Introduction

Thermal energy storage (TES) refers to technologies that provide long-term storage of heat or cold for later use. TES helps to balance energy demand between production and use of thermal energy to improve the overall energy efficiency of a system. By storing thermal energy, TES decouples the timing of energy production from energy use. This allows energy to be generated when it is convenient and/or economical to do so.

Types of Thermal Energy Storage
There are three main types of TES systems based on the medium being used to store heat or cold:

Sensible Heat Storage
Sensible heat storage utilizes the inherent heat capacity of a substance to store thermal energy by increasing its temperature. Common storage mediums include water, rocks, concrete, and phase change materials (PCMs). The amount of heat that can be stored is directly proportional to the mass of storage material and temperature change.

Latent Heat Storage
Latent heat storage relies on the heat released or absorbed during phase change of a substance between solid and liquid or liquid and gas states. PCMs like paraffin wax are commonly used as they allow for higher energy storage densities compared to sensible heat alone. As the storage medium changes phase, heat is either absorbed or released at a nearly constant temperature.

Thermochemical Storage
Thermochemical storage exploits reversible chemical reactions to store heat or cold. The reactions are driven by heat to product reactants that can recombine endothermically, releasing heat at a later time. Common thermochemical storage systems include sorption reactions using salts, zeolites or metal oxides/hydroxides. They offer very high storage densities but require more complex reaction equipment.

Applications of Thermal Energy Storage
TES plays an important role in balancing generation and demand in several applications:

Solar Thermal Power Plant Integration
Stored thermal energy allows solar thermal power plants to generate electricity even when the sun isn't shining. TES provides 8-15 hours of power generation beyond daylight hours.

District Heating & Cooling Systems
Off-peak or excess thermal energy from cogeneration plants or waste heat is stored for space heating or cooling buildings during peak demand periods.

Building Thermal Management
Buildings employ TES for space conditioning like preheating ventilation air or providing free cooling with stored coolth. Ice storage is popular for air conditioning load shifting.

Industrial Process Integration
Many industrial processes have thermal loads suitable for TES to improve overall plant efficiency by utilizing waste heat or better matching energy production to demand.

Seasonal Thermal Energy Storage
Large centralized TES reservoirs can store thermal energy on the scale of weeks or months to provide heating/cooling between different seasons. An emerging solution for decarbonizing heat supply.

Transport Refrigeration
Cold storage helps refrigerated vehicles provide consistent temperature-controlled transport by storing excess onboard refrigeration capacity when the vehicle is idle for later use.

As the share of renewable energy increases, TES provides a key enabling technology to ensure stable, reliable grid operation by balancing intermittent renewable generation with shifting thermal loads. Its diverse applications are expected to grow significantly in the coming decades as TES plays a more prominent role in energy systems worldwide.