Patent Application
20110180235
The present system relates generally to microscale heat transfer systems or heat and mass transfer systems, and more particularly to monolithic or integrated microscale heat or heat and mass transfer systems or apparatuses comprising a plurality of shims or layers, each shim including a plurality of microchannels for performing heat and/or mass exchange functions.
Traditionally, vapor-compression systems have been used in various heating and cooling applications, such as residential and commercial air conditioners, chillers, and heat pumps. These systems generally comprise four basic components—an evaporator, compressor, condenser, and expansion device. The evaporator and condenser comprise heat exchangers that evaporate and condense refrigerant while absorbing and rejecting heat. The compressor takes the refrigerant vapor from the evaporator and raises its pressure sufficiently to condense the vapor in the condenser. After exiting the condenser, the flow of condensed refrigerant at higher pressure is controlled by the expansion device back into the evaporator, and the cycle repeats to produce continuous heating or cooling effects.
Traditional vapor-compression systems, however, have several disadvantages. For example, most vapor-compression systems rely on synthetic refrigerants that have negative environmental impact. Also, most vapor-compression systems utilize expensive, high-grade electrical energy for power. Further, vapor-compressions systems are often loud and unreliable due to the use of a compressor, and often employ bulky overall system designs that prohibit small-scale or portable use.
An absorption heat pump (also referred to herein as an “absorption cooling and/or heating system”) can be considered an environmentally benign replacement for a traditional vapor-compression system. In principle, the compressor of a traditional vapor-compression system is replaced by a combination of a desorber, absorber, liquid solution pump, and recuperative solution heat exchanger to form an absorption heat pump. A benefit of absorption heat pumps is the reduced concern about reliability due to the absence of a major moving part, i.e., the compressor. The lack of a compressor in the absorption heat pump also implies much quieter operation as compared to a vapor-compression system. Further, unlike vapor-compression systems that utilize high-grade electrical energy as the input that drives the system, absorption heat pumps typically run on more readily available and low-grade thermal energy, which may be obtained from combustion of bio-fuels and fossil fuels, from largely untapped waste heat sources (e.g., automobile exhaust, excess manufacturing heat, etc.), from solar thermal energy, and other similar energy sources. In cooling mode operation, this thermal energy input is used to provide cooling and/or dehumidification, while in the heating mode, the heat input is used to pump ambient heat to higher temperatures.
Because the compressor of a vapor-compression system is replaced in an absorption heat pump by a combination of a desorber, absorber, liquid solution pump, and recuperative solution heat exchanger, absorption heat pumps are generally more heat and mass exchange intensive than vapor-compression systems, thereby requiring additional heat transfer surface area. Due to this comparatively larger surface area requirement, absorption heat pumps have typically been relegated to very large commercial and industrial chiller applications, and achieving compact designs while delivering high coefficients of performance (COPs) has been a major challenge. Additionally, several advanced absorption cycles, such as the double-effect, triple-effect, and Generator-Absorber Heat Exchange cycles developed to improve COPs, rely on additional internal recuperation to improve performance, further emphasizing the need for high heat and mass transfer rates per volume. In fact, these cycles have not been widely implemented primarily because of a lack of practically feasible and compact heat and mass exchange devices.
It is desirable, therefore, to achieve a compact absorption cooling and/or heating system that delivers outputs comparable to those of larger systems. However, in absorption systems that use the two most common working fluid pairs (i.e., lithium bromide-water and ammonia-water), processes such as absorption and desorption naturally involve coupled heat and mass transfer in binary fluids, leading to complexities and challenges in system design. Particularly, in ammonia-water systems, due to the presence of both absorbent (i.e., water) and refrigerant (i.e., ammonia) in the liquid and vapor phases throughout the system, such binary fluids processes occur in all components in the system (including the condenser, evaporator, rectifier, and recuperative heat exchangers). With other, less common working fluids (e.g., multi-component fluids), multi-component heat and mass transfer processes are required. For the implementation of absorption systems in compact, high-flux configurations that can take advantage of disperse availability of waste heat, solar thermal energy, or other energy in smaller capacities than at the industrial scales, the heat and mass exchanger designs should provide several features that are difficult to achieve simultaneously. For example, the systems should include low heat and mass transfer resistances for the working fluids, the requisite transfer surface area for the working fluids and the fluids that couple those working fluids to external heat sources and sinks in compact volumes, and low resistances for the coupling fluids, among other similar system properties.
Most of the available absorption component concepts fall short in one or more of these features essential for achieving compact, high-flux designs. For example, the primary configuration employed currently in commercial absorption chillers (i.e., absorption of vapor into solution films falling over tube banks carrying coolant liquid) suffers from high coolant-side resistances and poor wetting of the transfer surface by the liquid film. Additionally, some prior designs enhance absorption/desorption processes, but fail to reduce single-phase resistance on the other side (i.e., coupling fluid side), thereby requiring large system components, and resulting in high working fluid and coupling fluid pressure drop, which results in high parasitic power consumption and also results in losses in driving temperature differences due to decrease in saturation temperatures brought about by pressure drops within system components.
In addition to absorption cooling and/or heating systems, it is further desirable to provide various other heat transfer or heat and mass transfer systems for performing other functions, such as related cooling or heating functions, basic heat transfer, distillation, and other similar functionalities as will occur to one of ordinary skill in the art.
Therefore, there is a long-felt but unresolved need for a microscale heat or heat and mass transfer system or apparatus that provides compact, modular, versatile design that can be applied for high flux heat and mass transfer, both in individual system components and in the overall system assembly, while overcoming the weaknesses of currently-used configurations. There is a further need for a microscale, monolithic absorption heat pump that provides significant heating and cooling outputs from a portable, integrated system. The principal embodiments of the present system, and variants thereof, represent a miniaturization technology highly adaptable to a variety of design conditions, and also to several systems in multiple industries involved in binary, ternary, and other multi-component fluid heat and mass transfer.
Briefly described, and according to one embodiment, aspects of the present disclosure generally relate to systems and apparatuses for absorption cooling and/or heating, or performing other heat and/or mass transfer functions. More particularly, according to one aspect, an array of parallel, aligned alternating shims with integral microscale passages and voids, fluid inlet and outlet passages, and vapor-liquid spaces as necessary, enclosed between cover plates, define the heat and mass transfer system components of a thermally-activated absorption heat pump. The assembly of parallel shims with microscale features directs fluid flow through a defined absorber, recuperative solution heat exchanger, desorber, rectifier (in applications using a working fluid with a volatile absorbent), condenser, recuperative refrigerant heat exchanger, and an evaporator, which together comprise the heat and mass transfer system components of a single-effect absorption heat pump. As described in greater detail herein, in a particular embodiment, the heat and mass transfer components are defined within a microscale, monolithic apparatus or assembly via pairs of alternating shims. In embodiments in which a double-effect, triple-effect, generator-absorber-heat exchange (GAX) cycle, or other advanced absorption cycle is desired, additional microscale features arranged into additional, defined, heat and mass transfer system components are incorporated into the apparatus to accomplish the requisite recuperative heat and mass transfer.
According to one aspect, the absorption cycle working fluid flows in microscale and other passages incorporated into one side of a shim, while the high (heat source), medium (heat rejection), and low (chilled stream) temperature coupling fluids flow on the other side of the shim in thermal contact with the respective working fluid streams on the initial side. Therefore, sets of two shims (“shim pairs”), with somewhat differentiated microscale feature geometries, comprise building blocks of an entire absorption heat pump or other heat or heat and mass transfer system that are duplicated in numbers required to accomplish the desired overall cooling or heating load. The features incorporated into each shim are arrayed in groups, with each group representing the corresponding passages for each heat or heat and mass transfer system component in a heat pump (e.g., absorber, desorber, etc.). Fluid connections between the respective, defined heat and mass transfer system components is achieved through connecting fluid lines external to the system, or through specifically designed routing passages between different parts of the shims or cover plates, or via some other similar connection mechanism. Generally, the working fluid is largely contained within the assembly of shims, therefore reducing fluid inventories several fold over conventional heat pumps that deliver similar capacities.
