Energy Accumulated in Heated Water - kWh
Water is often used to store thermal energy. Energy stored - or available - in hot water can be calculated
E = cp dt m (1)
where
E = energy (kJ, Btu)
cp = specific heat of water (kJ/kgoC, Btu/lboF) (4.2 kJ/kgoC, 1 Btu/lbmoF for water)
dt = temperature difference between the hot water and the surroundings (oC, oF))
m = mass of water (kg, lbm)
Example - Energy stored in a 1000 liter water tank
Water is heated to 90 oC. The surrounding temperature (where the energy can be transferred to) is 20 oC.
The energy stored in the water tank can be calculated as
E = (4.2 kJ/kgoC) ((90 oC) - (20 oC)) (1000 liter) (1 kg/liter)
= 294000 kJ
= (294000 kWs) (1/3600 h/s)
= 81.7 kWh
- 1 J (Joule) = 0,1020 kpm = 2.778×10-7 kWh = 2.389×10-4 kcal = 0.7376 ft lbf = 1 (kg m2)/s2 = 1 watt second = 1 Nm = 1 ft lb = 9.478×10-4 Btu
- 1 Btu (British thermal unit) = 1055.06 J = 107.6 kpm = 2.92875×10-4 kWh = 251.996 calorie (IT - International Table calorie) = 0.252 kcal = 777.649
Example - Solar Energy stored in a 200 U.S. gallons Water Tank
A solar energy water buffer tank with 200 US gallons is heated 200 oF.
The solar energy stored can be calculated as
E = (1 Btu/lbmoF) (200 oF) (200 U.S. gallons) (8.3 lbm/US gallon)
= 332000 Btu
Hot Water Energy Calculator
Energy Stored in Water - Liters/kWh
Energy Stored in Water - U.S. gallons/Btu's
