JOURNAL OF APPLIED SCIENCES RESEARCH

Copyright © 2014, American-Eurasian Network for Scientific Information publisher
JOURNAL OF APPLIED SCIENCES RESEARCH
JOURNAL home page: http://www.aensiweb.com/JASR.
2014 May; 10(5): pages 516-522.
Published Online 2 2 June 2014.
Research Article
Increasing Advanced Rankine cycle efficiency Using Thermodynamics concept
Amir vosough
Young Researchers and Elite Club, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran.
Received: 15 April 2014; Revised: 2 0 May, 2014; Accepted: 25 May 2014
© 2014
AENSI PUBLISHER All rights reserved
ABSTRACT
Scin This study focuses on the thermodynamic Rankine cycle. Rankine cycle used in this study has one reheat and at least two
extracts, between the turbines and the feed water heaters. Concepts of energy and exergy are applied in analysis of thermodynamic cycles.
After analysis of energy and exergy of the cycle and determination of energy and exergy for each point and energy and exergy losses, the
cycle will be optimized. One of the ways of Rankin cycle optimization is to optimize the efficiency of the cycle with respect to
temperature, pressure, and mass flow rate of the turbine extracts in order to maximize the efficiency. To do this, the whole cycle must be
modeled by a computer code in terms of thermodynamic and the efficiency of the cycle must be examined by optimization algorithms such
as genetic algorithm. After optimization, the cycle is examined in terms of economy in order to determine how much will be the cost
savings. The purpose of economic evaluation is to allocate the price to per unit of energy and exergy. Then according to the useful output
of the cycle and its optimized efficiency and comparing it with the state before cycle optimization, the cost savings will be determined.
Key words:
INTRODUCTION
Rankine cycle is used in thermal power plants.
Steam power plants are one of the most important
thermal power plants which are responsible for a
significant share of electrical energy production in
most countries, includingIran. Paying attention to the
efficiency of power generation units in power plants
is very important because of fuel consumption
optimization, air pollution reduction, maintenance
costs
reduction,
and
production
increase.
Unfortunately, the efficiency of power plants has not
been paid enough attention in our country and there
have not been any special efforts to increase their
efficiency. However, due to increasing demand for
electrical energy and also saving fossil fuels, the
efficiency of power plants has been taken into
consideration in recent years. See more in [1-7].
Methods for increasing the efficiency of the Rankine
cycle:
Steam power plants produce most of the world's
electricity and even the smallest increase in their
thermal efficiency can save a lot of fuel. Therefore, it
is attempted to increase the efficiency of steam
power plant cycle. The main idea of all changes for
increasing thermal efficiency of the power cycle is
one thing: the increase in average temperature under
which the heat in the boiler is given to the working
fluid, or the decrease in average temperature under
which the heat in the condenser is excreted from the
working fluid. It means that the average temperature
of the fluid should be as high as possible in heat
absorption process and it should be as low as
possible in heat rejection process. Now, three ways
will be investigated to do this on a simple ideal
Rankine cycle.
Lowering the condenser pressure:
Water vapor in condenser is in the form of a
saturated mixture and its temperature is equal to the
saturation temperature corresponding to the
condenser pressure. Therefore, lowering the
condenser pressure causes a decrease in water vapor
temperature and the temperature under which the
heat is excreted. The effect of the condenser pressure
on the Rankine cycle efficiency is shown in T-S
Chart. Turbine inlet state is kept constant in order to
do the comparison. The shaded area in the chart
shows an increase in output net work based on
lowering condenser pressure from
p4
P
.The
to 4
required heat input, the area under the curve (
2  2
), increases, but this increase is very small.
Therefore, the overall effect of lowering the
condenser pressure is the increase in thermal
efficiency
of
the
cycle.
Corresponding Author: Amir vosough, Young Researchers and Elite Club, Mahshahr Branch, Islamic Azad University,
Mahshahr, Iran.
517
Amir vosough et al, 2014 /Journal Of Applied Sciences Research 10(5), May, Pages: 516-522
Fig. 1: Increasing Rankine cycle efficiency by lowering the condenser pressure
Super-heating the water vapor to high temperatures:
Super-heating the water vapor to high
temperatures can increase the average temperature
under which the water vapor is heated without
increasing the pressure in the boiler. The effect of
super-heating on the performance of steam power
cycles is shown in T-S Chart. The shaded area shows
increase in the net work and the total area under the
curve shows increase in input heat. Therefore, due to
super-heating the water vapor, the net work and input
heat increases. The overall effect of these increases is
the increase in thermal efficiency, because of the
increase in the average temperature under which the
heat is generated.
