Question

Bottle of tea: A student keeps tea in her refrigerator. The tea comes in half-liter bottles and she drinks one bottle of tea per day. The refrigerator uses 30 W of power, on average. The electricity is supplied by a power plant with a heat rate of 9000 Btu/kWh and that is cooled using once-through withdrawal cooling that increases the temperature of the water by 10°C on average. How much power plant cooling water is used per bottle of tea to keep the tea refrigerated?

Answer 1

Power required per bottle of tea = 30W

Assuming the bottle is refrigerated for one day.

Power required per day = 30*24 = 720Wh = 0.72 kWh

Heat rate of power plant = 9000 Btu/kWh

Heat required per day = 9000*0.72 = 6480 Btu = 6480*1.055 = 6836.4 kJ

Scpecific Heat of cooling water = 4.184 kJ/.kgK

Temperature difference = 10K

Heat = Mass of water * Specific Heat * Temp difference

Thus, Mass of water = 6836.4/ (4.184 * 10) = 163.4 kg

Therefore 163.4 kg power plant cooling water is used per bottle to keep the tea refrigerated.