Cryophase ­­Phase Change Materials- PCM Gel bricks for Temperature  Control

Cryphase_PCM_gel_bricks

Thanks to Global Warming and Stringent temperature control regulations around the world, the Cryolux research team are continuously working on research and development of  New Cryophase  Phase Change Materials. To understand the functionality and applications of Cryophase PCM Gel Bricks, the following terminologies need understanding.

Phase Change:

The four phases (state) of matter are solids, liquids, gases and plasma. Plasma is a significant number of electrons that have high energies that no nucleus can hold them.

In theory, any matter can be changed from one phase to another phase by heating or cooling. Following are the Phase Changes associated with matter and associated terminologies.

1)      Solid to Liquid -> Melting - by Heating

Example: Ice melts into water when heated above its freezing point (0 Deg C).

2)      Liquid to Solid –> Freezing (Solidification)- by cooling

Example: Freezing water below its freezing point (0 Deg C) to form Ice.

3)      Liquid to Gas  –> Evaporation (Vaporisation) – by heating

Example: When water is heated above its boiling point (100 Deg C) and water vaporises to steam.

4)      Gas to Liquid –> Condensation (Precipitation) – by cooling

Example: Formation of due in the morning by condensation.

5)      Solid to Gas –> Sublimation –by heating

Example: Dry ice (solid carbon dioxide) sublimates at ambient temperature when exposed to air.  Another example is Iodine which sublimates to form Gas at ambient temperature.

6)      Gas to Solid –> Deposition – by cooling

Example: Compressed Carbon dioxide in gas form in a Fire Extinguisher turns into solid form as a white powder (Dry ice) when released.

Sensible Heat Vs Latent Heat:

Water remains as solid (ice) at temperatures below 0 Deg C. It remains a liquid between 0 and 100 Deg C and vaporises to form steam above 100 Deg C (at sea level at 1 atmospheric pressure). When ice from low temperature say –30 Deg C is heated, the temperature of ice increases with the absorption of heat. Thermal Energy absorbed / Stored (TES) is calculated as

TES = m*Cp*(T2-T1)

Where m is mass of ice (g)

(T2 – T1) is the rise in temperature

Cp is the specific heat of water/ ice  i.e. Heat Energy required to raise the medium of unit mass by 1 unit of temperature = 1 Calorie per gm per Deg C.

This Thermal Energy is called Sensible Heat of Storage (SHS) since this internal energy can be sensed or felt. It is the thermal energy which is transferred to a substance resulting in a change in temperature.

When the temperature of ice increases from sub-zero and reaches 0 Deg C, ice starts melting with the associated absorption of a large amount of heat, without the change in temperature. This thermal energy is called Latent Heat of Fusion (LHF) and is calculated as m* Lf where m is the mass of ice and Lf is Latent Heat of Fusion.

For water, Lf is 79.72 Calories/ g

After the ice melts completely at 0 Deg C, it absorbs sensible heat from 0 Deg C till it reaches 100 Deg C. At 100 Deg C, water starts boiling converting to steam (water vapour) at a constant temperature.

Latent Heat of Vaporisation (LHV) is calculated as m*Lv where m is a mass of Liquid; Lv is the Latent heat of vaporisation.

For water Lv = 541 Calories/ g

In practice, sensible heat of absorption or release takes place faster while the Latent heat of freezing/fusion takes a longer period.

Super Cooling / Sub Cooling:

It is a state where liquids do not solidify even below their normal freezing point. Formation of Clouds at high altitudes with super cooled droplets of water at temperatures of -55c which is much below their freezing point is an example of this phenomenon.

Super Cooling is an undesirable feature in the case of PCMs. This is kept to a minimum by the addition of appropriate agents to the PCMs.

How do Cryophase Phase Change Materials (PCMs) Work?

PCMs are materials that change from one phase to another at a designated temperature with associated absorption or release of large amount heat energy. This is termed as Latent Heat of Storage (LHS). This process will be reversible.

LHS can be achieved through Liquid to Solid, Solid to Liquid and Solid to Gas phase changes. However, solid to liquid and liquid to solid phase changes are only practical for PCMs. Although liquid to gas phase change has a high latent heat than solid to liquid, liquid to gas phase changes are impractical as large volumes or high pressures are required to store the materials in their gas phase.

When the environment’s temperature is higher than that of the Cryophase Phase Change Materials, heat transfers from the surroundings to the material, which creates a cooling effect and changes PCM’s state from solid to liquid.  When the environment’s temperature is lower than that of the Cryophase Phase Change Materials, heat transfers from the PCM to the surroundings generating a warming effect and PCM changes back to its solid state. Thus, PCMs absorb and release/emit heat energy while maintaining a nearly a constant temperature by oscillating between solid and liquid phases within a narrow temperature range. Hence, PCMs are ideal passive materials for temperature control. Melting of PCMs takes place more often with a small volume change, usually less than 10%.

