Direct DC solar-electrolysis system

1 . A renewable energy system comprising:

a renewable energy source producing DC electricity;

an electrochemical device directly connected to receive said DC electricity without intermediate active power conversion,

wherein the renewable energy source comprises a plurality of photovoltaic solar panels wired in a series to form photovoltaic strings, a plurality of photovoltaic strings wired in parallel to form a photovoltaic field, and a plurality of photovoltaic fields wired in parallel to form a photovoltaic system, wherein the number of panels in each string is selected to achieve a predefined maximum bus voltage,

wherein the electrochemical device comprises a water electrolyzer stack module connected directly to the unregulated DC bus of the photovoltaic system comprising one or more groups of individual electrolyzer cells, wherein each group of individual electrolyzer cells contains a number of individual electrolysis cells connected in series, wherein the number of cells in each group is selected to (i) achieve a rated operating voltage of the electrolyzer cell groups less than or equal to the photovoltaic system bus voltage at one of the stack's a) beginning of life, b) end of life, or c) middle of life, (ii) maximize the lifetime hydrogen production of the renewable energy system, or (iii) minimize the levelized cost of hydrogen produced by the renewable energy system over its lifetime,

wherein each group of individual electrolyzer cells comprises two physically distinct stacks of cells, wherein the two stacks are wired in series with a mid-voltage connection to form branch pairs in the electrolyzer module, wherein one or more branch pairs are wired in parallel to accept the maximum total current produced by the photovoltaic system at the maximum solar irradiance expected at the installed location, wherein a positive bus and a negative bus of a direct current (DC) bus connecting the renewable energy source to the electrochemical device are symmetrically referenced to earth ground to achieve generally equal absolute values of DC operating voltage, relative to earth ground, for each bus.

2 . The system of claim 1 ,

wherein the total number of groups of individual electrolyzer cells in the water electrolyzer stack module is selected to accept the maximum total current produced by the photovoltaic system at the maximum solar irradiance expected at the installed location.

3 . The system of claim 1 ,

wherein the number of cells in each group is selected to achieve an electrolyzer rated voltage less than the bus voltage of the photovoltaic system according to the formula:

N

c

≤

V

PM

V

EB

,

 wherein

N C is number of individual electrolysis cells in each group

V EB is the rated beginning of life voltage for an individual electrolysis cell

V PM is the maximum bus voltage of the photovoltaic system.

4 . The system of claim 3 ,

wherein

V EB is selected from one of: 1.5V/cell, 1.6V/cell, 1.7V/cell, 1.8V/cell, or 1.9V/cell

V PM is selected from one of: 1000 VDC, 1,500 VDC, 2,000 VDC, or 3,000 VDC.

5 . The system of claim 1 ,

wherein the number of cells in each group is selected to achieve an electrolyzer rated voltage greater than a selected fraction of the bus voltage of the photovoltaic system according to the formula:

N

c

≥

f

MB

·

V

PM

V

EE

,

 wherein

N C is number of individual electrolysis cells in each group

V EE is the rated end of life voltage for an individual electrolysis cell

V PM is the maximum bus voltage of the photovoltaic system

f MB is a selected fraction of the maximum bus voltage of the photovoltaic system, wherein,

f MB is selected to maximize the lifetime hydrogen production of the system, or, minimize the levelized cost of hydrogen produced over the life of the system.

6 . The system of claim 5 ,

wherein

V EE is selected from one of: 1.5V/cell, 1.6V/cell, 1.7V/cell, 1.8V/cell, or 1.9V/cell

V PM is selected from one of: 1000 VDC, 1,500 VDC, 2,000 VDC, or 3,000 VDC

f MB is selected from one of: 0.95, 0.9, 0.85, or 0.75.

7 . The system of claim 1 ,

wherein the number of cells in each group is selected to achieve an electrolyzer rated voltage less than or equal to the maximum peak power voltage of the photovoltaic system according to the formula:

N

c

≤

V

MPP

V

EB

,

 wherein

N C is number of individual electrolysis cells in each group

V EB is the rated beginning of life voltage for an individual electrolysis cell

V MPP is the maximum peak power voltage of the photovoltaic system.

8 . The system of claim 1 ,

wherein each cell group is configured with one or more disconnect switches to enable the input impedance characteristic of the stack module to be changed dynamically during operation.

9 . The system of claim 1 ,

wherein the individual electrolyzer cells in each stack unit have a pitch less than or equal to 5 mm, 3 mm or 2 mm.

10 . The system of claim 1 ,

wherein the electrolyzer cells comprise an anode electrode configured to provide a low voltage decay rate when exposed to a high number of voltage cycles, and wherein the anode electrode provides a relatively poor performance at beginning of life, and wherein the anode electrode contains no precious group metals or rare earth metals.

11 . The system of claim 1 ,

wherein a water inlet connection for each physically distinct stack is made at the end of each stack that corresponds to the mid-voltage connection point of the pair.

12 . The system of claim 1 ,

wherein a hydrogen outlet connection for each physically distinct stack is made at the end of each stack that corresponds to the mid-voltage connection point of the pair.

13 . A method of producing hydrogen comprising:

providing the renewable energy system according to claim 1 , wherein the renewable energy source comprises the photovoltaic system having an unregulated DC bus with a predetermined maximum voltage, wherein the electrochemical device comprises the water electrolyzer stack module comprising one or more groups of electrolyzer cells connected in series;

directly connecting the electrolyzer stack module to the unregulated DC bus without intermediate voltage regulation;

selecting the number of electrolyzer cells in each group to achieve impedance matching between photovoltaic source polarization characteristics and electrolyzer impedance polarization characteristics;

operating the renewable energy system at natural electrical operating points determined by intersection of the source and impedance polarization characteristics; and

producing hydrogen through water electrolysis powered directly by the photovoltaic system.

14 . The method of claim 13 , further comprising:

monitoring solar irradiance conditions in real-time;

dynamically switching electrolyzer cell groups on or off in response to changing solar irradiance.

15 . The method of claim 13 , further comprising:

establishing symmetric ground referencing by connecting the positive bus to earth ground through a first reference resistor and the negative bus to earth ground through a second reference resistor; and maintaining the positive bus at a positive voltage relative to earth ground and the negative bus at a negative voltage relative to earth ground of substantially equal absolute magnitude.

16 . The method of claim 13 , further comprising:

configuring electrolyzer stacks in paired arrangements with opposite orientations; connecting water inlet and hydrogen outlet plumbing at mid-voltage connection points between paired stacks; and maintaining fluid connections at lowest possible voltage potentials relative to earth ground.

17 . A method of optimizing solar-to-hydrogen conversion efficiency comprising:

providing the renewable energy system according to claim 1 , wherein the renewable energy source comprises the photovoltaic system having an unregulated DC bus with a predetermined maximum voltage, wherein the electrochemical device comprises an electrolyzer system;

characterizing source polarization characteristics of the photovoltaic system across a range of solar irradiance conditions;

characterizing impedance polarization characteristics of an electrolyzer system across beginning-of-life to end-of-life operation;

determining optimal electrolyzer cell count by modeling intersection points of the polarization characteristics;

configuring the electrolyzer system with the determined optimal cell count; and

operating the directly-coupled system to achieve maximum annual hydrogen production.

18 . The method of claim 17 , further comprising:

simulating annual hydrogen production using historical solar irradiance data;

calculating levelized cost of hydrogen for different cell count configurations;

identifying cell count that minimizes levelized cost of hydrogen; and

implementing the identified optimal configuration.