U.S. patent application number 10/945714 was filed with the patent office on 2005-02-24 for high flux heat removal system using liquid ice.
Invention is credited to Frankel, Richard S., Kramer, Gary W..
Application Number | 20050039883 10/945714 |
Document ID | / |
Family ID | 32990958 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050039883 |
Kind Code |
A1 |
Kramer, Gary W. ; et
al. |
February 24, 2005 |
High flux heat removal system using liquid ice
Abstract
A cooling system for apparatus powered by electricity, that
generates a substantial amount of heat during operation, and the
heat must be dissipated to avoid failure of electrical and/or
electronic components, such as semiconductor devices and integrated
circuits, comprising the electrical apparatus. The cooling system
employs liquid ice impinged on a heat sink thermally coupled with
electrical apparatus. The attendant phase changes of the liquid ice
first to water and then to steam remove a substantial amount of
waste heat to prevent failure of the electrical apparatus.
Inventors: |
Kramer, Gary W.; (Auburn,
CA) ; Frankel, Richard S.; (Woodside, CA) |
Correspondence
Address: |
William C. Milks, III
RUSSO & HALE LLP
401 Florence Street
Palo Alto
CA
94301
US
|
Family ID: |
32990958 |
Appl. No.: |
10/945714 |
Filed: |
September 20, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10945714 |
Sep 20, 2004 |
|
|
|
10460351 |
Jun 12, 2003 |
|
|
|
6793007 |
|
|
|
|
Current U.S.
Class: |
165/80.3 ;
257/E23.1 |
Current CPC
Class: |
H05K 7/20345 20130101;
F25B 2339/047 20130101; Y02P 60/855 20151101; H01L 23/4735
20130101; F25C 1/14 20130101; Y10S 165/908 20130101; F25C 5/24
20180101; F25D 17/02 20130101; Y02P 60/85 20151101; H01L 2924/0002
20130101; C09K 5/10 20130101; F25C 1/16 20130101; F25C 2301/002
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
165/080.3 |
International
Class: |
F28F 007/00 |
Claims
1-18. (canceled).
19. A method for thermally conducting and removing high heat flux
waste heat from electrical apparatus, comprising: providing liquid
ice at a predetermined pressure; providing a heat transfer plate
having a first side thermally coupled with the electrical
apparatus, the heat transfer plate being constructed from a
thermally conductive material with high tensile strength to enable
efficient heat transfer by thermal conduction; and impinging the
liquid ice on a second side of the heat transfer plate opposite the
side on which the electronic apparatus is disposed.
20. The method according to claim 19 wherein the predetermined
pressure is a partial vacuum, whereby the temperatures associated
with phase changes of melting and boiling are lowered.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to cooling systems and, more
particularly, to cooling systems for electrical apparatus.
Generally, the present invention provides a cooling system for
apparatus powered by electricity, that generates a substantial
amount of heat during operation, and the heat must be dissipated to
avoid failure of electrical and/or electronic components, such as
semiconductor devices and integrated circuits, comprising the
electrical apparatus. Specifically, one embodiment of the present
invention provides a cooling system preferably employing liquid ice
jet impinged on a heat sink thermally coupled to electrical
apparatus, and the attendant phase changes of the liquid ice first
to water and then to steam to remove a substantial amount of waste
heat to prevent failure of the electrical apparatus.
[0003] 2. Description of the Prior Art
[0004] Cooling is an important process associated with operation of
high-density electronic devices. Existing waste heat removal
technology is limited to approximately 100 W/cm.sup.2.
[0005] In the next ten years, the power density of high-power
electronics is expected to increase and generate waste heat that
will exceed 1,000 W/cm.sup.2. Thermal management technology capable
of removing waste heat of 1,000 W/cm.sup.2 produced by advanced
power electronic devices is needed.
[0006] For example, the U.S. Department of Navy has reported that
the cooling requirements are expected to increase at least an order
of magnitude during the next decade. As stated in "Next Generation
Navy Thermal Management Program," CARDIVNSWC-TR-82-2002/12, by
Michael Kuszewski and Mark Zerby, Naval Surface Warfare Center:
[0007] "It is expected that heat fluxes for new technologies such
as Advanced Radar will exceed 1000 W/cm.sup.2, and some advanced
weapons may be higher. These heat fluxes are expected to be present
by the end of this decade. Heat fluxes are growing so fast in the
electronics arena that even Intel, who has been designing its
Thermal Management Systems to handle less than 100 W/cm.sup.2, has
extrapolated its increase of heat flux to reach 1000 W/cm.sup.2
before the end of this decade."
