Claims:CLAIMS
1. An energy recovery system, comprising:
a high temperature fluid expander configured to expand a first hot fluid into a first expanded fluid;
a high temperature recuperator configured to transfer heat from the first expanded fluid to a second partially hot fluid to generate a second hot fluid, wherein the high temperature recuperator cools the first expanded fluid to a first partially hot fluid;
a low temperature fluid expander configured to expand the second hot fluid into a second expanded fluid, wherein the second expanded fluid is combined with the first partially hot fluid to generate a combined fluid; and
a low temperature recuperator configured to transfer heat from the combined fluid to a second cold fluid to generate the second partially hot fluid, wherein the combined fluid is cooled and split to a first cold fluid and the second cold fluid.

2. The energy recovery system as recited in claim 1, comprising a heat exchanger configured to transfer waste heat to the first cold fluid to generate the first hot fluid.

3. The energy recovery system as recited in claim 1, wherein the low temperature recuperator cools the combined fluid to generate a cool fluid.

4. The energy recovery system as recited in claim 3, comprising a fluid cooling system coupled to the low temperature recuperator and configured to cool the cool fluid to ambient temperature fluid.

5. The energy recovery system as recited in claim 4, further comprising a pressure building system configured to pressurize the ambient temperature fluid into a pressurized fluid, wherein the pressurized fluid is split into the first cold fluid and the second cold fluid.

6. The energy recovery system as recited in claim 5, wherein the high temperature fluid expander, the low temperature fluid expander, and the pressure building system are driven by a single shaft.

7. The energy recovery system as recited in claim 6, wherein the single shaft drives a power generator.

8. The energy recovery system as recited in claim 1, wherein the high temperature fluid expander and the low temperature fluid expander are driven by a single shaft.

9. The energy recovery system as recited in claim 8, wherein the single shaft drives a power generator.

10. The energy recovery system as recited in claim 1, wherein the high temperature fluid expander comprises a power turbine.

11. The energy recovery system as recited in claim 1, wherein the low temperature fluid expander comprises a power turbine.

12. A method of operating an energy recovery system, comprising:
expanding a first hot fluid into a first expanded fluid;
transferring heat from the first expanded fluid to a second partially hot fluid for generating a second hot fluid, wherein transferring heat from the first expanded fluid comprises cooling the first expanded fluid to a first partially hot fluid;
expanding the second hot fluid to a second expanded fluid;
combining the second expanded fluid with the first partially hot fluid for generating a combined fluid; and
transferring heat from the combined fluid to a second cold fluid for generating the second partially hot fluid.

13. The method as recited in claim 12, wherein expanding the first hot fluid comprises expanding the first hot fluid via a high temperature fluid expander.

14. The method as recited in claim 12, wherein expanding the first hot fluid comprises generating power via a power generator.

15. The method as recited in claim 12, further comprising driving the high temperature fluid expander and the low temperature fluid expander via a single shaft.

16. The method as recited in claim 12, further comprising generating power via a power generator coupled to the high temperature fluid expander and the low temperature fluid expander via a single shaft.

17. The method as recited in claim 12, wherein transferring heat from the first expanded fluid comprises transferring heat from the first expanded fluid via a high temperature recuperator.

18. The method as recited in claim 12, wherein expanding the second hot fluid comprises expanding the second hot fluid via a low temperature fluid expander.

19. The method as recited in claim 12, wherein expanding the second hot fluid comprises generating power via a power generator.

20. The method as recited in claim 12, wherein transferring heat from the combined fluid comprises cooling the combined fluid.

21. The method as recited in claim 12, wherein transferring heat from the combined fluid comprises pressurizing the combined fluid.

22. The method as recited in claim 12, wherein transferring heat from the combined fluid comprises splitting the combined fluid to a first cold fluid and the second cold fluid.

23. The method as recited in claim 22, wherein splitting the combined fluid comprises heating the first cold fluid via a heat exchanger for generating the first hot fluid.

