U.S. patent application number 14/065696 was filed with the patent office on 2015-02-05 for regenerative electrocaloric cooling device.
This patent application is currently assigned to NASCENT DEVICES LLC. The applicant listed for this patent is NASCENT DEVICES LLC, THE PENN STATE RESEARCH FOUNDATION. Invention is credited to Ailan Cheng, Qiming Zhang.
Application Number | 20150033762 14/065696 |
Document ID | / |
Family ID | 52426408 |
Filed Date | 2015-02-05 |
United States Patent
Application |
20150033762 |
Kind Code |
A1 |
Cheng; Ailan ; et
al. |
February 5, 2015 |
REGENERATIVE ELECTROCALORIC COOLING DEVICE
Abstract
A regenerative electrocaloric (EC) device is provided. The
regenerative EC device uses a special configuration to expand the
temperature span T.sub.h-T.sub.c, thereby increasing the cooling
power and improving the efficiency thereof. The EC regenerative
cooling device includes two electrocaloric effect (ECE)
elements/rings in direct thermal contact with each other. The two
rings rotate in opposite directions and are divided into multiple
sections with an electric field or electric fields applied to every
other region/section and an electric field or electric fields
removed from remaining sections.
Inventors: |
Cheng; Ailan; (State
College, PA) ; Zhang; Qiming; (State College,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NASCENT DEVICES LLC
THE PENN STATE RESEARCH FOUNDATION |
State College
University Park |
PA
PA |
US
US |
|
|
Assignee: |
NASCENT DEVICES LLC
State College
PA
THE PENN STATE RESEARCH FOUNDATION
University Park
PA
|
Family ID: |
52426408 |
Appl. No.: |
14/065696 |
Filed: |
October 29, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61860452 |
Jul 31, 2013 |
|
|
|
Current U.S.
Class: |
62/3.1 |
Current CPC
Class: |
F25B 2321/001 20130101;
Y02B 30/66 20130101; F25B 21/00 20130101; Y02B 30/00 20130101 |
Class at
Publication: |
62/3.1 |
International
Class: |
F25B 21/00 20060101
F25B021/00 |
Claims
1. A regenerative electrocaloric (EC) cooling device comprising: a
first EC ring having N spaced apart first ring high electric field
regions (HEFRs) and N spaced apart first ring low electric field
regions (LEFRs); a second EC ring having N spaced apart second ring
HEFRs and N spaced apart second ring LEFRs, N being equal to or
greater than 1; said first EC ring rotating in an opposite
direction to said second EC ring and in sliding contact therewith,
said N first ring HEFRs, N first ring LEFRs, N second ring HEFRs
and N second ring LEFRs fixed in space during rotation of said
first EC ring and said second EC ring; at least one hot end located
between a first ring HEFR and a first ring LEFR; at least one cold
end located between a second ring HEFR and a second ring LEFR; an
electrical power supply in communication with said first and second
EC rings and producing an electric field thereacross, said electric
field across said first and second EC rings reducing an entropy and
increasing a temperature thereof such that said first EC ring is at
a higher temperature than said second EC ring and vice versa; and a
fixed ring in thermal contact with at least one of said first EC
ring and said second EC ring, said fixed ring having N hot ends at
a temperature of T.sub.h and N cold ends at a temperature of
T.sub.c, said fixed ring also having 2N spaced apart regions of
heat exchange at said N hot ends and said N cold ends; each of said
heat exchange regions made from a high thermal conductivity
material with a thermal conductivity >50 W/mK and separated from
adjacent heat exchange regions by regions of a low thermal
conductivity material with a thermal conductivity <0.3 W/mK;
said heat exchange regions at said N cold ends absorbing heat from
an outside source of heat and said heat exchange regions at said N
hot ends transferring heat to a heat sink such that heat is pumped
from the outside heat source at T.sub.c to the heat sink at
T.sub.h.
2. The regenerative device of claim 1, wherein each of said N first
ring HEFRs is oppositely disposed from and in sliding contact with
each of said N second ring LEFRs and each of said N first ring
LEFRs is oppositely disposed from and in sliding contact with each
of said N second ring HEFRs during said rotating of said first EC
ring relative to said second EC ring.
3. The regenerative device of claim 2, wherein heat passes from
said N first ring HEFRs to said N second ring LEFRs and from said N
second HEFRs to said N first ring HEFRs.
4. The regenerative device of claim 3, wherein said first EC ring
and said second EC ring rotate relative to a fixed ring, said fixed
ring having N hot ends at a temperature of T.sub.h and N cold ends
at a temperature of T.sub.c, said fixed ring also having 2N spaced
apart regions of heat exchange (at T.sub.h and T.sub.c), each of
said heat exchange regions made from a high thermal conductivity
material (thermal conductivity >50 W/mK) and separated from
adjacent regions of heat exchange by regions of low thermal
conductivity material (thermal conductivity <0.3 W/mK). The heat
exchange regions at T.sub.c absorb heat from an outside source of
heat and the heat exchange regions at T.sub.h transfer heat to a
heat sink such that heat is pumped from an outside heat source at
T.sub.c to a heat sink at T.sub.h.
5. The regenerative EC device of claim 4, wherein said first EC
ring has a plurality of first ring EC segments and subsets of said
plurality of first ring EC segments are located within each of said
N first ring HEFRs and other subsets of said plurality of first
ring EC segments are located within each of said N first ring LEFRs
as said first EC ring rotates; and said second EC ring has a
plurality of second ring segments and subsets of said plurality of
second ring EC segments are located within each of said N second
ring HEFRs and other subsets of said plurality of second ring EC
segments are located within each of said N second ring LEFRs as
said second EC ring rotates.
6. The regenerative device of claim 5, wherein a ring segment of
each of said subsets of said plurality of first ring EC segments
travels from a first ring HEFR into the adjacent first ring LEFR,
causing a lowering of temperature to T.sub.c as a ring segment of
each of said other subsets of said plurality of first ring segments
travels from a first ring LEFR into the adjacent first ring HEFR,
causing a rising in temperature to T.sub.h.
7. The regenerative device of claim 6, further comprising a low
thermal conductivity (thermal conductivity <0.3 W/mK) divider
between adjacent first ring EC segments and between each adjacent
second ring EC segments.
8. The regenerative device of claim 7, further comprising a ring
rotation source operable to rotate said first EC ring and said
second EC ring opposite to each other, said ring rotation source
also operable to rotate said first EC ring relative to said second
EC ring at a constant angular speed or rotate said first EC ring
relative to said second EC ring at a non-constant angular
speed.
9. The regenerative device of claim 7, wherein said ring rotation
source has a first angular speed when said plurality of first ring
segments are not aligned with said plurality of second ring
segments and a second angular speed when said plurality of first
ring segments are aligned with said plurality of second ring
segments, said first angular speed greater than said second angular
speed.
