Phase change material: Can it led a revolution in refrigeration tech?

One of the roadblocks to the penetration of perishable and dairy products in the rural segment is unavailability of reliable power. There is a large demand for refrigeration equipments that offer additional features to protect the products during uncertainties such as equipment failure and power outages. This article attempts to explain an innovative energy storage technology that has the potential to revolutionize refrigeration.
 
Phase change materials
Phase change materials (PCM) fall under the sub category of energy exchanging smart materials. Energy exchanging smart materials are materials which store latent and sensible energy in the form of light, heat, electricity or hydrogen and exhibit reversibility. A PCM has the ability to store and release large amounts of heat/energy while maintaining a constant temperature unlike conventional solutions as shown in the figure 1. Phase change materials have tremendous potential to fulfill the growing need of energy for cooling and heating applications across various industries. It finds its application globally in diverse sectors like cold chain, HVAC, refrigeration, automobile air-conditioning, apparels, waste heat recovery, consumer goods and more; wherever there is a need for thermal energy storage.
 
Difference between conventional eutectic mixtures
A generic term ‘eutectics’ are also used for referring any mixture of chemical compounds or elements that have a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients. This composition is known as the eutectic composition and the temperature at which it solidifies is known as the eutectic temperature. Whether it is salt water solution, glycol solution or PCMs all are referred to as eutectics; hence there is a need to understand the difference.
 
For an eutectic solution to qualify as a phase change material for the purpose of storage of thermal energy there are some important factors to be considered. Without the consideration of these factors a eutectic solution would find its application as an anti-freeze solution or simply as a sensible heat storage medium at various temperatures.
 
Phase change materials are engineered to change their phase (solid to liquid or vice versa) at a specific temperature and one should look at the following three factors to qualify a PCM;

• High thermal storage capacity in the form of latent heat (200KJ/Kg or above)
• Constant temperature maintenance during the release of stored energy
• Guaranteed repeatability in performance for over 3000 times 

The benefits of the above features are precise temperature control (allowing not more than +/- 1°C of error); longer duration of retention period (upto 18 hours due to high latent heat), and reduction in the overall weight of the freezer due to high energy storage to weight ratio of the PCMs.
 
Performance of PCM in commercial freezers or coolers
Freezers for ice-creams are designed to maintain a temperature between – 18°C to -23°C while beverages are maintained between 2°C to 8°C in a cooler. The PCM chosen for both of these applications are different due to the different temperature requirements. The selected PCM gets charged passively when the power is available within 9 -10 hours.
 
The selection of the phase change materials is important as one of the critical factor is the minimum temperature that the refrigeration unit can offer to ensure that the PCM is completely charged. Once fully charged the PCM is in ready state to release the energy in event of a power or equipment failure. This freezer is designed to maintain a temperature of -18°C to -19°C at an ambient of 40°C for a minimum of 16 hours and inspite of several opening and closing of the lid.
 
There are several refrigeration companies in India and outside which have now introduced deep freezers and chest coolers incorporated with these phase change materials to cater to the market in low and middle income countries where power availability is an issue. The variety of PCMs available now allows OEMs to manufacture a range of refrigeration equipment to cater to different temperature requirements. The introduction of these materials has proved to be a boon to the refrigeration industry.
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Samit Jain is the managing director of Pluss Polymers Pvt Ltd