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. Many energy-intensive industrial sectors such as cement, fertilizer, aluminum, textile, and paper industries generate large amounts of waste heat which is converted to usable electricity using steam turbines. However, working fluids more efficient than steam, such as carbon dioxide in its supercritical phase (sCO2), are also being considered. sCO2 power cycles have higher efficiencies and offer significantly simple and compact equipment layout making them potential replacements for steam Rankine cycle and air Brayton cycles for power generation. However, sCO2 cycles operate at high pressures and there are several technological hurdles. For example, high turbomachinery speeds, reliable dry gas seals, and compact high-efficiency heat exchangers are required to realize an sCO2 power generation unit. Compared to steam or a gas Brayton cycle, sCO2 cycles can be reconfigured to suit a wide range of operating conditions, making them particularly suitable for waste heat recovery.

[0003] While thermal efficiency is the principal driver in a standard power cycle, a waste heat recovery (WHR) cycle is designed to maximize heat recovery factor. Heat recovery factor of a WHR cycle is defined as the ratio between work output to available heat. Available heat is measured as the heat released by a heat source when brought to ambient conditions from a high temperature. Heat recovery factor can also be defined as thermal efficiency multiplied by recovery effectiveness, where recovery effectiveness is the ratio of actual heat captured by the WHR system to available heat, and thermal efficiency is the ratio of work output to captured heat. Apart from the thermodynamic requirement of maximizing heat recovery factor, another aim in waste heat recovery applications is to develop a compact bottoming power generation unit that can quickly adapt to transience in the topping cycle. Additionally, quicker startup and shutdowns are desirable, considering that WHR systems are prone to fluctuating heat load depending on the temperature and mass flow rate of the exhaust waste heat source. Therefore, the range of operation of a WHR system should be wide enough to capture various grades of heat load while maintaining a consistent power output regardless of the deviations in the operating conditions from the design point.

[0004] Earlier attempts, such as the simple recuperated sCO2 cycle, have suffered from low recovery effectiveness, which results in lower heat recovery factor. Other sCO2 cycles that have offered stable operation with reasonably good heat recovery factors have also suffered from low recovery effectiveness. Another attempt has been to use a dual recuperated dual expansion (DRDE) power cycle. It offers one of the highest heat recovery factors and great recovery effectiveness. However, in the above applications it is observed that power developed by the earlier cycles and by the DRDE cycle drops rapidly from their respective design points when there is a small change in the split ratio of the working fluid after compression.

[0005] Therefore, there is a need for an alternate cycle that is less sensitive to fluctuations in the split ratio for flow bifurcation, while using standard components employed in an sCO2 cycle.

SUMMARY
[0006] An energy recovery system is described, which includes a high temperature heat exchanger 18 that transfers heat from hot exhaust gases to a partially heated fluid (C) to generate a high temperature fluid (D). A first fluid expander 22 expands the high temperature fluid (D) and generates a first expanded fluid (E). A high temperature recuperator 24 transfers heat from the first expanded fluid (E) to a first partially hot fluid (F) and generates a first low temperature fluid (G), while the first partially hot fluid (F) gains heat to form the partially heated fluid (C). A combined low temperature fluid (I), which includes the first low temperature fluid (G) and a second low temperature fluid (H), is passed through a low temperature recuperator 28 to recover heat and generate a warm fluid (J). The warm fluid (J) is cooled via a fluid cooling system 30 to generate a cooled fluid K. The cooled fluid K is compressed via a pressure building system 32 to generate a high pressure low temperature fluid (L).

[0007] The high pressure low temperature fluid (L) is split via a second splitter 34 into a first high pressure low temperature fluid (M) and a second high pressure low temperature fluid (N). The low temperature recuperator 28 transfers heat from the combined low temperature fluid (I) to the first high pressure low temperature fluid (M) to generate a partially hot fluid (O). The partially hot fluid (O) is split via a third splitter 36 into the first partially hot fluid (F) and a second partially hot fluid (P). A low temperature heat exchanger 38 transfers heat from a warm waste heat stream (Q) to the second high pressure low temperature fluid (N) to generate a third partially hot fluid (R). The second partially hot fluid (P) and the third partially hot fluid (R) together form a combined partially hot fluid (S). A mid-temperature heat exchanger 42 transfers heat from a partially hot waste heat stream (B) to the combined partially hot fluid (S) to generate a hot fluid (T). A second fluid expander 44 expands the hot fluid (T) to form the second low temperature fluid (H). Thus, the high temperature heat exchanger 18, the mid-temperature heat exchanger 42 and the low temperature heat exchanger 38 together transfer heat from hot exhaust gases to the energy recovery system while the first fluid expander 22 and the second fluid expander 44 together convert this transferred heat to power. A method of operating the energy recovery system is also described.

