Patent Publication Number: US-2022235978-A1

Title: Evaporator assemblies and heat pump systems including the same

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to evaporator assemblies and, in particular, to air inlets for evaporator assemblies. 
     BACKGROUND 
     Decreasing the energy consumption of water heaters can have a large impact on the energy usage of an overall household or other building. Some studies have found the water heater to be the second-most energy consuming appliance in a typical household, trailing only the heating and air conditioning system in the home. Particularly in heat pump water heater systems, increasing the heat transfer coefficient of the heat exchangers is desirable because increased efficiency of the heat pump will lead to increased efficiency of the water heater overall. When ambient air enters the heat pump to exchange heat with a thermal working fluid, a large portion of the heat transfer efficiency can be lost due to uneven distribution of air. Air recirculation and general turbulent flow can reduce the contact area of the heat exchanger that is available for heat transfer, thus reducing the heat transfer coefficient and efficiency of the system. 
     What is needed, therefore, are heat pump units that improve the flow of ambient air entering the heat pump to improve the heat transfer coefficient of the heat pump. The present disclosure addresses this need as well as other needs that will become apparent upon reading the description below in conjunction with the drawings. 
     BRIEF SUMMARY 
     The present disclosure relates generally to evaporator assemblies and, in particular, to air inlets for evaporator assemblies. 
     The disclosed technology can include an evaporator assembly comprising a housing defining and interior chamber, an air inlet, an air outlet, and an evaporator unit within the interior chamber. The air inlet can have a substantially semi-circular cross section through which air enters the interior chamber. Air entering the interior chamber can transfer heat with the evaporator unit before flowing out of the air outlet. 
     The straight edge of the semi-circular air inlet can have a length from approximately 10 in to approximately 15 in. The air inlet can also be included in a top pan which defines a top side of the interior chamber. The top pan can be configured to engage a top end of the evaporator assembly. The air inlet can also include a grille. 
     The air outlet can be positioned on a side of the evaporator assembly. The air outlet can be configured such that an air flow path extends between the air inlet and the air outlet. The evaporator unit can be positioned in the air flow path, thereby creating a cross flow across the evaporator unit. The velocity magnitude of air flowing from the air inlet to the air outlet, particularly the air in contact with the evaporator unit, can deviate less than approximately 0.1 m/s across the exposed surface area of the evaporator. 
     Also disclosed herein are heat pump systems comprising the same. The heat pump systems can also comprise a condenser unit, a compressor, and a thermal expansion valve, all of which can form a fluid circuit. The fluid circuit can flow a heat transfer fluid therethrough. 
     The disclosed technology can also include a method of modeling an evaporator assembly. The method can comprise calculating a pressure drop across the evaporator assembly, modelling the evaporator assembly as a solid block comprising a porous medium, simulating a simulated air flow beginning at an air inlet and interacting with the solid block, and calculating a heat transfer coefficient for the evaporator assembly based at least partially on the simulated air flow. The porous medium can have characteristics such that the solid block creates a pressure drop corresponding to the pressure drop of the evaporator assembly. 
     The method can also maximize the heat transfer coefficient by modifying (or, depending on project constraints, restricting modification of) one or more of: an air flow rate, an air temperature, a size of the air inlet, a location of the air inlet, an orientation of the air inlet, a size of the air outlet, a location of the air outlet, an orientation of the air outlet, a porosity of the porous medium, and the volume of the porous medium. 
     These and other aspects of the present disclosure are described in the Detailed Description below and the accompanying figures. Other aspects and features of examples of the present disclosure will become apparent to those of ordinary skill in the art upon reviewing the following description of specific examples of the present disclosure in concert with the figures. While features of the present disclosure may be discussed relative to certain examples and figures, all examples of the present disclosure can include one or more of the features discussed herein. Further, while one or more examples may be discussed as having certain advantageous features, one or more of such features may also be used with the various examples of the disclosure discussed herein. In similar fashion, while examples may be discussed below as device, system, or method examples, it is to be understood that such examples can be implemented in various devices, systems, and methods of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate multiple examples of the presently disclosed subject matter and serve to explain the principles of the presently disclosed subject matter. The drawings are not intended to limit the scope of the presently disclosed subject matter in any manner. 
         FIG. 1  illustrates a front cross-sectional view of an evaporator assembly in accordance with the present disclosure. 
         FIG. 2  illustrates a top-down cross-sectional view of an evaporator assembly in accordance with the present disclosure. 
         FIG. 3  illustrates a top-down cross-sectional view of another evaporator assembly in accordance with the present disclosure. 
         FIG. 4A  illustrates a front cross-sectional view of an air flow profile of an evaporator assembly in accordance with the present disclosure. 
         FIG. 4B  illustrates a side cross-sectional view of an air flow profile of an evaporator assembly in accordance with the present disclosure. 
         FIG. 4C  illustrates an isometric cross-sectional view of an air flow profile of an evaporator assembly in accordance with the present disclosure. 
         FIG. 5A  illustrates a front cross-sectional view of an air flow profile of another evaporator assembly in accordance with the present disclosure. 
         FIG. 5B  illustrates a side cross-sectional view of an air flow profile of another evaporator assembly in accordance with the present disclosure. 
         FIG. 5C  illustrates an isometric cross-sectional view of an air flow profile of another evaporator assembly in accordance with the present disclosure. 
         FIG. 6  illustrates a system diagram of an example heat pump system in accordance with the present disclosure. 
