Abstract:
The invention provides an efficient way of cooling of the microelectronic devices and converting the heat back into the electrical power. With addition of the ambient-air heat exchanger the system generates enough power to completely satisfy the demand of the microelectronic device and replace the electric battery with the cryogenic storage vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    None 
       FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable 
       SEQUENCE LISTING OR PROGRAM 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of Invention 
         [0005]    The present application generally relates to the cooling systems, and in particular, the present invention relates to an electrical energy-generating cooling system and to a cryogenic cooling system. 
         [0006]    2. Prior Art 
         [0007]    Modern microelectronic devices generate substantial amounts of heat during their operation. This presents both the problem and the opportunity. 
         [0008]    The problem is the need to remove the heat from the device to avoid overheating. Usually the heat has to be dissipated into the ambient air with the temperature  20 - 30 K below the temperature of the device. This calls for massive heat exchangers with developed surfaces, pumps or fans for the forced convection. Many attempts have been made to cool the microelectronic device with the media colder than the ambient air. This requires a cooler or heat pump that consumes energy and generates even more heat in the vicinity of the device. 
         [0009]    The opportunity is to convert the heat back into electric energy and thus reduce the energy consumption from the outer source like battery or power network. For example, in U.S. Pat. No. 6,877,318 to Tadayon et al. (2005) a system is described that uses the micro-machined turbine in the Rankine cycle. The maximum (Carnot) efficiency of the cycle with the worker fluid cooled by ambient air is 9%. Because of the lower efficiency of the Rankine cycle and the losses inherent to the miniature turbines the real efficiency of the system is about 1%. The better efficiency cannot be achieved without introduction of the cryogenic coolants. 
         [0010]    Meanwhile significant progress has been made in using the cryogenic liquids and the liquid nitrogen (LN2) in particular for energy storage and generation. It is proven that the specific energy of the liquid nitrogen storage is more than the specific energy of electric batteries. The U.S. Pat. No. 5,390,500 to White et al (1995) describes a multipass heat exchanger that eliminates frost buildup harmful to electronics. The research has concentrated on the systems generating several kilowatts of power for car locomotion, as described for example in U.S. Pat. No. 3,681,609 to Boese et al (1972). No systems are known to use the cryogenic power cycle for micro-power generation. 
       SUMMARY 
       [0011]    In accordance with the present invention a two stage cooling system with electric energy-generating capability is described. A heat from a heat source, in particular an electronic chip, is converted into electric energy by a first stage conversion device. The residual heat is sunk by the cryogenic liquid that is thus evaporated, heated further by the ambient air heat exchanger and directed into a second stage expander turbine that drives a second stage electric generator. 
     
    
     
       DRAWINGS—FIGURES 
         [0012]      FIG. 1  is a block diagram showing a system for energy storage, power generation and cooling in one embodiment of the present invention. 
           [0013]      FIG. 2  is a block diagram showing a variation of the system with the ambient air heat exchanger moved outside the heat engine. 
           [0014]      FIG. 3  is a drawing showing the cross-section elevation of power generating unit with the standing-wave thermoacoustic engine. 
           [0015]      FIG. 4  is an enlarged cross-section elevation of the generator coil/heat exchanger combination. 
           [0016]      FIG. 5  is a cross-section elevation of the generator coil/heat exchanger combination used in the rotary-type Stirling engine. 
           [0017]      FIG. 6  shows the T-S diagram of the cryogenic Rankine cycle. 
           [0018]      FIG. 7  shows the power generating unit used as a part of the personal cooling system. 
           [0019]      FIG. 8  shows the array of the power generating units used for air cooling and water condensation. 
       