[0008] Accordingly, the U.S. Navy recently published RFQ N03-T022
Acquisition Program: DD(X); CVN(X) having the:
[0009] "OBJECTIVE: To develop advanced thermal management
technologies to improve high flux waste heat removal by a factor of
10.times. over existing technologies in electronic devices."
[0010] Also,
[0011] "The proposed solution must be able to keep the
semiconductor junction below 125F [sic, 125C]. . . . "
[0012] Considered in more detail, spray cooling with water is a
known cooling technique to remove heat from electronics relatively
efficiently. See, Kuszewski and Zerby, supra. In situations where
cooling very hot surfaces or protecting sensitive surfaces from
overheating is important, then the most effective technique
available is direct impact by impingement jets (not necessarily
sprays). The reverse side of a mounting plate, on which the
electronic devices are disposed, is sprayed by high velocity
impinging jets of water. The heat generated by the electronics is
removed at constant temperature by the liquid vapor phase of the
water.
[0013] The heat transfer processes involved in water sprays
impinging on hot surfaces have been studied by, among others,
Bernardin J D, and Mudawar I, "Film boiling heat transfer of
droplet streams and sprays," Intl. J. Heat Mass Transfer, 40 (11),
2579-2593 (1997). Rockwell has also published a paper that reports
having achieved removal of 1,000 W/cm.sup.2 using a water jet plus
boiling. However, Rockwell was only able to cool a very small area
(unspecified).
[0014] The challenge presented by the need to conduct waste heat
from electronic devices efficiently and to provide removal of waste
heat on the order of 1,000 W/cm.sup.2 at a rate that will maintain
the operating temperature of electronic devices at or below
125.degree. C. is imposing. The 125.degree. C. limit requires
efficient heat transfer to sink heat away from the electronic
apparatus. The high heat flux (1,000 W/cm.sup.2) furher requires an
effective heat removal process to maintain the operating
temperature of electronic devices at or below the 125.degree. C.
limit.
[0015] It would therefore be desirable to provide removal of waste
heat from electronic devices to maintain the operating temperature
of electronic devices at or below 125.degree. C. It would also be
desirable to remove waste heat at a rate to prevent the operating
temperature of electronic devices from exceeding the 125.degree. C.
limit. Furthermore, it would be desirable to achieve these
objectives for electrical apparatus that generates waste heat on
the order of 1,000 W/cm.sup.2.
SUMMARY OF THE INVENTION
[0016] One embodiment of the present invention provides a cooling
system for thermally conducting and removing high heat flux waste
heat. The cooling system in accordance with one embodiment of the
present invention employs a refrigerant or coolant, preferably,
liquid ice, and, preferably, at reduced pressure to improve high
heat flux waste heat removal by a factor of ten times over known
cooling techniques. One embodiment of the cooling system in
accordance with the present invention is especially suitable to the
challenge of removing high heat flux waste heat resulting from
operation of power electronics given the severe limitation on the
maximum operating temperature allowable for electronic devices.
[0017] One preferred embodiment of the cooling system in accordance
with the present invention provides a heat transfer plate
consisting of copper, aluminum, silver, or another suitable
thermally conductive material, such as beryllium oxide ceramic,
boron nitride, aluminum nitride ceramic, or diamond, with high
tensile strength to enable efficient heat transfer by thermal
conduction, in thermal contact with the electrical apparatus. The
heat transfer plate also serves as a structural component of a
circulation subsystem that contains the refrigerant or coolant.
Impinging jets deliver copious amounts of a refrigerant or coolant
to the hot surface of the heat transfer plate opposite the side on
which the electronic apparatus is disposed in thermal contact with
the heat transfer plate. In a preferred embodiment of the present
invention, jet impingement of a refrigerant or coolant in the form
of liquid ice is employed.
[0018] Jet impingement of liquid ice is provided on the heat
transfer plate at atmospheric pressure or at a reduced pressure.
Preferably, the liquid ice may be maintained at less than
atmospheric pressure, for example, in a partial vacuum, wherein the
temperatures associated with phase changes of melting and boiling
are lowered.
