Patent Publication Number: US-2011048961-A1

Title: System and Method for Refilling an Electrolyzer Tank from a Water Reservoir

Description:
This application claims priority to U.S. Provisional Application 61/236,828, which was filed on Aug. 25, 2009 and which is fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electrolyzer systems, and more specifically to systems for controlling fluid level within an electrolyzer, and most specifically to a system and method for maintaining electrolyte level and concentration within a water electrolyzer tank. 
     2. Description of Related Art 
     The recovery of hydrogen and oxygen gas by means of electrolysis of water has been practiced for over a century. More recently, water electrolyzers have been used for the purpose of generating and supplying a small stream of hydrogen and oxygen gas as a fuel supplement to internal combustion engines. The hydrogen/oxygen gas stream is usually only a fraction of a percent of the intake combustion air flow but evidence has been presented to show that this small stream can reduce emissions of particulates from diesel engines and in some cases also reduce emissions of NOx and provide small increases in engine fuel efficiency. Typically these water electrolyzers are powered by electrical current from the vehicle battery or alternator and are fitted to the back of the cab compartment of a truck or under the hood of a car. 
     To further improve the effectiveness of these devices it is necessary to minimize their volume and mass and maximize their energy efficiency for a given intended hydrogen or oxygen output. Furthermore it is important to operate an electrolyzer that can function reliably for thousands of hours in extremes of temperature, and in the presence of continuous shock and vibration and road grime and grit. 
     The purpose of this invention is to create a highly efficient water electrolyzer that, for a give amount of hydrogen produced, also minimizes both the mass and volume of the device. A related objective of the invention is to create an electrolyzer that will have no electro-mechanical moving parts and that will be constructed to ensure high reliability even under harsh road conditions. 
     Principles of Electrolyzers and of Electrolysis 
     The science and engineering principles behind the design and operation of water electrolyzers are well known and understood. Some general principles follow. 
     Electrolysis of water is the decomposition of water (H 2 O) into oxygen (O 2 ) and hydrogen gas (H 2 ) due to an electric current being passed through the water. An electrical power source is connected to two electrodes, or two plates (typically made from some inert metal such as platinum or stainless steel), which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons are transferred to water molecules), and oxygen will appear at the anode (the positively charged electrode where electrons are transferred from water molecules to the electrode). The generated amount of hydrogen is twice the amount of oxygen, and both are proportional to the total electrical charge transmitted through the water. Electrons carry the current in the circuit external to the electrolysis cell and in the electrodes, while charged ions carry electric current through the water or electrolyte solution. 
     In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e − ) from the cathode being given to water molecules to form hydrogen gas: 
     Cathode (reduction): 2H 2 O(1)+2e − →H 2 (g)+2OH − (aq) 
     At the positively charged anode, an oxidation reaction occurs, where water is oxidized to generate oxygen gas and giving electrons to the anode. 
     Anode (oxidation): H 2 O(1)→1/2O 2 (g)+2H + (aq)+2e −   
     Combining these two reactions with 
     H 2 O(1)→2H + (aq)+2OH − (aq) 
     yields the overall decomposition of water into oxygen and hydrogen: 
     Overall reaction: 2H 2 O(1)→2H 2 (g)+O 2 (g) 
     For every two electrons the number of hydrogen molecules produced is twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. In acid solution the reactions and standard electrode potentials are 
       2 H   + ( aq )+2 e   −   →H   2 ( g ) E   0 =0.00 V    
       1/2 O   2 ( g )+2 H   + ( aq )+2 e   −   →H   2   O (1) E 0 =1.23 V    
     giving a Standard EMF of 1.23V. 
     In base solution the reactions and standard electrode potentials are 
       2 H   2   O (1)+2 e   −   →H   2 ( g )+2 OH   − ( aq )E 0 =−0.83 V  
 
       1/2 O   2 ( g )+2 H   2   O (1)+2 e   − →2 OH   − ( aq )E 0 =0.40 V  
 
     giving a Standard EMF of 1.23V. 
     Electric current is carried through the electrolyte solution by way of movement of ions such as H + (aq) or OH − (aq). However in pure water these ions are in very low concentration so an additional electrolyte must be added to allow practical values of current to flow. Typically an alkalis such as Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) is added in quite high concentrations. A typical value for KOH would be about 30 wt %, the concentration at which the electrical conductivity reaches a maximum. 
     Faraday&#39;s Law provides the relationship between the current and the rate of electrolysis, 
     where N the number of moles of gas released by a current  1  in time t is given by 
         N=I*t /( n*F )  (1)
 
     N is the number of electrons required to deliver one mole of gas, for hydrogen n=2 for oxygen n=4. Thus the rate of hydrogen production is given by 
       Δ I/Δt=I/ (2 *F )  (2)
 
