Abstract:
One embodiment of the invention includes a PV array and an electrolyzer operatively connected together and each operatively connected to a utility power grid so that electricity produced by the PV array is selectively delivered to the utility power grid and the electrolyzer. The resulting process increases the efficiency of the solar-hydrogen production process, and results in lower-cost renewable hydrogen.

Description:
TECHNICAL FIELD 
       [0001]    The field to which the disclosure generally relates includes hydrogen generation by electrolyzers, and more specifically, reducing the cost of renewable hydrogen generation. 
       BACKGROUND 
       [0002]    Hydrogen generation devices use electricity to produce hydrogen (and oxygen) by electrolysis of water in an electrolyzer. The hydrogen generated is stored for use as a fuel, useable in fuel cells and internal combustion engines. The oxygen is vented to the atmosphere. Electrolyzers may be powered by solar energy. Solar hydrogen generation by a photovoltaic-electrolyzer (PV-electrolyzer) is a renewable and environmentally beneficial energy source. Converting U.S. fuel supplies to renewable energy sources is essential for sustainable transportation, sustainable economic growth, reducing greenhouse gas emissions, and for national energy security by replacing polluting fossil fuels imported from unstable regions overseas. 
         [0003]    It is not economical to directly connect solar electricity from photovoltaic arrays (PV arrays) alone to power an electrolyzer and produce exclusively renewable hydrogen fuel, although a direct connection method, with the maximum power point voltage of the PV array matching the electrolyzer operating voltage, would give the highest efficiency conversion of solar energy to hydrogen. This lack of economy arises because solar power from a fixed angle PV system is produced effectively for only about six hours per day on average, leaving 18 hours per day when solar electricity production is too little for hydrogen generation and the electrolyzer can not be used. Thus, the electrolyzer is idle most of the time, which increases the size and cost of electrolyzer required for a desired daily fuel production rate. In addition, the electrolyzer is currently the most expensive part of a hydrogen generator system. Therefore, other solutions are needed to make solar powered electrolytic hydrogen production economically viable. 
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       [0004]    One embodiment of the invention includes a PV array and an electrolyzer operatively connected together and each operatively connected to a utility power grid so that electricity produced by the PV array is selectively delivered to the utility power grid and the electrolyzer. 
         [0005]    Other exemplary embodiments of the invention will become apparent from the detailed description of exemplary embodiments provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the 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 
         [0006]    Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings. 
           [0007]      FIG. 1  is a block diagram of the power control system that selectively delivers electricity from the PV array to the utility power grid and the electrolyzer, according to one embodiment of the invention. 
           [0008]      FIG. 2  illustrates the relationship between hourly energy (kWh) and the time over a 24-hour period for the utility power grid, PV array, and electrolyzer. 
           [0009]      FIG. 3  illustrates the optimum operating current (I opt ) of the electrolyzer arrived at by plotting the hydrogen cost ($/kg) versus the electrolyzer operating current (A). 
           [0010]      FIG. 4  illustrates the relationship between hydrogen cost ($/kg) and the electrolyzer operating current (A) for electrolysis by solar power only, electrolysis by solar and grid power using net metering, and electrolysis by solar and grid power using variable electricity price. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0011]    The following description of embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0012]      FIG. 1  is a block diagram of a product  10  comprising a power control system  12 . The power control system  12  combines at least one of PV power (DC current) from a PV array  14  or AC power from a utility power grid  16  to power an electrolyzer  18 . The PV array  14  may be any suitable PV array, including, but not limited to crystalline silicon, amorphous silicon, cadmium telluride, and copper indium diselenide based PV modules. These PV modules can include, but are not limited to, the Sharp NT-S5E1U model manufactured by Sharp Electronics Corp., Huntington Beach, Calif., available from AAPS Alternative Power Systems, Carlsbad, Calif., USA; the Sanyo HIP-190BA3 module manufactured by Sanyo Solar (part of Sanyo Electric Co., Ltd., Japan), which is available from Alternative Energy Store, LLC, Hudson, Mass.; and other suitable PV modules. 