         FIG. 7  illustrates a flowchart of a method of modelling an evaporator assembly in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, a problem with current water heaters is that ambient air entering heat pump units, such as in the evaporator unit, is not evenly distributed across the heat exchanger. The fluid dynamics of current air inlets tend to cause turbulent flow, air recirculation, vortices, and other disruptive flow patterns. As a result, the amount of air contacting the heat pump working fluid is typically uneven, ineffective, or both. This can reduce the heat transfer coefficient of the heat exchanger and the overall efficiency of the heat pump, causing the system to waste additional time and energy to provide the necessary heat transfer. 
     Disclosed herein are heat pump units and evaporator assemblies for water heaters that can provide improved air flow, heat transfer, and overall efficiency. Such units can have semi-circular air inlets, which can guide air such that it flows freely and smoothly into contact with a heat transfer unit, such as an evaporator. Such air inlets can improve the smoothness of air flow and more evenly distribute air flow in contact with the evaporator. Not only can the even air flow improve the heat transfer coefficient of the evaporator, but the even air flow can do so while using less air. With the improved air inlet, the heat pump unit can provide the same or similar amount of heat transfer while intaking air at a lower volumetric flow rate as compared to traditional systems, thus reducing the energy consumption of the unit while also improving the efficiency of the unit. 
     While the present disclosure is described relating to heat pump units for water heaters and evaporators for heat pump units, it is understood that the technology described herein is not so limited. Indeed, unless otherwise explicitly stated, the present disclosure can be used in conjunction with any heat transfer unit configured to transfer latent heat (e.g., an evaporator or a condenser), sensible heat (a heat exchanger, a heater, or a chiller), or both from air to another working fluid. Additionally, unless otherwise explicitly stated, the present disclosure is not limited to use in water heating applications and can be used in heat pumps for any application. 
     Although certain examples of the disclosure are explained in detail, it is to be understood that other examples and applications are contemplated. Accordingly, it is not intended that the disclosure is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. Other examples of the disclosure are capable of being practiced or carried out in various ways. Also, in describing the disclosed technology, specific terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Herein, the use of terms such as “having,” “has,” “including,” or “includes” are open-ended and are intended to have the same meaning as terms such as “comprising” or “comprises” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” are intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such. 
     By “comprising” or “containing” or “including” is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named. 
     It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. 
     The components described hereinafter as making up various elements of the disclosure are intended to be illustrative and not restrictive. Many suitable components that would perform the same or similar functions as the components described herein are intended to be embraced within the scope of the disclosure. Such other components not described herein can include, but are not limited to, for example, similar components that are developed after development of the presently disclosed subject matter. 
     As used herein, the term “deviates approximately,” “deviates by,” and variations thereof are intended to refer to the absolute value of a difference between a given value and a deviation. In other words, a given value X deviating by approximately Y can be rewritten as X±Y. 
     Reference will now be made in detail to examples of the disclosed technology, some of which are illustrated in the accompanying drawings. Wherever convenient, the same references numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  illustrates a cross-sectional component diagram of an evaporator assembly  100  for a heat pump unit. As shown, the evaporator assembly  100  can comprise a housing  110 . The housing  110  can include a top pan of a water heater. The housing  110  can be of various sizes and can define an interior chamber  120  inside of which certain components of the evaporator assembly  100  (or the heat pump unit) can be housed. One such component housed within the interior chamber  120  can include an evaporator unit  130 . The evaporator unit  130  can be or include a heat exchanger configured to conduct a heat exchange between air in the interior chamber  120  and a working fluid flowing through the evaporator unit  130 . The heat exchanged by the evaporator unit  130  can be latent heat (e.g., heat to change the phase of working fluid from liquid to vapor), sensible heat (e.g., heat to change the temperature of the working fluid), or a combination thereof. 
     The evaporator assembly  100  can have an air inlet  140  which can be an aperture in the housing  110  allowing air to flow from the external environment into the interior chamber  120 . The evaporator assembly  100  can also include an air outlet  150  which can be another aperture in the housing  110  allowing air to flow out of the interior chamber  120 . The air outlet  150  can lead the air back out to the external environment or into other chambers and components of a water heater. 
     The air inlet  140  can be positioned on a top side of the evaporator assembly  100 , as shown. Such a top side can be referred to as a “top pan” that engages the evaporator assembly  100 . The top pan can also define the top side of the interior chamber  120  if the top side is not already defined by the housing  110 . The air outlet  150  can be positioned on a side of the evaporator assembly  100 , as shown. 
     The air inlet  140  and the air outlet  150  can form an air flow path  160  along which air entering the evaporator assembly  100  flows from the air inlet  140  to the air outlet  150 . The evaporator unit  130  can be positioned within the air flow path  160  to ensure that flowing air contacts the evaporator unit  130  to transfer heat. Increasing the average velocity along the air flow path  160 , and therefore across the heat exchanger, can increase the Reynolds number of the air in contact with the evaporator unit  130 . Without wishing to be bound by any particular scientific theory, increasing the Reynolds number of the air in contact with the evaporator unit  130  can increase the heat transfer coefficient of the evaporator unit  130 . 
     Alternatively, if the air along the air flow path  160  is disrupted or uneven, the Reynolds number will decrease, thus decreasing the heat transfer coefficient of the evaporator unit  130 . While uneven flow may result in higher local air velocities in certain locations along the evaporator unit  130 , due to turbulence and air recirculation, others locations along the evaporator unit  130  can receive very little air flow and/or air flow having a low local air velocity, resulting in the total average air velocity along the evaporator unit  130  being lower than the higher local air velocities. Thus, there is an opportunity for improvement in the heat transferability of evaporator units in heat pumps. It is desirable to improve the air velocity distribution to thereby increase the Reynolds number of the air contacting the evaporator unit, as shown in Equation 1: 
     
