What are PCMs and why do we use them?
Phase change materials (PCMs) are substances with a high latent heat of fusion.¹ They store latent heat during the transition phase. A PCM absorbs thermal energy and stores it during a solid-to-liquid phase transition, allowing the temperature to be maintained near melting point. Latent heat storage is beneficial as it requires a smaller temperature difference between the storage and releasing functions than sensible heat, making PCMs ideal for maintaining temperatures within a narrow temperature range.
PCMs are used due to their high latent heat capacity; they have the ability to maintain internal temperatures at whichever temperature is desired. Due to the increasing focus on climate change and the use of renewable energy, PCMs have been researched and developed a lot over the last decade. This is due to PCMs’ ability to maintain internal temperatures without the use of gas or fuel. They have a large energy storage density, which is available within a narrow temperature range. There are many types of phase change materials which all have their advantages and disadvantages. The three main types of PCMs are inorganic, organic and bio-based.
Inorganic PCMs
Inorganic PCMs are most commonly salt hydrates. The benefit of inorganic PCMs is that they are relatively cheap to produce and obtain. They have a high latent heat capacity and high thermal conductivity which are key performance characteristics of PCMs. However, the downside of inorganic PCMs is that salt hydrates are vulnerable to supercooling, which is when a liquid is chilled below its freezing point without it becoming a solid. This can cause problems in the system which will cause it to under-perform.
Organic PCMs
Organic PCMs are typically split into two main groups: paraffin and non-paraffin. One advantage of using organic PCMs is that they have a high latent heat capacity, which is a key factor when determining which type to use. Another advantage of organic PCMs is that, unlike inorganic PCMs, they will freeze without supercooling, which will result in a much more reliable system. However, organic PCMs also have disadvantages. These include the high cost of producing them. They are derived from crude oil which is not a renewable resource, therefore may be scrutinized. Organic PCMs are also extremely flammable, which seriously limits applications to construction due to the safety issues associated with flammable materials.
Bio-based PCMs
Bio-based PCMs are typically made from fatty acids derived from either plants or animals. Fatty acids are molecules made up of a hydrocarbon and a carboxyl group, therefore they are extremely environmentally friendly to use. They are non-toxic, cheap and have a high latent heat capacity. However, the main downside to using bio-based PCMs is that they are poor thermal conductors, which is a key characteristic of PCMs. Poor thermal conductivity is a bad characteristic to have; this is due to the PCM being heated/ cooled unevenly, and therefore only a small proportion of the material changing state. This can result in the material under-performing due to only a small proportion of the material absorbing/ releasing heat.
Applications of PCMs
PCMs have many potential applications in various sectors. They can be used for cooling electrical engines, for cooling food and drink, as thermal systems in spacecraft, for maintaining high temperatures in greenhouses, for conditioning of buildings and many other purposes. However, this article will discuss the potential applications in the construction industry.
The applications of PCMs in the construction industry are endless. They can be incorporated into every element of a building, from the roof to the floors to electrical appliances. They can be used to reduce rapid temperature fluctuations within internal environments by storing latent heat in the solid-liquid phase change of a material. Heat is absorbed and released almost isothermally, and is used to reduce the energy consumed by conventional heating and cooling systems by reducing peak loads.
One example of PCMs being used in construction is incorporating the material either side of a double facade. A double facade is a wall system consisting of two skins, or facades, constructed in such a way that air flows in the intermediate cavity. By placing a PCM with a specific melting or freezing point in the cavity, thermal energy can be absorbed and released by the PCM to reduce temperature fluctuations within the building.In a hot country the PCM will require a higher melting point, and in a cooler country it will require a lower freezing point. With a PCM placed in the cavity in a hot country, ventilation (cool air) through the cavity will freeze the PCM within the cavity overnight. Then throughout the day as the temperature rises to above the PCM’s melting point, air from outside will travel through the cavity, where the thermal energy from the air will be absorbed by the PCM, and where it will then radiate into the building, thus lowering the internal temperature. In a cooler climate a Trombe wall system can be used (which is a passive solar building design where a wall is built on the winter sun side of a building with a glass external layer and a high heat capacity internal layer separated by a layer of air, which can be implemented to heat the building). Single or double glazing is usually placed on the exterior surface of the massive heat storage module, with a thin air gap separating these two materials. The exterior surface of the wall is painted black to absorb the most solar energy to melt the PCM; this energy is then stored and conducted through the wall over the period of the day. In winter solar heating scenarios, when the temperature of the internal environment drops, heat from the wall storage radiates into the building from the Trombe wall over several hours, thus increasing the temperature of the building. This same concept can be incorporated into a roof as well, using a roof ventilation system with a PCM around the perimeter of the vent. The same concept can also be incorporated into an underfloor ventilation system. PCM use is equally effective in both hotter and cooler climates, due to the variation in temperature ranges of PCMs available.
An example of when PCMs have been implemented into a building and their performance reviewed was a study called the ‘EFdeN project’. This study was carried out at the Technical University of Civil Engineering in Bucharest. During the study, the team used a number of different active and passive strategies such as solar shadings, heat recovery, vacuum tube collectors and many more, which were designed in order to obtain the perfect indoor conditions with minimal energy consumption. PCMs were one of these strategies, and the results obtained from the study supported the use of PCMs. The team conducted tests with different PCM materials, and the best one was found to be the PCM type M182/Q23 which is a bio-based PCM, with a latent heat capacity of 574 Wh/m², which had a great impact on the energy reduction: cooling 48% and heating 11%. The PCM was incorporated into the walls of the building, where it would absorb energy in the morning and then radiate the heat into the internal environment when the internal temperature of the building dropped in the evening.
