Phase Change Materials (PCMs) are employed in various types of thermally regulated packaging as a means of protecting the payload against damaging temperature changes during transit.
PCMs are an appropriate solution for the shipping of thermally sensitive or perishable payloads such as pharmaceuticals, biologics, blood, organs, vaccines, food etc. PCMs improve over a basic insulation approach by passively maintaining the payload space within a controlled temperature range influenced by the properties of the PCM.
The simplest PCM utilised in some systems is water/ice operating at about 0oC that is often in the form of a gel. However, it is often necessary to insulate the payload from the ice pack to prevent damage by freezing and therefore alternative PCMs selected to operate at a suitable temperature are often preferred.
PCMs are available at operating temperatures to meet the needs of the various shipping applications requiring stable temperatures even under extreme ambient conditions. The phase change temperature dictates the controlled temperature range of the payload space and the PCM quantity and latent heat capacity determines the duration of protection (depending also on other factors such as ambient temperatures, insulation levels etc.). High performance PCMs deliver precise temperature transitions with high heat capacity. The best organic PCMs (that are crystalline waxes in their solid state) provide reliable and repeatable thermal performance with precise thermal transitions. These may be simple hydrocarbons (n-alkane type) or may be bio-based PCMs of, for example, vegetable origin and therefore renewable. Alternatively, packs of high quality salt hydrate PCMs can be suitable for certain packaging applications.
PCMs can be supplied in a diverse range of containers suitable for most shipping requirements such as pouches, rigid containers, mouldable bags, blankets etc.
PCM packs in their various formats can be reused for multiple shipments and in this way represent a very cost-effective thermal management solution.
Phase change materials in spacesuits for US astronauts were developed by NASA in the 1980s to protect against the wide variations in temperature that can occur in space. PCMs provided a thermal buffer which helped stabilize the temperature within the spacesuit. The technology had been developed earlier by Triangle Research & Development Corporation (North Carolina) for the US Air Force. Patents were filed which covered the use of microencapsulated PCMs for use both within fibres (“in-fibre” technology) and also in fabric coatings. This technology provides the wearer/user with proactive temperature regulation and a high degree of thermal comfort and its use has extended to various types of apparel, footwear, mattresses, mattress toppers and pillows.
Since the early-mid 2000’s when there was one dominant supplier the PCM textile market has developed significantly and continues to grow.
A suitable transition temperature for a comfortable microclimate can be provided by organic PCMs, for example, n-alkane (paraffin) PCMs with particular chain lengths and thermal properties.
Many textile applications require the PCM to be microencapsulated to enable it to be incorporated satisfactorily within the textile fibre, fabric coating or foam. Requirements of the microencapsulated PCM (mPCM) include:
- Particle-size suitable for the application e.g. small & uniform for fibres
- Durability and stability to the to the imposed mechanical and thermal stresses and to the chemicals used (which can be harsh)
- High PCM content (high core/shell ratio)
- Durability in use including resistance to washing and/or dry cleaning
The in-fibre route utilizes mPCM in their aqueous slurry form i.e. as individual microcapsules dispersed in water. Hence, the dope used to prepare the fibres must be compatible with aqueous mPCM slurries. So far this has led to the development of both acrylic and viscose mPCM-containing fibres.
mPCM for textile coating applications can be of larger particle size but still must be resistant to the coating chemicals and processes used by the industry. As many of the textile coatings are hydrophobic, or have low moisture vapour transfer properties, the coatings can be applied as a discontinuous layer i.e. the so-called dot matrix printing. This permits the transport of moisture through the uncoated fabric e.g. in bedding applications.
Not all textile applications require the use of microencapsulated PCMs. Several companies have investigated the use of macroencapsulating the PCM within e.g. a hollow fibre. This is of particular use when the fibre forming process is carried out at temperatures and pressures which may be too high for mPCMs to withstand. The use of polymeric PCMs (pPCM) as an alternative to mPCM has been the subject of investigation and may find use in the future.
As well as PCM fibres and fabric coatings, PCMs are also applied to foams such as polyurethane or latex foams that can be used in mattresses or furniture upholstery. Compression of the foam by the body facilitates heat transfer, accessing the PCMs temperature-specific heat capacity, providing the sensation of thermal comfort or coolness. Again, the use of microencapsulated PCMs is one of the common ways of producing suitable thermo-regulating foams. This is a developing market and there are already several suppliers of thermo-regulating foams and mattresses containing PCM.
