U.S. patent application number 12/865212 was filed with the patent office on 2012-07-05 for heat transfer system utilizing thermal energy storage materials.
This patent application is currently assigned to DOW GLOBAL TECHNOLOGIES INC.. Invention is credited to David H. Bank, Andrey N. Soukhojak.
Application Number | 20120168111 12/865212 |
Document ID | / |
Family ID | 43796127 |
Filed Date | 2012-07-05 |
United States Patent
Application |
20120168111 |
Kind Code |
A1 |
Soukhojak; Andrey N. ; et
al. |
July 5, 2012 |
HEAT TRANSFER SYSTEM UTILIZING THERMAL ENERGY STORAGE MATERIALS
Abstract
An enhanced heat transfer between stored thermal energy and a
heat recipient via a capillary pumped loop. The devices, systems
and methods employ a thermal energy storage material having a solid
to liquid phase transition at a temperature and a structure having
a plurality of capillaries.
Inventors: |
Soukhojak; Andrey N.;
(Midland, MI) ; Bank; David H.; (Midland,
MI) |
Assignee: |
DOW GLOBAL TECHNOLOGIES
INC.
Midland
MI
|
Family ID: |
43796127 |
Appl. No.: |
12/865212 |
Filed: |
December 14, 2009 |
PCT Filed: |
December 14, 2009 |
PCT NO: |
PCT/US09/67823 |
371 Date: |
July 29, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61245767 |
Sep 25, 2009 |
|
|
|
Current U.S.
Class: |
165/10 ;
165/104.17; 165/104.26; 165/104.28; 60/320 |
Current CPC
Class: |
F28D 20/023 20130101;
Y02E 60/14 20130101; Y02E 60/145 20130101; F28D 15/043 20130101;
F28D 20/028 20130101 |
Class at
Publication: |
165/10 ;
165/104.17; 165/104.28; 165/104.26; 60/320 |
International
Class: |
F28D 20/00 20060101
F28D020/00; F28D 15/04 20060101 F28D015/04; F01N 5/02 20060101
F01N005/02; F28D 15/02 20060101 F28D015/02 |
Claims
1. A device including i) one or more thermal energy storage
material compartments: ii) a thermal energy storage material
encapsulated in one or more thermal energy storage material
compartments, wherein the thermal energy storage material has
having a solid to liquid phase transition at a temperature; iii)
one or more working fluid compartments in thermal contact with the
thermal energy storage material; and iv) and a capillary structure
having a plurality of capillaries in the working fluid compartment;
wherein the device is a heat storage device.
2. The device of claim 1, wherein the thermal energy storage
material has a solid to liquid phase transition at a temperature
greater than about 50.degree. C.
3. The device of claim 2, wherein the thermal energy storage
material has a solid to liquid phase transition at a temperature
from about 90.degree. C. to about 300.degree. C.
4. (canceled)
5. The device of claim 2, wherein the thermal energy storage
material is encapsulated in a plurality of capsules.
6. (canceled)
7. (canceled)
8. The device of claim 2, comprising: i) one or more containers
each with at least one inlet and one outlet for a working fluid,
and at least one inlet and at least one outlet for a second fluid;
ii) one or more capsules containing the thermal energy storage
material in the container having at least a first outer surface,
wherein the thermal energy storage material is a phase change
material; iii) a first flow path for the flow of the working fluid
through the container wherein the flow path is at least partially
defined by the first outer surface of the capsule; iv) a capillary
structure having a plurality of capillaries capable of pumping a
working fluid through the first flow path, wherein the capillary
structure partially fills the first flow path and is at least
partially in contact with the first outer surface of the capsule,
so that when contacted with a working fluid on one end, the working
fluid is drawn into the capillary, and a second portion of the
first flow path that is free of a capillary; v) a second flow path
for the flow of a second fluid through the container; wherein the
first flow path is in a working fluid compartment, the second flow
path is in a heat transfer fluid compartment, and the phase change
material is in a phase change material compartment; the phase
change material is in thermal communication with the working fluid
compartment and the heat transfer fluid compartment; and wherein
the device is a heat storage device.
9. The device of claim 8, wherein the plurality of capillaries
includes capillaries having a pore diameter of less than about 200
.mu.m.
10. The device of claim 9, wherein the capsule includes a second
outer surface wherein the second outer surface at least partially
defines the second flow path.
11. (canceled)
12. (canceled)
13. (canceled)
14. The device of claim 9, wherein the container is at least
partially insulated.
15. (canceled)
16. A system for storing and transferring heat including: a. the
heat storage device of claim 2; b. a condenser having at least a
first inlet and at least a first outlet and a first flow path for a
working fluid; wherein the heat storage device is in fluid
connection with the condenser and the system comprises a capillary
pumped loop including the first flow path of the condenser and the
first flow path of the heat storage device.
17. The system of claim 16, wherein the system includes the working
fluid and the working fluid has a combined vapor pressure of all of
its components equal to 1 atmosphere at a temperature from about
0.degree. C. to about 250.degree. C.
18. (canceled)
19. The system of claim 16, wherein the working fluid includes one
or more alcohols, one or more ketones, one or more hydrocarbons, a
fluorocarbon, a hydrofluorocarbon, water, ammonia, or any
combination thereof.
20. The system of claim 19, wherein the working fluid includes a
solution of ammonia and water.
21. (canceled)
22. The system of claim 17, wherein the system includes a working
fluid valve to control the flow of the working fluid from the heat
storage device to the condenser.
23. The system of claim 16, wherein the system includes a vapor
tube connecting the outlet of the heat storage device and the inlet
of the condenser; a liquid tube connecting the outlet of the
condenser and the inlet of the heat storage device; and a working
fluid at least partially filling the condenser.
24. (canceled)
25. (canceled)
26. (canceled)
27. The system of claim 16, wherein the system includes a means of
heating the phase change material in the heat storage device, so
that when the heat storage device is at a temperature sufficient to
cause the combined vapor pressure of all components of the working
fluid to exceed 1 atmosphere and the valve is opened to allow flow
of the working fluid, the working fluid is a) pumped by the
capillary structure; b) at least partially vaporized; and c) at
least partially transported to the condenser; and d) at least
partially condenses in the condenser; so that heat is removed from
the heat storage device.
28. (canceled)
29. (canceled)
30. The system of claim 16, wherein the system is free of a pump
for pumping the working fluid other than the capillary pump.
31. The system of claim 16, wherein the condenser includes a second
flow path, a second inlet and a second outlet for transporting a
heat transfer fluid through the condenser so that the condenser can
transfer heat from the working fluid to the heat transfer
fluid.
32. (canceled)
33. (canceled)
34. The system of claim 16, wherein the system includes a working
fluid, and the ratio of the volume of the working fluid to the
volume of the phase change material is less than about 20:1.
35. (canceled)
36. The system of claim 16, wherein the second fluid is an exhaust
gas, and the system includes a valve for controlling the flow of
the exhaust gas through the second flow path of the heat storage
device.
37. The system of claim 16, wherein the system has an average power
density of at least about 1 kW per liter of the insulated
(internal) volume of the device averaged over the first 30 seconds
of operation, which begins when a valve is opened allowing for flow
of the working fluid through the first flow path of the heat
storage device, wherein at least 50% by volume of the phase change
material is in a liquid state at the time the valve is opened.
38. (canceled)
Description
CLAIM OF PRIORITY
[0001] The present application claims the benefit of the filing
date of US Provisional Patent Application No.: 61/245,767 (filed on
Sep. 25, 2009 by Soukhojak et al.), the contents of which is
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to enhanced heat transfer
between stored thermal energy and a heat recipient via a capillary
pumped loop.
BACKGROUND OF THE INVENTION
[0003] Industry in general has been actively seeking a novel
approach to capture and store waste heat efficiently such that it
can be utilized at a more opportune time. Further, the desire to
achieve energy storage in a compact space demands the development
of novel materials that are capable of storing high energy content
per unit weight and unit volume. Areas of potential application of
breakthrough technology include transportation, solar energy,
industrial manufacturing processes as well as municipal and/or
commercial building heating.
[0004] Regarding the transportation industry, it is well known that
internal combustion engines operate inefficiently. Sources of this
inefficiency include heat lost via exhaust, cooling, radiant heat
and mechanical losses from the system. It is estimated that more
than 30% of the fuel energy supplied to an internal combustion
engine (internal combustion engine) is lost to the environment via
engine exhaust.
[0005] It is well known that during a "cold start" internal
combustion engines operate at substantially lower efficiency,
generate more emissions, or both, because combustion is occurring
at a non-optimum temperature and the internal combustion engine
needs to perform extra work against friction due to high viscosity
of cold lubricant. This problem is even more important for hybrid
electric vehicles in which the internal combustion engine operates
intermittently thereby prolonging the cold start conditions, and/or
causing a plurality of occurrences of cold start conditions during
a single period of operating the vehicle. To help solve this
problem, original equipment manufacturers (OEM) are looking for a
solution capable of efficient storage and release of waste heat.
The basic idea is to recover and store waste heat during normal
vehicle operation followed by controlled release of this heat at a
later time thereby reducing or minimizing the duration and
frequency of the cold start condition and ultimately improving
internal combustion engine efficiency, reducing emissions, or
both.
