Design and Fabrication of Evaporators for Thermo-Adsorptive Batteries. Taylor A. Farnham

Transcription

1 Design and Fabrication of Evaporators for Thermo-Adsorptive Batteries by Taylor A. Farnham Submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Bachelor of Science in Mechanical Engineering at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2014 c Massachusetts Institute of Technology All rights reserved. Author Department of Mechanical Engineering May 18, 2014 Certified by Evelyn Wang Associate Professor of Mechanical Engineering Thesis Supervisor Accepted by Annette Hosoi Associate Professor of Mechanical Engineering Undergraduate Officer

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3 Design and Fabrication of Evaporators for Thermo-Adsorptive Batteries by Taylor A. Farnham Submitted to the Department of Mechanical Engineering on May 18, 2014, in partial fulfillment of the requirements for the degree of Bachelor of Science in Mechanical Engineering Abstract Current heating and cooling within electric vehicles places a significant demand on the battery, greatly reducing their potential driving range. An Advanced Thermo- Adsorptive Battery (ATB) reduces this load by storing thermal energy within a bed of adsorptive sheets. A phase change heat exchanger capable of delivering the required cooling via liquid-vapor phase change was designed and prototyped for ATB. The thermal performance and fluid flow within the phase change heat exchanger were characterized for both coolant and refrigerant. A full-scale and quarter-length prototype was designed within the desired geometric and operating condition constraints. In order to build the phase change heat exchanger, fabrication techniques, including brazing, copper sintering, and bonding porous media were explored and characterized. In addition, the quarter-length design was fabricated and insights from its construction are proposed as recommendations for future work. Thesis Supervisor: Evelyn Wang Title: Associate Professor of Mechanical Engineering 3

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5 Acknowledgments I would like to acknowledge my thesis supervisor, Evelyn Wang, for graciously providing this research opportunity. I would also like to thank Dr. Shankar Narayanan for directly supervising my work, providing invaluable insight, and assisting with numerous revisions. I would also like to thank Daniel Hanks and Jiansheng Feng for their assistance in various aspects of the experimental fabrication. 5

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7 Contents 1 Motivation Advanced Thermo-Adsorptive Battery Adsorption Phase change heat exchangers Pressure drop through porous media Copper sintering Thesis overview Physical System Overview Coolant Design Evaporator Design Porous Medium Design Fabrication Milling Flattening Brazing Tapping Sintering Bonding Final Assembly Conclusions 53 7

8 4.1 Future Work A G-Code for CNC Milling 55 8

9 List of Figures 1-1 [Reproduced from Narayanan [2]] Schematic diagram of the ATB system in each of its three modes with possible operational temperatures: (a) cooling mode for summer, (b) heating mode for winter, and (c) regeneration mode. In cooling mode (a) heat from the EV-cabin is transferred to the evaporator and dissipated by the evaporating working fluid. In heating mode (b), vapor from the evaporator is transferred to the adsorptive bed, where it is adsorbed in a highly exothermic reaction; this heat is then transferred to the EV-cabin. In the regenerative mode (c), the adsorptive bed is heated, driving vapor from the saturated bed and allowing the process to repeat again [Reproduced from Narayanan [3]] Fluid paths for cooling the EV-Cabin (dashed lines) and heating the EV-Cabin (solid lines) are shown. In either case, liquid refrigerant from a reservoir evaporates in the phasechange heat exchanger. This vapor passes upward through the adsorption bed, where its adsorption releases heat. The EV-Cabin can be heated by interfacing with the adsorptive bed, or cooled by interfacing with the phase-change heat exchanger

10 1-3 [Reproduced from Narayanan [2]] A rendering of the evaporator, as described above and shown schematically in Figures 1-1 and 1-2, is shown (in silver) in close proximity to the adsorptive bed (in brown), with holes shown for the installation of coolant lines. Darker gaps between the individual adsorptive sheets represent void channels through which vapor can pass. These channels allow for vapor diffusion across the entire length of the adsorptive sheet. Liquid refrigerant enters into the end of the evaporator, evaporates to vapor and passes out through the sides, diffuses through the gaps between adsorptive sheets, and adsorbs onto the sheet [Reproduced from Narayanan [2]] The repeating structure within the zeolite includes pores optimized for the adsorption of water molecules. Increasing the pore density allows for greater adsorption efficiency and ultimately contributes to an increase in released heat The operation of the evaporator is shown by the above schematic (left) and cross-section view (right). Two systems of small channels carry the two fluids - the coolant and the refrigerant. Heat from the coolant is transferred to the refrigerant, causing the refrigerant to evaporate and the coolant to drop in temperature. The newly-formed vapor leaves the evaporator to the adsorption bed. The length-wise center cross-section illustrates the location of channels on each of the two blocks and how the coolant channels form the interior, while the evaporator channels face the exterior The location of copper sinter panels on top of the evaporator with a top-down view (above) and cross-sectional view (below) is shown. An impermeable membrane (not shown) covers the initial inlet channel, while the sinter provides a porous medium through which the refrigerant flows from underneath

11 2-1 A schematic of the main heat transfer mechanisms during cooling is shown. Coolant, heated from the EV-Cabin, exchanges 2500W with the evaporator-side of the device. This heat transfer is sufficient to evaporate the water refrigerant after it experiences a reduction in pressure A preliminary render of the serpentine channel within one half of the evaporator is shown on the left (a). (b) shows a view of the crosssection of the coolant half with the width w and depth d labeled. Finally, (c) shows the top-down view of the coolant half with the width w labeled. Once the two half-pieces are assembled, it becomes clear the final channel dimensions are w x 2d The required channel depth is plotted for a range of channel widths. As the width of the channel increases the required depth decreases. The power, shown in red, represents the amount of power required to pump the coolant through the corresponding geometry. Finally, the black box represents the Operating Region for the evaporator based on geometric constraints The required contact area is shown as a function of heat transfer. As the total heat transfer increases the required area asymptotically approaches approximately 0.02m 2. As the total heat transfer rises, so does the required mass flow rate. The increase in mass flow rate increases the overall heat transfer coefficient through forced convection The geometry of the coolant channels for a quarter-scale prototype is shown, with all dimensions in millimeters. The pattern shown above would simply be repeated for a full-scale evaporator. Critical features are the inlet and outlet lengths (10mm), channel thickness (2.5mm), channel length (60mm), and channel spacing (3.5mm)

