Claims:CLAIMS
1. An energy recovery system, comprising:
a flow splitter 24 configured to split a fluid (L) into a first low temperature fluid (A) and a second low temperature fluid (D);
a heat exchanger 18 configured to transfer heat from a waste heat source 12 to the first low temperature fluid (A) to generate a first high temperature fluid (B);
a high temperature fluid expander 20 configured to expand the first high temperature fluid (B) to generate a first expanded fluid (C);
a high temperature recuperator 22 configured to transfer heat from the first expanded fluid (C) to the second low temperature fluid (D) to generate a second high temperature fluid (E);
a low temperature fluid expander 26 configured to expand the second high temperature fluid (E) to generate a second expanded fluid (F); and
a low temperature recuperator 28 configured to recover heat from the second expanded fluid (F) to generate the fluid (L).

2. The energy recovery system of claim 1, wherein the fluid (L) comprises supercritical carbon dioxide.

3. The energy recovery system of claim 1, wherein high temperature fluid expander 20 comprises a power turbine.

4. The energy recovery system of claim 1, wherein low temperature fluid expander 26 comprises a power turbine.

5. The energy recovery system of claim 1, wherein high temperature fluid expander 20 and the low temperature fluid expander 26 are configured to drive a power generator 16 to generate power.

6. The energy recovery system of claim 1, wherein the low temperature recuperator 28 is configured to generate a first partially hot fluid (G), and the high temperature recuperator 22 is configured to generate a second partially hot fluid (H).

7. The energy recovery system of claim 6, further comprising a mixer 30 configured to combine the first partially hot fluid (G) and the second partially hot fluid (H) to generate a combined fluid (I).

8. The energy recovery system of claim 7, wherein the combined fluid (I) is cooled and pressurized to generate a pressurized fluid (K), and wherein the low temperature recuperator 28 is configured to recover heat from the second expanded fluid (F) and transfer heat to the pressurized fluid (K) to generate the fluid (L).

9. A method of operating an energy recovery system, comprising:
splitting a fluid (L) into a first low temperature fluid (A) and a second low temperature fluid (D);
transferring heat from a waste heat source to the first low temperature fluid (A) for generating a first high temperature fluid (B);
expanding the first high temperature fluid (B) for generating a first expanded fluid (C);
transferring heat from the first expanded fluid (C) to the second low temperature fluid (D) for generating a second high temperature fluid (E);
expanding the second high temperature fluid (E) for generating a second expanded fluid (F); and
recovering heat from the second expanded fluid (F) for generating the fluid (L).

10. The method of claim 9, wherein splitting the fluid (L) comprises splitting the fluid (L) via a flow splitter 24 based on a desired flow split ratio.

11. The method of claim 9, wherein expanding the first high temperature fluid (B) comprises generating power via a power generator.

12. The method of claim 9, wherein expanding the second high temperature fluid (E) comprises generating power via a power generator.

13. The method of claim 9, wherein transferring heat from the first expanded fluid (C) to the second low temperature fluid (D) comprises generating a second partially hot fluid (H), and recovering heat comprises transferring heat from the second expanded fluid (F) for generating a first partially hot fluid (G).

14. The method of claim 13, further comprising combining the first partially hot fluid (G) and the second partially hot fluid (H) for generating a combined fluid (I).

15. The method of claim 14, wherein generating the combined fluid (I) comprises cooling and pressurizing the combined fluid (I) for generating a pressurized fluid (K), wherein recovering heat comprises transferring heat from the second expanded fluid (F) to the pressurized fluid (K) for generating the fluid (L). , Description:SUPERCRITICAL CARBON DIOXIDE BASED WASTE HEAT RECOVERY CYCLE
BACKGROUND
[0001] The invention relates generally to the field of heat engines and, more specifically, to waste heat recovery systems.
[0002] A device that converts heat to work is called a heat engine. Heat engines operate on a working fluid to convert heat to work. Attempts have been made to utilize high efficiency working fluids such as carbon dioxide in its supercritical phase (sCO2) in heat engines.
[0003] sCO2 power cycles have higher efficiencies and offer significantly simple and compact equipment layout. This makes them potential replacements for steam Rankine cycle and air Brayton cycles for power generation. Compared to steam or a gas Brayton cycle, sCO2 cycles can be reconfigured for a wide range of operating conditions. This makes them particularly suitable for waste heat recovery. A large amount of waste heat is released untapped in the existing simple recuperated architecture. This is because during internal recuperation prior to the waste heat exchange process, the lowest temperature to which the waste heat stream is cooled is limited by the temperature of high-pressure CO2 after recuperation. As the waste fluid stream is not cooled enough, it fails to capture waste heat efficiently. Therefore, a simple recuperated sCO2 cycle is not very effective in utilizing all the exhaust heat produced.
[0004] All existing attempts for sCO2 waste heat recovery use a splitter-based flow bifurcation in the cycle layout. In these attempts, the cycle may perform well at design point. However, the power developed drops rapidly with a small change in the split ratio of the cooled fluid stream. This makes the architecture quite sensitive to fluctuations in split ratio.
[0005] Therefore, an energy recovery system that is less sensitive to fluctuations in split ratio and capable of recovering maximum amount of heat is desirable.

