Patent Application
20240142143
This disclosure pertains to a heat pump cycle and, more particularly, to a heat pump cycle for high performance at high-temperature lift and high-temperature range.
In some industrial processes, heat needs to be applied to one or more materials at a relatively high temperature. The most common method of creating this heat is through the combustion of fossil fuels. However, as the world moves towards a carbon-free energy system, alternative techniques for providing industrial heat will be desired. While direct electrical heating with devices such as electric resistance heaters, electric arc heaters, or electrical induction heaters can attain the necessary temperatures, the coefficient of performance (“COP”) of these processes can never be greater than 1. As used in this context, the COP is defined as the amount of heat transferred to the process divided by the electrical power input.
These forms of direct electrical heating discussed in the paragraph immediately above are also potentially utilized in converting electrical power to thermal power, which can be stored for later use in so-called “electro-thermal energy storage” systems. In those cases, the round-trip efficiency of such a system is the mathematical product of the COP and the generating cycle efficiency. Because of the characteristics of the aforementioned heating system, the round-trip efficiency (“RTE”) of the overall storage process will be less than the efficiency of the power cycle used to convert the thermal energy back to electricity, typically ˜40% for temperatures in the 550-600° C. range.
In a first aspect, a dual rail heat pump cycle, comprises a low-temperature heat source; a two stage, high-temperature heat exchange process through which, in operation, heat is exchanged with a thermal medium; and a working fluid circuit. The working fluid circuit includes an expansion process; a compression process; a recuperation process, and a pair of parallel flow paths. The recuperation process is interposed between the expansion process and the compression process and has a high-pressure side defined by the compression process and a low pressure side defined by the expansion process. The pair of parallel flow paths between the recuperation process and the high-temperature heat exchange process on the high-pressure side of the recuperation process.
In another aspect, a dual rail heat pump cycle includes a low-temperature heat source and a working fluid circuit through which, in operation, a working fluid circulates. The working fluid circuit includes a recuperator, a compression device, an expansion device, a pair of high-temperature heat exchangers, and a pair of parallel flow paths. The expansion device is downstream from the high-pressure side of the first recuperator, upstream from the low-temperature heat source, and defines a low-pressure side of the recuperator. The pair of parallel flow paths extend between the recuperator and the high-temperatures heat exchangers on the high-pressure side of the recuperator.
The above presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the disclosure or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
The present disclosure is best understood from the following detailed description when read with the accompanying Figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
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.
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.
In the traditional heat pump cycle 100, the working fluid is compressed using a compression device 110, such as a compressor, from a first relatively low temperature, low pressure State 1 to one of higher temperature and pressure, State 2. The heat introduced to the working fluid by the compression can then be transferred to a thermal medium that receives and either uses or stores that heat. In
During the process of heating the thermal medium, the working fluid is cooled to State 3. The fluid is then expanded to State 4 using an expansion device 120. The expansion device 120 may be an expander such as an adiabatic expansion valve or a fluid expander that also extracts thermodynamic work from the fluid. The temperature and pressure of the working fluid decrease in the expansion device 120 to State 4. Low-temperature heat is then added to the working fluid from an external low-temperature source 125 to bring the working fluid back to State 1. In many cases, the low-temperature heat source may be ambient heat from the environment in which the heat pump 100 is operating.
The thermodynamic process for the heat pump cycle in
The net specific work required to transfer the heat can be represented by the enthalpy increase during the compression process (State 1 to State 2), less the enthalpy decrease during the expansion process (State 3 to State 4). The amount of heat transferred to the thermal medium is represented by the enthalpy change in the high-temperature heat exchanger 115 (State 2 to State 3). One performance parameter of a heat pump cycle previously introduced is the Coefficient of Performance (“COP”), which in this context is the heat transferred divided by the net specific work. Consistent with the previous definition of COP, one can represent the COP by following expression:
In the equation above, Qh is the heat transferred, Wnet is the net specific work, and h1-h4 are the specific enthalpy of the working fluid at States 1 through 4 respectively in
Other performance parameters are the temperature to which the thermal medium is heated (Th2), and the temperature “range”. The temperature range is the difference between the final temperature of the thermal medium and its initial temperature (Trange=Th2 Th1). Another parameter is “lift”, which may be mathematically represented as Lift=Th2−Tsource, where Tsource is the temperature of the heat source. Based on fundamental thermodynamic and heat transfer principles, for heat to transfer to the thermal medium, the working fluid temperature in State 2 (T2) must be greater than Th2, and the working fluid temperature in State 3 (T3) must be greater than Th1.