According to an additional aspect, cooling, heat rejection, and heat source fluid streams enter and leave the heat or heat and mass transfer apparatus through appropriate inlet and outlet connections, enabling versatile deployment of heating or cooling loads, irrespective of the physical location of the heat or heat and mass transfer apparatus. In one aspect, a working solution pump is provided external to the system assembly to pump the working fluid through the heat and mass transfer components and microchannels arrayed across each shim in the assembly. During the heat pump cycle, and according to a further aspect, expansion of the refrigerant stream and the refrigerant-absorbent solution from the low to high-side pressures (and intermediate pressures as necessary for advanced absorption cycles) is accomplished through integral tailored constrictions within the shims or through externally connected valves.
According to various aspects, the microchannels and other microscale passages in the shims comprise square, rectangular, semi-circular, semi-elliptical, triangular or other singly-connected cross-sections to enable fluid flow in single-phase or two-phase state, as necessary, with the microscale cross-section shape and dimensions determined based on heat and mass transfer requirements, operating pressures, structural strength of the assembled apparatus, manufacturing constraints for dimensional tolerances and bonding of the shims and cover plates, and other factors. Generally, the microscale channels in the shims are formed through processes such as lithography, etching, machining, stamping, or other appropriate processes based on overall assembly dimensions as well as microscale channel dimensions. Joining and assembly of a plurality of shim pairs and cover plates to form an embodiment of the microscale heat or heat and mass transfer system is accomplished through processes such as diffusion bonding and brazing for the most commonly utilized metallic assemblies, and if permitted or dictated by the working fluid, operating conditions, and desired loads, by gluing for plastic, ceramic, or other nonmetallic apparatus parts. Modularity in heat duties is achieved by varying the microscale channel dimensions, number of channels, length and width of the shims, and number of shim pairs.
According to another aspect, for large scale implementation of microscale heat or heat and mass transfer assemblies as described herein, multiple assemblies are connected in series and/or parallel arrangements through external plumbing to form a plurality of connected heat or heat and mass transfer assemblies. According to various aspects, for larger capacities, the shims are subdivided into individual assemblies representing each heat and mass transfer system component of the heat or heat and mass transfer system rather than a monolithic heat or heat and mass transfer assembly, to facilitate flexibility in connections, and largely unconstrained increases in delivered loads.
These and other aspects, features, and benefits of the claimed invention(s) will become apparent from the following detailed written description of the preferred embodiments and aspects taken in conjunction with the following drawings, although variations and modifications thereto may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
The accompanying drawings illustrate one or more embodiments and/or aspects of the disclosure and, together with the written description, serve to explain the principles of the disclosure. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment, and wherein:
Prior to a detailed description of the disclosure, the following definitions are provided as an aid to understanding the subject matter and terminology of aspects of the present systems and methods, are exemplary, and not necessarily limiting of the aspects of the systems and methods, which are expressed in the claims. Whether or not a term is capitalized is not considered definitive or limiting of the meaning of a term. As used in this document, a capitalized term shall have the same meaning as an uncapitalized term, unless the context of the usage specifically indicates that a more restrictive meaning for the capitalized term is intended. However, the capitalization or lack thereof within the remainder of this document is not intended to be necessarily limiting unless the context clearly indicates that such limitation is intended.
Absorbent: material or fluid that, either by itself or in multi-component form combined with ammonia or another refrigerant, comprises a working fluid or portion of a working fluid for performing the heat and mass transfer functions of a heat or heat and mass transfer system (e.g., an absorption heat pump) as described herein. Examples include, but are not limited to, water (in ammonia-water mixtures), lithium bromide (in lithium bromide-water mixtures), and other similar materials.
Coefficient of performance (COP): ratio of a desired output (i.e., cooling or heating) from a system embodiment as compared to the input energy.
Coupling fluid: fluid used to transfer heating and/or cooling to and from embodiments of the present system. Generally connects embodiments of the present system to one or more heat sources, heat sinks, ambient spaces, conditioned spaces, etc., generally via hydronic coupling. Examples include, but are not limited to, ethylene glycol-water solution, propylene glycol-water solution, calcium chloride-water solution, high temperature heat transfer fluids (e.g., synthetic oil), and other similar fluids. Sometimes referred to herein as coolant.
Cover plate: rigid outer layer on outer sides of embodiments of the present system to provide structure, support, and, in some embodiments, fluid transfer channels to shims contained between cover plates. Cover plates generally include holes or inlet and outlet openings for transferring coupling fluid and working fluid flow streams entering and exiting embodiments of the system.
Fluid distribution passage: channel or passage that transports fluid from voids formed by stacked-up shims (i.e., headers) to microchannels within heat and mass transfer system components or heat exchange components in embodiments of the present system. Generally synonymous with distribution passage, fluid passage, passage, or passageway.
Header: element within a heat and mass transfer system component that provides an opening or port to receive or expunge fluid. Generally formed by a plurality of stacked-up voids associated with individual shims that, when combined, form a passageway for fluid flow. Types generally comprise inlet headers and outlet headers.
Heat or heat and mass transfer system: a system for transferring heat or heat and mass comprising properties, features, dimensions, components, etc., as described herein. As will be understood and appreciated, generally describes a heat transfer system or a heat and mass transfer system formed by one or more heat and mass transfer system components, as described herein. Generally synonymous with heat or heat and mass transfer apparatus, heat or heat and mass transfer assembly, or heat and/or mass transfer system.
Heat and mass transfer system component: generic term used to describe any component capable of performing heat and/or mass transfer, generally (although not always) within a larger heat or heat and mass transfer system. Examples include, but are not limited to, an absorber, recuperative solution heat exchanger, desorber, rectifier, condenser, recuperative refrigerant heat exchanger, evaporator, or other similar component. Generally (although not always) includes or comprises at least one heat exchange component. Generally synonymous with heat and mass transfer component. Sometimes synonymous with heat exchanger.
Heat exchange component: generic term used to describe any component capable of performing heat transfer. May comprise a heat and mass transfer system component, or a sub-component thereof. Generally synonymous with heat exchanger.
Microchannel: channel or passage of microscale dimensions formed in a shim as described herein for transferring fluid in single-phase or multi-phase state to accomplish heat and/or mass transfer functionality. Generally characterized by circular (or non-circular) cross-sections with hydraulic diameters less than 1 mm (although, as will be understood, channels larger than 1 mm may exhibit fluid flow and heat and mass transfer phenomena similar to microchannels at somewhat larger hydraulic diameters, depending upon the given fluid properties and operating conditions). Generally synonymous with microscale passage or microscale channel.
Microscale: relatively smaller in size as compared to other systems or components of similar functionality and/or output. Generally miniature, as understood in the art.
Monolithic: constituting one, undifferentiated whole or unit. Generally synonymous with integrated.
Multi-component fluid: fluid comprising more than one discrete substance (i.e., more than one species). Examples include, but are not limited to, ammonia-water mixtures and lithium bromide-water mixtures. Generally synonymous with multi-constituent fluid, multi part fluid, binary fluid, ternary fluid, quaternary fluid, fluid pair, etc.
