Heat can be used as a means to store energy through various mechanisms. Here are a few commonly employed methods:
Thermal Energy Storage: Thermal energy can be stored in substances that have a high heat capacity, such as water, molten salts, or phase-change materials. These materials absorb heat when it is available and release it later when needed. Thermal energy storage systems are used in various applications, including solar thermal power plants, district heating, and HVAC systems. The stored heat can be used for generating electricity, heating buildings, or providing hot water.
Sensible Heat Storage: Sensible heat storage involves storing thermal energy by changing the temperature of a substance without changing its state. This can be achieved by using insulated containers or tanks filled with a high-heat-capacity material, like rocks or concrete. When heat is added to the system, the material's temperature increases, and when heat is needed, the material's stored energy is released by transferring it to a heat transfer fluid.
Latent Heat Storage: Latent heat storage involves storing thermal energy by utilizing the latent heat of a substance during a phase transition, such as melting or vaporization. Phase-change materials (PCMs) are used for this purpose. When the PCM absorbs heat, it undergoes a phase change (e.g., from solid to liquid), storing a significant amount of energy. When the stored heat is required, the PCM solidifies, releasing the stored energy.
Thermochemical Storage: Thermochemical storage systems store energy by using reversible chemical reactions. These reactions absorb heat when energy is supplied and release it when energy is demanded. For instance, the reaction between certain metal oxides and hydrogen can store and release heat. Thermochemical storage offers the advantage of high energy densities and the ability to store energy over long periods without significant losses.
These are just a few examples of how heat can be stored as energy. The specific method used depends on the application, available resources, and desired energy storage capacity and duration. Each method has its advantages and limitations, and their suitability varies depending on the context in which they are employed.