Latent heat storage is a type of thermal energy storage that involves storing heat by changing the phase of a material, typically using phase change materials (PCMs). During charging, the material absorbs heat and changes from solid to liquid or from liquid to gas, and during discharging, it releases heat as it solidifies or condenses. This phase change process allows PCMs to store and release a large amount of energy at a nearly constant temperature, making them efficient and effective for various applications. Here are some common latent heat storage materials and concepts:
1. Phase Change Materials (PCMs):
- Phase change materials are substances that undergo a phase transition (solid to liquid or liquid to gas) at a specific temperature, known as the melting or freezing point.
- PCMs have high latent heat of fusion, which is the amount of energy absorbed or released during the phase change process.
- The most commonly used PCMs for latent heat storage are organic compounds, inorganic salts, and eutectic mixtures.
2. Organic PCMs:
- Organic PCMs are typically hydrocarbons, fatty acids, or sugar alcohols.
- Examples of organic PCMs include paraffin wax (C16H34), which melts around 58-60°C, and polyethylene glycol (PEG), with various melting points depending on its molecular weight.
3. Inorganic Salt PCMs:
- Inorganic salt PCMs consist of salts such as nitrates, sulfates, and chlorides.
- Common inorganic salt PCMs include sodium sulfate decahydrate (Na2SO4 · 10H2O) with a melting point of 32.4°C and calcium chloride hexahydrate (CaCl2 · 6H2O) with a melting point of 29.4°C.
4. Eutectic Mixtures:
- Eutectic mixtures are combinations of different materials that exhibit a unique melting point lower than that of their individual components.
- Eutectic mixtures provide more tailored melting points and can be designed to match specific application requirements.
- An example is a eutectic mixture of sodium nitrate (NaNO3) and potassium nitrate (KNO3), which forms a mixture called solar salt with a melting point of around 220°C.
5. Concepts and Applications:
- Latent heat storage is widely used in solar thermal systems for space heating and cooling. The high-energy density and nearly constant temperature during charging and discharging make PCMs an excellent choice for storing thermal energy and providing stable temperature control.
- Building envelopes can be enhanced with PCM-enhanced materials to provide passive temperature regulation. These materials absorb excess heat during the day and release it at night, reducing heating and cooling loads.
- PCM-enhanced textiles have been developed to improve personal comfort, such as cooling vests or heated clothing.
- Latent heat storage is also used in some Concentrated Solar Power (CSP) plants as an alternative to molten salt for high-temperature storage.
Overall, latent heat storage materials and concepts offer a promising and efficient means of storing thermal energy, providing a significant advantage in terms of energy density and temperature control compared to sensible heat storage methods. They have applications in a wide range of industries and technologies seeking to harness the benefits of thermal energy storage.