    
    
     DRAWINGS—REFERENCE NUMERALS 
       [0020]      
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 100 
                 heat source/microchip 
               
               
                 101 
                 circuit board 
               
               
                 200 
                 cryogenic vessel 
               
               
                 201 
                 pump 
               
               
                 300 
                 heat conversion device 
               
               
                 301 
                 hot heat exchanger 
               
               
                 302 
                 ambient air heat exchanger 
               
               
                 303 
                 cold heat exchanger 
               
               
                 310 
                 thermoacoustic engine case 
               
               
                 311 
                 insulating chamber 
               
               
                 312 
                 coolant chamber 
               
               
                 313 
                 thermoacoustic stack 
               
               
                 400 
                 first stage electric generator 
               
               
                 410 
                 membrane 
               
               
                 411 
                 magnet 
               
               
                 412 
                 generator coil 
               
               
                 430 
                 hot engine part 
               
               
                 431 
                 cold engine part 
               
               
                 432 
                 insulating insert 
               
               
                 433 
                 hot heat exchanger/coil 
               
               
                 434 
                 cold heat exchanger/coil 
               
               
                 435 
                 flywheel 
               
               
                 436 
                 displacer 
               
               
                 437 
                 magnet/counterweight 
               
               
                 500 
                 expander turbine 
               
               
                 501 
                 turbine casing 
               
               
                 600 
                 second stage electric generator 
               
               
                 601 
                 adiabatic expansion process 
               
               
                   
                 diagram 
               
               
                 602 
                 isothermic expansion process 
               
               
                   