[0008] 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
[0009] FIG. 1 is an illustrative representation of a system including a novel energy recovery system in accordance with an embodiment of the invention.
[0010] FIG. 2 is a schematic representation of an exemplary energy recovery system in accordance with one aspect of the present technique.
[0011] FIG. 3 is another embodiment of the exemplary energy recovery system of FIG. 2 in accordance with an aspect of the present technique.
[0012] FIG. 4 is still another embodiment of the exemplary energy recovery system of FIG. 2 in accordance with another aspect of the present technique.
[0013] FIG. 5 is a graphical representation of the power output of the energy recovery system of FIG. 2 when the split ratio of the compressed fluid is varied in accordance with an aspect of the present technique.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0014] 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.

[0015] 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.

[0016] 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.

[0017] Referring generally to FIG. 2, an exemplary energy recovery system 14 is illustrated in accordance with one embodiment of the present technique. Hot exhaust gases flowing in direction A from a waste heat source 12 enters a high temperature heat exchanger 18 via a first splitter 20. The waste heat source 12 is thermally coupled to the heat exchanger 18 via the first splitter 20 so that the high temperature exhaust gases A pass through the heat exchanger 18. It may be noted that the first splitter 20 can be configured to operate in a manner that the flow of the hot exhaust gases is passed through the high temperature heat exchanger 18 entirely, partially or can be made to completely bypass the high temperature heat exchanger 18. The first splitter 20 can be controlled and operated either manually or electronically. 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 noted.

[0018] In the current embodiment, the entire flow of hot exhaust gases A passes through the high temperature heat exchanger 18 and exits from it as a partially hot waste heat stream generally shown by arrow B. The high temperature heat exchanger 18 receives a partially heated fluid, flowing in direction C, which is utilized to extract heat from the hot exhaust gases A in the high temperature heat exchanger 18. Subsequently, the partially heated fluid C having extracted the heat achieves a high temperature, flow of which is generally shown by arrow D.

[0019] Heat energy from the waste heat source 12, in the form of heat, is thus partially extracted by the heat exchanger 18 and the extracted heat in the form of the high temperature fluid, flowing in direction D, enters a first fluid expander 22. The first fluid expander 22 reduces the temperature and pressure of the high temperature fluid D which leaves the first fluid expander 22 at an intermediate temperature as first expanded fluid denoted by arrow E. In the process, the first fluid expander 22 utilizes the heat energy to generate power using the power generator 16.

[0020] The first expanded fluid E passes through a high temperature recuperator 24 in which a first partially hot fluid F is used to extract further heat from the first expanded fluid E. The first expanded fluid E reduces in temperature and exits the high temperature recuperator 24 as a first low temperature fluid G, while the first partially hot fluid F gains heat and exits as partially heated fluid C. A first mixer 26 combines the first low temperature fluid G with a second low temperature fluid H to generate a combined low temperature fluid I. The combined low temperature fluid I then passes through a low temperature recuperator 28, where it further loses heat to generate a warm fluid J.

[0021] After losing its heat in a fluid cooling system 30, the warm fluid J emerges as cooled fluid K. The fluid cooling system 30 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 30 may be configured to reduce the temperature of the warm fluid J 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 30 may be set up to reduce the temperature of the warm fluid J 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 30 may be designed to reduce the temperature of the warm fluid J closer to such temperatures, such as about 40°C to 60°C. In other words, the fluid cooling system 30 or the entire energy recovery system 14 may be configured to operate at any ambient conditions.

[0022] The cooled fluid denoted by K coming out from the fluid cooling system 30 is then fed into a pressure building system 32. The pressure building system 32 pressurizes the cooled fluid K into a high-pressure low temperature fluid L, 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.

[0023] The high-pressure low temperature fluid L is then split via a second splitter 34 into a first high-pressure low temperature fluid M and a second high-pressure low temperature fluid N. The first high-pressure low temperature fluid M passes through the low temperature recuperator 28 to extract heat from the combined fluid I while generating partially hot fluid O. The partially hot fluid O is then split via a third splitter 36 into a first partially hot fluid F and a second partially hot fluid P. The first partially hot fluid F passes through the high temperature recuperator 24 and generates the partially heated fluid C, as noted earlier.

