Abstract:
A magnetohydrodynamic energy conversion device with an electrically conductive working fluid flowing through a conduit in a magnetic field has permanent magnets aligned for maximum field density for inducing an electric current in the fluid and a multistage cooling system for cryogenically cooling the magnets whereby heat is removed from the device at successive cooling stages having respective different coolants, e.g., water, liquid nitrogen and liquid helium, to maintain the magnets at temperatures low enough to produce high tesla magnetic flux densities in the presence of a high temperature working fluid.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to magnetohydrodynamic (MHD) energy conversion devices, i.e., energy conversion devices capable of converting heat directly into electrical energy. 
         [0002]    As is known in the art, electricity can be produced in a MHD (magnetohydrodynamic) energy conversion device wherein a working fluid in the nature of a plasma, i.e., an electrically-conductive ionized gas flows through a magnetic field perpendicular to the direction of plasma flow. This action generates an electric current in the ionized gas which can be tapped by electrodes located in the magnetic field and in contact with the moving plasma. The stronger the magnetic field and the higher the temperature and velocity of the ionized gas, the greater is the relative amount of electricity that can be extracted from the MHD energy conversion device. Examples of MHD devices used to convert solar and laser energy into electricity are illustrated and described in U.S. Pat. Nos. 4,191,901 and 4,275,318. 
         [0003]    Electromagnets have proven impractical for use in MHD energy conversion devices because they require electricity to stay cool in order to sustain high magnetic flux densities necessary for efficient production of electrical energy. Prior art MHD energy conversion devices have been limited to the use of magnets having magnetic flux densities of no greater then 1-2 tesla which has made such energy conversion devices unsuitable for use in electric power generation for residential, remote, portable, commercial and industrial applications. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides for a high efficiency MHD energy conversion device which utilizes flux trapped in superconductors as permanent magnets having magnetic flux densities of up to 17 tesla, and multistage cooling systems which employ coolants, including liquids with cryogenic properties such as liquid helium and liquid nitrogen. An electrically conductive fluid, in the form of an ionized gas or plasma, flows through an electrically conductive fluid conduit in a magnetic field produced by the pole pieces of two permanent magnets aligned for maximum field density for inducing an electric current in the fluid by separating the positive charges from the negative charges. As used herein, the reference to “permanent magnets” includes superconductors having trapped flux as opposed to electromagnets in which a current is passed through a coil surrounding a ferromagnetic core. 
         [0005]    Each permanent magnet is disposed in a chamber containing a cryogenic liquid, e.g., helium, which has been circulated from the chamber through a heat exchanger, releasing the heat absorbed within the chamber before being pumped back into the chamber. 
         [0006]    The chamber containing each permanent magnet is sandwiched between two dewar vacuum containers in contact with the cooling chambers of two respective liquid nitrogen cooling systems. Other cryogenic liquids having similar cooling properties may be substituted for nitrogen. Each cooling chamber of one of a pair of inner liquid nitrogen cooling systems is separated from the cooling chamber of a respective microchannel water cooling chamber by another dewar vacuum container. Each microchannel water cooling chamber absorbs heat from a contacting radiation shield proximate the conductive fluid conduit. The foregoing arrangement enables the permanent magnets to operate at cryogenic temperatures in the presence of an electrically conductive working fluid having temperatures as high as 3,000 degrees centigrade for reaching magnetic flux densities of up to 17 tesla substantially increasing the efficiency of the MHD energy conversion device. Cryogenic temperatures below 4.2 degrees Kelvin are possible. 
     
    
     
       DESCRIPTIONS OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective schematic view a basic prior art magnetohydrodynamic (MHD) energy device. 
           [0008]      FIG. 2A  is a fragmented schematic diagram of an upper portion of an MHD energy conversion device in accordance with the invention. 
           [0009]      FIG. 2B  is a fragmented schematic diagram of a lower portion of the MHD energy conversion device of  FIG. 2A . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    Referring now to  FIG. 1  of the drawings, there is shown a basic MHD Energy conversion device known in the prior art. A working fluid in the nature of a plasma or gas is propelled through a nozzle (not shown) to accelerate the fluid, and then into a conduit situated within a magnetic field provided by magnets (not shown) above and below the conduit. 
         [0011]    A current transverse to the magnetic field and the direction of flow of the working fluid is tapped by electrodes mounted on opposite sides of the conduit in a plane transverse to the magnetic field. A load can be connected across the electrodes for receiving an electric output current from the MHD. 