[0006] To be a practical solution, the energy density and the
thermal power density requirements for a thermal energy storage
system are extremely high. Applicants have previously filed 1) U.S.
patent application Ser. No. 12/389,416 entitled "Thermal Energy
Storage Materials" and filed on Feb. 20, 2009; and 2) U.S. patent
application Ser. No. 12/389,598 entitled "Heat Storage Devices" and
filed on Feb. 20, 2009. These previous applications are herein
incorporated by reference in their entirety.
[0007] There are known exhaust heat recovery devices in prior art.
However, they do not provide a long term (>6 hr) heat storage
capability, which is desired to mitigate cold start conditions
immediately after or even prior to a cold start. Therefore, there
is a need for a system which can offer an unprecedented combination
of high energy density, high power density, long heat retention
time, and a simple mechanism of on-demand heat transfer in an
automotive exhaust heat recovery system.
SUMMARY OF THE INVENTION
[0008] One aspect of the invention is a device including a thermal
energy storage material having a solid to liquid phase transition
at a temperature greater than about 50.degree. C.; and a capillary
structure; wherein the device is a heat storage device.
[0009] Another aspect of the invention is a device comprising one
or more containers each with at least one inlet and one outlet for
a working fluid, and at least one inlet and at least one outlet for
a second fluid; one or more capsules containing a phase change
material in the container having at least a first outer surface; a
first flow path for the flow of the working fluid through the
container wherein the flow path is at least partially defined by
the first outer surface of the capsule; a capillary structure
having a plurality of capillaries capable of pumping a working
fluid through the first flow path, wherein the capillary structure
partially fills the first flow path and is at least partially in
contact with the first outer surface of the capsule, so that when
contacted with a working fluid on one end, the working fluid is
drawn into the capillary, and a second portion of the first flow
path that is free of a capillary structure; a second flow path for
the flow of a second fluid through the container; wherein the first
flow path is in a working fluid compartment, the second flow path
is in a heat transfer fluid compartment, and the phase change
material is in a phase change material compartment; the phase
change material is in thermal communication with the working fluid
compartment and the heat transfer fluid compartment; and wherein
the device is a heat storage device.
[0010] Another aspect of the invention is a system for storing and
transferring heat including a heat storage device such as one
described herein; a condenser having at least a first inlet and at
least a first outlet and a first flow path for the working fluid;
wherein the heat storage device is in fluid connection with the
condenser and the system comprising a capillary pumped loop
including the first flow path of the condenser and the first flow
path of the heat storage device.
[0011] Yet another aspect of the invention is a method of
discharging heat that includes a step of circulating a working
fluid through a heat storage device described herein, such as one
including a thermal energy storage material and a capillary
structure.
[0012] The present invention can be used for mitigating cold start
conditions in internal combustion engines and provide additional
steady-state coolant heating, if needed, for occupant comfort
heating and/or windshield defrosting. Other industrial applications
of the present invention can also include cooling systems, other
power producing applications such as Rankine cycle heat engine,
thermoelectric generator or other.
[0013] In other aspects of the invention, the present invention can
also be used to warm-up an electrochemical battery in a hybrid
electric, plug-in hybrid electric, extended range electric vehicle
or purely electric vehicle; comfort heating of vehicles capable of
electric-only propulsion; automotive air conditioning using
adsorption or absorption cycle refrigeration; a steady-state
exhaust heat recovery using a heat engine, e.g. Rankine cycle; and
industrial and residential heat storage.
BRIEF DESCRIPTION OF THE FIGURES
[0014] The present invention is further described in the detailed
description which follows, in reference to the noted plurality of
drawings by way of non-limiting examples of embodiments of the
present invention, in which like reference numerals represent
similar parts throughout the several views of the drawings, and
wherein:
[0015] FIG. 1 is a schematic showing some of the main components of
a heat storage device.
[0016] FIG. 2A is a cross-section of an illustrative heat storage
device. The cross-section illustrates the internal structure of a
three-chamber (exhaust gas, phase change material, and working
fluid) two-flow (exhaust gas and working fluid) heat storage device
containing thermal energy storage material and an evaporator.
[0017] FIG. 2B is another cross-section of an illustrative heat
storage device.
[0018] FIG. 3 is a schematic showing some of the main components of
a thermal energy storage system including a heat storage device and
a condenser.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0019] In the following detailed description, the specific
embodiments of the present invention are described in connection
with its preferred embodiments. However, to the extent that the
following description is specific to a particular embodiment or a
particular use of the present techniques, it is intended to be
illustrative only and merely provides a concise description of the
exemplary embodiments. Accordingly, the invention is not limited to
the specific embodiments described below, but rather; the invention
includes all alternatives, modifications, and equivalents falling
within the true scope of the appended claims.
[0020] As will be seen from the teachings herein, the present
invention provides a unique and unexpectedly efficient approach to
the packaging and containment of thermal energy storage materials
(which also includes what is commonly called "phase change
materials") for heat storage and discharge applications, and
particularly for applications requiring high power densities so
that heat can be quickly stored, quickly removed, or both. Thermal
energy storage systems herein exhibit extraordinary high power
density capabilities and may be used for removing heat from a heat
storage device (continuing the phase change material) in the system
at a rate of at least about 10 kW per liter of the heat storage
device. The teachings herein contemplate the packaging and
containment of thermal energy storage materials in a relatively
robust structure that will resist failure due to corrosion, due to
thermally induced strains from cyclical thermal loading, or both,
and will also yield a relatively high storage and discharge
capacity in relation to the overall volume occupied by such
structures and systems incorporating them. The teachings herein
also contemplate a flow path in the heat storage device for a
working fluid in which the flow path, partially includes a
capillary structure having a plurality of capillaries. The
capillary structure may be employed to at least partially pump the
working fluid. One of the advantages of the capillary structures
herein is that a relatively compact assembly is possible, which
exhibits unexpectedly large and rapid heat storage and discharge
capabilities. The system may be capable of pumping the working
fluid without the use of a pump other than the pumping of the
capillaries. As will be seen, the teachings herein contemplate a
manner of packaging discrete amounts of thermal energy storage
material in a plurality of capsular structures. The teachings
contemplate the assembly of such capsular structures for use in a
heat storage device. A number of applications made possible or more
efficient as a result of such structures, devices, and/or systems
are also contemplated as part of the teachings.
Heat Storage Device
[0021] As discussed above, the thermal energy storage system
includes a heat storage device (i.e., a thermal energy storage
device) capable of storing thermal energy. As such, the heat
storage device is capable of receiving heat (such as waste heat or
otherwise), storing the heat, and discharging the heat at a later
time so that it may be used to heat one or more objects.
Preferably, the heat storage device is capable of rapidly
discharging the heat. During the discharging of the heat, the heat
storage device may function as an evaporator, at least partially
converting a working fluid from a liquid phase to a vapor phase. As
such, the heat storage device includes a working fluid compartment
for containing the working fluid (the compartment may include one
or more flow paths), one or more working fluid inlets for receiving
the working fluid (e.g., in a liquid state) connected to the
working fluid compartment (e.g., at one side or one end of the
working fluid compartment), and one or more working fluid outlets
for expelling the working fluid (e.g., in a gaseous state), so that
the working fluid flows into the one or more working fluid inlets,
through the one or more flow paths of the working fluid compartment
and out of the one or more working fluid outlets. Preferably, at
least a portion of the flow path (e.g., a portion of each flow
path) includes a capillary structure (such as a structure having a
plurality of capillaries) capable of wicking the working fluid. The
working fluid compartment of the heat storage device may be a
section of a capillary pumped loop, and the capillary structure may
be used to at least partially pump the working fluid through the
loop.
[0022] In various aspects of the invention the heat storage device
may be relatively light weight, relatively small, or both. As such,
the heat storage capacity density (i.e., the maximum amount of heat
that can be stored in the heat storage device divided by the volume
of the heat storage device) may be relatively high, and the heat
storage capacity to mass ratio (i.e., the ratio of the amount of
heat that can be stored in the heat storage device and the mass of
the heat storage device) may be relatively high. To achieve these
efficiencies, the heat storage device may employ materials (such as
thermal energy storage materials, encapsulation materials,
container materials, materials for the capillary structure, and the
like) that are light weight.
[0023] A large portion of the heat storage device preferably
includes one or more thermal energy storage materials (preferably
one or more phase change materials) capable of efficiently storing
heat. The concentration of the thermal energy storage material in
the heat storage device may be maximized with the proviso that the
heat storage device has a working fluid compartment large enough
for the working fluid to flow and quickly transfer heat from the
device, and a heat transfer fluid compartment in thermal contact
with the thermal energy storage material and large enough for a
heat transfer fluid to flow through and efficiently transfer heat
into the device. The volume of the one or more thermal energy
storage materials may be greater than about 10% by volume,
preferably greater than about 20% by volume, more preferably
greater than about 30% by volume, even more preferably greater than
about 40% by volume, and most preferably greater than about 50% by
volume, based on the total volume of the container of the heat
storage device.
[0024] The heat storage device may have a sufficient number of
compartments so that the thermal energy storage material and one or
more fluids are separated from each other. The heat storage device
may have two or more (preferably three or more) compartments. The
compartments may be employed to separate (e.g., substantially or
entirely isolate) one, or all of i) the thermal energy storage
material, ii) a first fluid (such as a heat transfer fluid) for
charging (e.g., heating) the thermal energy storage material (e.g.,
for charging the phase change material), and iii) a second fluid
(such as a working fluid) for discharging (e.g., cooling) the
thermal energy storage material (e.g., the phase change material).
