HEAT PUMP STEAM GENERATOR

Abstract

The disclosure provides a heat pump cycle that allows for an improved matching of the T(Q) slopes of the heat pump cycle. More particularly, in the heat pump cycle, the high temperature heat exchange is separated into two stages. Furthermore, a portion of the working fluid that was cooled in the first stage, is further cooled by expansion before being mixed with a heated working fluid for input to the recuperating heat exchanger.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

This section introduces information from the art that may be related to or provide context for some aspects of the technique described herein and/or claimed below. This information is background information facilitating a better understanding of that which is disclosed herein. This is a discussion of “related” art. That such art is related in no way implies that it is also “prior” art. The related art may or may not be prior art. The discussion is to be read in this light, and not as admissions of prior art.

In many industrial processes, heat is applied to one or more materials (the “heat transfer target”) at a relatively high temperature. The most common method of applying this heat today is through the combustion of fossil fuels. However, as the world moves towards a carbon-free energy system, alternative means to provide industrial heat will become more desirable. While direct electrical heating with devices such as electric resistance heaters, electric arc heaters or electrical induction heaters can attain the desired temperatures, the coefficient of performance (“COP”) of these processes can never be greater than 1. The COP, as that term is used herein, is the amount of heat transferred to the process divided by the electrical power input.

A heat pump system refers to a thermodynamic device and process that extracts heat from a source (the “heat transfer source”), increases the temperature of that heat through thermodynamic processes, and applies that heat to the heat transfer target. One industrial application of a heat pump system is the generation of steam for process and heating uses. Steam can be characterized by its pressure and, if provided in a superheated state, by its superheat temperature. “Saturated” steam consists of steam that is fully in the vapor state and at the temperature that is dictated by the boiling point temperature at the given pressure. Today, most steam is provided by transferring heat from the products of combustion in a fired heater, whether fueled by natural gas, coal, biomass or other fuel. In those cases, other than non-carbon-containing fuels such as hydrogen or ammonia, significant greenhouse gas emissions may be generated in the steam generating process. In these cases, other harmful atmospheric emissions such as Nitrogen Oxide (NOx), unburned fuel, and particulate matter are generated.

Alternatively, steam can be generated by a so-called “eBoiler”, or electric boiler. In an eBoiler, electrical resistance, induction, or arc heating are used to add heat to liquid water, thus generating steam. As discussed previously, the best theoretical performance of these type of direct electrical heating methods is a COP of 1.

In contrast to direct electric heating, thermodynamic heat pump cycles can attain COP values well in excess of 1.0. However, cycle and working fluid limitations typically only permit modest heating temperatures in heat pumps. For instance, traditional hydrofluorocarbon (“HFC”) and hydrochlorofluorocarbon (“HCFC”) refrigerants thermally decompose at high temperatures, making them unsuitable for higher temperature heat pump applications. A transcritical CO2 heat pump can provide higher temperature than other refrigerants due to its thermal stability. However, the attainable COP of a simple or recuperated transcritical CO2 heat pump for steam generation purposes is relatively low due to the thermodynamics characteristics of a standard transcritical heat pump cycle.

SUMMARY

In one aspect, a closed-loop, transcritical, recuperated heat pump cycle, comprises a heat transfer source, a heat transfer target, a working fluid, and a working fluid circuit. The working fluid circuit transfers heat from the heat transfer source to the heat transfer target via the working fluid. The working fluid circuit includes a heat exchanger to transfer heat from the heat transfer source to the working fluid, a fluid compression process, a recuperator heat exchanger, a heat exchanger to transfer heat from the working fluid to the heat transfer target and two fluid expansion processes. The first fluid expansion process bypasses a portion of the working fluid around the recuperator to optimize the exergetic efficiency of the recuperator. This improves the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range. The second fluid expansion process expands the remainder of the working fluid after it has passed through the recuperator.

