Dear Muhsen,

Depending on the location you are going to place the PCM, you should expect different effects. For example, by placing it on the evaporator, one might get lower temperature fluctuations and reduced compressor start/stops. If placed on the condenser, an increase in start/stops may be achieved, but with an overall reduced compressor on-time. In any case, the PCM phase change temperature and PCM mass should be selected for each specific condition. You may consult the following paper for further information: https://doi.org/10.1016/j.rser.2023.113746

Best regards

When utilizing Phase Change Materials (PCMs) with a vapor compression refrigeration cycle, you are essentially creating a thermal energy storage (TES) system. By incorporating PCMs, the refrigeration unit can store excess cooling capacity when demand is low (e.g., at night) and release it when demand is high (e.g., during the day or a power outage). This reduces the frequency of the compressor turning on and off, leading to significant energy savings and improved temperature stability.

How PCMs can be utilized with a vapor compression cycle

The integration of PCMs can occur in several locations within the refrigeration system, each with distinct benefits.

1. At the evaporator (for cold storage)

This is the most common and effective application for improving energy efficiency and maintaining compartment temperature.

• Charging mode (compressor ON): Excess cooling capacity from the refrigerant is used to freeze the PCM, storing cold energy as latent heat. The compressor may run for a longer, more efficient period to fully "charge" the PCM.
• Discharging mode (compressor OFF): When the refrigeration load increases or the compressor is idle, the PCM melts, absorbing heat from the cabinet. This maintains a stable, low temperature inside the refrigerator for a prolonged period without the compressor running.
• Benefits: Reduces compressor on/off cycling, decreases temperature fluctuations within the compartment, and provides cooling during power outages.

2. At the condenser (for heat storage)

In this approach, the PCM is used to enhance the heat rejection process.

• Charging mode (compressor ON): The hot refrigerant vapor rejects heat to the PCM, causing it to melt. This can lead to a lower condensation temperature, decreasing the compressor's work and potentially improving the system's Coefficient of Performance (COP).
• Discharging mode (compressor OFF): The PCM releases its stored heat to the ambient air.
• Benefits: Improves heat transfer efficiency, especially in high ambient temperatures, and reduces the peak temperature of the condenser.

3. In a separate cold storage tank

For larger or more complex applications, a dedicated tank is often used.

• Operation: A secondary heat transfer fluid (like water or glycol) circulates between the evaporator and a PCM tank. The vapor compression cycle freezes the PCM in the tank during off-peak hours when electricity is cheaper. During peak hours, the stored cold from the PCM tank is used to cool the load, either by itself or in conjunction with the compressor.
• Benefits: Offers greater flexibility and storage capacity, reduces energy costs by shifting consumption to off-peak hours, and can provide backup cooling.

Important considerations for utilizing PCM

To successfully integrate PCMs, you must carefully consider and design for several factors.

PCM selection

The choice of PCM is critical and depends on the specific application's temperature range.

• Phase change temperature: For a household refrigerator, a PCM with a freezing point around the desired cabinet temperature (e.g., between -5°C and -20°C for a freezer) is needed.
• Latent heat: The PCM should have a high latent heat of fusion to store a large amount of energy per unit mass.
• Thermal stability: It must be chemically and thermally stable over a large number of freeze/thaw cycles.
• Physical properties: Consider its density, corrosiveness to containment materials, and volume change upon phase transition.

Encapsulation and integration

The physical design of the PCM unit is vital for effective heat transfer and long-term reliability.

• Containers: PCMs must be encapsulated in containers (e.g., plates, cylinders, or macro-encapsulated bottles) to prevent leakage.
• High thermal conductivity: Many PCMs have low thermal conductivity, which hinders heat transfer. Enhancing this can be done by using high-conductivity packaging or incorporating fins or nanoparticles.
• Installation: In a standard refrigerator, PCM plates are often installed on the interior walls or around the evaporator coil to maximize the surface area for heat exchange.

System design and control

Integrating PCMs changes the operational dynamics of the refrigeration system, requiring new control strategies.

• Larger thermal load: PCMs add a thermal mass to the system. The compressor will initially need to run for a longer period to freeze the PCM, which needs to be factored into the overall design.
• Optimized cycling: The control system must be tuned to optimize the charge/discharge cycles. The compressor's on-time will be extended to freeze the PCM, but its overall on-time ratio over a 24-hour period can be significantly reduced.
• Compressor wear: By reducing the number of start/stop cycles, PCM integration can reduce wear and tear on the compressor, extending its lifespan.