As such, the heat storage device may include a thermal energy
storage material compartment for the thermal energy storage
material (e.g., a phase change material compartment for the phase
change material), a compartment (e.g., a heat transfer fluid
compartment) for a first fluid, and a compartment (e.g., a working
fluid compartment) for a second fluid. The thermal energy storage
material compartment may be in thermal communication with the heat
transfer fluid compartment, the working fluid compartment, or
preferably both. It will be appreciated that the thermal energy
storage material compartment may share one or more walls with the
heat transfer fluid compartment, with the working fluid
compartment, or both. For example, the thermal energy storage
material may be stored in capsules having a first surface that at
least partially defines the heat transfer fluid compartment and a
second surface that at least partially defines the working fluid
compartment. The device may have one or more inlets and one or more
outlets for the first fluid, both attached to the compartment for
the first fluid, so that the first fluid (e.g., the heat transfer
fluid) may flow through the inlet and enter into the device, flow
into the compartment for the first fluid, and providing thermal
energy to the thermal energy storage material (e.g., to the phase
change material) and exit the device through an outlet. Similarly,
the device may have one or more inlets and one or more outlets for
the second fluid and attached to the compartment for the second
fluid, so that the second fluid (e.g., the working fluid) may flow
through the inlet and enter the device, flow into the compartment
for the second fluid removing thermal energy from the thermal
energy storage material (e.g., from the phase change material) and
exit the device through an outlet.
Capillary Structure
[0025] As described above, the heat storage device includes a
capillary structure that contains a plurality of capillaries.
Preferably, the working fluid compartment includes a capillary
structure. In general the wicking of a fluid into a capillary
increases as the radius of the capillary decreases. The capillary
structure may be any structure having a sufficient number of
capillaries with sufficiently small radii so that the capillary
structure is capable of pumping the working fluid. The capillary
structure may pump the working fluid when the heat storage device
(e.g., the thermal energy storage material) has a temperature at
which the working fluid has a pressure greater than about 1
atmosphere; the condenser has a temperature at which the working
fluid has a pressure less than about 1 atmosphere, or preferably
both. The capillary structure may be employed for pumping the
working fluid. Preferably, the capillary structure is employed as
the only means of pumping the working fluid. As such, it is
contemplated that the heat storage device may be employed in a
system having a working fluid loop that is free of any pump, other
than the capillary pump.
[0026] The capillary structure may of one or more objects having a
porous structure, by packing together a plurality of objects so
that the gaps between the objects forms the porous structure, or
both. The capillary structure (e.g., the wick structure) of the
heat storage device (e.g., of the evaporator of the heat storage
device) may contain one or more fibers or filaments, one or more
grooves, or one or more other porous structures having a generally
small pore size, so that the capillary structure is capable of
creating a capillary pressure on the working fluid that is great
enough to overcome gravitational forces, the gas pressure
difference between the evaporator and the condenser, or both. The
capillary structure may be any art known capillary structure (such
as those employed in heat pipes and capillary pumped loops, e.g.,
for cooling electronic devices). For example simple homogeneous
capillary structure such as a wrapped screen, sintered metal, or
axial groove may be employed. Other capillary structures that may
be employed include slab, pedestal artery, spiral artery, tunnel
artery, axial groove with a varying groove width, double wall
artery, monogroove, channel wick, and the like. Any of the above
structures may be adapted for a generally layered structure, such
as may be formed between the flat surfaces of two blister
packs.
[0027] The capillary structure has a pore size sufficiently small
to overcome gravitational forces, to overcome the gas pressure
difference between the evaporator and the condenser, or both. The
capillary structure has a pore size sufficiently high so that
liquid working fluid can enter the capillaries. The capillary
pressure is generally inversely proportional to the pore radius.
The capillary structure may have an average pore radius less than
about 2 mm, preferably less than about 1 mm, more preferably less
than about 400 .mu.m, even more preferably less than about 100
.mu.m, even more preferably less than about 30 .mu.m, even more
preferably less than about 20 .mu.m, and most preferably less than
about 10 .mu.m.
[0028] The capillary structure located in the working fluid
compartment shall fill a sufficient volume of the working fluid
compartment to overcome gravitational forces, the gas pressure
difference between the evaporator and the condenser, or both. The
capillary structure may fill greater than 1 volume %, preferably
greater than about 5 volume %, more preferably greater than about
10 volume %, and most preferably greater than about 25 volume %, of
the working fluid compartment of the heat storage device. The
capillary structure may fill less than about 95 volume %,
preferably less than about 90 volume %, more preferably less than
about 85 volume %, and most preferably less than about 75 volume %
of the working fluid compartment of the heat storage device. The
remaining volume of the working fluid compartment of the heat
storage device is preferably free of the capillary structure.
Thermal Energy Storage Material Compartment
[0029] As described above, the thermal energy storage material
preferably is isolated in one or more compartments in the heat
storage device. Typically, the thermal energy storage material has
a relatively low thermal diffusivity (e.g., compared with the
material of the compartment in which it is provided). Preferably
the shape and/or size of the one or more compartments are selected
so that thermal energy can rapidly transfer into and out of the
thermal energy storage material. As such, the heat storage device
may employ one or means for increasing the heat transfer. For
example, the one or more thermal energy storage materials may have
at least one dimension that is relatively small (e.g., compared
with one or more other dimensions), the thermal energy storage
material may be stored in a plurality of compartments, the interior
of the one or more compartments may have thermally conductive
objects (e.g., fins, wire, mesh, and the like), or any combination
thereof. For example, the thermal energy storage material may be
stored in at least about 5, 10, 15, or 20 compartments.
[0030] The thermal energy storage material preferably is in a
plurality of individually isolated cells (such as capsules), having
a total surface area of the plurality of cells that is relatively
high, a distance from a surface of a cell to the center of the cell
that is relatively low, or both. The plurality of cells (e.g.,
capsules) may be arranged in one or more layers of cells. For
example, the heat storage device may include a plurality of layers
of cells (e.g., capsules): Each layer of cells may contain a single
cell or a plurality of cells. It will be appreciated that a layer
of cells (e.g., a layer of capsules) may have a relatively low
thickness, a relatively high surface area to volume ratio, or both,
so that heat can be rapidly removed from the interior of the cells.
The cells may be in any arrangement in a layer. For example, the
cells may be of the same size and shape, the cells may have varying
sizes and shapes, the cells may be arranged in a repeating pattern
(e.g., a pattern that contains 1, 2, or more cells) or may be
arranged in a pattern that generally does not repeat. In a
preferred aspect of the invention, the cells are arranged as an
array of capsules (e.g., a 1-dimensional array, a 2-dimensional
array, or a radial array) in each layer of capsules.
[0031] The heat storage device may include a plurality of layers of
capsules with a spacing between one or more pairs of adjacent
layers of capsules. A spacing may be used as a portion of the
working fluid compartment or as a portion of the heat transfer
fluid compartment. Layers of capsules may have a spacing on one
side, have spacings on two opposing sides, have no spacing, or any
combination thereof. For example, there may be a spacing between
every pair of adjacent layers of capsules. Preferably, there is a
spacing between every pair of adjacent layers of capsules and the
spacings alternately are the working fluid compartments and the
heat transfer fluid compartments.
[0032] A layer of capsules may have a surface that is arcuate and
an opposing surface that is generally flat. A generally arcuate
surface may be particularly attractive for a heat transfer fluid,
where the arcuate path may increase the heat flow from the heat
transfer fluid into the capsules. A generally flat surface may be
particularly attractive for placing a capillary structure (and the
thickness of the capillary structure may determine the separation
between two layers of capsules on either side of a portion of the
working fluid compartment). Layers having opposing surfaces that
are both generally flat or both arcuate may also be employed. The
heat storage device may also employ two adjacent layers of capsules
that partially or substantially entirely nest together.
[0033] The size and shape of the capsules may be chosen to maximize
the transfer of heat to and from the phase change material
contained in the capsules. The average thickness of the capsules
(e.g., the layer of capsules) may be relatively short so that the
heat can quickly escape from the center of the capsule. The average
thickness of the capsules may be less than about 100 mm, preferably
less than about 30 mm, more preferably less than about 10 mm, even
more preferably less than about 5 mm, and most preferably less than
about 3 mm. The average thickness of the capsules may be greater
than about 0.1 mm, preferably greater than about 0.5 mm, more
preferably greater than about 0.8 mm, and most preferably greater
than 1.0 mm.
[0034] The capsules preferably have a relatively high surface area
to volume ratio so that the area of contact with the working fluid,
the area of contact with the heat transfer fluid, or both is
relatively high. For example, the capsules may have a surface that
maximizes the contact with a working fluid compartment; the
capsules may have a geometry that maximizes the transfer of heat
between the capsule and the working fluid compartment, or both. The
ratio of the total surface area of the interface between the
working fluid compartment and the phase change material compartment
to the total volume of the thermal energy storage material in the
heat storage device may be greater than about 0.02 mm.sup.-1,
preferably greater than about 0.05 mm.sup.-1, more preferably
greater than about 0.1 mm.sup.-1, even more preferably greater than
about 0.2 mm.sup.-1, and most preferably greater than about 0.3
mm.sup.-1.
[0035] The thermal energy storage material compartment may be in
the form of a blister pack or a stack of blister packs. For
example, the thermal energy storage material may be encapsulated
between an embossed metal layer and a flat metal layer which are
sealed together to form a plurality of isolated capsules. Without
limitation, the heat storage device may employ a capsule or an
arrangement of capsules (e.g., a blister pack or stack of blister
packs) described in U.S. patent application Ser. No. 12/389,598
entitled "Heat Storage Devices" and filed on Feb. 20, 2009.
