Phase Change Materials Technology

Introduction

Increasing demand for energy saving and environment friendly technology is driving the growth for the phase Change Materials market. Cold chain is the most widely used application requiring critical temperature control; hence there is a need for awareness about PCM in this segment to reduce several challenges presently faced. The term “cold chain” applies to temperature-controlled supply chain that requires the maintenance of a specified temperature range in order to retain their quality. In order to maintain this chain of uninterrupted movement of goods there is a need for innovative technologies which are cost effective, efficient and easy to use for the end consumer. We will see the What, Why and How of  Phase Change Materials Technology – A Material having ability to store high amount of thermal energy by virtue of change in its phase and be able to release this energy at a constant temperature.

Phase Change Materials

Phase Change Materials (PCM) fall under the sub category of energy exchanging smart materials. Energy exchanging smart materials is defined as those materials that are able to 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. PCM’s have tremendous potential to fulfill the growing need of energy for cooling and heating applications across various industries. 

A generic term “eutectics” are also used for describing PCMs but there is a subtle difference. Eutectic solution are 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. A eutectic solution to be qualified as a phase change material for the purpose of storage of thermal energy there are other important factors which are desired. Without the consideration of these factors a eutectic solution would find its application as an anti-freeze solution or to utilize it as a sensible heat storage medium at various temperatures..

The concept of PCM can be better explained by drawing an analogy with water which is a natural form of Phase Change Material (PCM) and most temperature control packaging use water by utilizing it high thermal energy storage capacity during its change of phase from liquid to solid or vice versa. The properties that make water an ideal material for storage & transport of materials for applications requiring temperature maintenance in the range of -2°C to 4°C are as follows,

·         Consistent, repeatable, reliable phase change material temperature of 0° C.

·         High latent heat of fusion.

·         Non-Toxic.

·         Easily packaged into pouches or bottles.

·         Inexpensive and easily available.

A good phase Change Material is a one which should mimic these properties and offer the most important ability which is of customizability to different temperature needs. The below figure 1 illustrates the behavior of phase change materials against non-phase change materials such as coolants, glycol solution, water based gels etc. used for the purpose temperature storage. The non-phase changing materials rely on the specific heat stored in them. The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The relationship between heat and temperature change is usually expressed in the form shown below where c is the specific heat.

 

This relationship does not apply if a phase change is encountered, because the heat added or removed during a phase change does not change the temperature. This is because the heat supplied to the object is used to overcome the forces of attraction that hold the particles together during change in the state. Hence, the formula to calculate heat added in independent of the temperature change which is Q = m L where L is specific latent heat. The figure 1 below illustrates the difference in the behavior of phase change materials against non-Phase changing materials such as water based gels, Glycol solutions, coolants etc. In case of PCMs the temperature stabilizes at the point where the phase starts changing liberating higher amount of energy.

Classification of Phase Change Materials

Phase Change Materials can be broadly classified as Organic and Inorganic type and the major difference between both the types are as follows;

Customizability

One of most widely used application of PCM is for requirement of storage and release of energy at 2 – 8 deg C. Water is the most affordable, easily available and widely adopted PCM to offer this temperature range. For example in central air-conditioning in large buildings which require water at 4 – 7 deg C, Pharmaceutical vaccines which requires temperature maintenance at 2 -8 deg C range. The challenge is when the application demands a very narrow or no deviation from the desired temperature or different temperature all together.

For applications which are very critical to temperatures such as pharmaceutical drugs, tissues, vaccines, perishable commodities there is a need to use a PCM for reliability & stability of the performance. Using water in such application can cause achieving no thermal backup leading to failure of performance due to super-cooling effect, which is undesirable leading to non-freezing of the water at its freezing point. PCMs are formulated with nucleating agents which suppress and prevent super cooling. Due to super cooling, water may not get frozen completely or partially at its desired temperature resulting in not contributing towards only specific heat. Secondly, for applications requiring different temperature range for instance -23 deg C for ice-creams, -10 deg C for tissues, 22 deg C for blood platelets, wine, 65 deg C for hot water storage etc. PCMs come in different melting/freezing point and hence can offer tailor made solution.

It is important to understand the following to arrive at the right PCM,

1)      The desired temperature at which the product/application needs to be maintained.

-          This will enable to select the right temperature PCM considering convective and conductive loses.

2)      The temperature available to charge the PCM

-          PCM require to be given energy(storage) in order to extract energy(release) out of it. Charging implies storage of energy in the PCM by changing its phase (Melt/freeze). If the desired charging temperature is not available then the PCM would not offer the backup.

