Thermal Energy Storage with Phase Change Materials
Kapil Gulati
Research Scholar,
Chemistry Department,
Kurukshetra University

Sohan lal
Assistant Professor,
Chemistry Department,
Kurukshetra University

Sanjiv Arora
Professor and Chairman
Chemistry Department,
Kurukshetra University

The materials which go through the process of phase change and having the unique property of large latent heat of fusion; melting and solidifying at almost constant temperature are accepted as phase change materials(PCMs). Phase change materials are the efficient sources of storing the thermal energy. PCMs are well recognized as "Latent Heat Storage Materials".

During the phase transitions PCMs charge and discharge the isothermal energy and are having the large latent heat capacity nearly 5 to 14 times higher than the ordinary thermal storage materials such as masonry and rock. At first National Aeronautics and Space Administration (NASA) introduced this technique of PCM. The NASA's space research program in the late 1970s and early 1980s included the abundant research and development on some new materials to protect astronauts and the delicate instruments from the extreme temperature fluctuations in space. Hence, a new technique of PCM was introduced to the world.

Methods of Energy Storage

The process of capturing the energy generated at once for its use at the later stage comes under the energy storage. Energy can be stored in multiple forms including electrical, mechanical and thermal energy. The technique of energy storage is important in preserving the available energy for improving its utilization.

Energy storage sources/methods can be classified as:
  • Short term behaviour
  • Long term behaviour
In most of the applications short term energy storage (of few hours) is required, while some other applications demands for the long term energy storage (of few months). For e.g. solar energy is available only during the day. So for its application storage of thermal energy is mandatory such that the stored heat may be used at the later stage.

The following index contains the types of energy storage:
  • Mechanical energy storage includes the pumped hydropower storage and flywheel energy storage.
  • Electrical and electrochemical energy storage includes the capacitor, flow batteries and rechargeable batteries.
  • Thermal energy storage (TES) contains the sensible heat and latent heat storage:
    • In Sensible heat storage (SHS) method, storing of energy is done by elevating the temperature of a solid or liquid i.e. by the process of heating or cooling. The drawback of SHS system is the large volume requirement even for the small temperature change.
    • In Latent heat storage (LHS) method, storing of energy is based on the changing of a phase i.e. by melting/vaporizing/solidifying /liquefying. So this method of storing the energy is made by heat absorbing or releasing during the phase change process for e.g. from solid to liquid or vice versa. Latent heat storage systems usually deal in the materials with a considerable capacity of heat of fusion. PCMs change their phase from solid to liquid absorbing a considerable amount of thermal energy for later use in a particular temperature range. This method of storing the energy is superior to SHS method because of its high storage density and isothermalnature during the phase change.
Latent Heat Storage Materials In the midst of the above methods latent heat thermal energy storage is a fascinating way. The phase change process can be made in the following ways: solid-solid, solid-gas, solid-liquid, liquid-gas and vice versa. In the solid-solid transitions heat storage is made during the material transformation from one crystalline form to another. These solid-solid transitions show small changes in volume & also associated with small latent heat than solid liquid transitions. These have the advantages of greater design flexibility and offer the advantage of less rigorous container requirements. However, these transitions are very slow. Higher latent heat of phase promising transitions i.e. liquid-gas and solid-gas transitions suffer from a disadvantage of showing very large changes in volume during phase transitions. These are also associated with container requirement problems which completely rule out their potential utility in thermal energy storage systems. No doubt solid-liquid transformations have comparatively smaller latent heat than liquid-gas; but these transitions are having advantage of involving only very less volume change. Hence, solid -liquid transitions have turned out to be very effective for use in thermal energy storage(TES) systems. PCMs themselves cannot be used as heat transfer medium because of their low thermal conductivity. Hence, there is a need of a separate heat transfer medium which must be hired with PCM in order to transfer the energy from the PCM to the substrate. Thus any latent heat storage system (PCM) should possess the following characteristics:
  • Melting temperature of the PCM should be in the desired operating temperature range.
  • An appropriate heat exchange surface.
PCM should be compatible with container

Pleaser refer Figure 1

Selection Criteria

For their application as PCM these must display the assured advantageous thermodynamic, kinetic and chemical properties. So any PCM should be chosen on the basis of the following selection criteria:

Thermodynamic properties
  • Melting temperature in the desired operating temperature range
  • Latent heat of fusion per unit volumeshould be high
  • High density, specific heat & thermalconductivity
  • Small volume changes on phasealteration
  • Congruent melting
Kinetic properties
  • Nucleation rate should be intensified
  • Crystal growth rate should be high so that large heat can be recovered from the storage system during the reverse process of phase change
Chemical properties
  • Absolutely reversible freeze/melt cycle
  • Non-corrosiveness, non-poisonous and non-combustible Economic properties
  • Availability
  • Low cost
Classification of Phase Change Materials

In general, phase change materials can be categorized as: organic and inorganic PCMs. Organic PCMs can be further divided into two classes: paraffins and non-paraffins. Inorganic PCMs can be subdivided into two classes: salt hydrates and metals.

Refer Figure 2.