Working Fluid Compartment And Heat Transfer Fluid Compartment
[0036] As discussed above, the heat storage device preferably
includes a working fluid compartment and a heat transfer fluid
compartment in thermal communication with the thermal energy
storage material compartment.
[0037] The thickness of the heat transfer fluid compartment is
chosen to facilitate the desired flow of the heat transfer fluid
through the flow path and to maximize the transfer of heat to the
phase change material. The average thickness of a layer of the heat
transfer fluid compartment may be less than about 20 mm, preferably
less than about 10 mm, more preferably less than about 5 mm, even
more preferably less than about 3 mm, and most preferably less than
about 2 mm. Higher thickness may be used when the rate at which
heat is stored from the heat transfer fluid to the thermal energy
storage material is not critical. The average thickness of a layer
of the heat transfer fluid compartment should be high enough so
that the pressure drop of the heat transfer fluid in the thermal
energy storage material device is low. Preferably, the pressure
drop between the heat transfer fluid inlet and the heat transfer
fluid outlet of the heat storage device is less than about 95%,
more preferably less than about 50%. The average thickness of a
layer of the heat transfer fluid compartment may be greater than
about 0.1 mm, preferably greater than about 0.2 mm, more preferably
greater than about 0.4 mm, and most preferably greater than about
0.6 mm.
[0038] The thickness of the working fluid compartment is chosen to
facilitate the desired flow of the working fluid through the flow
path and to maximize the transfer of heat from the phase change
material. The average thickness of a layer of the working fluid
compartment may be less than about 20 mm, preferably less than
about 10 mm, more preferably less than about 5 mm, even more
preferably less than about 3 mm, and most preferably less than
about 2 mm. The average thickness of a layer of the working fluid
compartment may be greater than about 0.1 mm, preferably greater
than about 0.2 mm, more preferably greater than about 0.4 mm, and
most preferably greater than about 0.6 mm.
[0039] The spacing between adjacent layers of capsules may be used
for the working fluid, the heat transfer fluid, or both. For
example, at least a portion (e.g., a layer) of the heat transfer
fluid compartment may be interposed between two adjacent layers of
capsules. At least a portion (e.g., a layer) of the working fluid
compartment may be interposed between two adjacent layers of
capsules and the average thickness of the working fluid compartment
may be defined by the distance (e.g., average distance) of
separation of the two layers of capsules. A layer of capsules may
have a layer of the working fluid compartment on one side of the
capsule layer and a layer of the heat transfer fluid compartment on
an opposing side.
[0040] The working fluid may be selected so that it flows into the
heat storage device as a liquid, is heated by the thermal energy
stored in the thermal energy storage material (e.g., the phase
change material) and vaporizes, and exits the heat storage as a
vapor. As such, it is preferable that the elevation of the working
fluid outlet is higher than the elevation of the working fluid
inlet.
[0041] As previously described, some of the working fluid
compartment typically includes a region having a capillary
structure for wicking the liquid into the compartment, and a region
that is free of a capillary structure for the working fluid (e.g.,
the gaseous working fluid). For example, within a single layer of
the working fluid compartment, there may be one or more regions
(such as a columnar region) that contain a capillary structure and
one or more regions (such as a columnar region) that are free of a
capillary structure.
[0042] A surface of the thermal energy storage material compartment
(e.g., an outer surface of a layer of capsules containing thermal
energy storage material) may generally define at least a portion of
the heat transfer fluid compartment. Similarly, a second surface of
the thermal energy storage material compartment (e.g., a second
outer surface of the layer of capsules containing thermal energy
storage material) may generally define at least a portion of the
working fluid compartment. It will be appreciated that one or more
additional materials (e.g., one or more additional layers) may
separate a layer of capsules from the working fluid compartment,
from the heat transfer fluid compartment, or both, provided that
the layer of capsules is in thermal communication with the working
fluid compartment, the heat transfer compartments, or preferably
both.
Thermal Energy Storage Materials
[0043] Without limitation, suitable thermal energy storage
materials for the heat storage device include materials that are
capable of exhibiting a relatively high density of thermal energy
as sensible heat, latent heat, or preferably both. The thermal
energy storage material is preferably compatible with the operating
temperature range of the heat storage device. For example, the
thermal energy storage material is preferably a solid at the lower
operating temperature of the heat storage device, is at least
partially a liquid (e.g., entirely a liquid) at the maximum
operating temperature of the heat storage device, does not
significantly degrade or decompose (e.g., during a time of at least
about 1,000 hours, preferably of least about 10,000 hours) at the
maximum operating temperature of the heat storage device, or any
combination thereof. The thermal energy storage material may have a
liquidus temperature e.g., a melting temperature greater than about
30.degree. C., preferably greater than about 50.degree. C., more
preferably greater than about 80.degree. C., even more preferably
greater than about 110.degree. C., and most preferably greater than
about 140.degree. C. The thermal energy storage material may have a
liquidus temperature less than about 400.degree. C., preferably
less than about 350.degree. C., more preferably less than about
290.degree. C., even more preferably less than about 250.degree.
C., and most preferably less than about 200.degree. C. The thermal
energy storage material may have a heat of fusion density greater
than about 0.1 MJ/liter, preferably greater than about 0.2 MJ/l,
more preferably greater than about 0.4 MJ/liter, and most
preferably greater than about 0.6 MJ/liter. The thermal energy
storage material may have a density less than about 5 g/cm.sup.3,
preferably less than about 4 g/cm.sup.3, more preferably less than
about 3.5 g/cm.sup.3, and most preferably less than about 3
g/cm.sup.3.
[0044] Other examples of suitable thermal energy storage materials
that may be employed in the heat transfer device include the
thermal energy storage materials described in U.S. patent
application Ser. No. 12/389,416 entitled "Thermal Energy Storage
Materials" and filed on Feb. 20, 2009; and U.S. patent application
Ser. No. 12/389,598 entitled "Heat Storage Devices" and filed on
Feb. 20, 2009.
[0045] The thermal energy storage material may include (or may even
consist essentially of or consist of) at least one first metal
containing material, and more preferably a combination of the at
least one first metal containing material and at least one second
metal containing material. The first metal containing material, the
second metal containing material, or both, may be a substantially
pure metal, an alloy such as one including a substantially pure
metal and one or more additional alloying ingredients (e.g., one or
more other metals), an intermetallic, a metal compound (e.g., a
salt, an oxide or otherwise), or any combination thereof. One
preferred approach is to employ one or more metal containing
materials as part of a metal compound; a more preferred approach is
to employ a mixture of at least two metal compounds. By way of
example, a suitable metal compound may be selected from oxides,
hydroxides, compounds including nitrogen and oxygen (e.g.,
nitrates, nitrites or both), halides, or any combination thereof.
One particularly preferred metal compound includes at least one
nitrate compound, at least one nitrite compound or a combination
thereof. It is possible that ternary, quaternary or other multiple
component material systems may be employed also. The thermal energy
storage materials herein may be mixtures of two or more materials
that exhibit a eutectic. A particularly preferred thermal energy
storage material includes a lithium containing compound, such as a
lithium salt. The thermal energy storage material may be a mixture
of two or more compounds (e.g., two or more salts) including at
least one compound containing lithium.
[0046] A large portion of the volume of the heat storage device may
be occupied by the thermal energy storage material so that the
power output of the heat storage device is relatively high, the
total volume of the heat storage device is relatively small, or
both. For example, the ratio of the volume of the working fluid
compartment to the volume of the thermal energy storage material
(e.g., the phase change material) in the heat storage device may be
less than about 20:1 (preferably less than about 10:1, more
preferably less than about 5:1, even more preferably less than
about 2:1 and most preferably less than about 1:1), the ratio of
the volume of the heat transfer fluid compartment to the volume of
the thermal energy storage material (e.g., the phase change
material) in the heat storage device may be less than about 20:1
(preferably less than about 10:1), more preferably less than about
5:1, even more preferably less than about 2:1 and most preferably
less than about 1:1), or both.
[0047] The heat storage device may contain a sufficient quantity of
the thermal energy storage material so that an object to be heated
(such as an internal combustion engine or a cockpit of a vehicle)
can be heated to a desired temperature. For example, the heat
storage device may contain sufficient quantity of thermal energy
storage material to increase the temperature of an internal
combustion engine by at least 10.degree. C., preferably at least
about 20.degree. C., more preferably at least about 30.degree. C.,
and most preferably at least about 40.degree. C.
Forming Capsules
[0048] The capsules of the thermal energy storage material may be
formed using any method that provides for the encapsulation of the
thermal energy storage material. Without limitation, the process
may employ one or any combination of the following: embossing or
otherwise deforming a thin material sheet (e.g., a foil) to define
a pattern in the sheet, filling depressions in the embossed sheet
with the thermal energy storage material, covering an embossed
sheet with a second sheet (e.g., a generally flat sheet), or
attaching the two sheets. The process of forming the capsules may
employ the processes described in U.S. patent application Ser. No.
12/389,598 entitled "Heat Storage Devices" and filed on Feb. 20,
2009.
