ENERGY CONVERSION

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• 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

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Motivation Recognizing that a phase change is
the most thermally effective way of transforming
heat, many large - scale ( gt500 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
largescale operations. Objective (1) Review of
basic Rankine Cycle. (2) Factors affecting
efficiency
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We shall be examining sub-system A
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4
2
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Components sub-systems of a simple vapour power
plant
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(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 to the
isentropic compression. (
) 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. ( )
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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 mixture with high quality. (
) 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 to enter the pump at state 1. (
)
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P2
2
3
Boiler
T
P
G
Condenser
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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))
P Pump T Turbine G Generator
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(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
Turbine
Boiler
Overall thermal
B
Qadd heat addition to the boiler
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(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.
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(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).
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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
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Example of Rankine cycle Consider a steam power
plant operating on the ideal Rankine cycle. The
steam enters the turbine at 3 MPa and 350C 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
600C instead of 350C. (c) the overall thermal
efficiency if the boiler pressure is raised to 15
MPa and the turbine inlet is maintained at
600C. Well see how the the thermal efficiency
increases from 33.5 to 43.0
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Case (a)
Case (b)
Case (c)
Steam is superheated to 600C instead of 350C
Steam is superheated to 350C
Steam is superheated to 600C boiler pressure
is raised to 15 MPa
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Part (a)
From tables
Superheated steam tables
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Part (a) - continued
Thus, the overall thermal efficiency is 33.5
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Part (b)
Thus, the overall thermal efficiency is increased
to 37.3
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Part (c)
Thus, the overall thermal efficiency is increased
to 43.0