       
         
           
             
               
                 
                   
                     R 
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                   = 
                   
                     
                       
                         ρ 
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                         u 
                         ⁢ 
                         L 
                       
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                         u 
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                         L 
                       
                       ν 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where Re is the Reynolds number, ρ is the fluid density, u is the fluid flow speed, L is the characteristic length, μ is the dynamic viscosity of the fluid, and ν is the kinematic viscosity of the fluid. 
     In the case of air flowing along the air flow path  160  and contacting the evaporator unit  130 , the average heat transfer coefficient for the evaporator unit can be calculated using Equation 2 for laminar flow and Equation 3 for turbulent flow. Laminar flow can be obtained with a Reynolds number at or below 2000, and turbulent flow can be obtained with a Reynolds number at or above 13000. 
     
       
         
           
             
               
                 
                   
                     
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     As used in Equations 2 and 3,  h   L-x     0    represents the average heat exchange coefficient over the characteristic length of the heat exchanger, L is the characteristic length, x 0  is the start of the characteristic length, Pr is the Prandtl number, and k is the thermal conductivity of the fluid, in this case air. 
     As shown, the relationship between the average heat transfer coefficient,  h   L-x     0   , and the Reynolds number, Re, is proportional. Furthermore, it can be seen that, for laminar flow (Equation 2), the Reynolds number has a greater effect over the average heat transfer coefficient compared to the effect of the Reynolds number under turbulent flow (Equation 3). Consequently, because the average heat transfer coefficient also has a proportional relationship with the rate of heat transfer ({dot over (Q)}) as shown in Equation 4, it follows that increasing the Reynolds number of the air flow path  160  can also increase the rate of heat transfer of the evaporator unit  130 . 
         {dot over (Q)}=hAΔT   (4)
 
