Phase change materials are matters that absorb and release heat energy (known as latent heat) when they change phase. When a material melts, it changes from the solid phase to the liquid phase. During phase transition, many materials can absorb a significant amount of heat energy. The opposite occurs when the material freezes and solidifies. When the material melts, it will give off the heat it has absorbed. Different materials melt and solidify at different temperatures and can absorb different amounts of heat energy.
Phase change materials are useful because they melt and solidify at certain, defined temperatures, making them suitable for controlling temperature in a variety of applications. Materials that melt to absorb heat are much more efficient at absorbing heat energy than sensible heat energy materials. This means that much less material is required to store the heat energy phase change material than to use a non-phase change material.
A PCM works via two different forms of heat energy: latent and sensible heat. Latent heat is the amount of energy required for matter to change from one state to another, e.g. from liquid to solid. The most common example of this type of heat energy is an ice cube. An ice cube will use its latent heat capacity and absorb the heat energy from the beverage. A fully melted ice cube means it has absorbed all the latent energy it can. On the other hand, sensible heat is the amount of energy required to change the temperature of a matter without changing its phase. An example of this is a kettle. When latent and sensible heat work together, they can maintain the required specific temperature over a long period of time.
Finally, PCM products typically last for the entire life of the product they are used in or applied to. The transition temperature and latent heat energy must remain the same throughout the melting and solidification cycles. Additionally, PCMs are covered with a shell to prevent leakage, deterioration and contamination.
Several new technologies are emerging to help meet the goal of reducing energy use in buildings. Some of these technologies are related to thermal insulation materials applied to the building envelope. PCMs work with the principle of latent heat thermal storage (LHTS) to absorb large amounts of energy when there is a surplus and release it when there is a shortage. Proper use of PCMs can reduce peak heating and cooling loads, thereby reducing energy use..
PCMs also gives the ability to provide a more comfortable indoor environment due to smaller temperature fluctuations. For building applications, PCMs offer numerous areas of use. Some of the areas studied to date are ventilation systems, passive heating and cooling systems, floors, roofs and wall coverings. PCMs can also be added directly into building materials such as concrete and wall panels, allowing them to be applied in structures with minimal change to the original design.
PROPERTIES OF PHASE CHANGE MATERIALS
Phase change materials (PCM) use the latent heat of phase change to control temperatures over a certain range. When the temperature rises above a certain point, the chemical bonds in the material will begin to break and the material will absorb heat in an endothermic process where it goes from solid to liquid state. As the temperature drops, the material will release energy and return to a solid state. Given that the phase change temperature is around the desired comfortable room temperature, the energy used to change the phase of the material will not only provide a more stable and comfortable indoor climate but also reduce peak cooling and heating loads. For this reason, phase change materials can increase the heat storage capacity, especially in buildings with low thermal mass. The temperature range varies depending on the materials used as the phase change material.
Schröder and Gawron summarize some of the desired properties of phase change materials as follows:
- High heat of fusion and high specific heat per unit volume and unit weight. This is desirable to achieve greater effect from latent heat storage with the smallest PCM volume possible.
- The phase change temperature is matched to suit the application. To use PCMs most effectively, the phase change temperature should be appropriate for the climate, its location in the system, or the type of system in which the PCM is used.
- Low vapor pressure at operating temperature. The vapor pressure should be as low as possible to avoid extra costs or danger of rupture due to pressure on the encapsulating material.
AREAS OF APPLICATION
Cold Storage for Solar Powered Cooling Applications
(pct around 5 -18°C)
PCM Included in Wall Material (Micro Capsules)
(pct around 22°C)
Heating Storage For Solar Energy and Longer Operating Time of Boilers
(pct around 60°c)
Hot Storage For Solar Powered Air Conditioner
(pct around 80°C)
In all cases, heat transfer must be made between the phase change material and the fluid cycle (charge, discharge).
Different techniques are used such as:
Direct contact between the phase change material and the heat transfer fluid: this requires materials that are chemically stable for long-term direct contact, and PCM solidifies in small particles, which then ensures adequate heat transfer during melting.
MACROSCOPIC-CAPSULES:
This is the most preferred encapsulation method. The most common approach is to use a plastic module that is chemically neutral to both the phase change material and the heat transfer fluid. Modules typically have a diameter of several centimeters.
MICRO-ENCAPSULATION
This is a relatively new technique in which PCM is encapsulated in a small shell of polymer material, which is a few micrometers in diameter (currently only for paraffins). A large heat exchange surface is achieved and the powder-like spheres can be integrated into many building materials or used as a pumpable slurry.
