:

Energy balances: For the turbines: and For the condenser: For the pump: For the boiler:

 

Ideal Regenerative Rankine Cycle

Ideal Regenerative Rankine Cycle

Regeneration Preheats steam entering boiler using a feedwater heater, improving efficiency Also deaerates the fluid and reduces large volume flow rates at turbine exit.

A more complicated cycle…

Reheat and Regenerative

As closing comment: You now have most of the tools you would need to analyze some relatively complicated vapor power generation units and be able to discuss their respective efficiencies or how to suggest improvements in their performance... (not that you would want to).

Combined Gas-Vapor Power Cycle Use of two cycles to maximize efficiency Gas power cycle topping a vapor power cycle Combined cycles have higher efficiency than either independently Works because: Gas turbine needs high combustion temp to be efficient, vapor cycle can effectively use rejected energy

Combined Cycle

Combined Cycle Combining Rankine and Brayton cycles

Combined Gas-Vapor Power Cycle Use of two cycles to maximize efficiency Gas power cycle topping a vapor power cycle Combined cycles have higher efficiency than either independently Works because: Gas turbine needs high combustion temp to be efficient, vapor cycle can effectively use rejected energy

Combined Cycle

Combined Cycle Combining Rankine and Brayton cycles