10. The regenerative devices of 7, wherein each of said plurality
of first ring segments and each of said plurality of second ring
segments is covered by a single electrode, said electrical power
supply applying an electric field of magnitude |E.sub.H-E.sub.L|
across each ring segment via said single electrode.
11. The regenerative device of 7, wherein each of said plurality of
first ring segments and each of said plurality of second ring
segments is covered by a M electrodes, said electrical power supply
applying an electric field of magnitude |E.sub.H-E.sub.L| at
increments of |E.sub.H-E.sub.L|/M across each ring segment via said
M electrodes.
12. The regenerative device of claims of 7, further comprising a
plurality of first EC ring-second EC ring-fixed ring units stacked
together to form a large cooling device.
13. The regenerative device of claim 7, further comprising a first
small OD EC ring and a second small OD EC ring, said first and
second small EC rings located and occupying an inner space within
an inner diameter of said first and second EC rings.
14. The regenerative device of claim 7, further comprising a
plurality of first EC ring-second EC ring units stacked together to
form a large cooling device and a plurality of first small OD EC
ring and a second small OD EC ring units, said plurality of first
and second small EC rings units located and occupying inner spaces
within inner diameters of said plurality of first EC ring-second EC
ring units.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
Application No. 61/860,452 filed on Jul. 31, 2013, which is
incorporated herein in its entirety by reference.
FIELD OF THE INVENTION
[0002] The present invention is related to regenerative
electrocaloric cooling devices and, in particular, regenerative
electrocaloric cooling devices using rotating electrocaloric
material rings.
BACKGROUND OF THE INVENTION
[0003] Most conventional heat pumps, refrigerators, air
conditioning, and climate control devices achieve cooling through a
mechanical vapor compression cycle (VCC). Such systems are known to
suffer from low efficiency and air conditioning is a major
contributor to electric utility peak loads. Another related problem
with today's VCC cooling technology is the adverse environmental
impact of the refrigerant gases employed, which are strong
greenhouse gases. These factors necessitate a search for new
cooling technologies for air-conditioning, refrigeration, heat
pumps, and climate controlling devices that possess improved energy
efficiency, low cost and are environmentally friendly.
[0004] The electrocaloric effect (ECE) is a result of a direct
coupling between thermal properties (such as entropy and
temperature) and electric properties (such as electric field and
polarization) in an insulation dielectric material. In this type of
material, a change in the applied electric field induces a
corresponding change in polarization, which in turn causes a change
in the dipolar entropy S.sub.p as measured by the isothermal
entropy change .DELTA.S in the dielectrics. If the field change is
carried out in an adiabatic condition, the dielectric material will
experience an adiabatic temperature change .DELTA.T. Recently, a
large electrocaloric effect has been discovered and developed
(Xinyu Li, Xiao-shi Qian, S. G. Lu, Jiping Cheng, Zhao Fang and Q.
M. Zhang. Tunable Temperature Dependence of Electrocaloric Effect
in Ferroelectric Relaxor P(VDF-TrFE-CFE) Terpolymer. Appl. Phys.
Lett. 99, 052907 (2011); Xinyu Li, Xiao-shi Qian, Haiming Gu,
Xiangzhong Chen, S. G. Lu, Minren Lin, Fred Bateman, and Q. M.
Zhang. Giant electrocaloric effect in ferroelectric
poly(vinylidenefluoride-trifluoroethylene) in copolymers near a
first-order ferroelectric transition, Appl. Phys. Lett. 101
132903(2012)) in modified polar-fluoropolymers such as
poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)
(P(VDF-TrFE-CFE)) terpolymer and polymer blends. In the EC
polymers, a temperature change of .DELTA.T as large as
.about.28.degree. C. can be induced near room temperature under an
applied field change of 180 MV/m (see FIG. 1).
[0005] A key component of a cooling device is the transportation of
entropy from the cold end to the hot end. The objective is to
transport entropy from one temperature level to another temperature
level in a reversible manner. This requires a substance whose
entropy depends on properties other than temperature. In the
cooling devices of this invention, this substance is the
electrocaloric (EC) material, whose entropy and/or temperature can
be changed by external electric fields.
[0006] All steady state converters must be cyclic since the entropy
carrying substance is not consumed. FIG. 2 illustrates an ideal
cooling cycle (Carnot cycle) which consists of two adiabatic and
two isothermal processes. For the Carnot cycle, the heat absorbed
from the cold source is Q.sub.c=T.sub.c(S.sub.c-S.sub.h) and the
coefficient of performance, COP=Q.sub.c/W (where W is the total
external work in the cooling cycle) can be expressed as:
COP=T.sub.c(T.sub.h-T.sub.c) (1)
[0007] In the cooling cycles of FIG. 2 (and cooling devices based
on the principle of cooling cycles in FIG. 2), the maximum
temperature span T.sub.h-T.sub.c is limited by the adiabatic
temperature change .DELTA.T of the EC material. In order to
increase the temperature span and also to improve the efficiency,
regenerative processes and/or regenerators are often introduced in
practical cooling devices such as refrigerators, air conditioning,
and heat pumps, and climate controlling devices such as
dehumidifiers. For example, illustrated in FIG. 3 is an ideal EC
Ericsson cycle in which a regenerator spans temperature between
T.sub.h and T.sub.c while there is no change in the electric field
to the EC material as a refrigerant. T.sub.h-T.sub.c can be much
larger than .DELTA.T in the EC material induced by the field change
of E.sub.h-E.sub.l. Several ECE based refrigeration cycles have
also been investigated. One example illustrated in FIG. 4 (a) is a
regenerative EC cooling device in which a heat exchange fluid is
employed to transfer heat from the cold end load (absorb heat
Q.sub.c) to the hot end (eject heat Q.sub.h). In this cooling
device, there are two EC beds and one example of the applied
electric fields at the two EC beds is presented in the FIG. 4 (b).
As the electric field (voltage) in one EC bed (for example, ECE1)
is changed from low to high, and hence causes a temperature
increase in that EC bed, the electric field (voltage) in the
another EC bed (for example, ECE2) is changed from high to low,
causing a decrease in EC bed temperature, all due to the ECE in the
two EC beds. The heat exchange fluid in this case will flow
counter-clock-wise to absorb heat at the cold end and eject heat at
the hot end. In the other half cycle, the electric fields in the
two EC beds are reversed and the heat exchange fluid flows in the
clock-wise direction. In this cooling device, heat exchange fluid
has a bi-directional flow in each EC bed.