  • Organic phase change materials cover a wide range of melting points between 0° and 200° C. However, because of the high content of carbon and hydrogen; these are not very stable at high temperatures. Also the thermal conductivity of organic PCMs is very low.
    • Paraffins: are usually applied to the paraffin waxes having the general chemical formula CnH2n+2, where (19 < n < 41). Crystallization of these long straight chains discharges the enormous amount of latent heat and with the increase in the number of carbon content in the chain there melting point also increase [10].
    • Non-paraffins: PCMs are associated with highly varied properties. Non-paraffin PCMs are having inimitable properties unlike the paraffins having very similar properties and covers a wide range of materials for phase change storage. Abhat et al. [11] & Buddhi & Sawhney [12] have examined a number of esters, fatty acids, fatty alcohols and glycol as the Non-paraffin PCMs.
  • Inorganic phase change materials are having melting enthalpies per unit mass similar to those of organic PCMs, but having larger enthalpies per unit volume because of their intensified densities.
  • Salt hydrates are considered as the mixture of inorganic salt and water with the formation of a crystalline solid with the general formula AB.nH2O. Their phase change is literally a dehydration of the hydration of the salt & resembles the process of melting/freezing thermodynamically.
  • a) Salt hydrates breakup into the lower hydrate and water AB.nH2O .AB.mH2O + (m-n).H2O
    b) Salt hydrates breakup into itsanhydrous form AB.nH2O .AB + n.H2O
The most common problem using these inorganic PCMs (salt hydrates) is their incongruent melting behaviour. As n moles of hydration water are not enough to dissolve the lower hydrates or anhydrous salts, as a result solution becomes supersaturated at the melting temperature. These salts settle down at the bottom of the container because of their high densities making these salts unavailable for recombination with water again during the reverse process of freezing. Another main disadvantage during working with salt hydrates as PCMs is super cooling of the liquid. Nucleation rate is generally very low at the temperature of fusion. That's why the solution needed to be super cooled to achieve a reasonable rate of nucleation. Hence, energy is being released at much lower temperature instead of being released at their fusion temperature.

Metals: are the broad categories of inorganic PCMs. Most significant properties of these materials are as follows:
  • Lower quantity of heat of fusion perunit weight
  • High amount of heat of fusion per unit volume
  • High value of thermal conductivity
  • Low value of specific heat
Measurements Techniques of the Latent Heat of Fusion and Melting Temperatures

The techniques generally used for calculating the temperature of melting and latent heat of fusion of PCMs are:
a) Differential thermal analysis (DTA)
b) Differential scanning calorimeter(DSC)

In these techniques specimen and reference material are heated at a constant rate say x°C/min. The value of temperature difference between them is noted which is being directly proportional to the difference in the heat flowing between two materials and the result is recorded as a DSC curve. The preferred reference material is alumina(Al2O3) which melts at near about 1150°C. By measuring the area underneath the DSC curve (peak) latent heat of fusion can be calculated and by drawing a tangent at the point of greatest slope on the face portion of the peak, melting temperature can be estimated. Both these techniques can be used with different inflowing atmospheres i.e. air, nitrogen and even argon.

Encapsulation of phase change materials

Organic PCMs have disadvantages in low degree of thermal conductivity, large volume change during phase change and their flammable character. When dealing with inorganic PCMs; show the high value of latent heat per unit volume, high thermal conductivity and non flammable character also, but they suffer from disadvantages of corrosiveness to most metals and also suffer from decomposition and super cooling which surely affect their phase change properties. So to rise above all these problems a new technique of employing the microencapsulated phase change materials in thermal energy storage has been introduced. Microencapsulation is described as the process of covering the small micron sized particles or droplets by coating/embedded in a homogeneous matrix to give small capsules offering many advantages such as an increase in the heat transfer area, minimizing the PCMs interaction with the outside environment and preventing the large volume changes during the phase alteration. The methods of microencapsulation are separated into three types (Refer Figure 3):

Applications of Microencapsulated Phase Change Materials

Since Microencapsulated phase change materials were introduced; it had been used in textile and building applications:

Textile Applications: Phase change technology in textiles means blending the microcapsules of PCM into the textile structures. Thermal performance of the textile is improved with the effect of the PCM treatment. During the phase transition from solid to liquid PCMs acquire the energy and during the reverse process of freezing energy is being discharged by them. In 1987 the triangle research and development corporation(Raleigh, USA) proved the feasibility of blended PCMs within textile fibers. For the application of PCMs in textiles the temperature of phase change materials should be in the comfort zone of human i.e. in a range from 15oC to 35oC. PCMs to be used in textiles should have a significant value of heat of fusion and specific heat per unit volume and weight; a high level of thermal conductivity;chemical stability and non corrosiveness. PCMs should have a reproducible crystallization without decomposition and should not affect the human's health. It should present a small super cooling degree and a high degree of crystal growth. Mostly used PCMs in textiles are the paraffin waxes with distinct phase change temperatures depending on the number of carbon atoms .

Please refer Table 1: Phase Change Material.

Hence, encapsulation of PCMs is mandatory before using in textiles in order to prevent the paraffin dissolution while it changes to the liquid state.

Building Applications: The capability to store thermal energy is the important factor for the effective use of solar energy in buildings. The problem affiliated with light weight building materials is their low thermal mass which tend to create the high temperature oscillations; resulting in high heating and cooling demand. Due to growth in the demand of thermal comfort of buildings the energy consumption is correspondingly increasing. PCMs are adjudged as the credible solution for minimizing the energy consumption of buildings. For raising the building passivity and to stabilize the indoor climate, PCMs are beneficial. The blending of PCMs in building walls is a way to intensify the storage capacity of the building envelope (walls, floors & ceiling). PCMs are executed in plasters, Gypsum wall boards and/or textured finishes. During the day time with high ambient temperature; PCMs blended in the building envelope melts storing the large amount of thermal energy. Thus, during the process of phase change heat gain into the buildings is reduced and hence lesser energy is required to keep the building cool. During the night time PCMs changes the phase back from liquid to solid discharging the excess heat into the buildings. This process is profitable during the winter time as the released heat is beneficial in warming the buildings.

Thus, phase change materials are the substances offering the storage of thermal energy by undergoing the process of phase change. There are plethora of techniques by virtue of phase change materials can be fabricated. Applications of phase change materials are not limited to the specific areas of textiles and buildings, but are having the vast number of other applications too.