In a second aspect, a method for generating steam, comprises: circulating a working fluid through a working fluid circuit of a closed-loop, transcritical, recuperated heat pump cycle, the heat pump cycle including a fluid expansion process and a recuperator; and bypassing a portion of the working fluid during the fluid expansion process around the recuperator to optimize the exergetic efficiency of the recuperator and improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

In a third aspect, a heat pump system, comprises a heat transfer source; a heat transfer target; a working fluid; and a working fluid circuit through which, when the heat pump system is operating, the working fluid circulates. The working fluid circuit includes a recuperator disposed on: a first flow path between a heat transfer source and a steam generation process; and a second flow path between the steam generation process and the heat transfer source, the second flow path including a fluid expansion process that bypasses a portion of the working fluid around the recuperator to optimize the exergetic efficiency of the recuperator, thereby improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

In a fourth aspect, a closed-loop, transcritical, recuperated heat pump cycle includes a fluid expansion process that bypasses a portion of a working fluid around a recuperator to optimize the exergetic efficiency of the recuperator, thereby improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred to the target over a limited temperature range.

The above presents a simplified summary in order to provide a basic understanding of some aspects of what is claimed below. This summary is not an exhaustive overview of the claimed subject matter. It is not intended to identify key or critical elements of the disclosure or to delineate the scope of the claims. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter disclosed below may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:

FIG. 1 is a schematic diagram of a conventional heat pump cycle.

FIG. 2 is a pressure-enthalpy (“PH”) diagram of the heat pump cycle of FIG. 1.

FIG. 3 is a simple recuperated cycle process flow diagram.

FIG. 4 is a temperature-heat flow (“TQ”) plot for the steam generator on a simple recuperated CO₂ heat pump cycle such as the simple recuperated cycle process flow of FIG. 3.

FIG. 5 is a PH diagram corresponding to the simple recuperated cycle process flow diagram of FIG. 3 and the TQ plot of FIG. 4.

FIG. 6 is a process flow diagram for a two-stage compression process with intercooling, where the intercooled heat is also used to generate steam.

FIG. 7 is a PH diagram for an intercooled-only cycle.

FIG. 8 is a TQ plot of the steam generator heat exchanger of the intercooled process flow of FIG. 6.

FIG. 9 is a TQ plot for the recuperator in the intercooled cycle in FIG. 6.

FIG. 10 conceptually illustrates a process flow diagram of a steam generating heat pump cycle in accordance with one or more embodiments of the presently claimed subject matter.

FIG. 11 is a TQ plot of the recuperator in the heat pump cycle of FIG. 10.

FIG. 12 is a PH diagram for the heat pump cycle of FIG. 10.

FIG. 13 is a TQ plot for the high temperature heat exchanger (“HTX”) of the heat pump cycle in FIG. 10.

FIG. 14A-FIG. 14E illustrate several embodiments for the steam generators of the heat pump shown in FIG. 10.

FIG. 15 conceptually illustrates an embodiment employing a multi-stage steam generation.

FIG. 16 is a block diagram of a control system including a programmed controller such as may be used to control fluid flow of the working fluid in some embodiments.

While the disclosed technique is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit that which is claimed to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.

DETAILED DESCRIPTION

Illustrative embodiments of the subject matter claimed below will now be disclosed. In the interest of clarity, not all features of an actual implementation are described in this specification. It will be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

In this disclosure, we describe a new heat pump cycle that provides a substantially higher COP than a standard or a simple recuperated heat pump can and can therefore provide steam over a range of pressures with a COP significantly greater than 1.0. More particularly, this disclosure provides a closed-loop, transcritical, recuperated heat pump cycle that includes a fluid expansion process that bypasses a portion of the working fluid around the recuperator to optimize the exergetic efficiency of the recuperator, thereby improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred to the target over a limited temperature range.

For instance, in a first example application a thermal storage reservoir may be heated to generate steam at a later time—such as where thermal energy is stored in sand at 300° C. to generate steam at 200° C. After generating steam from the sand reservoir, the sand temperature will still be >200° C., so the heat pump cycle only heats the sand from 200° C. back to 300° C. Another example would be a regeneratively heated process—for instance, a process that requires air at 300° C., but the process only cools the air to 220° C. The residual heat could then be used to preheat more air to, say, 200° C., and then the heat pump would boost it back up to 300° C.