Refrigerant: material or fluid that, either by itself or in multi-component form combined with water or another absorbent, comprises a working fluid or portion of a working fluid for performing the heat and mass transfer functions of a heat or heat and mass transfer system (e.g., an absorption heat pump) as described herein. Examples include, but are not limited to, ammonia (in ammonia-water mixtures), water (in lithium bromide-water mixtures), and other similar materials. Generally synonymous with ammonia, as used herein.
Shim: thin, rigid layer defining features associated with one or more heat or heat and mass transfer components as described herein. Generally includes a plurality of microchannels, fluid distribution passages, and voids for transferring working fluid and/or coupling fluid across the shim. Generally synonymous with layer or laminate.
Shim group: combination of a plurality of shim pairs bonded or otherwise combined together to define one or more heat or heat and mass transfer system components.
Shim pair: combination of two discrete shim types (e.g., A and B, described herein) bonded or otherwise combined together to enable heat and/or mass transfer between fluids flowing in microchannels, voids, and other passages in each shim.
Void: hole or space defined by a plurality of stacked-up shims that enables fluid flow in to our out of a heat or heat and mass transfer system component. Generally relates to the space within or formed by a header. Generally synonymous with stacked-up void or vapor-liquid space.
Working fluid: fluid transferred throughout embodiments of the present system to accomplish heat and/or mass transfer functions. At various stages in an absorption cycle process or other similar heat cycle, can be in liquid state, vapor state, or liquid-vapor mixture. Examples include, but are not limited to, ammonia-water mixtures and lithium bromide-water mixtures. Generally comprises multi-component fluids, but also comprises single-component fluid as needed.
For the purpose of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the disclosure is thereby intended; any alterations and further modifications of the described or illustrated embodiments, and any further applications of the principles of the disclosure as illustrated therein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. All limitations of scope should be determined in accordance with and as expressed in the claims.
Aspects of the present disclosure generally relate to heat or heat and mass transfer systems or apparatuses. Particularly, one embodiment of the present apparatus comprises a plurality of shims assembled or pressed between two outer plates that, when combined, form discrete but integrated heat and mass transfer system components that make up a microscale, monolithic absorption cooling and/or heating system or absorption heat pump, or other heat or heat and mass transfer system. The shims generally include a plurality of microchannels, voids, and other heat transfer features for transferring working fluids and coupling fluids between defined heat and mass transfer system components throughout the apparatus, and into and out of the apparatus to and from heating and cooling sources and sinks as needed. According to one embodiment, two distinct shim types are used (i.e., shims A and B, described in greater detail below), and the shims are combined (e.g., bonded together) as a plurality of shim pairs, wherein the two distinct shims in each pair comprise slightly different microchannel and fluid passage arrangements as compared to each other to enable thermal contact between the fluids flowing within the microchannels in each shim pair.
According to one aspect, each shim includes the geometries of all of the necessary heat and mass transfer system components that comprise an absorption cooling and/or heating system, namely, an absorber, recuperative solution heat exchanger, desorber, rectifier, condenser, recuperative refrigerant heat exchanger, and an evaporator. As will be appreciated, these heat and mass transfer components perform their conventional functions as understood by one of ordinary skill in the art. Thus, in one embodiment, when a plurality of shim pairs are combined, a microscale absorption heating and/or cooling system is formed. Further, as pairs of shims are stacked and combined together, the number of microchannels (and, hence, the overall heat exchange surface area) of each heat and mass transfer component increases, thereby increasing the heat exchange capacity of each component and the overall system. In this way, embodiments of the present apparatus comprise monolithic, microscale heat or heat and mass transfer systems that can be scaled to meet individual application requirements as desired.
As described in greater detail herein, embodiments of the present invention(s) yield compact overall geometries for absorption heat pumps and other heat or heat and mass transfer apparatuses, with several fold reductions in system volume as compared to conventional systems for equivalent cooling and/or heating loads. As described previously, conventional absorption heat pumps require additional heat and mass transfer components as compared to vapor-compression systems, leading to larger overall system sizes. Accordingly, it has previously not been possible to implement heat-driven absorption heat pumps in small geometries. Embodiments of the present invention(s), however, exploit the inherent and novel advantages of fluid flow and heat and mass transfer phenomena at microscales to enable high cooling and heating capacity systems in relatively small system packages. Therefore, embodiments of the present system are able to take advantage of: a) high heat and mass transfer coefficients in small hydraulic diameter microscale passages, b) large surface-to-volume ratios at small hydraulic diameters, c) flexibility of parallel flows in pluralities of microchannels in multiple parallel shim assemblies to achieve high heat and/or mass transfer rates with low pressure drops, and d) the ability to modify microchannel dimensions, the numbers of microchannels used in each heat and mass transfer component, the number of shims used in the system, and overall system envelope width and length to precisely tailor system size to desired loads. Further, as described in greater detail below, hydronic coupling and the absence of long interconnecting lines between heat and mass transfer system components (due to the relatively small size of system embodiments) minimizes working fluid inventories, overall system size and mass, fluid pressure drops, parasitic power requirements, and undesirable heat losses and gains to and from the ambient.
Generally, embodiments of the present system utilize thermal energy as the input energy source, such as waste heat, solar energy, energy from primary fuel combustion, etc. A wide range of source energy temperatures are utilized to provide cooling and/or heating, and a wide range of heating and cooling loads are supplied using the present system. Accordingly, embodiments of the system inherently allow for modular design of heating or cooling capacities ranging from a few Watts to Megawatts. Generally, the utilization of microscale fluid flow and heat and mass transfer principles enables the realization of compact system assemblies that deliver substantially higher cooling and heating capacities in equivalent system volumes as compared to conventional or prior systems. Embodiments of the system require relatively minimal use of electrical energy to pump the working fluids. Preferably, multi-component fluid mixtures are used as working fluids so that the system need not use synthetic fluids with ozone-depleting and global warming potential, and therefore the system has minimal adverse environmental impact.
As will be appreciated, embodiments of the present system are useful for a variety of commercial applications. Generally, embodiments of the present system can be implemented as replacements for conventional vapor-compression systems or absorption heat pumps in most applications, especially when small-scale applications are needed. However, as will be understood, embodiments of the present apparatus can be utilized in a variety of applications, including but not limited to, waste heat recovery and upgrade applications, heat-driven chillers and heating and air-conditioning systems, cogeneration systems, heat transformers, integrated cooling, heating, and power systems, vehicular, marine, naval, and stationary climate control systems, processing and refrigerated transport of food, medicines, vaccines, and other perishable items, harvesting of ambient moisture for potable water using thermal energy input, micro-reactors and combustors, and a variety of other applications as will occur to one of ordinary skill in the art.
For purposes of example and explanation of the fundamental functions and components of the disclosed systems and apparatuses, reference is made to
As shown, the heat or heat and mass transfer apparatus 10 includes two cover plates 110, 111, a shim group 108 (generally comprising a plurality of shims 102, 104, described in greater detail below) sandwiched between the cover plates, and a plurality of coupling fluid lines 120 for transferring coupling fluid into and out of the apparatus 10.
As shown, the cover plates comprise two cover plate types, namely, front cover plate 110 and back cover plate 111, that, depending on the particular embodiment, include various holes 122 for transporting fluid to and from the apparatus 10. As will be understood, the arrangement of holes 122 and overall structure of the cover plates 110, 111 may or may not vary between each cover plate 110, 111 depending on the particular system embodiment.