[0019] With the operating temperature of electronic devices
required to be at 125.degree. C. or below, and the temperature of
the liquid ice at approximately -2.degree. C., there is a large
temperature differential and two phase changes as the liquid ice
first transforms to water and then to steam to effect heat removal
and cooling as the steam is circulated by the circulation subsystem
away from the heat transfer plate. In addition, the super-cooled
liquid consisting of liquid ice maintains steam bubbles associated
with melted liquid ice boiling small, resulting in more effective
heat transfer. Finally, use of liquid ice as a refrigerant or
coolant is compatible with cooling systems aboard ships operated by
the U.S. Navy, thereby satisfying the apparent desirability and
advantage to integrate the cooling system in accordance with the
embodiments of the present invention with other cooling systems on
a ship (for example, air conditioning systems).
[0020] The foregoing and other objects, features, and advantages of
the present invention will become more readily apparent from the
following detailed description of various embodiments, which
proceeds with reference to the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
[0021] The various embodiments of the present invention will be
described in conjunction with the accompanying figure of the
drawing to facilitate an understanding of the present invention. In
the drawing:
[0022] FIG. 1 is a block diagram of one embodiment of the cooling
system in accordance with the present invention employing liquid
ice as a refrigerant or coolant.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The principle underlying the cooling system in accordance
with one embodiment of the present invention is the substantial
amount of heat that is required to transition ice to water and then
water to steam. The cooling system of the present invention
therefore pertains to two distinct operating regimes in cooling,
namely, the heat of fusion incident to the phase change of ice to
water at 0.degree. C. and the heat of evaporation incident to the
subsequent phase change of water to steam at 100.degree. C. These
phase changes occur very rapidly when liquid ice is employed as a
refrigerant or coolant exposed to waste heat on the order of 1,000
W/cm.sup.2.
[0024] Considered in more detail, at 0.degree. C., the temperature
of ice ceases to rise, even though heat continues to be applied and
absorbed. The reason for this phenomenon is added heat is used to
break crystal bonds in the ice. Melting one gram of ice requires 80
calories/gram.
[0025] Only when all of the ice melts does the temperature of the
resulting water begin to rise. Then, each calorie absorbed by the
water increases the water temperature by 1.degree. C. until the
boiling temperature, 100.degree. C., is reached. At this point, the
100.degree. C. temperature remains constant as heat is applied
until the boiling water is vaporized to steam.
[0026] One gram of liquid water at 100.degree. C. must absorb 540
calories of thermal energy in order to vaporize, forming steam. The
540 calories required to vaporize a gram of water at 100.degree. C.
is a relatively large amount of energy, much more than the amount
required to bring a gram of ice at absolute zero (-273.degree. C.)
to boiling water at 100.degree. C.
[0027] It should also be noted that evaporation occurs beneath the
surface of liquid water in the boiling process. Bubbles of vapor
typically form in the water and are buoyed to the surface, where
they escape. Bubbles form at the boiling temperature, when the
pressure of the vapor within them is great enough to resist the
pressure exerted by the surrounding liquid phase water, which is
determined in part by the atmospheric pressure. Lowering the
pressure lowers the boiling temperature.
[0028] The embodiments in accordance with the present invention
provide a cooling system implementing a novel thermal management
method to remove waste heat from electrical apparatus and/or
electronic devices, such as power semiconductors, with heat flux
densities on the order of 1,000 W/cm.sup.2. The embodiments of the
present invention also maintain the semiconductor junctions at
125.degree. C. or below with no waste heat added into the
workspace.
[0029] The cooling system for removing waste heat from electronic
devices using the refrigerant or coolant will be described below.
The refrigerant or coolant employed in accordance with the
embodiments of the present invention is generally referred to as
"liquid ice." The term "liquid ice" means liquid ice, slurry ice,
pumpable ice, slush ice, binary ice, and equivalent material.
Liquid ice typically consists of a suspension of relatively small
(100-700 microns effective diameter) ice crystals in a mixture of
water and freezing point depressant. Preferably, the ice in the
water comprises crystals no greater than 200 microns effective
diameter, although it is conceivable that many crystals of the ice
may form clusters, the aggregate size of which is greater. Liquid
ice, at concentrations in the range of 5-30% water by weight, has
flow properties approximating those of water, i.e., the liquid ice
can be handled, unlike normal ice, by standard equipment, such as
pumps, pipes, and nozzles. Unlike slowly melting normal ice, liquid
ice undergoes near instantaneous phase change because of the vast
surface area to volume ratio of the microscopic liquid ice
crystals.