SUMMARY
[0006] A stable energy recovery system is provided. The energy recovery system includes a flow splitter that splits a fluid into a first low temperature fluid and a second low temperature fluid. It further includes a heat exchanger that transfers heat from a waste heat source to the first low temperature fluid while generating a first high temperature fluid. A high temperature fluid expander expands the first high temperature fluid to generate a first expanded fluid. A high temperature recuperator transfers heat from the first expanded fluid to the second low temperature fluid to produce a second high temperature fluid. A low temperature fluid expander is used to expand the second high temperature fluid to generate a second expanded fluid, and a low temperature recuperator is utilized to recover heat from the second expanded fluid while generating the fluid that is split by the flow splitter to the first and the second low temperature fluids. A method of operating the energy recovery system is also provided.
[0007] These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an illustrative representation of a system including an energy recovery system in accordance with an embodiment of the invention.
[0009] FIG. 2 is a schematic representation of an exemplary energy recovery system in accordance with one aspect of the present technique.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0010] FIG. 1 is a diagrammatical view of a system 10 including a waste heat source 12 and an exemplary energy recovery system 14 in accordance with one aspect of the present technique. The waste heat source 12 provides a heated fluid from which heat is recovered by the energy recovery system 14. During this process, a part of the heat is rejected to the surroundings. The heat energy that is recovered by the energy recovery system 14 is converted to work, which is high-grade energy, via a power generator 16.
[0011] It may be noted that in various embodiments, the waste heat source may include hot gaseous or fluid flow from gas turbines, hot air evacuation chimneys, furnaces, boilers, heat exhausted from industrial processes such as steel plant operation, cement kilns, brick kilns or glass making, combustors, nuclear reactors, gas turbine exhaust from naval vessels, as a by-product of power plants and other manufacturing operations among other diverse waste heat or low-grade heat sources. Further, low-grade heat from solar thermal sources or geothermal sources can also be utilized in the system 10 while keeping within the scope of the techniques described herein. It would be appreciated by one of ordinary skill in the art that often such generated heat is wasted away or dissipated or removed and is therefore lost.
[0012] The energy recovery system 14 includes one or more heat exchangers configured to recover heat energy from the waste heat source 12 efficiently and feed it into a system of fluid expanders in order to convert the heat energy to work by the power generator 16. The power generator 16 uses the recovered heat to convert it into mechanical energy, or to drive electrical generator, or to store power in chemical or battery-based power storage systems. In various embodiments, the power generator 16 may include alternators as well.
[0013] Referring generally to FIG. 2, an exemplary energy recovery system 14 is illustrated in accordance with certain embodiments of the present technique. Hot exhaust gases from a waste heat source 12 enters a heat exchanger 18. The waste heat source 12 is thermally coupled to the heat exchanger 18 so that the high temperature exhaust gases pass through the heat exchanger 18. The heat exchanger 18 receives a first low temperature fluid, having a low temperature and flowing in direction A, which is utilized to extract heat from the high temperature exhaust gases in the heat exchanger 18. Subsequently, the first low temperature fluid having extracted the heat achieves a high temperature, flow of which is generally shown by arrow B. The hot exhaust gases that entered heat exchanger 18 from the waste heat source 12 leave the heat exchanger stack at a lower temperature. While the above waste heat source 12 has been described with hot exhaust gases, the waste heat source may include other fluids or other forms of heat energy, such as for example solar heat, which can be equally utilized by the heat exchanger 18 to extract heat, as previously described.
[0014] Energy from the waste heat source 12, in the form of heat, is thus extracted by the heat exchanger 18 and the extracted heat in the form of the first high temperature fluid, flowing in direction B, enters a fluid expander 20. The fluid expander 20 reduces the temperature and pressure of first high temperature fluid B which leaves the fluid expander 20 at an intermediate temperature and pressure as a first expanded fluid denoted by arrow C.
[0015] The first expanded fluid C then passes through a high temperature recuperator 22, which exchanges the heat energy by transferring the heat to a second low temperature fluid flowing as denoted by arrow D into the high temperature recuperator 22 from a flow splitter 24. Thus, the second low temperature fluid D gains in temperature in the high temperature recuperator 22 and subsequently flows as a second high temperature fluid, as shown by arrow E, into a second fluid expander 26. Fluid expander 26 reduces the temperature and pressure of the second high temperature fluid E and a second expanded fluid flowing out of the fluid expander 26 shown by arrow F is fed into a low temperature recuperator 28.
