The demand for improved performance and reduced environmental footprint in energy-efficient building systems is experiencing essentially unlimited growth. Thermal storage systems are, by definition, a transformative solution for managing heating, ventilation and air conditioning (HVAC) loads by buffering intermittent and price-variant energy sources, using high-capacity heat/cold reservoirs with prolonged storage endurance.

These systems decouple energy generation from usage with storage of hours to days, enabling buildings to translate thermal reserves from off-peak or solar-overcapacity hours to be deployed as demand spikes. This not only reduces operational costs but also supports grid stability and contributes to decarbonization goals.

While the concept of a ‘storage heater’ is far from new, the methods being used to drive the performance of this area of human-environment control are increasingly effective, scalable and practical.

An exploration of the key technologies behind thermal storage, their advantages, real-world applications, and how they are shaping the future of smart energy management in buildings is a useful primer in the technologies involved, their application and relative strengths.

What are thermal storage systems?

Thermal storage systems literally hold thermal energy as hot or cold sinks/reservoirs, for delayed use over potentially prolonged periods. In HVAC applications, this typically means storing chilled or hot water, ice or other thermally active materials, under heavy insulation, during low-price periods (e.g. nighttime or solar over-supply) and applying that stored energy to offset loads during peak demand. This strategy is known as load shifting and it is the leading edge of electricity grid stabilization.

By optimizing the times at which energy is consumed through alignment of high demand periods with high availability and low-cost periods, thermal storage lowers operational costs. A secondary and significant benefit is reduction in strain on HVAC equipment and utilities during peak hours, leading to operational savings and emissions reductions. When reliably integrated, thermal storage can even reduce system CAPX costs by reducing the peak capacity requirements of installations.

Key thermal storage technologies

Chilled water storage

Chilled water systems use large insulated tanks to store water that has been cooled during off-peak electricity periods. During peak hours, the stored chilled water is circulated through the building’s HVAC system to meet cooling needs.

The benefits that this delivers are simple and proven technology, unlimited scalability where footprint is available and extreme system simplicity, with few moving parts and no stressed components.

However, the thermal capacity of water, at 4.2 joules per kilogram per Kelvin (J/Kg/K) is relatively low, requiring large volumes of water for practical executions.

As a recent example, Stanford University greatly expanded its chilled water capacity. More than 20 miles of distribution piping connect buildings on the campus to help regulate temperatures. The efficiency of the system, and recent heat waves, prompted the university to upgrade.

Ice storage

In ice thermal storage, chillers operate at night to freeze water. The ice is stored in tanks and used the next day to cool buildings without running the chillers. The phase change to ice delivers another 300 J/Kg with no temperature change, but the solid medium and relative expansion that occurs with freezing delivers systemic challenges. When the temperature is reduced below zero, the additional thermal capacity drops to around 2.1 J/Kg/K, half that of liquid water.

This approach offers a higher energy density than chilled water, due to the latent heat of freezing, therefore requiring a smaller footprint for comparable capacity, suiting systems in buildings with space constraints. It delivers a high load capacity, which can be beneficial at startup, in hotter climates or higher heat source applications.

However, ice-based systems require more expensive and larger lower-temperature chillers and considerably more complex control systems for optimal operation.

The Ice Bear system offers a great example of the use of an ice storage system to manage cooling loads and reduce peak electricity usage, significantly cutting operational costs at 100 sites totaling 45 GWh with Southern California Edison.

Molten salt storage

This approach is used more often in industrial and utility-scale applications and it is only suited to the capture/storage of heat. Molten salt stores heat at high temperatures, typically over 500° C to allow the salt to fully liquify and transport relatively easily. It is particularly preferred for solar thermal plants, where reflector arrays focus solar irradiance onto a single point collector to deliver very high temperatures. This is typically used directly to generate electricity, but storage of the liquified salt is easily achieved, to offset supply to match demand. In HVAC, molten salt is still an emerging technology.

It offers high thermal efficiency and storage duration, when thoroughly insulated. It is a relatively safe method of storing high source-temperature energy, requiring no pressure containment and offering no flammability. However, it is costly to establish and maintain, due to the very high temperatures and the aggressive behaviors of most salts.

Thermal capacity is relatively limited, at around 1 J/Kg/K, though this can be boosted by use of tertiary salt blends — LiNO₃, NaNO₃ and KNO₃ — raising capacity to 1.6 J/Kg/K.

While primarily used in solar power (e.g., Crescent Dunes Solar Energy Plant), molten salt is being explored in large district heating systems, though not without challenges. Crescent Dunes has had a mixed history, with prolonged outages, but the principle of operation is still receiving commercial attention and technical investment, and this project is still live.

Phase change materials (PCMs)

PCMs are selected for the capacity to absorb or release large amounts of thermal energy when they change phase (solid to liquid, or vice versa). They are commonly used for thermal energy storage and management in various applications.

PCMs can be categorized into: paraffins (waxes), which are preferred for their low cost, chemical stability, and ease of availability; fatty acids — stearic acid and lauric acid; sugar alcohols such as erythritol for their low cost and low melting temperatures; salt hydrates, made by combining water and salts, such as sodium sulfate or calcium chloride.

At extreme temperatures, certain metals and their alloys can also be used as PCMs, though this is a niche that is not practical for normal HVAC applications.

Overall, these materials typically offer compact and lightweight storage solutions and they can be used either actively in centralized systems or passively, distributed into walls and ceilings to increase local environmental inertia.

This approach to thermal storage typically incurs higher upfront costs than various alternatives, and solutions are somewhat experimental and not yet widely adopted.

Sand and gravel beds

Sand-based thermal energy storage is gaining market traction for high-temperature heating applications. Sand has a high specific heat capacity and is relatively inexpensive.

This approach offers low material cost and easy availability in a hard to damage storage medium with very high temperature resilience and environmentally benign media. However, its use in HVAC is more speculative than exploited and its overall heat capacity is relatively low, requiring large media containers and high weight.

Case study: Siemens Gamesa developed a pilot sand-based thermal energy storage system in Germany capable of storing electricity as heat for up to a week.

Conclusion

Thermal storage systems are exiting the niche technologies realm, becoming central to future-ready HVAC strategies. By shifting loads, enabling smarter energy use and shaving peak electricity demand, these systems deliver economic and environmental benefits across all building types.

As the technologies mature and adoption increases, thermal storage will equip environment control systems to not just consume energy efficiently, but to contribute to the shaping and balancing of wider grid energy demand.

While still marginal in terms of power-capacity and grid scale impact, these approaches are slowly becoming normalized, suggesting their entry into mainstream is likely in the near future.