Note: Descriptions are shown in the official language in which they were submitted.
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TRIPLE EXPANSION WASTE HEAT RECOVERY SYSTEM AND
METHOD
BACKGROUND
[0001] The present application relates generally to power generation and,
more particularly, to a system and method for recovering waste heat from a
plurality
of heat sources having different temperatures for the generation of
electricity.
[0002] Many industrial power requirements could benefit from power
generation systems that provide electricity or mechanical power with minimum
environmental impact and that may be readily integrated into existing power
grids or
rapidly sited as stand-alone units. Combustion engines such as gas turbines or
large
reciprocating engines are suitable for power generation in industrial
applications but
rely on increasingly costly fuel and also generate emissions and waste heat.
One
method to generate electricity from the waste heat of a combustion engine
without
increasing the output of emissions and without requiring additional fuel is to
apply a
bottoming cycle. Bottoming cycles use waste heat from a heat source, such as
an
engine, and convert that thermal energy into electricity. Rankine cycles are
often
applied as the bottoming cycle for large combustion engines. Rankine cycles
are also
used to generate power from geothermal or industrial heat sources. A
fundamental
Rankine cycle includes a turbogenerator, a boiler, a condenser and a feed
pump.
[0003] In one conventional system provided to generate electricity from
waste
heat, a Rankine cycle system using carbon dioxide as working fluid is used
along with
a recuperator. However, the amount of heat that can be recovered from the
waste heat
source is limited as a boiler inlet temperature of the working fluid increases
after
passing the recuperator. The boiler efficiency declines and the heat input as
well as
power output is limited.
[0004] There is therefore a need for an efficient Rankine cycle system
that
utilizes the most waste heat and generates an increased net power output.
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BRIEF DESCRIPTION
[0005] In accordance with an embodiment of the invention, a waste heat
recovery system is provided. The waste heat recovery system includes a Rankine
cycle system for circulating a working fluid. The Rankine cycle system
includes at
least one first waste heat recovery boiler configured to transfer heat from a
heat
source to the working fluid. The Rankine cycle system also includes a first
expander
configured to receive the heated working fluid from the at least one first
waste heat
recovery boiler. Further, the Rankine cycle system includes a second expander
and a
third expander coupled to at least one electric generator. The waste heat
recovery
system also includes a condenser configured to receive the working fluid at
low
pressure from the first expander, the second expander and the third expander
for
cooling and a pump connected to the condenser for receiving a cooled and
condensed
flow of the working fluid from the condenser, wherein the pump is configured
for
pumping the condensed working fluid to a primary flow of the working fluid
into the
first waste heat recovery boiler, a secondary flow of the working fluid into
the second
expander and a tertiary flow of the working fluid into the third expander.
[0006] In accordance with an embodiment of the invention, a waste heat
recovery system is provided. The waste heat recovery system includes a Rankine
cycle system for circulating a working fluid. The Rankine cycle system
includes at
least one first waste heat recovery boiler configured to transfer heat from a
stream of
hot gases or flue gases to the working fluid. The Rankine cycle system also
includes
a first expander configured to receive the heated working fluid from the at
least one
first waste heat recovery boiler. Further, the Rankine cycle system includes a
second
expander coupled to the first expander and a third expander coupled to the
second
expander such that the first expander, the second expander and the third
expander are
coupled directly or indirectly to each other in series and further coupled to
a
generator. The waste heat recovery system also includes a condenser configured
to
receive the working fluid at low pressure from the first expander, the second
expander
and the third expander for cooling. Further, the waste heat recovery system
includes a
pump connected to the condenser for receiving a cooled and condensed flow of
the
working fluid from the condenser, wherein the pump is configured for pumping
the
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condensed working fluid to a primary flow of the working fluid into the first
waste
heat recovery boiler, a secondary flow of the working fluid into the second
expander
via a first recuperator and a tertiary flow of the working fluid into the
third expander
via a second recuperator. Furthermore, the waste heat recovery system includes
at
least one second waste heat recovery boiler configured for heating the
secondary flow
of the working fluid exiting the first recuperator prior to entering the
second expander.
[0007] In accordance with an embodiment of the invention, a method of
recovering waste heat for power generation using a working fluid in a Rankine
cycle
is provided. The method includes pumping a primary flow of the working fluid
though at least one first waste heat recovery boiler for transferring heat
from a stream
of hot gases or flue gases to the working fluid. The method also includes
expanding
the heated primary flow of the working fluid through a first expander.
