Patent Application
20050160752
The present invention relates to a system for dissipating heat from a high power density device (HPDD). More specifically, the invention relates to a system that helps in effective dissipation of heat at a distance away from the HPDD.
This application sets forth improvements to our previous apparatus which is described in U.S. Pat. No. 6,658,861 entitled “Cooling Of High Power Density Devices By Electrically Conducting Fluids” which issued Dec. 9, 2003.
Electronic devices such as central processing units, graphic-processing units, laser diodes etc. generate a lot of heat during operation. If the generated heat is not dissipated properly from high power density devices, temperature buildup may occur. The buildup of temperature can adversely affect the performance of these devices. For example, excessive temperature buildup may lead to malfunctioning or breakdown of the devices. So, it is important to remove the generated heat in order to maintain normal operating temperatures of these devices.
The heat generated by HPDD is removed by transferring the heat to ambient atmosphere. Several methods are available to transfer heat from a HPDD to the atmosphere. For example, an electric fan placed near a HPDD can blow hot air away from the device. However, a typical electric fan requires a large amount of space and thus it may not be desirable to place a fan near the HPDD due to space constraints in the vicinity of the HPDD. In case of notebook computers or laptops, there is additional constraint on the positioning of the fans due to the compact size of these devices. For at least the foregoing reasons, it would be desirable to provide heat dissipation (e.g. using a fan) at a location away from the HPDD.
Another way to dissipate heat from a HPDD involves the use of a large surface area heat sink. Essentially, the heat sink is placed in contact with the HPDD to transfer heat away from the HPDD into the heat sink. The transferred heat is then dissipated through the surface area of the heat sink, thereby reducing the amount of temperature buildup in the HPDD. In case a significant amount of heat is generated, a larger-sized heat sink is necessary to adequately dissipate the heat. Also in some cases the heat sink cannot be placed adjacent to HPDD due to form factor restriction. This may be due to non-availability of space near the HPDD or due to other devices/components located nearby that cannot withstand the rise in temperature due to dissipated heat. One way of dealing with the form factor limitation is to place a heat sink at a sufficiently large distance from the HPDD. In this case, heat has to be transferred from HPDD to the heat sink before being dissipated to the atmosphere.
A heat pipe is a device that can effectively transfer heat from one point to another. It consists of a sealed metal tubular container whose inner surfaces may also include a capillary wicking material. A heat transfer fluid flows along the wick structure of the heat pipe.
A Heat pipe is useful in transferring heat away from the HPDD when the form factor and other constraints limit dissipation of heat near the HPDD itself. Further, it has the ability to transport heat against gravity with the help of porous capillaries that form the wick.
Heat pipes exploit liquid-vapor phase change properties. Thus, maximum heat transfer is limited by the vapor/liquid nucleation properties. Interface resistance between the metal surface and the liquid layer also limits the maximum heat flow. Heat pipes do not solve the problems of interface resistances at the hot source end and the cold sink end. Interface resistance between the metal surface and the liquid layer also limits the maximum heat flow. It is also not possible to cool multiple hot sources using a single heat pipe. Often these heat pipes contain CFC fluids that are not environment-friendly. The performance of these heat pipes depend on the orientation of the heat pipe structure with respect to the gravitational forces, operating temperatures, and the nature of fluids in the loop. The dependence of performance on orientation restricts the flexible positioning of heat pipes.
The above-discussed limitations of heat pipes have made forced fluid cooling an attractive option. The forced fluid cooling is based on circulating water through a HPDD. Water carries away heat from the HPDD and dissipates the heat at a sink placed at a distance. The heat is dissipated at the sink using fluid-fluid heat exchangers such as finned radiators with natural or forced convection. In forced fluid cooling, more than one HPDD can be cooled in a single loop.
However, the use of water in forced fluid cooling has some limitations. The low thermal conductivity of water limits its effectiveness as a heat transfer fluid. So, in this case the only mode of transfer of heat is convection. Transfer of heat by conduction is negligible. Also, water is circulated using mechanically moving pumps that may be unreliable, occupy large volumes, and contribute to vibration or noise.
U.S. Pat. No. 3,654,528 entitled “Cooling Scheme For A High Current Semiconductor Device Employing Electromagnetically-Pumped Liquid Metal For Heat And Current Transfer” describes the use of liquid metal to spread heat uniformly in the heat sink placed in contact with a wafer. However, this patent describes heat dissipation in the proximity of the heat-generating device and does not address to the form factor limitation. Further, the use of electromagnetic (EM) pumps requires an extra power supply that generates heat. Removal of this additional heat adds to the burden.