         [0013]    The electrolyzer  18  produces hydrogen and oxygen by the electrolysis of water. The electrolyzer  18  may be any suitable electrolyzer. The electrolyzer  18  may be a proton exchange membrane (PEM) electrolyzer including an anode, a cathode and a membrane in between or any other suitable electrolyzer. The electrolyzer  18  may be a high-pressure electrolyzer. 
         [0014]    The hydrogen output  34  of the electrolyzer  18  is collected and stored at an appropriate pressure and used as a fuel. By combining the solar power with another source of electricity, such as grid electricity from utilities, the electrolyzer  18  can be operated 24 hours a day if desired, and solar powered electrolytic hydrogen production can be economically viable, because the cost to produce a desired amount of hydrogen fuel is reduced. 
         [0015]    In one embodiment, the PV power is routed to a variable DC DC converter  20 , where the current and voltage are converted to the predetermined optimum operating current (I opt ) and optimum operating voltage (V opt ) for the electrolyzer  18 . The variable DC DC converter  20  may be a variable output DC power supply circuit consisting of internal potentiometers, wire coils, semiconductors, and other electronics. The output of the variable DC DC converter  20  is electrically connected to the electrolyzer  18 . 
         [0016]    The variable DC DC converter  20  is controlled by a pre-programmed controller (logic system)  22 , including, for example, a mainframe computer or microprocessor and associated circuits, switches, and wiring. An ammeter  24  connected to the electrolyzer  18  and the controller  22  measures the total DC current input to the electrolyzer  18 . The controller  22  uses the signal from the ammeter corresponding to this total DC current input to the electrolyzer  18  to set the current flow originating from the PV array  14  (defined as I DC A) and the current flow originating from the utility power grid  16  (defined as I DC B) so that the total current flow to the electrolyzer  18  (I DC A+I DC B) equals the optimum operating current (I opt ) of the electrolyzer. The optimum current is based on cost, and I opt  is the operating current at which the solar powered electrolyzer produces hydrogen at the lowest cost. 
         [0017]    In another embodiment, the PV array  14  is routed directly to the electrolyzer  18  through a potentiometer  26  instead of through the variable DC DC converter  20 . The alternative circuit with direct connection has less resistance and allows greater system efficiency than the circuit with the variable DC DC converter  20 . A switch  32  may be manually operated and the potentiometer  26  may be controlled by the controller  22 . The switch  32  determines whether the current is routed through the potentiometer  26  or through the variable DC DC converter  20 . The direct connection through the potentiometer is used when the PV array  14  has been designed and constructed to supply the optimum operating voltage (V opt ) required by the electrolyzer  18  without using a variable DC DC converter  20 . When using this alternative (direct connection) circuit, the ammeter  24  measures the total DC current input to the electrolyzer  18 . The controller  22  uses this total DC current input to the electrolyzer  18  to set the current flow from the PV array  14  (defined as I DC A) and the current flow from the utility power grid  16  (defined as I DC B) so that the total current flow to the electrolyzer  18  (I DC A+I DC B) equals the optimum operating current (I opt ) of the electrolyzer. 
         [0018]    The PV array  14  is electrically connected to the input of a variable DC AC inverter  30 . The variable DC AC inverter  30  converts the solar DC electricity to AC electricity with current, voltage, and wave form required for electricity sold to the utility power grid  16 . The variable DC AC inverter  30  is a variable output AC power supply or transformer circuit consisting of internal potentiometers, wire coils, semiconductors, and other electronics. The output of the variable DC AC inverter  30  is connected to the utility power grid  16  in order to sell excess PV power not immediately required to run the electrolyzer  18 . This AC current output to the grid is also controlled by the controller  22 . 