[0049] Suitable sheets for encapsulating the thermal energy storage
material include thin metal sheets (e.g., metal foil) that are
durable, corrosion resistant, or both, so that the sheet is capable
of containing the thermal energy storage material, preferably
without leakage. The metal sheets may be capable of functioning in
a vehicle environment with repeated thermal cycling for more than 1
year and preferably more than 5 years. Without limitation,
exemplary metal sheets that may be employed include metal sheets
having at least one layer of brass, copper, aluminum, nickel-iron
alloy, bronze, titanium, stainless steel or the like. The sheet may
be a generally noble metal or it may be one that includes a metal
which has an oxide layer (e.g. a native oxide layer or an oxide
layer which may be formed on a surface). The metal sheet may
otherwise have a substantially inert outer surface that contacts
the thermal energy storage material in operation. One exemplary
metal sheet is an aluminum foil which comprises a layer of aluminum
or an aluminum containing alloy (e.g. an aluminum alloy containing
greater than 50 wt. % aluminum, preferably greater than 90 wt %
aluminum). Another exemplary metal sheet is stainless steel.
Suitable stainless steels include austenitic stainless steel,
ferritic stainless steel or martensitic stainless steel. Without
limitation, the stainless steel may include chromium at a
concentration greater than about 10 wt. %, preferably greater than
about 13 wt. %, more preferably greater than about 15 wt. %, and
most preferably greater than about 17 wt. %. The stainless steel
may include carbon at a concentration less than about 0.30 wt. %,
preferably less than about 0.15 wt. %, more preferably less than
about 0.12 wt. %, and most preferably less than about 0.10 wt. %.
For example, stainless steel 304 (SAE designation) containing 19
wt. % chromium and about 0.08 wt. % carbon. Suitable stainless
steels also include molybdenum containing stainless steels such as
316 (SAE designation).
[0050] The metal sheet has a thickness sufficiently high so that
holes or cracks are not formed when forming the sheet, when filling
the capsules with thermal energy storage material, during use of
the capsules, or any combination thereof. For applications such as
transportation, the metal sheet preferably is relatively thin so
that the weight of the heat storage device is not greatly increased
by the metal sheet. Suitable thicknesses of the metal sheet may be
greater than about 10 .mu.m, preferably greater than about 20
.mu.m, and more preferably greater than about 50 .mu.m. The metal
foil may have a thickness less than about 3 mm, preferably less
than 1 mm, and more preferably less than 0.5 mm (e.g., less than
about 0.25 mm).
Thermal Energy Storage System
[0051] The heat storage device may be employed in a thermal energy
storage system. The thermal energy storage material system may be
used in an operational cycle containing three phases, a charge
phase, a storage phase, and a discharge phase.
[0052] The thermal energy storage system preferably includes a
means of heating the phase change material in the heat storage
device, so that when the heat storage device is at a temperature
sufficient to cause the combined vapor pressure of all components
of the working fluid to exceed 1 atmosphere and the working fluid
valve is opened to allow flow of the working fluid, the working
fluid is a) pumped by the capillary structure; b) at least
partially vaporized; c) at least partially transported to the
condenser; and at least partially condenses in the condenser; so
that heat is removed from the heat storage device.
[0053] The thermal energy system of the present invention may
include: a heat storage device as described herein, a condenser
(e.g., having an inlet for the working fluid and an outlet for the
working fluid), a vapor line (e.g., a vapor tube) connecting the
working fluid inlet of the condenser to the working fluid outlet of
the heat storage device, and a working fluid liquid line (e.g., a
liquid tube) connecting the working fluid outlet of the condenser
to the working fluid inlet of the heat storage device. As described
hereinbefore, the working fluid compartment preferably includes a
capillary structure. As such the thermal energy storage system may
contain a capillary pumped loop including the working fluid
compartment in the heat storage device, a working fluid compartment
in the condenser, the working fluid vapor line, and the working
fluid liquid line. The condenser is capable of removing heat from
the working fluid so that the working fluid partially or preferably
entirely condenses. The vapor line is capable of containing the
working fluid (e.g., in a vapor phase), without leaking, as it
flows from the heat storage device to the condenser. The working
fluid liquid line is capable of containing the working fluid (e.g.,
in a liquid phase), without leaking, as it flows from the condenser
to the heat storage device.
[0054] The thermal energy storage system may also include a working
fluid reservoir that is capable of storing excess working fluid so
that when the fluid is being pumped by the capillary pump, the
liquid line is filled with the working fluid. The working fluid
reservoir may have a fill level that is higher in elevation than
the working fluid inlet of the heat storage device, lower than the
elevation of the working fluid inlet of the condenser, or both. The
capillary pumped loop may have one or more valves, such as a valve
in the working fluid liquid line. The valve in the working fluid
liquid line may be used to prevent the working fluid from
circulating in the capillary pumped loop when the heat storage
device is charging, when the heat storage device is storing heat,
or both. The valve may be opened when it is desired to discharge
heat from the heat storage device (e.g., to heat an internal
combustion engine).
[0055] The thermal energy storage system may include a heat
transfer fluid inlet line (which may be a tube, a pipe, or the
like) and a heat transfer fluid outlet line, respectively for
flowing the heat transfer fluid into and out of the heat storage
device. The heat transfer fluid inlet line and the heat transfer
fluid outlet line are capable of containing the heat transfer fluid
(e.g., while it flows) without leaking or cracking. For example the
heat transfer fluid lines preferably do not leak or crack at the
pressures of the heat transfer fluid. The thermal energy storage
system may also have a heat transfer fluid bypass line capable of
containing the heat transfer fluid so that it may flow unobstructed
outside of the heat storage device without leaking. The heat
transfer bypass line may be employed when the thermal energy
storage material in the heat storage device is at or above its
maximum nominal temperature, or when the temperature of the heat
transfer fluid is above a critical temperature at which degradation
of the thermal energy storage material may occur. The thermal
energy storage system may also include a valve such as a diverter
valve (e.g., a bypass valve) capable of controlling the amount of
the heat transfer fluid that flows through the heat storage device
and the amount of the heat transfer fluid that flows through the
bypass line. The diverter valve may be employed to divert some or
all of the heat transfer fluid to the bypass line (e.g., when the
heat storage device is fully charged, or when the temperature of
the heat transfer fluid is below a temperature of the thermal
energy storage material in the heat storage device). The diverter
valve may allow some or preferably all of the heat transfer fluid
to flow into the heat storage device when one or any combination
(e.g., all) of the following conditions are met: the temperature of
the thermal energy storage material in the heat storage device is
below the temperature of the heat transfer fluid, the heat storage
device is not fully charged, or the temperature of the heat
transfer fluid is below the maximum nominal temperature of the heat
storage device.
[0056] The heat transfer fluid used to heat the heat storage device
may be any liquid or gas so that the fluid flows through the heat
storage device (e.g., without solidifying) when it is cold. For
example, the heat transfer fluid may be a liquid or gas at a
pressure of about 1 atmosphere pressure and a temperature of about
25.degree. C., preferably about 0.degree. C., more preferably
-20.degree. C., and most preferably at about -40.degree. C. Without
limitation, a preferred heat transfer fluid for heating the heat
storage device is an exhaust gas, such as an exhaust gas from an
engine (e.g., an internal combustion engine).
[0057] The condenser of the thermal energy storage system may be a
heat exchanger capable of transferring thermal energy from the
working fluid to another fluid. For example, the condenser may be
employed for transferring heat from the working fluid to a heat
transfer fluid. The heat transferred in the condenser (e.g., in the
heat exchanger) preferably includes the heat of vaporization of the
working fluid. The thermal energy storage system may include a cold
line for providing a heat transfer fluid into the heat exchanger,
and a heat line for removing heat transfer fluid from the heat
exchanger. The cold line and the heat line preferably are capable
of containing the heat transfer fluid of the heat exchanger without
leaking as it is flows through a loop. The cold line and heat line
may be part of a heat transfer fluid loop. The heat transfer fluid
loop may be connected to an object to be heated. Without
limitation, the object to be heated may be an internal combustion
engine, a vehicle cockpit, an oil reservoir, or any combination
thereof. The heat transfer fluid used in the heat transfer fluid
loop may be a liquid or a gas. Preferably, the heat transfer fluid
is capable of flowing at the lowest operating temperature that it
may be exposed to during use (e.g., the lowest ambient
temperature). Any heat transfer fluid employed in heating the heat
storage device may also be employed in the heat exchanger.
Preferably, the heat transfer fluid of the heat exchanger is a
liquid. For example, any art known engine coolant may be employed
as the heat transfer fluid. A particularly preferred heat transfer
fluid is a mixture of a glycol and water.
[0058] As described above, the thermal energy storage system
includes a means of heating the phase change material in the heat
storage device. When the heat storage device (e.g., the phase
change material in the heat storage device) is at a temperature
sufficient to cause the combined vapor pressure of all components
of the working fluid to exceed about 1 atmosphere and the working
fluid valve is opened to allow flow of the working fluid, the
working fluid is a) pumped by the capillary structure; b) at least
partially vaporized; c) at least partially transported to the
condenser; and d) at least partially condenses in the condenser; so
that heat is removed from the heat storage device.
Working Fluids
[0059] Suitable working fluids (e.g., for the capillary pumped
loop) include pure substances and mixtures having one or any
combination of the following characteristics: a good chemical
stability at the maximum thermal energy storage system temperature,
a low viscosity (e.g., less than about 100 mPas), good wetting of
the capillary structure (e.g., good wick wetting), chemical
compatibility with (e.g., the working fluid causes low corrosion
of) the materials of the capillary pumped loop (such as the
container material, the materials employed to encapsulate the
thermal energy storage material, the materials of the vapor and
liquid lines, and the like), a temperature dependent vapor pressure
that is conducive to both the evaporator and the condenser
temperatures, a high volumetric latent heat of vaporization (i.e.,
the product of the latent heat of fusion and the density of the
working fluid at about 25.degree. C. in units of Joules per liter),
or a freezing point less than or equal to the freezing point of the
heat transfer fluid of the condenser (e.g., a freezing point less
than or equal to the freezing point of antifreeze, a freezing point
less than or equal to about -40.degree. C., or both). For example,
the equilibrium state of the working fluid may be at least 90%
liquid at a temperature of -40.degree. C. and a pressure of 1
atmosphere.
