Patent Application
20060060333
1. Field
Aspects of this disclosure generally relate to cooling systems and methods that employ a fluid near or above its critical pressure, and more particularly to a small-scale apparatus needed to operate such a cycle. Typical target applications include, for example, cooling of printed circuit boards, computers, computer components, analytical and laboratory equipment, lasers, and remote sensing equipment.
2. Background
The cooling of such devices as computers, servers, telecommunications switchgear and numerous other types of electronic and medical gear has been an intense area of research for quite some time. The need for increased performance, together with ever increasing compactness, has led to greatly increased levels of heat dissipation from these devices. Integrated circuits work faster, more reliably, and with less current leakage if their temperature is kept as low as possible. The heat emitted by these circuits can overwhelm the cooling capacity of conventional air-blown heat sinks, especially in the cases of thin-profile apparatuses such as laptop computers and stacked server boards. Another method of improving a heat sink is to construct it as a thermoelectric cooler, known as a Peltier cooler, which enables the temperature at the junction with the heat source to be substantially below the temperature of the heat source itself. Peltier coolers require more input power than can be dissipated and are therefore an inefficient means of refrigeration. The attachment of a heat pipe to an electronic component is another method of removing heat from a target device. Typically, one end of the heat pipe is exposed to a heat source while the other end is exposed to a heat sink. Evaporation of a working fluid inside the heat pipe at the exposed end allows for heat to be absorbed from the heat source. In the absence of a pumping means to speed the rate of mass transfer within the heat pipe, capacity is limited.
Microchannel heat exchangers through which a pumped fluid flows can take the place of conventional air-blown heat sinks. In such cases, the heat exchanger could be placed atop an integrated circuit so as to coot it directly. Liquid coolant within the microchannel heat exchanger would be impelled toward a secondary heat-rejecting heat exchanger by pumping, capillary action, thermo-syphoning, electrohydrodynamic or other means of fluid flow and returned to the microchannel heat exchanger as coolant. The small size of the channels allows for high pressure operation, which widens the possibilities for heat-transfer fluids to use in the system.
Alternatively, heat from integrated circuits can be drawn from within the printed-circuit boards upon which they are mounted by means of thin heat sinks that are laminated into the boards themselves. Until now, these heat sinks have been passive components that conduct heat to an external heat exchanger, which is typically cooled by countercurrent air, such that they supplement the function of a heat-sink that is mounted on top of an integrated circuit. Examples are found in U.S. Pat. No. 6,288,906 and others which describe the use of conductive vertical posts, or “thermal vias” to transport heat to metallic planes that are typically located on the top or back side of a printed circuit board. This metallic plane can serve a primary function as the electrical ground for the board. Blish et at (U.S. Pat. No. 6,518,661) take this concept a step further by connecting internal conductive planes first with one set of thermal vias that runs down from the heat-generating elements, then with another set of thermal vias that runs up to a air-blown heat sink that is mounted some distance away from the integrated circuit. All of these inventions are limited by the thermal conductivity of the transporting media, which in turn is limited by the temperature difference that can be achieved between the conductive media and cooling air. More heat-removal capacity can be achieved if the heat conducting media contained microchannels capable of transporting a coolant.
A key problem in bringing any microchannel structure to commercial reality is in connecting fluid inlets and outlets in a manner that is not so bulky as to defeat the design advantages of the thin structure itself. In U.S. Pat. No. 5,099,910, Walpole and Missaggia teach a means of installing one central inlet and two outlets on one level of a two-level structure, with holes arranged in such a way that fluid flows in opposite directions in adjacent channels, thereby reducing the temperature gradient for fluid entering and exiting the device. More elaborate manifolding systems have appeared since, but most employ a similar two- or even three-layer distribution system (U.S. patent application 2004/0104022, Kenny et at). As these manifolds become more complicated, however, heat exchangers tend to grow in thickness, and they would no longer be candidates for incorporation within laminated printed circuit boards.
An apparatus for cooling a device includes the following components: a fluid that is near or above its critical pressure, a heat exchanger, a pump for circulating the fluid, and a fluid connection between the heat exchanger and the pump. The device may be an electrical or electronic component that includes an integrated circuit or embedded control. The fluid may be carbon dioxide, water, air, or natural hydrocarbon. The pump may utilize electrical, electromechanical, mechanical, or magnetic means of fluid flow and the actuation of the pump may be electrohydrodynamic, magnetic, or electromechanical actuation. The heat exchanger is of microchannel type.