Phase Change Materials (PCMs) can help regulate the internal temperature of a room by their ability to absorb or release large amounts of heat energy when changing between solid and liquid states (or phases). For example, a PCM operating at 20 – 24oC will work to buffer the interior climate towards this temperature, helping to maintain cool and comfortable conditions and avoid overheating; hence eliminating or reducing the need for conventional mechanical cooling.
Alternatively, systems comprising PCM heat batteries can store renewable thermal energy at a particular temperature depending on the application, which can be accessed on demand for heating or cooling. PCMs can also store renewable heat efficiently in the temperature range required to provide hot water for buildings.
There are generally three approaches for using PCMs in buildings:
Thermal Mass (Passive)
Heating Systems (Active)
Cooling Systems (Active)
Thermal mass describes a part of the building fabric that has the capacity to store and release significant quantities of heat during a daily thermal cycle.
Thermal mass can minimise temperature peaks and troughs passively, to maintain the internal temperature within a comfortable range and reduce the risk of overheating. Thermal mass can be provided by high heat capacity construction materials that are thermally coupled to the internal space.
- Heavyweight concrete or masonry in thick sections
- Thin, physically lightweight but thermally heavyweight construction materials containing phase change material (PCM)
Theoretical heat capacities over 19oC – 24oC temperature range
The principle is very simple. In a building the basic thermal balance to maintain a constant, comfortable temperature is heat gains = heat losses. When heat gains are high then it follows that to avoid or reduce an increase in air and radiant temperature then heat has to be lost by e.g. conduction through the fabric, losses through controlled and uncontrolled ventilation, absorption by the building fabric or by mechanical cooling. To avoid or minimise the use of mechanical cooling then one or more of the other factors must play a significant part. A high level of insulation (low U-value necessary for winter energy efficiency) will limit conduction through the fabric. Good air tightness will limit uncontrolled ventilation losses. Controlled ventilation, natural or mechanical, will help when the external air temperature is a good deal lower than the internal temperature. Storing some of the heat in the fabric, and then getting rid of it at night, is therefore a fairly obvious thing to do and is why the use of thermal mass is not new. The benefit of storing heat in PCM thermal mass at a moderate constant temperature should also be clear.
Heavyweight construction with concrete slabs and/or masonry may provide sufficiently high levels of active thermal mass if the building is designed and operated appropriately. The mass should be in close contact with the interior space such that it is able to exchange thermal energy satisfactorily and, for summer cooling, heat should be purged from the mass as part of a 24 hour cycle.
This is often carried out through night time ventilation with cool external air. The Concrete Centre in the UK provides some good guidance on the use of concrete thermal mass, which can be readily found via an internet search (http://www.concretecentre.com/).
PCM-containing construction products in most cases resemble, and are used like, familiar building materials. Hence they are a very convenient and physically lightweight – though thermally heavyweight – form of thermal mass. Like conventional heavyweight thermal mass, heat stored during the day should be purged at night to recharge the cooling capacity for the following day. Several commercial products are now available including:
- Plasterboards for walls and ceilings
- Suspended ceiling tiles
- Wet-applied plaster
- Clay-based building boards
- Natural-fibre panels
- Aluminium panels
PCM products are ideal for modern, lightweight buildings where the level of thermal mass would otherwise be low, or in physically heavyweight buildings where the mass is not thermally accessible e.g. due to use of internal wall insulation on external solid walls or use of plasterboard with air gap behind (as happens with a “dot & dab” fixing method, for example). The table below shows some (approximate) calculations, based on the Arup/Concrete Centre U-value software. Whilst a nine inch, wet plastered solid brick wall has very poor insulation properties, nevertheless it possesses high admittance (ability to exchange heat with the room air) and a high Kappa (thermal mass) value. It will also allow moisture vapour transfer. Using internal wall insulation is extremely effective at improving the U-value but effectively decouples any thermal mass within the bricks/blocks away from the room air. As a result the admittance and the thermal mass values are reduced significantly. This also occurs if dry lining boards are used on battens or via a dot and dab system. As mentioned above, the air trapped behind the boards can provide effective insulation; leading to a structure which is structurally heavyweight but thermally lightweight.
What about cost? Firstly, it should be clear that PCM building materials shouldn’t really be compared in cost to “standard” (non-PCM) materials. Thermal mass, when operated correctly in combination with night ventilation, provides a building services function. Where the alternative would be to install conventional mechanical cooling then the cost of an alternative, thermal mass/ventilation strategy may be compared. In addition to savings in the capital cost of a mechanical cooling system sized for peak loads, operational cost savings are made due to energy savings and maintenance savings. This is in addition to the sustainability benefits of a low energy solution that works with nature to keep buildings comfortable. Phase Energy can freely assist in performance and economic assessment of a project using PCM – get in touch and let’s see how we can help.