[0060] The vapor pressure of the working fluid should be high
enough in the evaporator so that a vapor stream is produced that is
sufficient to pump the working fluid. Preferably, the vapor
pressure of the working fluid should be high enough in the
evaporator so that a vapor stream is produced that is sufficient to
carry the desired thermal power measured in watts from the
evaporator to the condenser. The vapor pressure of the working
fluid in the evaporator preferably is sufficiently low so that the
capillary pumped loop does not leak and does not rupture.
[0061] The wetting of the working fluid to the capillary structure
may be characterized by a contact angle of the working fluid on the
material of the capillary structure. Preferably, the contact angle
is less than about 80.degree., more preferably less than about
70.degree., even more preferably less than about 60.degree., and
most preferably less than about 55.degree..
[0062] The working fluid preferably condenses at moderate pressures
at temperatures below about 90.degree. C. For example, the working
fluid may condense at about 90.degree. C. at a pressure less than
about 2 MPa, preferably less than about 0.8 MPa, more preferably
less than about 0.3 MPa, even more preferably less than about 0.2
MPa, and most preferably less than about 0.1 MPa.
[0063] The working fluid preferably can flow at very low
temperatures. For example, the working fluid may be exposed to very
low ambient temperatures and preferably is capable of flowing from
the condenser to the heat storage device at a temperature of about
0.degree. C., preferably about -10.degree. C., more preferably
about -25.degree. C., even more preferably about -40.degree. C.,
and most preferably about -60.degree. C. The working fluid
preferably is in a gas state when it is at a temperature of the
fully charged heat storage device. For example, the working fluid
may have a boiling point at 1 atmosphere less than the phase
transition temperature of the thermal energy storage material in
the heat storage device, preferably at least 20.degree. C. less
than the phase transition temperature of the thermal energy storage
material, and more preferably at least 40.degree. C. less than the
phase transition temperature of the thermal energy storage
material. In various aspects of the invention, it may be desirable
for the working fluid to have a boiling point at 1 atmosphere (or
the temperature at which the combined vapor pressure of all of the
components of the working fluid is equal to 1 atmosphere may be)
greater than about 30.degree. C., preferably greater than about
35.degree. C., more preferably greater than about 50.degree. C.,
even more preferably greater than about 60.degree. C., and most
preferably greater than about 70.degree. C. (e.g., so that the
working fluid is a liquid at ambient conditions). In various
aspects of the invention, the boiling point at 1 atmosphere of the
working fluid may be (or the temperature at which the combined
vapor pressure of all of the components of the working fluid is
equal to 1 atmosphere may be) less than about 180.degree. C.,
preferably less than about 150.degree. C., more preferably less
than about 120.degree. C., and most preferably less than about
95.degree. C.
[0064] The working fluids may be any fluid that can partially or
completely evaporate in the heat storage device when the thermal
energy storage material is at or above its liquidus temperature.
Without limitation, exemplary working fluids may include or consist
essentially of one or more alcohols, one or more ketones, one or
more hydrocarbons, a fluorocarbon, a hydrofluorocarbon (e.g., an
art known hydrofluorocarbon refrigerant, such as an art known
hydrofluorocarbon automotive refrigerant), water, ammonia, or any
combination thereof.
[0065] A particularly preferred working fluid includes or consists
substantially of water and ammonia. For example, the combined
concentration of water and ammonia in the working fluid may be at
least about 80 wt. %, more preferably at least about 90 wt. %, and
most preferably at least about 95 wt. % based on the total weight
of the working fluid water and ammonia. The concentration of
ammonia may be sufficient to keep the boiling point of the working
fluid below the boiling point of water (e.g., at least 10.degree.
C. below the boiling point of water). The concentration of ammonia
may be greater than about 2 wt. %, preferably greater than about 10
wt. %, more preferably greater than about 15 wt. % and most
preferably greater than about 18 wt. % based on the total weight of
the working fluid. The concentration of ammonia may be less than
about 80 wt. %, preferably less than about 60 wt. %, more
preferably less than about 40 wt. % and most preferably less than
about 30 wt. % based on the total weight of the working fluid. The
concentration of water in the working fluid may be greater than
about 20 wt. %, preferably greater than about 40 wt. %, more
preferably greater than about 60 wt. % and most preferably greater
than about 70 wt. % based on the total weight of the working fluid.
The concentration of water in the working fluid may be less than
about 98 wt. %, preferably less than about 95 wt. %, more
preferably less than about 90 wt. %, even more preferably less than
about 85 wt. %, and most preferably less than about 82 wt. % based
on the total weight of the working fluid. For example, a solution
of about 21 wt. % ammonia and about 79 wt. % water have a liquidus
point of about -40.degree. C. and the upper limit of a boiling
range at 1 atmosphere of less than about 100.degree. C. This
solution may be stored (e.g., as a liquid) in a non-pressurized
container at room temperature.
[0066] Preferably, the working fluid has a combined vapor pressure
of all of its components equal to 1 atmosphere at one temperature
from about 0.degree. C. to about 250.degree. C.
[0067] The working fluid is capable of efficiently transferring
thermal energy from the heat storage device so that the amount of
working fluid needed to remove an amount of heat from the heat
storage device is relatively small (e.g., compared to a device that
uses a heat transfer fluid that is not a working fluid to remove
the heat). Preferably, a large portion of the heat transferred by
the working fluid is transferred in the form of heat of
vaporization. The volume of working fluid, the flow rate of the
working fluid, or both, may be relatively low in the thermal energy
storage compared to a system that employs a heat transfer fluid
that is not a working fluid and has the same initial power. The
flow rate of the working fluid (i.e., the working fluid in the
liquid state flowing into the heat storage device) per liter of the
container of the heat storage device may be less than about 5
liters/min, preferably less than about 2 liters/min, more
preferably less than about 1 liter/min, even more preferably less
than about 0.5 liters/min, and most preferably less than about 0.1
liters/min. The ratio of the volume of the working fluid (e.g., in
the system or in the capillary pumped loop) in the system to the
total volume of the container (i.e., the volume inside the
container) of the heat storage device (or even the ratio of the
volume of the working fluid in the system to the volume of the
thermal energy storage material in the heat storage device) may be
less than about 20; preferably less than about 10, more preferably
less than about 4, even more preferably less than about 2, and most
preferably less than about 1.
[0068] As described above, the working fluid may transfer some of
the thermal energy in the form of heat of vaporization. The working
fluid preferably has a high heat of vaporization so that the amount
of heat that can be transferred is high. Suitable working fluids
for the heat storage device may have a heat of vaporization greater
than about 200 kJ/mole, preferably greater than about 500 kJ/mole,
more preferably greater than about 750 kJ/mole, even more
preferably greater than about 1,000 kJ/mole, and most preferably
greater than about 1,200 kJ/mole.
[0069] In applications where the temperature of the working fluid
may be less than 0.degree. C., the working fluid preferably is not
water (e.g., so that the working fluid does not freeze, cause a
rupture, or both).
[0070] It will be appreciated that the materials that contact with
the working fluid may be resistant to corrosion from the working
fluid. For example, any one or all of the surfaces of the heat
storage device or thermal energy storage system that may come in
contact with the working fluid (e.g., the interior of the working
fluid vapor line, the interior of the working fluid liquid line,
the surfaces of the working fluid compartment of the heat storage
device, the interior surfaces of the working fluid valve, the
surface of a working fluid compartment in the condenser, the
interior surface of the working fluid reservoir, and the like) may
be made of stainless steel.
[0071] It will be appreciated that any of the working fluids or
heat transfer fluids employed in the thermal energy storage system
described herein may include an additives package. For example, the
additives package may include a stabilizer, a corrosion inhibitor,
a lubricant, an extreme pressure additive, or any combination
thereof.
Operation of A Thermal Energy Storage System
[0072] The thermal energy storage system has a plurality of
operational phases including a charging phase where heat from
outside the heat storage device is provided to the thermal energy
storage material, a storing phase where at least some of the heat
is stored in the thermal energy storage material, and a discharging
phase where at least some of the heat is removed from the thermal
energy storage material.
1. Charge Phase
[0073] The charge phase may occur when the temperature of the heat
storage device (is below its maximum nominal temperature and the
heat transfer fluid (e.g., exhaust gas)) has a temperature greater
than the temperature of the thermal energy storage material. During
the charge phase, the step of charging the thermal energy storage
material (e.g., the phase change material) may include a step of
transferring heat from the heat transfer fluid to the thermal
energy storage material. During the charge phase, the discharge
valve for the working fluid preferably is closed. Any residue of
liquid working fluid in the evaporator (i.e., the working fluid
compartment of the heat storage device) may boil off, enter the
condenser, become liquid in the condenser and enters the reservoir.
An exhaust gas bypass (such as a bypass activated by a valve shown
in FIG. 1) may be used to prevent overheating the heat storage
device when the thermal energy storage system is fully charged or
when the exhaust is hot enough to cause local phase change material
overheating, which may result in phase change material degradation.
