Thermal energy storage (TES) refers to the process of storing excess heat or cold for later use. This is often accomplished by using materials that change phase, such as water freezing into ice or melting back into liquid, to absorb or release energy as needed. TES systems help balance energy supply and demand, improve efficiency, and support renewable energy integration by storing energy during periods of low demand and releasing it when demand is higher. Common applications include building climate control, industrial processes, and district heating or cooling networks.