As used herein, the terms “transcritical” and “exergetic” take on their customary and usual meaning in the art. The term “transcritical” refers to a cycle that operates below the critical pressure between the expansion device outlet and the compression device inlet, and above the critical pressure between the compression device outlet and the expansion device inlet. The term “exergy” and the derivative “exergetic” refer to the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir.

The phrase “limited temperature range” as used herein refers to the difference between the temperature of the heat transfer target prior to application of heat by a heat pump or heat pump cycle and the temperature of the heat transfer target after application of said heat. As those in the art having the benefit of this disclosure will appreciate, a water/steam mixture nominally boils at constant temperature, so in this case the temperature range could theoretically be as small as zero. However, there may be cases in which a material might need to heat up from, say 150° C. to 200° C., or 150° C. to 250° C. Thus, a “limited temperature range” is a range of temperatures over which water, or a water/steam mixture, may be heated to generate steam.

The term “optimize” in the phrase “to optimize the exergetic efficiency of the recuperator” means to maximize the quantification of the COP for the recuperator of a heat pump given the particular hardware implementation. Note that the maximum achievable quantification may be limited by design and operational factors. For example, some hardware implementations may be able to achieve a higher quantification for the COP than other hardware implementations. Also, as a practical matter, operational considerations may impair the ability to actually achieve a maximal quantification of the COP in all conditions. For example, sensor calibrations may drift and/or equipment may fail or operate out of specification. To “to optimize the exergetic efficiency of the recuperator” therefore also encompasses the effort to maximize the quantification of the COP for the recuperator in light of these kinds of physical and operational limitations.

To facilitate an understanding of the presently disclosed technique, a brief discussion of some conventional heat pumps and heat pump cycles will now be presented. Referring to FIG. 1, in a traditional heat pump cycle, the working fluid is compressed from a relatively low temperature, low pressure state (1) to a state (2) of higher temperature and pressure. (States in the heat pump cycle of FIG. 1 are represented as Roman numeral digits in ovals or circles.) This heat can then be transferred to a medium that receives and either uses or stores that heat.

In FIG. 1, the medium starts at an initial temperature and is heated to a higher temperature. During the process of heating the medium, the working fluid is cooled to state (3). The fluid is then expanded to state (4). The expansion may be either through an adiabatic expansion valve or a mechanical expander that extracts enthalpy from the fluid by performing shaft work. The temperature and pressure of the fluid medium decrease in the expansion device. Low-temperature heat is then added to the fluid from an external source, in many cases from the environment, or from a medium such as water.

The thermodynamic process is shown on the pressure-enthalpy diagram of FIG. 2. For some applications, a conventional heat pump cycle provides a good combination of performance and simplicity. For instance, CO2 heat pumps are frequently used to heat water from ambient temperature to domestic hot water heating temperature.

In the particular case under discussion here, the medium to be heated is water, from a subcooled or saturated liquid state to a saturated vapor or superheated state.

In one example, the water is to enter the boiler as a saturated liquid and exit as a saturated vapor at atmospheric pressure. In this case, the steam is formed at a constant temperature of 100° C. For the case shown in FIG. 2, it is clear that the temperature at states (2) (“T2”) and (3) (“T3”) will be required to be higher than 100° C. to transfer heat to the steam. In addition, the temperature at state (1) is limited to less than the temperature of the heat transfer source, which also establishes the pressure at states (1) and (4) to be less than the saturation pressure of the refrigerant at the heat transfer source temperature.

Therefore, to achieve both T2 and T3 greater than 100° C., the compressor pressure ratio (“CPR”) to attain a temperature in excess of 100° C. would need to increase significantly. Given mechanical constraints on compressors and piping systems, the simple heat pump is an impractical solution for steam generation, especially at higher steam pressures where the water boiling point temperature increases, exacerbating the CPR and pressure issues.