Sustainability
In terms of sustainability and PCMs’ effects on the environment, they depend mainly on the type of PCM being used. If the PCM being used is bio-based (made from fatty acids) then this method is extremely environmentally friendly due to the substance being renewable, relatively easy to acquire and biodegradable, unlike paraffin (which take decades to completely decompose). PCMs are relatively sustainable; due to the many types of PCMs, they can be acquired from many places. Many PCMs can be derived from plants; for example, the PCM manufacturer PureTemp uses natural sources such as palm oil, palm kernel oil, rapeseed oil, coconut oil and soybean oil. However, some PCMs such as paraffin wax are derived from crude oil, which is not a renewable resource.
Legal frameworks in place for PCMs
The RAL Quality Assurance Association provides documentation of regulations and standards that companies must comply with in order to be able to use the quality mark on their product. The RAL Quality Mark allows customers to have faith that a product is of a high standard; this is due to the extensive testing that the product must undergo before the quality mark can be used on a product. More than 9000 manufacturers and service providers use the RAL quality mark to show customers that their product is of a high standard. The RAL provides a detailed document named RAL GZ 896 which includes all the required testing and standards that any PCM product must comply with. It also provides a detailed product data sheet that provides guidance to companies seeking to achieve the minimum performance requirements. This includes:
- Whether PCM is encapsulated or not encapsulated.
- Operating range.
- Maximum and minimum permissible temperature.
- Specific weight.
- Information on permissible applications.
- Phase transition temperature.
- Stored heat.
- Reproducible phase transition (cycle category).
- Thermal conductivity.
Specifying PCMs
In terms of specifying PCMs, the key criteria to include are latent heat capacity, phase transition temperature, thermal conductivity and cycling stability. These four key criteria together dictate the PCM’s overall performance. The latent heat capacity of the material, which is measured in kJ kg−¹, is used to determine the amount of energy released or absorbed during the change of phase of the substance. This ultimately dictates the extent to which the material can heat/ cool a building. The phase transition temperature refers to the temperature in which the material changes from one phase to another, which is a vital piece of information to have in order to ensure that a suitable temperature is selected for a specific building. Thermal conductivity is another key factor that is required in the specification of the PCM: thermal energy must flow either in to or out of the PCM in order for the phase change to occur. Therefore, the time required for a PCM to completely charge or discharge is directly dependent on its thermal conductivity. Cycling stability of the PCM must be specified under standards set by the RAL; this involves thermal cycle testing of the material to determine whether the material will maintain a constant phase transition temperature after many cycles. This is required in order to provide information on the product’s life cycle, which is a vital piece of information for consumers.
In the UK market for PCMs, there are two main manufacturers. The first is DuPont, who have developed a PCM technology called ‘Energain’. Using this technology, DuPont have developed a product called the DuPont™ Energain® thermal mass panel, which is a panel with a core material consisting of a copolymer and a paraffin wax phase change material. At temperatures below 18°C, the wax remains in a solidified state. Once the temperature inside a room reaches 22°C, due to solar gains and/ or external temperatures, the phase change takes place and the paraffin wax melts, thus absorbing thermal energy. When specifying their products, DuPont work in accordance with RAL standards and regulations. DuPont provide all the required information for their products such as thermal conductivity, latent heat capacity, cycling stability and phase transition temperature. They also provide information related to the product’s parameters such as thickness, width and length. DuPont have different product options available to meet the client’s requirements in terms of parameters.
The other main manufacturer in the UK is BASF, with their line ‘Micronal PCM’, which consists of microscopic particles of wax encased in a tough polymer shell. An example of where BASF have integrated this technology into a product is their Knauf PCM SmartBoard®, which is a wallboard containing three kilograms of Micronal PCM per square metre capable of reducing temperature fluctuations in a building. BASF also complies with regulations and standards set by the RAL Quality Assurance Association in order to use the RAL Quality Mark on their products.
DuPont Energain was added to a sustainable building design by Richard Hawkes. His sustainable building design called ‘Crossway’ incorporated Energain PCM into the wall system using PCM thermal mass board. The use of PCM was very uncommon in 2009 when Crossway was constructed, suggesting that it was ahead of its time. The house received an A-A rating on its energy performance certificate.
One example of software used for specifying materials is our own ‘NBS Create’ product. NBS Create is designed to make it easy to collate the data that you need to produce specifications for materials. NBS Create can be used to specify PCMs, and will make the whole process of a lot easier. This is due to the mix of a user-friendly layout and a powerful, complex piece of software. NBS Create will allow the user to input data such as the PCM’s thermal conductivity, latent heat capacity, cycling stability and phase transition temperature. The software will then take this information and display it in a user-friendly format which is easy to understand for consumers, helping them select a suitable PCM which meets their requirements.
Conclusion
In conclusion, PCMs have endless potential applications in a range of different industries. The use of phase change materials for thermal storage has attracted researchers all over the world, mainly due to their large energy storage density, which is available within a narrow temperature range. Work on PCMs has been ongoing for decades, but their commercial use is still limited due to their high production and encapsulation costs. However, recent developments such as the Knauf PCM smartboard have provided positive results and hope for the future of PCM. Case studies such as the EFdeN project have proved the energy saving potential of PCMs and their potential benefits across a range of industries. Research and development of PCMs will continue, and they will hopefully be implemented more and more over the coming years.
¹ The specific latent heat of fusion, l, of a substance is the heat needed to change a mass of 1 kg of the substance from a solid at its melting point into liquid at the same temperature.