In light of the above discussion it is clear that methods provided by the prior art do not satisfactorily address the issue of removal of heat at a desirable distance away from a high power density device. Thus there is a need for a flexible method for managing dissipation of heat at a distance away from the high power density device.
It is an object of the invention to provide an efficient and flexible method of dissipating heat away from a high power density device.
It is another object of the invention to provide a system for effective dissipation of heat from high power density devices, while taking care of the form factor limitations.
It is another object of the invention to provide a system for the effective transfer of heat from high power density devices at a predefined distance away from the device.
It is another object of the invention to provide a system that uses electromagnetic pumps for circulating liquid metal for carrying away the heat generated by high power density device.
It is another object of the invention to provide electromagnetic pumps that use polymers or refractory metals as the material of construction of the conduit and gallium indium alloy as the liquid metal.
It is yet another object of the invention to provide an electromagnetic generator to power the electromagnetic pumps.
It is yet another object of the invention to enable the use of fluid-fluid heat exchangers (including condensers, radiators etc.) for dissipating heat generated by high power density device.
The invention provides a system for effective removal of heat from a high power density device and dissipating the heat at a distance. The system in accordance with the invention has a liquid metal chamber mounted on a high power density device. The liquid metal chamber can be either a solid-fluid heat exchanger or a sealed liquid metal container allowing direct contact of the liquid metal with the high power density device. a conduit carrying liquid metal passes through the liquid metal chamber. The liquid metal is circulated in the conduit. The liquid metal carries away the heat generated by the high power density device and dissipates it at a heat exchanger or heat sink provided at a predefined distance away from the device. This system is highly flexible and can be used in different embodiments depending on form factor limitations. The same conduit (carrying the liquid metal) can be used for carrying heat away from multiple devices. multiple pumps arranged in series or parallel arrangements may also be provided. two or more loops (of the conduit) can use a common pump or common s liquid metal chamber. A loop can dissipate heat, which is further carried away by another loop, more complex networks of loops can also be formed.
The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the invention, wherein like designations denote like elements, and in which:
The present invention provides a system for dissipating heat from high power density devices. The system has a liquid metal chamber placed adjacent to a high power density device. In an embodiment, the liquid metal chamber is a solid-fluid heat exchanger. In another embodiment, the liquid metal chamber is a sealed liquid metal container allowing direct contact of the liquid metal with the high power density device.
Heat sink 209 is constructed of a low thermal resistance material. Examples of such materials include copper and aluminum. Heat sink 209 has a large surface area for effectively dissipating heat to the atmosphere. Heat sink 209 may dissipate heat by natural convection or by forced convection with the use of a fan. A finned structure (as shown in the figures) is sometimes used as a heat sink. In fact, the finned structure may also have liquid metal circulating through its fins. It will be apparent to one skilled in the art that any heat sink structure (used for transferring heat to the atmosphere) may be employed in the system without departing from the scope of the invention.
Conduit 203 is constructed of polymer materials such as teflon or polyurethane. Alternatively, refractory metals such as vanadium or molybdenum may also be used as the material of construction of conduit 203. Polymers like teflon prove to be good conduit materials as they are inert to most chemicals, provide low resistance to flow of liquids and are resistant to high temperature corrosion. Solid-fluid heat exchanger 201 comprises a thermally conducting surface closely attached to the high power density device and a housing containing the liquid metal. For processor chip cooling applications, the thermally conducting surface could be a thin-film tungsten, nickel layer on the backside of the processor or a discrete surface of tungsten, nickel, anodized aluminum or nickel-coated aluminum soldered to the backside of the chip. The housing material could be an inert polymer (Teflon, polyurethane, etc.), glass or thermally conductive material such as tungsten, nickel, nickel-coated aluminum, anodized aluminum, nickel-coated copper etc.
System 200 may be used for dissipating heat from a wide variety of devices. For example high power density device 202 of
In certain applications, the system may need to be provided with electromagnetic interference (EMI) shielding to shield the high power density device from electromagnetic radiations generated by the pump. These electromagnetic radiations, if not shielded, might adversely affect the performance of the high power density device or its components. Accordingly, the electromagnetic pump is enclosed within a housing that shields the high power density device. This EMI shielding may be provided using standard methods such as magnetic shields and EMI shielding tapes. As shown in
In the preferred embodiment, tube 309 is constructed of polymer materials such as Teflon or polyurethane. Teflon has the advantage that it can be easily machined. Alternatively, refractory metals such as tungsten or molybdenum may also be used as the material of construction of tube 309. Ultra-thin anodized aluminum or nickel-coated aluminum or copper can also be used.