Compared to conventional water storage techniques, phase change storage has the following main advantages:
Higher thermal energy storage capacity than sensible energy storage (smaller storages), if at least only small useful temperature differences can be achieved.
Relatively constant temperature during charging and discharging.
Burner cycles and therefore CO and HC emissions can be reduced for the auxiliary heat generation unit.
Phase Change Material
Solar Powered Heating System With and Without PCM (Reference: Course.lumenlearning.com)
PHASE CHANGE MATERIAL
Building Applications
Paraffin wax based PCMs embedded in a copolymer and placed between two metal sheets are used in some construction materials to absorb heat gains and release heat at night. During the charging process, PCM cards on one wall lower the inner wall surface temperature, but during the heat dissipation process the PCM wall surface temperature is higher than the other walls. In the melting area, the heat flux density of a PCM wall is almost twice that of a normal wall. Also, in the charging phase, a PCM wall performs better than an ordinary wall in terms of thermal insulation, but in the heat dissipation phase, the PCM wall discharges more heat energy.
Unlike structural insulated panels, which have quite consistent thermal features, the characteristics of a PCM fluctuate depending on the environment. The structural insulated panel is always operational, preventing thermal movement from hot to cold. The temperature variation throughout the structural insulated panel insulation determines the thermal flux.
The value of the PCM is displayed when in-wall temperatures cause the PCM to change its state. It can be concluded that the higher the temperature difference between day and night, the more reliably the PCM works to reduce the heat flow. Using a PCM structural insulated panel wall would be ideal for climates with large temperature fluctuations, such as hot during the day and cool at night.
PCMs can be used for temperature regulation, high density heat or cold storage, and thermal comfort in buildings with a limited temperature range. As a result, if solar energy is stored efficiently, it can be used to combat the nighttime cold. The use of PCMs provides the possibility of meeting the heating demand. It helps to store the energy produced during the day and to maintain a comfortable building temperature.
Vehicle Applications
The number of studies on the applicability of PCM in automobile applications is rising. There is ongoing research on the use of PCMs for refrigerated vehicles designed to carry perishable cargo at sensitive temperatures. Refrigerated trucks are controlled by small refrigeration units mounted outside the vehicle to maintain a constant temperature and relative humidity inside the trailer. They run on gas, so temperature fluctuations in the trailer have a significant impact on shipping costs.
Maximum heat transfer rates and total heat fluxes in a refrigerated trailer are reduced with the use of PCM. As a heat transfer reduction technology, Ahmed, Meade, and Medina (2010) updated the traditional approach to refrigerated truck trailer insulation using paraffin based PCMs in standard trailer walls. When all walls (south, east, north, west and top) were analyzed, the peak heat transfer rate decreased by 29.1% on average, but for single walls the peak heat transfer rate decreased by 11.3-43.8%.
There was 16.3% reduction in the average daily heat flow to the refrigerated compartment. These findings can lead to energy savings, reduced emissions from diesel fueled cooling units, smaller refrigerated equipment and longer equipment operating life.
Gasoline is the most common fuel used in vehicles (i.e. gas or petroleum). Other forms of liquids used in automobiles include liquefied petroleum gases and diesel. Hybrid vehicles have recently gained popularity among consumers for their ability to drastically reduce hazardous exhaust pollutants when driven in electric mode. Li-ion batteries have long been used in electrical devices (mobile phones, laptops and portable devices).
Many experts, especially in the United States, are investigating the use of Li-ion batteries in transportation applications to double the fuel economy of hybrid cars and minimize emissions. To start the vehicle in electric mode, Li-ion battery modules can be installed to match the vehicle’s rated voltage.
However, this raises an important problem that prevents Li-ion batteries from being used in many applications: during discharge, Li-ion batteries release energy as a result of exothermic electrochemical processes. The energy generated by the battery must be redistributed to the environment. If the transfer rate is insufficient, some of the gelled phase components become gaseous, increasing the internal pressure of the cell.
As a result, energy should be released from the cell as quickly as possible or the temperature of the cell should not rise. According to Sveum, Kizilel, Khader, and Al-Hallaj (2007), thermally managed Li-ion batteries using PCM minimize the need for additional cooling systems and increase available power. The researchers used efficient thermal management to keep the battery packs at the right temperature, and the PCM’s high latent heat of fusion allowed it to remove excessive heat.
Phase Change Material
With the proper use of a passive thermal management system, a pack of Li-ion batteries can be kept within a small temperature range. (Reference: allcelltech.com)
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