Abstract:
Exemplary embodiments include an apparatus, and method associated therewith, for recovering the compression energy stored in hydrogen gas and oxygen gas generated by the electrolysis of water in a high-pressure water electrolyzer. The restored compression energy may be recovered and converted to a useable form to provide power to the high-pressure water electrolyzer, or alternatively to provide usable power to a coupled system that uses high-pressure hydrogen gas or oxygen gas such as a fuel cell for an electric vehicle, or both for use in providing power to the electrolyzer and to the fuel cell electric vehicle.

Description:
TECHNICAL FIELD 
       [0001]    The field to which the disclosure relates generally to energy recovery systems and, in particular, to the recovery of compressive energy generated during a high-pressure water electrolysis process. 
       BACKGROUND 
       [0002]    Electrolyzers convert abundant, low-energy content chemicals into more valuable ones by using electricity to break down compounds into elements or simpler products. A water electrolyzer is a system of cells in which each cell contains two electrodes. In each cell water is oxidized at one electrode (called the cell anode), to produce oxygen gas, and reduced at the other electrode (called the cell cathode), to produce hydrogen gas. The oxidation-reduction reactions are driven by a direct current (DC) power source. Oxygen and hydrogen are generated in a stoichiometric ratio—two volume units of hydrogen for every one of oxygen—at a rate proportional to the applied cell current. 
         [0003]    Water electrolysis appears to be ideally suited to making and storing hydrogen needed to power fuel cells, including specifically fuel cell powered electric vehicles. In a high-pressure water electrolyzer, hydrogen gas can be produced at sufficiently high-pressures (up to about 10,000 pounds per square inch, psi) for storage without the need for mechanical compression. Such systems, however, require significant energy input to drive the high-pressure electrolysis process. In addition, oxygen that is generated in this process generally goes unutilized, and is typically vented to the atmosphere. 
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       [0004]    One exemplary embodiment includes a method and apparatus for recovering the compression energy stored in hydrogen gas and oxygen gas generated by the electrolysis of water in a high-pressure water electrolyzer. 
         [0005]    In one exemplary embodiment, the potential energy in compressed oxygen gas generated as a by-product of electrolytic hydrogen production via water electrolysis in a high-pressure electrolyzer may be used to drive a pneumatic engine. The pneumatic engine can then drive an electrical generator to produce electricity, and the electricity generated may be used to partially power the electrolyzer that originally made the oxygen gas and hydrogen. 
         [0006]    In another exemplary embodiment, the potential energy in compressed hydrogen gas may be recovered as expansion energy that in turn may drive an electrical generator. This electrical energy may then be used to partially power the high-pressure electrolyzer that originally made the oxygen and hydrogen gas. 
         [0007]    In a related exemplary embodiment, the potential energy from both the compressed hydrogen gas and oxygen gas generated within the high-pressure water electrolyzer may be recovered as expansion energy that in turn may drive one or more electrical generators. This electrical energy may then be used to partially power the high-pressure water electrolyzer that originally made the oxygen and hydrogen gas. 
         [0008]    In yet another exemplary embodiment, the expansion of hydrogen gas may also be used aboard a fuel cell electric vehicle. In this embodiment, the compressed hydrogen gas may be recovered as expansion energy that in turn may drive a mechanical electrical generator. This electrical energy may be used to partially power the fuel cell. 
         [0009]    In still another exemplary embodiment, the expansion energy of hydrogen gas may be used directly as mechanical energy from a pneumatic engine to help propel the fuel cell electric vehicle. 
         [0010]    In a further exemplary embodiment, the expansion energy of hydrogen gas may both be used in a hybrid fuel cell/pneumatic vehicle as both mechanical energy from a pneumatic engine to help propel the vehicle and further may be used to drive a mechanical electrical generator and may be used to power a fuel cell electric vehicle. 
         [0011]    Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a schematic flow chart of a system used to generate high-pressure hydrogen and oxygen gases using a high-pressure water electrolyzer and using the hydrogen in a fuel cell electric vehicle or stationary fuel cell with recovery of both the chemical energy of the hydrogen and the compression energy stored in the high-pressure gases in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0014]    The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
         [0015]    Referring now to  FIG. 1 , a system  10  that may generate high-pressure hydrogen gas and oxygen gas via high-pressure water electrolysis is provided in one exemplary embodiment. A portion the hydrogen gas generated may be used by a fuel cell electric vehicle  11  (or stationary fuel cell) is also illustrated within the exemplary embodiment. 
         [0016]    The system  10  may include a high-pressure water electrolyzer  12  that may be used to generate high-pressure hydrogen gas and oxygen gas from water. The electrolyzer  12  may be powered by electricity from a solar system grid  14  or other conventional electrical powering devices (not shown). 
         [0017]    By definition, a high-pressure electrolyzer is a water-based electrolyzer that is capable of producing hydrogen gas and oxygen gas at pressures up to about 10,000 pounds per square inch. One example of a conventional high-pressure electrolyzer  12  that may be utilized in the exemplary embodiment is the Avalance high-pressure electrolyzer (available from Avalance LLC of Milford, Conn.), which uses a unipolar alkaline (KOH) electrolyte system with cylindrical steel electrolysis cells and includes structure for balancing the hydrogen gas and oxygen gas levels and electrolyte levels to keep the gases and electrolytes separate, as well as preventing the mixing of the hydrogen gas and oxygen gas. 