[0030] Typical freezing point depressants include salts (for
example, NaCl), ethanol, methanol, propylene glycol, or ethylene
glycol. Other contemplated freezing point depressants include
sodium bicarbonate, sugar, acetic acid, citric acid, and Talin. A
mixture of any of the freezing point depressants can also be used.
In the case of a ship, the seawater, which already contains salt,
can be pumped through an icemaker to create ice crystals within the
seawater. The substance used as the freezing point depressant and
the concentration in water largely determine the characteristics of
the resulting liquid ice. See, ASHRAE Research Project 1166,
"Behavior of ice slurry in thermal storage systems," 2002.
[0031] The substance used as a freezing point depressant and its
concentration in water largely determines the particular nature of
the resulting liquid ice. In general, X.sub.ice=1-X.sub.0/X.sub.cf
where X.sub.ice is the equilibrium ice concentration in suspension,
X.sub.cf is the mass concentration of freezing point depressant in
liquid phase, and X.sub.0 is the overall initial concentration of
depressant. Thus, a 20% solids concentration of ice in liquid ice
made from brine can be produced at -6.degree. C. using a starting
solution with 8% by weight of salt.
[0032] Ice crystals in a liquid ice mixture are formed from pure
water. As the number and size of ice crystals increase, the
concentration of freezing point depressant in the remaining fluid
increases until equilibrium is reached where no further ice
formation at the operating temperature of the cooling system
occurs. Since the ice generated consists of pure water only, the
concentration of freezing point depressant in the remaining mixture
rises and acts to "lubricate" the liquid ice, preventing localized
melting and re-freezing that would otherwise cause agglomeration.
At concentrations of up to 5-40% water by weight, liquid ice is
relatively easy to produce (at approximately -2.degree. C.) and
store and has flow properties approximating those of water,
allowing the liquid ice to be handled by standard equipment, such
as pumps, pipes, and nozzles.
[0033] Preferably, the percentage of ice to water by weight is
above 20% and below 40%. However, any percentage of ice will be
effective as long as the liquid ice is pumpable.
[0034] At a 20% concentration, liquid ice has approximately six
times the cooling capacity of water at 6.degree. C., and the latent
heat needed to melt the ice content is about 70 kJ per kilogram of
mixture. When used as a heat exchange fluid in contact with a solid
surface, the volumetric flow rate of liquid ice having a 20%
concentration of solids may be three to eight times lower than that
of chilled water to obtain the same cooling effect. See, Bellas J,
Chaer I, and Tassou S A, "Heat transfer and pressure drop of ice
slurries in plate heat exchangers," Proc. 6.sup.th UK Heat Transfer
Conference, I. Chem. E., Nottingham, England, 2001. At the highest
concentrations compatible with pumpability (approximately 40%),
pressure drops of three times those of chilled water have been
reported. Very high energy transfer rates (above 3
kWm.sup.-2K.sup.-1) are ensured by the vast quantities,
omnipresence, and large surface area of the microscopic ice
crystals, which undergo nearly instantaneous phase change at
constant temperature on receipt of the enthalpy of the crystal.
[0035] The high flux heat removal cooling system in accordance with
various embodiments of the present invention will now be described
in conjunction with FIG. 1. The basic vapor-compression cycle
provided by the cooling system shown in FIG. 1 is similar
regardless of the concentration of the liquid ice mixture. In
accordance with various embodiments of the present invention,
liquid ice is the refrigerant or coolant, at atmospheric pressure
or at a subatmospheric pressure.
[0036] The preferred embodiment in accordance with the present
invention makes use of liquid ice at a subatmospheric pressure. The
particular vapor-compression cycle of the cooling system shown in
FIG. 1 is a high waste heat removal cycle employing active
components including a chiller, evacuator, an array of spray
nozzles, and condensers.
