Documentation for electrolyzer equipment in NeqSim process simulation.

Table of Contents

Overview

Location: neqsim.process.equipment.electrolyzer

Classes:

Class Description
Electrolyzer Water electrolysis unit (green hydrogen)
ElectrolyzerTechnology Enum of technology defaults (PEM, ALKALINE, SOEC, AEM)
ElectrolyzerIVCharacteristic Polarisation (I-V) model driving cell voltage from current density
CO2Electrolyzer CO₂ electrolysis unit

Electrolyzers convert electrical energy into chemical energy through electrochemical reactions. Key applications:

The water Electrolyzer reaction is:

\[2\,H_2O \rightarrow 2\,H_2 + O_2\]

The hydrogen and oxygen products are exposed as getHydrogenOutStream() and getOxygenOutStream(), and the electrical demand is published on the unit’s energy stream (getEnergyStream().getDuty()).

Electrolyzer Class

Basic Usage (water-feed mode)

In the default water-feed mode the inlet water molar rate sets production; the electrical power is computed as an output.

import neqsim.process.equipment.electrolyzer.Electrolyzer;
import neqsim.process.equipment.stream.Stream;
import neqsim.thermo.Fluid;
import neqsim.thermo.system.SystemInterface;

// Water feed
SystemInterface water = new Fluid().create("water");
Stream waterFeed = new Stream("water", water);
waterFeed.setPressure(1.0, "bara");
waterFeed.setTemperature(298.15, "K");
waterFeed.setFlowRate(2.0, "mole/sec");
waterFeed.run();

// Electrolyzer (default 1.23 V reversible cell voltage)
Electrolyzer electrolyzer = new Electrolyzer("PEM Electrolyzer", waterFeed);
electrolyzer.run();

// Products and power
double h2Rate = electrolyzer.getHydrogenOutStream().getFlowRate("kg/hr");
double o2Rate = electrolyzer.getOxygenOutStream().getFlowRate("kg/hr");
double power = electrolyzer.getEnergyStream().getDuty();   // W
double stackPower = electrolyzer.getStackPower();          // W

System.out.println("H2 production: " + h2Rate + " kg/hr");
System.out.println("Stack power:   " + power / 1.0e3 + " kW");

Key Methods

Method Description
getHydrogenOutStream() Hydrogen product stream
getOxygenOutStream() Oxygen product stream
setCellVoltage(double) / getCellVoltage() Fixed operating cell voltage (V)
setCurrentDensity(double) / getCurrentDensity() Current density (A/cm²), used with an I-V model
setFaradaicEfficiency(double) / getFaradaicEfficiency() Current efficiency (0..1]
getStackPower() Stack (DC) power from the last run (W)
getSpecificEnergyConsumption_kWh_per_kg_H2() Stack specific energy (kWh/kg H₂)
getMassBalance(String unit) Mass balance check

Technologies and I-V Models

Select a technology to apply cell voltage, current density and nominal Faradaic efficiency. Set inlet temperature and pressure separately; the enum’s temperature and pressure getters are reference values. Calling setTechnology overwrites previous voltage/current-efficiency settings, so apply measured overrides afterwards:

import neqsim.process.equipment.electrolyzer.ElectrolyzerTechnology;
import neqsim.process.equipment.electrolyzer.ElectrolyzerIVCharacteristic;

electrolyzer.setTechnology(ElectrolyzerTechnology.PEM);

Available technologies: PEM, ALKALINE, SOEC, AEM.

Current-efficiency basis and remaining qualification

Faradaic efficiency is $\eta_F=2F\dot n_{H_2}/I_{eq}$, a charge-utilization ratio. An LHV/HHV energy efficiency also depends on voltage, heating value and the stack/system boundary. External SOEC heat does not establish current efficiency.

