TRIPLE EXPANSION WASTE HEAT RECOVERY SYSTEM AND METHOD

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. 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 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. [0010] 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 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 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- 4I-5I-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 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 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. [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.