In the preferred embodiment, the liquid metal carried by tube 309 is an alloy of gallium and indium. Preferred compositions comprise 65 to 75% by mass gallium and 20 to 25% indium. Materials such as tin, copper, zinc and bismuth may also be present in small percentages. One such preferred composition comprises 66% gallium, 20% indium, 11% tin, 1% copper, 1% zinc and 1% bismuth. Some examples of the commercially available Gain alloys include galistan—a concoction popular as a substitute for mercury (Hg) in medical applications, and newmerc. The various properties of Ga—In alloy make it desirable liquid metal for use in heat spreaders. The Ga—In alloy spans a wide range of temperature with high thermal and electrical conductivities. It has melting points ranging from −15° C. to 30° C. and does not form vapor at least upto 2000° C. It is not toxic and is relatively cheap. It easily forms alloys with aluminum and copper. It is inert to polyimides, polycarbonates, glass, alumina, Teflon, and conducting metals such as tungsten, molybdenum, and nickel (thereby making these materials suitable for construction of tubes).
However, it is apparent to one skilled in the art that a number of other liquid metals may be used without departing from the scope of the invention. For example, liquid metals having high thermal conductivity, high electrical conductivity and high volumetric heat capacity can be used. Some examples of liquid metals that can be used in an embodiment of the invention include mercury, gallium, sodium potassium eutectic alloy (78% sodium, 22% potassium by mass), bismuth tin alloy (58% bismuth, 42% tin by mass), bismuth lead alloy (55% bismuth, 45% lead) etc. Bismuth based alloys are generally used at high temperatures (40 to 140° C.). Pure indium can be used at temperatures above 156° C. (i.e., the melting point of indium).
In accordance with another embodiment of the invention, the present invention provides a system for dissipating heat from a high power density device in a folding microelectronic device. This embodiment is shown in
The system consists of a solid-fluid heat exchanger 201, a conduit 203, at least one electromagnetic pump 211 and a heat sink 209. Solid-fluid heat exchanger 201 is filled with liquid metal that absorbs heat from high power density device 202. Conduit 203 passes through solid-fluid heat exchanger 201 and carries the heated liquid metal away. The liquid metal is pumped by at least one electromagnetic pump 211.
Conduit 203 comprises a portion 404 that carries the heated liquid metal from base member 402a across the bend of folding microelectronic device 402 to folding member 402b. Further, portion 404 allows folding member 402b to bend with respect to base member 402a. Portion 404 is made of a flexible material that is inert to liquid metal. Exemplary materials include rubber, elastomer and Teflon™. Alternatively, entire conduit 203 is made of the flexible material such that there is no need of the flexible portion.
Conduit 203 carries the liquid metal into folding member 402b of folding microelectronic device 402. Heat from the liquid metal in conduit 203 is transferred to heat sink 209, which is located in folding member 402b. Heat sink 209 then releases the heat to the atmosphere. After transferring heat to heat sink 209, the liquid metal returns to base member 402a through conduit 203 to complete the closed loop.
Another embodiment of the invention for dissipating heat from a high power density device in a folding microelectronic device is shown in
Further, in folding member 402b, the liquid metal transfers heat to heat sink 209, which rejects heat to the atmosphere. Cold liquid metal returns to hinge 502 and flows through it to reach base member 402a through conduit 203. Further, the liquid metal flows to solid-fluid heat exchanger 201, hence completing a closed loop.
Referring to
The arrangement described with respect to
The embodiments described with the help of
As described with respect to
As will be apparent to those skilled in the art, the system as described above may be used with the flexible conduit as shown in
Yet another embodiment of the invention is shown in
Heated liquid metal in liquid-heat pipe heat exchanger 808 is carried away by conduit 810. Electromagnetic pump 812 pumps the liquid metal through conduit 810. The liquid metal transfers heat to heat sink 814. Heat sink 814 rejects the heat to the atmosphere. Cooled liquid metal returns to liquid-heat pipe heat exchanger 808 through conduit 810, hence forming a closed loop.
The systems of liquid metal and heat pipes described above may be used for effective heat dissipation over large distances without requiring a large amount of liquid metal. This reduces the overall weight and the cost of the heat dissipation system.
