Water from Air

MOFs (metal–organic frameworks — the aluminum fumarate / MOF-303 class used here) are crystalline sponges whose pores grab water vapor at low humidity and release it when heated. The cycle: the sorbent bed breathes night air (highest relative humidity, a small fan drawing airflow), then desorbs at midday — a glazed solar-thermal box at 55% efficiency supplies primary heat, and the induction coil tops up through the cartridge's ferromagnetic steel shell — and the vapor condenses on an ambient radiator into the treated-water chain.

Working capacity per cycle depends on night humidity: roughly 0.22 L per kg of sorbent at 20% RH, 0.32 at 30%, 0.42 at 50%. Passive (solar-thermal-only) operation gets one cycle a day; active operation with fans, electric top-up, and heat recovery gets three. Desorption enthalpy is ~3.1 MJ per liter — thermodynamics, not engineering, so it does not improve with scale — and with 40% heat recovery the active-mode cost lands at 0.62 kWh of electricity per liter.

Ranges below span the two dominant uncertainties: night humidity (20–50% RH) and MOF sorbent price ($30–80/kg today; ~$10/kg projected at scale would cut capex 20–40%).

Lifetime harvested water costs $67–290/m³ over 10 years, against $5–30/m³ for water trucked into camps — and purifying water you already have costs 310× less energy per safe liter than harvesting it (0.002 kWh/L for UV-C against 0.62). Air-harvest never beats a working borehole. Its case is the places the benchmark breaks: saline or contested aquifers, trucking routes cut by insecurity, camps paying the top of the trucking range with no endgame. That is why the module is dashed lines on the system schematic: shipped only where source water genuinely fails, on the expansion headroom (third PV module, extra battery) the power system already carries.

The flywheel got the same treatment in chapter 1; this design's credibility rests on letting arithmetic kill its own most photogenic features.