Further, the
method includes pumping a secondary flow of the working fluid through a second
expander and pumping a tertiary flow of the working fluid through a third
expander.
Finally, the method includes passing a combination of the primary flow of the
working fluid, the secondary flow of the working fluid and the tertiary flow
of the
working fluid exiting the first expander, second expander and the third
expander
respectively through an auxiliary precooler and a condenser for condensing the
combination of the working fluid and further passing to a pump.
DRAWINGS
[0008] These and other features, aspects, and advantages of the present
invention will become better understood when the following detailed
description is
read with reference to the accompanying drawings in which like characters
represent
like parts throughout the drawings, wherein:
[0009] FIG. 1 is a diagrammatical representation of a cycle of a
recuperated
waste heat recovery system in accordance with an embodiment of the present
invention.
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[0 0 1 0] FIG. 2 is an illustrative diagram of the cycle shown in FIG. 1 as
represented by a temperature-entropy diagram in accordance with an embodiment
of
the present invention.
[0011] FIG. 3 is a diagrammatical representation of a cycle of a
recuperated
waste heat recovery system in accordance with another embodiment of the
present
invention.
[0012] FIG. 4 is a flow chart illustrating exemplary steps involved in a
method
of recovering waste heat for power generation using a working fluid in a
Rankine
cycle in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION
[0013] When introducing elements of various embodiments of the present
invention, the articles "a," "an," "the," and "said" are intended to mean that
there are
one or more of the elements. The terms "comprising," "including," and "having"
are
intended to be inclusive and mean that there may be additional elements other
than the
listed elements. Any examples of operating parameters are not exclusive of
other
parameters of the disclosed embodiments.
[0014] FIG. 1 is a diagrammatical representation of a cycle of a
recuperated
waste heat recovery system 10 in accordance with an embodiment of the present
invention. The waste heat recovery system 10 includes a Rankine cycle system
12 for
circulating a working fluid 14. In one embodiment, the working fluid is a
supercritical carbon dioxide. The Rankine cycle system 12 includes at least
one first
waste heat recovery boiler 16 configured to transfer heat from a heat source
to the
working fluid 14. The Rankine cycle system 12 also includes a first expander
18
configured to receive the heated working fluid 14 from the at least one first
waste heat
recovery boiler 16. Further, the Rankine cycle system 12 includes a second
expander
20 coupled to the first expander 18. Furthermore, the Rankine cycle system 12
includes a third expander 22 coupled to the second expander 20 such that the
first
expander 18, the second expander 20 and the third expander 22 are coupled
directly or
indirectly to each other in series and further coupled to a generator 24. Non-
limiting
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example of the expanders 18, 20, 22 include a gas turbine. In one embodiment,
each
of the first expander 18 or second expander 20 or the third expander 22 may be
coupled independently to different generators. In another embodiment, the
first
expander 18, second expander 20 and the third expander 22 may be coupled
through
gearboxes. The waste heat recovery system 10 also includes a condenser 26
configured to receive the working fluid 14 at low pressure stage 6 from the
first
expander 18, the second expander 20 and the third expander 22 for cooling. In
one
embodiment, the condenser 26 utilizes a flow of cold fluid 27 for cooling the
working
fluid 14. Further, the waste heat recovery system includes a pump 28 connected
to
the condenser 26 for receiving a cooled and condensed flow of the working
fluid 14
from the condenser 26. The pump 28 is configured for pumping the condensed
working fluid 14 to a primary flow (indicated by arrow 30) of the working
fluid 14
into the first waste heat recovery boiler 16, a secondary flow (indicated by
arrow 32)
of the working fluid 14 into the second expander 20 and a tertiary flow
(indicated by
arrow 34) of the working fluid 14 into the third expander 22. Since the
working fluid
carbon dioxide has a rather low critical temperature, condensation like in a
normal
Rankine cycle may not be attainable under warm ambient conditions. It needs to
be
understood that in this system the condenser 26 shall not be strictly limited
to a device
that fully condenses the working fluid to a liquid state but can also be a
device that
may only cool the gas to dense, supercritical state. Likewise the pump 28 may
not
only pump a liquid but also transfer and pressurize a gas leaving the
condenser 26.