     The minimum voltage required to electrolyze water is 1.23 V but higher voltages must be applied in order to increase the current. Voltage drops occur at the electrodes due to overpotential and across the electrolyte gap between to two electrodes. 
     Overpotential (η) refers to the difference between the applied potential necessary to produce a current i and the equilibrium potential E 0  at zero current, 
       η= E−E   0   (3)
 
     For the anode where oxygen is produced the overpotential is related to the current density by 
         i=i   0 exp(− b η)  (4)
 
       where 
         b=αnF/RT  and  i=I/A   (5)
 
     and where i 0  is a constant relating to the particular electrode reaction and the surface on which it occurs, e.g. platinum in KOH, α is a constant usually with a value of 0.5, F is the Faraday constant, R the gas constant and T temperature in K, A is the active area of the electrode and I is the actual measured current. A similar equation exists for the cathode but with different values of i 0  and α. These equations can also be expressed in terms of the overvoltage as 
       η= B 1 n i   0   −B 1 ni=B 1 n i   0   /I   (6)
 
     where B=1/b. 
     The gap between to two electrodes is filled with conducting electrolyte but does incur a potential drop. This potential drop is given by 
         V   electrolyte   =I*R   (7)
 
     where I is the current and R the resistance of the electrolyte, and where 
         R=A/d*κ   (8)
 
     where A is the effective electrode area and d the electrode separation, κ is the conductivity of the electrolyte which is a function of the electrolyte composition and concentration and temperature. 
     The Current Voltage Characteristic for a single cell is therefore given by 
         V=E   0 +η anode −η cathode   +IR   (9)
 
         V=E   0   +B   anode 1 n i   0   /i−B   cathode 1 n i   0   /i+IR   (10)
 
     It can be seen that for a given current the voltage can be reduced by increasing the effective surface area of the electrodes, reducing the electrode separation, increasing the concentration and temperature of the electrolyte and by catalyzing the electrodes which has the effect of increasing the value of i 0 . 
     The maximum efficiency of an Electrolyzer c, is given by, 
       ε=Δ H   0   /ΔG   0  
 
     where ΔG 0  and ΔH 0  are the standard Cibbs Energy Change and standard Enthalpy change for the electrolysis reaction. For water electrolysis the maximum efficiency is 120%. This is greater than 100% because in principle the reaction can extract heat from the surroundings. In practice, however, the efficiency is below 100% because the driving voltage is always greater than 1.23 V. 
     The actual efficiency is given by 
       ε=Δ H   0 /( nFV )
 
