PEM fuel-cell CHP with cathode water management

A PEM fuel-cell stack (real Butler-Volmer + Nernst polarization model, not an efficiency shortcut) runs on H2 with a compressed cathode air supply — the reaction water genuinely splits between vapor (carried out in the exhaust air) and condensed liquid via a real (T,P) VLE flash, not a fixed assumption. A downstream cooler + knockout drum recovers additional water from the exhaust before venting. Honesty note: this models the electrochemistry and water management rigorously, but stack thermal management (active cooling to remove waste heat) isn't modeled as a duty here — electrolyzer_rigorous.py reports electrical power only, not a separate thermal-loss term, so there's no literal 'coolant loop' node; the downstream cooler condenses the exhaust, which is the real, honest analog available today.