Fig. 2: Increasing Rankin cycle efficiency by super heating the water vapor to high temperatures
Super-heating the water vapor has another
desirable effect: As it can be seen from TS Chart, it
reduces the moisture content of water vapor in
turbine exhaust (the quality in mode
than the quality in mode 4).
4
is greater
Increasing the boiler pressure:
Another way to increase the average temperature
in the process of heat absorption is to increase the
function pressure of the boiler. This causes an
increase in boiling temperature. Therefore, it
increases the efficiency of the cycle and the average
temperature under which the water vapor is heated.
The effect of increasing the boiler pressure on the
performance of steam power cycles is shown T-S
chart. Note that for a given temperature entering the
turbine, the cycle shifts to the left and the moisture
content of the water vapor increases in the turbine
output. However, as we will explain in the next
518
Amir vosough et al, 2014 /Journal Of Applied Sciences Research 10(5), May, Pages: 516-522
section, this undesirable side effect can be eliminated
by reheating the water vapor.
Fig. 3: Increasing Rankin cycle efficiency by increasing the boiler pressure
In the previous section we saw that increasing
the boiler pressure increases the thermal efficiency of
the Rankine cycle. However, it increases the
moisture content of water vapor to an unacceptable
extent. Obviously, we will ask: How can we use the
increased efficiency in high pressure of the boiler
without encountering the problem of excess moisture
in the final stages of the turbine?
There are two solutions:
1 - Super-heating the water vapor to high
temperatures before it enters the turbine. This is a
desirable solution, because the average temperature
under which the heat is given increases and hence the
cycle efficiency also increases. However, this is not
Fig. 4: Rankin cycle with a reheat
the final solution, because it requires an increase in
water vapor temperature to the extent that is
metallurgically unreliable.
2- Expanding the water vapor of turbine in two
stages and its reheating between these stages. In
other words, the simple ideal Rankine cycle should
be modified with reheating process. Reheating is a
practical approach for the problem of excess
moisture in turbines and often is used in modern
steam power plants.
T-S chart shows an ideal Rankine cycle with
reheating. The scheme of a power plant working on
the basis of this cycle is shown in the figure.
519
Amir vosough et al, 2014 /Journal Of Applied Sciences Research 10(5), May, Pages: 516-522
Fig. 5: Schematic of a power plant using Rankine cycle with a reheat
The expansion process in the simple ideal
Rankine cycle with reheat occurs in two stages; in
this regard, it is different from the simple ideal
Rankine cycle.In the first stage (high pressure
turbine), water vapor is expanded to a mediumpressure in a single-entropy process and returns to
the boiler. Then, it is reheated in a constant pressure
usually to the inlet temperature of the first floor of
the turbine. In the second stage (low pressure
turbine), water vapor is expanded to the condenser
pressure in a single-entropy process. Therefore, the
total heat input and total work output of the turbine
for a cycle with reheat are as follows:
Where z is the fraction of steam extracted from


the turbine (  m9 / m5 ) at the second stage. Solving
for z:
z
h3  h2   yh7  h2 
h10  h2
(6)
qin  h5  h8
qout  (1  y  z )(h11  h1 )
wnet  qout  qin
(7)
(8)
(9)
qin  qBoiler  qRe heat  (h3  h2 )  (h5  h4 )
W
m  net
w net
(10)
wTturb .  wT 1  wT 2  (h3  h4 )  (h5  h6 )
q
 th  1  out
q in
(11)
(1)
Energy Analyse
Result:
The fraction of steam extracted is determined
from the steady- flow energy balance equation
applied to the feed water heaters. Noting that
Q  W  ke  pe  0
Since we know, the external irreversibility of
Rankine cycle is due to temperature difference in
heat transfer surfaces and the main part of which is
related to heat transfer in the boiler and especially in
Economizer. It means that temperature difference
between the combustion gases and the water entering
the boiler increases the irreversibility. To reduce this
irreversibility, the water should be heated gradually
using a fluid with a temperature close to the water
temperature. Feed water heaters are used for this
purpose and turbine extract steam is used for heating
the water. Steam heat is given to water through a heat
exchanger.
y
h5  h4
h9  h6
(2)
For the open FWH,
E in  E out  E system  0
 m h   m h
i i
(3)
e e
 m 7 h7  m 2 h2  m 10 h10  m 3 h3
(4)
yh7  (1  y  z )h2  zh10  1h3
(5)
There are three types of conventional feed water
heaters:
1 - Open or direct-contact
520
Amir vosough et al, 2014 /Journal Of Applied Sciences Research 10(5), May, Pages: 516-522
2 - Closed with retrograded discharge
3 - Closed with progressive discharge
Large power plants usually employ one open
heater and 6 to 8 closed heaters.
