Temperature regulation: Phase change materials (PCMs)

Transcription

1 Temperature regulation: Phase change materials (PCMs) PGMs Passive temperature regulation. Reduced heating and cooling demand. Regulating the temperature of buildings consumes vast quantities of energy for both heating and cooling, in the process producing CO2 emissions. With the help of nanotechnology, the energy consumption can be significantly reduced. Latent heat storage, also known as phase change material (PCM), can be used as an effective means of regulating indoor room temperatures. The good thermal retention of PCM can be used both in new and existing buildings as a passive means of evening out temperature fluctuations and reducing peak temperatures. It can be used both for heating as well as cooling (e.g. to protect against overheating). A good example that illustrates the high thermal capacity of latent heat stores is an ice cube that begins to change to its liquid state at 0 C. The liquid state also begins at 0 C but the energy required for this change of state is equivalent to that required to heat liquid water from 0 C to 80 C. This "hidden" thermal buffer, or the latent thermal storage, is correspondingly large, and this principle can be used for the insulation of buildings using PCM. The word "latent" can be regarded as meaning "hidden" - it exists but is not evident. The use of phase change materials is not new. In ancient Baghdad, rooms were kept cool with the help of a natural PCM: ice. Research into PCMs has been undertaken for many years. In the 1940s, first attempts were made to use PCM in buildings in the USA, and in 1953 the first microcapsule was patented, also in the USA. The widespread application of the material has only become feasible since the production of microcapsules (more precisely micro-encapsulated phase change material, MPCM), which represented a milestone in the development of PCMs. In the 1980s the NASA undertook basic research and development into PCMs, enter-

2 Close-up of a phase change material embedded in glazing. Crystallisation of a salt phase change material. Wax droplets with an acrylic glass sheathing that is practically Indestructible, even by sawing or drilling. An image of an opened microcapsule embedded in a concrete carrier matrix, taken using scanning electron microscopy. An image of minute paraffin-fiiled capsules in their solid state, taken using light microscopy. They exhibit an exceptionally high thermal capacity and during a phase change turn to liquid.

3 ing into partnership with industry from 1988 onwards. Thermally adaptable spacesuits and gloves for astronauts were developed that enabled the wearer to withstand the extreme temperatures of outer space. Important patents followed in the early 1990s. In the context of building and construction, the main application area is for conserving energy. PCMs are invariably made from paraffin and salt hydrates. Minute paraffin globules with a diameter of between 2 and 20 nm are enclosed in a sealed plastic sheathing. These can be integrated into typical building materials, whereby around 3 million such capsules fit in a single square centimetre. As PCM is able to take up energy (heat) without the medium itself getting warm, it can absorb extremes in temperature, allowing indoor areas to remain cooler for longer, with the heat being retained in the PCM and used to liquefy the paraffin. As the temperature rises, melting the waxy contents of the microcapsule, the paraffin changes from solid to liquid. The same principle also functions in the other direction: rooms that are cooling down stay warm for longer, while the molten paraffin gradually hardens, before losing warmth. The temperature level of the materials remains constant. The amount of energy that is taken up or released is considerable so that even a comparatively small mass has a large thermal retention capacity, with which temperatures inside buildings can be regulated. During a phase change, the warmth is retained latently for as long as is required to change from one physical state to another. During this process, the PCM absorbs a particular amount of heat, the specific latent heat, equivalent to the amount of energy required to melt the paraffin. Instead of rising, the temperature of the PCM remains constant. The process functions according to the same principle in the opposite direction - during a phase change PCMs are able to store warmth as well as cold (known as the "free-cooling principle"). Energy is therefore stored latently when the material changes from one physical state to another, whether from solid to liquid or from liquid to gaseous. The latent warmth or cold, which effectively fulfils a buffer function, can be used for temperature regulation. The predefined, so-called switching temperature, in which the phase change from one physical state to another occurs in latent heat storing materials designed for construction, is defined as 25 C, as above this temperature the indoor air temperature is generally regarded as being unpleasantly warm (as given in the German regulations for workplaces). Depending upon the PCM used, to regulate a 5 C increase in temperature only 1 mm of phase change material is required in comparison to mm of concrete. The PCM has a far greater thermal capacity: a concrete wall warms

4 PCM plaster applied on Interior walls provides thermal insulation. Up much more quickly whilst the temperature of a PCM remains unchanged. PCMs are available with different switching temperatures for different application areas. In areas other than construction, switching temperatures can vary considerably, ranging from minus degrees to 100 C. In the meantime, PCMs have become available in the form of additives that can be integrated into conventional building materials such as plasters, plasterboards or aerated concrete blocks with specific retention properties. The first two materials can be used to add a temperature regulatory function to an interior, the latter to equip a building with latent heat storage properties from the outset. Most importantly, the materials need to be exposed to warmth. For example, it would not make sense to internally insulate an aerated concrete block PCM wall, as it would hinder the effectiveness of the PCM. From a fire safety perspective, it is important to note that paraffin, as used in PCMs, is flammable and PCM products are not therefore classified as flame-resistant. In contact with other building materials, paraffin-based latent heat storage products do not cause any (undesirable) chemical reactions, such as corrosion. An added advantage, particularly on-site, is that due to their small size, the globular PCM contained in materials are practically resistant to damage. As such mater- Temperature regulation: Phase change materials (PCMs)

5 Layer composition of a decorative PCM gypsum plaster applied to a masonry substrate. Although only 15 mm thick, this plasterboard panel contains 3 kg of micro-encapsulated latent heat storage material per square metre. ials can be mechanically worked without damaging the temperature regulation function. During the phase change, a change in volume occurs, which needs to be considered when materials are used in enclosed containers. Materials containing PCM should have a good thermal conductivity to ensure rapid transfer to and from the PCM. Paraffin has a relatively low thermal conductivity, but is compensated for by a higher reactivity resulting from the large surface area of the minute particles. To benefit from a continually functional PCM, the thermal capacity of the PCM should not change over its lifetime. In addition to conserving energy by reducing the energy demand for heating and cooling, PCMs are also recyclable and biologically degradable. As with other innovative insulation materials, there is a large market for the use of PCMs in the construction industry, as they improve indoor climates, reducing costs and in some cases even obviating the need for air conditioning. With regard to the need to reduce CO2 emissions, PCMs offer a further nanotechnology-based opportunity to achieve this aim. Latent heat storage systems are already successfully used in transport containers for sensitive materials, in outdoor clothing, as a base component for creams as well as for food wrappers. Their use could become widespread in the construction industry.