[0024] The second high-pressure low temperature fluid N passes through a low temperature heat exchanger 38, to extract heat from a warm waste heat stream Q, and generate a third partially hot fluid R. Meanwhile, the warm waste heat stream Q that entered the low temperature heat exchanger 38 leaves the low temperature heat exchanger stack at a lower temperature. A second mixer 40 combines the second partially hot fluid P and the third partially hot fluid R to generate a combined partially hot fluid S. The combined partially hot fluid S then passes through a mid-temperature heat exchanger 42. In the mid-temperature heat exchanger 42, the partially hot waste heat stream B loses further heat to form the warm waste heat stream Q while the combined partially hot fluid S gains heat to generate a hot fluid T. The hot fluid T expands in a second fluid expander 44 to generate the second low temperature fluid H. Thus, the high temperature heat exchanger 18, the mid-temperature heat exchanger 42 and the low temperature heat exchanger 38 together transfer heat from hot exhaust gases to the energy recovery system while the first fluid expander 22 and the second fluid expander 44 together convert this transferred heat to generate power.

[0025] As would be appreciated by one of ordinary skill in the art, fluid expanders 22 and 44 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, a transcritical carbon dioxide (tCO2) 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 transcritical or supercritical conditions. While sCO2 is preferred as a working fluid, other pure fluids, such as but not limited to, hydrocarbons (such as methane, ethane or propane), Nitrogen, Helium or fluid mixtures such as a mixture of hydrocarbons and CO2 may also be used with appropriate operating pressures. As illustrated in FIG. 2, fluid expanders 22 & 44 may drive a power generator 16 to produce any form of high-grade energy as described with reference to FIG. 1.

[0026] The fluid cooling system 30 may include a low temperature heat exchanger, a gas cooler, a condenser, or the like. Similarly, in various embodiments, the pressure building system 32 may include a compressor or a pump depending upon the phase of fluid used and may be driven by an electrical motor 46. 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.

[0027] Referring now to FIG. 3, an alternate embodiment based on the energy recovery system described with reference to FIG. 2 is illustrated in accordance with another embodiment of the present technique. In this embodiment, the first splitter 20 is configured to bypass the high temperature heat exchanger 18 by directing the entire flow of the hot exhaust gases directly to the mid-temperature heat exchanger 42. Thus, the combined partially hot fluid S passes through the mid-temperature heat exchanger 42 and extracts heat from the hot exhaust gases A to generate the hot fluid T. In the mid-temperature heat exchanger 42, the waste heat stream A loses heat to the combined partially hot fluid S and generates the warm waste heat stream Q. Hot fluid T expands in the second fluid expander 44 to generate the second low temperature fluid H. In this embodiment, the first mixer 26 is configured to direct the entire flow of the second low temperature fluid H through the low temperature recuperator 28. Thus, the low temperature recuperator 28 transfers heat to the second low temperature fluid H to form the warm fluid J.

[0028] The warm fluid J passes through the fluid cooling system 30 and loses heat and emerges as cooled fluid K. As noted earlier, the fluid cooling system 30 rejects heat to the surroundings bringing down the temperature to near ambient levels. The cooled fluid K coming out from the fluid cooling system 30 is then fed into the pressure building system 32. The pressure building system 32 pressurizes the cooled fluid K into a high-pressure low temperature fluid L. The high-pressure low temperature fluid L is then split via the second splitter 34 into a first high-pressure low temperature fluid M and a second high-pressure low temperature fluid N.

[0029] The first high-pressure low temperature fluid M passes through the low temperature recuperator 28 to extract heat from the second low temperature fluid H and generates partially hot fluid O. The second high-pressure low temperature fluid N passes through a low temperature heat exchanger 38, to extract heat from the warm waste heat stream Q, and generate a third partially hot fluid R. The warm waste heat stream Q that entered the low temperature heat exchanger 38 leaves the low temperature heat exchanger stack at a lower temperature. In this embodiment, the third splitter 36 directs the entire flow of the partially hot fluid O towards the second mixer 40. The second mixer 40 combines the partially hot fluid O and the third partially hot fluid R to generate the combined partially hot fluid S, completing the fluid flow cycle. This simplifies the fluid flow path, and is desirable when the fluid volume is low or the fluid temperature is not very high, and a partial operation of the energy recovery system is achieved.

