HydroXpandHandbook

← HXS-2 handbook

Quick start

HXS-2 · updated 2026-10-01

Nine steps from the ports to your first hydrogen, then a baseline record: the BoP you need, the loop and line sizes, filling electrolyte and the current ramp.

The sequence from delivery to your first hydrogen. Nine steps, and half a day is enough once the external BoP is ready.

HXS-2 is a stack. You build the power supply, electrolyte circulation, gas handling, instrumentation and interlocks around it. The sizing rationale for that design is in the stack integration guide.

  • TimeHalf a day if the BoP is ready
  • PrerequisiteAn external BoP and a DC supply
  • PPEEye protection · alkali-resistant gloves
  • Prior knowledgeHydrogen safety and DC power handling
How to read this chapter

First timeWork through the nine steps in order. This is a bare stack, though, so the BoP has to exist first. Step 1 lists what you need, and the reasoning behind the numbers is in the stack integration guide.

Done this beforeRe-check the ports and the loop, then jump to the first run. The operating range and the shutdown sequence are in the operation and shutdown chapter.

On this page1. What you need in place

1. What you need in place

  • 1 × DC power supplyRecommended output 0–45 V and 0–50 A. A wider range lets you run the stack over a wider window. It needs a constant-current mode.
  • 1 × electrolyte loopA tank, a circulation pump and a heat exchanger, all in KOH-compatible materials. A tank heater is optional, but you need one for extended low-current running.
  • Three linesThe electrolyte inlet, the hydrogen outlet, and the outlet that carries oxygen and electrolyte together. That third one is particular to this product.
  • Electrolyte and DI waterFor the first fill, 0.1 M KOH in DI water; for make-up during operation, DI water. Keep the DI water in its own reservoir.
  • Valves, sensors and safety devicesWhatever your site and system design require, provided by you. Where they go is covered in the stack integration guide.

Photos show the type of item only. Tubing photo by Best Tech Nick 25 (Wikimedia Commons, CC BY-SA 4.0, background removed and edited). The edited version is released under CC BY-SA 4.0.

2. Identify the ports

Find the labels on the stack and confirm the three kinds below. Naming can vary by revision, so follow the labels on the unit you received.

Front and isometric line drawings of HXS-2. The end plate carrying the ports has four of them. Upper left, H2: 1/4-inch double ferrule, plugged if unused. Upper right, OUT: oxygen and electrolyte together, 3/8-inch PEEK tube. Lower left, IN: electrolyte in, 3/8-inch PEEK tube. Lower right, H2: 1/4-inch double ferrule, recommended as the outlet because condensate drains easily. The isometric view marks the two current-collector tabs on top of the stack as the DC terminals: M6, 4–6 N·m, polarity as marked on the stack.
Figure 3-1 HXS-2 ports and DC terminals. Drawn from the 3D model; port positions match the front view of the outline drawing. They can differ between revisions, so follow the markings on your stack.
PortQtyPosition (front)What passes throughFitting
IN1Lower leftElectrolyte in3/8-inch PEEK tube
OUT1Upper rightOxygen and electrolyte out, together3/8-inch PEEK tube
H22Upper left · lower rightHydrogen out. Use one and plug the other1/4-inch double ferrule
DC terminals1 set—DC power inM6 threaded tabs · torque 4–6 N·m
HXS-2 outline drawing (PDF)Port coordinates and sizes on the front view

3. Connect the electrolyte loop

A typical arrangement leaves the tank and comes back to it.

