Keeping Medicines Cold Without a Refrigerator: A Passive Phase-Change Cold Box for a Household Medicine Supply During a Blackout

SIMULATED, NOT MEASURED. A blackout removes the refrigerator on which many households depend for medicines labelled for storage at 2-8 C. This paper specifies a passive cold box built from 50 mm rigid polyisocyanurate board and 5.0 kg of phase-change panels melting at 5 C (latent heat 190 kJ/kg, total 950 kJ), sized for a household medicine supply (8.51 L inner cavity, 4.67 L for medicines), and evaluates it with a time-step thermal model. It holds the interior inside 2-8 C for 78.2 h at 25 C ambient, 62.6 h at 30 C, 52.1 h at 35 C, 52.6 h under a diurnal heatwave of mean 35 C and peak 38 C, and 44.7 h at 40 C. It therefore fails a 48 h outage at 40 C and fails a 72 h outage at every ambient at or above 30 C. Door openings at two per day cost 1.56 kJ per day at 30 C against 392.3 kJ per day by conduction. Water ice lasts longer against the upper edge but sits below 2 C for most of its melt. Total cost in household materials is $121. The 5 C panels regenerate in an ordinary refrigerator in 8.5 h in the model, with no freezer required. All numbers are reproducible from the deposited compute.py and results.json.