24. An energy recovery system, comprising:
a heat exchanger configured to transfer waste heat to a first cold fluid to generate a first hot fluid;
a high temperature fluid expander configured to expand the first hot fluid into a first expanded fluid;
a high temperature recuperator configured to transfer heat from the first expanded fluid to a second partially hot fluid to generate a second hot fluid, wherein the high temperature recuperator cools the first expanded fluid to a first partially hot fluid; and
a low temperature fluid expander configured to expand the second hot fluid into a second expanded fluid, wherein the second expanded fluid is combined with the first partially hot fluid to generate a combined fluid.

25. The energy recovery system as recited in claim 24, wherein the high temperature fluid expander and the low temperature fluid expander are driven by a single shaft.

26. The energy recovery system as recited in claim 25, wherein the single shaft drives a power generator.

27. The energy recovery system as recited in claim 24, further comprising a fluid cooling system.

28. The energy recovery system as recited in claim 24, further comprising a pressure building system.

29. The energy recovery system as recited in claim 28, wherein the low temperature fluid expander drives the pressure building system via a single shaft. , Description:BACKGROUND
The invention relates generally to the field of heat engines and, more specifically, to low temperature and waste heat recovery systems.

A form of energy can be called high grade energy based on the ease of its conversion to other forms of energy, while low grade energy is one that is difficult to be converted to other forms of energy. Work can be completely converted to other forms of energy, including heat, but thermodynamically, heat cannot be entirely converted to work, and it is generally difficult to covert heat to work. Therefore, heat is a form of low-grade energy, while work is a form of high-grade energy.

In industrial processes, heat is a common byproduct and is produced at varying temperatures. Depending on the temperature at which heat is available, it can be classified as low-grade heat (at low temperature) or high-grade heat (at high temperature). In various industrial processes, the heat generated may range from low-grade heat to high-grade heat. It is desirable to convert this heat into work.

A device that converts heat to work is a called a heat engine. Since thermodynamically, it isn’t feasible to convert entire heat to work, some amount of heat is rejected to the surroundings by the heat engine during the process of conversion of heat to work. The potential for heat recovery is dependent on ambient temperatures. This is because temperature differential (?T) governs the quantity of heat that can be recovered or transferred. So, large amounts of heat can be transferred if the temperature differential is large. At lower ambient temperatures, heat at high temperatures (high-grade heat) has more potential for conversion to work than low temperature heat (low-grade heat), as ?T is large. However, at higher ambient temperatures, even high-grade heat requires a lot of effort and more efficient means for recovery, as the temperature differential is not enough to reject heat to the surroundings, thereby reducing the quantum of heat converted to work. Thus, the generated heat is often allowed to dissipate into the surroundings and is generally rendered unusable.

Heat engine operates 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. sCO2 as a working fluid offers faster start-up and run times and is therefore ideally suited for waste heat recovery. However, while operating at higher ambient temperatures, it needs high-grade heat for its operation, and is therefore not considered suitable for waste heat recovery systems at temperatures below 500°C. As a result, such attempts have failed to successfully recover low-intensity heat in locations where the ambient temperature is high.

Therefore, an energy recovery system that is capable of recovering low-intensity heat is desirable.

SUMMARY
An energy recovery system including a high temperature fluid expander, a high temperature recuperator and a low temperature fluid expander is provided. The high temperature fluid expander expands a first stream of hot fluid into a first stream of expanded fluid. The high temperature recuperator transfers heat from the first stream of expanded fluid to a partially hot fluid to form a second stream of hot fluid, while cooling the first stream of expanded fluid to a first stream of partially hot fluid. The low temperature fluid expander expands the second stream of hot fluid into a second stream of expanded fluid, which is combined with the first stream of partially hot fluid to generate a combined fluid. The combined fluid is cooled and split to a first stream of cold fluid and a second stream of cold fluid. A heat exchanger may be used to transfer waste heat to the first stream of cold fluid to generate the first stream of hot fluid. A low temperature recuperator may be further used to transfer heat from the combined fluid to the second stream of cold fluid to generate the partially hot fluid. A method of operating the energy recovery system is also provided.

