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 2014. 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
2
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 ThermoAdsorptive 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
4
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
6
Contents
1 Motivation
19
1.1
Advanced Thermo-Adsorptive Battery . . . . . . . . . . . . . . . . .
22
1.2
Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
1.3
Phase change heat exchangers . . . . . . . . . . . . . . . . . . . . . .
23
1.4
Pressure drop through porous media . . . . . . . . . . . . . . . . . .
24
1.4.1
Copper sintering . . . . . . . . . . . . . . . . . . . . . . . . .
25
Thesis overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
1.5
2 Physical System Overview
27
2.1
Coolant Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.2
Evaporator Design . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.3
Porous Medium Design . . . . . . . . . . . . . . . . . . . . . . . . . .
37
3 Fabrication
39
3.1
Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.2
Flattening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3.3
Brazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
3.4
Tapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
3.5
Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
3.6
Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
3.7
Final Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
4 Conclusions
53
7
4.1
Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A G-Code for CNC Milling
53
55
8
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. . . . . . . . . . .
20
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 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. . . . . . . . . . . . . . . . . .
9
21
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. . . . . . . . . . . . . . . . . . . . . . . . . . .
22
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. . . . . . . . . .
23
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
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 . . . . . . . . . . . . . . . . . . . . . . . .
10
25
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
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 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. . . . . . . . . . . . . . . . .
29
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. . . . . . . . . . . . . . . . . . . . . . . . . .
30
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 0.02m2 . 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
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). . . . . . . . .
11
33
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. . . . . . . . . . . . . . . . . . . . . . . . .
34
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. . . . . . . . . . .
35
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) . . . . . . . . . . . . . . . . . . . . . .
36
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 . . . . . . . . . . . . . . . . . .
37
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 38-75 microns.
The red shaded region shows the permeability range for sinter made
from a powder of less than 10 microns. . . . . . . . . . . . . . . . . .
12
38
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
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. .
42
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. . . . . .
43
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.
3-5 A
1
”
16
. . . . . . . . . . . . .
44
NPT to 14 ” 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. . . . . . . . . . . . . .
13
44
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 0-180 minutes. Shrinkage
tends to increase linearly with sinter duration, while the permeability
shows a more sporadic behavior at shorter durations. . . . . . . . . .
46
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 cross-sectional area of
the channel and could interfere with the ability to form a continuous,
stable copper sinter. . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
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 0.445 inches across, while the sintered
block has reduced to 0.435 inches – a reduction of 2.2%.
. . . . . . .
48
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. . . . . . . . . . . . . . .
14
48
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
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. . . . . . .
52
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
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.
15
. . .
54
16
List of Tables
2.1
Dimensions and Pressure Drops for Critical Features . . . . . . . . .
37
3.1
Milling Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
17
18
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 EVCabin. 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
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
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
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
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
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 2.2.
1.4
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
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.
1.4.1
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 10−12 m2 . A review of
literature finds that typical copper foams may have a permeability within the range of
10−7 to 10−9 m2 [4]. By contrast, copper sinters may have a permeability within the
range of 10−11 to 10−13 m2 [5]. The required permeability for the reliable operation
25
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
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
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 (𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡 )
(2.1)
where 𝑄 is the total heat transfer, 𝑚
˙ is the coolant mass flow rate, 𝑐𝑝 refers to the
𝐽
), and 𝑇𝑖𝑛 and 𝑇𝑜𝑢𝑡 refer to the bulk coolant temspecific heat of the mixture (3988 𝑘𝑔𝐾
28
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∘ ,
for the coolant mixture.
Equation 2.1 provides a required mass flow rate of 0.031 𝑘𝑔
𝑠
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
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
𝐷ℎ =
4𝑤(2𝑑)
4𝐴
=
𝑃
2(𝑤 + 2𝑑)
(2.2)
The Reynolds and Prandtl numbers were calculated as
𝑅𝑒 =
4𝑚
˙
2𝜇(2𝑑 + 𝑤)
(2.3)
𝑐𝑝 𝜇
𝑘
(2.4)
𝑃𝑟 =
where 𝜇 and 𝑘 are the viscosity and thermal conductivity of the mixture, respectively.