As described herein, shim group 108 generally includes a plurality of shims 102, 104. Referring briefly to
According to one embodiment, the shims comprise two shim types (i.e., shims A 102 and B 104, described in greater detail below), such that the shims are stacked and aligned in alternating fashion within the heat transfer apparatus 10 to form a plurality of shim pairs (each pair including one of each shim type, A and B). In the embodiment shown in
Returning to
According to various embodiments, the shims 102, 104 are manufactured from steel or other thermally conductive metals, ceramics, plastics (in low temperature applications), and other similar materials as will occur to one of ordinary skill in the art. The cover plates 110, 111 are manufactured from materials similar or dissimilar to those of the shims, as long as the resulting cover plates have adequate strength and rigidity characteristics to hold the assembly 10 together during operation. The microchannels (discussed below) in the shims 102, 104 are generally formed via a photochemical etching process or other etching process, lithography, stamping or machining during shim manufacturing, or other similar micro-cutting technique. Once manufactured, the shims 102, 104 and cover plates 110, 111 are bonded together via diffusion bonding, brazing, or gluing (in low temperature applications), or combined via a bolted or clamped assembly, or otherwise assembled via similar bonding or assembly techniques, to form a monolithic, microscale heat or heat and mass transfer system 10.
As shown in
As listed in
As shown in
Alternatively, in applications in which size is a relatively insignificant factor, and greater heating and cooling loads and capacities are needed, embodiments of the present system can be scaled to large-scale apparatuses limited only by available storage space and manufacturing constraints. Further, according to various embodiments, the individual heat and mass transfer system components are removed from the overall assembly 10 (i.e., the shims define a singular heat and mass transfer component instead of a plurality of components) to enable modularity in overall system design (described in greater detail below in conjunction with
The coupling fluid lines 120 in
As will be understood, as more shims (and shim pairs) are combined in the shim group 108, the corresponding number of microchannels increases, as does the resulting surface area for thermal contact associated with the microchannels in each heat and mass transfer system component within the apparatus 10 (described in greater detail below). Thus, for applications that require greater cooling and or heating outputs, greater numbers of (and/or larger) shim pairs are needed. For example, in its most basic implementation, a single shim pair comprising one shim A 102 and one shim B 104 may be adequate to form the shim group 108 to perform the necessary heat and mass transfer functions of a given application. In other embodiments, tens, hundreds, or more shim pairs may be used. As will be understood, the number of shims used and overall shim and apparatus size depends on the particular use and application of each particular system embodiment.
Still referring to
With reference to the schematic representation 500 shown in
Upon expansion in the expansion valve 512 to the low-side pressure of the system, the dilute solution exiting through line 514 enters the absorber component 1500, where it absorbs refrigerant (i.e., ammonia) vapor arriving from the recuperative refrigerant heat exchanger 1300 through line 516 (described in greater detail below). As shown, dashed lines (e.g., line 516) represent a vapor phase of the working fluid, whereas solid lines (e.g., line 510) represent a liquid phase. As referred to herein and as understood in the art, when describing ammonia-water working fluid, “ammonia” is generally synonymous with “refrigerant”, and “water” is generally synonymous with “absorbent” (although, as is understood, refrigerant may not comprise pure ammonia, as some relatively minimal or trace amounts of water may be present, and vice versa). Alternatively, when describing lithium bromide-water working fluid, “lithium bromide” is generally synonymous with “absorbent”, and “water” is again generally synonymous with “refrigerant”. These terms are understood in the art as applicable to any refrigerant-absorbent working fluid pair.
Still referring to
Returning to discussion of the desorber 1100, heat of desorption is conveyed to the desorber by a high-temperature heat transfer fluid line 522, which is in turn connected to the heat source 130 that drives the system (i.e., fluid coupling with heat source). Ammonia-water vapor leaving the desorber component 1100 (described previously) enters the rectifier component 1150, wherein a cooling fluid line 524 is employed to rectify the ammonia-water vapor to a higher concentration of ammonia. As shown, the rectifier 1150 and desorber 1100 are combined in a single component; however, as will be understood, these components may be separated according to various embodiments as desired. Depending on the particular embodiment, the cooling fluid employed in cooling fluid line 524 is a medium temperature hydronic fluid, or the concentrated solution exiting the solution pump 502, or some other fluid depending on the particular system design and operating conditions.
Reflux ammonia-water solution from the rectifier 1150 returns to the desorber 1100, where it is expunged through fluid line 508 (described previously). High concentration ammonia (i.e., refrigerant) vapor exiting the rectifier 1150 is conveyed to the condenser component 1200 via fluid line 526. In the condenser 1200, the concentrated ammonia vapor is condensed and subcooled to liquid refrigerant (i.e., ammonia) by medium temperature hydronic fluid line 528 that eventually rejects heat of condensation to the ambient (e.g., heat rejection 140). Liquid refrigerant leaving the condenser 1200 through fluid line 530 enters the previously-mentioned recuperative refrigerant heat exchanger 1300, where it is further cooled by vapor-phase refrigerant exiting the evaporator component 1400 (described below). The cooled liquid refrigerant exits the recuperative refrigerant heat exchanger 1300 through fluid line 532, which carries it to the refrigerant expansion valve 534. Upon expansion to the system low-side pressure, the resulting two-phase refrigerant mixture is conveyed to the evaporator component 1400 by fluid line 536.
In the evaporator component 1400, vaporization of the two-phase refrigerant mixture effects cooling of the low-temperature coupling fluid entering through line 538. Fluid line 538 is eventually connected (via hydronic coupling) to the conditioned space where the desired cooling (e.g., space-conditioning 150) is achieved. Vaporized refrigerant exits the evaporator 1400 through line 540 and flows to the previously-discussed recuperative refrigerant heat exchanger 1300, where it serves as coolant for the liquid (high pressure) refrigerant exiting the condenser 1200 and entering the recuperative refrigerant heat exchanger 1300 through line 530. The heated refrigerant vapor exits the recuperative refrigerant heat exchanger 1300 through line 516 and flows to the absorber component 1500 (as described previously) to complete the cycle.
As described previously, minor modifications to the system shown in
As also described previously,
Still referring to
As described previously, for some heat and mass transfer components, the features (e.g., microchannel arrangement, etc.) of shim A 102 as compared to shim B 104 vary within each individual heat and mass transfer component. According to one embodiment, these differences enable the desired fluid flows and heat transfer functions between the working fluid and coupling fluids exchanging heat therein (e.g., one shim type carries working fluids, whereas the other shim type carries coupling fluids). These differences are shown and described in greater detail below and in subsequent figures. Specifically, the heat and mass transfer components that include internal shim differences to achieve necessary heat transfer functions are the recuperative solution heat exchanger 800a, 800b, the condenser 1200a, 1200b, the recuperative refrigerant heat exchanger 1300a, 1300b, the evaporator 1400a, 1400b, and the absorber 1500a, 1500b.
Alternatively, the features within each of shims A and B for the desorber 1100 and rectifier 1150 are the same as compared to each other (e.g., the arrangement of microchannels and other microscale passages are similar). Based on the functions of these heat exchange components, arrangement of internal shim features, and the fluid flows within the shims, disparate arrangements of shim features are not necessary (in one embodiment) for these shim types. Accordingly, shims A 102 and B 104 are identical for the exemplary embodiment shown for the desorber 1100a, 1100b, and rectifier 1150a, 1150b portions of the shims.
Additionally, alignment notches 602 and 604 in each shim A 102 and B 104 provide holes in the shims to facilitate precise alignment, assembly, and joining of the plurality of shim pairs and cover plates 110, 111 as desired within each heat transfer apparatus. As shown, exemplary notches 602 and 604 have varying cross sections as compared to each other (i.e., notch 602 is circular-shaped, whereas notch 604 is square-shaped) to enable easy joining and alignment of the shims within the overall assembly (e.g., such that shims are not accidentally reversed during system assembly). As will be understood, depending on the particular embodiment, notches 602, 604 define virtually any cross-sectional shape, or, in some embodiments, are entirely unnecessary and thus not included.