[0016] In the low temperature recuperator 28, the second expanded fluid F further transfers heat energy to pressurized fluid K entering it and the resulting first partially hot fluid denoted by arrow G leaves the low temperature recuperator 28. In the high temperature recuperator 22, the first expanded fluid C, after losing heat flows out of the high temperature recuperator 22 as a second partially hot fluid H. A mixer 30 combines the first partially hot fluid G with second partially hot fluid H and the combined fluid is denoted by arrow I. This combined fluid I passes through a fluid cooling system 32, which rejects heat to the surroundings bringing down the temperature to near ambient levels. It may be noted at this point that the fluid cooling system 32 may be configured to reduce the temperature of the combined fluid I depending on the ambient temperatures. For example, in regions with low ambient temperatures such as between 18°C and 25°C, the fluid cooling system 32 may be set up to reduce the temperature of the combined fluid I to temperatures close to that, such as about 20°C to 28°C. In tropical regions with higher ambient temperatures, say between 35°C and 50°C, the fluid cooling system 32 may be designed to reduce the temperature of the combined fluid I closer to such temperatures, such as about 40°C to 60°C. In other words, the fluid cooling system 32, and therefore the entire energy recovery system 14, may be configured to operate at any ambient conditions.
[0017] After losing much of its heat in the fluid cooling system 32, it emerges as cooled fluid J. The cooled fluid denoted by J coming out from the fluid cooling system 32 is then fed into a pressure building system 34. The pressure building system 34 pressurizes the cooled fluid J into high-pressure low temperature pressurized fluid K, which may be in a liquefied state, a gaseous state or supercritical state, in various embodiments, depending upon the fluid and the phase of fluid used. As earlier noted, this pressurized fluid K is utilized to reduce the temperature of the second expanded fluid F in the low temperature recuperator 28. After gaining temperature, the pressurized fluid K emerges from the low temperature recuperator 28 as fluid L. Flow splitter 24 is utilized to split fluid L into two streams, the first low temperature fluid A which is used to recover heat from the waste heat source 12 using the heat exchanger 18, and the second low temperature fluid D flowing into the high temperature recuperator 22.
[0018] In various embodiments, the fluid L flowing out from the low temperature recuperator 28 is split into the first low temperature fluid A and the second low temperature fluid D based upon the heat source, ambient temperature and the quantum of heat available at source. As previously explained, cooled fluid J flowing out from fluid cooling system 32 is close to ambient temperatures. This ambient temperature cooled fluid J is pressurized by the pressure building system 34 and the pressurized fluid K flowing out from the pressure building system 34 is therefore at a temperature as close as possible to the ambient temperatures. After fluid L is split into the first low temperature fluid A and the second low temperature fluid D to a desired flow split ratio, the first low temperature fluid A is able to extract the maximum possible heat through the heat exchanger 18. As would be appreciated by one of ordinary skill in the art, the desired flow split ratio of the first low temperature fluid A and the second low temperature fluid D may vary during operation, due to various reasons. However, the effect of the variations in the flow split ratio for a given mass flow rate, through the pressure building system 34, on the net power produced is minimal and therefore, the net power output is stable. This is because the fluid that is split in flow splitter 24 does not directly enter the fluid expanders 20 and 26, but instead passes through another set of heat exchangers 18 and 22 prior to expansion in the fluid expanders 20 and 26. Thus, this configuration provides a much more stable power output during operation compared to configurations known in the art where the possibility of variations in the flow split ratio is higher.
[0019] As would be appreciated by one of ordinary skill in the art, fluid expanders 20 and 26 may be power turbines, such as for example, a steam power turbine wherein the fluid is steam operating at superheated temperatures, a gas power turbine, or a supercritical carbon dioxide (sCO2) power turbine, where the operating fluid is carbon dioxide (CO2) with pressure and temperature optimized for maintaining CO2 at supercritical conditions. Other suitable pure fluids such as propane or fluid mixtures such as a mixture of propane and CO2 may also be used. As illustrated in FIG. 2, fluid expanders 20 and 26 may drive one or more power generators 16 to produce any form of high-grade energy as described with reference to FIG. 1. Two separate power generators 16 coupled with the shafts of fluid expanders 20 and 26 may be used. Alternatively, a single power generator 16 coupled via gears with the shafts of fluid expanders 20 and 26 may also be used.
[0020] The fluid cooling system 32 may include a low temperature heat exchanger, a gas cooler or a condenser, etc. Similarly, in various embodiments, the pressure building system 34 may include a compressor or a pump depending upon the phase of fluid used and may be driven by an electric motor 36. For example, in case the fluid is in gaseous or supercritical phase, a compressor may be used, while if the fluid is in liquid phase, a pump may be used.
[0021] Applications for embodiments of the invention may be found in combined cycle systems or waste heat recovery systems where waste heat is recovered and utilized to drive power generators in order to convert the low-grade energy (heat) to high-grade energy (work). Teachings of the present techniques can be used to build power systems and power plants where waste heat is available at any ambient temperature. In the techniques noted above, changes in the fluid flow split ratio at the flow splitter 24 will not introduce significant variations in the power output. Thus, the energy recovery system 14 will be stable regardless of any changes in the fluid flow split ratio
[0022] While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.