                 diagram 
               
               
                 603 
                 actual process diagram 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0021]    I propose a system that uses a vessel with cryogenic liquid for energy storage, cools the microelectronic device with the liquid and generates electric energy by utilizing the heat from the device and from the environment. 
         [0022]    The main components of the system are shown on  FIG. 1 . Reversible heat engine (e.g. Stirling cycle engine)  300  is equipped with two “hot” heat exchangers. Heat exchanger  301  absorbs heat from the microelectronic device  100  and heat exchanger  302  absorbs heat from the ambient air. Heat is partially converted into the mechanical energy and then into the electrical energy by the electric generator  400 . The residual heat is sunk at the “cold” heat exchanger  303 . Pump  201  forces the cryogenic liquid from the heat insulated vessel  200  through the heat exchanger  303 . There the liquid evaporates and the vapor is superheated to the ambient air temperature. The vapor is directed into the expander type turbine  500  connected to the electric generator  600 . 
         [0023]    Since only the residual heat from the first stage (heat engine) reaches the cryogenic liquid the specific energies in this binary cycle are very high. For example the available work Q for the liquid nitrogen (LN2) in the Rankine cycle is 769 kJ/kg. The specific energies of LN2 in the open Rankine cycle may reach 300 kJ/kg, which is already comparable with the best available battery technology and is well above the specific energy of the lead-acid or Ni—Cd batteries at  110  kJ/kg. Given Etha1 is the thermal efficiency of the heat engine and Etha2 is the thermal efficiency of the Rankine cycle the specific energy of LN2 “fuel” in binary cycle is 
         [0000]        Qe=Q ( Etha 1/(1 −Etha 1)+ Etha 2) 
         [0024]    Assuming the thermal efficiency of the heat engine is the same as that of the Rankine cycle the specific energy of LN2 in the binary cycle is 792 kJ/kg. 
         [0025]    To reduce the complexity, size or cost of the system at the expense of giving up some thermal efficiency, one of the “hot” heat exchangers may be placed outside the heat engine and deliver heat directly to the cryogenic liquid.  FIG. 2  shows a variation of the system where the ambient air heat exchanger  302  is placed outside the engine and is used to superheat the vapor evaporated at the heat exchanger  303 . 
         [0026]    When the microelectronic device is connected to the power grid it is desirable to have an option to conserve the cryogenic liquid “battery” and switch to the main power supply. When the system switches from generation mode to mains powered mode the pump  201  is shut off. As a result turbine  500  and generator  600  halt. Generator  400  is connected to the mains power as a motor and delivers mechanical energy to the reversible heat engine  300  which now operates as a cooler. The heat from the microelectronic device is sunk at the heat exchanger  301  and dissipated from the heat exchanger  302 . When the peak cooling performance is required the system may switch back to generating mode. 
         [0027]      FIG. 3  shows one possible embodiment of such a system. Microelectronic device  100  is mounted on the circuit board  101 . For better heat transfer the top of the device may be equipped with grooves  301 . On top of the device sits the tube  310  with double walls divided into two chambers. Lower chamber  311  is evacuated for the purpose of heat insulation. Pump  201  supplies the cryogenic liquid from insulated vessel  200  into the upper chamber  312 . Inside the tube rests a stack  313  made of a porous material. Tube and stack form a thermoacoustic engine with the device surface serving as a hot heat exchanger and the inner walls of the upper chamber and coil  412  as a cold heat exchanger. Heat removed from the device is partially converted into the mechanical energy of acoustic wave and partially absorbed by the cryogenic liquid through the walls of the upper chamber. 
         [0028]    The mechanical energy is converted to electricity by means of the linear generator. The acoustic wave drives the flexible membrane  410  with the magnet  411  attached to it. Motion of the magnet induces an electric current in the coil  412 . 
         [0029]    The heat absorbed by the cold heat exchanger causes the liquid to evaporate. The vapor is then directed into the multiple pass heat exchanger  302 . The exchanger design prevents frost buildup. The vapor heated to the ambient-air temperature is directed into the expander type microturbine  500  combined with the electrical generator. The expanded vapor (gas) is then released into the ambient air. 
         [0030]    In the mains powered mode the alternating electric current in coil  412  causes the magnet  411  and membrane  410  to vibrate. The resulting acoustic wave cools the microchip device. 
         [0031]    In the design depicted on  FIG. 3  the coil  412  serves both as a part of the electric generator and as a heat exchanger.  FIG. 4  shows a detailed view of the coil/tube assembly. The windings of the coil work as heat conductors and the large surface of the windings facilitates the heat exchange. The combination of generator windings and heat exchanger reduces weight, size, and cost of the system. 
         [0032]    The generator/heat exchanger combination can be used in many different types of heat engines.  FIG. 5  shows a cross-section elevation of the displacer chamber of the rotary Stirling cycle engine combined with the electric generator. The cylindrical chamber is divided into cold part  430  and hot part  431  by the heat insulating insert  432 . Coils  433  and  434  are threaded through the walls of the cold part and hot part respectively thus enhancing the heat exchange. The displacer  436  and the counterweight magnet  437  are attached to the flywheel  435 . During the engine operation the flywheel rotates and the motion of the magnet induces an electric current in coils  433  and  434 . 
         [0033]    The coil windings may also be used as a regenerator type heat exchanger for example as a thermoacoustic engine stack. 
         [0034]    Exposure of the coil windings to the cryogenic temperatures makes possible to use the high temperature superconducting wire and further improve generator efficiency. 
         [0035]    One more distinctive feature of the device on  FIG. 3  is a turbine casing  501  manufactured from the heat-conductive material. Since the vapor in the turbine is colder than the ambient air then the heat from the ambient air is sunk at the turbine housing thus improving the turbine efficiency. 
         [0036]    On  FIG. 6  is a T-S diagram of the cryogenic Rankine cycle. The area of the closed loop determines the efficiency of the cycle. If no heat exchange occurs in the turbine then the expansion process is adiabatic and is presented by line  601 . At the maximum possible heat exchange rate the gas in the turbine is always at the temperature of the ambient air and the expansion process becomes isothermal (line  602 ) bringing the performance of the cycle to the maximum possible level. So the heat sinking to the turbine should be maximized. 
         [0037]    The high expansion ratio typical for cryogenic vapors will normally require a multitude of micro-turbines connected sequentially. The gas is warmed in between the expansions and the ambient air heat is sunk at every stage. The curve  603  describing the actual process is in between the lines  601  and  602 . 
         [0038]    The power generating unit is compact and well suited for the mobile applications. It is capable of adjusting the power output to the demands of the microelectronic device. When the power consumption increases the amount of heat sunk at the cold heat exchanger of the engine increases as well. More liquid is evaporated increasing the amount of gas available for the second stage operation. The throughput of the second stage increases and so is the power generated by the second stage. The drop in the power consumption will decrease the throughput of the second stage and conserve the cryogenic liquid. 
         [0039]    The fact that the unit generates power by sinking the ambient heat allows for using it in the personal cooling system.  FIG. 7  shows the unit combined with the cooling vest or collar. The array of the devices shown on  FIG. 8  produces electricity, cools a room or tent and also produces distilled water via condensation.