Sealed liquid metal container 902 may be sealed in a number of ways depending on the nature of high power density device 202 to be cooled. A seal may be made using an interference fit between sealed liquid metal container 902 and high power density device 202. A seal may also be made using compressed o-rings or similar compression seals. The o-rings may be made of materials such as Teflon™, Buna-n, and Viton™. Addition of a bonding agent or a sealant, such as epoxy, may also be used to seal sealed liquid metal container 902. Sealed liquid metal container 902 may also be soldered or welded onto high power density device 202.
Furthermore, sealed liquid metal container 902 can be shaped according to the distribution of heat generated by high power density device 202, to enhance the heat transfer to the liquid metal. For example, if the heat generated at a specific part of high power density device 202 is more than the heat generated at other parts, sealed liquid metal container 902 can be shaped such that the volume of liquid metal that flows over this specific part is more than the volume of liquid metal that flows over other parts of high power density device 202. In this way, the total amount of liquid metal required for the system may be reduced. This would lead to a reduction in the weight and the cost of the system.
Liquid metal in liquid metal chamber is carried away by conduit 203. The liquid metal is pumped by at least one electromagnetic pump 211. Conduit 203 carries the liquid metal to heat sink 209. Heat sink 209 dissipates the heat from the liquid metal to the atmosphere. Cooled liquid metal is carried back to sealed liquid metal container 902 through conduit 203.
This embodiment increases the efficiency of heat transfer to the liquid metal by avoiding the use of a solid-fluid heat exchanger. When a solid-fluid heat exchanger is used, an interface exists between a high power density device and the solid-fluid heat exchanger. Air gaps may exist on this interface due to the roughness of the surfaces of the solid-fluid heat exchanger and the high power density device. Air gaps reduce the heat transfer between the high power density device and the liquid metal. Removing the solid-fluid heat exchanger eliminates with the interface and hence increases the efficiency of heat transfer.
A face 1001a of thermoelectric generator 1001 is placed in contact with section 203a of conduit 203. Section 203a carries hot liquid metal to heat sink 209 and has a high temperature. A Face 1001b of thermoelectric generator 1001 is placed in contact with section 203b of conduit 203 that carries liquid metal (that has been cooled after dissipating heat) away from the heat sink 209 to solid-fluid heat exchanger 201. Face 1001b is thus at a relatively low temperature. The temperature difference between the two faces of thermoelectric generator 1001 is utilized to produce potential difference for powering electromagnetic pump 211. Thus, in this case there is no need of external power source to run electromagnetic pump 211. The external power supply, if used, generates heat that has to be removed. This adds to the burden of heat removal from the system. By using potential difference generated by thermoelectric generator 213 to run electromagnetic pump 211; this added burden is done away with.
Thermoelectric generator 1001 is comprised of series of p type semiconductor members and n type semiconductor members sandwiched between thermally conducting, electrically-insulating substrates such as oxide-coated silicon wafers, aluminum nitride (AIN) and other thin ceramic wafers. Thermoelectric generator 1001 utilizes the “Seebeck effect” to convert the temperature difference between the hot section 203a and the cold section 203b of conduit 203 to electrical energy in the form of a potential difference. The voltage generated by thermoelectric generator 1001 depends on the temperature difference between the sections 203a and 203b. The performance (i.e. the ratio of electrical power to the heat flow into the hot end) of thermoelectric generator 1001 is governed by the Seebeck coefficient and thermal conductivity of p and n type semiconductor members used to form the device. Alloys of bismuth (Bi), tellurium (Te), antimony (Sb) and selenium (Se) are the most commonly used materials for manufacturing the semiconductor members of thermoelectric generator 1001 for devices operating near room temperature.
The use of thermoelectric generators provides sufficient power to drive the electromagnetic pumps. This may be illustrated using the following representative example:
The power requirement is dependent on the distance the fluid needs to move. Typically, this power requirement may range from few milli-watts (say for moving the fluid a distance of 10 cm in case of a laptop), to a watt (say for moving the fluid several meters in a server rack).
The coefficient of performance of a thermoelectric generator i.e. the ratio of electrical power to the heat flow into the hot end, is roughly:
η=ε(ΔT/Th)
where ε is the thermodynamic conversion efficiency, ΔT is the temperature differential between the hot and cold ends, and Th is the temperature of the hot end. The value of Δ is 0.1 for conventional Bi/Sb/Te/Se alloys and Pb/Te/Se alloy materials. The typical temperature differential across the two ends of thermoelectric generator would be around 15-40K (i.e., Kelvin). Assuming ΔT=30 K and Th=358 K (85° C.) the coefficient of performance η of the thermoelectric generator comes out to be 0.0084. If the high power density device dissipates 100 W, the electrical power generated by the thermoelectric generator will be 0.84 W, which is sufficient for driving the electromagnetic pump. Of course, better thermoelectric generators can easily double the performance.