         [0018]    Water may be introduced to the electrolyzer  12  from a holding tank  16 ; through the use of a high-pressure pump (not shown). The water may undergo a oxygen evolution reaction (oxidation reaction) at the electrolyzer anode (not shown) and may undergo a hydrogen evolution reaction (reduction reaction) at the electrolyzer cathode (not shown) according to the general formula: 
         [0000]      H 2 O→H 2 +½O 2    
         [0019]    The high-pressure hydrogen gas  18  and oxygen gas  20  produced within the electrolyzer  12  may be separately removed under pressure to a hydrogen gas storage tank  22  and oxygen gas storage tank  24 , respectively. In one exemplary embodiment, the pressure of hydrogen gas  18  that is removed may approach about 10,000 pounds per square inch. 
         [0020]    The high-pressure oxygen gas  20  may then be introduced from the storage tank  24  into an oxygen gas expansion engine  26  (pneumatic engine). The expanding oxygen gas within the oxygen expansion engine  26  may then drive an electrical generator  28  to produce electricity, and the electricity generated may be used to partially power the electrolyzer  12 . The expanded gas from the pneumatic engine  26  may then vented to the atmosphere  30 . 
         [0021]    The storage of high-pressure electrolytically-produced oxygen, along with recovery of the compression energy using a oxygen gas expansion engine  26  as mechanical energy, followed by conversion of the mechanical energy into electrical energy, may increase the efficiency of a solar electrolysis process by utilizing much of the energy stored in the high-pressure oxygen. It is estimated that an energy savings of up to about three percent of the lower heating value (LHV) energy of the hydrogen gas produced by electrolysis in the electrolyzer  12  may be recovered as electrical energy by using the compression energy in the stored oxygen in the exemplary embodiment described herein (10,000 psi of stored O 2 ). 
         [0022]    The hydrogen gas  18  generated in the electrolyzer  12  may be introduced from the hydrogen gas storage tank  22  to a hydrogen gas expansion engine  32  (pneumatic engine). The expansion of hydrogen gas within the hydrogen expansion engine  32  may then drive an electrical generator  36  to produce electricity, and the electricity generated may be used to power the electrolyzer  12 . The expanded hydrogen gas may then be transferred to a fuel cell electric vehicle holding tank  40 . 
         [0023]    The storage of high-pressure electrolytically-produced hydrogen, along with recovery of the compression energy using a hydrogen gas expansion engine  32  as mechanical energy, followed by conversion of the mechanical energy into electrical energy, may increase the efficiency of a solar electrolysis process by utilizing much of the energy stored in the high-pressure hydrogen. It is estimated that an energy savings of up to about six percent of the lower heating value (LHV) energy of the hydrogen gas produced by electrolysis in the electrolyzer  12  may be recovered as electrical energy by using the compression energy in the stored hydrogen in the exemplary embodiment described herein (10,000 psi of stored H 2 ). 
         [0024]    A fuel cell electric vehicle holding tank  40  for a fuel cell electric vehicle  11  may also be filled with expanding hydrogen gas from the hydrogen gas storage tank  22  through the gas expansion engine  32  until such time as there is an equilibrium state in hydrogen gas pressure between the hydrogen gas storage tank  22  and the holding tank  40 . This equilibrium state may preferably be tied to a predetermined hydrogen gas pressure within the holding tank  40 , corresponding to a predetermined quantity of hydrogen gas. In this equilibrium state, there is little conversion of compression energy to mechanical energy occurring in the hydrogen gas expansion engine  32 . The subsequent release of hydrogen gas from the holding tank  40  to the fuel cell  54  as described below allows additional hydrogen gas to be filled from the hydrogen gas storage tank  22  through the engine  32  to maintain the equilibrium state. In the exemplary embodiment shown herein, the hydrogen gas pressure in the holding tank  40  may be maintained at about 10,000 psi. 
         [0025]    The holding tank  40  may hold the compressed hydrogen gas on a vehicle  11  until such time as it is needed in the fuel cell  54  to generate electric power to propel the vehicle  11  and/or provide power to a particular vehicle component. When needed, the compressed hydrogen gas contained in the holding tank  40  may be expanded within the second hydrogen expansion engine  50  and released to the fuel cell  54 . 
         [0026]    In fuel-cell conversion, the hydrogen gas entering the fuel cell  54  is reacted with oxygen (which may enter the fuel cell  54  from a storage tank  58  or from an ambient setting), in a stoichiometric ratio, to produce water and electricity, the latter of which may be used to power an electric traction motor  62 . The electric traction motor  62  may convert the electrical energy to mechanical energy to propel the vehicle  11  again as shown in box  60 . Additional electrical energy for the electric traction motor  62  may be provided by the pneumatically-powered electrical generator  56 . 
         [0027]    The expanding hydrogen gas entering the second hydrogen expansion engine  50  from the holding tank  40  may also be used to drive an electrical generator  56  and/or may also be fed, in the form of mechanical energy, to the wheels of the fuel-cell electric vehicle to propel the vehicle  11 , as shown in box  60 . 
         [0028]    Thus, the exemplary embodiment illustrated herein provides a method and apparatus for increasing the efficiency of the high-pressure hydrogen generation and utilization process by recovering and utilizing the compression energy stored in high-pressure hydrogen gas and oxygen gas in ways to reduce energy costs associated with their production and end use. 
         [0029]    The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.