Applying Thermal Storage to Offset Peak Demands

As energy demands continue to rise, buildings are increasingly focused on energy efficiency, sustainability, availability of services, and cost-effective solutions for heating and cooling. One powerful strategy gaining traction is thermal energy storage (TES), a system that allows buildings to store thermal energy for later use. Thermal storage solutions help buildings handle peak heating and cooling demands with reduced energy costs, reduced service sizes, enhanced comfort, and lower environmental impact.
In this post, we’ll explore the benefits of thermal storage in managing peak demands, break down the two main types—sensible and phase-change thermal storage—and explain how materials with high specific heat play a crucial role in optimizing storage efficiency.

What is Thermal Energy Storage?
Thermal energy storage systems capture thermal energy produced during periods of low demand and store it for release when demand peaks. The stored energy can be used for either heating or cooling and can take various forms depending on the material used for storage. TES systems may use sensible heat storage or phase-change materials (PCMs) for energy storage, each with unique properties, benefits, and limitations. The thermal storage media used in TES systems often have a high specific heat. This enhances storage capacity by absorbing and releasing more energy per unit of mass with minimal temperature change.

1. Sensible Thermal Storage

Sensible thermal storage works by changing the temperature of a material such as water or a solid like concrete, to store heat. The thermal energy is stored by raising or lowering the temperature of the material.
Materials with high specific heat capacity, like water and certain salts, are excellent choices for sensible thermal storage because they can store significant amounts of energy with relatively small temperature shifts. Additionally, architectural phase change materials can be incorporated into the construction materials of the building.

Pros of Sensible Thermal Storage:

• Cost-Effective: Typically more affordable because it uses commonly available materials.
• Simple Technology: Easily implemented and understood technology, making it widely accessible and reliable.
• Wide Range of Applications: Can be used in both cooling (chilled water storage) and heating (hot water tank) applications.
  
  

Cons of Sensible Thermal Storage:

• Lower Energy Density: Requires large volumes of material to store significant amounts of energy, which means more space is needed.
• Temperature Control Limitations: Sensible storage systems only store energy based on temperature changes, which can limit their energy density and efficiency for certain applications.

Example: In a chilled water storage system, chillers produce cold water during off-peak hours, which is stored in large tanks. During peak demand, the stored chilled water is circulated through the building’s HVAC system, eliminating the need for chillers to operate at high-cost times.

2. Phase-Change Thermal Storage
   Phase-change materials (PCMs) store energy by changing phases—for instance, from solid to liquid or vice versa. The energy stored in phase changes is called latent heat, and it allows PCMs to store and release energy more efficiently at a constant temperature. Common PCMs include ice (for cooling applications), as well as certain salts and waxes designed for specific temperature ranges.
   
   

Pros of Phase-Change Thermal Storage:

• High Energy Density: PCMs can store more energy in a smaller volume than sensible storage, which reduces space requirements.
• Constant Temperature Release: PCMs release energy at a constant temperature, making them ideal for applications that need stable temperature control.
• Efficient for Peak Shaving: Because of their high energy density, PCMs are ideal for buildings with limited space that still need effective peak demand management.

Cons of Phase-Change Thermal Storage:

• Higher Costs: Specialized PCMs are generally more expensive to produce compared to sensible thermal storage.
• Complexity in Design and Maintenance: Some PCMs require specific storage and handling considerations, which can add complexity to the system design and maintenance.
• Potential Environmental and Safety Concerns: Some PCMs, such as certain salts and waxes, may pose environmental or safety concerns if not properly managed.

Example: An ice-based storage system cools a building by using chillers to freeze water at night. During the day, the ice melts, absorbing heat and providing cooling without running additional chillers, which significantly reduces peak energy costs.

Comparing Sensible vs. Phase-Change Thermal Storage for HVAC Applications

Both sensible and phase-change thermal storage contribute to energy savings and efficiency improvements, but their differences are important when determining the right fit for a building:

Benefits of Thermal Storage for Peak Heating and Cooling:

• Reduced Energy Costs: By storing energy during low-cost, off-peak hours, TES systems reduce peak demand charges and optimize energy spending. Sensible and phase-change storage both allow HVAC systems to operate less during high-cost hours, though the choice of type may depend on building needs, available space, and specific heat media.
• Availability of Services: By utilizing stored energy during peak hours, TES systems can reduce the required electrical/gas service size by reducing overall equipment sizes.
• Enhanced System Efficiency: Thermal storage systems reduce strain on HVAC systems, allowing them to operate more efficiently. Phase-change systems particularly excel here, as they provide steady temperature release without rapid cycling, enhancing equipment longevity. High specific heat materials in sensible storage allow for longer-lasting energy supply as well.
• Lower Environmental Impact: Reducing peak energy use supports sustainability goals by lowering overall energy consumption and carbon emissions. Both types of TES can integrate well with renewable energy sources, helping buildings reach net-zero or carbon-neutral targets.
  
  

Is Thermal Storage Right for Your Building?
Thermal energy storage can be a valuable addition for buildings looking to manage energy use, reduce costs, and improve comfort. Whether choosing sensible storage for its simplicity or phase-change storage for high-density applications, both offer advantages that can be tailored to a building’s unique needs.

Closing Thoughts
Thermal storage offers an immediate and lasting solution for peak load management, enhancing a building’s energy efficiency and comfort while contributing to a more sustainable future. Embracing thermal energy storage—whether sensible, with high specific heat media, or phase-change—puts buildings on a path to optimized performance and environmental responsibility. Get in touch with our team to explore how!