Week 8. Vapor And Combined

Power Cycles III

GENESYS Laboratory

Objectives

1. Analyze vapor power cycles in which the working fluid is alternately vaporized and condensed.

2. Investigate ways to modify the basic Rankine vapor power cycle to increase the cycle thermal efficiency

3. Analyze the reheat and regenerative vapor power cycles 4. Analyze power cycles that consist of two separate cycles known as

combined cycles and binary cycles 5. Analyze power generation coupled with process heating called

cogeneration

GENESYS Laboratory

The Ideal Regenerative Rankine Cycle

The first part of the heat-addition process in the boiler takes place at relatively low temperatures

GENESYS Laboratory

• Shortcoming of basic Rankine cycle: The low average heat addition temperature reduces the cycle efficiency • Solution: Need to raise the temperature of the liquid leaving the pump before it enters the boiler • The steam is used as feedwater instead of expanding further in the turbine • Regeneration - Improves cycle efficiency - De-aerates the feedwater to prevent corrosion in the boiler - Helps control the large volume flow rate of the steam at the final stages of the turbine • Open feedwater heater vs. closed feedwater heater

Open Feed-water Heaters

• An open feedwater heater is basically a mixing chamber, where the steam extracted from the turbine mixes with the feedwater exiting the pump • The heat and work interactions are

The ideal regenerative Rankine cycle with an open feedwater heater

in 5 4

6out 7 1

5

turb,out 5 6 6 7

pump,in pump ,in pump ,in pump ,in 1 2 1 pump ,in 3 4 3

7 1outth,reg

in 5 4

1 (fraction of steam extracted)

1

1 ,

11 1

q h h

mq y h h y

m

w h h y h h

w y w w w v P P w v P P

y h hq

q h h

Ⅰ Ⅱ Ⅰ Ⅱ

GENESYS Laboratory

Closed Feed-water Heaters

• The heat is transferred from the extracted steam to the feedwater without any mixing taking place

The ideal regenerative Rankine cycle with a closed feedwater heater

56

18

in

outregth,

111

hh

hhy

q

q

GENESYS Laboratory

Open vs Closed Feed-water Heaters

A steam power plant with one open and three closed feedwater heater

• Open feed-water heater - simple and inexpensive - good heat transfer characteristics - pumps are required to handle the feedwater

• Closed feed-water heater - complex and expensive due to the internal tubing network - less effective since the two streams are not allowed to be in direct contact - do not require a separate pump for each heater

GENESYS Laboratory

The condensed steam is then pumped to the feedwater line or routed to another heater or to the condenser through a device called a trap, which allows the liquid to be throttled to a lower pressure region but traps the vapour.

Ex 5) The Ideal Regenerative Rankine Cycle

GENESYS Laboratory

Ex 5-1) The Ideal Regenerative Rankine Cycle

Consider a regenerative cycle using steam as the working fluid. Steam leaves the boiler and enters the turbine at 4 MPa, 400C. After expansion in the turbine to 400 kPa, some of the steam is extracted from the turbine to heat the feedwater in an open feedwater heater. The pressure in the feedwater heater is 400 kPa, and the water leaving it is saturated liquid at 400 kPa. The steam not extracted expands to 10 kPa. Determine the cycle efficiency.

GENESYS Laboratory

Ex 6) The Ideal Reheat-Regenerative Rankine Cycle

GENESYS Laboratory

Cogeneration

• The production of more than one useful form of energy (such as process heat and electric power) from the same energy source -Process heat: require energy input in the form of heat • The process heat has high quality of energy that can be used for certain industrial process, for example, textile, food processing, steel making, and refining industries

An ideal cogeneration plant

GENESYS Laboratory

Cogeneration

• All the energy transferred to the steam in the boiler is utilized as either process heat or electric power • Utilization factor: how much energy transferred to the steam in the boiler is utilized as either process heat or electric power

net P outu

in in

Net work output+Process heat delivered or 1

Total heat input

W Q Q

Q Q

Cogeneration tradition

Electricity

Heat

Loss Loss

Boiler

Korean Electricity

Energy saving

GENESYS Laboratory

87/100 87/159

Combined Gas-Vapor Power Cycles

• The gas-turbine (Brayton) cycle topping a steam turbine (Rankine) cycle • It has a higher thermal efficiency than either of the cycles executed individually

Combined gas-steam power plant

GENESYS Laboratory

Binary Vapor Cycles

• Water is the best working fluid presently available, but it is far from being the ideal one • The binary cycle is an attempt to overcome some of the shortcomings of water and to approach the ideal working fluid by using two fluids • A combination of two cycles, one in the high-temperature region and the other in the Low-temperature region

Mercury-water binary vapor cycle

GENESYS Laboratory