HXS-2 is a stack. You build the power supply, electrolyte circulation, gas handling, instrumentation and interlocks around it. This document gives you the numbers and the structure that design needs.
Which document to read when
| Datasheet | Specifications and measured performance curves. While you are evaluating the purchase |
|---|---|
| This guide | BoP sizing rationale, instrumentation and interlock design. While you are designing |
| User manual | Port connections, commissioning, maintenance, troubleshooting. After delivery. ver2.2, 48 pages |
| Assembly guide | Unpacking, disassembly and re-assembly bolting procedure. 8 pages |
| Warranty and service policy | Warranty scope and exclusions, MEA replacement criteria, handling procedure |
Design inputs
| Item | Recommended | Limit |
|---|---|---|
| Operating current | 50 A | 80 A maximum |
| Stack voltage | approx. 40 V (BOL) | 50.6 V maximum · 2.2 V/cell × 23 cells |
| Electrolyte | 0.3 M KOH in DI water | KOH purity 95% or higher · no fixed replacement interval |
| Electrolyte temperature | 60 °C | room temperature to 80 °C |
| Circulation flow | 4 L/min | 1 to 10 L/min |
| Hydrogen-side pressure | 0 barg | 10 barg maximum |
| Feed water | ASTM D1193 Type II | 1.0 µS/cm or below · make-up 0.42 L/h @ 50 A |
| Cells · active area | 23 cells | anode 100 cm² · cathode 79 cm² |
Performance curves are measured at 0.1 M KOH and 50 °C. If we have given you project-specific conditions, apply those for the first test instead.
Process schematic
The minimum arrangement of the electrolyte loop and the hydrogen line. This is a concept diagram and does not replace a P&ID.
- Feed runs from the tank through the filter, pump and heat exchanger into the stack IN port. Keep the pump below the tank liquid level.
- OUT is a wet discharge carrying oxygen and electrolyte together, so it passes through gas-liquid separation and returns to the tank. Measure anode pressure and outlet temperature on this run.
- The two hydrogen ports are joined and routed to the back-pressure regulator. We recommend a pressure gauge upstream and downstream of the BPR.
- Do not place any valve that can shut off on the tank vent or the O₂ vent.
Ports and connections
| IN | Electrolyte inlet · 3/8″ double-ferrule |
|---|---|
| OUT | Oxygen and electrolyte wet discharge · 3/8″ double-ferrule |
| H₂ (two ports) | Hydrogen outlet · 1/4″ double-ferrule each |
| DC terminals | M6 threaded terminals · tightening torque 4 to 6 N·m |
| Rated output | Hydrogen approx. 500 NL/h @ 50 A. Oxygen is half that |
| Make-up water | approx. 0.42 L/h @ 50 A |
| Size · weight | 190 × 210 × 215 mm · 19 kg |
- Port markings follow the engraving on the stack. Layout can differ between revisions, so check the physical engraving first.
- Support piping and cables separately so their weight does not rest on the ports and terminals.
- Size DC cable for 50 A continuous and keep terminal torque between 4 and 6 N·m. Over-torque damages the terminal.
- The stack weighs 19 kg. Use two people or a lift.
Electrolyte loop
Three things have to be settled in the loop: flow, pump head and wetted-part materials.
| Recommended flow | 4 L/min (allowed 1 to 10 L/min) |
|---|---|
| Stack pressure drop | approx. 0.11 bar at 4 L/min. Measured on the stack alone |
| Pump selection basis | Variable-speed pump with at least approx. 0.3 bar available head at 4 L/min |
| Speed command | A 24 VDC BLDC driven by 0–5 V or PWM is sufficient. Equivalent schemes are fine |
- 0.11 bar covers the stack only. Add filter, heat exchanger, flow meter, valve and piping losses to arrive at the final head.
- 0.3 bar is a preliminary selection basis, used before the P&ID and the full loss calculation. It is not a guaranteed final head.
- Install the pump below the tank liquid level for flooded suction. Do not use it to pressurise the anode loop.
- Wetted-part materials must be confirmed compatible by the component supplier at your concentration, temperature and design margin.
- Electrolyte is distributed in parallel to each cell from a common anode manifold. It is not in series.
- If you run fewer cells, scale the flow proportionally: 4 L/min for 23 cells means roughly 2 L/min for 12. The per-cell current range is unchanged.
Raising the flow increases the pressure drop across the stack, widens the anode-to-cathode pressure difference and pushes crossover up. The bottleneck is the internal flow field, which is multi-channel and does not reduce to a single number. Splitting the electrolyte loop is the alternative. This is outside our validated range, so we do not commit to flow or performance figures there.
The bipolar plate is formed in a die, so changing the flow field means cutting a new die. It is not a practical option. If you need to explore it, we will share drawings and go through it on a call.
Heat to remove
Heat load is the input power minus the chemical energy carried away as hydrogen. The figures below are calculated from Faraday's law and the higher heating value. They are not measured.
| Rated | 50 A · 40 V | input 2.0 kW · hydrogen 43 g/h · 1.7 kW on an HHV basis |
|---|---|---|
| Rated heat load | approx. 0.3 kW | the difference between the two figures above |
| Operating limit | 80 A · 50.6 V | input 4.1 kW · hydrogen 69 g/h · 2.7 kW on an HHV basis |
| Limit heat load | approx. 1.3 kW | a 1.5 kW class heat exchanger leaves margin |
As the stack degrades, the voltage at a given current rises and the heat load rises with it. Base your design margin on the voltage limit.