[0030] In another variant of the energy recovery system illustrated in FIG. 4, the pressure building system 32 and the second fluid expander 44 are coupled together with a common shaft 48. The shaft 48 synchronizes the rotation of the pressure building system 32 and the second fluid expander 44. The second splitter 34 is utilized to control the fluid mass flow rate through the second fluid expander 44 in such a way that the power generated by the second fluid expander 44 is the same as the power consumed by pressure building system 32 under all operating conditions. The split ratio of the second splitter 34 may be dynamically governed by a control system. This ensures that the coupling provides a compact system and reduces capital cost by eliminating one power generator 16. The fluid flow path and the entire operation of the energy recovery system of FIG. 4 follow the same manner of operation as described with reference to FIG. 2.

[0031] Turning now to FIG. 5, a graphical representation of the power output of the energy recovery system of FIG. 2 is illustrated when the split ratio of the compressed fluid is varied by the second splitter 34. The x-axis represents the ratio of splitting of the high-pressure low temperature fluid L by the second splitter 34 into the first high-pressure low temperature fluid M and the second high-pressure low temperature fluid N. The y-axis represents the power output of the energy recovery system described with reference to FIG. 2 at the various split ratios at the second splitter 34. As described before, the second splitter 34 splits the high-pressure low temperature fluid L into the first high-pressure low temperature fluid M and the second high-pressure low temperature fluid N. During the splitting process, the second splitter 34 distributes fluid mass flow of the high-pressure low temperature fluid L among the high temperature heat exchanger 18, the mid-temperature heat exchanger 42 and the low temperature heat exchanger 38. However, because the energy recovery system is a closed loop cycle, the total mass flow rate coming in contact with the hot exhaust gases remains constant regardless of the split ratio. For example, if the fluid mass flow of the first high-pressure low temperature fluid M is 70%, then the fluid mass flow of the second high-pressure low temperature fluid N is 30%. Further, in this example, if this 70% fluid M is then split at the third splitter 36 into 10% fluid P and 60% fluid F, then the 30% fluid N or correspondingly 30% fluid R combines with the 10% fluid P to form 40% fluid S. The second fluid expander 44 generates 40% power from this 40% fluid S or correspondingly 40% fluid T. Similarly, the 60% fluid F then flows through the high temperature recuperator 24, and subsequently as 60% fluid C through the high temperature heat exchanger 18, and further as 60% fluid D through the first fluid expander 22, which generates 60% power from the 60% fluid D. Thus, the total heat transferred from the hot exhaust gases in the energy recovery system or the total power output of the energy recovery system remains nearly constant as illustrated in FIG. 5. Furthermore, as the total power output of the energy recovery system remains nearly constant, it is possible to couple the pressure building system 32 and the second fluid expander 44, as described in FIG. 4. As the total power output of the energy recovery system remains almost invariant with the split ratio of the second splitter 34, coupling of the pressure building system 32 with the second fluid expander 44 is possible without sacrificing the power output.

[0032] 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. , Claims:CLAIMS
1. An energy recovery system, comprising:
a high temperature heat exchanger 18 configured to transfer heat from hot exhaust gases (A) to a partially heated fluid (C) to generate a high temperature fluid (D);
a first fluid expander 22 configured to expand the high temperature fluid (D) to generate a first expanded fluid (E);
a high temperature recuperator 24 configured to transfer heat from the first expanded fluid (E) to a first partially hot fluid (F) to generate a first low temperature fluid (G), wherein the first partially hot fluid (F) gains heat to generate the partially heated fluid (C);
a low temperature recuperator 28 configured to recover heat from a combined low temperature fluid (I) to generate a warm fluid (J), wherein the combined low temperature fluid (I) comprises the first low temperature fluid (G) and a second low temperature fluid (H), and wherein the warm fluid (J) is cooled and compressed to generate a high pressure low temperature fluid (L);
a second splitter 34 configured to split the high pressure low temperature fluid (L) into a first high pressure low temperature fluid (M) and a second high pressure low temperature fluid (N), wherein the low temperature recuperator 28 transfers heat from the combined low temperature fluid (I) to the first high pressure low temperature fluid (M) to generate a partially hot fluid (O);
a third splitter 36 configured to split the partially hot fluid (O) into the first partially hot fluid (F) and a second partially hot fluid (P), wherein a low temperature heat exchanger 38 transfers heat from a warm waste heat stream (Q) to the second high pressure low temperature fluid (N) to generate a third partially hot fluid (R);
a mid-temperature heat exchanger 42 configured to transfer heat from a partially hot waste heat stream (B) to a combined partially hot fluid (S) to generate a hot fluid (T), wherein the combined partially hot fluid (S) comprises the second partially hot fluid (P) and the third partially hot fluid (R), and wherein the partially hot waste heat stream (B) loses heat to generate the warm waste heat stream (Q); and
a second fluid expander 44 configured to expand the hot fluid (T) to generate the second low temperature fluid (H).