Also shown in
As noted in the glossary, for example, even though an exemplary embodiment utilizes microchannels comprising hydraulic diameters of 306 μm, microchannel fluid flow and heat and mass transfer phenomena may be exploited in channels with hydraulic diameters ranging from 1 μm to about 1 mm (and greater). In fact, channels may exhibit fluid flow and heat transfer phenomena specific to microchannels at somewhat larger hydraulic diameters, even up to about 3 mm, depending upon the fluid properties and operating conditions, the corresponding vapor bubble formation phenomena and critical bubble diameters, and varying effects of surface tension, gravity, and inertial forces in these channels at different pressures and temperatures for different fluids and fluid mixtures.
Further, according to one embodiment, the microchannel sizes are the same (e.g., hydraulic diameter of 306 μm) throughout each heat and mass transfer system component in the system. In other embodiments, the microchannel dimensions vary on a per-component basis (e.g., microchannels in the absorber 1500 may comprise different dimensions than those in the condenser 1200). Additionally, in still other embodiments, microchannel dimensions may vary for shim A 102 as compared to shim B 104, even within the same heat and mass transfer component. As will be understood and appreciated, various microchannel dimensions are used according to various system embodiments as needed.
Additionally, according to various embodiments, the microchannels are formed via photochemical etching, stamping, cutting, or other machining techniques. Further, the cross-sectional shapes of the microchannels in the shims comprise, depending on the embodiment, square, rectangular, semi-circular, semi-elliptical, triangular, or other singly-connected cross-sections to enable fluid flow in single-phase or two-phase state, as necessary, wherein the microchannel cross-section shape and dimensions are determined based on heat and mass transfer requirements, operating pressures, structural strength of the assembled apparatus, manufacturing constraints for dimensional tolerances and bonding of the shims and cover plates, and other similar application-specific factors.
As described, embodiments of the present system generally comprise microscale heat or heat and mass transfer systems or heat-driven cycle apparatuses. More particularly, exemplary embodiments comprise monolithic, microscale absorption heating and/or cooling apparatuses including discrete but integrated heat and mass transfer system components, such as recuperative solution heat exchangers, desorbers, rectifiers, condensers, recuperative refrigerant heat exchangers, evaporators, absorbers, and other similar components. The particular architectures and functions of these discrete components and the operative connections between the components as represented by an exemplary embodiment (e.g., absorption heat pump) of the present system are described in greater detail below.
Recuperative Solution Heat Exchanger
The dilute ammonia-water solution enters the recuperative solution heat exchanger 800 via external tubing (not shown) from solution pump 502 (generally initiated from absorber 1500). In the exemplary embodiment shown in
Still referring to
As shown in
According to various embodiments of the present system, voids 802a, 802b, 812a, 812b, and other voids associated with the recuperative solution heat exchanger 800, as well as other heat and mass transfer system components of the apparatus described herein, comprise varying cross-sections as desired or necessitated by the particular embodiment. For example, voids 812 comprise a square cross-section, whereas voids 802 comprise a circular cross-section for the embodiment shown in
As briefly discussed previously,
Desorber/Rectifier
Referring to the embodiment of the desorber 1100 shown in
Slot voids 1110 in shims A and B near the inlet headers 1102 provide thermal isolation between the recuperative solution heat exchanger component 800 and the desorber heat source voids 1106, so that the external heat is maximally applied to the concentrated solution. According to other embodiments and aspects, similar voids are used in various locations throughout the present system to effect thermal separation between heat exchange components that should be maintained at hot and cold temperatures. The dilute ammonia-water solution and ammonia-water vapor mixture exiting the desorber passages 1104 collect in the desorber outlet headers 1108 formed by the stacked-up voids defined by the plurality of shim pairs A and B, and subsequently flow into the rectifier 1150.
Generally, the ammonia-water vapor from the desorber outlet headers 1108 flows into the rectifier vapor space 1122 formed by rectifier trays 1112 on shims A and B. As the vapor proceeds along the rectifier 1150, cooling by a coupling fluid flowing in counterflow orientation to the ammonia-water vapor through passages 1116 along the side walls of the vapor space chamber 1122 effects rectification of the vapor. Depending on the particular embodiment, this coupling fluid comprises medium temperature coupling fluid or concentrated ammonia-water solution exiting the solution pump 502 (described previously). The coupling fluid enters the assembly 10 at inlet headers formed by stacked-up voids 1118 in shims A and B, and exits from outlet headers formed by stacked-up voids 1120 in shims A and B. During the rectification process, reflux liquid (i.e., dilute ammonia-water solution) collects in trays 1112 and flows back into the desorber exit headers 1108 where it mixes with the dilute ammonia-water solution therein before exiting the desorber. According to one embodiment, the dilute ammonia-water solution exits the desorber exit headers 1108 via a hole in a cover plate (not shown). Rectified, high concentration ammonia-water vapor exits the rectifier vapor space 1122 through vapor outlet headers formed by stacked-up voids 1114 in shims A and B, and is subsequently transferred to the condenser 1200.
According to one embodiment, coupling fluid flowing between stacked-up voids 1118 and 1120 via passage 1116 in shims A and B is in forced-convective flow. On the other hand, as the ammonia-water vapor passes through the rectifier 1150 and is rectified, reflux liquid flows back down the rectifier and collects at exit header 1108. This counterflow of vapor and reflux liquid within the rectifier 1150 comprises a gravity/buoyancy-driven flow (unlike the forced-convective flow on the coupling fluid side) that further enhances the rectification of the vapor from the dilute ammonia-water solution. The varying geometries possible due to the shim, passage, and microchannel geometries incorporated into various embodiments of the present system enable the combination of co- and counterflow forced-convective and gravity/buoyancy-driven flows for different fluid streams, as desired, in the various heat and mass transfer system components, such as the rectifier 1150 and desorber 1100, in embodiments of the heat or heat and mass transfer system. As will be understood and appreciated, for this flow to occur, the overall system 10 should be oriented such that the rectifier 1150 is aligned vertically above the desorber 1100. Thus, for example, when in use, the embodiment of the system herein described should be oriented similarly to that shown in
Condenser
As shown, ammonia-water vapor from the rectifier 1150 enters the condenser component 1200 through inlet headers formed by the stacked-up voids 1210a, 1210b in shims A 102 and B 104, respectively. Void 1210a in shim A leads to a plurality of microchannels 702, which enable flow of condensing vapor in counterflow orientation to, and in thermal contact with, the coupling fluid flowing through similar microchannels 702 on shim B. The condensed and subcooled refrigerant liquid exits microscale channels 702 in shim A and flows into outlet headers formed by the stacked-up voids 1212a, 1212b in shims A and B, respectively. Variations, options, and other details associated with microchannel geometries, coupling fluid inlet and outlet passages 1204, 1206, and voids for the condenser 1200, including shapes, cross-sections, and dimensions, apply equally and are similar to those described previously in conjunction with the recuperative solution heat exchanger 800.
Recuperative Refrigerant Heat Exchanger
Low-pressure vapor from the evaporator 1400 enters the recuperative refrigerant heat exchanger 1300 through inlet headers formed by the stacked-up voids 1310a, 1310b in shims A 102 and B 104, respectively. Void 1310a in shim A leads to a plurality of microscale passages 702, which enable flow of low-pressure refrigerant vapor as coolant for, in counterflow orientation to, and in thermal contact with, high-pressure refrigerant liquid flowing through similar microscale passages 702 on shim B. The refrigerant vapor exits microscale passages 702 and flows into outlet headers formed by the stacked-up voids 1312a, 1312b in shims A and B, respectively. Variations, options, and other details associated with microchannel geometries, high-pressure refrigerant liquid inlet and outlet passages 1304, 1306, and voids for the recuperative refrigerant heat exchanger 1300, including shapes, cross-sections, and dimensions, apply equally and are similar to those described previously in conjunction with the recuperative solution heat exchanger 800.