     As shown, {dot over (Q)} represents the heat transfer rate, h represents the average heat transfer coefficient, and ΔT represents the temperature difference of the air between the air inlet  140  and the air outlet  150 . Additionally, as illustrated by Equation 4, the rate of heat transfer of the evaporator unit  130  can also be increased by increasing the heat transfer area (A). The heat transfer area can decrease if the air flow path  160  comprises flow disruptions, such as recirculation or vortices. 
       FIG. 2  illustrates a top-down cross-sectional view of the evaporator assembly  100  showing the air inlet  140 . Compared to the designs having a circular air inlet  140  such as the one shown in  FIG. 3 , the presently disclosed air inlet  140  design shown in  FIG. 2  has a semi-circular profile. The air inlet  140  can have a straight edge  210  and a curved edge  220 . Although the air inlet  140  is described herein as being semi-circular, it is not required that the radius of the curved edge  220  be such that the curbed edge forms half of a perfect circle. Rather, the curved edge  220  can be any length or radii that intersects with the straight edge  210  at two points. 
     It is to be understood that the air inlet  140  described herein can have shapes other than those shown and expressly described with respect to  FIG. 2 . For instance, the air inlet  140  can be trapezoidal, pentagonal, triangular, or have any number of sides that need not be equidistant. Furthermore, the particular air inlet  140  described in  FIG. 2  can be modified. For instance, the curved edge  220  need not be a continuously smooth curve. Rather, the curved edge  220  can comprise a plurality of straight-line segments interconnected to form an overall arc. As would be appreciated, increasing the number of straight-line segments in the curved edge  220  can increase the smoothness of the curved edge. The curved edge  220  can also be modified as desired to alter and/or finely tune air flow. For instance, the curved edge  220  can include a variety of scallops, fins, waves, and the like. Likewise, the straight edge  210  need not necessarily be precisely straight, although it can. As alternatives, the straight edge  210  can have a curve (e.g., with less arc than the curved edge  220 ) and/or can have multiple segments (e.g., multiple straight segments. 
     The straight edge  210  can have a length from 5 in to 20 in (e.g., from 6 in to 19 in, from 7 in to 18, from 8 in to 17 in, from 9 in to 16 in, or from 10 in to 15 in). The curved edge  220  can have any suitable length to intersect the straight edge  210  at both ends of the straight edge  210 . 
     The air inlet  140  can also include a grille, mesh, or other such protective cover to keep debris out of the air inlet  140  while still allowing for air flow through the air inlet. 
     The orientation of the air inlet  140  in  FIG. 2  is also not intended to be limiting. In fact, the air inlet  140  can be oriented in a number of ways. For example, the straight edge  210  and the curved edge  220  can be flipped opposite to the orientation shown in  FIG. 2 . Alternatively, the air inlet  140  can be rotated at any angled as desired. The position of the air inlet can also be altered. For instance, the air inlet  140  and the air outlet  150  can be switched (e.g., the air inlet  140  is on a side of the evaporator assembly  100  and the air outlet  150  is on a top surface). The air inlet  140  can also be positioned on any side surface of the evaporator assembly  100  so long as the air outlet  150  is positioned on an opposite side of the evaporator unit  130 . 
     As shown, the semi-circular air inlet  140  can provide air into the interior chamber  120  and through the air flow path  160  that has a more even distribution with fewer instances of recirculation and/or vortices. In such a manner, the air inlet  140  can provide an evenly distributed air profile in the interior chamber  120  that can increase the average velocity of air in contact with the evaporator unit  130  and increase the Reynolds number of the air flow path  160 . These increases can thereby increase the heat transfer coefficient of the evaporator unit  130  and the overall efficiency of the evaporator assembly  100 . 
       FIGS. 4A and 4B  show the air flow profiles from a front cross-sectional view and a side cross-sectional view of the evaporator assembly  100 , respectively. Additionally,  FIG. 4C  illustrates the same air flow profile from an isometric cross-sectional view. As shown, the velocity magnitude surrounding the evaporator unit  130  remains substantially consistent throughout the interior chamber  120 . 