12 2-6 A shortened internal view of the proposed evaporator design is shown. Refrigerant enters from a reservoir (not shown), flows through the Inlet Channel, distributes longitudinally through the Main Artery, then distributes transversally through the individual Distribution Channels. A porous medium, not shown, covers the Main Artery and Distribution Channels and acts as the final pressure drop before the vapor reaches the adsorptive bed The pressure drop, shown in parentheses, across each of the major evaporator elements is presented. Arrows indicate the flow of the refrigerant within the evaporator. The addition of a non-porous barrier above the inlet and Inlet Channel prevents the refrigerant from circumventing the pressure drop in the Inlet Channel A dimensioned drawing of the evaporator for a quarter-length prototype is shown, with all dimensions in millimeters. The pattern shown above would simply be repeated for a full-scale evaporator. Critical features are the dimensions of Distribution Channels, as well as the space between channels (7mm) Fluid within the channel (position a) is forced through the length of porous media (L) across a cross-sectional area (A) to position b. As the volume of fluid passes through the porous media its pressure drops, proportionally to the permeability of the material, and inversely proportional to the viscosity of the fluid The required permeability is shown for varied sample thickness, from 0 to 10mm, for both the full-scale (left) and quarter-length prototypes (right). The blue hatched region shows the permeability range readily available for copper sinter made from a powder with size microns. The red shaded region shows the permeability range for sinter made from a powder of less than 10 microns

13 3-1 The designed pattern for the interior and exterior faces of each copper block is shown. The interior face contains a serpentine channel with a thickness of 2.5mm for coolant flow. The exterior face contains a series of channels, from 2 to 5mm in thickness, through which the refrigerant flows A schematic is shown of potential arrangements between the two copper blocks prior to brazing, and their predicted success or failure. Blocks on the left (a) are predicted to have a successful braze, defined by a vacuum-tight seal, while blocks on the right (b) are predicted to fail The material layers during a brazing operation are presented. The outside steel blocks provide structural integrity and allow for bolts to hold the system together while Belleville washers act as springs to provide a compressive force. The graphite layer provides a boundary between the steel and the target copper blocks. This boundary prevents the copper from bonding to the steel and allows for easier removal of the device from the assembly. The thin silver sheet is placed between the copper blocks so that once heated the molten silver will wet along the copper-copper contact area forming a bond as it cools A representative temperature profile during the heating cycle. The assembly is initially heated at a rate of 600 C/hr until it reaches 1000 C, a temperature between the melting points of silver (962 C) and copper (1085 C). This temperature is held for 18 minutes to ensure the entire assembly reaches a uniform temperature. Finally the assembly shows an exponential temperature decay as it cools A 1 NPT to 1 compression fitting is placed next to a sample device 16 4 (a). The device has been tapped to allow for the male NPT fitting to connection (b). In this example a short piece of plastic tubing has been added to the compression fitting. The plastic allows for positively pressuring the internal channel with a syringe

14 3-6 [Reproduced from Espinosa [5]] The above plots show the linear shrinkage (left) and permeability (right) for copper particles in the range of 38-75um after sintering at 650 to 950 C for minutes. Shrinkage tends to increase linearly with sinter duration, while the permeability shows a more sporadic behavior at shorter durations The potential for copper powder to intrude into the evaporator channels during sintering is demonstrated. While the device may initially rest on top of a layer of powder (a), if the device settles or sinks into the copper powder, the displacement would likely rise into the channel (b). This rise would decrease the available cross-sectional area of the channel and could interfere with the ability to form a continuous, stable copper sinter A copper sinter sample is seated in a graphite mold. Due to shrinkage during the sintering process there is a consistent gap between the sinter and mold. The mold cavity is inches across, while the sintered block has reduced to inches a reduction of 2.2% Potential sinter panels are shown along a full-scale evaporator model. While minimizing the number of individual panels and the number of sealant lines, it is important to consider the manufacturability of these sinter panels. As the panel length increases it becomes more likely that their properties are not uniform throughout

15 3-10 The potential effects of misapplication of thermal adhesive are presented. The correct application (a) shows a uniform, thin layer of thermal adhesive between the evaporator and the copper sinter. This layer allows for minimal thermal resistance while maintaining integrity. Over-application of thermal adhesive (b) could result in a thicker layer between the evaporator and the copper sinter. The added length would add to the overall thermal resistance. In addition, the thermal adhesive may extend outward and block pores on the copper sinter. Finally, under-application (c) could result in poor contact between the two surfaces. This could increase the interfacial resistance and could dramatically increase the thermal resistance between the evaporator and the copper sinter A copper sinter block is shown above the evaporator surface. Shown without thermal adhesive, a small gap can be seen between the sinter and the exterior surface. The sinter remains clear of the Inlet Channel to allow for the installation of a non-permeable membrane The final assembly is shown prior to the installation of the non-permeable membrane inlet connections. Once installed the device will be ready for further testing and performance characterization A linear combination of four quarter-scale units is shown. Red lines indicate the boundary of each original quarter-scale unit, while blue lines indicate the boundary of each sinter block. Shading has been used to identify alternate blocks. By staggering the location of the sinter blocks, there is no boundary continuous through the entire device. This is likely increase stability as it removes a single point of failure

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17 List of Tables 2.1 Dimensions and Pressure Drops for Critical Features Milling Parameters

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19 Chapter 1 Motivation The recent surge in electric vehicles has in large part led to an increase in energy efficiency goals within modern automobiles. Lacking the conventional internal combustion engine, many processes must be adapted to reduce their demand on the electric battery. Heating and cooling the cabin of an electric vehicle can have a significant impact on battery life, and can decrease the driving range by over 30%. [1] One proposal integrates an Advanced Thermo-Adsorptive Battery (ATB) to reduce the load on the electric battery. The ATB system is capable of both heating and cooling the cabin; its overall process is illustrated in Figure 1-1. The system operates in one of three modes to either cool the cabin, heat the cabin, or recharge the system. To cool the cabin, heat is absorbed through the evaporation of a working fluid. To heat the cabin, the same evaporated vapor is adsorbed by an adsorbent bed, which releases heat. Finally, to recharge the system the adsorbent bed is heated directly expelling any trapped vapor. The ATB relies on two fluid systems to provide heating and cooling to the EV- Cabin. The first system uses a water and ethylene glycol mixture as a coolant to thermally interface with the actual EV-Cabin. By bringing warm or cold coolant in thermal contact with the EV-Cabin the cabin temperature can be controlled. The second fluid system uses water as an evaporating working fluid to cool the first system. The water, acting as a refrigerant, is subject to a significant pressure drop which reduces its saturation temperature. Consequently, it is capable of evaporating at a 19

20 Figure 1-1: [Reproduced from Narayanan [2]] Schematic diagram of the ATB system in each of its three modes with possible operational temperatures: (a) cooling mode for summer, (b) heating mode for winter, and (c) regeneration mode. In cooling mode (a) heat from the EV-cabin is transferred to the evaporator and dissipated by the evaporating working fluid. In heating mode (b), vapor from the evaporator is transferred to the adsorptive bed, where it is adsorbed in a highly exothermic reaction; this heat is then transferred to the EV-cabin. In the regenerative mode (c), the adsorptive bed is heated, driving vapor from the saturated bed and allowing the process to repeat again. lower temperature, which allows heat from the coolant line to cause it to evaporate. During cooling, refrigerant and coolant enter independent channels within the evaporator. As the refrigerant evaporates it absorbs heat from its surroundings and chills the coolant line. The evaporated vapor then passes through the bed of adsorptive sheets and is adsorbed by the zeolite material. This process releases heat and is used to heat the alternate coolant line. By controlling which line (cool from the evaporator or warm from the adsorptive bed) interfaces with the EV-Cabin one can control whether to heat or cool the electric vehicle, as shown in Figure 1-2. In the first mode of operation the ATB utilizes the heat released by evaporating refrigerant to cool the EV-Cabin. A stream of coolant passes from the EV-Cabin and into the evaporator. In this same unit a supply of water refrigerant enters and undergoes a significant drop in pressure. This drop in pressure allows the refrigerant to evaporate at a lower temperature, using the heat from the coolant flow. The coolant temperature drops and chills the EV-Cabin as it cycles back. In the second mode of operation the ATB system utilizes the heat released by the adsorptive bed to warm the EV-Cabin. As vapor gets adsorbed by the bed, its heat 20