Gas handling and venting
At low current, electrolysis alone does not raise the electrolyte to the recommended temperature. If you plan long periods at low current, fit a KOH-compatible heater in the tank and put temperature monitoring and over-temperature protection in your BoP.
- OUT is a wet discharge carrying oxygen and electrolyte together. Provide gas-liquid separation and return the electrolyte to the tank.
- Do not put any valve or device that can shut off on a vent line. The tank vent must also be non-isolable.
- Place drain and sample ports at the lowest point of the tank.
- Wherever a hydrogen volume can be closed off or pressurised, provide a dry nitrogen purge connection and a safe discharge path. Set purge flow and duration after reviewing the piping volume and the P&ID. Never purge with compressed air or oxygen.
First commissioning conditions
| Pressure | Atmospheric, or close to atmospheric, on both sides |
|---|---|
| Hydrogen outlet | Unrestricted. Free vent |
| BPR | Fully open or bypassed |
What a pressurised build must include
- Independent anode and cathode pressure transmitters, plus a calculated differential
- Independent high-pressure and differential-pressure shutdown interlocks
- Automatic product-gas isolation on high pressure
- A verified safe discharge or depressurisation path
- A manual BPR is an adjustment component only. We do not approve it as a standalone pressure control or safety device.
Pressure limits and instrumentation
10 barg is the maximum hydrogen-side operating pressure. It does not mean an allowable differential of 10 bar. The anode-to-cathode pressure difference is a separate item — read it together with the undetermined section below.
Measured at full load under pressure
The control limit is 2 vol% hydrogen in oxygen (H₂-in-O₂). The figures below are measured over the window of a separate 10 barg run for which gas analysis records exist.
| Recorded window | 189.5 h | the window for which gas analysis records exist |
|---|---|---|
| H₂-in-O₂ average | 1.006 vol% | 1.435 vol% maximum over the same window |
| Control limit | 2 vol% | we recommend stopping above this |
Crossover rises at part load
These multipliers come from 49 measured points between ambient pressure and 6 barg. The multiplier held consistent regardless of pressure.
| Full load | 1× reference | control limit is 2 vol% H₂-in-O₂ |
|---|---|---|
| 60% load | 1.25 to 1.43× | |
| 40% load | 1.67 to 1.94× | below 40–50%, 2 vol% becomes the practical design limit |
Mitigate with at least one of: a minimum-load limit, reduced pressure at low load, or a 2 vol% interlock.
There is no numerical differential-pressure limit for a standalone HXS-2 that covers both steady and transient states. Pressurised full-stack validation is in progress. Trip settings are fixed per project after reviewing transmitter range and response time, valve fail position, relief arrangement and enclosed gas volume against the P&ID. Do not carry system-level (HXB-V1) alarm values over to a standalone stack as differential-pressure limits.
Minimum instrumentation by location
| Location | Instrumentation |
|---|---|
| Tank | Level, temperature, non-isolable vent, drain and sample at the lowest point |
| Stack inlet | Temperature downstream of the heat exchanger, flow |
| Stack outlet | Outlet temperature, KOH-compatible anode pressure |
| Hydrogen outlet | Cathode pressure immediately after the stack and upstream of any regulator or valve. We recommend a secondary pressure downstream of the BPR as well |
| External controller | Flow, both pressures, stack current and voltage, tank/inlet/outlet PT100, level, leak, emergency stop and valve feedback, pump and power commands, isolation and vent outputs, hard-wired emergency stop |
HXS-2 has no onboard PLC and no standard communication interface. An external controller provides the functions above.
Commissioning sequence
Detailed procedures for each step and the full first-run checklist are in chapter 6 of the user manual.
- Confirm atmospheric pressure on both sidesCheck that the hydrogen outlet is open and the BPR is open or bypassed.
- Fill electrolyte and check for leaksFill the tank, circulate, and inspect every joint.
- Set flow and temperatureSet 4 L/min and verify the recommended temperature. Use a heater if you are running at low current.
- Connect DC and ramp up from low currentRaise the current in steps while watching voltage and temperature.
- First-run checksCheck voltage behaviour, leaks, temperature and gas discharge. Follow the items in manual section 6-5.
Where maintenance ends and disassembly begins
- MEA replacement is a return service. Send us the stack and we replace the MEA only. End plates, bipolar plates, cell frames and fasteners are reused.
- Disassembling it yourself requires a 5-tonne hydraulic press. The bolting procedure is in the assembly guide.
- We do not warrant performance or leak-tightness after disassembly. Read this together with the warranty and service policy.
- There is no fixed electrolyte replacement interval. What is consumed in operation is water; the KOH itself is not consumed. Stop and inspect, then replace, when H₂-in-O₂ exceeds the control limit or when electrolyte condition and performance trend change.
- Other periodic inspection items are in chapter 7 of the manual.
Take the PDF
The same content as a document, Korean and English, no email form. Eight pages.
HX-SIG-HXS2-EN-001 · Rev 02 · 2026-08-23
This document is a design reference. Where an individual contract specifies something different, the contract prevails.
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