To achieve this combination of performance parameters, the baseline heat pump cycle described above must use an extremely high compressor pressure ratio (“CPR”). This adds significantly to the cost of the system, and beyond certain pressure ratios (typically around 10:1), the solution becomes impractical. In the case shown in
The heat pump cycle illustrated in
The performance of the recuperated heat pump cycle is better than the simple heat pump cycle for Th2 values greater than approximately Th2=200-250° C. For these simulations used to generate
However, another factor in heat pump performance is the COP at a combined high Trange and Th2. Here, the simple recuperated cycle is limited in its ability to provide this combination of attributes.
As
Illustrative examples 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 implementation, numerous implementation-specific decisions may 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.
This disclosure describes a “dual rail” heat pump cycle that flexibly allows for a flowing thermal medium to be heated over a wide range of temperatures and obtain excellent heat pump performance values. The dual rail heat pump cycle increases the heat pump's capability to increased Th2, while also keeping a large Trange, and still maintaining a good COP. A second embodiment that is a variant of the first is then disclosed. In the next, more complex, heat pump cycle, two internal heat exchangers, each called a “recuperator”, are used to transfer some of the residual heat downstream of the heat exchanger to preheat the working fluid prior to entering the compression device.
As used herein, the terms “low temperature” and “high temperature” in the context of heat exchange are defined relative to one another and relative to the temperature of the circulating working fluid. In the embodiments disclosed herein, “low temperature” can mean temperatures in the range of −25° C. to 50° C. and “high temperature” can mean 80 C to 600 C. However, in other embodiments, these numbers might differ. For example, in some embodiments, the heat source may be the ambient environment. In other embodiments, the low-temperature heat source may be a waste heat source rather than the ambient environment, in which case the low temperatures and the high temperatures may be higher than for the embodiments disclosed herein. A waste heat source will nevertheless still be “low temperature” in the sense that it is lower temperature than the target heat sink temperature (Th2). However, it will generally be a higher temperature than the ambient environment.
Thus, as used herein, a “low-temperature” heat exchanger (where used) transfers heat from a heat source into the circulating working fluid. The heat is therefore transferred from a “higher” temperature heat source to a “lower” temperature working fluid. Conversely, a “high-temperature” heat exchanger transfers heat from a “higher temperature”, circulating working fluid into a relatively “lower temperature” thermal medium. The temperatures at which the heat exchange occurs in the “high-temperature” heat exchanger occurs at temperatures greater than those at which the heat exchange occurs in the “low-temperature” heat exchanger.
The terms “high pressure” and “low pressure” are similarly defined relative to one another. For example, it may be said that the recuperation process 733 has a “high-pressure side” and a “low pressure side” that are defined by the expansion process 725 and the compression process 730. The working fluid in States 7-8 and 1 is at lower pressure due than the pressure of the working fluid in States 3B, 4B, and 4-5. (This is due to the actions of the expansion process 725 and the compression process 730, respectively.) The “side” of the recuperation process 733 in which the working fluid is in States 7-8 and 1 may therefore be referred to as the “low pressure side” and the “side” I which the working fluid is in States 3B, 4B, and 4-5 may therefore be referred to as the “high-pressure side”.
Thus, precise numerical quantification for the terms “high temperature”, “low temperature”, “high pressure” and “low pressure” in the context of heat exchange will depend on implementation specific details of any given embodiment. While representative numerical quantifications may be provided herein for the illustrated embodiments, it is likely that other embodiments not illustrated may employ other numerical quantifications. Those in the art having the benefit of this disclosure will be able to readily determine suitable numerical quantifications for a particular embodiment.
The dual rail heat pump cycle 700 includes a heat exchange process (“HTX”) 720, an expansion process 725, a compression process 730, a recuperation process 733, and a low-temperature heat source 735. The working fluid circulates through the heat exchange process 720, expansion process 725, compression process 730, and low-temperature heat source 735, through a working fluid circuit 740. The working fluid in this particular embodiment is Carbon dioxide (CO2). Alternative embodiments may employ other working fluids. The thermal medium heated by the heat exchange process 720 may be, for example and without limitation, any flowing thermal medium such as air, water, heat transfer fluid, or molten salt. The working fluid circuit 740 also includes, in this particular embodiment, a generator 745 driven by the expansion process 725 and a motor 750 driving the compression process 730.
The expansion process 725 and the compression process 730 employ at least one expansion device 726 and at least one compression device 731, respectively. The compression device 731 may be a compressor, such as a fully hermetic reciprocating compressor or a scroll compressor or a centrifugal compressor. The expansion device may be an expander, such as an adiabatic expansion valve or a fluid expander. The fluid expander may be a turbine, or a reciprocating expander, or a scroll expander, for example. Those skilled in the art having the benefit of this disclosure may appreciate still other implementations.
Referring now to both
In the first stage, the working fluid transfers heat to the thermal medium in the heat exchanger 710, heating the thermal medium and cooling the working fluid. The working fluid enters the heat exchanger 710 in a State 2, whereupon heat is exchanged between the working fluid and the thermal medium. This heat exchange heats the thermal medium and cools the working fluid.