     As shown, the velocity magnitude in the interior chamber  120  can differ by a value of 1 m/s or less (e.g., 0.9 m/s or less, 0.8 m/s or less, 0.7 m/s or less, 0.6 m/s or less, 0.5 m/s or less, 0.4 m/s or less, 0.3 m/s or less, 0.2 m/s or less, or 0.1 m/s or less). Across the surface of the evaporator unit, the velocity magnitude of air in the interior chamber  120  can differ by a value of 0.5 m/s or less (e.g., 0.4 m/s or less, 0.3 m/s or less, 0.2 m/s or less, or 0.1 m/s or less). That is to say, if the average air velocity in the air flow path  160  is, for example, 0.9 m/s, then the velocity magnitude of air at any given point in contact with the evaporator unit  130  can be from 0.4 m/s to 1.4 m/s. In such a manner, the air inlet  140  can achieve uniform and evenly distributed air, thereby increasing the heat transfer coefficient of the evaporator unit  130 . 
     In contrast, the air flow profile for a standard circular air inlet is shown in  FIGS. 5A-C .  FIGS. 5A and 5B  illustrate front and side cross-sectional views, respectively, while  FIG. 5C  illustrates an isometric cross-sectional view. As shown, the air velocity magnitude in the interior chamber  120  swings wildly. For an average velocity magnitude of 0.7 m/s, the velocity within the interior chamber reaches extremes such as 3 m/s and 0.05 m/s. Both of these extremes occur near the evaporator unit  130 . As a result, not only is the average velocity (and therefore the Reynolds number) of the air flow path  160  decreased, but the heat transfer coefficient is also decreased. In contrast, the air inlet  140  in  FIGS. 4A-C  can see a higher average velocity of 0.9 m/s, thereby increasing the Reynolds number and the heat transfer coefficient. 
       FIG. 6  illustrates an example heat pump system  600 . As shown, the heat pump system  600  can comprise an evaporator assembly  100  (including an evaporator unit  130 ), a compressor  610 , a condenser assembly  620 , and a thermal expansion valve  630 . The evaporator assembly  100 , the condenser assembly  620 , the compressor  610 , and the thermal expansion valve  630  can form a fluid circuit including various additional pipes, valves, and other fitments. The heat pump system  600  can also include components to encourage fluid flow along the fluid circuit, such as a pump  640 , and the heat pump system  600  can also include components to encourage air flow, such as a fan  650 . A heat transfer fluid can be configured to flow through the fluid circuit and undergo heat transfer at both the evaporator assembly  100  and the condenser assembly  620 . 
     Also disclosed herein are methods of modelling an evaporator assembly. Although the methods described below are described with respect to the evaporator assembly  100 , it is understood that the methods and methodologies described herein can be used to model any evaporator assembly, unless explicitly stated otherwise. 
     As will be understood by one having skill in the art, modeling heat exchanger systems is commonly accomplished using various computational fluid dynamics (CFD) methods. However, due to the intricate nature of heat exchangers (e.g., due at least in part to the numerous fins attached to the heat exchanger tubes), it is notoriously difficult to construct accurate CFD models of air flowing across a heat exchanger. 
       FIG. 7  illustrates a method  700  of modelling the evaporator assembly  100 . As shown, in block  710 , the pressure drop across the evaporator assembly  100  can be calculated. As would be appreciated, the various components within the interior chamber  120  (e.g., the evaporator unit  130 ), as well as the various fittings and other operational components (e.g., the fan  650 ) can cause a pressure drop between the air inlet  140  and the air outlet  150 . This pressure drop can be further influenced by the size and shape of both the air inlet  140  and the air outlet  150 . This pressure drop an influence how the air flow path  160  behaves in the interior chamber  120 . The method  700  can then proceed on to block  720 . 
     In block  720 , the evaporator assembly  100  can be modeled as a solid block comprising a porous medium. The porous medium can be modified in the CFD model to create a pressure drop corresponding to the calculated pressure drop from block  710 . That is, instead of modeling the intricacies of the heat exchanger&#39;s fins and other components, the impact of the heat exchanger on air flow can be approximated by using a solid block having the characteristics of a porous medium. The solid block can have dimensions corresponding to a desired size of the evaporator assembly  130 . This can ensure that the velocity distribution across the surface area of the solid block is accurately modeled. The method  700  can then proceed on to block  730 . 
     In block  730 , the air flow path  160  from the air inlet  140  to the air outlet  150  can be simulated as flowing over and/or through the porous medium. The air flow path  160  can be simulated to model the operating conditions of air flowing through the interior chamber  120  and contacting the evaporator unit  130 . To aid in calculating air flow velocities through the air flow path  160 , Equation 5 and Equation 6, and Equation 7 can be used. 
     