3)      The compatible encapsulation

-          Depending on the type of PCM the right encapsulation needs to be designed considering the materials resistance to corrosion, temperature and mechanical stresses. Various different encapsulations are explained below,

      i)                    Spherical / cuboid encapsulations: These are largely used in applications requiring very     large bulk storage of thermal energy and also allow flexibility of discharging this energy in controlled manner. These are widely used solar, building air-conditioning, process cooling and milk chilling application. These encapsulations can be customized to various sizes and the material depending on the temperature that these encapsulations would be subjected to. Eg: HDPE ( >70 deg C), PE (<70 deg C>90 deg C), SS (< 90 deg C).

ii)                  Pouches: These are largely used as an inner lining within insulated panels used in deep freezers, refrigerators, coolers for beverages, transport boxes. Pouches provide a larger surface area in application in applications in which PCMs are lined beneath the surface requiring cooling. Pouches are customized in shapes and the material used depending on the application and the easier integration.

There are various innovative ways of encapsulating PCMs other than the above two methods to make the end product integration more aesthetically appealing and for enhancement in the performance. At this stage its is very critical for the original equipment manufacturer of the product and PCM manufacturer to closely work together to understand the limitations and advantages of the various possibilities.

Applications

PCM has its application across various industries where there is a need for temperature control and regulation. Example of its application in other industries is summarized below

1.      Cold Chain Industry – PCMs have been proven to be very useful in cold chain industry. The major applications are to maintain temperatures during power failures. Major deep freezer manufacturers have started using phase change materials in their freezer to avoid the spoilage of products during power cut-offs, which is a major problem in India. Many FMCG companies in the dairy industry have started fast adopting phase change materials for last mile distribution into rural areas.  The PCM based eutectic plates offer solution to existing challenges in the cold supply chain issues such as power outages, poor infrastructure, high fuel costs and critical last mile etc. When compared with traditional reefer trucks this technology offer up to 80% savings in operating cost in virtue of the reduction in diesel consumption. In addition PCMs enables multi-temperature transport improving the payload factor thereby improving feasibility of part load transportation.

PCM integrated into the end product – Deep freezers at the retail

2.      Building HVAC - PCM based thermal energy storage systems help in reducing capital costs, operating cost and increase efficiency by utilizing off peak electricity during the night hours. PCMs help store this energy during the off peak hours in large insulated tanks, which can be harnessed during the day time for serving the cooling needs of the building. PCM are used in commercial, residential and industrial buildings.

3.      Solar Applications- PCMs are being used to store the solar energy in the form of thermal energy which can be used in the absence of sunlight. This enables the solar based systems to work at night without use of external sources of heat/electricity. Solar applications in driers, water heater and even air-conditioners can make use of PCMs in improving the utilization factor. The figure below illustrates a solar domestic water heater connected to a insulated tank filled with spherical encapsulations filled with high temperature PCM. The selected PCM should ensure that temperature provided by the water melts the PCM thereby also charging it. When the sun’s energy is not available, the water in the tank would start to gradually lose the heat. As soon as the temperature of the water lowers down below the point of phase change temperature, the PCM begins to solidify releasing it energy and maintaining the temperature of the water hot.

PCM application to store Solar energy

4.      Healthcare manufacturers need to safeguard their products and offer evidence of complying

      to maintain a certain temperature range throughout the movement of goods. Biopharmaceuticals and vaccines contribute to a majority of shipments of temperature-sensitive pharmaceuticals, which are typically high in monetary value and have low product volume. With the surge in mail order shipments, increasing scrutiny by regulatory agencies and the desire to lower costs , the need to develop an effective and reliable cold chain management protocol has become a necessity of utmost importance.

5.      Temperature-controlled clothing- PCMs can be incorporated with fabrics in various ways and can be used to provide comfortable temperature even in extreme temperature conditions.

Many more possibilities are yet to be exploited with PCMs extending its application into retail products, home appliances, automobiles etc.

 About Pluss Polymers Pvt. Ltd.

Pluss is a venture funded organization with equity stake being held by Tata Capitals premium innovation fund promoted by the Tata. Pluss is engaged in R&D and manufacturing of Specialized polymers and Phase change materials (PCMs). PCM technology finds its application globally in diverse sectors like cold supply chain, Building HVAC, refrigeration, automobiles, thermal wear, waste heat recovery, consumer goods and more; wherever there is a need to store thermal energy. Pluss has pioneered in the PCM technology in India and is presently the only company in India with a wide range of indigenously manufactured PCMs with many patents. For more information, please visit www.pluss.co.in.

This article was published in the Cold Chain supplement of Air Conditioning and Refrigeration Journal, Mar-Apr 2014. Copyright 2014 Air Conditioning and Refrigeration Journal. Reprinted here by permission from Air Conditioning and Refrigeration Journal. This article may not be copied nor distributed in either paper or digital form by other parties without permission from Air Conditioning and Refrigeration Journal.