9.2 Thermal Energy Storage (TES): Chilled Water, Ice Storage, and Phase Change Materials

Thermal Energy Storage (TES)

Fundamentals of Thermal Energy Storage

Thermal Energy Storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used at a later time for heating and cooling applications or power generation. In commercial and institutional facilities, TES is primarily used for space cooling. By operating chillers during off-peak hours (usually at night) to chill water or make ice, facilities can shift their major cooling electrical loads away from expensive daytime peak periods. This strategy, known as load shifting, is a powerful tool for reducing peak demand charges and taking advantage of time-of-use (TOU) electricity rates.

TES systems can be broadly categorized by the physical state of the storage medium: Sensible Heat Storage and Latent Heat Storage.

Sensible Heat Storage: Chilled Water Systems

Sensible heat storage relies on changing the temperature of a storage medium without changing its phase. Water is the most common medium due to its high specific heat capacity, widespread availability, and low cost. In a chilled water TES system, chillers cool water to around 39°F to 42°F during the night. This chilled water is stored in a large, insulated tank. During the day, the cold water is circulated through the building's air handling units to provide space cooling, while the chillers remain off or operate at reduced capacity.

Sizing Equations for Sensible Heat Storage The amount of thermal energy stored in a chilled water system is governed by the sensible heat equation: Formula: Q = m × C_p × ΔT Where:

• Q = Total heat transferred or stored (Btu)
• m = Mass of the water (lbs)
• C_p = Specific heat capacity of water (1 Btu/lb·°F)
• ΔT = Temperature difference between the stored chilled water and the return water from the building (°F)

Example Calculation: A facility has a 100,000-gallon chilled water storage tank. The chillers cool the water to 40°F, and the return water from the building is 56°F. How many ton-hours of cooling are stored?

1. Calculate the mass of the water: 100,000 gallons × 8.33 lbs/gallon = 833,000 lbs.
2. Calculate the temperature difference (ΔT): 56°F - 40°F = 16°F.
3. Calculate the total energy stored (Q): 833,000 lbs × 1 Btu/lb·°F × 16°F = 13,328,000 Btu.
4. Convert Btu to ton-hours (1 ton-hour = 12,000 Btu): 13,328,000 Btu / 12,000 Btu/ton-hour = 1,110.67 ton-hours.

Chilled water systems require a relatively large footprint because water only stores 1 Btu per pound per degree Fahrenheit of temperature change. However, they integrate seamlessly with standard chilled water HVAC systems and offer excellent operational flexibility.

Latent Heat Storage: Ice Storage Systems

Latent heat storage relies on the phase change of a material—typically the transition of water from liquid to solid (ice). Ice storage systems use chillers (specifically, ice-making chillers) to freeze water inside tanks or coils during off-peak hours. During peak daytime hours, warm return liquid from the building melts the ice, providing cooling without running the energy-intensive chillers.

The primary advantage of ice storage over chilled water storage is its compact size. This is due to the latent heat of fusion.

Sizing Equations for Latent Heat Storage When water turns to ice at 32°F, it releases a massive amount of energy compared to simple sensible temperature changes. Formula: Latent Heat of Fusion for Water = 144 Btu/lb

This means that freezing 1 pound of water into ice stores 144 Btu of cooling energy. By comparison, raising the temperature of 1 pound of liquid water by 16°F (as in our chilled water example) only yields 16 Btu. Therefore, an ice storage system can hold roughly 9 times more energy in the same volume as a chilled water system, making it ideal for facilities with limited space.

Example Calculation: A facility needs to store 1,200 ton-hours of cooling capacity using an ice storage system. Assuming only latent heat is utilized, how many pounds of ice must be created?

1. Convert ton-hours to Btu: 1,200 ton-hours × 12,000 Btu/ton-hour = 14,400,000 Btu.
2. Calculate the mass of ice required: 14,400,000 Btu / 144 Btu/lb = 100,000 lbs of ice.
3. Convert to gallons (approximate volume of liquid water before freezing): 100,000 lbs / 8.33 lbs/gallon = 12,004 gallons. This volume is significantly smaller than the 100,000+ gallons required for a similar capacity chilled water system.

It is important to note that making ice requires chillers to operate at lower evaporating temperatures (typically around 20°F to 24°F), which reduces the chiller's efficiency (kW/ton) compared to producing standard 42°F chilled water. The economic viability of ice storage depends on the off-peak electricity rates being low enough to offset this penalty in chiller efficiency.

Phase Change Materials (PCMs)

While water/ice is the most common phase change material, other Phase Change Materials (PCMs) are engineered to change phase at specific temperatures suited for different applications. For example, eutectic salts or organic paraffins can be formulated to melt and freeze at 45°F, 55°F, or even room temperature (72°F).

PCMs can be encapsulated in spheres or panels and placed in standard chilled water tanks to increase their thermal capacity without freezing the entire tank. They can also be integrated into building materials (like drywall or ceiling tiles) to increase the thermal mass of the building envelope, absorbing excess heat during the day and releasing it at night.

Operating Strategies

TES systems generally operate under one of two control strategies:

1. Full Storage (Load Shifting): The chillers run only at night to charge the storage. During the day, the chillers are turned off completely, and the entire building cooling load is met by the TES. This requires a very large storage capacity but provides the maximum peak demand reduction.
2. Partial Storage (Load Leveling): The chillers run continuously for 24 hours. At night, they charge the TES. During the day, the chillers and the TES operate simultaneously to meet the building's cooling load. This requires a smaller, less expensive storage tank and smaller chillers, minimizing capital costs while still achieving significant demand savings.

Thermal Energy Storage remains a proven, robust, and cost-effective strategy for managing HVAC electrical demand, often yielding paybacks of 3 to 7 years in regions with high demand charges.