       
         
           
             
               
                 
                   
                     V 
                     . 
                   
                   = 
                   vA 
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     . 
                   
                   = 
                   
                     
                       m 
                       . 
                     
                     ρ 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     In Equation 5 and Equation 6, {dot over (V)} represents the air volumetric flow rate, ν represents the flow velocity, A represents the cross-sectional area of the flow, ρ represents the air density, and {dot over (m)} represents the mass flow rate. The method  700  can then proceed on to block  740 . 
     In block  740 , the heat transfer coefficient for the evaporator assembly  100  can be calculated based at least partially on the air flow from block  730 . Equation 5 and Equation 6 can be combined to yield Equation 7 to aid in the calculation. 
         {dot over (m)}=ρνA   (7)
 
     Upon obtaining the mass flow rate from the simulation in block  730 , the heat transfer rate and/or the heat transfer coefficient can be calculated using Equation 8. 
         {dot over (Q)}={dot over (m)}cΔT   (8)
 
     In Equation 8, {dot over (Q)} represents the heat transfer rate, c represents the specific heat capacity of air, and ΔT represents the temperature difference of the air between the air inlet  140  and the air outlet  150 . Using the heat transfer rate, any of the preceding equations, such as Equation 4, can be used to calculate the heat transfer coefficient. The method  700  can terminate after block  740  or proceed on to other method steps not shown. For example, the method  700  can then maximize the heat transfer coefficient by modifying (or, depending on project constraints, restricting modification of) one or more of: an air flow rate, an air temperature, a size of the air inlet, a location of the air inlet, an orientation of the air inlet, a size of the air outlet, a location of the air outlet, an orientation of the air outlet, a porosity of the porous medium, and the volume of the porous medium. 
     While the present disclosure has been described in connection with a plurality of example aspects, as illustrated in the various figures and discussed above, it is understood that other similar aspects can be used, or modifications and additions can be made to the described aspects for performing the same function of the present disclosure without deviating therefrom. For example, in various aspects of the disclosure, methods and compositions were described according to aspects of the presently disclosed subject matter. However, other equivalent methods or composition to these described aspects are also contemplated by the teachings herein. Therefore, the present disclosure should not be limited to any single aspect, but rather construed in breadth and scope in accordance with the appended claims. 
     EXAMPLES 
     An evaporator can have an 11-inch long semi-circular air inlet with a grille in the top pan of a heat pump system. The heat pump system can be sized for a 50-gallon water heater. Upon flowing air through the evaporator at a volumetric flow rate of 160 cfm, the average air velocity within the evaporator can be 0.9 m/s. Due to the semi-circular air inlet, the air in contact with the evaporator can differ from no less than 0.6 m/s to no greater than 1.2 m/s.