Fig. 6: T-s diagram of Rankine cycle
Figure 7, shows the effects of condenser
pressure on the cycle performance. It is evident that
the efficiency decrease with an increase in the
condenser pressure parameters. Decreasing the cycle
Condenser pressure and temperature will result to
higher power output for the same mass flow rate of
steam and fuel input into the boiler. Figure 8
depicted the work output of turbinesvs condenser
pressure.
Figure 9 show the effects of the boiler pressure
and temperature on the cycle performance. It is
evident that the efficiency rises with an increase in
the superheated steam parameters. Increasing the
cycle steam pressure and temperature will result in a
higher power output for the same mass flow rate of
steam and fuel input into the boiler. The steam has
higher energy/exergy content, resulting in higher
work output of the turbine. This option however
should be balanced with higher capital costs of the
equipment, due to material considerations. The boiler
tubes and turbine blades are made of higher cost steel
alloys. The benefits of higher revenues from the
excess power output should be balanced against the
increase in capital cost to ensure that the „„payback
period” on the investment is favorable.Figure 10 and
11 shows the effects of Thermal efficiency vs pump
and turbine efficiency.
0.444
th
0.44
0.436
0.432
0.428
0.424
4
6
8
10
12
14
Condenser pressure (kPa)
Fig. 7: Thermal efficiency vs condenser pressure
16
18
20
521
Amir vosough et al, 2014 /Journal Of Applied Sciences Research 10(5), May, Pages: 516-522
1240
wturb (kW)
1230
1220
1210
1200
1190
1180
4
6
8
10
12
14
16
18
20
Condenser pressure (kPa)
Fig. 8: work output vs condenser pressure
0.47
0.46
th
0.45
0.44
0.43
0.42
0.41
4000
6000
8000
10000 12000 14000 16000 18000 20000
boiler pressure (kPa)
Fig. 9: Thermal efficiency vs boiler pressure
0.4536
0.4534
th
0.4532
0.453
0.4528
0.4526
0.4524
0.4522
0.452
0.7
0.75
0.8
0.85
pump
Fig. 10: Thermal efficiency vs pump effisioncy
0.9
0.95
1
522
Amir vosough et al, 2014 /Journal Of Applied Sciences Research 10(5), May, Pages: 516-522
0.46
0.45
th
0.44
0.43
0.42
0.41
0.4
0.39
0.7
0.75
0.8
0.85
0.9
0.95
1
turb
Fig. 11: Thermal efficiency vs turbine efficiency
Conculation:
Reference
Efficiency increase and pollutant emission
control are the most significant projects of the world.
The present investigation shows the effect of boiler
and condenser pressure to Rankine cycle efficiency.
Also an optimization has done to Rankine cycle to
increase plant efficiency. The result shows: 1Decreasing the cycle Condenser pressure and
temperature will result to higher power output for the
same mass flow rate of steam and fuel input into the
boiler.
2- The Cycle efficiency rises with an increase in
the superheated steam parameters. Increasing the
cycle steam pressure and temperature will result in a
higher power output for the same mass flow rate of
steam and fuel input into the boiler.
3- Increasing inPump and turbineefficiency due
to increasing in thermal efficiency of cycle.
1.
2.
3.
4.
5.
6.
Acknowledgments
7.
Financial support for this research entitled
“Increasingadvcanced Rankine cycle efficiency” was
provided by the Mahshahr Branch, Islamic Azad
University,
Mahshahr,
Iran”
gratefully
acknowledgment.
Kotas, T.J., 1985. The Exergy Method of
Thermal Plant Analysis, Butterworths, Essex.
Al-Muslim, H., I. Dincer, S.M. Zubair, 2003.
Exergy analysis of single- and two-stage crude
oil distillation unit, Journal of Energy Resources
Technology, 125(3): 199-207.
Dincer, I., M. Rosen, 2007. Exergy: Energy,
Environment and Sustainable Development
Amazon.com.
Hajidavalloo, E., A. Vosough, 2012. Energy and
exergy analyses of a supercritical power plant,
Int. J. of Exergy, 9: 435-452.
Ghorbani, B., G.R. Salehi, M. Amidpour, M.H.
Hamedi, 2012. Exergy and exergoeconomic
evaluation of gas seperation process, Journal of
Natural Gas Science and Engineering, 9: 86-93.
Vosough, A., 2012. Thermodynamic Parametric
Study of a Supercritical Powerplant, Journal of
Thermophysics and Heat Transfer, 26: 629-637.
Vosough, A., M. Bakhshesh, 2012. Boiler
parametric study to decrease irreversibility,
Indian Journal of Science and Technology, 5:
2534-2539.