Power Stations, Cheat Sheet of Engineering

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Power Stations

MPE 316

Studied hours/week

Lecture Lab/Tutorial Total

Assis. Prof- Mahmoud Nasef

1

Fayoum University Mechanical Department Faculty of Engineering

2

Grade description marks Class work 20 Mid-term 20 Lab & oral 15 final 70 total 125

• Introduction - Electrical Thermal Energy Generation

Over 65 % of the world's electrical energy used today is generated by steam/gas

turbine generators burning fossil fuels as their source of energy

Seam boiler

4^ Gas turbine

-Rankine Cycle

5

Analyzing Vapor Power Systems — Rankine Cycle

• Process 1–2: Isentropic expansion of the working fluid through the turbine from saturated vapor at state 1 to the condenser pressure.
• Process 2–3: Heat transfer from the working fluid as it flows at constant pressure through the condenser with saturated liquid at state 3.
• Process 3–4: Isentropic compression in the pump to state 4 in the compressed liquid region.
• Process 4–1: Heat transfer to the working fluid as it flows at constant pressure through the boiler to complete the cycle.

TURBINE

Condenser

Pump

Boiler

Example

• Steam is the working fluid in a Rankine cycle. Saturated vapor enters the turbine at 8.0 MPa and saturated liquid exits the condenser at a pressure of 0.008 MPa. The net power output of the cycle is 100 Mw. The turbine and the pump each have an isentropic efficiency of 85%. Determine for the modified cycle
• the thermal efficiency,
• the mass flow rate of steam, in kg/h, for a net power output of 100 MW,
• the rate of heat transfer into the working fluid as it passes through the boiler, in MW,
• the rate of heat transfer from the condensing steam as it passes through the condenser, in MW,
• the mass flow rate of the condenser cooling water, in kg/h, if cooling water enters the condenser at 15C and exits as 35C.
• Discuss the effects on the vapor cycle of irreversibilities within the turbine and pump.

h 1 = 2758.0 kJ/kg s 1 = 5.7432 kJ/kg K h 2s = 1794.8 kJ/kg h 3 = 173.88 kJ/kg.

Solution

13 Assoc.Prof. Abd El-Hamied

HOW CAN WE INCREASE THE EFFICIENCY OF THE

RANKINE CYCLE?

• Lowering the Condenser Pressure (Lowers Tlow,avg)
• Decreasing the condenser pressure tends to increase the thermal efficiency.
• It cannot be lower than the saturation pressure corresponding to the temperature of the cooling medium.

Disadvantages 1- Air leakage into the condenser 2- It increases the moisture content of the steam at the final stages of the turbine

14 Assoc.Prof. Abd El-Hamied

• Increasing the boiler pressure

increasing the boiler pressure of the ideal Rankine cycle tends to increase the thermal efficiency

Disadvantages

the moisture content of steam at the turbine exit increases

16 Assoc.Prof. Abd El-Hamied

• How can we take advantage of the increased efficiencies at higher boiler pressures without facing the problem of excessive moisture at the final stages of the turbine?

1. Superheat the steam to very high temperatures before it enters the turbine. This is not a viable solution, however, since it requires raising the steam temperature to metallurgically unsafe levels.
2. Expand the steam in the turbine in two stages, and reheat it in between. In other words, modify the simple ideal Rankine cycle with a reheat process. Reheating is a practical solution to the excessive moisture problem in turbines, and it is commonly used in modern steam power plants.

17 Assoc.Prof. Abd El-Hamied

Ideal Reheat Cycle

19 Assoc.Prof. Abd El-Hamied

• If the turbine inlet pressure is not high enough, double reheat

would result in superheated exhaust.

• This is undesirable as it would cause the average temperature for

heat rejection to increase and thus the cycle efficiency to

decrease.

• Therefore, double reheat is used only on supercritical-pressure (P

  22.06 MPa) power plants. A third reheat stage would increase

the cycle efficiency by about half of the improvement attained by

the second reheat. This gain is too small to justify the added cost

and complexity.

20 Assoc.Prof. Abd El-Hamied

EXample

• Steam is the working fluid in a Rankine cycle with superheat and reheat. Steam enters the first-stage turbine at 8.0 MPa, 480C, and expands to 0.7 MPa. It is then reheated to 440C before entering the second-stage turbine, where it expands to the condenser pressure of 0.008 MPa. The net power output is 100 MW. If the turbine stages and pump are isentropic, determine
• the thermal efficiency of the cycle,
• the mass flow rate of steam, in kg/h,
• the rate of heat transfer from the condensing steam as it passes through the condenser, in MW. Discuss the effects of reheat on the vapor power cycle.
• If each turbine stage has an isentropic efficiency of 85%, determine the thermal efficiency.
• Plot the thermal efficiency versus the turbine stage efficiency ranging from 85 to 100%.