         [0019]    The potentiometer  26  may be used when necessary to control the ratio of current flowing to the electrolyzer  18  and current flowing to the utility power grid  16  through the variable DC AC inverter  30  so that the predetermined I opt  is maintained. The potentiometer  26  can be adjusted through a range of resistance to control the percentage of the total PV generated current that flows directly to the electrolyzer  18  and the percentage of the PV generated current that flows to the variable DC AC inverter  30 . The potentiometer  26  can be set to approximately zero ohms resistance so that a direct connection is established between the PV array  14  and the electrolyzer  18 . Alternatively, the potentiometer  26  can be adjusted to any value of resistance needed to split the PV generated current in any desired ratio between the electrolyzer  18  and the variable DC AC inverter  30 . In one embodiment, about 75% of the total PV electric power is routed to the variable DC AC inverter  30  and about 25% of the total PV current is sent directly to the electrolyzer  18 . This split ratio may provide sufficient renewable PV power to the grid during daytime to equal and balance the grid power taken from the electric utility and used to operate the electrolyzer when PV power is not available due to insufficient sunlight. To minimize power losses, the variable resistance of the potentiometer  26  is kept as small as possible consistent with the need to maintain the desired splitting ratio between the electrolyzer  18  and the variable DC AC inverter  30 . 
         [0020]    Because the PV array  14  can only produce solar electricity to operate the electrolyzer  18  in daylight hours, power from the utility power grid  16  is required for cost effective hydrogen fuel generation. However, since the primary reason for making and using hydrogen fuel is environmental, the use of grid electricity preferably should be avoided unless it is generated renewably. To make the hydrogen production completely renewable, an oversized PV array  14  may be used to produce sufficient solar power during the daylight hours to operate the electrolyzer  18  and to produce surplus solar power to be transmitted (sold) to the utility power grid  16 . This surplus solar energy can be seen as energy stored for later use in the utility power grid like a bank deposit. The utility power grid  16  then sells an equal amount of power back to the hydrogen generation system at night to operate the electrolyzer  18  during the hours of darkness. This power from the utility power grid  16  which is sold back to the hydrogen generation system is considered “renewable” or “green” energy because it is equally balanced by the solar PV electricity sold to the utility power grid in the daytime. During full daylight, the PV output may be larger than the optimum current required by the electrolyzer: part of the PV output goes through the variable DC DC converter to operate the electrolyzer, and the rest of the PV output goes through the variable DC AC inverter to be sold to the utility power grid. Thus, the hydrogen produced by the electrolyzer can be completely classified as a renewable fuel. In one embodiment, the total renewable electricity flow from the PV system to the utility grid corresponds to the amount of renewable electric energy required to operate the electrolyzer and to produce a desired amount of renewable hydrogen fuel. 
         [0021]      FIG. 2  illustrates a combination of solar power and grid power supplying the electrolyzer with power 24 hours a day. As shown in  FIG. 2 , during the middle of the 24-hour period, excess solar power (from the PV array) is sold to the utility grid. At the beginning and end of the 24-hour period, power is sold from the utility grid to the system to power the electrolyzer when the PV array is incapable of producing enough power. 
         [0022]    In one embodiment, power from the utility power grid enters the power control system  12  at night and at other times of low sunlight. The utility power grid  16  is connected to a variable AC DC converter  28  that converts AC power from the grid (I AC ) to the optimum DC current and voltage to operate the electrolyzer  18  when combined with any PV current available at the time. The variable AC DC converter  28  is a variable output DC power supply circuit consisting of internal potentiometers, wire coils, semiconductors, and other electronics. The variable AC DC converter  28  is also controlled by the controller  22  so that the combined PV current (I DC A) and grid current input (I DC B) to the electrolyzer  18  equals the optimum operating current for the electrolyzer (I opt ). 
         [0023]    Thus, the PV system may be built large enough to generate enough electricity to operate the electrolyzer at its optimum current 24 hours per day and 365 days per year based on minimizing hydrogen cost. Since the number of hours of sunshine per day varies seasonally and can vary hourly with weather conditions, the total area of the PV array can best be sized for average conditions. The total annual PV output predicted for the location and PV module orientation should equal the total annual power input of the electrolyzer at its optimum current. Thus, in a given year, there may be a surplus or deficit in the renewable solar energy produced by the PV system, but the long term renewable energy supply will average out to equal the electrolyzer input required for 100% renewable hydrogen fuel production. The annual average number of peak sun hours of insolation (incident solar radiation) for various PV systems in numerous U.S. locations has been measured and published in standard tables by the U.S. Department of Energy National Renewable Energy Laboratory. Annual PV module output can be determined by power output measurements under various seasonal conditions, or manufacturers provide data for the power, current, and voltage output of their products under standard solar radiation which can be corrected for temperature under normal operating conditions. The average daily PV system output (kWh) can be estimated by multiplying the average peak sun hours times the average PV module power output if both averages are determined for all seasons. 
