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PCM FACT

Frequently asked questions about phase change materials

 

1.  What are Phase Change Materials?
Phase change materials (PCM) are substances that absorb and release thermal energy during the process of melting and freezing. When a PCM freezes, it releases a large amount of energy in the form of latent heat at a relatively constant temperature. Conversely, when such material melts, it absorbs a large amount of heat from the environment.


The most commonly used PCM is water/ice. Ice is an excellent PCM for maintaining temperatures at 0C. But waters freezing point is fixed at 0C (32F), which makes it unsuitable for most thermal energy storage applications.


To address that limitation, PCMs have been developed for use across a broad range of temperatures. They typically store 5 to 14 times more heat per unit volume than materials such as water, masonry or rock. Among various heat storage options, PCMs are particularly attractive because they offer high-density energy storage and store heat within a narrow temperature range.


To help understanding, PCM can be described as mixtures of chemicals having freezing and melting points above or below the water freezing temperature of 0C (32F). They are ideal products for thermal management solutions as they store and release thermal energy during the process of melting & freezing (changing from one phase to another).  When such a material freezes, it releases large amounts of energy in the form of latent heat of fusion, or energy of crystallization. Conversely, when the material is melted, an equal amount of energy is absorbed from the immediate environment as it changes from solid to liquid.


2.  What are the important features of Industrial Grade PCM?
To be a useful industrial grade PCM, a material has to meet several criteria:

Large latent heat of fusion 

Release and absorb large amounts of energy when freezing and melting; this requires the PCM to have a large latent heat of fusion and to be as dense as possible.

Fixed phase change temperature

Have a fixed and clearly determined phase change temperature (freeze/melt point); the PCM needs to freeze and melt cleanly over as small temperature range as possible. Water is ideal in this respect, since it freezes and melts at exactly 0C (32F). However, many PCMs freeze or melt over a range of several degrees, and will often have a melting point that is slightly higher or lower than the freezing point. This phenomenon is known as hysteresis.

Avoid excessive supercooling

Supercooling is observed with many eutectic solutions and salt hydrates. The PCM in its liquid state can be cooled below its freezing point whilst remaining a liquid. Some salt hydrates can be cooled to +50C (122F) below their freezing point without crystallization occurring. This can be beneficial, for example in hot packs where a +48C (118.4F) PCM is kept as a supercooled liquid at room temperature until the hot pack is required and supercooling is broken by mechanical or chemical nucleation. However for most applications, supercooling must be kept to a minimum by the addition of suitable nucleating agents to the PCM.

Remain stable and unchanged over many freeze/melt cycles

PCMs are usually used many times over, and often have an operational lifespan of many years in which they will be subjected to thousands of freeze/melt cycles. It is very important that the PCM is not prone to chemical or physical degradation over time which will affect the energy storage capability of the PCM. Some eutectic solutions may be susceptible to microbiological attack, so must be protected with biocides. Long term stability can be a problem in some salt hydrate PCMs, unless they are modified to prevent separation of the component materials over successive freeze/melt cycles.

Non-hazardous

PCMs are often used in applications whereby they could come in contact with people, for example in food cooling or heating applications, or in building temperature maintenance. For this reason they should be safe. Ideally a PCM should be non-toxic, non-corrosive, non-hazardous and non-flammable. There are many substances that behave excellently as PCMs but cannot be used due to issues over safety.

Economical 

It doesn't matter how well a substance can perform as a PCM is if is prohibitively expensive. PCMs can range in price from very cheap (e.g. water) to very expensive (e.g. pure linear hydrocarbons). If cost outweighs the benefits obtained using the PCM, its use will be very limited.


3.  How many types of PCM?

Generally speaking, there are two types:

Organic PCMs C mostly composed of organic materials such as paraffin, wax, fatty acids and sugar alcohols.

Inorganic PCM C a common example of which is salt hydrates


4.  What is latent heat?

There are two kinds of heat energy: sensible and latent.

Sensible heat is heat supplied or taken away and causes an immediate change in temperature without changing the state.

While latent heat is heat supplied or taken away and causes a change in state without change in temperature.

Most common heat storage systems, such as a conventional water heater, use sensible heat, the energy needed to alter the temperature of a substance with no phase change.

Latent heat, which can be 100 times that of sensible heat, is the amount of energy required to change matter from one state to another, liquid to solid or vice versa.

Sensible heat and latent heat work together in thermal storage materials. This results in the ability to maintain specific temperatures for extended periods of time.

Latent heat is measured in joules per gram. Preferred latent heats are greater than 150 J/g and preferably greater than 180 J/g. A high latent heat of fusion means 
that a lesser amount of material is needed to store a given amount of energy.





5
What is the temperature range of commercially available PCMs?

See chart below:



6. How are PCM being used? What are the applications?

      The global PCM market is experiencing high growth as a result of increasing demand for eco-friendly, energy-saving materials in industries such as building/construction, refrigeration, HVAC, thermal energy storage, textiles and electronics. PCM applications include:


       Refrigerators and freezers that use less energy.

       High-performance textiles that provide relief from hot and cold conditions.

       Shipping containers that maintain goods at the desired temperature for longer periods.

 Construction material that keeps living spaces comfortable and reduces heating and cooling costs.

 Thermal energy storage systems that shift a buildings cooling and heating needs to cheaper, off-peak hours.


7.  What is a thermal cycle test?

In a thermal cycle test, a PCM is frozen and melted multiple times. The test is an important measure of a PCMs durability. RAL, an independent quality assurance association based in Germany, has set standards for PCM quality. The RAL's Quality and Testing Regulations for Phase Change Materials specifies these increments for thermal cycling testing: 50, 100, 500, 1,000, 5,000 and 10,000.


8.  Are PCM corrosive?

A few PCMs may interact with some metals and should be tested on a case-by-case basis. Some are not compatible with copper, aluminum or carbon steel but are compatible with 316 stainless steel and perhaps 304 stainless steel.

But note that compatibility is not an issue for encapsulated PCMs, which do not come into contact with the environment.