21 Figure 1-2: [Reproduced from Narayanan [3]] Fluid paths for cooling the EV-Cabin (dashed lines) and heating the EV-Cabin (solid lines) are shown. In either case, liquid refrigerant from a reservoir evaporates in the phase-change heat exchanger. This vapor passes upward through the adsorption bed, where its adsorption releases heat. The EV-Cabin can be heated by interfacing with the adsorptive bed, or cooled by interfacing with the phase-change heat exchanger. of adsorption defines how much heat is released to the environment. In this case, the heat is captured by a heat exchanger that directs the warmed water to the EV-Cabin. Similar to during warming, the coolant passes through the EV-Cabin, but in this case the fluid warms the overall cabin. The ATB system allows for energy storage, in the form of potential released heat, within a bed of adsorptive material. As vapor is adsorbed into the bed, heat is released to the environment. Eventually, the vapor will saturate the adsorptive bed and the process will cease. By heating the adsorptive bed in the regeneration stage, the third mode of operation, the vapor is expelled from the bed for a subsequent adsorptive cycle. Through the above modes of operation, the ATB system can address all of the heating and cooling needs of the electric vehicle in a system that is reusable and has a significantly reduced demand for electricity. The regenerative step can occur when the vehicle is connected to an external power source (grid power), resulting in a greatly decreased load on the electric battery during operation. Such improvements may increase the overall driving range and accelerate their adoption by the public. 21

22 Figure 1-3: [Reproduced from Narayanan [2]] A rendering of the evaporator, as described above and shown schematically in Figures 1-1 and 1-2, is shown (in silver) in close proximity to the adsorptive bed (in brown), with holes shown for the installation of coolant lines. Darker gaps between the individual adsorptive sheets represent void channels through which vapor can pass. These channels allow for vapor diffusion across the entire length of the adsorptive sheet. Liquid refrigerant enters into the end of the evaporator, evaporates to vapor and passes out through the sides, diffuses through the gaps between adsorptive sheets, and adsorbs onto the sheet. 1.1 Advanced Thermo-Adsorptive Battery The Advanced Thermo-Adsorptive battery controls the temperature within an electric vehicle by either supplying heat from adsorption, or cooling from evaporation. This can be controlled by directing coolant, which is in thermal contact with the EV-Cabin, to interface with coolant from the adsorptive bed (shown in brown) or through coolant from the evaporator evaporator (shown in silver) in Figure 1-3, 1.2 Adsorption The Advanced Thermal Battery primarily captures and releases heat through the use of an adsorptive bed. Adsorption is the process by which refrigerant molecules, or adsorbate, adhere to the surface of a highly porous adsorbing material. In the case of the ATB, the adsorbate (water) is adsorbed into an advanced porous adsorbent called zeolite. The storage capacity of the micro-porous structure of the adsorbent permits a greater uptake of water vapor, as shown in Figure 1-4 illustrating the crystal structure of zeolite 13X. As the adsorbate enters the porous structure it reaches a lower energy state. This difference can be captured by the adsorbate s heat of adsorption, which 22

23 Figure 1-4: [Reproduced from Narayanan [2]] The repeating structure within the zeolite includes pores optimized for the adsorption of water molecules. Increasing the pore density allows for greater adsorption efficiency and ultimately contributes to an increase in released heat. is then released to the environment. As water begins to occupy a greater number of pores, the zeolite begins to saturate and stop accepting additional vapor. Once saturated, the zeolite must be recharged by applying heat and forcing the vapor to evacuate the zeolite through desorption. After the zeolite bed is fully desorbed the system is ready to re-adsorb liquid and release heat again. 1.3 Phase change heat exchangers Phase change heat exchangers are currently used in a number of industrial and commercial applications, including power plants, refineries, and refrigerators. Heat transfer during a phase change, most commonly evaporation or condensation, can be orders of magnitude greater than for single phase flow. Since the design temperature of the system (30-40 C) is insufficient to boil the water refrigerant at atmospheric pressure, the refrigerant will require a series of pressure drops. By dropping the pressure of the refrigerant, its saturation temperature also decreases. Figure 1-5 shows the operation of the evaporator, Common approaches to reduce pressure in refrigerator systems include passing the fluid through an expansion valve or through a narrow channel. The sudden expansion of a fluid can lead to a significant drop in pressure, but also a drop 23

24 Figure 1-5: The operation of the evaporator is shown by the above schematic (left) and cross-section view (right). Two systems of small channels carry the two fluids - the coolant and the refrigerant. Heat from the coolant is transferred to the refrigerant, causing the refrigerant to evaporate and the coolant to drop in temperature. The newly-formed vapor leaves the evaporator to the adsorption bed. The length-wise center cross-section illustrates the location of channels on each of the two blocks and how the coolant channels form the interior, while the evaporator channels face the exterior. in temperature. When the system is running near the triple point of the fluid, such drops in temperature may increase the risk of freezing. One alternative is to rely on the frictional losses of the fluid flowing through a narrow channel. It is desirable that the fluid distributes evenly across the entire surface of the phase change heat exchanger. This distribution allows for increased thermal contact between the two fluids and allows for improved heat transfer, increasing the efficiency of the device. To achieve this level of fluid distribution, a network of distribution channels is required. Strategic design of distribution channels can have two-fold benefits: even distribution, and gradually achieving the required pressure drop. By manipulating the geometric constraints of the evaporation channels, the required pressure drop can be achieved for the required mass flow rates, as discussed in Chapter Pressure drop through porous media Another method of decreasing fluid pressure is to force it through a semi-permeable or porous medium. The design of the porous medium geometry and permeability has a significant impact on its performance. If the porous media is made too thick, the pressure differential will be insufficient to drive the flow. If the fluid were to 24