2. The energy recovery system of claim 1, comprising a fluid cooling system 30 configured to cool the warm fluid (J) to generate a cooled fluid (K).

3. The energy recovery system of claim 2, comprising a pressure building system 32 configured to compress the cooled fluid (K) to generate the high pressure low temperature fluid (L).

4. The energy recovery system of claim 3, wherein the pressure building system 32 and the second fluid expander 44 are couplable via a common shaft 48.

5. The energy recovery system of claim 1, wherein the second splitter 34 is configured to vary the splitting of the high pressure low temperature fluid (L) into the first high pressure low temperature fluid (M) and the second high pressure low temperature fluid (N).

6. The energy recovery system of claim 1, further comprising:
a first splitter 20 configured to regulate the flow of hot exhaust gases (A) between the high temperature heat exchanger 18 and the mid-temperature heat exchanger 42; and
a first mixer 26 configured to mix the first low temperature fluid (G) and the second low temperature fluid (H) either partially or entirely to generate the combined low temperature fluid (I).

7. The energy recovery system of claim 1, wherein the third splitter 36 is configured to split the partially hot fluid (O) either partially or entirely as the first partially hot fluid (F) or the second partially hot fluid (P).

8. A method of operating an energy recovery system, comprising:
transferring heat from hot exhaust gases (A) to a partially heated fluid (C) for generating a high temperature fluid (D);
expanding the high temperature fluid (D) to a first expanded fluid (E);
transferring heat from the first expanded fluid (E) to a first partially hot fluid (F) for generating the partially heated fluid (C), while generating a first low temperature fluid (G);
generating a combined low temperature fluid (I) by combining the first low temperature fluid (G) and a second low temperature fluid (H);
recovering heat from the combined low temperature fluid (I) for generating a warm fluid (J);
cooling and compressing the warm fluid (J) for generating a high pressure low temperature fluid (L);
splitting the high pressure low temperature fluid (L) into a first high pressure low temperature fluid (M) and a second high pressure low temperature fluid (N);
transferring heat from the combined low temperature fluid (I) to the first high pressure low temperature fluid (M) for generating a partially hot fluid (O);
splitting the partially hot fluid (O) into the first partially hot fluid (F) and a second partially hot fluid (P);
transferring heat to the second high pressure low temperature fluid (N) for generating a third partially hot fluid (R);
combining the second partially hot fluid (P) and the third partially hot fluid (R) for generating a combined partially hot fluid (S);
transferring heat from a partially hot waste heat stream (B) to the combined partially hot fluid (S) for generating a hot fluid (T); and
expanding the hot fluid (T) to the second low temperature fluid (H).

9. The method of claim 8, comprising regulating the flow of hot exhaust gases (A) for transferring heat from hot exhaust gases (A) either partially or entirely to the partially heated fluid (C).

10. The method of claim 8, comprising regulating the flow of hot exhaust gases (A) for transferring heat from hot exhaust gases (A) either partially or entirely to the combined partially hot fluid (S).

11. The method of claim 8, wherein expanding the high temperature fluid (D) comprises generating power via a first fluid expander 22.

12. The method of claim 8, wherein generating a combined low temperature fluid (I) comprises combining the first low temperature fluid (G) either partially or entirely with the second low temperature fluid (H).

13. The method of claim 8, comprising cooling the warm fluid (J) for generating a cooled fluid (K).

14. The method of claim 13, comprising compressing the cooled fluid (K) for generating the high pressure low temperature fluid (L).

15. The method of claim 14, comprising synchronizing compressing of the cooled fluid (K) and expanding of the hot fluid (T).

16. The method of claim 8, wherein splitting the high pressure low temperature fluid (L) comprises varying a split ratio of the first high pressure low temperature fluid (M) to the second high pressure low temperature fluid (N).

17. The method of claim 8, wherein splitting the partially hot fluid (O) comprises splitting the partially hot fluid (O) either partially or entirely into the first partially hot fluid (F).

18. The method of claim 8, wherein splitting the partially hot fluid (O) comprises splitting the partially hot fluid (O) either partially or entirely into the second partially hot fluid (P).

19. The method of claim 8, wherein expanding the hot fluid (T) comprises generating power via a second fluid expander 44.