[0008] FIG. 5 illustrates the temperature profile at one EC bed at
different stages of the cooling cycle of such an active
electrocaloric regenerative refrigeration (AERR) cooling device as
shown in FIG. 4 when the device reaches a steady state operation
(maintaining a temperature difference between the hot end T.sub.H
and cold end T.sub.C). In step 1, application of a voltage raises
the EC bed temperature. In step 2, the heat exchange fluid is
pumped from x=0 to x=L, the EC bed length, through the EC bed and
heat is ejected at the heat sink (T.sub.H). In step 3, the removal
of the electric field causes cooling of the ECE bed (there is no
heat exchange fluid flow) and in the step 4, the heat exchange
fluid will be pumped in the opposite direction (from x=L to x=0)
and heat absorbed from the cold end (at TO. This process is
repeated during the cooling device operation. Two EC beds are
employed for the AERR device shown in FIG. 4. Each EC bed consists
of EC plates stacked in a parallel configuration with channels
(spaces) between EC plates for the bi-directional flow of heat
exchange fluid. The channel width is dictated by the thermal
diffusion length of the heat exchange fluid, which typically is
between 0.1 and 0.5 mm. However, such narrow channels between the
EC thin plates can cause high flow resistance which limits the
cooling power and reduces efficiency.
[0009] Moreover, a comparison of devices in FIG. 4 (and FIG. 5)
with the Ericsson cycle in FIG. 3 indicates that the regenerative
process in the device of FIG. 4 is not ideal since there is a
temperature change in the regenerator (also in the heat exchange
fluid) as the electric field is changed. More recently, a solid
state EC refrigerator was introduced and demonstrated, however, the
regenerator temperature of that cooling device still changes with
the applied field [Haiming Gu, Brent Craven, Xiaoshi Qian, Xinyu
Li, Ailan Cheng, S. C. Yao, Q. M. Zhang. Simulation of
electrocaloric refrigerator with high cooling-power density. Appl.
Phys. Lett. 102, 112901 (2013)]. In these regenerative
refrigerators, the electric field along the length of the
regenerator changes with time, which causes change of the
regenerator temperature. The bi-directional flow of the heat
exchange fluid in the cooling device of FIG. 4 is also not
convenient.
[0010] Even though the EC materials have great potential for
cooling devices with high cooling power and high cooling
efficiency, the current ECE cooling device designs are not
convenient for practical operation and cannot fully utilize the
superior performance of the EC materials. Therefore, new cooling
system design and now cooling control method are highly desirable
to achieve high cooling power and high efficiency.
SUMMARY OF THE INVENTION
[0011] A regenerative electrocaloric (EC) device is provided. The
regenerative EC device uses a special configuration to expand the
temperature span T.sub.h-T.sub.c, thereby increasing the cooling
power and improving the efficiency thereof. One embodiment of the
EC regenerative cooling device of the instant invention includes
two electrocaloric effect (ECE) elements/rings in direct thermal
contact with each other. The two rings rotate in opposite
directions and are divided into multiple sections with an electric
field or electric fields applied to every other region/section and
an electric field or electric fields removed from remaining
sections.
[0012] The regenerative EC device can include a first EC ring and a
second EC ring, the first EC ring rotating in an opposite direction
to the second EC ring and in sliding contact therewith. The EC
device has a hot end and a cold end, and an electrical power supply
(or supplies) in communication with the two EC rings. The
electrical power supply produces an electric field across a portion
of the first EC ring, the electric field reducing an entropy of the
subjected portion of the first EC ring. The reduced entropy results
in an increase in temperature of the selected portion of the first
EC ring such that the portion with the electric field thereacross
has a higher temperature than another portion of the EC ring which
does not have the electric field thereacross. The cold end of the
regenerative EC device absorbs heat from an outside source of heat
and the hot end of the EC device transfers or expels heat to a heat
sink. In addition, rotation of the first EC ring relative to the
second EC ring pumps heat from the cold end to the hot end via the
rotation and applied electrical field and thereby provides a
regenerative EC cooling device.
[0013] The first EC ring has a first ring high electric field
region (HEFR) and a first ring low electric field region (LEFR),
and the second EC ring has a second ring HEFR and a second ring
LEFR. The regenerative EC device is arranged and operates such that
the first ring HEFR is oppositely disposed from and in direct
sliding thermal contact with the second ring LEFR. Also, the first
ring LEFR is oppositely disposed from and in sliding contact with
the second ring HEFR. During operation of the regenerative EC
device, heat passes from the first ring HEFR to the second ring
LEFR and from the second ring HEFR to the first ring LEFR.
[0014] In some instances, the regenerative EC device includes a
frame and the first EC ring and the second EC ring rotate relative
to the frame. In such instances, the first ring HEFR, first ring
LEFR, second ring HEFR, and second ring LEFR are stationary
relative to the frame and the first EC ring and second EC ring have
portions that rotate into and out of their respective HEFR and
LEFR.
[0015] The first EC ring can have a plurality of first ring
segments, a first subset of the first ring segments located within
the first ring HEFR and a second subset of the first ring segments
located within the first ring LEFR. In addition, the second EC ring
can have a plurality of second ring segments, a first subset of the
second ring segments located within the second ring HEFR and a
second subset of the second ring segments located within the second
ring LEFR. Similarly as stated above, rotation of the first EC ring
and/or the second EC ring affords for a ring segment of the first
subset of the first ring segments to travel from the first ring
HEFR into the first ring LEFR as a ring segment of the second
subset of the first ring segment travels or passes from the first
ring LEFR into the first ring HEFR. The same can be true for the
second ring, that is as one ring segment of the second EC ring
passes or travels into the second ring HEFR, another ring segment
passes or enters into the second ring LEFR.
[0016] The segments of the first EC ring and/or second EC ring are
naturally made from an EC material and are divided from each other
with a low thermal conductivity divider. In this manner, heat
conduction within a given ring from T.sub.h to T.sub.c can be
reduced since the heat conduction is proportional to the thermal
conductivity of the material between T.sub.h and T.sub.c. Stated
differently, the low thermal conductivity dividers cause a low
thermal conductivity of the ring. Also, the overall thermal
conductivity of the ring between T.sub.h and T.sub.c should be less
than 0.3 W/mK and most plastics have thermal conductivity <0.3
W/mK and thus can be selected for the dividers.
[0017] A process for removing heat from a heat source is also
included, the process including providing a regenerative EC device
as disclosed herein and contacting the cold end of the EC device
with an object having heat and contacting the hot end of the
regenerative EC device with a heat sink. The cold end of the
regenerative EC device absorbs heat from the object, the EC device
pumps the heat to the hot end of the regenerative EC device, and
the hot end expels the heat to the heat sink as the first EC ring
rotates in an opposite direction to the second EC ring. In this
manner, a cooling device is provided to remove heat from heat
sources or objects such as electronic devices, engine components, a
room, cold chamber of a refrigerator and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Reference is made to the attached drawings, wherein elements
having the same reference numeral designations represent similar
elements throughout.