Most commonly used PCM is water which has a freezing point of 0 Deg C which makes it unsuitable for most Thermal Energy Storage applications. Hence Cryolux has developed Phase Change Materials for a wide range of temperatures from -30 C  to more than 100 C wherein each of those PCMs will have a specific/discrete temperature of phase change, typically within 1-degree variation. Cryolux has developed formulations which combine over 200 different PCMs to cover the entire range between -30 C and 100 C. These  PCMs store up to 15 times more heat per volume than water.  PCMs are attractive because they provide a high energy density of storage and store heat within a narrow temperature range.

Types of Phase-change Materials:

 - Water based PCMs (Ice, Ice Pack, Gel packs)

 - Inorganic PCMs (Salt Hydrate, Metallics)

 - Organic PCMs (Paraffin’s, Fatty Acids / Bio based Vegetable oils)

 - Eutectic Solutions (Organic, Inorganic)

Water based PCMs: (Ice, Gel Packs, Dry Ice Packs ) at 0 C

Ice and Gel Packs are very popular to keep products cold around 0 C. To obtain a  water based PCM lower than 0 C, salt can be added to water to lower the freezing point. These low-cost refrigerants are non-toxic, non-flammable, environmentally friendly and easy to use. Their demerit is that they are confined to a temperature of 0 C. Gel packs / Ice packs / Dry Ice packs are usually made from a salt solution. The predominant ingredients in Gel packs are Sodium Polyacrylate [-CH2-CH(CO2Na)-]n. The Addition of salt changes the crystal structure of ice and reduces its effectiveness as a Phase Change Material by reducing the latent heat.

Inorganic PCMs:

An Inorganic compound is any compound that lacks carbon atom.  Sodium chloride (household salt), water, hydrochloric acid, salt hydrates are some examples of inorganic Phase Change Materials.

Salt Hydrates PCMS:  (-35 Deg C to 100 Deg C)

These PCMs are water based but they freeze/ melt at temperatures above 0 Deg C. They also use different salts to vary phase change temperature.  In some cases, the same salt can be used in different concentrations in water to provide both positive temperature and negative temperature PCMs. The positive temperature PCMs utilise the ability of some salts to form complexes with water and to form water of crystallisation, hence ‘salt hydrates’. This means that although the salt crystal is solid, it may contain over  50% water. When the crystal is heated, it melts with release water of crystallisation and allowing the salt to dissolve in this water. In this process, it absorbs a large amount of latent heat, which is reversed when the solution freezes.

The most attractive characteristics of salt hydrates are:

(i)               High latent heat of fusion per unit volume

(ii)              Sharp melting point

(iii)             Relatively high thermal conductivity which is the ability to absorb/ store or release the latent heat in a given volume of storage material in a short period (almost double that of paraffin.

(iv)             Small volume changes on melting.

(v)           They are not very corrosive, compatible with plastics and only slightly toxic

(vi)             Sufficiently inexpensive.

Limitations of Salt Hydrate PCM ice bricks:

(i)                Limited  temperature range

(ii)        Change of volume is relatively high (up to 10%) thus requiring special packaging

(iii)            Phase Change Ice Bricks melt incongruently i.e. they melt to a saturated aqueous phase and the solid phase which is a lower hydrate of the same salt. Due to higher density, salt phase settles down at the bottom of the container, a phenomenon called decomposition, also referred as segregation. The solid phase does not mix with the saturated solution to form the original salt hydrate. This results in an irreversible melting–freezing of  salt hydrate which goes on with each charge/discharge cycle.

The problem of incongruent melting can be handled by one of the following methods  :

(a)      By addition thickening agents having a crystal structure similar to that of parent substance which prevents settling of solid salts

(b)      By encapsulating the PCM to reduce separation.  

(iv)     Another important problem common to salt hydrates is that of supercooling.

(v)     These  are also very corrosive when they come in contact with metals. Sometimes corrosion inhibitors are added to limit this problem.

Metallic’s PCMs:

This category of PCM includes low melting metals and metal eutectics.

The properties include

(i)                 High thermal conductivity

(ii)                Low latent heat of fusion per unit volume

(iii)               High latent heat of fusion per unit weight

(iv)               Low specific heat

(v)                Relatively low vapor pressure.

These categories have not been seriously considered for PCM due to weight penalties.

Organic Phase Change Materials – PCM Bricks (Paraffin’s and Fatty Acids):

Organic compounds consist of molecules with carbon atoms.  These are either naturally available or man-made (synthetic).

Some examples : Glucose, Cellulose, Insulin, Glyceride, fat, Proteins, Alcohols, Urea

Paraffin’s : (- 8 Deg C to +40 Deg C)

These are hydrocarbons with a waxy consistency at room temperature.   Paraffin’s are of subgroups - Straight Chain (n-Paraffin) and Branched chain (iso-Paraffin). Isoparaffin don’t make good PCMs. The presence of any iso-paraffin in a Phase Change Brick can severely degrade the thermal performance of the PCM. Hence, only n-paraffins are considered.