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
FIG. 1 is an illustrative representation of a system including an energy recovery system in accordance with an embodiment of the invention.
FIG. 2 is a schematic representation of an exemplary energy recovery system in accordance with one aspect of the present technique.
FIG. 3 is another embodiment of the energy recovery system in accordance with aspects of the present technique.
FIG. 4 is still another embodiment of the energy recovery system in accordance with aspects of the present technique.
FIG. 5 is yet another embodiment of the energy recovery system in accordance with aspects of the present technique.
FIG. 6 is a different embodiment of the energy recovery system in accordance with aspects of the present technique.
FIG. 7 is a graphical representation of the temperature and pressure behaviour of the fluids during an exemplary operation of the energy recovery system of FIG. 2.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
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.

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, industrial processes such as steel plant operation or glass making, combustors, nuclear reactors, 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.

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.

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 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 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.

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 high temperature fluid, flowing in direction B, enters a fluid expander 20. The fluid expander 20 reduces the temperature and pressure of fluid B which leaves the fluid expander 20 at an intermediate temperature and pressure as fluid denoted by arrow C.

Fluid C then passes through a high temperature recuperator 22, which significantly extracts the heat energy by transferring the heat to fluid flowing as denoted by arrow D into the high temperature recuperator 22 from a low temperature recuperator 24. Thus, fluid D gains in temperature in the high temperature recuperator 22 and subsequently flows, as shown by arrow E, into a second fluid expander 26. Fluid expander 26 reduces the temperature and pressure of fluid E and the fluid flowing out of the fluid expander 26 shown by arrow F is combined with the fluid flowing out of the high temperature recuperator 22 shown by arrow G, and the combined fluid H is fed into the low temperature recuperator 24.

In the low temperature recuperator 24, fluid H further transfers the heat energy to low temperature fluid entering it as denoted by arrow I. It is this fluid I that gains heat and flows out as shown by arrow D, and enters the high temperature recuperator 22, as noted previously. Fluid flowing out of the low temperature recuperator 24, shown by arrow J, has much reduced temperature, as would be apparent to one of ordinary skill in the art. Fluid J is fed into a fluid cooling system 28, 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 28 may be configured to reduce the temperature of fluid J depending on ambient temperatures. For example, in regions with low ambient temperatures such as between 18°C and 25°C, the fluid cooling system 28 may be set up to reduce the temperature of the 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 28 may be designed to reduce the temperature of the fluid J closer to such temperatures, such as about 40°C to 60°C. In other words, the fluid cooling system 28, and therefore the entire energy recovery system 14, may be configured to operate at any ambient conditions.

The cooled fluid denoted by K coming out from the fluid cooling system 28 is then fed into a pressure building system 30. The pressure building system 30 pressurizes fluid K into higher pressure 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. In a specific embodiment, the ratio of expansion of fluid B to fluid C in the fluid expander 20 is same as the ratio of compression of fluid K to fluid L in the pressure building system 30. However, in various other embodiments, the ratio may be maintained differently, as would be appreciated by one of ordinary skill in the art. Fluid L flowing out from pressure building system 30 is split into fluid A and fluid I flowing into the heat exchanger 18 and the low temperature recuperator 24 respectively. The ratio of splitting the fluid L may be based upon the heat source, ambient temperature and the quantum of heat available at source. As previously explained, fluid K flowing out from fluid cooling system 28 is close to ambient temperatures. This ambient temperature fluid K is pressurized by the pressure building system 30 and the pressurized fluid L flowing out from the pressure building system 30 is therefore at a temperature as close as possible to the ambient temperatures. As such, after fluid L is split into fluid A and fluid I, 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, 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.

