Sand-Battery Thermal Storage for Home Heating: A First-Principles Residential Design for Three Sunless Days in a Cold Climate

SIMULATED, NOT MEASURED. A residential sand thermal store is designed from first principles for a 140 m2 cold-climate house (5.01 kW average load at -10 C) to deliver 360.7 kWh over three sunless days. It requires 4,514 kg of sand in 2.82 m3 heated from 75 C to 450 C, in a vessel 2.33 m across with 0.40 m insulation and a standing loss of 0.41 kW (retention 0.924 at 3 days, 0.243 at 30 days). At $0.17/kWh and 6,000 kWh/yr delivered with free curtailed charging, delivered heat costs $0.082/kWh versus $0.17 for direct electric heating and $0.068 for a heat pump at COP 2.5. Losing cases are reported in full, including winter rooftop charging, which would take 78.1 days of surplus to fill the store once.