Melting Point of paraffin’s  is  between 6 and 180 Deg C. Paraffin waxes have lower melting point and poorer latent heat than pure paraffin’s.  Paraffin’s have good thermal storage capacity and freeze without super cooling. They have chemical stability over many heating and freezing cycles. Paraffin PCMs have a high heat of fusion and are non-corrosive and compatible with most materials and non-reactive with most materials of encapsulation. Commercially cost effective paraffin’s are mixtures of alkenes and therefore do not have well-defined melting points. 98% hexadecane has a latent heat of 230 J/g and a melting point of 18 Deg C. This material is available as 1% in petroleum crude oil. Hence, the cost of extraction of 98% pure product is expensive.

Vegetable based Oils / Fatty Acids PCMs: ( -90 Deg C to +150 Deg C)

Around 300 different ranging from -90 Deg C to +150 Deg C  with latent heats between 150 and 220 J/g have been developed. The advantages of Vegetable based fatty acids are that they are non-toxic, non-corrosive, have  infinite life cycles, flammable only at high temperatures (high flash points). Many vegetable PCMs are food grade and have no effects when ingested. These PCMs can be derived from agricultural crops with a stable price. These PCMs have superior price performance characteristics compared to salt hydrates  and paraffin based PCMs. They are compatible with wall boards that absorb water and capable of being micro encapsulated unlike salt hydrates. Fats and oils melt near ambient temperature and are ideal for 0 Deg C to 110 Deg C. Beyond 110 Deg C, fatty acid PCMs are less ideal due to thermal degradation. Fatty acids and their eutectic mixtures perform better over inorganic materials with little super cooling, high latent heat, less volume change, good thermal and chemical stability even after repeated cycles.

Eutectic Phase Change Materials :

Eutectic PCM is a mixture of two or more chemicals (either Organic or Inorganic) that  dissolve in one another as liquids that all of such mixtures liquefy at the lowest temperature. Temperature at which the mixture solidifies is called Eutectic Temperature.  If the original liquid had the eutectic composition, no solid would separate until the eutectic temperature is reached. Also, both solids would separate in the same ratio as that in the liquid. A true eutectic  is one in which chemicals are mixed in a particular ratio resulting in freezing / melting point of the mixture being  lower than the corresponding points of component chemicals.

Below are some of the Eutectics :

Examples :

Inorganic-Inorganic : CaCl2.6H2O+MgCl2.6H2O (-65 Deg C to 0 Deg C)

Organic-Organic           : Capric –Lauric acid

                                         : Lauric - Palmitic acid

                                         : Lauric - Slearic acid

Organic-Inorganic         : Urea - NH4Br

Eutectics have a sharp melting point and their volumetric thermal storage density is slightly above organic compounds. Their disadvantage is that their cost is over three  times that of Organic / Inorganic PCMs.

Eutectics of salt in water provide melting point in the range of -65 Deg C to 0 Deg C

Solid – Solid PCMs  : (+25 Dec C to +180 Deg C)

A new class of phase-change material i.e. Solid – Solid phase change materials have been developed. They have a large heat of phase transition. These materials change their crystalline structure from one lattice configuration to another, at a well-defined temperature. Their transformation can involve latent heat comparable to the most effective solid / liquid PCMs, a range of solutions from +25 Deg C to +180 Deg C.

Characteristics / Properties of ideal  Phase Change Materials – PCM bricks :

An ideal phase change material would be specifically calibrated to suit the specific temperature requirement which means the phase change brick would have the longest lifespan at the desired temperature. The time duration of the temperature it maintains is also subject to external factors such as ambient temperature, insulation properties, humidity and thermal mass of the refrigerant and payload. The best PCM bricks would have a sharp melting point without hysteresis. A good PCM brick should have high latent heat per unit mass per unit volume. They will also have high thermal conductivity in both solid and liquid phases thereby assisting fast charging and discharging of heat energy storage. They also have high thermal stability ie retention of thermal properties mainly melting point and latent heat over large temperature cycles. They should have high nucleation rate with little or no super cooling (sub cooling) during freezing process, completely reversible freeze/ melt cycle, congruent melting to avoid segregation, Low density and volume variation to avoid problems with a storage tank. They should have long term chemical stability. An ideal PCM has small volume change on phase transformation. Ideally the phase change bricks should be nontoxic, non-corrosive, non-poisonous, non-flammable and non-explosive. Good phase change materials must be abundantly available and preferably at low cost. They should also be compatible with the container material. The bio degradability is an important aspect. At the end of the lifecycle, the PCM should be  safe enough to be landfilled and degrade naturally in about six months.

Applications  of Phase Change Materials (PCM)

PCMs find a variety of applications, both in positive and negative temperature ranges.

Positive Temperature applications

Negative Temperature Applications

Chilled  food transport

Temperature controlled packaging

Water Heaters

Ice cream vending

Electronic / Battery Cooling

Frozen Food transport

Cooling Buildings (Passive)

Medicine and vaccine transport

Air-conditioning

Marine Refrigeration

Blood storage and transport

Cold Transport

Medicine and vaccine transport

Cooling Human body & Cryotherapy applications

Drink cooling & vending

Refrigerator energy Saver

Cryolux Continue to develop the Cryophase PCM range and we welcome any inputs and suggestions in this regard . Please email us at enquiry@cryolux.com.au