[0015] In one embodiment, the first waste heat recovery boiler 16
includes a
heat exchanger section configured to transfer heat from a first stream of hot
gases or a
first flow of flue gases 17 to the primary flow (indicated by arrow 30) of the
working
fluid 14 entering the first expander 18. As shown in FIG. 1, the Rankine cycle
system
12 also includes a first recuperator 36 configured to transfer heat from the
primary
flow 30 of the working fluid 14 exiting the first expander 18 to the secondary
flow 32
of the working fluid 14 prior to entering into the second expander 20. In one
embodiment, the first recuperator 36 is an intermediate temperature
recuperator.
Further, the Rankine cycle system 12 includes a second recuperator 38
configured to
transfer heat from a secondary flow 32 of the working fluid exiting the second
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expander 20 to the tertiary flow 34 of the working fluid 14 prior to entering
into the
third expander 22. In one embodiment, the second recuperator 38 is a low
temperature recuperator.
[0016] Furthermore, in one embodiment, the Rankine cycle system 12
includes an auxiliary cooler 40 for precooling a combined flow of the primary
flow 30
of working fluid 14, the secondary flow 32 of working fluid 14 and the
tertiary flow
34 of the working fluid 14 after exiting from the first expander 18, the
second
expander 20 and the third expander 22 respectively prior to entering the
condenser 26.
In a combined heat and power (CHP) system, the heat attained in the auxiliary
cooler
40 from precooling may be used for an external process. In one embodiment, the
auxiliary cooler 40 utilizes the heat attained from precooling in the Rankine
cycle
system 12 by transferring the heat to the primary flow 30 of the working fluid
14 for
preheating prior to entering the waste heat recovery boiler 16.
[0017] As shown in FIG. 1, the cycle of the waste heat recovery 10
includes
one main loop cycle 42 indicated by stages 1, 2, 3H, 4H, 5H, and 6. The waste
heat
recovery system 10 also includes a second loop cycle 44 and a third loop cycle
46 that
are parallel to the main loop cycle 42. Such cascading of the second and third
loop
cycles 44, 46 efficiently harnesses additional remaining superheat using the
first
recuperator and second recuperator from the expanded carbon dioxide (working
fluid
14) after expansion in first and second expanders 18, 20. As shown in FIG. 1,
the
second loop cycle 44 is indicated by stages 1, 2, 31, 41, 51, 6 and the second
loop cycle
46 is indicated by stages 1,2, 3L, 4L, 6.
[0018] FIG. 2 is an illustrative diagram of the cycle 10 shown in FIG. 1
as
represented by a temperature-entropy diagram 50 in accordance with an
embodiment
of the present invention. The temperature (degree Celsius) is shown on the
vertical
Y-axis and the entropy (kilojoules per Kelvin) on the horizontal X-axis. The
temperature-entropy diagram 50 clearly indicated the main loop cycle 42
(indicated
by stages 1-2-3H-4H-5H-6-1), the second loop cycle 44 (indicated by stages 1-2-
31-
41-51-6-1), and the third loop cycle 46 (indicated by stages 1-2-3L-4L-6-1).
In the
main loop cycle 42, the liquid working fluid 14 (shown in FIG. 1) coming from
the
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condenser 26 is pumped to a very high pressure (e. g. 300 bar) at stage 2 and
subsequently heated in the waste heat recovery boiler 16. After being heated
to a
temperature approaching that of the waste heat source, the working fluid 14
generates
power in a first expander 18 (shown in FIG. 1). The working fluid 14 undergoes
an
expansion process during which the temperature and pressure of the working
fluid 14
drop in the stage 3H to 4H. Further, the low pressure working fluid 14 exiting
the
first expander 18 is cooled in the first recuperator 36 (shown in FIG. 1)
where the
working fluid transfers heat to the secondary flow 32 of working fluid 14 (as
shown in
FIG. 1) that is diverted from the primary flow 30 of the working fluid 14
after the
pump. This secondary flow 32 also expands in the second expander 20 (stage 31
to
41) that is operating at lower temperature and again heats the tertiary flow
34 of the
working fluid (shown in FIG. 1) in the same manner in a second recuperator 38,
where the temperature further drops from state 41 to 51. In one embodiment,
the
secondary flow 32 can optionally be heated further in an additional heat
exchanger
section in a waste heat recovery boiler to a higher temperature, possibly as
high as the
first stream. The tertiary flow 34 of the working fluid 14 (shown in FIG. 1)
is also
diverted from the high pressure line (primary flow 30) after the pump and
after being
heated by the secondary flow 32 in the second recuperator 38 (as shown in FIG.
