A sodium acetate heating pad. When the sodium acetate solution crystallises, it becomes warm.

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A video showing a "heating pad" in action

A phase change material (PCM) is a substance which releases/absorbs sufficient energy at phase transition to provide useful heat/cooling. Generally the transition will be from one of the first two fundamental states of matter - solid and liquid - to the other. The phase transition may also be between non-classical states of matter, such as the conformity of crystals, where the material goes from conforming to one crystalline structure to conforming to another, which may be a higher or lower energy state.

The energy released/absorbed by phase transition from solid to liquid, or vice versa, the heat of fusion is generally much higher than the sensible heat. Ice, for example, requires 333.55 J/g to melt, but then water will rise one degree further with the addition of just 4.18 J/g. Water/ice is therefore a very useful phase change material and has been used to store winter cold to cool buildings in summer since at least the time of the Achaemenid Empire.

By melting and solidifying at the phase change temperature (PCT), a PCM is capable of storing and releasing large amounts of energy compared to sensible heat storage. Heat is absorbed or released when the material changes from solid to liquid and vice versa or when the internal structure of the material changes; PCMs are accordingly referred to as latent heat storage (LHS) materials.

There are two principal classes of phase change material: organic (carbon-containing) materials derived either from petroleum, from plants or from animals; and salt hydrates, which generally either use natural salts from the sea or from mineral deposits or are by-products of other processes. A third class is solid to solid phase change.

PCMs are used in many different commercial applications where energy storage and/or stable temperatures are required, including, among others, heating pads, cooling for telephone switching boxes, and clothing.

By far the biggest potential market is for building heating and cooling. PCMs are currently attracting a lot of attention for this application due to the progressive reduction in the cost of renewable electricity, coupled with limited hours of availability, resulting in a misfit between peak demand and availability of supply. In North America, China, Japan, Australia, Southern Europe and other developed countries with hot summers peak supply is at midday while peak demand is from around 17:00 to 20:00. This creates a lot of demand for storage media.

Solid-liquid phase change materials are usually encapsulated for installation in the end application, to contain in the liquid state. In some applications, especially when incorporation to textiles is required, phase change materials are micro-encapsulated. Micro-encapsulation allows the material to remain solid, in the form of small bubbles, when the PCM core has melted.

Characteristics and classification[edit]

Latent heat storage can be achieved through changes in the State of matter from liquid→solid, solid→liquid, solid→gas and liquid→gas. However, only solid→liquid and liquid→solid phase changes are practical for PCMs. Although liquid–gas transitions have a higher heat of transformation than solid–liquid transitions, liquid→gas phase changes are impractical for thermal storage because large volumes or high pressures are required to store the materials in their gas phase. Solid–solid phase changes are typically very slow and have a relatively low heat of transformation.

Initially, solid–liquid PCMs behave like sensible heat storage (SHS) materials; their temperature rises as they absorb heat. Unlike conventional SHS materials, however, when PCMs reach their phase change temperature (their melting point) they absorb large amounts of heat at an almost constant temperature until all the material is melted. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. A large number of PCMs are available in any required temperature range from −5 up to 190 °C.^[1] Within the human comfort range between 20 and 30 °C, some PCMs are very effective, storing over 200 kJ/kg of latent heat, as against a specific heat capacity of around one kJ/(kg*°C) for masonry. The storage density can therefore be 20 times greater than masonry per kg if an temperature swing of 10 °C is allowed.^[2] However, since the mass of the masonry is far higher than that of PCM this specific (per mass) heat capacity is somewhat offset. A masonry wall might have a mass of 200 kg/m², so to double the heat capacity one would require additional 10 kg/m² of PCM.

^[3] Example Organic Bio-based PCM in a poly/foil encapsulation for durability in building applications, where it works to reduce HVAC energy consumption and increase occupant comfort.

Organic PCMs[edit]

Hydrocarbons, primarily paraffins (C_nH_2n+2) and lipids but also sugar alcohols.^[4][5][6]

• Advantages
  • Freeze without much supercooling
  • Ability to melt congruently
  • Self nucleating properties
  • Compatibility with conventional material of construction
  • No segregation
  • Chemically stable
  • Safe and non-reactive
• Disadvantages
  • Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle. Nano composites were found to yield an effective thermal conductivity increase up to 216%.^[7][8]
  • Volumetric latent heat storage capacity can be low
  • Flammable. This can be partially alleviated by specialised containment.