Evaporator
As shown, ammonia-water two-phase mixture from fluid line 536 (see
Absorber
In the embodiment shown, dilute ammonia-water solution from fluid line 514 (see
According to one embodiment, ammonia-water vapor from fluid line 516 (see
According to the embodiment shown, microchannels 702 on shim A extend in length further toward inlet header 1510a than do microchannels 702 on shim B for purposes of enabling entry of incoming ammonia-water vapor into the microchannels on shim A through vapor inlet holes 1518. The mixed two-phase flow of dilute ammonia-water solution entering microchannels 702 on shim A through header 1510a and the ammonia-water vapor entering these same microchannels through inlet holes 1518, upon absorption, exits the microchannels as concentrated solution into outlet headers formed by voids 1512a, 1512b in shims A and B, respectively. As will be understood, the vapor enters microchannels 702 via inlet holes 1518 based on forced convective flow, which also prevents the dilute solution from flowing into inlet holes 1518. Variations, options, and other details associated with microchannel geometries, coupling fluid inlet and outletpassages 1504, 1506, 1516, and voids for the absorber 1500, including shapes, cross-sections, and dimensions, apply equally and are similar to those described previously in conjunction with the recuperative solution heat exchanger 800.
FIGS. 16A and 16B are enlarged perspective views of portions of shims A 102 and B 104, respectively, associated with an absorber 1500 according to an embodiment of the present apparatus 10. Specifically,
To avoid unnecessary cluttering, the fluid lines and connections to coupling fluids, etc., are not shown in
The discussion below relates to specifics for a particular, exemplary embodiment of the present system described herein. Specifically, described below are calculations, manufacturing processes, design details, dimensions, feature arrangements, exemplary working fluids and coupling fluids, and other similar details associated with the described, exemplary embodiment and methods of making the same. As will be understood and appreciated, the specific embodiment and application described below is but one embodiment of the present system, and is not intended to limit the scope of the present disclosure, or the invention(s) and systems described herein, in any way.
Specifically, the discussion below describes the design and fabrication of a miniaturized (i.e., microscale), monolithic absorption heat pump system utilizing microchannel heat and mass transfer system components. An exemplary embodiment of the present system was built according to the specifications, parameters, etc., outlined below, and the achieved performance results for the exemplary embodiment under specified parameters are provided herein.
Manufacturing Techniques
The manufacturing techniques used to build the exemplary apparatus allow multiple microchannel heat and mass transfer components or heat exchange components (i.e., heat exchangers) to be fabricated at the same time in a single, monolithic structure. For this exemplary embodiment, the microchannels 702 are first formed on stainless steel shims 102, 104 by a wet chemical etching process. The shims are then diffusion bonded together to form the overall apparatus 10. By placing shims with different microchannel configurations in an alternating pattern, the fluid streams of each heat and mass transfer component are allowed to come into close thermal contact. The steps according to one embodiment of the microchannel manufacturing process are outlined in greater detail below.
Photochemical Etching
A photosensitive material (photoresist) is then applied to both sides of the given shim 102, 104. The photo resist material used in the production of the exemplary apparatus is a dry film, negative resist. The portions of the resist exposed to UV light cure and protect the underlying steel during the etching process.
A mask containing the image of the required flow channels (i.e., microchannels) is created for both sides of each of the two shim designs (i.e., shims A 102 and B 104). The mask is a film with opaque sections representing the areas to be etched and transparent sections representing areas wherein the photo resist should remain to protect the base material from the etching chemicals. The masks are mounted to both sides of the shim and aligned to ensure features match up on both sides of the steel.
The arrangement of the steel, photoresist, and mask is then exposed to ultra violet light to cure the photoresist. The uncured photoresist is then removed in a developing process. The metal with the cured photoresist is then passed through the etching process, wherein a ferric chloride solution (i.e., acid solution) is used as the etchant. This acid solution removes the exposed metal and forms the microchannels and holes in the steel shim.
Once the shim is removed from the etching process, the remaining photoresist material is similarly removed. During the etching process, the shims remain connected to the process sheets by several tabs. Leaving the shims tabbed to the sheets ensures consistent etching. After the photoresist material is removed, the individual shims are removed from the process sheet. The photoresist application and the etching process are generally conducted in a clean room to reduce the risk of dust contamination which could cause manufacturing defects during the etching process.
Diffusion Bonding
In the exemplary embodiment described herein, the shims are joined using a diffusion bonding process. As will be understood, the shims may be combined via other bonding or combination processes according to various embodiments, as described previously. The diffusion bonding process begins with cleaning the shims 102, 104 and an inspection to ensure there are no burrs or foreign objects on the shim material. The shims are coated with a nickel plating in an electroless nickel plating procedure. The nickel coating is applied to aid in creating a hermetic seal during the diffusion bonding process.
The shims 102, 104 and cover plates 110, 111 are then arranged in the correct order (e.g., alternating shims A and B) and proper alignment of the shims is carefully monitored (e.g., via alignment notches 602, 604 described previously). Two pins are inserted into alignment notches in the front plate 110, end plate 111, and shim group 108, respectively. In this particular embodiment, all shims and the back end cover plate 111 each have at least one alignment notch. This alignment scheme enables the steel shims to lie flat, even if there are minor inconsistencies in the position of the alignment notches. It also allows the steel to expand and contract due to thermal expansion during the bonding process without causing buckling or delimitation while achieving alignment tolerances of ±0.05 mm.
The assembled system 10 is then placed in a hot press vacuum furnace 1900, illustrated and represented by
During the bonding process, the surface asperities on contacting surfaces begin to deform plastically. The deformation continues until the pores between the surfaces have been eliminated. The atoms from adjacent surfaces can then diffuse across the interface, allowing the grain boundaries to reorganize in the interface region. This process forms a bond with a strength approaching the yield strength of the bulk material.
Cycle Design Calculations
A thermodynamic model for the exemplary system was developed by choosing representative design conditions for the operation of a single-effect absorption cycle in the cooling mode. Throughout this section and for ease of reference, previously-presented reference numerals are used to identify various system components. Particularly, reference is made to
These specified representative external conditions for heat source and sink, combined with allowances for temperature differences between the external conditions and the working fluid that yield reasonable component surface area requirements, result in high and low side operating pressures of approximately l 1600 and 400 kPa, respectively. Thus, the high-side pressure is established by the choice of a driving temperature difference between the condensed refrigerant (i.e., ammonia) and an ambient sink in the condenser 1200. Similar consideration of the driving temperature difference at the desorber component 1100 and the already established high-side pressure yields a dilute solution outlet temperature and concentration, i.e., fraction of ammonia in an ammonia-water solution. Using the corresponding concentrated solution inlet temperature at the desorber 1100, the high-side pressure, and the equilibrium properties of ammonia-water mixtures, a solution inlet enthalpy is obtained. Coupled with the dilute solution outlet enthalpy at the desorber 1100, the heat input is related to the concentrated solution flow rate through an energy balance between the heat source and the working fluid. For the representative design point calculation, the resulting concentrated solution mass flow rate and ammonia mass fraction are 2.7×10−3 kg/s and 0.37, respectively. The energy balance calculation and the equilibrium relationships also yield the vapor quality (ratio of ammonia-water vapor mixture to total ammonia-water two-phase flow rate) and concentration at the desorber 1100 outlet. A summary of key operating conditions for this representative, exemplary cycle is provided in Table 1.