Thermoelectric cooler 1003 provides a first stage spot cooling of the high power density device 202. Thermoelectric cooler 1003 utilizes the “Peltier effect” to cool the high power density device 202. The construction of thermoelectric cooler 1003 is similar to thermoelectric generator 1001. A direct current supplied to the thermoelectric cooler 1003 produces a temperature difference between its two surfaces. Thus, surface of thermoelectric cooler 1003 in contact with high power density device 202 is at low temperature (with respect to high power density device 202) and surface of thermoelectric cooler in contact with solid-fluid heat exchanger 201 is at higher temperature (with respect to solid-fluid heat exchanger 201). The amount of cooling provided by thermoelectric cooler 1003 is a function of the current supplied to it. The use of thermoelectric cooler 1003 is desirable in cases where surface of high power density device 202 has uneven temperature distribution with some regions having temperature much greater than other regions. The first stage spot cooling provided by thermoelectric cooler 1003 helps to make temperature distribution uniform on the surface of high power density device 202.
Fluid-fluid heat exchangers make use of transfer of heat between two fluids over a common surface. Thus, use of liquid metal in the invention makes it possible to use a heat exchanger for dissipating heat. Fluid-fluid heat exchanger 1101 provides controlled cooling such that the rate of cooling may be regulated depending on requirements. The regulation of cooling rate may be achieved by varying the flow rate or temperature of the fluid in fluid-fluid heat exchanger 1101.
The fluids that are most commonly used in heat exchangers are water, air or freon. Fluid-fluid heat exchanger 1101 can be tubular shell and tube type of heat exchanger with counter or concurrent flow. Heat exchanger 1101 can also be a plate type heat exchanger. Fluid-fluid heat exchanger 1101 may be replaced by multiple heat exchangers connected in series or parallel. In fact, in place of the combination of heat exchanger 1101 and heat sink 209, heat exchanger 1101 alone can be used to dissipate heat. It will be apparent to one skilled in the art that any device that can dissipate/extract heat from liquid metal (e.g. thermoelectric cooler, vapor compression cooler) can replace heat exchanger 1101 without departing from the scope of the invention.
It will be apparent to one skilled in the art that the abovementioned embodiments may be combined in many ways to achieve flexibility in construction of heat dissipation systems.
This embodiment demonstrates the flexibility achieved by using liquid metal as a heat transfer medium. Closed conduits 1303 and 1309 (where liquid metal absorbs heat directly from high power density device) can be seen as primary closed conduits. Closed conduit 1317 can be seen as a secondary closed conduit (where liquid metal absorbs heat dissipated by other closed conduits). Thus, liquid metal in primary closed conduits 1303 and 1311 dissipates heat at common fluid-fluid heat exchanger 1305. This heat is carried away by liquid metal in secondary closed conduit 1317 and dissipated at heat sink 1319. It will be apparent that more complex networks of primary closed conduits and secondary closed conduits may be provided without departing from the scope of the invention. For example a network may have a plurality of primary and secondary closed conduits that dissipate heat at a common heat exchanger.
Besides physical flexibility, the use of liquid metal also provides design flexibility. As a result of the design flexibility, design of circuits based on electric considerations can be first worked out. Once the electric circuits have been designed, the liquid loops can be designed based on the form factor limitations (due to the circuit components). This approach enables the design of a circuit without taking thermal considerations in account in the first place.
The system may further include a heat spreader positioned adjacent to the high power density device. The heat spreader can include a plurality of cooling chambers containing liquid metal and a plurality of electromagnetic pumps arranged in a configuration so as to circulate the liquid metal in the cooling chambers.
From the above discussion it is evident that liquid metal heat transfer provides a highly flexible method of heat removal. The various embodiments provided by the invention may be used in computational devices such as laptops to dissipate heat generated by the central processing unit. The flow of liquid metal in conduits (made of polymers) provides a lot of flexibility to carry away the heat and dissipate it at a heat sink placed at bottom or screen of the laptop. The fluid conduit can be flexed, or bent, allowing the flow of liquid metal to be routed across hinges (in a laptop).
The networks of primary and secondary closed conduits provided by the invention can be used for cooling multiple processors in a server where several discrete high power density devices are located in close physical proximity. Primary closed conduits may be used to dissipate heat locally while secondary closed conduits can carry away this heat and dissipate it at distant less-populated areas on the board.
While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.