Upon exiting the first stage of the heat exchange process 720 in State 3, the working fluid is then divided into two portions at a point 755. The first portion in state 3A enters a second stage of the heat exchange process 720. In the second stage, represented by the heat exchanger 711, the working fluid is further cooled, which preheats the thermal medium. Note that the terminology “first” and “second” relative to the heat exchange process 720 is from the perspective of the working fluid circulation. The thermal medium is flowing in the opposite direction of the working fluid. Thus, from the perspective of the thermal medium, the thermal medium is preheated in the second heat exchanger 711 before a second heating in the first heat exchanger 710, the working fluid being cooled in both exchanges. The second portion of the working fluid in State 3B enters a second recuperator 716 (“RCX2”), where the second portion is cooled and preheats the working fluid on the low-pressure side of the system (State 8 to State 1).
Upon the first portion exiting the second stage of the heat exchange process 720, represented by the heat exchanger 711, in State 4A and the second recuperator 716 in State 4B, the flow recombines the first and second portions at point 760. The recombined working fluid in State 4 then enters into an additional, first recuperator (“RCX1”) 715 for further cooling of the high-pressure working fluid (State 4 to State 5) and preheating the low-pressure working fluid (State 7 to State 8). The high-pressure working fluid then enters the expansion device 726 in State 5, where work may be extracted, and the working fluid pressure and temperature are reduced to State 6. Heat is then added from the environment or similar source (i.e., the low-temperature heat source 735) as the working fluid is brought from State 6 to State 7.
The division of the working fluid into two portions allows for better matching of the heat capacities, defined as the mathematical product of the mass flow rate and the specific heat capacity, in the two sides—i.e., the high-pressure side (State 3B to State 4B) and the low pressure side (State 8 to State 1)—of the second recuperator (716). In addition, the working fluid can then be cooled to a lower temperature than it can in the single recuperated cycle, which also allows its TQ slope to better match the thermal medium thermal slope as shown in
In the extended dual rail heat pump cycle 1000, the working fluid may be additionally split at State 4, where a first portion (State 4C) enters an additional lower-temperature stage of the heat exchange process 1005 (“HTX”) represented by a high-temperature heat exchanger 1010 (“HTX3”). The working fluid then is further cooled while further preheating the thermal medium in the third high-temperature heat exchanger. The second portion (State 4D) of the working fluid enters the first recuperator 1015 (“RCX1”), where it is cooled, while heating the low-pressure working fluid (State 7 to State 8).
Note the presence of the third and fourth parallel flow paths 1020, 1021. The third parallel flow path 1020 More particularly, the third rail 1020 comprises the flow path from the split at the point 1030 in which the working fluid is in States 4C and 4D as well as the third high temperature heat exchanger 1010. The fourth rail 1021 comprises the flow path from the split at point 1030 in which the working fluid is in States 4D and 5D and as well as the recuperator 1015. Those in the art having the benefit of this disclosure will appreciate that it is possible to conceive additional configurations that repeat the flow split and parallel HTX and RCX flow paths any number of times, with the purpose of further matching the heat capacities of the fluids on either side of the heat exchangers.
As was mentioned above in the discussion of
As those in the art having the benefit of this disclosure will appreciate, the heat pump cycles of
Although such control systems are readily known to those in the art, one such control system 1200 is shown in
The controller 1210 includes a processor-based resource 1220 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 1210 may also include a memory 1225 encoded with instructions (not shown) executable by the processor-based resource 1220 to implement the functionality of the controller 1210. Again, depending on the implementation of the processor-based resource 1220, the memory 1225 may be a part of the processor-based resource 1220 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 1220—e.g., an ASIC—the memory 1235 may be omitted altogether.
A flow control system may be used, more particularly, to control the flow of the working fluid between the two parallel paths, or rails. Referring to
As used herein, the article “a” is intended to have its ordinary meaning in the patent arts, namely “one or more.” Herein, the term “about” when applied to a value generally means within the tolerance range of the equipment used to produce the value, or in some examples, means plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless otherwise expressly specified. Further, herein the term “substantially” as used herein means a majority, or almost all, or all, or an amount with a range of about 51% to about 100%, for example. Moreover, examples herein are intended to be illustrative only and are presented for discussion purposes and not by way of limitation.
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the disclosure. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the systems and methods described herein. The foregoing descriptions of specific examples are presented for purposes of illustration and description. They are not intended to be exhaustive of or to limit this disclosure to the precise forms described. Obviously, many modifications and variations are possible in view of the above teachings. The examples are shown and described in order to best explain the principles of this disclosure and practical applications, to thereby enable others skilled in the art to best utilize this disclosure and various examples with various modifications as are suited to the particular use contemplated. It is intended that the scope of this disclosure be defined by the claims and their equivalents below.
