Thermal energy storage is a way to save heat or cold for later use, much like a battery stores electrical energy. It helps renewable energy systems work when the Sun is not shining, the wind is not blowing, or demand changes during the day. Buildings, power plants, and industrial systems use thermal storage to reduce wasted energy and lower peak electricity use.
The main idea is simple: collect thermal energy when it is available, hold it in a storage material, then release it when it is needed.
Key Facts
- Stored sensible heat: Q = mcΔT, where m is mass, c is specific heat, and ΔT is temperature change.
- Stored latent heat during a phase change: Q = mL, where L is latent heat of fusion or vaporization.
- Thermal power rate can be estimated by P = Q/t, where t is the charging or discharging time.
- Common storage media include water, molten salt, rocks, concrete, ice, and phase change materials.
- Insulation reduces heat loss by slowing conduction, convection, and radiation between the tank and surroundings.
- Thermal energy storage can shift energy use from high-demand times to low-demand times, improving grid stability.
Vocabulary
- Thermal energy storage
- A system that captures heat or cold, stores it in a material, and releases it later for heating, cooling, or power production.
- Sensible heat
- Thermal energy stored by changing a material's temperature without changing its phase.
- Latent heat
- Thermal energy absorbed or released when a material changes phase at nearly constant temperature.
- Phase change material
- A material chosen to melt, freeze, evaporate, or condense at useful temperatures while storing or releasing latent heat.
- Heat exchanger
- A device that transfers thermal energy between two fluids or between a fluid and a storage material without mixing them.
Common Mistakes to Avoid
- Confusing thermal energy with temperature: temperature tells how hot something is, while thermal energy also depends on mass and material properties.
- Forgetting the mass in Q = mcΔT: a large tank with a small temperature change can store more energy than a small object with a large temperature change.
- Using Q = mcΔT during a phase change: when melting or freezing occurs, the correct relation is Q = mL because temperature stays nearly constant.
- Ignoring heat loss to the surroundings: real storage units need insulation because stored heat or cold gradually leaks out over time.
Practice Questions
- 1 A water tank contains 500 kg of water and is heated from 25°C to 75°C. Using c = 4186 J/(kg·°C), how much thermal energy is stored?
- 2 An ice storage system freezes 80 kg of water at 0°C. If the latent heat of fusion of water is 334,000 J/kg, how much energy is stored as latent heat?
- 3 Explain why a phase change material that melts at 22°C can be useful for cooling a building during a warm afternoon, even if its temperature changes very little.