[0019] FIG. 1 is a graphical representation of an adiabatic
temperature change as a function of applied electric field for an
EC material such as a polymer;
[0020] FIG. 2 is a graphical representation of a thermodynamic
refrigeration cycle (Carnot cycle) based on the electrocaloric
effect, where S is entropy, T is temperature, E is applied electric
field, subscripts h and c refer to high and low end temperatures,
and the arrows in boxes near the labels `A` and `C` represent
disordered dipolar and ordered dipolar states of an EC
material;
[0021] FIG. 3 is a graphical representation of an ECE Ericsson
cooling cycle with: (a) electric field (E) versus temperature (T);
and (b) entropy (S) versus temperature (T);
[0022] FIG. 4 is: (a) schematic diagram of a regenerative EC
cooling device employing two EC beds with a heat exchange fluid;
and (b) an example of an electrical field pattern applied to the
two EC beds (ECE1 and ECE2) for the cooling device shown in 4(a)
and with heat exchange fluid in the device flowing in two opposite
directions and provided by the bidirectional fluid pump shown in
the figure;
[0023] FIG. 5 is a graphical representation of a cooling cycle and
temperature profile for one EC bed shown in FIG. 4(a) and with
steady state having been obtained by the EC bed;
[0024] FIG. 6 is: (a) a perspective view of an embodiment of the
present invention; and (b) a cross-sectional view of section B-B
shown in FIG. 6(a);
[0025] FIG. 7 is: (a) a temperature-entropy diagram for an ideal
cooling cycle for the cooling device shown in FIG. 6(a); and (b) a
temperature-entropy diagram for a non-ideal cooling cycle for the
cooling device shown in FIG. 6(a), where in the illustration,
E.sub.L=0 but for the general case, E.sub.L<E.sub.H and may not
be=0;
[0026] FIG. 8 is: (a) a schematic illustration of two EC rings
aligned with each other such that EC material segments and gaps of
low conductivity material between the EC material segments are
aligned with each other; (b) a schematic illustration of the
segments shown in FIG. 8(a) in misalignment; (c) an expanded view
of the two rings shown in FIG. 6; and (d) a schematic illustration
of position versus time for segments of the rings shown in FIGS.
8(a), (b), and (c);
[0027] FIG. 9 is: (a) a side cross-sectional view of the section
9A-9A shown in FIG. 6; (b) a side cross-sectional view of section
9B-9B shown in FIG. 6; and (c) a side cross-sectional view of
section 9C-9C shown in FIG. 6;
[0028] FIG. 10 is a schematic illustration of a regenerative EC
unit having a plurality of EC ring units (FIG. 9) stacked on top of
each other.
[0029] FIG. 11 is a schematic illustration of a regenerative EC
cooling device with more than two regions for applying an electric
field to a top ring and more than two regions for applying an
electric field to a bottom ring;
[0030] FIG. 12 is a schematic illustration of a regenerative EC
cooling device with coaxial rings;
[0031] FIG. 13 is: (a) a schematic illustration of an EC ring
having a plurality of EC material segments and each segment being
covered by a single electrode; and (b) a schematic illustration of
the EC ring shown in FIG. 12(a) with each EC material segment
covered by multiple smaller electrodes; and
DETAILED DESCRIPTION OF THE INVENTION
Definitions
[0032] The following terminology and definitions are used
throughout the specification and are provided here for clarity.
[0033] T.sub.h (T.sub.H): Temperature of the heat sink (hot end) of
the EC cooling device.
[0034] T.sub.c (T.sub.L): Temperature of the cold load (cold end)
of the EC cooling device.
[0035] E.sub.H (E.sub.h): The maximum electric field applied to a
cooling device of an embodiment of the present invention.
[0036] E.sub.L (or E.sub.L=0): The low electric field applied to an
EC cooling device of an embodiment of the present invention
(E.sub.H>E.sub.L, and in some cases, E.sub.L=0).
[0037] .DELTA.E=E.sub.H-E.sub.L is the change of electric
field.
[0038] EC ring: A ring shaped EC module of outer-diameter (OD),
inner diameter (ID) and thickness d, with multiple EC segments
separated by gaps consisting of low thermal conductivity (k<0.2
W/mK).
[0039] Regions with or without electric fields: Regions fixed in
the space. As the EC rings rotate, the EC segments will leave a
region without electric field and enter a region with electric
field and vice versa.
[0040] Thermal diffusion length: .delta.= {square root over
(2.alpha./.omega.)}, where co is the angular frequency and
.alpha.=k/(.rho.c) is the thermal diffusivity.
[0041] The heat exchange layer: consisting of high thermal
conductivity segments (such as Al, with thermal conductivity
>100 W/mK) and low thermal conductivity segments filling the
spaces between the high thermal conductivity segments. The high
thermal conductivity segments are in thermal contact with the EC
rings at T.sub.h and T.sub.c areas to exchange heat between the EC
rings and external heat sink and cold load,
[0042] Aligned position of the two EC rings: the EC segments and
gaps in one ring overlapped (aligned) with the EC segments and gaps
in the other ring, respectively.
[0043] The present invention provides a regenerative electrocaloric
(EC) cooling device. As such, the invention has use for
refrigeration or cooling of devices, objects, rooms (e.g. air
conditioning), etc.
[0044] The regenerative EC device includes two EC material rings
that rotate in opposite directions of each other. The two EC rings
are also in direct thermal contact with each other. In this manner,
heat flows between the two EC rings and affords for regeneration
thereof. In addition to the two EC rings, an electrical power
supply affords for applying an electrical field across at least a
portion of one of the EC rings. As such, the portion of the EC ring
that has the electrical field applied thereacross exhibits a
rearrangement of dipolar states, i.e. from a disordered dipolar
state to an ordered dipolar state. In addition, and with an
adiabatic system, the ordering of the dipolar states reduces the
entropy and increases the temperature of the material. Likewise,
the removal of the electric field from the portion of the EC
material results in a disordered dipolar state and an associated
increase in entropy and decrease in temperature.
[0045] With the electrical field applied across a portion of at
least one of the EC rings, in combination with rotation of the two
EC rings, the regenerative EC device has a cold end and a hot end.
In addition, the cold end absorbs heat from an object having heat,
and which is desired to be cooled, and the EC device pumps the heat
to the hot end where it is expelled to a heat sink.
[0046] It is appreciated that when a given EC ring portion has an
electric field applied thereacross, the corresponding and
oppositely disposed portion of the other ring does not have an
electric field applied thereacross. As stated above, the
temperature of the EC ring portion with the electric field applied
thereacross increases due to the ordering of the dipolar states and
reduction in entropy, and this portion has a higher temperature
than the corresponding and oppositely disposed EC ring portion
without the electric field. As such, heat is transferred from the
EC ring having the electric field applied thereacross to the
oppositely disposed EC ring that does not have the EC ring applied
thereto. In this manner, the cooling device is regenerated.
[0047] In some instances, a pair of oppositely disposed and
oppositely rotating EC rings are in thermal contact with each other
and each ring has a plurality of EC material segments separated by
gaps of low thermal conductivity material. Also, each of the rings
has a high electric field region and a low electric field region.