25 Figure 1-6: The location of copper sinter panels on top of the evaporator with a top-down view (above) and cross-sectional view (below) is shown. An impermeable membrane (not shown) covers the initial inlet channel, while the sinter provides a porous medium through which the refrigerant flows from underneath evaporate completely within the porous media, the liquid transport challenges may be avoided. However, excessively large porous beds could still be inefficient and increase the weight, and size of the device. Conversely, if the porous bed is made too thin, the fluid would pass through without complete phase-change, which could be detrimental to the performance of the ATB Copper sintering There exist many potential material choices for the porous media. Primary considerations included metallic foams and metal sinters. In considering the required permeability, thermal conductivity, and metal-refrigerant compatibility, copper sinter was determined to be a suitable option. It is desirable that the porous medium make a strong and structurally-reliable interface with the rest of the device. The porous medium s primary contact is with the copper channels, and so the ability to form a strong bond with copper was required. In addition, the porous medium needs to have a high thermal conductivity to facilitate evaporation of liquid. From these requirements, copper was chosen as the best material for use in the porous medium. In order to maintain a relatively small thickness (under 5mm) the permeability of the porous medium needs to be especially low, on the order of m 2. A review of literature finds that typical copper foams may have a permeability within the range of 10 7 to 10 9 m 2 [4]. By contrast, copper sinters may have a permeability within the range of to m 2 [5]. The required permeability for the reliable operation 25

26 of the evaporator in the ATB is within the range of the sintered copper permeability, which can be controlled by manipulating the sintering manufacturing process. 1.5 Thesis overview My thesis project investigates a prototype design for the evaporator unit as part of the Advanced Thermo-Adsorptive Battery system. A full-scale evaporator unit was designed in accordance to the required thermal performance and physical limitations of the system. From this design a quarter-length prototype was derived and subsequently fabricated. Learnings from this fabrication are presented for application in the full-scale unit. Chapter 2 describes the Advanced Thermo-Adsorptive Battery as a system and presents the various design constraints and considerations. Thermal calculations are presented to form the basis for the design of the evaporator and coolant path. Chapter 3 discusses the various fabrication methods employed for the construction of the prototype. Finally, Chapter 4 presents insight into the scaling from prototype to full-scale unit. 26

27 Chapter 2 Physical System Overview The Advanced Thermo-Adsorptive Battery (ATB) system faces numerous design criteria which are relevant in the design and construction of the phase change heat exchanger. The most relevant criteria are: the required heat transfer performance, the allotted physical space, and the operational conditions. The ATB system is designed to provide 2500W of heating or cooling for the cabin of an electric vehicle, over the course of one hour. For compactness, the heat exchanger must fit within an eight centimeter by eighty centimeter rectangle bound, and the thickness should be minimized. Overall cost, weight, and manufacturability were also considered. Combined with the system operational parameters, these criteria were critical in determining design and process conditions for the system. While the ATB system is cooling the EV-Cabin, the evaporator works to capture heat from a coolant stream that interfaces with the cabin. The heat from the coolant stream passes to the evaporator-side of the device, where it is used to evaporate lower pressure liquid, as shown in the schematic in Figure 2-1. For actual construction, the coolant stream and evaporation-side are milled into opposite faces of a copper block. A second copper block is milled to be the mirror image of the first. When bonded together the two coolant streams form a single, continuous stream through the larger block. Both exterior surfaces of the block are designed to allow evaporation of the refrigerant. This allows for vapor coming from the evaporator to quickly disperse to the adsorptive bed surrounding the unit. 27

28 Figure 2-1: A schematic of the main heat transfer mechanisms during cooling is shown. Coolant, heated from the EV-Cabin, exchanges 2500W with the evaporatorside of the device. This heat transfer is sufficient to evaporate the water refrigerant after it experiences a reduction in pressure. 2.1 Coolant Design A critical design constraint was that the coolant must be able to dissipate 2500W of heat with an initial temperature of 25 C to a final temperature of 5 C. Given the low temperature region in which it would be operating, it was also desirable to have some mixture of ethylene glycol to prevent freezing of coolant at low temperature. An iterative approach yielded 90 weight-percent water, 10 weight-percent ethylene glycol to be an optimal mixture. Heat transfer and fluid dynamic performance has been calculated using a linear interpolation of the fluid properties. In order to enhance the contact area between the coolant liquid and the evaporator face, a serpentine channel was chosen for the coolant path. For the purpose of manufacturing and assembly, it was decided to mill the serpentine channel into opposite faces of copper blocks then bond the blocks together. Figure 2-2 shows the overall design of each half of the coolant channel. The first requirement of the system was to determine the required mass flow for the coolant. From conventional heat transfer this was calculated using the following equation Q = mc p (T in T out ) (2.1) where Q is the total heat transfer, m is the coolant mass flow rate, c p refers to the specific heat of the mixture (3988 J kgk ), and T in and T out refer to the bulk coolant tem- 28

29 Figure 2-2: A preliminary render of the serpentine channel within one half of the evaporator is shown on the left (a). (b) shows a view of the cross-section of the coolant half with the width w and depth d labeled. Finally, (c) shows the top-down view of the coolant half with the width w labeled. Once the two half-pieces are assembled, it becomes clear the final channel dimensions are w x 2d. perature at the inlet and outlet of the evaporator, respectively. Given the requirement of 2500W of heat transfer, an inlet temperature of 25 and outlet temperature of 5, Equation 2.1 provides a required mass flow rate of kg s for the coolant mixture. Once the mass flow rate was fixed, an iterative approach was required to assess the thermal and dynamic performance of the system. The following shows the methodology employed as well as the resulting values. First: the channel geometry, width (w) and depth (d), were assumed. Through multiple iterations a width of 2.5mm and depth of 2.5mm were found to be satisfactory. In order to form a relatively thin evaporator, channels were milled on a piece of stock approximately 6mm thick. This constrained the channel thickness to less than 2.5mm since deeper channels would likely cause structure instabilities and potential cross-flow from the coolant to the refrigerant. In addition, certain contact area was required to ensure adequate heat transfer. This led to a limit on the width of the channels if the channels were too wide there would be insufficient gaps between the channels. The lack of sufficient spacing could cause flow to cross from one channel to the next, instead of following the serpentine path. It was found that channels 4mm 29

30 Figure 2-3: The required channel depth is plotted for a range of channel widths. As the width of the channel increases the required depth decreases. The power, shown in red, represents the amount of power required to pump the coolant through the corresponding geometry. Finally, the black box represents the Operating Region for the evaporator based on geometric constraints. approached the limit for sufficient gap spacing. Figure 2-3 shows the required channel depth for a given width, as well as the required power to pump the coolant through a channel of that size. The hydraulic diameter was calculated through the relation D h = 4A P = 4w(2d) 2(w + 2d) (2.2) The Reynolds and Prandtl numbers were calculated as Re = 4 m 2μ(2d + w) P r = c pμ k (2.3) (2.4) where μ and k are the viscosity and thermal conductivity of the mixture, respectively. Through this the hydraulic diameter was found to be m, and the flow was characterized with a Reynolds number of 3634 and a Prandtl number of 16.8 From the Reynolds number criteria (Re > 2300) it was determined the flow would be turbulent within the channel. This was assumed to be beneficial for both heat transfer and mixing, and also resulted in a slight reduction in friction factor compared 30