A low vapor concentration exiting the desorber 1100 would cause severe temperature glide penalties in the evaporator 1400, resulting in rising refrigerant temperatures that would unduly restrict cooling. To ensure an adequately pure ammonia refrigerant stream, the vapor stream should be cooled in the rectifier 1150 to strip off extra water vapor. Accordingly, the design outlet temperature of the saturated vapor stream leaving the rectifier is set to provide a minimum ammonia concentration of 98% for this exemplary embodiment.
Generally, the reflux liquid that is condensed out of the refrigerant stream in the rectifier 1150 flows back into the separation chamber where it mixes with the dilute solution before exiting the desorber 1100. An energy balance on the rectifier vapor inlet and outlet and liquid streams yields the rectifier cooling load.
In this embodiment, for the design calculations to provide the required cooling, the concentrated solution stream leaving the solution pump 502 (see
The refrigerant vapor leaving the rectifier 1150 flows to the condenser 1200. With an assumption of a subcooled liquid refrigerant outlet from the condenser, the refrigerant concentration and the high-side pressure, and the condenser refrigerant outlet temperature are established. The condenser heat load is also calculated using an energy balance. In addition, this condenser heat load is used in combination with a set coolant (i.e., coupling fluid) flow rate to determine the coolant outlet temperature from the condenser (i.e., via medium temperature fluid line 528). After leaving the condenser 1200, the refrigerant flows through the recuperative refrigerant heat exchanger 1300 where it is further cooled by the refrigerant leaving the evaporator.
The expansion of the refrigerant exiting the recuperative refrigerant heat exchanger 1300 through the expansion valve 534 (see
The calculated cooling load, in conjunction with a set chilled water (i.e., coupling fluid) flowrate and inlet temperature, yields the chilled water outlet temperature (i.e., to desired cooling 150) from an energy balance. A similar energy balance is conducted on the recuperative refrigerant heat exchanger 1300 to obtain the low pressure vapor state at the outlet of this heat exchanger, which is also the absorber refrigerant inlet condition for this exemplary embodiment.
Returning to the solution circuit, according to the described exemplary embodiment, the concentrated solution inlet to the recuperative solution heat exchanger 800 is determined by the outlet of the rectifier 1150. An energy balance on the recuperative solution heat exchanger yields the dilute solution and concentrated solution outlet conditions from this heat exchanger, as well as its heat load. The solution expansion valve 534 is assumed to be isenthalpic. Regarding the absorber 1500, with the inlet conditions to the absorber fixed (as described above), and with an assumed solution subcooling at the absorber outlet, the absorber heat load is calculated from an energy balance.
Once the state points are fixed (as described above) using mass, species and enthalpy balances for each exemplary heat and mass transfer system component, the heat transfer rates of each component necessary to yield the desired cooling load are also fixed. Subsequent calculations are conducted to obtain the required heat and mass transfer component surface area requirements based on these desired heat loads and the relevant heat and mass transfer models and correlations. Varying levels of detail may be incorporated into such component design calculations, but for the fabrication of this particular exemplary embodiment, the component heat and mass transfer calculations are conducted by treating each component as one single, integrated component, with the fluid properties averaged over the component. Thus, the heat and mass transfer component sizes are obtained based on the component heat loads, the coupled heat and mass transfer resistances, and the driving log-mean temperature differences. This technique is valid for heat exchangers where the heat capacity rates of the two fluid streams (e.g., the working fluid and coupling fluid) are constant along the length of the heat exchanger. For ammonia-water systems, in some components, the thermal capacities vary along the length, but this technique may be applied to obtain reasonable estimates of component sizes, with proper accounting of the driving temperature differences. The heat and mass transfer component geometries are generally determined based on these heat and mass transfer calculations to satisfy the required heat loads, as well as on dimensional requirements based on manufacturing techniques discussed herein.
Component Design Calculations
As described above, according to the described exemplary embodiment, the required size (overall heat transfer conductance, UA) of each heat transfer component is determined from the cycle model. The specific fluid channel configuration (i.e., microchannels, voids, and distribution passages) of each heat and mass transfer component is determined by estimating the overall heat transfer resistance of each individual component.
As shown in
According to the described embodiment and as shown in
According to the described, exemplary embodiment, each heat and mass transfer component in the system is modeled by computing the thermal resistance presented by each fluid flowing through the respective microchannels in shims A 102 and B 104, in addition to the conductive thermal resistance presented by the intervening metal wall between the shims (as illustrated in
Based on the heat exchange component heat and mass transfer design approach outlined above, the microchannel width, microchannel length, and number of microchannels for each component are determined to satisfy the design heat loads at the pertinent operating conditions computed from thermodynamic cycle analyses. As will be understood, however, the approach outline above is but one approach to determine appropriate microchannel dimensions, and other approaches are used in other embodiments as will occur to one of skill in the art.
In the described, exemplary embodiment, the flows of all coupling fluids are in single-phase laminar liquid flow. Similarly, working fluid flows in the recuperative solution heat exchanger 800 and the recuperative refrigerant heat exchanger 1300 are in single-phase laminar flow. According to the described embodiment, the heat transfer coefficient and friction factor for such single-phase flows in the exemplary microchannel shape (shown in
According to one embodiment, for vapor-to-liquid phase change processes such as condensation, correlations described in Shah, M. M., A General Correlation for Heat Transfer During Film Condensation Inside Pipes, International Journal of Heat and Mass Transfer, Vol. 22(4), pp. 547-556 (1979), and Kandlikar, S., Garimella, S., Li, D., Colin, S. and King, M. R., Heat Transfer and Fluid Flow in Minichannels and Microchannels, Elsevier Science (2005), are used, as applicable, for each particular phase change process. Other guidance for addressing single-component and multi-component phase-change heat and mass transfer in condensers, absorbers, evaporators, desorbers, and rectifiers is taken from the models, correlations, and techniques outlined in Carey, V. P., Liquid-Vapor Phase-Change Phenomena: An Introduction to the Thermophysics of Vaporization and Condensation Processes in Heat Transfer Equipment, Washington, D.C., Taylor & Francis Series, Hemisphere Pub. Corp. (1992), and Hewitt, G. F., Shires, G. L. and Bott,
T. R., Process Heat Transfer, Boca Raton, CRC Press, Begell House (1994). Two-phase pressure drops are estimated using the two-phase pressure drop multiplier approach of Mishima et al., Some Characteristics of Air-Water Two-Phase Flow in Small Diameter Vertical Tubes, International Journal of Multiphase Flow, Vol. 22(4), pp. 703-712 (1996). Nonlinear variations of vapor quality with heat exchanger length, even in these integrated analyses, are accounted for by evaluating these correlations at the integrated average properties along the component length. Conservative estimates of two-phase pressure drops are obtained by computing the greatest pressure gradient in the heat and mass transfer component as a function of vapor quality or component length and applying that to the total length of the heat and mass transfer component.
Generally, evaporation heat transfer coefficients are calculated using the correlation from Kandlikar et al., Predicting Heat Transfer During Flow Boiling in Minichannels and Microchannels, Chicago, IL, Soc. Heating, Ref. Air-Conditioning Eng. Inc., pp. 667-676 (2003) and Kandlikar et al., An Extension of the Flow Boiling Correlation to Transition, Laminar, and Deep Laminar Flows and Microchannels, Heat Transfer Engineering, Vol. 25(3), pp. 86-93 (2004). The mean heat transfer coefficient for the evaporating stream is calculated at a representative integrated average vapor quality along the evaporator length to account for the non-linear variation of vapor quality with evaporator length.