ALKALINE uses a nominal screening estimate from Ulleberg (2003), DOI 10.1016/S0360-3199(02)00033-2, Eq. (9), Table 3, PHOEBUS: 80 °C, reported 7 bar, stationary 30 wt% KOH. The paper reports pressure without establishing a separate gauge/absolute conversion; the existing NeqSim reference remains 7 bara. With $j_m$ in mA/cm², the published expression is $\eta_F=0.96j_m^2/(250+j_m^2)$; 250 has units (mA/cm²)². At the existing nominal 0.4 A/cm² this gives 0.9585023401. This evaluates the fitted expression beyond the current-efficiency measurements in Fig. 6. It is an explicit extrapolation for screening, not a measured 0.4 A/cm² benchmark, a universal alkaline constant or a qualified operating range. The source provides no uncertainty interval for that extrapolation.

At fixed 1.85 V this changes native stack SEC from 79.33746 to 51.31884 kWh/kg on the getter’s 0.002016 kg/mol H₂ basis. The reference is frozen after technology selection: changing load, temperature or pressure does not update current efficiency. Supply an equipment-specific value with setFaradaicEfficiency after selecting the technology; low-load/crossover behavior needs separate evidence.

The existing coarse alkaline I–V coefficients instead give 1.7064354 V at 0.4 A/cm² and 80 °C, hence 47.33637 kWh/kg with this same current efficiency. The two voltage models are different screening cases; neither is validated by agreement with a rounded industry SEC. AlkalineFaradaicDefaultTest executes both native POWER and matched WATER_FEED cases against independent Faraday-law, stoichiometry, power and SEC calculations.

PEM (0.65), SOEC (0.85) and AEM (0.60) still have unqualified legacy current efficiencies. #4682 remains open for their primary-source basis, uncertainty/applicability, off-design behavior and coherent voltage/SEC qualification. The material-balance repair in #4681 is a separate completed change.

For a current-dependent screening voltage, attach an ElectrolyzerIVCharacteristic. When set, the cell voltage is computed from the current density and temperature through the polarisation model:

electrolyzer.setIVCharacteristic(
    new ElectrolyzerIVCharacteristic(ElectrolyzerTechnology.PEM));
electrolyzer.run();
double v = electrolyzer.getCellVoltage();   // e.g. ~1.85 V for PEM at 2 A/cm2, 80 C

Operating Modes

The Electrolyzer supports two operating modes via setOperationMode(OperationMode):

Mode Driver Output
WATER_FEED (default) Inlet water molar rate Electrical power
POWER Available electrical power Water demand and H₂/O₂ production

WATER_FEED is the default. The inlet must contain pure water: every mole supplied is consumed and produces one mole of hydrogen and half a mole of oxygen. Faradaic efficiency is a current-utilization factor, not a water conversion fraction. There is no residual-water or electrolyte outlet. setAvailablePower(...) automatically switches the unit into POWER mode.

Both modes use the same Faraday-law balance:

\[\dot n_{H_2}=\dot n_{H_2O}=\frac{\eta_F I_{eq}}{2F},\qquad \dot n_{O_2}=\frac{\dot n_{H_2}}{2},\qquad P_{DC}=V_{cell}I_{eq}\]

Here flows are in mol/s, $F=96485.3329$ C/mol, voltage is in V and power is in W. $I_{eq}$ (getStackCurrent(), A) is the sum of cell currents; with cells in series it is the terminal current multiplied by the number of cells. It is paired with the cell voltage, not the total stack voltage.

In WATER_FEED, the inlet flow, composition, pressure and temperature remain specified by the caller. Current is $2F\dot n_{H_2O}/\eta_F$. When stack area is supplied, current density is derived from current and total active area; otherwise the explicitly configured density drives an attached I-V model. In POWER, run() deliberately sets and runs the connected inlet stream at the actual stoichiometric water demand, including zero demand at zero power or standby. Use a dedicated demand stream when connecting to a larger water-supply model. Pressure, temperature and pure-water composition are retained. Mixed feeds are rejected before changing their flow; electrolyte circulation, impurities, excess water and residual-water separation require a separate model.

At fixed voltage, the stack SEC is proportional to $1/\eta_F$ in both modes:

\[\mathrm{SEC}_{DC}=\frac{2FV_{cell}}{\eta_F M_{H_2}\,3.6\times10^6}\]

SEC is in kWh/kg and the native getter uses $M_{H_2}=0.002016$ kg/mol. At 18 MW DC, 1.85 V and $\eta_F=0.62$, either mode at matched production consumes 31.260877955 mol/s water, produces the same hydrogen rate and 15.630438977 mol/s oxygen, and reports 79.337462426 kWh/kg. ElectrolyzerFaradaicBalanceTest checks these balances, matched fixed-voltage and I-V cases, repeated runs and changing operating conditions.