A temperature sensor is preferably embedded in the vicinity of the
phase change material to prevent overheating it by triggering the
exhaust bypass valve. Other control strategies may preferably be
used to prevent phase change material overheating.
2. Storage Phase
[0074] When the internal combustion engine is shut down, for
example, when the vehicle is parked, the discharge valve remains
closed. The heat stored in the thermal energy storage system is
slowly lost to the environment. Therefore, some form of insulation
is preferably used in the present invention. The better the
insulation of the system is, the longer is the storage time.
[0075] Any known form of insulation which prevents loss of heat by
the heat storage device may be utilized. For example, any
insulation as disclosed in U.S. Pat. No. 6,889,751, incorporated
herein of its entirety by reference, may be employed. The heat
storage device preferably is (thermally) insulated container, such
that it is insulated on one or more surfaces. Preferably, some or
all surfaces that are exposed to ambient or exterior will have an
adjoining insulator. The insulating material may function by
reducing the convection heat loss, reducing the radiant heat loss,
reducing the conductive heat loss, or any combination. Preferably,
the insulation may be through the use of an insulator material or
structure that preferably has relatively low thermal conduction.
The insulation may be obtained through the use of a gap between
opposing spaced walls. The gap may be occupied by a gaseous medium,
such as an air space, or possibly may even be an evacuated space
(e.g., by use of a Dewar vessel), a material or structure having
low thermal conductivity, a material or structure having low heat
emissivity, a material or structure having low convection, or any
combination thereof. Without limitation, the insulation may contain
ceramic insulation (such as quartz or glass insulation), polymeric
insulation, or any combination thereof. The insulation may be in a
fibrous form, a foam form, a densified layer, a coating or any
combination thereof. The insulation may be in the form of a woven
material, an unwoven material, or a combination thereof. The heat
transfer device may be insulated using a Dewar vessel, and more
specifically a vessel that includes generally opposing walls
configured for defining an internal storage cavity, and a wall
cavity between the opposing walls, which wall cavity is evacuated
below atmospheric pressure. The walls may further utilize a
reflective surface coating (e.g., a mirror surface) to minimize
radiant heat losses.
[0076] Preferably, a vacuum insulation around the system is
provided. More preferably, a vacuum insulation as disclosed in U.S.
Pat. No. 6,889,751, incorporated herein of its entirety by
reference, is provided.
3. Discharge Phase
[0077] When the heat stored in the thermal energy storage system
needs to be transferred to the object to be heated, the discharge
valve opens to a desired degree depending on the required discharge
power (e.g., in units of Watts (W)). Liquid working fluid stored in
the reservoir enters the evaporator driven by gravity, wets the
wick, flows along the wick upward driven by capillary pressure and
gets evaporated by heat flowing from the phase change material. The
vapor flows along the gaps between the wicks and then into the
condenser, where it releases its heat stored as both the latent
heat of vaporization and sensible heat to coolant, which circulates
between the condenser and a cold internal combustion engine and/or
air heater core. During a high-power discharge the vapor pressure
in the evaporator can substantially exceed that in the condenser.
This pressure difference tries to push the liquid out of the
evaporator along the liquid line. Without being bound by theory, it
is believed that the capillary pressure formed by microscopic
menisci of liquid filling the pores of the capillary structure
(e.g., the wick) is what holds this pressure and keeps "pumping"
liquid into the evaporator. The capillary pressure is inversely
proportional to the wick's pore size (Young-Laplace equation). The
vapor has no other option to release its pressure but to flow into
the condenser through the vapor line. This establishes a circular
flow pattern inside a capillary pumped loop. When a desired amount
of heat has been transferred by the capillary pumped loop the
discharge valve closes.
[0078] During the discharge phase, the device and system of the
present invention may have a relatively high power output (in units
of Watts), a relatively high power output density (e.g., in units
of Watts per liter of volume in the heat storage device), or both.
The power output, the power output density, or both may be greater
than (e.g., at least 20% greater than, more preferably at least
about 100% greater than) that of an identical device or system with
the exception that it does not have a capillary structure. For
example, the device, system, or both may have an average power
density of at least about 1 kW/liter, preferably at least about 10
kW/liter, more preferably at least about 25 kW/liter, even more
preferably at least about 30 kW/liter, and most preferably at least
about 50 kW/liter based on the total internal volume of the
insulated volume of the heat storage device (e.g., the sum of the
volumes of the thermal energy storage material compartment, the
heat transfer fluid compartment and the working fluid compartment),
where the power is averaged over initial discharge operation (e.g.,
the first 30, 60, or 120 seconds of the discharge operation), which
begins for example, when a valve is opened allowing for flow of the
working fluid through the first flow path of the heat storage
device, and when a substantial portion (e.g., when at least 50% by
volume, or at least 75% by volume) of the phase change material is
in a liquid state at the start of the discharge operation (e.g. at
the time the valve is opened).
[0079] It should be appreciated that the thermal energy storage
system of the present invention may also be operated in a
"combination" mode (e.g., as a steady-state mode) by simultaneously
charging and discharging the thermal energy storage material (i.e.,
by simultaneously performing both the charge and the discharge
phases). In the combination mode of operation, both the discharge
valve of the working fluid is open and heat transfer fluid (e.g.,
hot exhaust gas) flows through the heat storage device. The
"combination" mode of operation may establish a continuous (e.g., a
constant) flow of heat from the exhaust gas to a heat recipient. An
advantage of the present system over other prior art steady-state
exhaust heat recovery devices is its ability to levelize the
fluctuations of thermal power of the exhaust stream (which is very
common in urban traffic) and deliver a more stable thermal power to
the recipient by using the large heat storage capacity of the phase
change material, essentially acting as a heat buffer between the
exhaust gas and the heat recipient. The levelization of the heat
flow can be very beneficial in ensuring optimal operation of
devices powered by the exhaust heat, e.g. Rankine cycle heat engine
(a.k.a. turbo-steamer), absorption or adsorption cycle
refrigeration system or simply a cabin air heater.
[0080] The thermal energy storage system may employ one or more
means to minimize the heat losses from the thermal energy storage
system to the environment. Exemplary means of minimizing heat
losses include insulating one or more components of the thermal
energy storage system (e.g., the heat storage device, a line, the
evaporator, or any combination thereof), use of low thermal
conductivity materials, use of geometries and/or coatings that
reduce radiative heat losses or heat flow distances, or any
combination thereof.
[0081] Without limitation, any of the insulation means disclosed in
U.S. patent application Ser. No. 12/389,598 entitled "Heat Storage
Devices" and filed on Feb. 20, 2009 may be employed.
[0082] As an example, the heat storage device may employ a vacuum
(e.g., a high-vacuum) jacket insulation, optionally with thin
internal radiative screens to slow down radiative heat transfer
between the internal and the external walls of the vacuum jacket.
The rate of radiative heat transfer is roughly inversely
proportional to the number of vacuum gaps between radiative screens
along the heat flow path. This approach is analogous to a double
Dewar flask. As such, the insulation may employ a one, two, three,
or more vacuum gaps.
[0083] The heat storage device may employ one or more materials
having a relatively low thermal conductivity to reduce or minimize
the heat losses to the ambient. For example, the heat storage
device may employ one or more materials having a thermal
conductivity less than 50% of the thermal conductivity of low
carbon steel (e.g., A36 grade), preferably less than 30% of the
thermal conductivity of low carbon steel, more preferably less than
20% of the thermal conductivity of low carbon steel, and most
preferably less than 10% of the thermal conductivity of low carbon
steel. Exemplary low thermal conductivity materials that may be
employed include, without limitation stainless steel, titanium
alloys, silica-based glass, or any combination thereof. For
example, low thermal conductivity materials may be employed for
lines (e.g., tubes) that connect the inlets and outlets of the heat
storage device to the condenser, to a heat source (e.g., an exhaust
pipe), or both.
[0084] The heat losses may be reduced by selecting a geometry of
one or more (e.g., even all) of the connecting lines (e.g., tubes
or pipes) that increases the heat path distance. For example, the
geometry of a line may employ thin-walled bellows instead of smooth
(e.g., cylindrical) walls. The lines may be curved (e.g., have a
substantially curved center line), so that direct "line-of-sight"
radiative heat transfer between the heat storage device and a
portion of the thermal energy storage system that is not insulated
is substantially reduced or even eliminated. As such, the geometry
may be selected to reduce the radiative aperture of a line without
substantially increasing the hydraulic resistance of the line.
Additionally, one or more sides of the line may be coated with a
coating capable of reducing indirect radiative heat losses. Such
coatings are generally a reflective coating such as silver.
[0085] In a preferred embodiment of the present invention, the
system has the following characteristics. The encapsulant sheet
thickness will be between 0.01-2 mm. The phase change material
capsule size is between 0.5-100 mm. Fluid gap between capsules is
between 0.1-10 mm. Dimensions of the blister pack depend on the
dimensions of the heat exchanger, which will vary a lot depending
on the application. It can be as small a single capsule dimension
or as large as a few meters for heating and air conditioning of
large buildings.
