Onsite Hydrogen for Power Plants: PEM vs Alkaline Electrolysis (2025 Guide)
By Green Gas Turbines Team · Published November 15, 2025 · 13 min read
Why Make Hydrogen Onsite?
Onsite electrolysis avoids trucked deliveries, reduces supply risk, and enables fast, flexible hydrogen for peaker/hybrid gas turbines. It can soak up low-price or curtailed electricity, support black-start, and provide oxygen byproduct for industrial users. Two technologies dominate: PEM (proton exchange membrane) and alkaline electrolysis.
PEM vs Alkaline: Quick Comparison
| Attribute | PEM Electrolysis | Alkaline Electrolysis | So What for a Power Site? |
|---|---|---|---|
| Dynamic response / turndown | Fast ramp (sub-seconds to seconds), wide turndown (≈10–100%) | Slower ramp (minutes), narrower turndown (≈30–100% typical) | PEM suits peakers, hybrids, and variable renewables; alkaline favors steady baseload production |
| Whole-system electricity use | ≈50–55 kWh/kg H2 (LHV basis), + compression | ≈49–54 kWh/kg H2 (LHV basis), + compression | Similar in practice; site parasitics and compression often dominate differences |
| Outlet pressure (without compressor) | Typically 20–30 bar | Typically 10–30 bar | Either way you’ll size compression to storage/crossover pressure |
| Water quality | High-purity deionized water (≈≤1 μS/cm) | Demineralized water; less stringent but polishing still needed | Plan a robust water plant either way (see below) |
| Hydrogen purity (after dryers/polishers) | Up to 99.999% | 99.8–99.999% (depends on system) | Both meet gas turbine needs with proper drying and filtration |
| Stack life (typical) | ≈40–80k operating hours | ≈60–90k operating hours | Budget stack replacements; alkaline often longer life at steady duty |
| CAPEX (electrolyzer module) | Higher $/kW; compact footprint | Lower $/kW; larger footprint | PEM pays for flexibility; alkaline wins on steady, low-cost kgs |
| Materials/catalyst | Precious metals (iridium/platinum) | Nickel-based |
How to Size an Onsite Electrolyzer for a Gas Turbine
Work backwards from turbine hydrogen consumption and your storage strategy.
Quick Sizing Steps for Onsite H2
-
Estimate H2 use at the turbine
- Simple cycle (100% H2): ~75 kg/MWh
- Combined cycle (100% H2): ~52 kg/MWh
- For blends: multiply by H2% (vol ≈ mass for screening)
-
Define duty
Required kg per event = (MW × hours) × (kg/MWh) × H2%
Continuous daily need = sum of events + reserve -
Decide make-rate vs store-rate
Electrolyzer kg/h × run hours + storage draw ≥ event need
Storage (kg) ≥ peak deficit between consumption and make-rate -
Add efficiency & compression
Electrical load (kW) ≈ kg/h × (50–55 kWh/kg + compression allowance)
Example: 50 MW simple-cycle peaker, 2 hours at 30% H2 → H2 need ≈ 75 × 50 × 2 × 0.30 = 2,250 kg. If you run a 1 MW PEM (≈20 kg/h) for 24 h you make ≈480 kg, so you’d either upsize to ≈5 MW or add storage (or both).
Planning Heuristics (Screening-Level)
Net load = Load − (Wind + Solar)
Required ramp (MW/min) ≈ max{Δ Net load / Δt}
Peaker coverage:
- Use batteries to cover first 0–2(–4) hours of the evening ramp.
- Size peakers to cover residual ramp + extended events (e.g., multi-hour/low-renewable periods).
- Ensure N−1 security: largest unit outage still covered by remaining flexible fleet.
ELCC note:
- As renewables share increases, peaker ELCC remains high if fuel-secure and fast-start.
- Battery ELCC declines with duration < worst-hour length; increase duration or pair with peakers/LDES.
Why not just overbuild renewables?
Overbuild helps energy adequacy but doesn’t reliably solve operability in worst hours or stability. A diversified stack (renewables, storage, peakers, transmission, demand flexibility) is cheaper and more reliable.