Note that the heat pump of FIG. 1-FIG. 2 is not a recuperated heat pump-i.e., there is no recuperator in that heat pump design. A recuperated heat pump is shown in FIG. 3. A recuperated heat pump adds an internal heat exchanger to transfer residual heat after the high-temperature heat exchanger (“HTX”) to preheat the fluid exiting the low-temperature heat exchanger (“LTX”), prior to entering the compressor. This internal heat exchanger is the “recuperator” (“RCX”). By preheating the fluid medium, a higher post-compressor temperature can be attained at a reasonable pressure ratio.

However, for the steam generator application, the recuperated heat pump performance is still constrained by the small temperature rise in the boiling water heat transfer problem. For instance, a simple recuperated heat pump, when generating 16.5 bar steam (Tsat=202° C.), is required to operate at a very high compressor discharge temperature to avoid a pinch restriction at the right-hand side of the TQ plot shown in FIG. 4.

Another potential issue with this configuration is the discharge state of the expander, state (5), which can be seen on the PH diagram of FIG. 5 to fall squarely within the vapor dome. Thus, a significant fraction of the expanded fluid will have flashed to the vapor state in the later stages of the expander, which could substantially affect both the performance and the durability of this device and is generally avoided in practice.

Finally, the COP of this cycle is limited as the average temperature of the high temperature heat addition process is relatively high (using the Lorenz mean temperature

$T_{m,h}=\frac{T_{h,\max }-T_{h,\min }}{\ln \left(T_{h,\max }/T_{h,\min }\right)}).$

For this example, T_m,h=321° C. and the predicted COP is 1.48.

To reduce T_m,h, the compression processes can be split into two or more separate stages and extract heat between each as shown in FIG. 6 and FIG. 7.

While this approach does reduce T_m,h to 261° C., the COP only raises slightly to 1.50, and the expansion into the dome persists. The fundamental problem with this cycle (at least while using CO₂ as the working fluid) is that the large dependence of working fluid specific heat capacity on temperature and pressure causes a mismatch in the temperature “glide” between the low-pressure and high-pressure sides of the recuperator. This causes a large temperature difference between the high-pressure CO₂ exit and the low-pressure CO₂ entrance of the recuperator, which in turn represents a large exergy destruction in the recuperator, and thus a loss in performance (FIG. 9).

In the subject matter claimed below, the heat capacity mismatch between the two fluid streams is addressed by dividing the flow at the outlet of Steam Generator 2 and expanding a portion directly through a turbine (expander). The fraction of the total flow at state 60 is roughly proportional to the ratio of the heat capacities at states 60 and 70, such that

m₆₀c_g,60=₇₀C_p,70.

This cycle increases the COP markedly over the previous two cycles. For this set of assumptions, the COP is 1.63, vs 1.50 for the previous best prediction.

The improvement in performance is due to several factors. The mean temperature of the high-temperature process T_m,h is further reduced to 256° C. using the same steam conditions as the previous analysis. The heat capacity of the two flow streams in the recuperator is now well-matched, significantly reducing the exergy destruction in that heat exchanger. And, the work recovered in the expander offsets a portion of the compressor work.

By better matching the heat capacities of the two streams, a much lower temperature at state 61 can be achieved. Thus, the lower-temperature expander T1 exit is now a single-phase liquid, or low-vapor content mixture of liquid and vapor. The higher-temperature expander T2 exit is also single-phase, on the vapor side of the two-phase region.

Other versions of this cycle could be envisioned with larger numbers of compression and intercooling stages, which could further improve performance by reducing T_m.h. Eliminating the intercooling (i.e., single-stage compression) also produces a cycle that works, but the COP is lower than with the multi-stage/intercooled configuration.

The two steam generators could be combined in a single unit with a combined “boiler” on the water side of the heat exchanger, with separate coils or other heat exchanger fluid passages for the two working fluid streams (20 and 40) that are at different pressures.

Rather than producing steam, the same heat pump could be used to transfer heat to other media over a relatively small temperature range defined as the difference between the maximum and minimum temperatures of the media during the heat transfer process). For instance, this heat pump could be used to increase the temperature of a thermal storage medium, such as sand, concrete, heat transfer fluid or oil, molten salt, etc.

