Transcript Slide 1

ENERGY CONVERSION
ES 832a
Eric Savory
www.eng.uwo.ca/people/esavory/es832.htm
Lecture 12 – Large-scale plants
Department of Mechanical and Material Engineering
University of Western Ontario
Motivation:
Recognizing that a phase change is the most
thermally effective way of transforming heat,
many large - scale ( >500 MW ) plants use steam
cycles (Rankine Cycles). The drawback is the
need for large (bulky) equipment and additional
safety concerns. Economically, these additional
costs can be justified only for very large–scale
operations.
Objective:
(1) Review of basic Rankine Cycle.
(2) Factors affecting efficiency
We shall be examining
sub-system A
3
4
2
1
Components & sub-systems of a simple vapour power plant
(1) Simplified Rankine Cycle:
1 - 2 Isentropic compression in pump :
Water enters state 1 as saturated liquid and is
compressed to the operating pressure in the
boiler. A slight increase of temperature occurs due
 m
to the isentropic compression. ( W
 (h2  h1 ) )
P
2 - 3 Constant pressure heat addition in boiler:
Water enters as compressed liquid at state 2 and
exits as super heated vapour at state 3. The boiler
(steam generator) is a heat exchanger transferring
heat (from combustion of coal as propane, or heat
of a nuclear reaction) to the water.
 m
 (h3  h2 ) )
(Q
b
3 - 4 Isentropic expansion through a turbine:
The superheated vapour at state 3 loses energy
through the turbine and exits at state 4. At this
state, the steam is usually a saturated liquid-vapour
T m
 (h3  h4 ))
mixture with high quality. ( W
4 - 1 Constant pressure rejection of heat in
condenser:
Steam is condensed and heat is rejected to the
environment. (Note: this heat can be reintroduced
into the cycle to increase overall efficiency. This
recuperation of heat is also called regeneration).
The water leaves the condenser as saturated liquid
 m
to enter the pump at state 1. ( Q
 (h4  h1 ))
c
P2
2
Boiler
P
1
3
T
Condenser
Actual cycle includes pressure losses:
P2 – P3’ = Pressure loss in boiler
P4 – P1 = Pressure loss in condenser
and entropy increases (dependant on
the isenthalpic efficiencies (12, 34))
G
4
P = Pump
T = Turbine
G = Generator
(2) Increasing the efficiency of the Rankine cycle
Given the large energy production rate, even a
small increase in efficiency will result in economic
benefits.
Basic efficiencies of concern are the isentropic
efficiencies:
Pump
Boiler
Turbine
B
Overall thermal
Qadd = heat addition to the boiler
(a) Lowering the condenser pressure, P1
- The steam enters the condenser as a saturated
mixture corresponding to the pressure inside the
condenser. Thus, lowering P1 automatically
lowers the temperature at which heat is rejected.
Typically, P1 is selected close to Patm.
(b)
Increasing the temperature of the
superheated steam, T3 at constant P3
- Increases temperature drop through turbine.
- Increases quality of vapour at exit.
(c)
Increasing boiler pressure, P3
This increases the temperature at which boiling
takes place (i.e. increases heat transformed to
steam). An undesirable effect is that the vapour
quality can drop at outlet of turbine. This is
usually remedied by reheating the steam.
Reminder: Steam quality = proportion of saturated steam
in a saturated water/steam mixture. A steam quality of 0
means 100% water while a steam quality of 1 means 100%
steam. Steam quality is useful in determining enthalpy of
saturated water/steam mixtures since the enthalpy of
steam (gaseous state) is many orders of magnitude higher
than the enthalpy of water (liquid state).
SUMMARY:
For large-plants, phase changes can effectively be
used to increase overall thermal performance.
Typically, for steam cycles, th ~ 40% - 50% whereas
for gas cycles th ~ 30% - 40%.
The extra cost of large installations can only be
justified by the large power outputs. Much of the
cost arises from:
(1) Higher operating pressures.
(2) Fuel preparation.
(3) Closed cycle operation.
Thermal efficiency is increased through increasing
the boiler pressure and temperature, while reducing
the condenser pressure. The following example
demonstrates some of these efficiency gains:
Example of Rankine cycle
Consider a steam power plant operating on the
ideal Rankine cycle. The steam enters the turbine
at 3 MPa and 350°C and is condensed in the
condenser at 10 kPa (all pressures are gauge).
Determine:
(a) the overall thermal efficiency of the plant
(b) the overall thermal efficiency if the steam is
superheated to 600°C instead of 350°C.
(c) the overall thermal efficiency if the boiler
pressure is raised to 15 MPa and the turbine inlet
is maintained at 600°C.
We’ll see how the the thermal efficiency increases
from 33.5% to 43.0%:
Case (a)
Steam is
superheated
to 350°C
Case (b)
Case (c)
Steam is
superheated to
600°C instead
of 350°C
Steam is
superheated to
600°C + boiler
pressure is
raised to 15 MPa
Part (a):
From
tables
Superheated
steam tables
Part (a) - continued:
Thus, the overall thermal efficiency is 33.5%
Part (b):
Thus, the overall thermal efficiency is increased to 37.3%
Part (c):
Thus, the overall thermal efficiency is increased to 43.0%