31 to the equivalent laminar case. The following correlations were used to determine the Darcy-Weisbach friction factor (f D ) and Nusselt number (Nu), f D = (0.79ln(Re) 1.64) 2 (2.5) Nu Dh = ( f 8 )(Re D h 1000)P r ( ( f 8 ) 1 2 (P r 2 3 1) (2.6) This provided a friction factor of and a corresponding Nusselt number of From the Darcy-Weisbach equation, the pressure drop in a channel can be calculated as, L ρv 2 P = f D D h 2 (2.7) where f D is the Darcy Friction Factor, L is the length of the channel, D h is the hydraulic diameter of the channel, velocity. is the density of the fluid, and V is the fluid From the following basic definition of the Nusselt number, Nu = hd h k (2.8) the heat transfer coefficient (h) was determined to be 6374 W. However, the contact m 2 K area between the coolant and evaporator must still be calculated with the equation A = Q h T (2.9) where Q is the total heat transfer and T is the temperature difference between the coolant and the surface. Figure 2-4 shows the relation between the total heat flux and the required area for a given temperature drop of 20 C. In order to increase the available contact area, a serpentine channel as shown in Figure 2-2 was chosen. A number of conservative estimates were made to ensure there was sufficient contact area for heat transfer. Only the area directly beneath the channel was considered, while the curved regions were assumed to have negligible contribution to the contact area, and the contact area was designed with a factor of 31

32 Figure 2-4: The required contact area is shown as a function of heat transfer. As the total heat transfer increases the required area asymptotically approaches approximately 0.02m 2. As the total heat transfer rises, so does the required mass flow rate. The increase in mass flow rate increases the overall heat transfer coefficient through forced convection. 32

33 Figure 2-5: The geometry of the coolant channels for a quarter-scale prototype is shown, with all dimensions in millimeters. The pattern shown above would simply be repeated for a full-scale evaporator. Critical features are the inlet and outlet lengths (10mm), channel thickness (2.5mm), channel length (60mm), and channel spacing (3.5mm). safety of four. With these considerations, excluding the safety factor, a total area of approximately 0.02m 2 was found to be sufficient for heat transfer. This area corresponded to a channel with a total arc length of just under 1.3m. The actual proposed design includes a net channel length of 7.5m, corresponding to a contact area of 0.11m 2. The total contact area from the serpentine design should ensure that the entire available area of the heat exchanger is utilized, and that heat transfer from the coolant is not a limiting factor in the process. An additional consideration was the pressure drop of the coolant and the required pumping power. The Darcy-Weisbach equation (Equation 2.7) was used to calculate the overall pressure drop of the coolant. While this only accounts for frictional losses of the fluid within the channel, form losses were found to be negligible. Finally the required pumping power was calculated by, P pump = P m ρ (2.10) While the increased channel length increases the overall pressure drop, the pumping 33

34 Figure 2-6: A shortened internal view of the proposed evaporator design is shown. Refrigerant enters from a reservoir (not shown), flows through the Inlet Channel, distributes longitudinally through the Main Artery, then distributes transversally through the individual Distribution Channels. A porous medium, not shown, covers the Main Artery and Distribution Channels and acts as the final pressure drop before the vapor reaches the adsorptive bed. power required was determined to be under 10W and was deemed sufficiently minor. 2.2 Evaporator Design Dissipation of heat from the coolant stream comes from the evaporating refrigerant. Similar to the coolant stream, the refrigerant should spread evenly across the entire available area to maximize heat transfer. However, utilizing a serpentine channel brings the risk that evaporation may be concentrated near the hot coolant inlet. Even if this were to achieve similar performance, the unequal distribution may extend downstream to the adsorptive bed. To help achieve a more even distribution an inlet channel was designed to distribute the refrigerant to the middle of the evaporator, as shown in Figure 2-6. Key design considerations for the evaporator also include the temperature of the refrigerant at the inlet, and the desired evaporation temperature, as well as their associated saturation pressures. The evaporator was designed assuming 40 C refrigerant at the inlet, with a saturation pressure of 7385Pa. In order to maximize cooling, evaporation should occur at the lowest temperature. However, 34

35 Figure 2-7: The pressure drop, shown in parentheses, across each of the major evaporator elements is presented. Arrows indicate the flow of the refrigerant within the evaporator. The addition of a non-porous barrier above the inlet and Inlet Channel prevents the refrigerant from circumventing the pressure drop in the Inlet Channel. approaching the triple point for water (0.01 C, 612Pa) increases the risk that the refrigerant will freeze. Freezing within the evaporator has the potential to dramatically decrease performance, as well as damage the porous medium. As such, an evaporation temperature of 3 C was deemed an acceptable tradeoff between performance and the risk of freezing. However, the saturation pressure of water at 3 C is 758Pa, forcing a total pressure drop of 6627Pa across the system. The target 6627Pa pressure drop occurs across four regions: the Inlet Channel, Main Artery, Distribution Channels, and porous medium. While it is possible to change the geometry of each feature, and its subsequent pressure drop, there are important external considerations. The refrigerant is likely to follow the path of least resistance and minimize its total pressure drop. Since the porous medium is designed to contain the liquid refrigerant, its pressure drop must be larger than any channel to which it is directly connected. The Main Artery and Distribution Channels hold the dual-purpose of reducing pressure, but also distributing the refrigerant across the entire evaporator. The pressure drop must be relatively minor to ensure adequate distribution. Through an iterative approach similar to the one modeled in Section 2.1 for the coolant line, final pressure drops for each region were generated and shown in Figure 2-7. In order to match the existing heat requirement, the evaporator side must also 35

36 Figure 2-8: A dimensioned drawing of the evaporator for a quarter-length prototype is shown, with all dimensions in millimeters. The pattern shown above would simply be repeated for a full-scale evaporator. Critical features are the dimensions of Distribution Channels, as well as the space between channels (7mm) dissipate 2500W of heat. This heat loss comes as the product of refrigerant mass flow ( m r ) and enthalpy of vaporization (h fg ), Q = m r h fg (2.11) By constraining the refrigerant to be water, the required mass flow can easily be calculated to roughly kg s. Once the mass flow is known, iteration can be employed to determine desirable dimensions for the channels within the evaporator. Unlike the coolant stream, the much lower mass flow rate places the refrigerant in a purely laminar regime (Re < 700). While a new correlation was used to determine the friction factor, f Dl am = 64 Re (2.12) the remaining calculations were identical to the coolant stream, as discussed in Section 2.1. Through multiple iterations, the proposed quarter-length evaporator design is shown in Figure 2-8 and full-scale design summarized in Table