The high electric field region of one ring is oppositely disposed
and in thermal contact with a low electric field region of the
other ring. It is appreciated that a given ring can have more than
one high electric field region and more than one low electric field
region. In such cases, the corresponding and oppositely disposed
ring would likewise have more than one high electric field region
and low electric field region.
[0048] In order to apply the electric field across a portion of an
EC ring, electrodes are in contact therewith. In addition, each EC
material segment can have one electrode thereacross or, in the
alternative, have more than one electrode thereacross. Finally, a
regenerative EC device can have a plurality of EC rings stacked on
top of each other in order to increase the cooling efficiency
and/or cooling power of the device.
[0049] Turning now to the figures, one embodiment of an EC device
70 is schematically illustrated in FIG. 6 in which a top EC ring
700 and a bottom EC ring 705 are stacked together. In addition, the
top and bottom EC rings 700, 705 are thermally coupled to each
other along the z-direction (the direction perpendicular to the
plane of the ring). The two rings rotate about the z-axis but in
opposite directions along a central axis and/or shaft 810. Although
shown to rotate about the z-axis via an arm or brace, it is
appreciated that the invention includes rotation of the EC rings
700, 705 about the z-axis via any method, attachment device, etc.,
known to those skilled in the art. Also, it is appreciated that the
orientation of the z-axis shown in FIG. 6a is for illustrative
purposes only, i.e. the z-axis and the EC rings 700, 705 can be
oriented in any direction so long as the rings rotate opposite to
each other relative to a fixed space.
[0050] As illustratively shown in the FIG. 6a, the top EC ring 700
rotates clockwise 780 and the bottom ring 705 rotates
counter-clockwise 790. It is appreciated the terms "top", "bottom",
"clockwise" and "counter-clockwise" are used for descriptive
purposes only and that the orientation of the EC rings can be
sideways, inverted, etc. Also, the two EC rings can rotate in
opposite directions respective to what is shown in the figure.
[0051] The top EC ring 700 and bottom EC ring 705 are made from a
plurality of EC material segments 710 and 730, respectively. Also,
and in a first example of the invention, an electric field
(E=E.sub.H) is applied to half of the ECE segments 710 in top EC
ring 700 as indicated by the cross-hatching on six of the segments
710 on the top EC ring 700. Also, the other half of the EC ring
segments 710 have no field electric field applied thereto
(E=0).
[0052] The region under the electric field region for the top ring
700 is 180 degrees apart from the region under electric field for
the bottom ring 705. Stated differently, there is no electric field
(or lower electric field) in the EC segments 730 of the bottom ring
705 when the corresponding (right above) EC segments 710 of the top
ring 700 are under electric field (E.sub.H). Furthermore, as the
top ring 700 rotates clockwise 780, EC segments 710 near T.sub.h
(high temperature end) move from the region of no-electric-field
(E=0) or low field (E=E.sub.L) to the region of high-electric-field
E.sub.H. Such rotation causes an entropy reduction and heat
ejection from the EC segments that have "crossed-over" from the
no-electric-field or low-electric-field region to the
high-electric-field region. Likewise, as the bottom EC ring 705
rotates counter-clockwise 790, EC segments 730 near T.sub.h move
from the no-electric-field (E=0) or low-electric-field E.sub.L to
high field E.sub.H and eject heat.
[0053] At T.sub.c, the EC segments 710, 730 of the top ring 700 and
bottom ring 705, respectively, move from high-electric-field
regions to no-electric-field or low-electric-field regions, thereby
affording an entropy increase in the EC segments, a reduction in
temperature and heat absorption from the cold end. Therefore, as
the top ring rotates clockwise from T.sub.h to T.sub.c (along the
path T.sub.h-B-T.sub.c) in the E.sub.H region, the temperature of
the EC segments will decrease from T.sub.h to T.sub.c. At the same
time, the bottom ring rotates counter-clockwise from T.sub.c to
T.sub.h in the no-field (or low field) region (along the path
T.sub.c-B-T.sub.h), the temperature of EC segments will increase
from T.sub.c to T.sub.h. Through the heat exchange between the two
rings, a regenerative process occurs via heat flow from the EC
segments 710 in the top ring 700 to EC segments 730 in the bottom
ring 705 as indicated by the arrows pointing "up" and "down" along
the z-direction.
[0054] In the other half-rings, i.e. the half rings shown on the
right hand side of the figure, heat flows from the EC segments 730
in the bottom ring 705 to the EC segments 710 in the top ring 700.
FIG. 6b is an illustration of the EC rings 700, 705 rotating from
E=0 to E.sub.H regions and affording regenerative heat flow in the
EC cooling device.
[0055] In a steady state operation, as the EC segments 710 in the
top ring 700 enter the E.sub.H region, the ECE causes a temperature
increase, resulting in heat ejection to a heat sink (not shown) at
T.sub.h. Also, as the EC segments 710 in the top ring 700 rotate
clockwise 780 from T.sub.h towards T.sub.c, the EC elements 730 in
the bottom ring 705 rotate counterclockwise 790 from T.sub.c
towards T.sub.h. The heat transfer between the EC elements 710 from
the top ring 700 under high-electric-field to the EC elements 730
of the bottom ring 705 under no- or low-electric-field as indicated
by the vertically oriented arrows in FIG. 6 provides a heat
regenerative process similar to that illustrated FIG. 3 and FIG. 7
to be discussed below.
[0056] A similar process occurs in the other half of the rings with
the functions of top and bottom rings are reversed. In particular,
and referring to the ring halves shown on the right-hand-side of
FIG. 6a and the vertically oriented arrows in FIG. 6b, the
temperature of the EC segments 730 of bottom ring 705 will be
slightly higher than the EC segments 710 of the top ring 700. Thus,
the heat will flow from the bottom ring 705 to the top ring
700.
[0057] When an EC segment moves near T.sub.c, the electric field
for the EC segment is reduced from E.sub.H to E.sub.L, the entropy
of the segment increases, the temperature of the EC segment
decreases, and the EC segment absorbs heat at the cold end and
thereby affords cooling of heat source (not shown). In order for
the cooling device in FIG. 6 to function effectively, i.e. with
high cooling power and high efficiency, the thermal conductivity of
the EC segments 710, 730 along the z-direction should be as high as
possible so that the regenerative process between the two rings can
occur with a very small temperature gradient (much less than
1.degree. C.) along the z-direction, and within a very short time.
Such a high thermal conduction rate affords the two rings to rotate
at high speed and increase the cooling power of the EC device.
[0058] Looking now at FIG. 7, a pair of temperature-entropy
diagrams for the half of the top ring 700 under high-electric-field
(E.sub.H) and the half of the bottom ring 705 under no- or
low-electric-field (E.sub.L). It is appreciated that the other two
halves of the rings have the same temperature-entropy diagram,
except that the electric field is applied to opposite halves of the
respective rings. Also, direct thermal coupling between the two
rings affords for heat ejected from EC segments under the
high-electric-field to be absorbed by oppositely disposed EC
segments under the low-electric-field (a regenerative process).