A closed-loop, transcritical, recuperated heat pump cycle 1000 in accordance with one or more embodiments is shown in FIG. 10. The heat pump cycle 1000 includes two expansion devices 1002, 1004, a heat transfer source 1006, two steam generators 1008, 1010, two compression devices 1012, 1014 and a recuperator 1016. The steam generators 1008, 1010 are more generally a set of heat exchangers and are used to generate steam from the input condensate in this particular embodiment. As mentioned above, however, other embodiments may operate on alternative input media to generate alternative output media or even for an alternative end use. Thus, although the heat transfer target in this embodiment is a condensate, the subject matter claimed below is not so limited. Note that alternative embodiments may include different equipment in different configurations so long as their performance is consonant with that disclosed herein.

The expansion devices 1002, 1004 may be expanders such as an adiabatic expansion valve or a mechanical expander. The compression devices 1012, 1014 may be compressors, such as a centrifugal compressor, an axial compressor, a piston compressor, a screw compressor, a sliding-vane compressor or a scroll compressor. The expansion devices 1002, 1004 and compression devices 1012, 1014 can be independent machines, or assembled as combined machines where two or more of the devices are disposed on a common mechanical shaft (not shown).

More particularly, in this embodiment, a working fluid circuit 1020 comprises the two expansion devices 1002, 1004, the heat transfer source 1006, the two steam generators 1008, 1010, the two compression devices 1012, 1014 the recuperator 1016. A working fluid (not shown) circulates through the working fluid circuit 1020 responsive to the motive force imparted by the compression devices 1008, 1010. The working fluid may be, for example, Carbon dioxide (CO₂), although the working fluid may be implemented using other fluids in other embodiments.

Heat is transferred to the working fluid in the heat transfer source 1006 from a heat source 1022 and is recaptured in the recuperators 1012. The heat is transferred from the working fluid to a condensate or other cold heating medium in the heat exchangers 1008, 1010 to generate steam or other hot heating medium. State points are presented in FIG. 10 as numerals in circles. The operating parameters of the heat pump cycle 1000 in this particular embodiment at the various state points are presented in Table 1 below.

TABLE 1
State point table for example case.
Flow rates are for Qh = 27 MWth, where Qh represents the heat
State	T	P	h	w
point	(° C.)	(MPa)	(kJ/kg)	(kg/s)
1	181.63	3.76	635.72	77.50
2	351.80	14.88	799.68	77.50
3	207.87	14.78	625.31	77.50
4	307.25	32.63	716.05	77.50
5	185.46	32.43	542.03	77.50
10	185.46	32.43	542.03	28.32
11	21.05	3.86	456.96	28.32
6	185.46	32.43	542.03	49.18
7	20.81	32.33	233.03	49.18
8	3.83	3.96	209.37	49.18
12	4.51	3.86	429.66	49.18
9	9.90	3.86	439.63	77.50

The heat pump cycle 1000 employs, in part, a fluid expansion process 1030 and a fluid compression process 1040. The fluid expansion process 1030 includes the expansion devices 1002, 1004 and the fluid compression process 1040 includes the compression devices 1012, 1014. Note that the fluid expansion process includes a path 1033 that bypasses a portion of the working fluid around the recuperator 1016 to optimize the exergetic efficiency of the recuperator 1016. The bypass 1033 also improves the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

As a result of the bypass 1033, the recuperator 1016 is disposed on a first flow path and a second flow path. The first flow path runs between the heat transfer source 1006 and the steam generation process 1008. The second flow path runs between the steam generation process 1010 and the heat transfer source 1006. This second flow path includes the portion of the fluid expansion process 1030 that bypasses a portion of the working fluid around the recuperator 1016 to optimize the exergetic efficiency of the recuperator 1016. Again, this placement of the recuperator 1016, because of the bypass 1033, improves the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