37 Table 2.1: Dimensions and Pressure Drops for Critical Features Inlet Channel Main Artery Distribution Channels Length (mm) Width (mm) Depth (mm) Pressure Drop (Pa) Figure 2-9: Fluid within the channel (position a) is forced through the length of porous media (L) across a cross-sectional area (A) to position b. As the volume of fluid passes through the porous media its pressure drops, proportionally to the permeability of the material, and inversely proportional to the viscosity of the fluid 2.3 Porous Medium Design A porous medium may constrict the flow and relies on a pressure differential as described by Darcy s Law [6], P = QμL ka (2.13) where Q is the volumetric flow rate, k is the permeability of the porous media, A is the area through which the fluid travels, μ is the viscosity of the fluid, and L is the length of the porous media. The case of the evaporator channels is shown by the schematic in Figure 2-9. The final pressure drop on the evaporative side of the device comes as the refrigerant passes through a porous medium. As will be discussed in Section 3.5, the particular porous medium consists of sintered copper copper powder that has been heated to near-melting temperatures to form a single, solid body. By controlling the temperature and duration of the sintering process the physical properties of the sinter can be altered. This section will discuss the optimization of copper sinter for this application. 37

38 Figure 2-10: The required permeability is shown for varied sample thickness, from 0 to 10mm, for both the full-scale (left) and quarter-length prototypes (right). The blue hatched region shows the permeability range readily available for copper sinter made from a powder with size microns. The red shaded region shows the permeability range for sinter made from a powder of less than 10 microns. The entire pressure drop across the evaporator is 6627Pa, with 6301Pa occurring as friction losses through channels. The remainder of pressure drop occurs as the fluid passes through the porous medium. Considering the existing constraints: geometry, fluid properties, and flow rate, Equation 2.15 can be simplified to P = 6.28 * 10 8 ( L k ) (2.14) This allows for a simplified relationship between the pressure drop across the porous medium and its thickness L and permeability k. Theoretically the sinter could be of any thickness; however, maintaining a smaller thickness reduces the volume and overall weight of the device. In practice, sintered metals tend to be fragile and could lack structural integrity if made too thin. In addition, an additional safety factor is required to prevent liquid breakthrough across the medium. By contrast, the permeability can only be controlled within a finite range. Through iteration a sintered copper design was found for a minimal suitable thickness and within the permeability-range of the copper powder. The final design consists of a 3mm thick sintered layer with a permeability of 2.7 * m 2 38

39 Chapter 3 Fabrication The overall design presents multiple difficulties, including an internal serpentine channel for the coolant, vacuum-tight connections, and high-temperature surroundings. The internal channel was addressed by producing two half-device mirrored copies and brazing the halves together. Vacuum-tight fittings were achieved through the combination of threaded connections and solder around the joint. High temperature operation constrained material selection for potential seals and solders. In order to assess the performance and manufacturability of the design, a quarterlength prototype was constructed. While the full-scale model design has dimensions of roughly 80cm x 8cm x 1.3cm, the prototype has dimensions of 20cm x 8cm x 1.3cm. Maintaining two of the same dimensions allows for better assessment of critical features fluid distribution and heat transfer through the device. Decreasing the length of the device should have a predictable effect on performance and allow for fewer repetitions of the coolant-side and evaporator patterns. Construction of the prototype can be divided into the following stages: milling, flattening, brazing, tapping, sintering, and bonding. 3.1 Milling To create the internal channel for the coolant, the actual device was formed from two blocks of copper. Half of the channel was milled on each piece so that the 39

40 Figure 3-1: The designed pattern for the interior and exterior faces of each copper block is shown. The interior face contains a serpentine channel with a thickness of 2.5mm for coolant flow. The exterior face contains a series of channels, from 2 to 5mm in thickness, through which the refrigerant flows. pieces could be aligned to form a complete channel. The evaporation distribution channels were milled into the faces opposite the internal channel. Figure 3-1 shows the designed pattern for the interior and exterior faces of each copper block. The interior face contains a serpentine channel with a thickness of 2.5mm for coolant flow. The exterior face contains a series of channels, from 2 to 5mm in thickness, through which the refrigerant flows. Milling was performed with a HAAS Super Mini CNC Mill, using commercially available 2mm, 2.5mm, and 5mm endmills. The smaller endmills proved to be more easily broken and in the case of the 2.5mm endmill, required a carbide tip to preserve durability for extended cutting. Material properties, particularly thermal conductivity and ductility, of the copper blocks also presented challenges. Milling copper too aggressively would result in poor chip removal that deteriorated the quality of the cut s finish and greatly reduced the life of the tool. Manipulating the stock feed rate, spindle speed, and depth of cut led to acceptable performance. These parameters are 40

41 Table 3.1: Milling Parameters Endmill Size Feed Rate Spindle Speed Max Depth of Cut mm (inch) inch/min rev/min mm (inch) 2 (0.0787) (0.020) 2.5 (0.0984) (0.033) 5 (0.1969) (0.030) summarized in Table 3-1 Table 3-1 shows the operation parameters for the different-sized drill bits. These parameters were found to produce a consistent chip, leave an acceptable finish on the material, and avoid excessive tool breakage. G-Code for the CNC mill program is provided in Appendix A 3.2 Flattening In preparation for brazing, the copper blocks were flattened using milling and sandpaper lapping. The blocks were first placed in the mill for a facing routine. A large endmill was passed along the surface of the part to remove any macro-scale peaks or burs. Sandpaper was then placed on a surface known to be acceptably flat and the blocks were slid across the abrasive surface in a figure-eight pattern. Repeated translation across the rough surface removed any remaining peaks in the surface of copper block. Sandpaper lapping also creates surface roughness which aids in wetting the braze to the copper block during brazing. By using successively finer sandpaper the height variation within the part can be more closely controlled. The surface compatibility of the two halves is essential to the quality of the silver braze. Surface roughness or incongruent curvature of the two halves could form a gap wider than the thickness of the silver braze. This gap might not be sealed and could compromise success of the braze. Figure 3-2 shows common differences across the two surfaces and the predicted success or failure of the braze. The successful samples (i-iii) all show a flat bonding surfaced between the two blocks. Since the braze material is thin (0.002 ) there is relatively little compensation for surface roughness or imperfections. However, it is predicted that the samples could withstand macro-scale 41

42 Figure 3-2: A schematic is shown of potential arrangements between the two copper blocks prior to brazing, and their predicted success or failure. Blocks on the left (a) are predicted to have a successful braze, defined by a vacuum-tight seal, while blocks on the right (b) are predicted to fail. deformation and maintain a seal, as long as the deformation is congruent between the two blocks. In case (ii) a linear deformation is shown, while case (iii) shows a more complex change. While such deformations may have impacts on other aspects of the design, the braze should still form successfully. By contrast, examples (iv, v) show predicted braze failures. Case (iv) may be the most common, with overall surface roughness greater than The braze material may not be able to compensate for such changes in surface height, which could allow a direct path to the exterior of the fused blocks. Case (v) shows an identical deformation to case a, but with a different orientation. Here the two convex faces oppose each other, creating a wider gap. This gap may not be filled with the braze material and is a likely source of leaks. 3.3 Brazing Brazing is a high temperature process that uses a filler material to bond metals. The filler material is chosen to have a lower melting point that the bonding metals so that when heated only the filler material melts. Once cooled the filler material forms a bond between the other metals. In this prototype, two copper blocks were bonded through the use of a silver braze as a filler material. A schematic of the brazing 42