Migration from the pre-#4681 calculation: for nonunity efficiency, water-feed mode now consumes all specified water and requires current divided by $\eta_F$; power mode now requests only the water represented by the products. Unity-efficiency material and electrical results retain their previous basis. The technology defaults themselves remain under separate qualification in #4682; selecting a technology does not establish a measured current efficiency or a validated nominal SEC.

Power-Driven Mode and Stack Sizing

In POWER mode the unit is given an available electrical power and inverts the stack power balance

\[P_{stack} = j \cdot A_{stack} \cdot V_{cell}(j, T)\]

for the current density $j$, then computes the hydrogen production and water demand. This requires the stack geometry to be defined first.

Sizing the stack

// Size a 1 MW stack at the nominal current density (technology default) and inlet T
electrolyzer.setTechnology(ElectrolyzerTechnology.PEM);
electrolyzer.setIVCharacteristic(
    new ElectrolyzerIVCharacteristic(ElectrolyzerTechnology.PEM));
electrolyzer.sizeStack(1.0e6);   // rated power 1 MW

// Or specify nominal current density and temperature explicitly
electrolyzer.sizeStack(1.0e6, 2.0, 353.15);  // 1 MW, 2 A/cm2, 80 C

// Or define geometry directly
electrolyzer.setActiveCellArea(1000.0);   // cm2 per cell
electrolyzer.setNumberOfCells(200);
// (or) electrolyzer.setStackActiveArea(200000.0);  // cm2 total

Running on a power budget

electrolyzer.setAvailablePower(0.5e6);   // 500 kW (switches to POWER mode)
electrolyzer.run();

double h2 = electrolyzer.getHydrogenOutStream().getFlowRate("kg/hr");
double current = electrolyzer.getStackCurrent();   // A
double stackPower = electrolyzer.getStackPower();  // W

If the stack geometry has not been configured, run() throws an IllegalStateException describing how to size the stack.

Method Description
setAvailablePower(double) Available electrical (AC) power (W); switches to POWER mode
sizeStack(double ratedPowerW) Size area from rated power and nominal current density
sizeStack(double ratedPowerW, double nominalJ, double tempK) Size area explicitly
setActiveCellArea(double) / setNumberOfCells(int) Per-cell area (cm²) and cell count
setStackActiveArea(double) / getStackActiveArea() Total active area (cm²)
setRatedPower(double) / getRatedPower() Nameplate stack power (W)
setNominalCurrentDensity(double) Nominal current density (A/cm²)
getStackCurrent() Total stack current from the last run (A)

Turndown, Standby and Curtailment

Electrolyzer stacks have a minimum turndown below which they idle, and an upper limit at their rated power. In POWER mode:

electrolyzer.setMinimumLoadFraction(0.10);   // 10 % minimum load
electrolyzer.setStandbyPowerFraction(0.02);  // 2 % idle draw

electrolyzer.setAvailablePower(0.05e6);       // below 10 % of 1 MW
electrolyzer.run();
boolean idle = electrolyzer.isStandby();       // true, no H2

electrolyzer.setAvailablePower(1.5e6);         // above rated
electrolyzer.run();
double curtailed = electrolyzer.getCurtailedPower();  // ~0.5 MW
Method Description
setMinimumLoadFraction(double) Minimum load as a fraction of rated power (0..1)
setStandbyPowerFraction(double) Idle auxiliary draw as a fraction of rated power (0..1)
isStandby() Whether the last run left the stack idling
getCurtailedPower() Electrical power curtailed above rated (W)

Ramp-Rate Limits and Transient Operation

Real stacks cannot follow instantaneous power steps. Set a maximum ramp rate (as a fraction of rated power per second) and drive the unit with runTransient(dt, id). The operating power ramps from a cold start toward the commanded availablePower, clamped by maxRampRate × rated × dt each step.

