Electrolyzer
Electrical load for the electrolyser.
Description
The Electrolyzer unit represents the electrical load associated with an electrolyser. The electrolyser consists of an anode and a cathode separated by an electrolyte. The unit calculates the amount of hydrogen produced based on the electrical energy provided and the temperature of the water in the tank.
In polymer electrolyte membrane (PEM) electrolysers, these chemical reactions determine the anodic and cathodic conversions:
An electrolyser consists of several individual electrolyser cells connected in series. Use the Assumption pH parameter to model the pH dependence either with a constant pH.
Equations
The Electrolyzer block calculates the electrical power using:
where is the voltage and is the total resistance. The total resistance, , is defined as:
where
-
- is the external resistance;
-
- this value is the resistive term obtained from the cross-sectional area of the hole ( ), the distance between anode and cathode ( ) and the resistivity of the solution .
The molar energy stored in the electrical load provides the process of electrolysis of water and is calculated from this equation:
The electrolyser requires a minimal amount of energy to carry out electrolysis:
where
-
- enthalpy of water;
-
- entropy of the reaction;
-
- water temperature;
-
- reference temperature.
If the electrical energy is insufficient to dissociate water, the electrolyser does not produce hydrogen. Therefore, the Electrolyzer block calculates the molar velocity of the electron, , and the molar velocity of hydrogen, :
where
-
- Avogadro’s constant;
-
- electron energy;
-
- number of cells;
-
- temperature-dependent electrolysis efficiency.
The current flowing through the electrolyser tank is calculated as:
where is Faraday’s constant.
The Electrolyzer block calculates the mass fractions of water consumed and hydrogen produced using these equations:
where is the molar mass of water and is the molar mass of hydrogen.
To calculate the pH of the water in the tank, the unit uses
Where:
-
traps hydrons floating in solution ( ); -
- number of moles of water;
-
- volume;
-
- the density of the water in the tank.
Residual protons in solution inside the electrolyser naturally increase the pH and electrical conductivity of the tank water over time. The increase in electrical conductivity results in fleeing electrons. To prevent this phenomenon, it is necessary to purify the water in the tank by removing the entire volume and filling it with clean water.
Ports
Non-directional
+ - positive
electricity
A non-directional port associated with the anode of the electrolyser tank.
- - negative
electricity
A non-directional port associated with the cathode of the electrolyser tank.
Input
purge - purge input
scalar
Input port associated with purge, dimensionless value. This signal allows the water in the tank to be replaced.
vol - volume of water, m^3
scalar
Input port related to the volume of water in the electrolyser tank, in m^3.
T is the temperature of the water, K
scalar
Input port associated with water temperature, in K.
Output
volPurging - volume of water treated, m^3
scalar
Output port associated with the volume of water used for purging, in m^3.
pH - pH of water
scalar
Output port related to the pH of the water in the electrolyser tank, a dimensionless value.
mdotH2 is the mass flow rate of hydrogen, g/sec
scalar
Output port associated with hydrogen mass flow rate, in g/s.
mdotH2O is the mass flow rate of water consumed, in g/sec
scalar
Output port associated with the mass flow rate of water consumed, in g/s.
Parameters
Main
Assumption pH - constant or dynamic pH
Constant (by default) | Dynamic
Specify whether the pH in the electrolyser tank is constant or dynamic.
Resistance - external resistance
10 Ohm (by default) | `non-negative scalar'.
The value of the external resistance.
Cross-sectional transport area - cross-sectional area of the hole
0.04 m^2 (by default) | positive scalar
The cross-sectional area perpendicular to the vector connecting the anode and cathode. Positively charged ions move across this area from the anode to the cathode, where they are reduced and hydrogen is formed.
Distance anode-cathode - distance between anode and cathode
0.01 m (by default) | positive scalar
Distance between anode and cathode immersed in an electrolytic solution.
Number of cells - number of cells
50 (By default) | Positive scalar
The number of cells in the electrolyser.
Initial pH - initial pH
0 (by default) | scalar
Initial pH value of the water in the tank.
Constant pH - constant pH
6 (By default) | scalar
Constant pH value of the water in the tank.
Dependencies
To enable this parameter, set the Assumption pH parameter to Constant.
pH vector, pH_vec is the pH vector
[5, 5.5, 6, 6.5, 7] ( by default) | ` vector of positive scalar strings'.
A vector of pH values. This parameter must have the same number of elements as the Electrical resistivity vector, rho_vec.
Dependencies
To enable this parameter, set the Assumption pH parameter to Dynamic.
Electrical resistivity vector, rho_vec - vector of electrical resistivity values
[.2, .75, 2.5, 8, 18.2] MOhm*cm (by default) | scalar vector `
A vector of electrical resistance values. This parameter must have the same number of elements as the parameter value vector pH vector, pH_vec.
Dependencies
To enable this parameter, set the Assumption pH parameter to Dynamic.
Temperature vector for efficiency, T_vec - temperature vector for efficiency
[273.0, 298.0, 323.0, 348.0, 373.0] K (by default) | vector of scalars
A vector of temperatures to determine the electrolysis efficiency. This parameter must have the same number of elements as Electrolysis efficiency vector, efficiency(T).
Electrolysis efficiency vector, efficiency(T) is the electrolysis efficiency vector
ones(5)(by default) | `scalar vector'.
A vector of electrolysis efficiency values. This parameter must have the same number of elements as the parameter Temperature vector for efficiency, T_vec.