43 Figure 3-3: The material layers during a brazing operation are presented. The outside steel blocks provide structural integrity and allow for bolts to hold the system together while Belleville washers act as springs to provide a compressive force. The graphite layer provides a boundary between the steel and the target copper blocks. This boundary prevents the copper from bonding to the steel and allows for easier removal of the device from the assembly. The thin silver sheet is placed between the copper blocks so that once heated the molten silver will wet along the copper-copper contact area forming a bond as it cools. assembly is shown in Figure 3-3. Before the braze assembly can be assembled, components must be cleaned to prevent contamination. The high temperature of the furnace has the potential to vaporize many organic contaminates that may affect the braze or the furnace itself. Aside from the graphite, each component was washed in acetone and then ethanol. Acetone is an effective solvent for many contaminates but may leave a residue on the component. The sequential cleaning with ethanol removes this residue and any excess fluid is dried. Once the braze assembly was clean and assembled it was loaded into a tube furnace. To prevent oxidation, a protective atmosphere was used. The assembly was left undisturbed for at least two hours in a stream of 4 liters per minute of nitrogen gas. The nitrogen stream was maintained until the tube temperature exceeded 200 C, at which point it was replaced with a forming gas consisting of 95% nitrogen, 5% hydrogen. The forming gas further inhibits growth of oxide on the copper, which has the potential to interfere with the braze. Figure 3-4 shows the approximate temperature profile over time. 43

44 Figure 3-4: A representative temperature profile during the heating cycle. The assembly is initially heated at a rate of 600 C/hr until it reaches 1000 C, a temperature between the melting points of silver (962 C) and copper (1085 C). This temperature is held for 18 minutes to ensure the entire assembly reaches a uniform temperature. Finally the assembly shows an exponential temperature decay as it cools. Figure 3-5: A 1 NPT to 1 compression fitting is placed next to a sample device (a) The device has been tapped to allow for the male NPT fitting to connection (b). In this example a short piece of plastic tubing has been added to the compression fitting. The plastic allows for positively pressuring the internal channel with a syringe. 3.4 Tapping After brazing, the two half units become a single piece with both the coolant inlet and outlet and refrigerant inlet on the short sides of the device. Connectors are mounted directly to the device to facilitate attaching sensors and tubing. To assist with general compatibility, standard connections were used with either a 1 or 1 male NPT thread 16 8 connection to a 1 compression fitting. Figure 3-5 shows a commercially-available 4 fitting and how it connects to the device. During operation the evaporator will be placed into a low-pressure vacuum environment. Performance of the evaporator is 44

45 dependent on the ability to form vacuum-tight seals at each connection. Compression fittings use a deformable ferule to ensure the connection can withstand the pressure difference. Alternative fittings may utilize rubber or polymeric O-rings to form a similar seal. In this particular application, the device may be heated to an excess of 250 C during the regeneration state. This high-temperature operation may degrade the plastics and could compromise the seal. NPT threaded fittings are an industry-standard and have an acceptable sealing capability. By using entirely metal components the fitting can withstand a higher operating temperature without degradation. Metal threaded connections have the added benefit of being either removable or reinforced with solder. When the connection is first installed it is simply screwed in, and can be removed just as easily without damaging the part. However, since the connector is designed to be permanently fixed, it may be beneficial to use solder to reinforce the connection. A silver solder could easily surround the fitting and copper block and may help ensure the vacuum-tight seal. In order to accurately position the connector a 1 endmill was used to locate the 4 center of the channel and drill the initial tap hole. A standard NPT taps was used to tap threads into the device. By positioning the tap with a drill press the alignment of the tap was ensured throughout the entire process. 3.5 Sintering Sintering copper powder was a critical process in the assembly of the evaporator. To form a solid copper sinter, copper powder is placed into a graphite mold and heated in a protective atmosphere to prevent oxide formation. As the powder heats it fuses into a solid, porous block. By adjusting the sintering temperature and duration, or by using differently sized particles, the properties of the sinter can be controlled. The evaporator requires a porous medium to provide an additional pressure drop for the refrigerant, as well as containing liquid refrigerant while allowing vapor to diffuse. Previous work has characterized the permeability of a copper sinter for varying 45

46 Figure 3-6: [Reproduced from Espinosa [5]] The above plots show the linear shrinkage (left) and permeability (right) for copper particles in the range of 38-75um after sintering at 650 to 950 C for minutes. Shrinkage tends to increase linearly with sinter duration, while the permeability shows a more sporadic behavior at shorter durations. particle sizes, sinter temperatures, and sinter durations [5]. For this work the critical parameters were linear shrinkage and permeability; the relevant figures are included below. As discussed in Section 2.3, the desired parameters for the copper sinter are a thickness of 3mm and a permeability of 2.7 * m 2. Following the chart to the left of Figure 3-5, the sinter should be heated to 750 C and held at that temperature for 15 minutes. This corresponds to a linear shrinkage of approximately 2%. This increase can be accounted for by increasing the dimensions of the graphite mold. 3.6 Bonding The final step of assembly is to attach the sintered porous medium to the base of the evaporator unit. While it would be desirable to simply sinter directly to the evap- 46

47 Figure 3-7: The potential for copper powder to intrude into the evaporator channels during sintering is demonstrated. While the device may initially rest on top of a layer of powder (a), if the device settles or sinks into the copper powder, the displacement would likely rise into the channel (b). This rise would decrease the available crosssectional area of the channel and could interfere with the ability to form a continuous, stable copper sinter. orator itself, this presents several difficulties. The principle challenge is performing the sinter without obstructing the channels on the exterior faces of the evaporator. Simply pouring sinter onto the face of the device would fill the channels with copper powder. This would then sinter when heated and impinge upon the normally clear channels. While material could be added to fill the channels, such as graphite powder, it may be impractical to remove graphite powder after the sinter is formed. Any graphite remaining in the channel may obstruct flow into the sinter and could negatively impact performance. One potential alternative is to perform the sinter upside-down with the evaporator resting on top of a bed of copper powder. While this may not initially fill the channels with copper powder, as the device settles in the furnace there is the possibility that it will sink into the powder. This could result in copper sinter intruding into the channel and a loss of control of channel depth, as shown in Figure 3-7. One difficulty with sintering directly to the evaporator is the physical shrinkage the sinter undergoes. As the copper powder compresses to form a single, solid block it shrinks. While the shrinkage may only be on the order of 2%, the shifting can cause the sinter to dislodge from whatever it is resting on. Figure 3-8 shows a small sample immediately after sintering. Another consideration is the construction of a copper sinter for the full-scale 80cm device. In order to achieve a consistent permeability the entire sinter 47