         [0012]    Referring now to  FIGS. 2A and 2B  of the drawings there is shown an MHD energy conversion device which includes a conduit  1  for an electrically-conductive ionized gas (not shown) which is heated, e.g., by solar energy, and flows through a magnetic field provided by permanent magnets  3 ,  3 ′ disposed on opposite sides of the MHD energy conversion device in a direction parallel to a longitudinal axis of the conduit  1 . In the preferred embodiment of the invention, the magnets  3 ,  3 ′ form an ellipse with the MHD energy conversion device passing through the center of the ellipse. 
         [0013]    For simplicity the invention has been illustrated in  FIGS. 2 and 2B  with one magnet  3 ,  3 ′ on each side of the MHD conduit  1  to provide a magnetic field intersecting the electrically conductive fluid conduit axis for inducing an electric current in said fluid. However, it is to be appreciated that multiple magnets may be provided on either side of the MHD energy conversion device. A pair of electrodes (not shown) are arranged on opposite sides of the electrically conductive fluid conduit  1  in a plane transverse to a plane of the magnetic field and adapted to be connected for applying a voltage to a load. 
         [0014]    The magnets  3 ,  3 ′ should be placed as close as possible to the high temperature MHD working fluid (plasma) conduit in order to provide the strongest possible magnetic field and, therefore, the greatest electrical output. 
         [0015]    The magnets  3 ,  3 ′ can be formed from specially treated rare earth superconductors like those developed by a company named Magnifye in Cambridge, England. In order to produce magnetic flux densities of as much as 17 tesla, these magnets can be cooled to cryogenic temperatures. The MHD energy conversion device which is operated in the magnetic field produced by these magnets employs a plasma working fluid with operating temperatures of about 3,000 degrees centigrade. The present invention solves the problem of employing magnets operating at cryogenic temperatures, e.g., below 4.2 degree Kelvin, and an MHD energy conversion device with a working fluid at about 3,000 degrees centigrade in close proximity, in the same apparatus, for achieving the efficiencies that can be obtained with magnets having flux densities as high as 17 tesla. 
         [0016]    Due to the high levels of heat convection and radiation from the MHD plasma and the very low thermal capacity of the permanent magnets at operating temperatures (magnet performance deteriorates at elevated temperatures) staged radiation shielding incorporating multiple cooling systems is provided between the MHD energy conversion device and magnets. 
         [0017]    Radiation shields  5 ,  5 ′ are provided between the MHD conduit  1  and each of the magnets  3 ,  3 ′. Each radiation shield  2 ,  2 ′ has an outer reflective surface and a black body inside. The radiation shields  5 ,  5 ′ are preferably fabricated from a highly reflective material that is transparent to a magnetic field, for example, an electropolished nonmagnetic metal. 
         [0018]    The conduit walls and the radiation shields  5 ,  5 ′ are cooled by microchannel water cooling systems  7 ,  7 ′ in which cooling water, a liquid at ambient temperature and pressure, is circulated through a channel  9 ,  9 ′ in housings having chambers disposed between the radiation shields  5 ,  5 ′ and magnets  8 ,  8 ′ to provide local cooling. Other coolants may be substituted for water as will be known to those skilled in the art. Heat transfer coefficient is inversely proportional to the diameters of the cooling channels. Hence, microchannels having diameters as small as possible are employed to maintain the conduit walls at temperatures low enough to preserve the structural integrity of the system. 
         [0019]    The microchannel cooling water is circulated by pumps  11 ,  11 ′ through heat exchangers  13 ,  13 ′ for dissipating the heat absorbed from the conduit&#39;s walls and the residual heat absorbed from the radiation shields  5 ,  5 ′ into the ambient environment. The microchannel water cooling systems can reduce the heat transfer from the MHD plasma chamber, which operates at approximately 2,000 degrees centigrade, into the rest of the magnet packaging  19 ,  25 ,  17 ,  39 ,  41 ,  29 ,  33 ,  31 ,  19 ′,  25 ′,  17 ′,  39 ′,  41 ′,  29 ′,  33 ′,  31 ′ and maintain a temperatures of less than 100 degrees centigrade. 
         [0020]    Although water is used for microchannel cooling due to its desirable thermal properties, other liquids can be used for microchannel cooling adjacent the radiation shields  5 ,  5 ′. 