Through this the hydraulic diameter was found to be 0.0033m, 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
to the equivalent laminar case. The following correlations were used to determine the
Darcy-Weisbach friction factor (𝑓𝐷 ) and Nusselt number (𝑁 𝑢),
𝑓𝐷 = (0.79𝑙𝑛(𝑅𝑒) − 1.64)−2
𝑁 𝑢 𝐷ℎ =
( 𝑓8 )(𝑅𝑒𝐷ℎ − 1000)𝑃 𝑟
1
2
(1 + 12.7( 𝑓8 ) 2 (𝑃 𝑟 3 − 1)
(2.5)
(2.6)
This provided a friction factor of 0.0427 and a corresponding Nusselt number of
38.8. From the Darcy-Weisbach equation, the pressure drop in a channel can be
calculated as,
∆𝑃 = 𝑓𝐷
𝐿 𝜌𝑉 2
𝐷ℎ 2
(2.7)
where 𝑓𝐷 is the Darcy Friction Factor, L is the length of the channel, 𝐷ℎ is the
hydraulic diameter of the channel, is the density of the fluid, and 𝑉 is the fluid
velocity.
From the following basic definition of the Nusselt number,
𝑁𝑢 =
ℎ𝐷ℎ
𝑘
(2.8)
the heat transfer coefficient (ℎ) was determined to be 6374 𝑚𝑊2 𝐾 . However, the contact
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
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 0.02m2 . 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
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 0.02m2 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 0.11m2 . 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,
𝑃𝑝𝑢𝑚𝑝 =
∆𝑃 𝑚
˙
𝜌
(2.10)
While the increased channel length increases the overall pressure drop, the pumping
33
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
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
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 (ℎ𝑓 𝑔 ),
𝑄˙ = 𝑚˙ 𝑟 ℎ𝑓 𝑔
(2.11)
By constraining the refrigerant to be water, the required mass flow can easily
. Once the mass flow is known, iteration can be
be calculated to roughly 0.001 𝑘𝑔
𝑠
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,
𝑓𝐷𝑙 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 2-1.
36
Table 2.1: Dimensions and Pressure Drops for Critical Features
Inlet Channel Main Artery Distribution Channels
Length (mm)
680
780
51.5
Width (mm)
2
5
2.5
Depth (mm)
1
2
2
Pressure Drop (Pa)
6147
154
1
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
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 38-75 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
𝐿
∆𝑃 = 6.28 * 10−8 ( )
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 * 10−12 m2
38
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
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
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)
1.5
6000
0.508 (0.020)
2.5 (0.0984)
1.5
6000
0.838 (0.033)
5 (0.1969)
1.5
6000
0.762 (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
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 0.002”. 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
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
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.
1
” NPT to 14 ” compression fitting is placed next to a sample device (a).
Figure 3-5: A 16
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
”
16
or 18 ” male NPT thread
connection to a 14 ” compression fitting. Figure 3-5 shows a commercially-available
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
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 41 ” endmill was used to locate the
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
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 0-180 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 * 10−12 𝑚2 . 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 evap46
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
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 0.445 inches across, while the sintered block has reduced to 0.435
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
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 3-10.
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 = 𝑄𝑅𝑡ℎ𝑒𝑟𝑚𝑎𝑙
𝑅𝑡ℎ𝑒𝑟𝑚𝑎𝑙 =
𝐿
𝑘𝐴
(3.1)
(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, 0.008m2 . From Equations 3.1 and 3.2,
49
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
an adhesive with a thermal conductivity of 5
𝑊
𝑚𝐾
could experience a temperature
difference of 2.5∘ C. [7]
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
.
𝑚𝐾
Using a similar approach, the temperature difference
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 3-12.
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
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
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
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
Appendix A
G-Code for CNC Milling
Enclosed is the G-Code used to generate the milled patterns for the coolant and
refrigerant flow paths.