The fluid cooling system 28 may include a low temperature heat exchanger, a gas cooler or a condenser, etc. Similarly, in various embodiments, the pressure building system 30 may include a compressor or a pump depending upon the phase of fluid used, and may be driven by an electric motor 32. 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. It may also be noted that the work or energy required for pumping liquid is much less than the work or energy required for pressurizing a gaseous fluid. Thus, it is desirable to use a fluid that is in the liquid phase and can be pumped.

Furthermore, in another embodiment of the technique described hereinabove, both the fluid expanders 20 and 26 may be engaged via a single shaft to a power generator 16. This is illustrated in FIG. 3, wherein fluid expander 20 is coupled to fluid expander 26 via a single shaft as fluid expander arrangement 34, and the same shaft drives a power generator 16. Therefore, this fluid expander arrangement 34 obviates the need for a second power generator. Similarly, the fluid expanders 20 and 26 may also be designed to be different stages of a single fluid expander, as would become apparent to one of ordinary skill in the art.

In still another embodiment of the energy recovery system 14 described earlier with reference to FIG. 2, FIG. 4 illustrates a different arrangement of the two fluid expanders 20 and 26. The two fluid expanders 20 and 26 in this embodiment are coupled together to each other as well as to the power generator 16 via a single shaft in order to drive the power generator. Additionally, the same shaft drives the pressure building system 30 as well. This fluid expander arrangement 36 not only eliminates the need for two power generators 16, but also eliminates the need for an electrical motor to drive the pressure building system 30.

In yet different embodiment of the energy recovery system 14 described earlier with reference to FIG. 2, the low temperature recuperator 24 may be eliminated, keeping well within the scope of the teachings of the present techniques. This has been illustrated in FIG. 5, wherein the high pressure fluid L flowing out from pressure building system 30 is split into fluid A and fluid D. Whereas fluid A flows into the heat exchanger 18, fluid D flows directly into the high temperature recuperator 22, without passing through the low temperature recuperator. Similarly, fluid F coming out of the fluid expander 26 and fluid G flowing out of the high temperature recuperator 22 is combined as fluid H and is fed directly into the fluid cooling system 28 for cooling. This configuration may also be replicated in the embodiments described with reference to FIG. 3 and FIG. 4 apart from the embodiment described hereinabove.

In another embodiment of the energy recovery system 14 previously described with reference to FIG. 5, the pressure building system 30 is driven by the fluid expander 26, and is illustrated in FIG. 6. In this configuration, fluid B after expansion in fluid expander 20 flows as fluid C into the high temperature recuperator 22. Fluid C thus transfers heat to fluid D entering the high temperature recuperator 22, and while fluid C flows out as fluid G at reduced temperature, fluid D gains in temperature and flows out of high temperature recuperator 22 as fluid E. Fluid E flows into fluid expander 26 and expands into fluid F, which combines with fluid G and this combined fluid H is cooled by the fluid cooling system 28. Fluid cooling system 28 cools fluid H to fluid K, which is pressurized by the pressure building system 30. This pressure building system 30 is driven by a shaft coupled with the fluid expander 26, thereby obviating the need for an electrical motor and reducing the drive transmission losses. The pressure building system 30 pressurizes fluid K to fluid L which is further split into fluid A and fluid D, as noted earlier.

The functioning of the energy recovery system 14 can be described using a pressure enthalpy phase diagram or graph 38 illustrated in FIG. 7 in conjunction with the energy recovery system 14 shown in FIG. 2. The pressure enthalpy phase diagram 38 illustrates the temperature and pressure behaviour of a fluid during an exemplary operation of the energy recovery system 14. In the phase diagram 38, the alphabetical arrow references previously used to denote the flow directions of the fluid in FIG. 2 have been generally used to depict co-ordinates or points illustrating the temperatures and pressures of the fluid on a Cartesian plane at the corresponding location in the system 14. Therefore, in the description that follows, alphabetical directions used in FIG. 2 can be interchangeably read as co-ordinates in FIG. 6, and vice versa. For example, the phrase, “sCO2 flowing in the direction A” can also be read as “sCO2 at co-ordinate A”.