import java.util.UUID;

electrolyzer.setMaxRampRate(0.1);          // 10 % of rated per second
electrolyzer.setCalculateSteadyState(false);
electrolyzer.setAvailablePower(1.0e6);     // commanded power

UUID id = UUID.randomUUID();
electrolyzer.runTransient(1.0, id);        // first second: reaches 0.1 MW
electrolyzer.runTransient(1.0, id);        // second: reaches 0.2 MW
double delivered = electrolyzer.getOperatingPower();   // 0.2 MW
Method Description
setMaxRampRate(double) Maximum ramp rate (fraction of rated power per second; 0 = unlimited)
runTransient(double dt, UUID id) Advance one time step with ramp limiting
getOperatingPower() System power delivered after the last transient step (W)

Balance-of-Plant Losses

The system (AC) power exceeds the stack (DC) power because of rectifier losses and auxiliary loads (pumps, thermal management):

\[P_{system} = \frac{P_{stack}}{\eta_{rect}} + f_{aux}\,P_{stack}\]

Defaults (rectifierEfficiency = 1.0, auxiliaryLoadFraction = 0.0) make system power equal stack power, preserving backward compatibility.

electrolyzer.setRectifierEfficiency(0.95);    // 95 % AC/DC
electrolyzer.setAuxiliaryLoadFraction(0.05);  // 5 % of stack power

electrolyzer.setAvailablePower(0.5e6);
electrolyzer.run();

double stackSec  = electrolyzer.getSpecificEnergyConsumption_kWh_per_kg_H2();
double systemSec = electrolyzer.getSystemSpecificEnergyConsumption_kWh_per_kg_H2();
double systemPower = electrolyzer.getSystemPower();   // W (includes BoP)
Method Description
setRectifierEfficiency(double) Rectifier AC/DC efficiency (0,1]
setAuxiliaryLoadFraction(double) Auxiliary load as a fraction of stack power (≥ 0)
getSystemPower() System-level electrical power (W)
getSystemSpecificEnergyConsumption_kWh_per_kg_H2() System specific energy (kWh/kg H₂)

Thermal Model and Water Consumption

The screening heat balance uses electrical input minus hydrogen chemical power, with fixed $V_{tn}=1.481$ V on a 25 °C HHV basis:

\[Q_{out}=\max(0,\;P_{DC}-2FV_{tn}\dot n_{H_2})=\max(0,\;(V_{cell}-\eta_F V_{tn})I_{eq})\]

This includes the heat associated with unproductive current. It assumes that charge which does not yield exported hydrogen dissipates its electrical input inside the stack; no side-product chemical energy is exported. It excludes sensible/latent enthalpy, temperature-dependent reaction enthalpy, gas crossover and external heat integration. For a negative signed balance the getter returns zero; external heat demand is $\max(0,2FV_{tn}\dot n_{H_2}-P_{DC})$ on this same reference basis. This is especially relevant to SOEC and is not a complete high-temperature steam-electrolysis heat balance.

Water consumption is stoichiometric — one mole of water per mole of hydrogen:

double wasteHeat = electrolyzer.getWasteHeat();              // W
double water_mol = electrolyzer.getWaterConsumption("mole/sec");
double water_kgs = electrolyzer.getWaterConsumption("kg/sec");
double water_kgh = electrolyzer.getWaterConsumption("kg/hr");
Method Description
getWasteHeat() Nonnegative stack heat output on the fixed-HHV screening basis (W)
getWaterConsumption(String unit) Stoichiometric water demand (mole/sec, kg/sec, kg/hr)

Hydrogen Delivery Pressure

Set a hydrogen delivery pressure to leave the product at elevated pressure; the ideal isothermal compression power from the stack pressure is then available:

\[W = \dot{n}\,R\,T\,\ln\!\left(\frac{P_2}{P_1}\right)\] 
electrolyzer.setHydrogenDeliveryPressure(30.0);   // bara (0 keeps inlet pressure)
electrolyzer.run();

double pOut = electrolyzer.getHydrogenOutStream().getPressure("bara");  // 30 bara
double wComp = electrolyzer.getHydrogenCompressionPower();              // W (ideal)
Method Description
setHydrogenDeliveryPressure(double) Delivery pressure (bara); 0 keeps inlet pressure
getHydrogenCompressionPower() Ideal isothermal compression power (W)

Energy Calculations

Faradaic Efficiency

The fraction of electrical current that drives the desired reaction:

\[\eta_F = \frac{n \times F \times \dot{n}_{product}}{I}\]

Where $n$ = electrons per molecule, $F$ = Faraday constant (96485 C/mol), $\dot{n}_{product}$ = molar production rate, $I$ = current.