     where V is the cell operating voltage at a given current. V is given by equation (10). 
     At a practical current density of about 2 A cm −2 , the cell voltage is about 2 V, and this would give an efficiency of about 74%. 
     Electrolyzer Design 
     From the above descriptions and equations it can be shown that the most energy efficient electrolyzer is one that minimizes the overall cell impedance. For an electrolyzer supplied with current from a vehicle alternator operating at a constant voltage of approximately 13 V and with a current draw limited to 20 to 30 A, the impedance of the electrolyzer is minimized by reducing the electrode gap, increasing the electrolyte conductivity and increasing the number of cells in series, usually to six. Under these circumstances the electrode area is optimized to reduce cell impedance but to remain within the chosen current draw from the alternator. 
     If the electrolyte is potassium hydroxide the maximum conductivity occurs at about 28% by weight potassium hydroxide. An electrolyzer with 6 cells in series with stainless steel electrodes of 200 cm 2  and a spacing of 1 cm immersed in 28% KOH will operate at 12 V and about 30 A and produce about 1.3 L/min of hydrogen. This electrolyzer would require a minimum volume of about 1.5 L of electrolyte or about 1 L of water. This amount of water would be consumed in about 16.5 hours. Clearly the electrolyzer would cease to function as designed well before the 1 L of water was consumed. 
     To facilitate long periods of operation between additions of water and to maintain electrolyzer energy efficiency it is necessary to add water as required to the electrolyzer, for example, from a water reservoir. In conventional practice, this requires a means for detecting the loss of water from the electrolyte and a means of transporting water from the reservoir to the electrolyte and a means of detecting when sufficient water has been added to replenish the water content. An electrolyzer is inherently a very robust device. Provided that the electrodes are not corroded by the applied potentials or reaction with the electrolyte or produced gasses then operational reliability is very high. However, the introduction of electro-mechanical level detectors, valves and pumps can severely reduce reliability, especially when exposed to the harsh environment experienced by devices operating large diesel engines such as long haul trucks, bull dozers, cranes, etc. Ideally both the means of detection of water content and of transporting water between the two reservoirs should contain no moving parts, electrical contacts, switches or require any external control circuitry. 
     SUMMARY OF THE INVENTION 
     The present invention discloses exemplary systems and methods for refilling an electrolyzer tank from a water reservoir. In one embodiment, a water electrolyzer system according to the invention includes an electrolysis chamber for production of hydrogen and oxygen and a water reservoir from which make-up water is transported to the electrolysis chamber to maintain a constant hydrogen output. 
     In another embodiment, a system for controlling electrolyte level and concentration within a water electrolyzer includes an electrolysis chamber containing electrolyte for production of hydrogen and oxygen, a water reservoir containing make-up water and separated from the electrolysis chamber through a check valve that opens only when electrolyte level drops to a predetermined level, and a gas lift pump within the water reservoir connected to the electrolysis chamber through the check valve, the gas lift pump having electrodes immersed in the make-up water. Energization of the electrodes creates bubbles that transport the make-up water to the electrolysis chamber to maintain a desired concentration of the electrolyte during the production of hydrogen and oxygen. 
     A method according to the invention maintains concentration of an electrolyte in a water electrolyzer during electrolysis of water. The method prescribes steps for containing the electrolyte within an electrolysis chamber, providing a water reservoir containing make-up water, connecting the make-up water to the electrolyte through a check valve that opens only when the electrolyte in the electrolysis chamber drops below a predetermined level, immersing a gas lift pump within the water reservoir so that the gas lift pump is in fluid communication with the electrolysis chamber through the check valve, and energizing the gas lift pump to transport the make-up water to the electrolysis chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. Component parts shown in the drawings are not necessarily to scale, and may be exaggerated to better illustrate the important features of the invention. In the drawings, like reference numerals designate like parts throughout the different views, wherein: 
         FIG. 1  is a schematic diagram of one embodiment of a system according to the invention for maintaining concentration of an electrolyte in a water electrolyzer during electrolysis of water. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following disclosure presents an exemplary embodiment of the invention for refilling an electrolyzer tank from a water reservoir. The invention provides a highly reliable, robust and passive control system (and related methods) for maintaining electrolyte level and electrolyte concentration within an electrolyzer tank. This technology rectifies the limitations of prior fluid level control systems, particularly those employed in water electrolyzers designed for use with internal combustion engines. Using the technology of the present invention, an electrolyzer may be optimized for efficiency and operational life. 
     Referring to  FIG. 1 , an exemplary embodiment of the invention is depicted in the form of a schematic for a water electrolyzer. In normal operation, a container  2  holds an amount of water  4 , the free surface of which is labeled  3  and container  23  holds an amount of electrolyte  21 , the free surface of which is  19 . In one embodiment, containers  2  and  23  may be formed within a common outer shell and separated by an inner wall  24 . Water may be added to container  2  via the filling port  1 , which may be closed during normal operation. The water may be distilled water, de-ionized water, bottled water, tap water or water containing some deliberately added chemical such as KOH. The electrolyte may be added to container  23  via the filling port  16 . The electrolyte may be added as solid KOH, NaOH, K 2 CO 3 , Na 2 CO 3  or as soluble salts, carbonates, or hydroxides with Group 1A or Group 2A metals from the periodic table. Alternatively the solid alone may be added through port  16 , then water may be added from container  2  via gas lift pump  11  to achieve or maintain a desired electrolyte concentration. 