%
O16004
(Code for serpentine channels on coolant flow path)
(2.5MM ENDMILL)
(origin at upper left corner of piece)
#2 = -0.0324
(z-coord of the tool)
#3 = 2.3623
(length of one section, 60.0032mm = 2.3623")
#4 = 0.1181
(rounding radius, 3.0mm = 0.1181")
#5 = 0.3937
(length of entrance, 10mm = 0.3937")
#6 = -0.0984 (depth of channel, 2.5mm = 0.0984")
#7 = 1.5
(feed rate)
#8 = 1
(section count)
T1 M06
G54
G94
M03 S6000
WHILE [ #2 GE #6 ] DO2
G90
G00 Z1.0
G00 X-0.1 Y-0.2756
G00 Z0.2
G01 Z#2 F#7
G01 X#5 F#7
M97 P1000
G01 X0.5 F#7
#2 = #2 - 0.033
55
END2
G00 Z2.0
M05
M02
N1000
#8 = 1
WHILE [ #8 LE 15 ] DO3
G91
G02 R#4 X#4 Y-#4 F#7
G01 Y-#3 F#7
G03 R#4 X#4 Y-#4 F#7
G03 R#4 X#4 Y#4 F#7
G01 Y#3 F#7
G02 R#4 X#4 Y#4 F#7
#8 = #8 + 1
END3
M99
%
56
%
O16005
(Code for Main Artery channels on evaporator)
(5.0MM ENDMILL)
(origin at upper left corner of piece)
#4 = 0.3346
(x-coord of 1st cut-in point, 8.5mm = 0.3346")
#5 = 0.6890
(x-coord of 2nd cut-in point, 17.5mm = 0.6890")
#6 = -0.5906 (y-coord of both cut-in points, 15.0mm = 0.5906")
#7 = -0.0187 (z-coord of the tool)
#8 = -0.0787 (depth of channel, 2.0mm = 0.0787")
#9 = 0.5
(feed rate)
G94
G90
G54
G00 Z1.0
M03 S6000
G00 X#4 Y#6
G00 Z0.2
WHILE [ #7 GE #8 ] DO2
G90
G01 Z#7 F#9
G91
G01 X6.8504 F#9
G01 Y-2.2244 F#9
G01 X-6.8504 F#9
G01 Y2.2244 F#9
#7 = #7 - 0.03
END2
G90
G00 Z3.0
M05
M02
%
57
%
O16006
(Code for Distribution Channels on evaporator)
(2.0MM ENDMILL)
(origin at upper left corner of piece)
#2 = 0.7677
(x-coord of 1st cut-in point, 19.5mm = 0.7677")
#3 = 1.1220
(x-coord of 2nd cut-in point, 28.5mm = 1.1220")
#4 = 0.3740
(spacing between sections, 9.5mm = 0.3740")
#5 = 2.2244
(length of one section, 56.5mm = 2.2244")
#6 = -0.0187 (z-coord of the tool)
#7 = -0.0787 (depth of channel, 2.0mm = 0.0787")
#8 = 0.5
(feed rate)
#9 = 1
(section count)
G94
G90
G54
G00 Z1.0
M03 S6000
WHILE [ #6 GE #7 ] DO2
G90
G00 X#2 Y-0.5906
G00 Z0.2
G01 Z#6 F#8
M97 P1000
G00 Z1.0
#6 = #6 - 0.03
END2
G90
G00 Z2.0
M05
M02
N1000
#9 = 1
WHILE [ #9 LE 8 ] DO3
G91
G01 Y-#5 F#8
G01 X#4 F#8
G01 Y#5 F#8
G01 X#4 F#8
#9 = #9 + 1
END3
G01 Y-#5 F#8
M99
%
58
%
O16007
(Code for Inlet Channel on evaporator)
(2.0MM ENDMILL)
(origin at upper left corner of piece)
#2 = 3.7598
(x-coord of cut-in point, 95.5mm = 3.7598")
#3 = -0.5807 (y-coord of cut-in point, 12.75mm+2mm = 0.5807")
#6 = -0.0200 (z-coord of the tool)
#7 = -0.0394 (depth of channel, 1.0mm = 0.0394")
#8 = 1.5
(feed rate)
G94
G90
G54
G00 Z1.0
M03 S6000
WHILE [ #6 GE #7 ] DO2
G90
G00 X#2 Y#3
G00 Z0.2
G01 Z#6 F#8
G91
G01 Y0.1870 F#8
G01 X-2.3425 F#8
G01 Y0.1575 F#8
G01 X6.1417 F#8
G01 Y-1.3386 F#8
G90
G00 Z1.0
#6 = #6 - 0.0194
END2
G90
G00 Z2.0
M05
M02
%
59
60
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