Latent Heat Storage | Material, Systems, and Applications

Introduction

Researchers are exploring new and renewable energy sources to reduce CO₂ emissions from fossil fuels, especially for low-temperature applications. Solar energy has great potential for heating and cooling buildings, providing hot water for homes and industries, cooking, and warming greenhouses. However, solar energy is not consistent, as it is only available during the day and can be unpredictable. We need efficient thermal energy storage systems to use solar energy effectively to save excess heat collected during the day for use at night.

Similar issues occur in heat recovery systems, where the timing of waste heat availability and usage does not always match, requiring thermal energy storage. In these systems, the useful energy from the collector is transferred to a storage medium and transformed into internal energy, which can be in the form of latent heat, sensible heat, or both.

Latent heat storage is more appealing than sensible heat storage because it has a higher storage density and smaller temperature change. However, there are practical challenges with latent heat storage, such as low thermal conductivity, changes in properties over time, phase segregation, subcooling, irregular melting, volume changes, and high costs. These issues must be resolved before latent heat storage can be widely adopted. This field is relatively new and still under research.

Latent Heat Storage Materials

Phase Change Materials (PCM) are used for latent heat storage. When the temperature rises, the chemical bonds in the PCM break, changing it from solid to liquid. This phase change absorbs heat (endothermic process). As the PCM melts, it absorbs heat and the temperature stays constant until the melting is complete. This absorbed heat is called latent heat.

Advantages of Latent Heat Storage:

1. High Storage Density: It stores large amounts of heat with small temperature changes.
2. Temperature Stability: The phase change occurs at a constant temperature, smoothing out temperature variations.

Compared to sensible heat storage, latent heat storage can have 5 to 10 times higher storage densities. PCM storage also requires less volume than water—about half the size. It works over a wide temperature range, as many PCMs can melt with a suitable heat of fusion for various applications.

Key Properties for PCMs in Thermal Storage Systems:

• Thermophysical Properties: Must handle the required temperatures effectively.
• Kinetics: Should have an appropriate rate of phase change.
• Chemical Properties: Must be stable and safe for long-term use.

Classification of PCM

There are many types of Phase Change Materials (PCMs), including organic, inorganic, and eutectic materials, which can be chosen based on their melting temperature and latent heat of fusion. However, most PCMs don’t meet all the necessary criteria for effective heat storage. Since no single material has all the ideal properties, we use available materials and improve their physical properties through system design.

Solutions to Improve PCM Performance:

• Metallic Fins: Increase thermal conductivity.
• Nucleating Agents: Prevent super-cooling.
• Suitable Thickness: Inhibit incongruent melting.

Types of PCMs:

1. Paraffin
2. Non-paraffins
3. Fatty Acids
4. Salt Hydrates
5. Eutectics
6. Cross-linked Polyethylene
7. Polyalcohols

Each of these groups has different thermal and chemical behaviors that affect the design of latent heat storage systems.

Latent Heat Storage Systems

Solar energy is inconsistent and varies throughout the day, but we need a steady and continuous energy supply for most applications. This makes storage systems crucial for using solar energy effectively. Phase Change Materials (PCMs) are often used in solar energy storage systems for various purposes, including:

1. Solar Water Heaters
2. Space Heating
3. Space Cooling
4. Greenhouse Heating
5. Solar Cooking
6. Waste Heat Recovery Systems

These applications help store and use solar energy efficiently.

Disadvantages of Latent Heat Storage

1. Low Thermal Conductivity: Heat transfer is often slow, reducing efficiency.
2. Variation in Properties: Physical properties can change over time and with repeated use.
3. Phase Segregation: Components can separate, leading to inconsistent performance.
4. Subcooling: Difficulty in solidifying after melting, which can hinder heat release.
5. Irregular Melting: Melting may not be uniform, affecting heat storage and release.
6. Volume Change: Expansion or contraction during phase change can cause material containment issues.
7. High Cost: Some PCMs can be expensive to produce and maintain.

Applications of Latent Heat Storage

1. Solar Water Heaters: Stores heat from the sun to provide hot water even when it’s not sunny.
2. Space Heating: Keeps buildings warm by storing and releasing heat.
3. Space Cooling: Absorbs excess heat to cool down spaces when needed.
4. Greenhouse Heating: Maintains a warm environment for plants by storing solar heat.
5. Solar Cooking: Uses stored solar heat to cook food, even when the sun isn’t shining.
6. Waste Heat Recovery: Captures and stores heat from industrial processes for later use.

Frequently Asked Questions (FAQs)

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