[0037] For applications requiring extremely high heat removal rates
in a continuous duty cycle, the preferred embodiment of the active
high flux heat removal cooling system in accordance with the
present invention illustrates an application using liquid ice. In a
preferred embodiment, the liquid ice is made from a mixture of
water and ethanol, because ethanol reduces the boiling point as it
depresses the freezing point of the binary solution. A reduced
boiling point allows more flexibility in choosing the material used
to shield the electrical apparatus from direct contact with the
liquid ice. The cooling system preferably employs a 30% ice mixture
concentration to provide more than 1,000 BTU/hr per pound of liquid
ice flowing with an inlet temperature of -2.degree. C. and a
boiling point of +80.degree. C. in a partial vacuum of 6.72 psia,
as shown in FIG. 1. Liquid ice in bulk form or in a fluidized bed
provides exceptionally high cooling rates at solid surfaces.
[0038] As shown in FIG. 1, the cooling system comprises a heat
transfer plate 10 consisting of copper, aluminum, silver, or
another suitable thermally conductive material, such as beryllium
oxide ceramic, boron nitride, aluminum oxide ceramic, or diamond,
with high tensile strength to enable efficient heat transfer by
thermal conduction. The external electrical apparatus, for example,
electronic devices, that generate the input heat flux indicated in
FIG. 1, are disposed in heat transfer relationship with the heat
transfer plate 10, for example, the electrical apparatus can be
mounted in thermal contact with the heat transfer plate.
Preferably, the heat transfer plate 10 is thin to maximize the
thermal gradient and avoid heat stored in the heat transfer plate.
Preferably, the thickness of the heat transfer plate 10 is on the
order of approximately one to two millimeters and has a thickness
that is the minimum thickness that enables the heat transfer plate
to provide the structural portion of the circulation subsystem at
the location at which the liquid ice removes high heat flux from
the heat transfer plate.
[0039] In the preferred embodiment in accordance with the present
invention in which the cooling system is operated at partial
vacuum, the heat transfer plate 10 comprises all or a portion of a
wall of a vacuum tank 12. The heat transfer plate 10 must have
sufficient structural integrity to withstand external atmospheric
pressure. Because the heat transfer plate 10 forms a portion of the
cooling system, in order to have a heat transfer plate thin enough
to provide effective heat transfer, the heat transfer plate can be
internally reinforced, for example, by means of a honeycomb
material 14, to prevent implosion under atmospheric pressure if the
cooling system is operated at subatmospheric pressure. If the
honeycomb material 14 is used for reinforcement, jets of liquid ice
are preferably directed at the centers of the honeycomb openings to
optimize heat transfer efficiency.
[0040] Honeycomb material 14 attached to the heat transfer plate
10, if designed with optimal jet spacing as the controlling factor,
may help increase the heat transfer rate via the finning effect and
by controlling the allocation of a portion of the hot surface to
each jet flow. A series of parallel slots may also be beneficial,
with some slots dedicated to vapor removal and sized
appropriately.
[0041] As shown in FIG. 1, the cooling system further comprises a
liquid ice generator 16. The liquid ice generator 16 can be a
commercially available scraped surface ice slurry generator (for
example, a Sunwell ice generator), an orbital rod system, or a
vacuum ice system chiller. The latter type is shown in FIG. 1 and
can provide up to 10,000 tons/day of liquid ice. The liquid ice
generator 16 can be modulated from idle to full power either
proportionally or by on/off cycling determined by a controller 18.
A cooling water supply 20 is connected to the liquid ice generator
16 to provide cooling for the internal compressor of the liquid ice
generator (not shown).
[0042] The liquid ice generator 16 is connected by a pipe 22 to an
inlet located proximate the top of a liquid ice reservoir 24. An
outlet located proximate the bottom of the liquid ice reservoir 24
is connected through a pipe 26 to a first variable frequency drive
(VFD) controlled liquid pump 28 that returns liquid ice to the
liquid ice generator 16 to maintain the temperature of the liquid
ice in the liquid ice reservoir at a predetermined temperature, for
example, -2.degree. C.
[0043] During conditions of light or low heat input loads, a
stirring pump 30 maintains the liquid ice mixture uniformly
isothermal. The stirring pump 30 recirculates the liquid ice
mixture through a pipe 32 connected between an outlet proximate the
bottom of the liquid ice reservoir 24 and an inlet proximate the
top of the liquid ice reservoir.