1),
expands in the third expander 22 from state 3L to 4L, and is subsequently
combined
with the primary flow 30 and the secondary flow 32 at low pressure at stage 6.
In one
embodiment, the combined flow of working fluid 14 can be further cooled in a
CHP
cooler or in a recuperator by heating one of the other flows of working fluid
30, 32 or
34, before being cooled and condensed. For condensation, the carbon dioxide
working fluid 14 is cooled below a critical temperature of 30 C, otherwise a
cooled,
dense gas is formed in the condenser 26 to be supplied to the feed pump.
[0019] FIG. 3 is a diagrammatical representation of a cycle of a
recuperated
waste heat recovery system 70 in accordance with another embodiment of the
present
invention. The waste heat recovery system 70 is similar to the waste heat
recovery
system 10 as shown in FIG. 1, except that the waste heat recovery system 70
includes
a second waste heat recovery boiler 21. In this embodiment, the second loop
cycle 44
includes the second waste heat recovery boiler 21 that utilizes a flow of hot
flue gases
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or fluids 19 to further heat the secondary flow 32 of the working fluid 14,
after being
heated first in the first recuperator 36, to a temperature equivalent to the
primary flow
30 of working fluid in the first waste heat recovery boiler 16. The heating of
the
secondary flow 32 of the working fluid 14 in the second waste heat recovery
boiler 21
can lead to a thermodynamic advantage that allows for higher efficiency at
lower peak
temperature of the waste heat recovery system 70.
[0020] FIG. 11 is flow chart illustrating steps involved in method 100 of
recovering waste heat for power generation using a working fluid in a Rankine
cycle.
At step 102, the method includes pumping a primary flow of the working fluid
though
at least one first waste heat recovery boiler for transferring heat from a
stream of hot
gases or flue gases to the working fluid. At step 104, the method includes
expanding
the heated primary flow of the working fluid through a first expander.
Further, at step
106, the method includes diverting a secondary flow of the working fluid from
the
primary flow through a second expander. At step 108, the method includes
diverting
a tertiary flow of the working fluid from the primary flow through a third
expander.
Finally, at step 110, the method includes passing a combination of the primary
flow of
the working fluid, the secondary flow of the working fluid and the tertiary
flow of the
working fluid exiting the first expander, second expander and the third
expander
respectively through an auxiliary precooler and a condenser for condensing the
combination of the working fluid and directing the condensed working fluid to
a
pump.
[0021] Advantageously, the present invention utilizes carbon dioxide as
the
working fluid which can be heated to very high temperatures, leading to high
efficiency of the waste heat recovery system. Also, carbon dioxide is non-
toxic and
thermally stable working fluid. The present system and method using a triple
expansion process using three expanders with cascaded recuperators extracts
maximum power out of the available waste heat directed in the present system.
Moreover, the heating of the secondary flow of the working fluid in the second
waste
heat recovery boiler can lead to a thermodynamic advantage that allows for
higher
efficiency at lower peak temperature of the waste heat recovery system.
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[0022] Furthermore, the skilled artisan will recognize the
interchangeability of
various features from different embodiments. Similarly, the various method
steps and
features described, as well as other known equivalents for each such methods
and
feature, can be mixed and matched by one of ordinary skill in this art to
construct
additional systems and techniques in accordance with principles of this
disclosure. Of
course, it is to be understood that not necessarily all such objects or
advantages
described above may be achieved in accordance with any particular embodiment.
Thus, for example, those skilled in the art will recognize that the systems
and
techniques described herein may be embodied or carried out in a manner that
achieves
or optimizes one advantage or group of advantages as taught herein without
necessarily achieving other objects or advantages as may be taught or
suggested
herein.
[0023] While only certain features of the invention have been illustrated
and
described herein, many modifications and changes will occur to those skilled
in the
art. It is to be understood that the appended claims are intended to cover all
such
modifications and changes as fall within the true spirit of the invention.
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