FIG. 14A-FIG. 14 E illustrate several embodiments for the steam generator 1008 and steam generator 1010 of FIG. 10. FIG. 14A, a partially-sectioned, plan, side view, and FIG. 14B, a partially-sectioned, plan, end view, of a traditional kettle reboiler. Liquid water enters the shell from the bottom nozzle, boils when it contacts the exterior surface of the tubes (collectively known as a “tube bundle”) that are filled with a circulating, hotter fluid. Steam exits the top nozzle. In the presently disclosed technique, CO2 would be the fluid that circulates through the tube bundle and there potentially are several hot CO2 streams that would be used as the heating fluid. Since these streams are at different pressures, they would need to be kept separate in the boiler. In one option, multiple separate boilers could be used. In another option, shown in FIG. 14C, a single shell could house multiple tube bundles that would be supplied with CO₂ each at different pressures.

FIG. 15 illustrates an embodiment employing a three-stage compression system with steam generation after each stage. In this particular embodiment, the recuperated heat pump cycle 1500 comprises an expansion cycle 1503 including expansion devices 1503, 1509; a compression process 1512 including compression devices 1515, 1518, and 1521; a heat source 1524; a recuperator 1527; and the steam generators 1530, 1533, and 1536. As with the embodiment of FIG. 10, the expansion process 1503 includes a bypass 1539 and the recuperator 1527 lies on two flow paths.

As alluded to above, the recuperated heat pump cycle 1500 uses a three-stage compression system. Each stage is represented by a respective one of the compression devices 1515, 1518, 1521. The steam generators 1530, 1533, and 1536 provide steam generation after each of the three stages of the compression process 1512.

As those in the art having the benefit of this disclosure will appreciate, the heat pumps of FIG. 10 and FIG. 15, as well as other embodiments, may also include thermal reservoirs, other heat exchangers, piping, pumps, valves and other controls not separately shown. For example, the flow of the working fluid through the working fluid circuit is generally a function of programmed control of fluid flow valves. These other components are not shown in FIG. 10 and FIG. 15 for the sake of clarity and so as not to obscure that which is claimed below within the present discussion.

Although such control systems are readily known to those in the art, one such control system 500 is shown in FIG. 16 for the sake of completeness. The control system 500 may include a plurality of fluid flow valves 505 and a controller 510 sending control signals over electrical lines 515. A controller such as the controller 510 may send control signals to the fluid flow valves 505 to control the working fluid flow as described above.

The controller 510 includes a processor-based resource 520 that may be, for example and without limitation, a microcontroller, a microprocessor, an Application Specific Integrated Circuit (“ASIC”), an Electrically Erasable Programmable Read-Only Memory (“EEPROM”), or the like. Depending on the implementation of the processor-based resource, the controller 510 may also include a memory 525 encoded with instructions (not shown) executable by the processor-based resource 520 to implement the functionality of the controller 510. Again, depending on the implementation of the processor-based resource 520, the memory 525 may be a part of the processor-based resource 520 or a stand-alone device. For example, the instructions may be firmware stored in the memory portion of a microprocessor or they may be a routine stored in a stand-alone read-only or random-access memory chip. Similarly, in some implementations of the processor-based resource 520—e.g., an ASIC—the memory 535 may be omitted altogether.

In the embodiments described above, the heat transfer source may be implemented in a variety of forms. On one form, the heat transfer source may be waste heat and, more particularly, a low grade waste heat. However, it is contemplated that other forms such as heat gathered from the ambient environment or locally sourced water (fresh or salt) may be more typical and more advantageous. Some embodiments may even chill water (say ˜4° C.), which is then used by a process and returned to the heat pump system at a higher temperature, which can then be used as the heat transfer source for the heat pump.

This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the claimed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the claims. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

1. A closed-loop, transcritical, recuperated heat pump cycle, comprising: a heat transfer source;a heat transfer target;a working fluid; anda working fluid circuit transferring heat from the heat transfer source to the heat transfer target via the working fluid, the working fluid circuit including: a recuperator; anda fluid expansion process that bypasses a portion of the working fluid around the recuperator to optimize the exergetic efficiency of the recuperator, thereby improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

2. The closed-loop, transcritical, recuperated heat pump cycle of claim 1, wherein the fluid expansion process includes a first expansion device and a second expansion device.