48 Figure 3-8: A copper sinter sample is seated in a graphite mold. Due to shrinkage during the sintering process there is a consistent gap between the sinter and mold. The mold cavity is inches across, while the sintered block has reduced to inches a reduction of 2.2%. Figure 3-9: Potential sinter panels are shown along a full-scale evaporator model. While minimizing the number of individual panels and the number of sealant lines, it is important to consider the manufacturability of these sinter panels. As the panel length increases it becomes more likely that their properties are not uniform throughout. must be held at a designated temperature for a prescribed amount of time. Given that many furnaces may have a significant temperature gradient, it may be challenging to fully control a larger scale sintering process. To address these issues, the copper sinter was made in smaller blocks and each of the individual blocks was attached to the device using a thermal adhesive. Smaller blocks are more easily manufactured and replaced if damaged. By having a smaller length it is easier to create uniform properties across the entire block. However, the introduction of multiple discrete sinter blocks creates a number of seams along the length of the device, as shown in Figure 3-9. These seams will be sealed with a thermal adhesive, but will require additional testing to make sure they do not provide a less resistive path for the refrigerant. If 48

49 the refrigerant can simply flow through the seams then the porous medium may not create a sufficient pressure drop. This could result in the refrigerant evaporating at a higher pressure, and therefore higher temperature, decreasing available heat transfer. Alternatively, leaking across the seam may permit liquid refrigerant to flood out of the porous medium. Escaping liquid will not have evaporated and would greatly impact the performance of the device. An additional consideration is the thermal resistance of thermal adhesive. The porous medium is heated primarily through conduction from the evaporator. Since heat transfer will need to go through the thermal adhesive it is important to choose an adhesive with a high thermal conductivity. In addition, care should be exercised to ensure the amount of adhesive is well-balanced. Too much adhesive may increase the thermal resistance and may also obstruct part of the copper sinter. Alternatively, too little adhesive could result in a high interfacial resistance between the sinter and evaporator because of poor contact. A summary of the potential effects is shown in Figure Over or under-application of thermal adhesive could cause significant changes in temperature for the same heat flux. In steady-state conditions the difference in temperature between two points is directly proportional to the heat flux and thermal resistance between these points. If one considers points directly on either side of the thermal adhesive, the temperature difference can be described using, T = QR thermal (3.1) R thermal = L ka (3.2) where L is the length, k is the thermal conductivity, and A is the interfacial area. In considering just a first approximation, one might consider the length to the be solely caused by the roughness of the sintered copper layer. This roughness is likely on the same order as the particle size used in the copper powder, around 40 microns. The area in question is the entire contact area, which is roughly the area of the evaporator excluding the area of the channels, 0.008m 2. From Equations 3.1 and 3.2, 49

50 Figure 3-10: The potential effects of misapplication of thermal adhesive are presented. The correct application (a) shows a uniform, thin layer of thermal adhesive between the evaporator and the copper sinter. This layer allows for minimal thermal resistance while maintaining integrity. Over-application of thermal adhesive (b) could result in a thicker layer between the evaporator and the copper sinter. The added length would add to the overall thermal resistance. In addition, the thermal adhesive may extend outward and block pores on the copper sinter. Finally, under-application (c) could result in poor contact between the two surfaces. This could increase the interfacial resistance and could dramatically increase the thermal resistance between the evaporator and the copper sinter. 50

51 an adhesive with a thermal conductivity of 5 W mk difference of 2.5 C. [7] could experience a temperature By contrast, if thermal adhesive were lacking, the area would likely form a gap, which would fill with vapor. Water vapor has a considerably lower thermal conductivity, on the order of 0.02 W. Using a similar approach, the temperature difference mk could theoretically reach 625 C. This would indicate that the sinter is in thermal isolation from the evaporator and there is still a constant heat flow provided from underneath. This is clearly a non-physical answer, and instead suggests that either the overall heat transfer would be limited (reducing performance), or heat would find an alternate path into the evaporator. Heat would likely convect through the refrigerant, although there may still be reductions in performance. The presence of an air gap has significantly more impact than a thick layer of thermal adhesive. However, excessive thermal adhesive may interfere with the ability of the water to enter the porous medium. This could in turn cause a mass transfer limitation, which would likely also decrease the performance of the device. Finally, this first approximation neglects the effects of convection, and further study would be needed to properly assess convection in the narrow gap between the evaporator body and the copper sinter. 3.7 Final Assembly The results of the above fabrication processes are shown in Figures 3-11 and Perhaps most notable is the yellow discoloration of the copper evaporator, which may be the result of minor contamination during the bonding stage. Despite the change in appearance, the mechanical integrity of the device does not appear to have been compromised. This discoloration appears only on the exterior faces and may be removed through abrasive sanding. 51

52 Figure 3-11: A copper sinter block is shown above the evaporator surface. Shown without thermal adhesive, a small gap can be seen between the sinter and the exterior surface. The sinter remains clear of the Inlet Channel to allow for the installation of a non-permeable membrane. Figure 3-12: The final assembly is shown prior to the installation of the non-permeable membrane inlet connections. Once installed the device will be ready for further testing and performance characterization. 52

53 Chapter 4 Conclusions Heating and cooling the cabins of electric vehicles consumes a significant portion of battery power, greatly reducing potential driving range. The absence of a conventional internal combustion engine places a greater premium on electric power and energy efficiency. To address these concerns novel solutions have been proposed. A prototype phase change heat exchanger and evaporator has been designed and fabricated as part of one such design. Design rationale and lessons learned through fabrication are shared, and recommendations for further scaling and manufacture are provided. The proposed Advanced Thermo-Adsorptive Battery requires several well-engineered components, including an efficient phase change heat exchanger. While this technology may be implemented in other scales and applications, its integration into the broader system provides a unique solution to energy loss within electric vehicles. Multiple fluid dynamic, heat transfer, and material problems were addressed and innovated in the creation of an evaporator prototype. 4.1 Future Work The work completed here includes the design of a full-scale phase-change heat exchanger and the design and fabrication of a quarter-length unit. The most immediate future work may be the full characterization of the heat transfer performance of the quarter-scale unit. Full characterization may reveal unanticipated results and could 53

54 Figure 4-1: A linear combination of four quarter-scale units is shown. Red lines indicate the boundary of each original quarter-scale unit, while blue lines indicate the boundary of each sinter block. Shading has been used to identify alternate blocks. By staggering the location of the sinter blocks, there is no boundary continuous through the entire device. This is likely increase stability as it removes a single point of failure. lead to full design and engineering constraints. Following this characterization an extension to a full-scale prototype may be in order. Many of the design and fabrication techniques provided for the quarter-scale prototype will likely be applicable to the full-scale unit. However, a modular approach may be required for the unit to achieve its full length. Just as the copper sinter in the quarter-scale prototype was formed in individual blocks and later attached, it may be necessary to use a similar technique to combine smaller evaporator units. In such an expansion, the full-scale evaporator could be formed through the linear combination of four quarter-scale units. By bonding additional units to the end of the last, the overall length could be increased to the desired amount. Such an implementation would require a minor restructuring of just the refrigerant Inlet Channel, as there would only be one refrigerant inlet on the end of the assembled device. Copper sinter panels are staggered such that their seams do not align with the divides between evaporator units. The overlap may provide structural support as it becomes less likely the entire unit will break at a common joint. In addition, the overlap may reduce the likelihood of a complete leak through the entire system. 54