Thermal modelling? Neither SAP nor SBEM can be used for calculations involving PCM products as they rely on the specific heat of the building components remaining constant across the temperature range employed. Certain dynamic simulation software e.g. EnergyPlus and DesignBuilder can be used as they contain a PCM module. Contact us for information on PCM data input into these software systems.
Also Contact us for:
- Information on commercial products and other free assistance
- Presentation on PCMs in buildings: “Soaking up the Heat”
Renewable heating systems can benefit from the use of PCM where compact and modular thermal energy storage is beneficial. Heat storage is usually needed when the availability of thermal energy is intermittent. The obvious example is solar thermal energy, which requires storage for later use in space and water heating.
Latent heat storage modules with PCM can take the form of heat exchanger cylinders or plates for example, and can be an efficient and space-saving alternative to the conventional approach of using a stratified water tank as a thermal store.
Cooling & Ventilation Systems
PCM cooling systems provide both ventilation & cooling of a space. They can (in principle) be either decentralised (room) or centralised (building). Cooling systems utilise PCM latent heat storage modules to store “coolth” (low temperature thermal energy) from e.g. cool outside air at night or a chiller unit operating off-peak periods.
The free cooling concept operates by storing colder thermal energy in external night time air in PCM storage modules, which are accessed for daytime cooling. When the cooling of recirculated air (or mixed recirculated/outside air) is triggered at a particular internal temperature set-point then air is channelled through a PCM unit to provide cooled supply air. PCM within the thermal storage unit melts at a constant (pre-selected) temperature as it absorbs heat from the air and so provides cooling. The heat collected by the PCM is purged at night using cool outside air and/or can be used for internal heating in the evening or the following morning, thus providing a heating as well as a cooling benefit. The purging of heat recharges (re-freezes) the PCM for the next cooling cycle.
An example of a ceiling mounted system, available in the UK, is Cool-phase from Monodraught.
PCMs may also be employed with a chilled ceiling circuit to provide a radiant cooling surface. By storing the low temperature thermal energy in the PCM multiple room systems can be charged from a single source. The system may also be operated using PCM-containing ceiling boards/panels cooled by circulating water via e.g. pipes or a capillary system.
Most batteries perform best when operating at moderate temperatures. Too cold, and the battery efficiency is reduced, leading to lower energy output. If the battery temperature rises too high, then degradation of the performance (and the battery) can occur. One of the problems found with e.g. Li-ion batteries was the potential for thermal runaway to occur when either the ambient temperature and/or the energy demand on the battery was too high.
The use of PCMs to moderate the battery temperature in the automotive industry is the subject of many patents, with PCM melting point temperatures typically in the 35oC – 55oC range. The PCMs may be incorporated into a porous graphite matrix to increase the rate of heat transfer.
Hybrid vehicles, and those fitted with a stop/start function can lose their air conditioning once the engine stops at traffic lights etc. Most of the major automotive companies, and their suppliers, are developing so-called cold storage evaporator units.
These use a phase change material (PCM) to store cold, from the A/C unit, when the vehicle engine is running and then deliver this to the vehicle’s interior, e.g. via a low powered fan, when the engine and the A/C stop (at traffic lights etc.).
When electric vehicles are used, during cold winter conditions, it has been found that a significant proportion of the battery’s energy can be used to heat the cabin and defrost the windscreen. This can reduce the vehicles driving range considerably. A PCM-based unit has been developed which can be heated whilst the car battery is being charged, and which then delivers most of the heating energy required by the car.
Other uses include passive thermo-regulating upholstery through the use of PCM fabrics or foams, cold-start improvement of diesel engines and more. Vehicles for transporting temperature sensitive goods can utilize PCMs to save energy, reduce energy costs, protect against cooling system failure or as an off-grid solution where the provision of energy is limited.
The refrigeration cycle generates both cold and heat. Refrigerators and freezers can store cold, via a PCM module, to use as additional thermal mass and/or to prevent the contents of the unit from spoiling during e.g. a power outage.
PCMs can also be used for portable, field refrigeration units designed to operate for several hours without power.
Refrigeration units reject heat at the condenser side as part of the refrigeration cycle. A PCM can be employed to store this rejected heat from the unit. This can then be used to help in the defrost cycle where the ice on the evaporator can be removed with the minimum of downtime in the cooling cycle. Normally this ice is removed using electrical heating and, during this time, cooling is suspended, and the use of PCMs can therefore provide a significant performance improvement and energy saving.
Refrigeration circuits in other applications, e.g. air source heat pumps, have also used PCMs to store heat to minimise the downtime due to the defrost cycle.