Water as a Seasonal Heat Store: A First-Principles Design for Half of a Cold-Climate Winter, Its Six-Month Loss, and the Scale at Which the Loss Becomes Acceptable

A buried insulated water tank that banks summer solar heat for winter space heating is designed from first principles for a well-insulated 160 m2 detached house in a cold Great Lakes climate with a yearly heating demand of 15,120 kWh. To deliver half that demand (7,560 kWh) from storage between 85 C and 35 C requires a 202.3 m3 cylindrical tank holding 11,764 kWh gross. Its standing loss is 0.973 kW, totalling 4,204 kWh over a six-month storage season: 35.7% of the stored heat is lost and 64.3% delivered. Charging takes 29.5 m2 of solar collectors. Delivered heat costs $1.167 per kWh over 25 years against $0.148 per kWh for an air-source heat pump, a factor of 7.91 against the store. Sharing one store among houses reduces the loss fraction: at or below 20% at 10 houses, 15% at 20 houses, and 10% at 100 houses, though delivered cost remains above the heat pump figure at every scale calculated. Design status: SIMULATED, NOT MEASURED. Nothing was built or hardware-tested.