TES for cooling

A thermal energy storage tank (TES tank) in cooling/HVAC systems is used to store “cooling energy” usually in the form of chilled water or ice so the building can meet cooling loads more efficiently, reduce operating costs, and improve system reliability.

Below is a clear, practical explanation:

What Is a TES tank

A TES tank is a large insulated vessel that stores cooling energy produced during off-peak hours (typically night). This stored energy is then used during the day when cooling demand and electricity costs are higher.

It is most common in:

District cooling systems
High-rise buildings
Hospitals and airports
Large commercial/industrial facilities

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Types of Cooling Thermal Storage:

A) Chilled Water Storage

A large tank filled with chilled water (typically 4–6 °C).
At night, chillers operate to cool the water and “charge” the tank.
During the day, the tank discharges this cold water to the building.

Pros
✔ Simple design
✔ No special heat exchangers
✔ Good for large capacity
Cons
✘ Large tank size
✘ Needs good stratification (temperature layers)

B) Ice Storage

Ice is created at night and melted during the day to supply cooling.

Two main technologies:

Ice-on-coil
Ice harvester

Pros
✔ Much smaller footprint than chilled water
✔ Very high energy storage density
✔ Takes advantage of low night electricity tariffs
Cons
✘ Requires low-temperature chillers (-5 to -7 °C)
✘ More complex controls

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Benefits:​

Operational benefits

Reduces peak daytime electricity consumption
Smaller chiller size (lower capital cost)
Improves chiller efficiency (night ambient temperatures are cooler)
Adds redundancy/back-up cooling

Financial benefits

Uses low-tariff night electricity
Reduces demand charges by shifting daytime load
In district cooling, reduces plant size and improves efficiency

Environmental benefits

Less stress on electrical grids
Lower CO₂ emissions (if grid is cleaner at night)

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How a TES Tank Works in Chilled Water Systems

Night (Charging Mode):

1. Chillers cool water down to ~4–5 °C.
2. Cold water enters the bottom of the tank.
3. Warm return water exits from the top.
4. Tank develops stratification (cold at bottom, warm at top).

Day (Discharging Mode):

1. Warm return water enters the top of the tank.
2. Cold water exits the bottom to the cooling coils.
3. Tank provides chilled water and reduces chiller operation.

Stratification is critical → The tank must maintain clear layers of cold and warm water without mixing.

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Typical Applications

Shopping malls
Airports
Hospitals
District cooling plants
University campuses
Commercial towers

Data centers .

High-rise commercial buildings

Hotels and resorts

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Design Features of a TES Tank

Internal diffuser ports to maintain stratification
Insulation (to minimise thermal loss)
Large diameter/height ratiod epending on flow stability
Temperature sensors to track stratification layer
Level indicators
Vortex-breakers
Access hatches

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Typical Temperatures

Chilled Water Storage: 4–12 °C (ΔT = 6–8 °C)
Ice Storage: Uses glycol or brine, supply around -5 to -7 °C

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Control Strategies

Full storage: TES covers almost all daytime load
Partial storage: TES supports peak hours only
Load shifting: Uses TES to reduce electricity demand
Demand limiting: Keeps plant within energy limits​​