3. The closed-loop, transcritical, recuperated heat pump cycle of claim 2, wherein the fluid expansion process further includes a respective flow control valve for each of the first expansion device and the second expansion device.

4. The closed-loop, transcritical, recuperated heat pump cycle of claim 1, further comprising a working fluid that, when the heat pump cycle is operating, circulates through the working fluid circuit.

5. The closed-loop, transcritical, recuperated heat pump cycle of claim 4, wherein the working fluid is Carbon dioxide (CO2).

6. The closed-loop, transcritical, recuperated heat pump cycle of claim 1, wherein the heat pump cycle is a steam generating system and the heat transfer target is water.

7. The closed-loop, transcritical, recuperated heat pump cycle of claim 1, further comprising a programmed control system.

8. The closed-loop, transcritical recuperated heat pump cycle of claim 1, further comprising a compression process, the compression process including a plurality of compression stages with steam generation after each stage.

9. A method for generating steam, comprising: circulating a working fluid through a working fluid circuit of a closed-loop, transcritical, recuperated heat pump cycle, the heat pump cycle including a fluid expansion process and a recuperator; andbypassing a portion of the working fluid during the fluid expansion process around the recuperator to optimize the exergetic efficiency of the recuperator and improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

10. The method of claim 9 wherein circulating the working fluid through the working fluid circuit includes circulating Carbon dioxide (CO2) through the working fluid circuit.

11. The method of claim 9, further comprising generating steam from a heat transfer target during the steam generating process.

12. The method of claim 9, further comprising controlling the method through a programmed control system.

13. A heat pump system, comprising: a heat transfer source;a heat transfer target;a working fluid; anda working fluid circuit through which, when the heat pump system is operating, the working fluid circulates, the working fluid circuit including: a recuperator disposed on: a first flow path between a heat transfer source and a steam generation process; anda second flow path between the steam generation process and the heat transfer source, the second flow path including a fluid expansion process that bypasses a portion of the working fluid around the recuperator to optimize the exergetic efficiency of the recuperator, thereby improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred from the heat transfer source to the heat transfer target over a limited temperature range.

14. The heat pump system of claim 13, wherein the heat transfer source includes waste heat, heat from an ambient atmosphere, or water.

15. The heat pump system of claim 13, wherein the heat pump system is a steam generating system and the heat transfer target is water.

16. The closed-loop, transcritical recuperated heat pump cycle of claim 13, further comprising a compression process, the compression process including a plurality of compression stages with steam generation after each stage.

17. A closed-loop, transcritical, recuperated heat pump cycle that includes a fluid expansion process that bypasses a portion of a working fluid around a recuperator to optimize the exergetic efficiency of the recuperator, thereby improving the coefficient of performance of the heat pump for certain types of applications in which heat is transferred to the target over a limited temperature range.

18. The closed-loop, transcritical, recuperated heat pump cycle of claim 17, wherein the closed-loop, transcritical, recuperated heat pump cycle comprises: a heat transfer source;a heat transfer target;a working fluid; anda working fluid circuit transferring heat from the heat transfer source to the heat transfer target via the working fluid, the working fluid circuit including:the recuperator; andthe fluid expansion process.

19. The closed-loop, transcritical, recuperated heat pump cycle of claim 18, wherein the fluid expansion process includes a first expansion device and a second expansion device.

20. The closed-loop, transcritical, recuperated heat pump cycle of claim 18, further comprising a working fluid that, when the heat pump cycle is operating, circulates through the working fluid circuit.

21. The closed-loop, transcritical, recuperated heat pump cycle of claim 18, wherein the heat pump cycle is a steam generating system and the heat transfer target is water.

22. The closed-loop, transcritical, recuperated heat pump cycle of claim 18, further comprising a programmed control system.

23. The closed-loop, transcritical recuperated heat pump cycle of claim 18, further comprising a compression process, the compression process including a plurality of compression stages with steam generation after each stage.