The disclosure further relates to the apparatus recited above, where an absence of lubricants increases the performance of the apparatus. Control may be provided by software, hardware, or other method. A sensor may be used to monitor and control temperature and temperature-related phenomena. Power may be derived from a public power network of the device or an independent source.
The disclosure further relates to the apparatus recited above, where the heat exchanger and the pump are contained in the apparatus package. A heat exchanger may be external to the apparatus package. The external heat exchanger is connected to the apparatus by a fluidic connection. The heat exchanger is integrated into a package of the device. The external heat exchanger is in thermal contact with the device.
Further aspects of the apparatus as recited include the fluid comprising thermally conductive nanoparticles to increase cooling performance and an additional effect of electrohydrodynamic or magnetic effect may be used to increase cooling performance.
A method for cooling a device that consists of the following: providing a fluid near or above its critical pressure, transferring heat from the device to the fluid, transferring heat from the fluid to an external environment, and providing a pump for fluid flow. The details of the disclosure mentioned in the previous paragraphs can also be applied to this method as well.
Another aspect of the invention comprises an apparatus for cooling a device includes the following components: a fluid that is near or above its critical pressure, two heat exchangers and a fluid connection between the heat exchangers. The details of the disclosure mentioned in the previous paragraphs can also be applied to this apparatus as well. In addition, the apparatus provides for a density difference to be maintained between the heat exchangers. Additionally, nanomaterials with high heat capacity may be added to the fluid to reduce the flow rate. Alternatively, the fluid may be a highly conducting fluid.
A method for cooling a device that consists of the following: providing a fluid near or above its critical pressure, transferring heat from the device to the fluid and transferring heat from the fluid to an external environment. The details of the disclosure mentioned in the previous paragraphs can also be applied to this method as well.
Additionally, a method is disclosed for removing heat from printed circuit boards by means of a heat exchanger that is laminated to the top or bottom sides of, or within, the structure of the board. An apparatus for executing this method is further disclosed. Said apparatus includes a thin heat-accepting heat exchanger, though which at least one array of microchannels is constructed. Said thin heat exchanger is capable of withstanding internal pressure of up to 6,000 psi, which permits the use of a wide variety of heat-transfer fluids, including such fluids as water, carbon dioxide, hydrocarbons, chlorofluocarbons, fluorinated hydrocarbons, ammonia and sulfur dioxide. The heat exchanger is formed from the materials from the group consisting of metallic, ceramic, polymeric or combination thereof.
An apparatus for cooling a device that includes a mechanism to impel a heat-transfer fluid, which can be a pump or compressor; at least one heat-rejecting heat exchanger; at least one heat-accepting heat exchanger that is laminated to a printed circuit board and which may be in contact with thermal vias that extend to the heat emitting device on the board surface; and fluid connections among these elements.
The printed circuit board itself may hold more than one heat emitting device, many, if not all, of which are cooled by the laminated heat acceptor.
The drawings are provided to illustrate some of the embodiments of the disclosure. It is envisioned that alternate configurations of the embodiments of the present disclosure maybe adopted without deviating from the disclosure as illustrated in these drawings.
A detailed description of the disclosure follows with reference to the following drawings:
Description
The present disclosure provides novel methods and apparatuses for cooling using a fluid near or above its critical pressure. The cooling methods herein relate to a sealed, closed loop for circulation of a fluid. The cooling system is comprised of at least one heat exchanger and may include a pump. All components may be connected within a closed circuit and may be integrated into one package or distributed throughout the device. The apparatus provides a means of cooling devices, including, but not limited to, electrical and electronic devices, and other devices and components having at least an integrated circuit or embedded control. Examples of such devices include electrical, electronic or optical elements within an appliance or the appliance itself, with at least an integrated circuit or embedded control that generates heat, including computers, servers, telecommunications switchgear, radio frequency devices, lasers and numerous other types of electronic equipment, medical equipment, military hardware and many more items that are generally compact in design.