[0059] Regarding the specific diagrams in FIG. 7, FIG. 7a is for an
ideal thermodynamic cycle, i.e. quasi-static with perfect heat
exchange. However, in a practical situation the EC cooling devices
are not operated at very low speed and a temperature difference
exists between the EC segments near the T.sub.h region and T.sub.c
region. Also, the temperature of the EC segments near the T.sub.h
region will be higher than the external heat sink and the
temperature of EC segments near the T.sub.c region will be lower
than the external cold load temperature. Both of these effects are
shown in the temperature-entropy diagram of FIG. 7b.
[0060] Although the two EC rings rotate in opposite directions, a
temperature distribution will not change with time once a steady
state condition is reached. The temperature gradient from T.sub.h
to T.sub.c along the plane of the rings, hereafter referred to as
the .phi.-direction, causes heat conduction from the T.sub.h end to
the T.sub.c end. Such heat conduction along the .phi.-direction is
a heat loss and lowers the cooling power and cooling device
efficiency. Therefore, it is important to reduce or eliminate the
thermal conduction along the .phi.-direction.
[0061] In order to reduce the thermal conductivity of the EC rings
along the .phi.-direction, the EC rings are divided into segments
as illustrated in FIG. 6 with neighboring EC segments 710, 730
separated by the gaps 720, 740, respectively. The narrow gaps 720,
740 between the neighboring EC segments 710, 730 are filled with
electrically insulating low thermal conductivity materials (or
epoxy) (k<0.2 W/mK). The electrically insulating low thermal
conductivity materials in gaps 720 and 740 can also be porous
materials. The cavity in the porous materials can be either air,
special gas such as SF6, or vacuum. It is appreciated that porous
materials typically have lower thermal conductivity than non-porous
materials.
[0062] From the above discussion, it is appreciated that the EC
rings of the cooling device should have a high thermal conductivity
along the z-direction and a low thermal conductivity along the
.phi.-direction. For the cooling device of FIG. 6, the EC segments
can be EC polymers or EC ceramics. In general, the thermal
conductivity k of an EC polymer is low and typically less than 0.3
W/mK. In contrast, EC ceramics usually have a higher thermal
conductivity (e.g., k>5 W/mK). Also, by embedding electrically
insulating high thermal conductivity fillers within EC polymers, EC
composites can be fabricated and the thermal conductivity of an EC
segment can be improved to greater than 1 W/mK.
[0063] The width of the gaps 720, 740 is generally small compared
to the overall width and/or length of the EC segments. For example,
the gaps 720, 740 can be less 5%, less than 10% or less than 15% of
the corresponding length of an EC segment. In addition, the width
of the gaps 720, 740 can determined by or be a function of a device
performance or cost parameter such as cooling power, temperature
span T.sub.h-T.sub.c, COP, manufacturing cost, etc. Finally an EC
ring structure is easily fabricated.
[0064] Typical dimensions of an EC ring be on the order of a 5 cm
diameter and a 0.2 mm thick. In addition, such a ring can be
fabricated into at least two segments using conventional
fabrication techniques. For example, the top EC ring 700 and bottom
EC ring 705 shown in FIG. 6 each have 12 EC segments with six of
the segments having a high-electric-field applied thereto and six
of the segments having no- or low-electric-field applied
thereto.
[0065] Not being bound by theory, the EC properties of EC rings can
be used to derive the performance of an EC device such as the one
illustrated in FIG. 6. Taking an EC polymer as the EC material, an
electric field change .DELTA.E=100 MV/m affords and entropy change
of .DELTA.S=0.081 J/cm.sup.3K and a heat absorbed from the cold end
of Q.sub.c=24.3 J/cm.sup.3. In the cooling cycle, the heat exchange
between the EC ring and external cold load occurs at the interface
within the thermal diffusion length, .delta.= {square root over
(2.alpha./.omega.)}, where .omega. is the angular frequency and
.alpha.=k/(.rho.c) is the thermal diffusivity. For EC polymer
composites, adding small amounts of high thermal conductivity
fillers can provide a thermal conductivity increase to greater than
0.5 W/mK, or even greater than 1 W/mK, without affecting the
ECE.
[0066] In addition, although the cooling device with two rings
rotating at a constant angular speed can work, a variable angular
velocity, such as a stepwise rotation, can be used to reduce
conduction heat loss between the hot and cold ends and thereby
improve the heat exchange and regenerative process between the two
rings along the z-direction. Naturally, improving heat exchange
between the two rings improves the cooling power and efficiency of
an EC device.
[0067] Referring to FIG. 8, as the two EC rings 700, 705 rotate in
opposite directions, the two rings can be in an aligned position as
illustrated in FIG. 8a or in an unaligned position as illustrated
in FIG. 8b. In the aligned position, the gaps 720 of the top ring
700 align with the gaps 740 of the bottom ring 705 and the EC
segments 710, 730 of the top and bottom rings 700, 705 completely
overlap each other. In contrast, the unaligned position results in
the gaps 720 in the top ring 700 to be in direct contact with EC
segments 730 of the bottom ring 705. It is appreciated that the
unaligned position results in higher thermal conductivity along the
0-direction since the EC segments 710, 730 thermally short circuit
the thermal conduction gaps 720, 740. Stated differently, the
thermal conduction path through the gaps 720, 740 is by-passed when
the EC rings are in the unaligned position.
[0068] Given the above, it is desirable that the time period which
the two EC rings 700, 705 are in an unaligned position be reduced
to as low or little as practically possible. As such, another
embodiment of the present invention rotates the two EC rings 700,
705 opposite to each other, but not at constant angular velocity.
Instead, drivers rotate the two EC rings 700, 705 such a transient
time (t.sub.trans) during which the two rings are unaligned with
each other is less than (FIG. 8b), and preferably mush less than, a
static time (t.sub.stat) during which the two rings are aligned
with each other (FIG. 8a). Preferably, t.sub.trans<0.3
t.sub.stat, and in some instances t.sub.trans<0.2 t.sub.stat,
and in other instances t.sub.trans<0.15 t.sub.stat, and in still
other instances t.sub.trans<0.05 t.sub.stat. It is appreciated
that during the static time, the two rings do not necessarily have
relative motion to each and are in thermal contact and exchanging
heat along the z-direction.
[0069] As mentioned above, step motors drive or move the two EC
rings such that the transient time (t.sub.trans) during which the
two rings are in unaligned position is much shorter than the
stationary time t.sub.stat, during which the two rings are in the
aligned position. In particular, FIG. 8d graphically illustrates
how an EC segment U1 of the top ring 700 shown in FIG. 8c moves and
overlaps with EC segments B1, B2, and B3 of the bottom ring 705 as
a function of time.
