A Paraffin Phase-Change Store for Building Temperature Control: Wax Mass, Melt-Front Thickness, Diurnal Cycling and Fire-Safe Enclosure for Three Hundred Usable Thermal Kilowatt-Hours

This paper sizes a paraffin phase-change storage system for an office building requiring daytime heat absorption and night-time release without mechanical cooling at peak hours. The duty calls for 300 usable thermal kilowatt-hours delivered at a rated power of 40 kilowatts. Working from a stated specific energy of 55 watt-hours per kilogram, an energy density of 110 watt-hours per litre, a usable depth-of-discharge fraction of 0.85, and a round-trip efficiency of 90 percent, the calculation gives a gross stored energy of 353 kilowatt-hours, an active-store mass of 6.42e+03 kilograms, a complete system mass of 9.3e+03 kilograms, and a complete system volume of 4.65e+03 litres (4.65 cubic metres). Full discharge at rated power lasts 7.5 hours, and a full recharge takes 10.6 hours at the stated charge power. Losses amount to 37.2 kilowatt-hours per complete cycle, appearing as heat at average rates of 1.75 kilowatts during charging and 2.48 kilowatts during discharge. Standing loss is 0.05 percent of the stored energy per day. Life is limited by the calendar limit, giving 7.5e+03 effective full cycles, about 25 years at the assumed cycling rate, and a lifetime delivered throughput of 2.25e+03 megawatt-hours. All values are computed from the stated assumptions; the system is a design study and has not been built or measured. Status: SIMULATED, NOT MEASURED.