     Electrodes  22  in the electrolysis chamber  23  are immersed in the electrolyte  21  and are connected to a supply of electric current  13  by wires  14 . The schematic depicts two unipolar electrodes  22  although three or more are possible in a serial array of electrodes, including one or more bipolar electrodes in the array that occupy intermediate positions between unipolar electrodes at either end of the array. When electric current flows from the power supply  13  to the electrodes  22  an electrochemical reaction will occur that will produce hydrogen at the negative electrode (or cathode) and oxygen at the positively charged electrode (or anode). The mixture of these gasses will form bubbles that rise to the surface  19  of the electrolyte  21 . The gasses will then escape through vent  15 . As the electrolysis proceeds, water in the electrolyte is converted into hydrogen and oxygen and the volume of the electrolyte in container  23  diminishes. The height of the electrolyte surface  19  will drop to below a predetermined minimum level, causing a float ball  20  to drop and open a valve port  18 . This allows make-up water from container  2  to flow into container  23  via the gas lift pump  11 . As water flows into container  23 , the level of the electrolyte will rise, causing ball  20  to rise and seat against the valve port  18 , thereby shutting off the flow of water from gas lift pump  11 . In this respect, float ball  20  and port  18  form a check valve that allows make-up water to flow into container  23 , but prevents the leakage of electrolyte  21  past port  18 . By this means the electrolyte level and composition is maintained constant, within some acceptable level of tolerance. Other check valve designs are possible within the scope of the invention. The check valve, and any parts thereof, preferably operate as passive components, and should be selected for high reliability. 
     The gas lift pump  11  utilizes gas bubbles produced by electrolysis of water  4  by electrodes  7 . Only two electrodes  7  are required for the gas lift pump to operate, although more than two electrodes may be utilized, if desired. A shroud  8  may completely or partially surround the electrodes  7 . Bubbles of hydrogen and oxygen that form on electrodes  7  rise and are trapped by the shroud  8  to form a larger or aggregate bubble  6 . As the bubble  6  grows in size it will periodically release a rising bubble  12  into gas lift pump tube  11 . As the rising bubbles  12  rise in tube  11  they will push water from within shroud  8  into tube  11  and lift the water to the point  9  where it may flow into container  23  via an open valve  18 . In the event that valve port  18  is closed, then the water may exit tube  11  via an overflow opening  10  and fall back into the water container  2 . The electrolysis gasses of hydrogen and oxygen will also exit via opening  10  and then pass into container  23  via the gas pressure equalization port  17 . 
     In one embodiment, a horizontal segment  25  of tube  11  may be level at zero degrees with respect to the container bottom. In another embodiment, the angle of segment  25  may depart slightly from zero (positively or negatively), or the cross sectional shape of segment  25  may be conical, to promote water flow into port  18 , or to promote the continued travel of bubbles  12  away from the water to prevent obstruction of the flow path. In another embodiment, opening  10  may be designed to promote the escape of the electrolysis gases. For example, opening  10  may be enlarged beyond the diameter of tube  11 , or it may include one or more sharp edges for dividing or bursting bubbles  12  that rise to the top of the conduit. 
     In another embodiment, a lower end segment  26  of tube  11  may extend into and below the top of shroud  8 , as shown. In this way, segment  26  may form a primary chamber  27  wherein a bubble  6  may expand until it achieves a size sufficient to allow a portion of the bubble to wrap around the outer wall of segment  26  and break away as a rising bubble  12 . In another embodiment, a segment  26  may be configured to promote the release of an entire bubble  6  into tube  11 . 
     For proper operation, the size and frequency of rising bubbles  12 , and the configuration of tube  11  and segment  26  should provide a supply of make-up water to container  23  that compensates for water lost from container  23  through electrolysis, evaporation and droplet formation. In other words, the gas lift pump  11  should be designed to cause a flow rate of water into container  23  in an amount equal to or greater than the actual rate of water loss from container  23 . In one mode of operation, when the float ball closes the check valve, some amount of make-up water supplied by the gas lift pump will remain within horizontal segment  25  in contact with the float ball, poised to replenish container  23  as soon as the level of electrolyte  21  drops below the predetermined minimum level. Replenishment of the make-up water through the check valve will raise the float ball, causing re-closure of the check valve. In this manner, transfer of make-up water into container  23  according to the invention guarantees a passive, self-governing control of electrolyte volume at or near the predetermined minimum level. 
     Given a steady-state operation of the gas lift pump, the size and frequency of rising bubbles  12  may be determined by adjusting the current and the size of segment  6  and inner chamber  27 . The gas lift pump may be energized using power supply  13 , or it may be energized from an independent power source. In the case where the power supply provides a constant voltage or current, the operation of the gas lift pump may be controlled by selecting the size and composition of electrodes  7 . 
     In another embodiment of the invention, gas lift pump  11  may produce bubbles by a means other than electrolysis. For example, gas lift pump  11  may be a motorized pump that pumps air into the make-up water to produce bubbles, much like a fish tank water pump used to oxygenate an aquarium. 
     A water electrolyzer system generally configured as disclosed herein optimizes electrolyzer operation in a number of ways. With respect to weight, the overall weight of the electrolyzer is minimized by limiting the volume of the heavier electrolyte to the smallest volume (container  23 ) required for producing a desired amount of electrolysis gasses. In container  2  the total volume of make-up water may be selected to achieve a desired operating time between refills. Because water is lighter than the electrolyte, the separation of make-up water from the electrolyte allows the total liquid weight of the electrolyzer (water plus electrolyte) to be minimized. With respect to power, electrical efficiency of the electrolyzer is maximized by maintaining a constant volume and concentration of electrolyte between the electrodes  22 . The ability to maintain a constant electrolyte concentration allows a designer to minimize heat losses for the duration of the electrolyzer duty cycle by optimizing electrolyzer performance based on power supply and electrode configuration. And with respect to reliability, the absence of moving parts, and the absence of control electronics, reduces the overall probability of system failure. 
     Exemplary embodiments of the invention have been disclosed in an illustrative style. Accordingly, the terminology employed throughout should be read in a non-limiting manner. Although minor modifications to the teachings herein will occur to those well versed in the art, it shall be understood that what is intended to be circumscribed within the scope of the patent warranted hereon are all such embodiments that reasonably fall within the scope of the advancement to the art hereby contributed, and that that scope shall not be restricted, except in light of the appended claims and their equivalents.