In one embodiment described below, the fluid used is CO2 at supercritical phase or simply noted as sCO2. sCO2 flowing in the direction A into the heat exchanger 18 is heated by the hot exhaust gases from the waste heat source 12 to a higher temperature, and this is denoted by arrow B. This high temperature sCO2 (B) flows into a power turbine 20 and expands while the temperature drops to an intermediate temperature and is generally shown by arrow C flowing into the high temperature recuperator 22. The high temperature recuperator 22 further extracts heat from the sCO2 (C) with the fluid sCO2 flowing out (G) at a lower temperature. The heat extracted by the high temperature recuperator 22 is transferred to fluid sCO2 (D) flowing into the high temperature recuperator 22. Thus, fluid sCO2 flowing in direction D after emerging in direction E attains a temperature around the same as fluid sCO2 flowing in direction C. This fluid sCO2 (E) enters a second power turbine 26. In the power turbine 26, the sCO2 expands and its temperature drops and this fluid sCO2 flows in direction F and combines with fluid sCO2 flowing in direction G, which is also around the same temperature. The combined fluid H flows into a low temperature recuperator 24, where it loses part of its heat to fluid I which gets heated and flows out of the low temperature recuperator 24 as denoted by D to enter the high temperature recuperator 22, as explained before. After losing heat, the combined sCO2 flow H drops in temperature and is denoted by arrow J. This sCO2 flowing in direction J enters a gas cooler 28, and the temperature of the sCO2 flowing out of the gas cooler 28 reduces to temperature near ambient levels and is shown by arrow K. Fluid K enters a pump 30 which pressurizes fluid K to fluid L which also increases the temperature of the fluid due to compression. This fluid sCO2 (L) is split into fluid A and fluid I. While fluid A flows into the heat exchanger 18, fluid I flows into the low temperature recuperator 24, where its temperature increases further and is shown by arrow D.

In other words, sCO2 at near ambient temperature or an ambient temperature fluid K is pressurized by a pressure building system 30 to a pressurized fluid L, which is split into a first cold fluid A and a second cold fluid I. The first cold fluid A gains heat in the heat exchanger 18 to generate a first hot fluid B, while the second cold fluid I gains heat in the low temperature recuperator 24 to generate a second partially hot fluid D. The first hot fluid B expands in a high temperature fluid expander 20 to generate a first expanded fluid C. This first expanded fluid C transfers heat to the second partially hot fluid D thereby reducing its temperature and generating a first partially hot fluid G, while in the process, the second partially hot fluid D gains temperature to generate a second hot fluid E. The second hot fluid E expands in a low temperature fluid expander 26 to generate a second expanded fluid F. The second expanded fluid F and the first partially hot fluid G are combined to form a combined fluid H, which transfers its heat to the second cold fluid I to generate a cool fluid J, while increasing the temperature of the second cold fluid I to the second partially hot fluid D. The cool fluid J is cooled further to ambient temperature fluid K via a fluid cooling system 28. This completes the sCO2 fluid loop which flows in different streams heating and expanding at various points to extract heat from a waste heat source 12 and convert it into energy using a power generator 16.

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). Furthermore, the teachings of the present techniques can be used to build power systems and power plants which can utilize waste heat that is available at any ambient temperature. As compared to waste heat recovery (WHR) systems known in the art, where higher ambient temperatures inhibit the operation of the WHR systems, the system described hereinabove is configured to operate in a wide range of ambient temperatures. Thus, the energy recovery system described above can be used in any ambient conditions. This in turn, overcomes the challenges posed by hot tropical environments, where setting up a WHR system based on fluids such as sCO2 was difficult.

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.