Inorganic[edit]

Salt hydrates (M_xN_yH₂O) ^[9]

• Advantages
  • High volumetric latent heat storage capacity
  • Availability and low cost
  • Sharp melting point
  • High thermal conductivity
  • High heat of fusion
  • Non-flammable
• Disadvantages
  • Difficult to prevent incongruous melting and phase separation upon cycling, which can cause a significant loss in latent heat enthalpy.^[10]
  • Corrosive to many other materials, such as metals.^[11][12][13] This can be overcome by encapsulation in small quantities in non-reactive plastic.
  • Change of volume is very high in some mixtures
  • Super cooling can be a problem in solid–liquid transition, necessitating the use of nucleating agents which may become inoperative after repeated cycling
    

    Example: eutectic salt hydrate PCM with nucleation and gelling agents for long-term thermal stability and thermoplastic foil macro-encapsulation physical durability. Applied for passive temperature stabilization to result in building HVAC energy conservation.^[14]

Hygroscopic materials[edit]

Many natural building materials are hygroscopic, that is they can absorb (water condenses) and release water (water evaporates). The process is thus:

• Condensation (gas to liquid) ΔH<0; enthalpy decreases (exothermic process) gives off heat.
• Vaporization (liquid to gas) ΔH>0; enthalpy increases (endothermic process) absorbs heat (or cools).

Whilst this process liberates a small quantity of energy, large surfaces area allows significant (1–2 °C) heating or cooling in buildings. The corresponding materials are wool insulation and earth/clay render finishes.

Solid-solid PCMs[edit]

A specialised group of PCMs that undergo a solid/solid phase transition with the associated absorption and release of large amounts of heat. These materials change their crystalline structure from one lattice configuration to another at a fixed and well-defined temperature, and the transformation can involve latent heats comparable to the most effective solid/liquid PCMs. Such materials are useful because, unlike solid/liquid PCMs, they do not require nucleation to prevent supercooling. Additionally, because it is a solid/solid phase change, there is no visible change in the appearance of the PCM, and there are no problems associated with handling liquids, e.g. containment, potential leakage, etc. Currently the temperature range of solid-solid PCM solutions spans from -50 °C (-58 °F) up to +175 °C (347 °F).^[15]

Nano Enhanced Phase change Materials (NePCM)[edit]

Dispersing thermally conductive nanostructures is an effective method to improve the thermal performance of phase change materials (PCMs). For this purpose, nanocarbons, nanometals, and nano metal oxides have been used to develop nano-enhanced phase change materials (NePCMs) with unique thermal properties. In 2007, Khodadadi and Hosseinizadeh firstly reported on the accelerated freezing process of water enhanced by Cu nanoparticles. They reported that the freezing time of water was shortened from 3000 s to 1400 s by dispersing 20 vol.% of nanocopper. After that research, various aspects of NePCMs were investigated by other researchers.

Triplex shell-and-tube systems filled with NePCM, adding porous copper foam to NePCM, the effect of cavity aspect ratio on NePCM are some of these field that have been investigated yet.

Selection criteria[edit]

The phase change material should possess the following thermodynamic properties:^[16]

• Melting temperature in the desired operating temperature range
• High latent heat of fusion per unit volume
• High specific heat, high density, and high thermal conductivity
• Small volume changes on phase transformation and small vapor pressure at operating temperatures to reduce the containment problem
• Congruent melting
• Kinetic properties
• High nucleation rate to avoid supercooling of the liquid phase
• High rate of crystal growth, so that the system can meet demands of heat recovery from the storage system
• Chemical properties
• Chemical stability
• Complete reversible freeze/melt cycle
• No degradation after a large number of freeze/melt cycle
• Non-corrosiveness, non-toxic, non-flammable and non-explosive materials
• Economic properties
• Low cost
• Availability

Thermophysical properties[edit]

Common PCMs[edit]

Volumetric heat capacity (VHC) J·m⁻³·K⁻¹

${\displaystyle \mathrm {VHC} =\rho c_{p}}$

Thermal inertia (I) = Thermal effusivity (e) J·m⁻²·K⁻¹·s^−1/2

${\displaystyle I={\sqrt {k\rho c_{p}}}=e={(k\rho c_{p})}^{\frac {1}{2}}}$

Commercially available PCMs[edit]