[0086] The thermal energy storage system may be employed in a
transportation vehicle (e.g., an automotive vehicle) for storing
energy from an engine exhaust gas. When the engine produces exhaust
gas, a bypass valve may either direct the flow of the gas through
the heat storage device so that the heat storage device is charged,
or through a bypass line to prevent the heat storage device from
overheating. When the engine is shut down, e.g., during a period
when the vehicle is parked, a substantial portion of the heat
stored in the heat storage device may be retained for a long time
(e.g., due to vacuum insulation surrounding the heat storage
device). Preferably, at least 50% of the thermal energy storage
material in the heat storage device remains in a liquid state after
the vehicle has been parked for 16 hours at an ambient temperature
of about -40.degree. C. If the vehicle is parked for a long enough
time (e.g., at least two or three hours) for the engine to cool
down substantially (e.g., so that the difference in temperature
between the engine and the ambient is less than about 20.degree.
C.), the heat stored in the heat storage device may be discharged
into the cold engine or other heat recipient indirectly by flowing
a heat transfer fluid (such as the engine coolant) through the heat
exchanger that includes the condenser for the working fluid. The
working fluid is circulated in a capillary pumped loop using the
capillary structure inside the heat storage device where the
working fluid is vaporized. The heat from the working fluid is
transferred to the engine coolant in the heat exchanger. By
employing the heat storage device, heat that otherwise would be
wasted may be captured during a previous trip to mitigate cold
start and/or provide instant cockpit heating.
[0087] The transfer of heat using the working fluid may begin by
opening the working fluid valve (i.e., the discharge valve). The
sealed working fluid reservoir connected to the loop via an
additional liquid line serves to accommodate changes in the working
fluid liquid volume inside the loop without substantial pressure
changes. Once sufficient or all useful heat is transferred from the
heat storage device, the discharge valve may close. The remaining
working fluid in the heat storage device may evaporate (e.g., from
heat remaining in the heat storage device or when the heat storage
device begins to charge) and then condenses in the condenser. As
the heat storage device becomes evacuated of the working fluid, the
liquid level of the working fluid level may change (e.g.,
rise).
[0088] The heat storage device may be a cross-flow heat exchanger
(i.e., having a flow direction for the working fluid and a
perpendicular flow direction for the flow of the exhaust gas).
During operation, the heat storage device may include three
chambers occupied by 1) exhaust gas; 2) stagnant phase change
material (e.g., inside capsules, such as a blisters pack); and 3)
working fluid. All three chambers are kept separate by thin walls
made of an appropriate material, preferably stainless steel.
Exhaust gas may flow between the surfaces (e.g., the curved
surfaces) of the capsules of phase change material inside blisters,
and the working fluid may flow between different surfaces (e.g.,
flat surfaces) of the capsules of phase change material inside
blisters in a direction that is generally perpendicular to the
exhaust gas flow direction. The liquid working fluid entering its
chamber preferably wets a capillary structure (e.g., a metal wick)
and gets transported up against the combined forces of gravity and
vapor pressure by the capillary forces acting upon the working
fluid liquid menisci formed inside the capillaries. This flow is
sustained by continuous evaporation of the liquid using the heat
drawn from the phase change material inside blisters. The vapor of
working fluid leaves the capillary structure and escapes to the top
of the device via vapor channels which may be interdigitated
between columns of the capillary structure squeezed between the
surfaces (e.g., the flat surfaces) of the capsules of phase change
material inside blisters. The vapor of working fluid flows into the
condenser where it transfers its heat of vaporization and sensible
heat to the cold coolant and becomes liquid again to return to the
heat storage device and continue its circulation in the loop, being
pumped only by the capillary forces existing inside the capillary
structure (e.g., metal wick) that is partially impregnated by
liquid working fluid. All columns of the capillary structure may be
connected to a common porous base. Such a porous base may be
employed to distribute the liquid working fluid entering from the
bottom of the device to the different columns.
[0089] Furthermore, the present invention may be used in
combination with additional elements/components/steps. For example,
absorption or adsorption cycle refrigeration system for air
conditioning may be used as the heat recipient instead of or in
addition to the cold coolant (e.g., the condenser may serve also as
an evaporator for the refrigerant circulating inside an air
conditioner's fluid loop). In another application, a steady-state
waste heat recovery system using a heat engine, e.g., a Rankine
cycle, can be constructed so that it uses the same or different
capillary pumped loop working fluid and adds a mechanical power
generating turbine to'the vapor line between the heat storage
device and the condenser, (e.g., to overcome high vapor pressure
upstream from the turbine), and/or adds a liquid pump to the liquid
line between the condenser and heat storage device. The above
turbine can convert a part of the captured from the exhaust gas
waste heat into useful mechanical or electrical work and thus
improve the overall fuel efficiency of the vehicle.
[0090] While the present invention may be susceptible to various
modifications and alternative forms, the exemplary embodiments
discussed above have been shown by way of example. However, it
should again be understood that the invention is not intended to be
limited to the particular embodiments disclosed herein. Indeed, the
present techniques of the invention are to cover all modifications,
equivalents, and alternatives falling within the spirit and scope
of the invention as defined by the following appended claims.
[0091] FIG. 1 is a thermal energy storage heat storage device of
the present invention including an inventive heat storage device.
As illustrated in FIG. 1, the heat storage device 10 may include a
container 12 having a working fluid inlet 14, a working fluid
outlet 16, a heat transfer fluid inlets 18 and a heat transfer
fluid outlets 20. The volume of the heat storage device is about 1
liter and the thermal energy storage material fills more than about
60 volume % of the heat storage device.
[0092] FIG. 2A is an illustrative cross-section (i.e., the
cross-section as shown in FIG. 1) of the heat storage device 10 of
FIG. 1. The heat storage device includes layers of capsules 32,
with spacing between each adjacent layer of capsules. The layers of
capsules 32 each have a surface that is arcuate 34 and an opposing
surface that is generally flat 36. At least a portion (e.g., a
layer) of the heat transfer fluid compartment 26 is interposed
between two layers of capsules. A portion (e.g., a layer) of the
working fluid compartment 22 is interposed between two adjacent
layers of capsules and the average thickness of the working fluid
compartment, defined by the distance (e.g., average distance) of
separation of the two layers of capsules, is about 1 mm. The layers
of capsules generally have a layer of the working fluid compartment
22 on one side of the capsule layer and a layer of the heat
transfer fluid compartment 26 on an opposing side. The average
thickness of a layer of the heat transfer fluid compartment is
about 1 mm.
[0093] As shown in FIGS. 1, 2A and 2B, the heat storage device has
an inlet 14 for flowing a working fluid into the working fluid
compartment of the heat storage device and an outlet 16 for flowing
the working fluid out of the heat storage device. The outlet is at
a higher elevation than the inlet, so that the flow of the working
fluid includes a generally vertical component. The working fluid is
a mixture of about 79 wt. % water and about 21 wt. % ammonia. The
working fluid is in thermal contact with the thermal energy storage
material 30. At least some of the working fluid compartment
includes 5 mm strips of a metal wick that forms a capillary
structure. The cross-section of FIG. 2A shows a region of the
working fluid compartment 22 in which all of the layers of the
working fluid compartments have the metal wick. Between the 5 mm
strips of wick are 10 mm wide sections that are free of metal wick,
as shown in the cross-section FIG. 2B. As illustrated in FIG. 2A,
the capillary structure (i.e., metal wick) may extend the length of
the working fluid compartment. A portion of the capillary structure
is in thermal contact with each of the capsules containing the
phase change material (e.g., the capillary structure is in contact
with a portion of the generally flat outer surface next to each
capsule).
[0094] The thermal energy system of the present invention is shown
in FIG. 3. The thermal energy storage system 50 includes the heat
storage device, a condenser 52 having an inlet 56 for the working
fluid 54 and an outlet 58 for the working fluid, a vapor tube 60
connecting the working fluid inlet 56 of the tube connecting the
working fluid outlet 58 of the condenser 52 to the working fluid
inlet of the heat storage device. The thermal energy storage system
contains a capillary pumped loop including the working fluid
compartment in the heat storage device, a working fluid compartment
in the condenser, the working fluid vapor tube, and the working
fluid liquid tube. The thermal energy storage system also includes
a working fluid reservoir 74. The working fluid reservoir has a
fill level that is higher in elevation than the working fluid inlet
of the heat storage device and lower than the elevation of the
working fluid inlet of the condenser 58. The capillary pumped loop
may have a valve 72 in the working fluid liquid tube 62. The valve
is used to prevent the working fluid from circulating in the
capillary pumped loop when the heat storage device is charging and
when the heat storage device is storing heat. The valve is opened
when it is desired to discharge heat from the heat storage
device.
[0095] Referring again to FIG. 3, the thermal energy storage system
includes a heat transfer fluid inlet line 64 and a heat transfer
fluid outlet line 66, for flowing the heat transfer fluid into and
out of the heat storage device. The thermal energy storage system
also has a heat transfer fluid bypass line 68. The thermal energy
storage system also includes a diverter valve (e.g., a bypass
valve) 70 to divert some or all of the heat transfer fluid to the
bypass line 68 (e.g., when the heat storage device is fully
charged, or when the temperature of the heat transfer fluid is
below a temperature of the thermal energy storage material in the
heat storage device).
[0096] The condenser 52 of the thermal energy storage system is a
heat exchanger The thermal energy storage system includes a cold
line 80 for providing a heat transfer fluid into the heat
exchanger, and a heat line 78 for removing heat transfer fluid from
the heat exchanger. The cold line 80 and heat line 78 are part of a
heat transfer fluid loop 84. The heat transfer fluid loop contains
an engine coolant, is connected to an internal combustion engine 76
and is used to heat the internal combustion engine with the energy
stored in the heat storage device.
* * * * *