According to the described exemplary embodiment, desorption (vapor generation from the concentrated solution) is achieved in the desorber 1100 using eight 150 W electrical cartridge heaters for a maximum heat input of 1200 W. Heat fluxes supplied by the heaters at a design desorber heat input rate of 800 W were found to be well below critical heat flux limitations estimated using the correlation for parallel mini/microchannels from Qu et al.,Measurement and Correlation of Critical Heat Flux in Two-Phase Micro-Channel Heat Sinks, International Journal of Heat and Mass Transfer, Vol. 47(10-11), pp. 2045-2059 (2004).
For the exemplary rectifier 1150 design, the liquid reflux and the vapor stream are assumed to be in thermal equilibrium so that the temperature of the reflux leaving the rectifier is equal to the temperature of the vapor entering the rectifier. In order to facilitate the approach to this equilibrium, four trays are included in the exemplary rectifier to hold the liquid reflux and allow heat and mass transfer interaction with the counterflow vapor. The heat transfer coefficient on the refrigerant side is estimated using the laminar film condensation correlation from Sadasivan et al., Sensible Heat Correction in Laminar Film Boiling and Condensation, Journal of Heat Transfer, Transactions ASME, Vol. 109(2), pp. 545-547 (1987). Only the area of the single wall in thermal contact with the concentrated solution is used for heat transfer estimation in the rectifier for the particular exemplary embodiment of the present system. The heat and mass transfer area associated with the trays is not included in this calculation in order to produce a more conservative result. The additional area of the trays further enhances the performance of this heat and mass transfer component.
Further details of the correlations mentioned above in reference to various articles and texts can be found in the cited literature. Representative dimensions of the exemplary heat and mass transfer system components resulting from the calculations described above are presented in Table 2. As will be understood and appreciated, the dimensions and geometric details shown in Table 2 are presented for illustrative purposes only, and pertain to the specific, described, exemplary embodiment of a single-effect absorption cycle, and are in no way intended to limit or exclude other combinations of system geometries, arrangements of various heat exchange components or heat and mass transfer components, channel hydraulic diameters, numbers of channels, shim thicknesses, numbers of shims, etc., used in other embodiments of the present system.
indicates data missing or illegible when filed
Packaging and Bonding Considerations
As described previously, preferred embodiments of the present system comprise microscale, monolithic heat or heat and mass transfer systems. Because it is often desirable, depending on the particular embodiment, to include more than one heat and mass transfer system component within an integrated, monolithic structure, system embodiments should account for extraneous heat transferred between internal heat and mass transfer system components. To account for this extraneous heat transfer, certain factors are taken into account in various embodiments, such as overall size of the microscale heat or heat and mass transfer system, spacing between heat and mass transfer components within the system, arrangement and type of fluid connections between various system components, and other similar factors as will occur to one of ordinary skill in the art.
As will be understood and as described previously, the exemplary embodiment described herein, and its associated operating parameters, temperature ranges, etc., are provided for illustrative purposes only, and are not intended to limit the scope of the present systems or apparatuses in any way. Generally, various operating temperature and pressure ranges are envisioned depending on the application under consideration. Thus, for example, when applied for waste heat recovery from high temperature combustion processes such as automotive exhaust, the heat source temperature could range from 300° C. to 900° C., while for low temperature waste heat recovery, the source temperature could be as low as 40° C. Similarly, for chiller applications, the cooled fluid temperature is typically about 5-15° C., whereas for refrigeration applications, the temperature could be well below 0° C. For heat rejection temperatures in air conditioning applications, ambient temperatures in 20-55° C. are contemplated. However, as will be understood, the specific values of these individual external temperatures are less important than the relationship between the heat source, heat sink, and the cooling temperature. Because a thermally-activated heat pump is generally known as a three temperature (i.e., low temperature cooling, medium temperature heat rejection, and high temperature input heat source) system, the temperatures for which the subject heat and mass transfer system may be applied should provide at least a minimal lift, i.e., temperature difference, between the low and medium temperatures to effect the desired output, and the medium and high temperatures to provide the driving force necessary to effect the desired output.
As described previously, one exemplary embodiment of the present system comprises a microscale, monolithic absorption cooling and/or heating system. The following section provides a comparison of the described, exemplary embodiment of the present system in the form of such an absorption cooling and/or heating system to a traditional vapor-compression system used for residential cooling. This section also provides representative parameters associated with the exemplary embodiment described herein. As will be understood and appreciated, the following discussion is provided for illustrative purposes only, and is in no way intended to limit the scope of the present disclosure, or the invention(s) and systems described herein.
The exemplary embodiment of the present system described above (i.e., a microscale, monolithic absorption heat pump) was manufactured and tested under realistic ambient, chilled fluid, and heat source conditions on a breadboard test facility.
During testing of the exemplary embodiment, nominal 300 W of cooling were delivered for an 800 W heat input at representative ambient and chilled fluid conditions. Furthermore, the exemplary system was demonstrated to operate in cooling mode over a wide range of ambient temperatures (i.e., 20-35° C.) and at different heat input rates (i.e., 500-800 W). A nominal coefficient of performance (COP) of 0.375 was achieved in a system volume of 200×200×34 mm3, representing a volumetric cooling capacity of 221 kW/m3. With a system mass of 7 kg, the corresponding specific cooling capacity is 0.043 kW/kg. The exemplary embodiment was purposely designed with all heat and mass transfer system component fluid inlets and outlets external (i.e., outside) to the apparatus, with wide spacing to enable installation of temperature and pressure instrumentation at the inlet and/or outlet of each component. An alternate embodiment, with internal flow passages and elimination of extra space for instrumentation, comprises projected dimensions of 120×120×25 mm3, with a mass of 3 kg. The corresponding volumetric cooling capacity of such an embodiment comprises 833 kW/m3, while the specific cooling capacity is 0.10 kW/kg.
By comparison, conventional 10.55 kW cooling capacity residential electric-vapor compression systems are on the order of 0.91×0.91×0.91 m3 and weigh about 225 kg, representing volumetric cooling capacities on the order of only 13.8 kW/m3and specific cooling capacities of 0.047 kW/kg. Therefore, on both volumetric and mass bases, the subject exemplary system embodiment represents a substantial reduction in size of cooling systems, while providing a similar cooling capacity to those of much larger, conventional vapor-compression systems.
When compared to conventional absorption cooling systems (as opposed to vapor-compression systems), the benefits of the exemplary system embodiment described above become even more evident. As described previously, absorption cooling systems are generally much larger than vapor-compression systems due to the additional heat and mass transfer components needed in absorption systems. Thus, as will be understood, comparison of the exemplary, described embodiment to a conventional 10.55 kW cooling capacity absorption cooling system indicates advanced volumetric or specific cooling capacities of the exemplary embodiment (due to the larger size and weight of conventional absorption cooling systems exhibiting an unchanged cooling capacity, as compared to the vapor-compression system discussed above). Additionally, because many fluid connections between heat and mass transfer system components are included within a monolithic, microscale structure of a system embodiment, leak reduction is enhanced, and required fluid inventory is substantially lower than that of conventional systems.
The foregoing description of the exemplary embodiments has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the inventions to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to explain the principles of the inventions and their practical application so as to enable others skilled in the art to utilize the inventions and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present inventions pertain without departing from their spirit and scope. Accordingly, the scope of the present inventions is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.
This application claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 61/085,192, filed Jul. 31, 2008, and entitled “Thermally Activated Cooling System”, which is incorporated by reference as if set forth herein in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2009/052362 | 7/31/2009 | WO | 00 | 2/8/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61085192 | Jul 2008 | US |