         [0021]    Disposed between each permanent magnet  3 ,  3 ′ and the microchannel cooling systems is a cryogenic liquid nitrogen cooling system  15 ,  15 ′ formed between dewar vacuum containers  17 ,  17 ′ for inner dewar vacuums and dewar vacuum containers  19 ,  19 ′ for outer dewar vacuums. Disposed between, and in contact with adjacent walls of the containers  17 ,  17 ′ and  19 ,  19 ′ are housings  25 ,  25 ′ having chambers through which liquid nitrogen is circulated from pumps  21 ,  21 ′ to cryogenic heat exchangers  23 ,  23 ′ where heat absorbed in the chambers of the housings  25 ,  25 ′ by the liquid nitrogen is expelled to the ambient environment. The liquid nitrogen continuously reenters the pumps  21 ,  21 ′ and is circulated through the chambers of the housings  25 ,  25 ′ to the heat exchangers  23 ,  23 ′ to remove heat transferred from the MHD energy conversion device through the conduit walls and radiation shields  5 ,  5 ′. The liquid nitrogen can reduce the temperatures of the components into which it comes in contact to approximately 77 degrees Kelvin. 
         [0022]    Cryogenic liquid nitrogen cooling systems  27 ,  27 ′ similar to the ones between the dewar vacuum containers  19 ,  19 ′ and  17 ,  17 ′ including pumps  21 ,  21 ′, conduit  25 ,  25 ′ and cryogenic heat exchanger  23 ,  23 ′ are provided between the magnets  3 ,  3 ′ and the ambient environment in order to dissipate the heat transferred from the ambient environment into the magnets  3 ,  3 ′ and sustain the low temperatures at which the magnets  3 ,  3 ′ must be maintained to provide high magnetic flux densities of as much as 17 tesla. These liquid nitrogen cooling systems are disposed between containers  29 ,  29 ′ for inner dewar vacuums and containers  31 ,  31 ′ for outer dewar vacuums. 
         [0023]    Sandwiched between adjacent walls of the containers  29 ,  29 ′ and  31 ,  31 ′ are housings  33 ,  33 ′ having chambers through which liquid nitrogen is pumped from pumps  35 ,  35 ′ to cryogenic heat exchangers  37 ,  37 ′ which transfer heat absorbed by the liquid nitrogen to the ambient environment through a cryogenic refrigerator. The liquid nitrogen continuously reenters the pumps  35 ,  35 ′ and is cycled back into the heat exchangers  37 ,  37 ′ to minimize the heat transfer from the ambient environment to the magnets. 
         [0024]    Primary cooling of the magnets  3 ,  3 ′ from the approximately 77 degrees Kelvin achievable by the liquid nitrogen cooling systems to the desired magnet operating temperature of less than 4.2 degrees Kelvin is achieved by liquid helium cooling systems  38 ,  38 ′ in which liquid helium is circulated by pumps  43 ,  43 ′ around the magnets  3 ,  3 ′. For this purpose the magnets  3 ,  3 ′ are disposed in respective chambers  39 ,  39 ′ in housings  41 ,  41 ′ having inlets and outlets connected to pumps  43 ,  43 ′ and cryogenic heat exchangers  45 ,  45 ′. Liquid helium is pumped through the heat exchangers  45 ,  45 ′ into the chambers  39 ,  39 ′ and back into the pumps  43 ,  43 ′ thereby maintaining the permanent magnets  3 ,  3 ′ at cryogenic operating temperatures, e.g. 4.2 degrees Kelvin and below. 
         [0025]    The MHD energy conversion device of the invention provides an effective solution to the problem of converting heat and, in particular, solar energy to electricity. Moreover, the MHD energy conversion device of the invention is scalable and can be constructed in various sizes depending on energy and space requirements. For example, a micro version of the MHD energy conversion device of the invention it could be placed at the focal point of a parabolic solar collector or any type of concentrating solar collector. 
         [0026]    It is to be appreciated that the foregoing is a description of a preferred embodiment of the invention to which alterations and modifications may be made without departing from the spirit and scope of the invention. For example, each of the cryogenic heat exchangers  23 ,  23 ′,  37 ,  37 ′.  45 . 45 ° can be provided with a cryogenic refrigerator  47 ,  47 ′ to enhance the removal of heat from the respective cooling systems or cryogenic refrigerators can be substituted for one or more respective cryogenic cooling systems to cool the respective magnets.