[0044] A second VFD controlled liquid pump 34 delivers the liquid
ice mixture through a pipe 36 from the liquid ice reservoir 24 to
an array of orifices 38 that cause jet impingement of the liquid
ice on the heat transfer plate 10. The heat transfer plate10 is
constructed of a suitable material with high heat conductivity.
Materials such as copper or aluminum are preferred. If, however, an
electrically insulating material is needed, then diamond, beryllium
oxide ceramic, boron nitride, or aluminum nitride ceramic can be
used. The material may be chosen by tensile strength to heat
conductivity ratio. The thicker the heat transfer plate 10, the
lower the temperature on the cooled side must be. For example, for
copper having a 1.8 millimeter thickness, the temperature of the
side that is cooled would need to be +80.degree. C. in order to
conduct 1,000 W/cm.sup.2 through the heat transfer plate 10, if the
temperature on the side heated by the electrical apparatus were to
be maintained at 125.degree. C. When using a higher boiling point,
the thickness of the heat transfer plate 10 would be
correspondingly reduced. Maximizing exposed surface area of the
cooled side of the heat transfer plate 10 by judicious use of
finning is preferred in order to increase surface area and increase
cooling efficiency.
[0045] The array of orifices 38 can comprise relatively large
aperture nozzles through which the liquid ice can be pumped. For
example, the array of orifices 38 can comprise a matrix of pressure
jet nozzles having a diameter of approximately 2 millimeters. For
flat surfaces, a carefully arranged array of round nozzles can be
more effective than a slot nozzle system for a given mass flow of
liquid ice, but depending on the geometry of the surface to be
cooled, slots can be superior.
[0046] The second VFD controlled liquid pump 34 can be controlled
by the controller 18 to provide continuous or pulsating jet
operation by the array of orifices 38. It should be noted that
high-speed jets can cause cavitation and erosion of metals. (The
leading edges of aircraft wings are damaged in this way when they
fly through rainstorms.) This sets an upper limit on jet speed and
has material property implications such that the choice of material
for the heat transfer plate 10 and the design of the heat transfer
plate are both important when using liquid ice.
[0047] After the liquid ice is impinged on the heat transfer plate
10, the spent liquid ice mixture that has not vaporized falls to
the bottom of the vacuum tank 12 as a liquid. The first VFD
controlled liquid pump 28 pumps the spent liquid through a pipe 40
that preferably connects to the pipe 26 so that the liquid is
returned to the liquid ice generator 16. Preferably, a heat
exchanger 42 is interconnected in the pipe 40, and the heat
exchanger is connected to a cooling water supply 44. The heat
exchanger 42 cools the liquid with cooling water to reduce the load
of the liquid ice generator 16.
[0048] Additionally, at the top of the vacuum tank 12, a vacuum
pump 46 preferably evacuates the vacuum tank 12 to a predetermined
reduced pressure, for example, a partial vacuum of approximately
6.72 psia. A consequence of employing liquid ice as the refrigerant
or coolant is that the compressor (not shown) of the vacuum pump 46
shown in FIG. 1 should not use oil as a lubricant. The water will
wash the oil away. Dry lubricants or water/steam should be
considered as alternative lubricants.
[0049] The steam resulting from boiling of the melted liquid ice is
discharged by the vacuum pump 46 and is preferably condensed by a
condenser heat exchanger 48 connected to a cooling water supply 50.
From there, the condensate is pumped by the first VFD controlled
liquid pump 28 through a pipe 52 that preferably connects to the
pipe 26 so that the condensate is returned to the liquid ice
generator 16.
[0050] The controller 18 controls the VFD controlled liquid pumps
28 and 34 to adjust the flow from the liquid ice generator 16 and
the liquid ice reservoir 24 for varying thermal loads. Temperature
sensors 54, 56, 58, and 60 provide feedback information to the
controller 18 to allow the variable load adjustments. The
temperature sensors 54, 56, 58, and 60 can be RTDs (Resistance
Temperature Devices), thermocouples, or thermistors, for example.
Finally, an inlet 62 is provided at the top of the liquid ice
reservoir 24 to initially charge the cooling system with water and
freezing point depressant or to add water and/or freezing point
depressant during operation. If the cooling system is installed on
board a ship, seawater can be supplied through the inlet 62 for use
in the production of liquid ice.
