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# 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/lb oF) (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 90oC. The surrounding temperature (where the energy can be transferred to) is 20oC.

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 = 2.778x10-7 kWh = 2.389x10-4 kcal = 0.7376 ft lbf = 1 (kg m2)/s2 = 1 watt second = 1 Nm = 1 ft lb = 9.478x10-4 Btu
• 1 Btu (British thermal unit) = 1055.06 J = 107.6 kpm = 2.92875x10-4 kWh = 251.996 calorie (IT - International Table calorie) = 0.252 kcal = 777.649

### Example - Solar Energy stored in a 200 US gallons Water Tank

A solar energy water buffer tank with 200 US gallons is heated 200oF.

The solar energy stored can be calculated as

E = (1 Btu/lbmoF) (200 oF) (200 US gallons) (8.3 lbm/US gallon)

= 332000 Btu

## Related Topics

### • Thermodynamics

Work, heat and energy systems.

## Related Documents

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Biomass fuels and their energy content.

### Energy Conversion Factors

Convert between energy units

### Energy Storage Density

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### Energy Transfer Equation

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### Fixtures - Hot Water Content

Content of hot water in some common used fixtures - basins, sinks and baths.

### Heat Capacity

The amount of heat required to change the temperature of a substance by one degree.

### Heat, Work and Energy

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### Hot Water Circulation System - Return Pipes

Hot water can be circulated through a return pipe if it's instantly required at the fixtures.

### Hot Water Heating System Temperatures vs. Outdoor Temperatures

Hot water heating temperatures adapts to outdoor temperatures.

### Hydropower

Power potential vs. head and flow rate.

### Pressure Energy

Calculate the potential of pressure energy in a incompressible fluid.

### Salt Hydrates - Melting points and Latent Melting Energy

Melting points and latent energy of salt hydrates.

### Steam and Vapor Enthalpy

Vapor and steam enthalpy, specific enthalpy of saturated liquid, saturated vapor and superheated vapor.

### Storing Thermal Heat in Materials

Energy stored as sensible heat in materials.

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