Innovative Polymeric Materials for Thermal Energy Storage

20th century has witnessed a rapid increase in the scale of industrialization all over the world. This has led to a significant increase in the number of commercial buildings for industrial purposes. According to a recent scientific research, these commercial buildings account for almost 41% of the world’s energy consumption, which constitutes 30% of the annual greenhouse gas emissions. It is expected that the energy demand in the building sector will rise by about 50% in 2050, and the space cooling demand will triple between 2010 and 2050 [1]. Hence, there is an urgent need to decrease the energy consumption of such building spaces. Form-Stable Phase change materials (PCMs) have proven to be a viable solution to reduce the energy load for such buildings.

Before exploring more about form-stable PCMs, let’s understand what is a PCM? PCMs are chemical compounds that can store/release large amount of energy in the form of latent heat. This process occurs at a constant temperature (Melting & Freezing Point). Paraffin, organic fatty acids and esters, inorganic salts & their eutectic mixtures can be efficiently used as PCMs because of their large latent heat (above 200 J/g). Fig.1 shows the phase diagram during the melting of ice to water. As we can see, latent heat contributes to the majority of the thermal energy storage compared to the sensible heat during solid-liquid phase change.

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Fig.1 Phase diagram during solid-liquid conversion [2]

According to a recent case study conducted by Pluss® in Gurgaon, temperature profile of a building wall (with & without PCM) is measured with respect to ambient. During the day, ambient temperature is around 43°C and at night it goes down to 27°C. We wish to maintain thermal comfort inside the building approximately between 33-37°C. This can be accomplished by using a PCM which has melting and freezing temperature around 35°C. When the environmental temperature rises, phase change material melts and maintains constant temperature in the range of 33-35°C. During night when the ambient temperature falls, PCM solidifies thereby keeping the temperature constant around 35-37°C. Fig.3 presents the comparative temperature profile inside the building with and without PCM when compared to the ambient temperature. We can clearly observe that with respect to ambient, temperature variation is significantly reduced by the use of PCM in the building walls.

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Fig.2 Building walls layered with PCM

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Fig.3 Internal wall surface temperature profile

Conventionally, PCMs have been in the form of liquid, which are encapsulated in drums, pouches, or panels. PCMs in liquid state bear the risk of leaking out in case of packaging defects or mishandling. Pluss® has come up with a method for preparation of form-stable PCMs (FSPCMs) via combining polymers with PCMs to store thermal energy and has introduced these materials under the brand name savE®. The blends formed using polymers act as shape stabilizers and do not allow the PCM to flow from its matrix. PCM is entrapped in the polymer matrix because of mechanical interaction with the polymer chains. These innovative materials for storing energy over conventional PCMs bear the advantage that any mishandling or packaging defect will not lead to leakage of the PCMs at application site. These materials are nontoxic in nature, flexible, have good latent heat, high tensile or tearing strength both above and below the melting point of the PCM. Further, it is so stable that it can easily be heated and cooled without leaking out its Phase Change Material (PCM). The form stable PCM thus provide stored heat that has improved safety and handling ability.

There are several methods to prepare FSPCMs:

1. Encapsulating solid-liquid PCMs into a polymeric structure via blending
2. Absorbing solid-liquid PCMs into porous materials like expanded perlite, vermiculite, graphite, etc.
3. Grafting solid-liquid PCMs onto the skeleton of high melting temperature polymers
4. Microencapsulating solid-liquid PCMs with different shells

These methods of FSPCM preparation have successfully prevented the leakage of solid-liquid PCMs, however their application is still restricted due to following reasons:

1. The latent heat is reduced by introducing large amount of supporting materials
2. Some of FSPCMs cannot be processed repeatedly once prepared
3. Complex process of preparation will increase the cost
4. Thermal conductivity is also low resulting in poor heat transfer from the surrounding to the PCM

With continuous scientific research in the field of FSPCMs, the drawbacks associated with the form stable PCMs are being overcome. Low thermal conductivity can be enhanced via addition of carbon-based compounds such as graphene and graphite due to their high thermal conductivity. Also, it is now possible to prepare FSPCMs with more than 99% PCM content and just 1% additives thereby retaining the large latent heat of the PCM and along with the form stabilized characteristics [3]. Since, FSPCMs have practically no leakage from the packaging material, they can be successfully used in the medical applications & food packaging industry without any risk of exudation of PCM to the outer surface and getting in contact with the surrounding.

Pluss® has developed “MiraCradle Neonate Cooler” based on the FSPCM technology. Pluss® has filed patent for this cascaded technology under PCT (Patent Corporation Treaty) in June 2014 [4]. This device aims to provide thermal incubation for the newborn infants suffering from Asphyxia (condition of severe deficiency in supply of oxygen). Therapeutic hypothermia induced by cooling a baby to around 33°C and maintaining the temperature between 33°C and 34°C for three days after birth has been considered the only medical intervention which helps during birth asphyxia. MiraCradle does exactly this by providing a precise temperature control between 33°C – 34°C for more than 72 hour. Fig.4 demonstrates the schematic design of this device.

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Fig.4 MiraCradle Neonate Cooler

The cradle is lined with insulating material to prevent the heat loss to ambient. 3 units of savE® FS29 is charged and kept in the bottom part of the cooler. Then the conduction mattress and a thin piece of cloth is placed on the top to commence “Therapeutic Cooling”. Once the baby has been placed in the cradle for 40-45 minutes, the baby’s core body temperature falls from around ~36.7°C to 33.5°C. In case the baby’s core body temperature does not fall down to 33.5°C within 40-45 minutes, 2 units of savE® FS 21 is introduced between the conduction mattress and the savE® FS 29 units for 15 minutes. This ensures that the temperature rapidly falls and stabilizes at 33.5°C thus providing the desired hypothermal effect. The success of MiraCradle can be attributed from the fact that it is being used at 39 hospitals across 12 states in India and over 110 babies have been treated so far. It has also won various prestigious awards such as CII Industrial Innovation Award 2014, Plasticon Award 2014 & Kirloskar Technology Award 2015.

Form-Stable PCM has a distinct advantage over regular PCMs in a number of ways. When used in any application, the system involved in thermal storage remains unaffected due any damage to encapsulation of the PCM. Moreover, the additives used in this technology do not affect the performance of the regular PCM. In addition to this, there is no chemical degradation to the PCM present in the polymer matrix. Thus, these can be easily incorporated in various energy storage applications such as building applications, solar energy applications, transportation of goods, cold storage, medical packaging, therapeutic Pads, HVAC (Heating, Ventilation & Cooling) systems, etc. With such a wide range of applicability and scope in the future of energy storage, form-stable PCMs can indeed be called need for the hour.

Works Cited:

1. Farah Souayfane, F. F.-H. (2016). Phase Change Materials (PCM) for cooling applications in buildings: A review. Energy and Buildings .

2. http://sustainabilityworkshop.autodesk.com/buildings/heat-energy-flows-buildings

3. One-Step Preparation of Form-Stable Phase Change Material through Self-Assembly of Fatty Acid and Graphene, J. Phys. Chem. C, 2015, 119 (40), pp 22787–22796

4. Pluss Polymer Pvt. Ltd, Life cradle device for inducing neonatal hypothermia, PCT/IN2014/000400

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