[0051] In operation, the active vapor-compression cycle cooling
system shown in FIG. 1, employing low-pressure liquid ice as a
refrigerant or coolant, provides an efficient high flux heat
removal system. Heat is removed from the primary heat source, such
as semiconductor modules, in thermal contact with the heat transfer
plate 10 using liquid ice impinged on the heat transfer plate by
the array of orifices 38. Since approximately 70% of the liquid ice
is water, the impinging jet is believed to consist of droplets of
water/freezing point depressant, larger particles of ice with most
of the liquid stripped from them during jet impingement, and
droplets of water containing the smallest ice particles. The ice
particles and ice-containing droplets are most likely to reach the
heat transfer plate 10 to provide improved cooling, while the
liquid-only droplets will be the first to vaporize. The liquid ice
is produced by the liquid ice generator 16, using water and a
freezing point depressant, preferably ethanol, and stored in the
liquid ice reservoir 24 from which the liquid ice is pumped by the
second VFD controlled liquid pump 34 as and when required. The heat
absorbed by the liquid ice, causing the liquid ice to undergo phase
changes first to liquid and then to steam, is dissipated by the
heat exchanger 42 and condenser 48, and residual liquid and
condensate are recirculated to the liquid ice generator 16.
[0052] In accordance with another embodiment of the present
invention, instead of the cooling system operating at partial
vacuum, the cooling system can be operated at atmospheric pressure
(1 bar). Operating at atmospheric pressure has advantages including
obviating the need for the honeycomb material 14 and enabling the
thickness of the heat transfer plate 10 to be decreased for
improved thermal conduction. Inclusion of the vacuum pump 46 is
preferred even if the cooling system is operated at atmospheric
pressure. Unless the steam is drawn out of the vacuum tank 12, the
pressure will rise and directly raise the liquid boiling
temperature, reducing the cooling efficiency.
[0053] As shown in FIG. 1, the heat exchanger 42 is cooled by the
cooling water supply 44, and the condenser 48 is cooled by the
cooling water supply 50. In the application in which the cooling
system is installed aboard ship and the cooling water supplies 44,
50 employ seawater, variable performance can result because of the
non-constant temperature of seawater. Consequently, one
contemplated modification to the embodiments of the present
invention is to substitute a liquid ice cooling supply for one or
both of the cooling water supplies 44, 50. The liquid ice can be
fed from the liquid ice reservoir 24 or from an auxiliary liquid
ice reservoir. Nevertheless, the final step in dissipating the heat
would continue to use cooling water, for example, seawater, either
directly in the condenser of the liquid ice generator 16, or
through heat transfer panels built into the side of a ship, for
example, assuming that sufficient surface area exists. Note that by
using liquid ice as the primary cooling medium for the electrical
apparatus and also for the condensation of the resulting steam and
cooling of the liquid effluent, the cooling system can
advantageously control thermal load shifting and storage for the
cooling system.
[0054] The cooling system in accordance with the present invention
enables improved cooling of electrical apparatus that generates
substantial waste heat, for example, waste heat on the order of
1,000 W/cm.sup.2. While various embodiments of the cooling system
of the present invention and various contemplated modifications
have been described above, other modifications and variations will
likely occur to those persons skilled in the art. For example,
rather than using continuous or pulsating jet impingement, the
liquid ice can be atomized by passing it through a conventional
pressure jet nozzle, in a manner similar to water mist systems. For
example, an ice mist can be created by pumping liquid ice having a
20-25% concentration through a 500-micron nozzle at 70 bar. In
addition to high-density heat removal, liquid ice is also a
superior cooling fluid for both medium- and low-density heat
exchange applications. As described above, for high-energy heat
removal, the liquid ice is applied directly to a thermally
conducting barrier. For medium energy devices, the liquid ice can
be used to chill a secondary liquid in which the hot components of
a conventional fluorochemical closed loop cooling system are
immersed. For air-cooled low energy devices and general air
conditioning, the liquid ice can be used for air chilling. Thus,
the cooling system can integrate within the overall cooling
services aboard a ship via a liquid ice generator and thermal
energy storage tank. The foregoing description of the embodiments
of the present invention is therefore exemplary and not limited to
the specific embodiments that are disclosed above. The scope of the
invention can only be ascertained with reference to the appended
claims and the equivalents thereof.
* * * * *