In its basic operation, the apparatus causes a cooling fluid to circulate between a heat acceptor, where heat is absorbed from the device being cooled, and a heat rejecter, where the absorbed heat is discharged from the apparatus, thereby cooling the fluid so that it can re-circulate to the heat accepter. The fluid flows through small channels of less than 1 millimeter in length, width or diameter. Because of frictional forces within these channels, the fluid must be impelled in some manner. The present disclosure describes two methods of impelling the fluid—one by means of a pump, the other by utilizing density differences and thereby doing without a pump. The heat rejecter in this apparatus is a type of heat exchanger that causes heat from the apparatus' fluid to transfer to an ambient fluid, typically air. The heat accepter in this apparatus is a heat exchanger than is in direct or indirect contact with the device.
According to the present disclosure, a pump may be used in the system to circulate a fluid around the closed circuit and through at least one or more heat exchangers for accepting and rejecting heat. The pump may be used for circulation of the fluid through the loop. Many types of pumps can be used for the purpose. The pump can be electrical in nature, meaning that the driving force is strictly electrical in nature and does not involve mechanical moving parts. Specific examples of electrical pumps include electrohydrodydnamic (EHD). In EHD pumps, an electric field is applied to a dielectric fluid, inducing an electric charge in the fluid and dragging fluid along with it as the electric field is made to travel down the flow path. The effect can be enhanced with the use of small particles in the fluid, which can become charged and move with the field, dragging fluid along with them and thus actuating a pumping effect. If the electric field were made static, i.e., it does not travel along the flow path, then the electric pump might take the form of electrokinetic pumps, such as electro-osmotic pumps.
Alternatively, the pump can be mechanical in nature, wherein the immediate driving force that impels the fluid is mechanical, such as the action of a reciprocating piston or a rotating-vane impeller. The force that drives a mechanical pump can itself be electrical in nature, such as an electric motor, in which case the combination of pump and motor can be described as actuated by electromechanical means.
A further means of fluid flow is magnetic in nature, as in the case of pumping element that moves in response to a changing magnetic field. An example is a piston impeller that moves back and forth with the changing direction of a magnet field. The magnetic field may result from electrical current flowing through a coil. As the current reverses direction, so does the magnetic field and the impeller. Such pumps can be described as magnetically actuated, because the means for actuating the driving element is magnetic.
The present disclosure also includes a method and apparatus for cooling without the use of a pump. In such case, the heat absorbed from the heat exchangers alters the characteristics of the fluid. Such an alteration—a change in density or viscosity—drives the flow of the fluid between the heat exchangers.
The present disclosure exploits some of the properties of a fluid near or above its critical pressure, which enable a reduction in the size of such components as heat exchangers and a pump. These reductions also allow for the process to use less energy. The said fluid is may be carbon dioxide, water, air or a natural hydrocarbon.
Heat transfer can be further improved with the addition of additives to fluid, such as thermally conductive nanoparticles. Such additives improve the heat transfer characteristics of the fluid, such as thermal conductivity. Nanoparticles may also provide a mechanism for inducing fluid flow in EHD devices. In addition, additives can be added to increase the heat capacity of the fluid which helps in reducing the flow rate of the fluid required to cooling certain heat load.
Another way to improve heat transfer is to limit or eliminate lubricants that might be contained in the fluid. Such lubricants would normally leak from the pump or be added to the fluid to increase the mechanical performance of the system. Such lubricants may coat the heat transfer area effectively reducing the heat transfer efficiencies. In a preferred embodiment of this disclosure, the pump—if used at all—is operated without the aid of lubricants and lubricants in the fluid are avoided.
All of the components and interconnections of the apparatus may be connected and sealed into one package. The entire package is contacted with the external surface of a device element and heat is transferred between the device element and the apparatus. In some cases, the components of the cooling apparatus may also be distributed across more than one device element rather than sealed into a single package. For example, a single heat-rejecting heat exchanger might serve all sub-assemblies of an apparatus in a device, not just one of them.
In one preferred embodiment,
The apparatus attaches to the packaging of the integrated circuit and at least one heat exchanger is near or in contact with said device. The heat accepting exchanger 11 of the system faces toward said packaging of the device and is directly in thermal contact with it. Heat exchanger 11 may be located in any position relative to the device, for example above or below the heat source 15, and it may have any suitable configuration. The heat rejecting exchanger 14 faces away from said device. Heat exchanger 14 may be located in any position relative to the device and may have any suitable configuration. A fan that is directed toward heat exchanger 14 may be used to discharge heat from the closed loop.