  • Tank → pump → heat exchanger → stack IN → stack OUT → tank
Minimum HXS-2 electrolyte loop and hydrogen line. Electrolyte leaves the tank, passes a circulation pump placed below the tank level and a heat exchanger, and enters stack IN. Oxygen and electrolyte leave together at stack OUT and return to the tank in a line of 3/8 inch or larger without a big climb and fall; gas separates in the tank and the oxygen vent goes to its own safe discharge point. A tank heater is optional, for long low-current runs. Hydrogen leaves the lower H2 port for a safe discharge point or approved downstream equipment, and the upper H2 port is plugged. The hydrogen line and the oxygen vent are never joined.
Figure 3-2 Minimum electrolyte loop and hydrogen line. The stack is the front view from figure 3-1, with the ports in their true positions. Valves, instruments and interlocks are in the stack integration guide.
  1. Connect the tank outlet to the circulation pump inlet.
    • Place the pump below the tank liquid level. With the suction side always flooded, it does not draw in air.
    • Do not use the pump to pressurise the anode loop. It is there to circulate.
  2. Connect the pump outlet to the heat exchanger inlet.
  3. Connect the heat exchanger outlet to IN on the stack.
  4. Return stack OUT to the tank.
    • The return has to flow smoothly. A line that climbs and falls traps gas and interrupts the return.
    • Keep the return line at 3/8 inch or larger. Do not neck it down below the port size.

4. Connect the hydrogen line

  1. Use the lower of the two H2 ports as the outlet.
    • Condensed moisture drains out more easily, which is why the lower one is recommended.
  2. Fit a plug on the H2 port you are not using.
  3. Route the hydrogen outlet to a safe, well-ventilated discharge point, or to approved downstream equipment.

5. Fill the electrolyte

Before filling, check the loop connections and the tightness of the fittings again.

  1. Make up 0.1 M KOH in DI water.
    • KOH of 95% purity or better, and water to ASTM D1193 Type II (1.0 µS/cm or below at 25 °C) recommended. The maximum is 30 µS/cm.
  2. Fill the tank to a level that suits your circulation loop.

6. Set flow and temperature

  1. Start circulation and set the flow to 4 L/min.
  2. Control the electrolyte temperature to 50 °C.
    • At high current it rises on its own from stack heat. At low current you need the tank heater.

7. Connect the DC supply

  1. Connect supply + to stack + and supply − to stack −.
    • Size the cable for 50 A continuous.
  2. Tighten the M6 terminals to 4–6 N·m.
    • Do not go past 6 N·m. Over-torque damages the terminal.
  3. Verify the polarity again before switching on.

8. Start the first run

  1. Confirm circulation is running and the flow is stable before applying current.
  2. Set the DC supply to constant current.
  3. Ramp the current in steps.
    • 0 to 10 A for two minutes, then 20 A, 30 A and 40 A for two minutes each, then the rated 50 A.
    • Stepping up on the first run lets you confirm flow, temperature control and voltage response at each level.
Current steps for the first run. First get circulation running with a stable flow and set the DC supply to constant current. Go from 0 to 10 A for two minutes, then 20 A, 30 A and 40 A for two minutes each, and at eight minutes step up to the rated 50 A. At each level, check flow, temperature control and voltage response.
Figure 3-3 First-run current steps. After the ramp and a short hold, make the three checks in step 9.

9. First-run checks

After the ramp and a short hold, confirm three things.

  • No electrolyte leaking at the stack ports and fittings, by eye.
  • The electrolyte flow is stable.
  • Stack voltage rises sensibly with current: about 40 V at 50 A at beginning of life.

What to record

Once the first run is stable, write that day's values down once. It becomes the starting point when you read the voltage trend later.

ItemWhat to write down
IdentificationModel, serial number and revision, from the label
Operating pointStack current and voltage, electrolyte flow and temperature, at the operating point you have chosen
If instrumentedHydrogen flow and pressure, hydrogen purity, oxygen purity
Electrolyte and waterMake-up date and concentration, feed-water quality
Loop layoutTank, pump and heat exchanger arrangement; routing of the IN, OUT and H2 lines and the vents
Operation and shutdownWhat to watch in normal operation, and how to stopMaintenance: reading the performance trendHow to read this record over timeStack integration guideBoP sizing rationale, instrumentation and interlocks

What this chapter is based on

Resource library →

TroubleshootingA find-by-symptom table, the order of checks when the voltage behaves oddly, causes and actions for two symptoms, and what to send us.

Ask the engineer who built it

If your case is not covered here, send us the operating condition and what you observed. The engineer who designed the system answers directly.

Technical enquiry