[0070] Although two EC rings of an EC device rotate in opposite
directions, a temperature profile of the device does not change
with time once a steady state condition is established. Such a
feature or aspect of the inventive EC device disclosed herein makes
easily affords for an external thermal load to exchange heat with
the EC segments at both the hot and cold ends. For example, direct
contact of an aluminum (Al) plate (k.sub.Al=205 W/mK at room
temperature) with EC rings affords heat exchange between the rings
and external thermal loads as schematically shown in FIG. 9a at
reference numeral 920. Because of the very high thermal
conductivity of Al, the thickness of Al plate here can be in 0.1 mm
or any thickness which is determined by the requirement of the
device. Also, the lateral area of the Al heat exchange plate in
contact with the rings can be the same as the area of one segment
of the EC ring.
[0071] It is another embodiment, heat exchange plate(s) are
stationary do not rotate with the EC rings. The heat exchange
plate(s) are in direct thermal contact with the EC rings at the hot
end and cold end in order to provide effective heat exchange
between the EC segments and cold load T.sub.c at the cold end and
between EC segments and heat sink at the hot end T.sub.h. The
remaining areas between the two high thermal conductivity heat
exchange plates can have a low thermal conductivity layer 930
thereon in order to prevent the heat conduction loss.
[0072] Friction due to rotation of the two rings 700, 705 in the EC
device 70 shown in FIG. 6 can cause damage to the EC segments and
can be reduced or alleviated by an optional thin protective layer
910. Examples of a thin protective layer includes, but is not
limited to, a 50 micron thick layer of stainless steel, nickel
alloy, copper alloy, cobalt, cobalt alloy, SiC, AlC, etc. The
contact surfaces of the protective layers should be smooth to
reduce friction and a thin lubricant layer (such as liquid or
grease) may or may not be present between the protective layers
910, thereby improving thermal contact and reducing friction
between two protective layers. The thin coupling layers 910 are
firmly attached to the EC rings 700, 705 and do not have
significant effect in causing heat loss between T.sub.h and
T.sub.c.
[0073] As stated above, the EC segments can be made from EC polymer
composites. Such ECE composites have an EC response of .DELTA.T=9 K
and Q.sub.c=24.3 J/cm.sup.3 under an electric field change of 100
MV/m at 300 K.
[0074] The EC segments in two EC rings can also be an EC ceramic,
e.g. Ba(Ti.sub.0.8Zr.sub.0.2)TiO.sub.3 and other EC ceramics with
high EC responses induced by a change of electric field. Such
ceramics can have an EC response of |.DELTA.T|>5 K and
Q.sub.c=15.4 J/cm.sup.3 under an electric field change of less than
20 MV/m at room temperature.
[0075] Such relatively high thermal conductivity EC ceramics (k=6
W/mK) make it possible to operate an EC device illustratively shown
in FIG. 6 at higher frequencies than EC devices that use EC
polymers. Also, for a ring of thickness=0.2 mm, the time for heat
to conduct completely through two EC rings along a z-direction is
about 0.11 seconds. Such a cooling device can achieve a cooling
power greater than 90 W/cm.sup.3 with a temperature span
T.sub.h-T.sub.c=20 K and a COP>8.5.
[0076] Taking into consideration the brittleness of a ceramic EC
segment, the thickness of a ceramic EC ring needs to be greater
than the thickness of a polymer EC ring, e.g. 0.4 mm for a ceramic
ring compared to 0.2 mm for a polymer ring. Such an increase in
thickness will reduce the lower limit of the cooling power to 23
W/cm.sup.3. However, it should be appreciated that this is still a
high cooling power. By reducing the lateral dimensions and
assembling the EC segment using polymer and epoxy to bond the small
ceramic sections of EC elements into large area EC cooling
elements, one can improve the fracture resistance of the ceramic EC
segments.
[0077] A plurality of two EC ring units or pairs can be stacked
together to form a bulk cooling device (or heat pump) as
illustrated in FIG. 10 where 3 EC units or pairs are stacked
together. It is appreciated that the number of EC units in the
cooling device can vary, e.g. from 1 to 5 units, 5 to 10 units, 10
to 15 units, 15 to 20 units, 20 to 30 units, 30 to 40 units, 40 to
60 units, 60 to 80 units, 80 to 100 units or more than 100. In this
manner, a plurality of first EC rings 700 and a plurality of second
EC rings 705 are stacked alternately on top of each other. Also,
each of the first EC rings 700 rotate in an opposite direction to
and are in sliding contact with a paired second EC ring 705.
[0078] A method, apparatus and/or embodiment for increasing the
cooling power of the cooling device of FIG. 6 is also provided.
Specifically, the way the electric field is applied is modified by
applying electric field to several smaller regions on a particular
EC segment instead of the enter EC region as illustrated in FIG. 6.
For example and for illustrative purposes only, an electric field
can be applied to two regions as illustrated in FIG. 11. The two
regions with an electric field and the two regions without an
electric field are arranged alternatively. Also, such a design can
increase, e.g. double, the cooling power of an EC device compared
with that in FIG. 6. Hence, the cooling power of the cooling device
is increased by increasing the number of regions with and without
an applied electric field.
[0079] The heat exchange of the device shown in FIG. 11 is
accommodated by increasing the number of the heat exchange plates
accordingly. For example, the cooling device of FIG. 6 has one
region with an applied electric field and one region without an
electric field. Thus, two high thermal conductivity plates are
used--one at T.sub.h end and one T.sub.c end--as shown in FIGS. 9b
and 9c. However, the cooling device shown in FIG. 11 has two
regions of high electric field and two regions without electric
field. Therefore, four high thermal conductivity plates are
used--one for each T.sub.h end and one for each T.sub.c end.
[0080] In general, the number of applied electric field regions
depends on the size of the device. The cooling device in FIG. 6 has
one pair of applied electric field regions while the example in
FIG. 11 illustrates two pairs of applied electric field regions. A
large size device can be divided into a large number of regions. It
also depends on the thermal conduction loss between the hot and
cold ends. As the number of regions for applying and removing
electric field increases, the distance between the hot and cold
ends will be reduced, which, in turn, will increase the heat
conduction loss due to the increased temperature gradient between
T.sub.h and T.sub.c in the device. Moreover, a large number of
regions for applying and removing electric field in the cooling
device may increase the design and manufacturing complication.
Hence, design of the cooling devices takes into account some or all
of the above mentioned factors into consideration.
[0081] In another embodiment, a method of increasing the cooling
power of an EC device is provided. In particular, if the rings of
the cooling devices of FIGS. 6, 8, and 11 have a large OD and ID, a
relatively large empty space inside the ID of rings will be
present. As such, the cooling power of the device is increased by
adding co-axial smaller rings (small OD EC rings) that utilize the
relatively large empty space as illustrated in FIG. 12. It is
appreciated that the device configuration illustrated in FIG. 12
can be used when a cooling device such as shown in FIG. 6 has a
large outer diameter (e.g., 50 cm) and a large inner diameter
(e.g., 30 cm).