Specific Energy

Theoretical minimum: 39.4 kWh/kg H₂ (HHV basis). Typical actual consumption:

Technology Energy (kWh/kg H₂)
Alkaline 50–55
PEM 50–60
SOEC 35–45

getSpecificEnergyConsumption_kWh_per_kg_H2() returns the stack value; getSystemSpecificEnergyConsumption_kWh_per_kg_H2() adds balance-of-plant losses.

CO₂ Electrolyzer

The CO2Electrolyzer converts CO₂ to valuable products (CO, formic acid, methanol, ethylene, syngas) through electrolysis. Refer to the class for the full API (CO₂ conversion, product selectivity, faradaic efficiency per product).

import neqsim.process.equipment.electrolyzer.CO2Electrolyzer;

CO2Electrolyzer co2Elec = new CO2Electrolyzer("CO2 electrolyzer", co2Stream);
co2Elec.run();

double power = co2Elec.getEnergyStream().getDuty();   // W

CO₂ electrolysis can produce various products depending on the catalyst: carbon monoxide (CO), formic acid (HCOOH), methanol (CH₃OH), ethylene (C₂H₄), and syngas (CO + H₂).

Usage Examples

Green hydrogen with variable renewable power

import java.util.UUID;
import neqsim.process.equipment.electrolyzer.Electrolyzer;
import neqsim.process.equipment.electrolyzer.ElectrolyzerTechnology;
import neqsim.process.equipment.electrolyzer.ElectrolyzerIVCharacteristic;

// 1 MW PEM stack with realistic balance-of-plant losses
Electrolyzer el = new Electrolyzer("H2 Electrolyzer", waterFeed);
el.setTechnology(ElectrolyzerTechnology.PEM);
el.setIVCharacteristic(new ElectrolyzerIVCharacteristic(ElectrolyzerTechnology.PEM));
el.sizeStack(1.0e6);
el.setRectifierEfficiency(0.95);
el.setAuxiliaryLoadFraction(0.05);
el.setMinimumLoadFraction(0.10);
el.setStandbyPowerFraction(0.02);
el.setMaxRampRate(0.1);
el.setCalculateSteadyState(false);

double[] powerProfileMW = loadRenewablePowerProfile();   // hourly MW
UUID id = UUID.randomUUID();
double totalH2 = 0.0;
for (int hour = 0; hour < powerProfileMW.length; hour++) {
  el.setAvailablePower(powerProfileMW[hour] * 1.0e6);
  el.runTransient(3600.0, id);
  if (!el.isStandby()) {
    totalH2 += el.getHydrogenOutStream().getFlowRate("kg/hr");   // kg over the hour
  }
}
System.out.println("Total H2: " + totalH2 + " kg");

A complete, executed example is available as a notebook: examples/notebooks/green_hydrogen_variable_power.ipynb.

Green hydrogen with elevated delivery pressure

import neqsim.process.processmodel.ProcessSystem;

ProcessSystem process = new ProcessSystem();
process.add(waterFeed);

Electrolyzer electrolyzer = new Electrolyzer("H2 Electrolyzer", waterFeed);
electrolyzer.setTechnology(ElectrolyzerTechnology.PEM);
electrolyzer.setHydrogenDeliveryPressure(350.0);   // deliver at 350 bara
process.add(electrolyzer);

process.run();

double h2Output = electrolyzer.getHydrogenOutStream().getFlowRate("kg/hr");
double wComp = electrolyzer.getHydrogenCompressionPower();   // ideal compression duty
System.out.println("H2 output: " + h2Output + " kg/hr");