         [0024]    In another embodiment, the amount of PV energy generation capacity (kWh) that will be stored in the utility power grid  16  and recovered for later use may be increased by 10-20% to account for greater losses in the two power conversion steps and longer wiring required. The expected power loss from each DC DC or AC DC converter or DC AC inverter in a circuit is expected to be 5-10%. 
         [0025]    The optimum operating current (I opt ) of the electrolyzer is the electrolyzer operating current (I oper ) corresponding to the minimum hydrogen cost per unit amount. The value of I opt  is used as the constant operating current for the 24-hour per day operation of the solar and grid powered electrolyzer system. As illustrated in  FIG. 3 , for a given electrolyzer, it is possible to determine the optimum operating current (I opt ) for the electrolyzer by plotting the unit hydrogen cost ($/kg) against I oper  and choosing the I oper  value that minimizes cost. The cost curve can pass through a minimum because the mass of hydrogen produced per day increases with increasing I oper , but the efficiency of the electrolyzer, measured by the mass of hydrogen produced per unit of electrical energy input to the electrolyzer (kg/kWh), decreases with increasing I oper . 
         [0026]    The cost of hydrogen production with solar electricity sold to the utility power grid is calculated by equation 1, where P opt  is the optimum power of the electrolyzer: 
         [0000]      H 2  production cost ($/kg)=[PV system cost+electrolyzer cost+(Electricity buying price 0   24 ∫Grid power bought× dt )−(Electricity selling price 0   24 ∫Grid power sold× dt )]×33.35 kWh/kg/( P   opt ×Electrolyzer Efficiency×24 h)  (equation 1) 
         [0027]    Using the lower heating value (LHV) of hydrogen (33.35 kWh/kg), the total hydrogen production is calculated using equation 2, where I opt  is the optimum current, V opt  is the optimum voltage of the electrolyzer, and P opt  is the optimum power of the electrolyzer: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0028]    The electrolyzer efficiency must be determined consistently using the water electrolysis potential of 1.23 volts at the LHV, where V oper  is the operating voltage of the electrolyzer (equation 3): 
         [0000]      Electrolyzer Efficiency ( LHV )=100%×no. of electrolyzer cells in series×1.23 volts/ V   oper   (equation 3) 
         [0029]    In one embodiment of the invention, solar electricity produced by a solar and grid powered hydrogen generator is sold to a utility power grid with a constant electricity price ($/kWh). When the electricity price is the same for buying or selling electricity to the utility power grid at any time of day, the policy is called net metering. Under a utility policy of net metering, which already exists in many U.S. states, the electric meter charges the PV system owner for the net difference between the grid electricity bought and PV electricity sold, i.e., electricity bought minus electricity sold at the current price set by the utility. 
         [0030]    In another embodiment, the solar electricity is sold to a utility power grid with a variable electricity price ($/kWh). If the electricity price for buying or selling electricity to the utility power grid can be negotiated with the utility company based on the electricity demand at the time of day, the policy may be called variable pricing. Peak electrical demand, with a potentially high electricity price, generally occurs about midday in cool weather and in the hottest part of the afternoon in summer in warm climates. Electricity demand is lowest at night, with a potentially reduced electricity price. Under a hypothetical “variable rate plan”, the PV system owner would need to petition the utility to pay a higher rate than the net metering rate for the PV power sold to the utility during peak demand times, and to sell the PV owner power at a lower rate than the net metering rate for grid power to operate the electrolyzer during nighttime and any other low demand times. 
         [0031]    As illustrated in  FIG. 4 , the unit hydrogen cost ($/kg) varies depending on whether the power control system  12  uses only solar power or uses a combination of solar power and power from the utility grid. In addition,  FIG. 4  illustrates that the unit hydrogen cost decreases when the PV system uses a variable electricity price based on the changing electrical demands during the day versus net metering. The embodiment in  FIG. 4  is based on a 20-cell electrolyzer operated 24 hours a day in Las Vegas using electricity from a fixed angle PV array. 
         [0032]    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.