The heat exchanger, or exchangers, used in the apparatus are preferably of a microchannel type, in which case the channel dimensions are less than 1 millimeter in cross-sectional length, width or diameter. The smaller the channel dimension, the larger the wall surface area can be, and hence, the more area there is for heat transfer. Within limits determined by the manufacturability of the channels and the increase in pressure drop, and with it power to drive the pump, channels should be as small as possible.
The heat exchanger may be integrated into the device, typically as part of the device “package,” i.e., components, adhesives and sealants that hold the device together as a single unit. For example, the heat rejecter may be mounted atop an integrated circuit, with a fan, and continuously blow heat away from the device package. The heat accepter, meanwhile, by be contained within the device package in the form of a microchannel heat exchanger that is in direct contact with the integrated circuit itself, or more likely, in direct contact with a heat sink that is itself in contact with the integrated circuit.
The pump can be selected from commercially available models such as Thar Technologies' P-10, P-50 or P-200 Series pumps, or can be designed to suit the specific cooling application.
In another preferred embodiment, there is a heat exchanger in addition to the one or more heat exchangers within the single unit packaging of the apparatus. Said additional heat exchanger is external to the apparatus but still is connected to the loop of the components within the single apparatus. Piping connects said external heat exchanger to the components within the apparatus packaging, providing a means for fluid flow between components of the cooling apparatus. The external heat exchanger faces away from the device. A fan may be attached to an external heat-rejecting heat exchanger is used to discharge heat from the closed loop.
In electronic devices such as microcomputers, the heat dissipated from an integrated circuit can range from 25 to 1,000 watts, and more typically between 50 and 200 watts. The area available for contact by the heat accepter against such an integrated circuit can rage from between 0.1 square inches and nearly 4.0 square inches. This combination of heat dissipation and area available calls for heat acceptors that are capable of removing as much as 1,000 watts per square inch, but typically in a range of 50 to 300 watts per square inch. The flow rate for a fluid above the critical point that is removing heat at this rate can be measured in milliliters per minute. For carbon dioxide, the rate is between 200 and 1,000 milliliters per minute.
In another preferred embodiment,
The apparatus attaches to the packaging of the integrated circuit and at least one heat exchanger is near or in contact with said packaging. The heat-accepting heat exchanger 15 of the system faces toward said packaging of the device and is directly in contact with it. The heat-rejecting heat exchanger 14 faces away from said packaging. A fan attached to the heat-rejecting heat exchanger 14 is used to discharge heat from the closed loop.
The heat exchanger, or exchangers, used in the apparatus are preferably of a microchannel type, in which case the channel dimensions are less than 1 millimeter in cross-sectional length, width or diameter. The smaller the channel dimension, the larger the wall surface area can be, and hence, the more area there is for heat transfer. Within limits determined by the manufacturability of the channels and the increase in pressure drop, and with it power to drive the pump, channels should be as small as possible.
The heat exchanger may be integrated into the device, typically as part of the device “package,” i.e., components, adhesives and sealants that hold the device together as a single unit. For example, the heat rejector may be mounted atop an integrated circuit, with a fan, and continuously blow heat away from the device package. The heat accepter, meanwhile, by be contained within the device package in the form of a microchannel heat exchanger that is in direct contact with the integrated circuit itself, or more likely, in direct contact with a heat sink that is itself in contact with the integrated circuit.
In another preferred embodiment, there is a heat exchanger in addition to the one or more heat exchangers within the single unit packaging of the apparatus. Said additional heat exchanger is external to the apparatus but still is connected to the loop of the components within the single cooling apparatus. Piping connects said external heat exchanger to the other components within the apparatus, providing a means for fluid flow between said components of the cooling apparatus. The external heat exchanger faces away from the device packaging. A fan attached to the heat-rejecting heat exchanger is used to discharge heat from the closed loop.
There is a plurality of advantages that may be inferred from the present disclosure arising from the various features of the apparatus, systems and methods described herein. It will be noted that other embodiments of each of the apparatuses, systems and methods of the present disclosure may not include all of the features described yet still benefit from at least some of the inferred advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of an apparatus, system and method that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the disclosure.