[0082] By using the inner space, i.e. the space within the inner
diameter, with rings of smaller outer diameter, the total cooling
power per unit volume is increased. In such a cooling device
design, the large and small diameter rings can rotate at the same
angular velocity and the number of segments at each ring and number
of regions of applying and removing electric field are optimized to
achieve high performance in terms of the cooling power density,
coefficient of performance, and temperature span between T.sub.h
and T.sub.c. In the alternative, the small diameter rings do not
rotate at the same angular velocity and/or do not have the same
number of ring segments as the larger diameter outer rings.
[0083] In general, reducing the cooling power will increase the
temperature span T.sub.h-T.sub.c. In the adiabatic condition, i.e.,
there is no heat exchange between the EC devices with external load
and heat sinks at T.sub.h and T.sub.c, T.sub.h-T.sub.c then will be
totally determined by the thermal conduction heat loss through the
devices (from T.sub.h and T.sub.c) and by the operation temperature
range of EC material. For the current designs of the cooling
devices using the EC materials in consideration, a
T.sub.h-T.sub.c>40 K can be obtained.
[0084] In the cooling devices illustrated in the figures, an
electric field is applied to the EC segments and as EC rings rotate
into and out of the region of high electric field temperature
changes of the EC segments occur. In order to apply an electric
field to EC segments, the EC segments are coated with thin films of
an electrical conductor material in order to form or serve as an
electrode. Such electrodes can be made from a thin layer of Al or
gold (Au) have a typical thickness of less than 10 nm, less than 20
nm, less than 30 nm, less than 40 nm, less than 50 nm . . . less
than 1 micron.
[0085] Turning now to FIGS. 13a and 13b, two different electrode
patterns on a top EC ring 700 are shown. The bottom EC ring 705 has
a similar electrode pattern and is not shown. In FIG. 13a, each EC
segment 710 is covered by a single thin electrode U1 . . . U12 with
no electrodes one the gaps 730. It is appreciated that such an
electrode pattern an electric field change will occur at one EC
segment at a time since neighboring EC segments are separated by
the electrically insulating low thermal conducting gaps 730. For
example, as the top ring 700 rotates clockwise, the EC segment U1
will enter the region of high electrical field, then U2 will enter
the high electric field region, then U3, and so on. Analogously,
the EC segment U7 will leave the region of high electric field,
then U8, then U9, and so on.
[0086] However, each EC segment can be further divided into two or
more electric sections as illustrated in FIG. 13b. Correspondingly,
as each EC segment enters the high electric field, the electric
field of each EC segment is increased at a smaller increment when
compared to the design illustrated in FIG. 13a. For the case of
FIG. 13b, in which there are two electrode sections, the increment
of the electric field applied or removed is divided by 2, i.e. from
E.sub.H to 1/2 (E.sub.H-E.sub.L) or 1/2 E.sub.H when E.sub.L=0.
[0087] As the top ring rotates, the EC section U1a will enter the
field region 1/2 E.sub.H, and then E.sub.H, at this time, U1b will
enter 1/2 E.sub.H, etc. A smaller field increment, in this case,
1/2 E.sub.H, will improve the EC cooling device reliability and
also the efficiency. In general, it is preferred that the electrode
at each EC segment is divided into N sections, with N>1, and
correspondingly, the electric field increment should also become
E.sub.H/N when E.sub.L=0 or (E.sub.H-E.sub.L)/N when
E.sub.L.noteq.0. It is appreciated that the reduced electric field
increment as the EC segment enters the high electric field region
can improve the EC device performance since a large and sudden
escalation of electric field will increase the probability of
electric breakdown of the EC material.
[0088] In order to better embody the present invention but not
limit its scope in any way, examples are provided below.
Examples
[0089] Using a device performance model, an EC ring having an
anisotropic thermal conductivity is assumed. The thermal
conductivity along and perpendicular to the z-direction are k.sub.Z
and k.sub..phi., respectively. Also, by preparing EC polymers
differently, the k.sub.Z and k.sub..phi. can be varied. A base EC
polymers having a thermal conductivity k=0.2 W/mK was assumed and
the following two device parameters were used: Case (1):
k.sub.z=0.5 W/mK and k.sub..phi.=0.2 W/mK, and Case (2): k.sub.z=1
W/mK and k.sub..phi.=0.2 W/mK.
[0090] For simulation purposes, an EC cooling device as illustrated
in FIG. 6 was used, and the EC ring thickness was set at 0.2 mm,
the outer diameter was 5.5 cm and the inner diameter was 3 cm.
Given the EC ring thickness, a desirable thermal diffusion length
of 0.2 mm was calculated. Each EC ring had 12 segments that were
spaced apart from each other and joined together with a low thermal
conductivity polymer of k=0.2 W/mK.
[0091] The cooling device illustratively shown in FIG. 6 was
simulated to be subjected to a high electric field of 100 MV/m with
the EC segments having a specific heat c=1.5 J/gK and a density
.rho.=1.8 J/cm.sup.3. The results showed the EC device to exhibit a
temperature change of .DELTA.T=9 K, an entropy change of
.DELTA.S=0.081 J/cm.sup.3K, and heat absorption from the cold end
of Q.sub.c=24.3 J/cm.sup.3. Also, and during operation of the
cooling device of FIG. 6, when each segment undergoes an electric
field change of E=100 MV/m, a total heat Q.sub.c=8.11 J is
absorbed, assuming the heat in each segment has enough time to be
transported to the cold load via heat exchange. In Case (1), the
thermal diffusion time was 1.3 seconds and this the time (1(2f)
needed one EC segment to go through the change of electric field.
When the electric field changes from high electric field E.sub.H to
low electric field, one EC segment will absorb 8.11 J heat from the
cold load. In Case (2), the thermal diffusion time was 0.65
seconds.
[0092] In Case (1), and considering the fact that there are two
rings in the EC device, the cooling power at the cold end is
W.sub.c=2.times.8.11/0.5=12.5 W. In Case (2), the total cooling
power is 25 W. Also, and considering the EC device has EC rings
with dimensions of 0.4 mm ring thickness and 5.5 cm OD, the cooling
device in FIG. 6 exhibits a rather high cooling power density of
greater than 26 W/cm.sup.3.
[0093] These modeling results have indeed been confirmed by a
finite element simulation of a real device in which a temperature
span T.sub.h-T.sub.c=20 K, a cooling power >20 W/cm.sup.3, and a
coefficient of performance (COP) larger than 9 (Carnot COP is 15)
for k=1 W/mK for EC segments were obtained. The temperature
gradient between T.sub.h and T.sub.c will cause a heat conduction
loss, which reduces the cooling power and efficiency. Hence the EC
rings in the cooling device(s) illustrated in the figures are
divided into segments with a low thermal conductivity gap between
two neighboring segments.
[0094] It is appreciated that the disclosed examples and
embodiments are presented for illustrative purposes only and are
not meant to limit the scope of the invention. As such, it is the
claims, and all equivalents thereof, that define the scope of the
invention.
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