In the case of carbon dioxide, the fluid would be maintained at a pressure above 1,070 pounds per sq. in. (absolute). Heat capacity is typically between 0.4 and 1.0 Btu per pound-° R, except near the critical point, at which it can jump up to 30 Btu per pound-° R, Thermal conductivity increases by a factor of almost four around the critical temperature. These conditions promote efficient heat acceptance and rejection when heat is exchanged against ambient air. The pressure difference between the heat rejecter and heat accepter is that which corresponds to the pressure drop of the apparatus, and can be as low as a few pounds per square inch. This difference is small enough to be overcome with a small pump.
In the case of a pumpless scheme, as shown in
Referring to
One embodiment of the current invention is shown in
The heat exchanger that accepts heat may be positioned within a printed circuit board as any layer in the lamination process. Furthermore, it may be added to the top or bottom sides of a printed circuit board. Thermal vias contribute greatly to the transference of heat from the heat-emitting devices to the heat exchanger that accepts heat within the board. Thermal vias, as shown in
The cooling system of the present invention is capable of withstanding internal pressures that might be encountered with heat-transfer fluids undergoing evaporation as they absorb heat, or might be encountered as a result of a pressure differential that develops between the inlet and the outlet of the microchannel array. Included in this group of fluids are such environmentally benign materials as water and carbon dioxide.
A heat exchanger that accepts heat measuring 100×100 millimeters square and 0.010 inches in thickness is constructed with an array of 80 microchannels running through the center of the lamination, from one side to the opposite side, and laminated into a printed circuit board configuration of layers with FR-4 epoxy insulation. The channels measured nominally 200 microns wide by 100 microns deep and were separated by a distance of nominally 100 microns. In the center of the board was a heat source measuring 27×27 millimeters. The microchannel heat exchanger that accepts heat was separated from direct contact with the heat source by the distance of one layer of FR-4 but was in indirect contact with the heat source through a square array of 81 thermal vias.
Heat emitted by the heater was from 15-55 watts. Into this heat exchanger was passed carbon dioxide at 84 bar and 37° C. inlet temperature, and at flowrates ranging between 0.18 and 1.70 gm per sec. As shown in
A single run of microchannels in the heat exchanger that accepts heat may serve more than one integrated circuit on a printed circuit board, however.
The heat exchanger that accepts heat is connected to the rest of the cooling system first by connectors that either deliver and distribute fluid to the microchannels at one end of the channel array and then gather the fluid at the discharge end. Connectors lead into pipes or tubes that direct flow to other system components: a pump or compressor to impel the fluid throughout the system; a heat exchanger that rejects heat for purposes of expelling heat to the environment; and in the case of sub-ambient refrigeration, an expansion device to relieve the pressure of the heat-transfer fluid and drive its temperature lower with little change in enthalpy. The heat-rejecter need not be of a microchannel design and typically uses blown air as a medium for cooling the heat-exchange fluid. The location of the pump or compressor depends on the thermodynamic conditions desired at different points in the loop.
The connector is joined to the heat exchanger that accepts heat in any suitable manner, including but not limited to clamping or soldering. One way to expose the microchannels outward of a printed circuit board is by means of connection tabs 61 and 64 (inlet and outlet). These are positioned at opposite edges of the heat exchanger that accepts heat because the single run of microchannels is spread directly across the printed circuit board. It is also possible to direct the microchannels in a broad 90-degree sweep across the board, such that the connection tabs are on adjacent edges. Alternatively, the microchannels could be turned around by 180 degrees and exit along the same edge as the inlet. Connecting through tabs on edges of the heat exchanger that accepts heat is just one method for passing fluid to and from the heat exchanger that accepts heat. Edge connection tabs do have the advantage of taking up only a small amount of space and allow installation onto boards that are typically placed in slots on mounting racks. Connection tabs can also be mounted on the surface.
This application is a continuation-in-part of U.S. patent application Ser. No. 10/702,396, which itself claims priority to U.S. provisional patent application Ser. No. 60/424,142 filed Nov. 5, 2002, and also claims priority to U.S. provisional patent application 60/619,504 filed Oct. 15, 2004, teachings of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60424142 | Nov 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10702396 | Nov 2003 | US |
| Child | 11198889 | Aug 2005 | US |
