Phase change materials for thermal energy storage

Progress in Materials Science 65 (2014) 67–123

Contents lists available at ScienceDirect

Progress in Materials Science journal homepage: www.elsevier.com/locate/pmatsci

Phase change materials for thermal energy storage Kinga Pielichowska a,⇑, Krzysztof Pielichowski b a AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Biomaterials, Al. Mickiewicza 30, 30-059 Krakow, Poland b Cracow University of Technology, Department of Chemistry and Technology of Polymers, ul. Warszawska 24, 31-155 Krakow, Poland

a r t i c l e

i n f o

Article history: Received 2 August 2013 Received in revised form 11 March 2014 Accepted 21 March 2014 Available online 2 April 2014

a b s t r a c t Phase change materials (PCMs) used for the storage of thermal energy as sensible and latent heat are an important class of modern materials which substantially contribute to the efficient use and conservation of waste heat and solar energy. The storage of latent heat provides a greater density of energy storage with a smaller temperature difference between storing and releasing heat than

Abbreviations: ABS, acrylonitrile–styrene–butadiene terpolymer; AC, active carbon; AMPL, 2-amino-2-methyl-1,3-propanediol; APP, ammonium polyphosphate; AS, acrylonitrile–styrene copolymer; BDO, 1,4-butanediol; CA, capric acid; CAC, cellulose acetate; CAlg, calcium alginate; CDA, cellulose diacetate; CEL, cellulose; CET, cellulose ether; CHS, composite heat sink; CLHS, cascaded latent heat storage; CMC, carboxymethyl cellulose; CNF, carbon nanofibre; CNT, carbon nanotube; DMAC, N,Ndimethylacetamide; DMSO, dimethylsulfoxide; DSC, differential scanning calorimetry; EDA, ethylenediamine; EG, expanded graphite; EGDS, ethylene glycol distearate; EP, epoxy resin; EPDM, ethylene propylene diene monomer rubber; EVA, ethylenevinyl acetate; FTIR, Fourier transform infrared spectroscopy; GNF, graphite nanofibre; HB-PUPCM, hyperbranched polyurethane PCM; HDPE, high-density polyethylene; HSU, heat storage unit; HTESS, hybrid thermal energy storage system; HVAC, heating, ventilation, air conditioning; IFR, intumescent flame retardant system; LA, lauric acid; LDPE, low-density polyethylene; LFTF, latent functional thermal fluid; LHS, latent heat storage; LHTES, latent heat thermal energy storage; LPG, liquefied petroleum gas; MA, myristic acid; MAPCM, molecular alloy PCM; MDI, 4,40 -diphenylmethane diisocyanate; MEPCM, microencapsulated phase change material; MF, melamine–formaldehyde resin; MMT, montmorillonite; NPG, neopentylglycol; OMT, organicallymodified montmorillonite; PA, palmitic acid; PANI, polyaniline; PCL, poly(e-caprolactone); PCM, phase change material; PD, pentadecane; PEG, poly(ethylene glycol), OH-terminated poly(ethylene oxide); PERT, pentaerythritol; PET, poly(ethylene terephthalate); PF, paraformaldehyde; PG, pentaglycerine; PMMA, poly(methyl methacrylate); PEO, poly(ethylene oxide); PPO, poly(propylene oxide); PS, polystyrene; PTHF, polytetrahydrofuran; PU, polyurethane; PV, photovoltaics; PVA, poly(vinyl alcohol); PVC, poly(vinyl chloride); RP, red phosphorus; SA, stearic acid; SAT, sodium acetate trihydrate; SEGS, solar thermal electricity generating system; SEM, scanning electron microscopy; SHS, sensible heat storage; SHTES, sensible heat thermal energy storage; SMA, styrene–maleic anhydride copolymer; SSPCM, shape-stabilized phase change material; SWCNT, single walls carbon nanotube; TD, 1-tetradecanol; TDI, toluene diisocyanate; TGA, thermogravimetric analysis; TEOS, tetraethoxysilane; TES, thermal energy storage; TESA, thermal energy storage aggregate; TESC, thermal energy storage concrete; TESM, thermal energy storage material; TPB, 1,4-polybutadiene; TRIS, tris(hydroxymethyl)aminomethane; UF, urea–formaldehyde resin; VMT, vermiculite; WAXD, wide angle X-ray diffraction. ⇑ Corresponding author. E-mail address: [email protected] (K. Pielichowska). http://dx.doi.org/10.1016/j.pmatsci.2014.03.005 0079-6425/Ó 2014 Elsevier Ltd. All rights reserved.

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Keywords: Phase change materials (PCMs) Thermal energy storage Encapsulation Shape stabilization Thermal conductivity Applications

the sensible heat storage method. Many different groups of materials have been investigated during the technical evolution of PCMs, including inorganic systems (salt and salt hydrates), organic compounds such as paraffins or fatty acids and polymeric materials, e.g. poly(ethylene glycol). Historically, the relationships between the structure and the energy storage properties of a material have been studied to provide an understanding of the heat accumulation/emission mechanism governing the material’s imparted energy storage characteristics. This paper reviews the present state of the art of PCMs for thermal energy storage applications and provides an insight into recent efforts to develop new PCMs with enhanced performance and safety. Specific attention is given to the improvement of thermal conductivity, encapsulation methods and shape stabilization procedures. In addition, the flame retarding properties and performance are discussed. The wide range of PCM applications in the construction, electronic, biomedical, textile and automotive industries is presented and future research directions are indicated. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal energy storage (TES) methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sensible TES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Latent TES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Solid–liquid PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Inorganic PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Organic PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Solid–solid PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Polyalcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PCMs encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Coacervation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Suspension polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Emulsion polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Polycondensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Polyaddition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Other methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Form–stable (shape-stabilized) PCMs (SS-PCMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. SS-PCMs with a polymer matrix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Polyethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Acrylics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3. Poly(vinyl chloride) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4. Polyurethanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5. Other polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. SS-PCMs with expandable graphite matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Other SS-PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Thermal storage in buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. Passive storage systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2. Active storage systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69 69 70 70 71 71 71 75 81 81 82 82 86 86 87 87 88 90 91 91 91 91 92 92 92 93 94 95 97 97 97 98

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6.2. Heating/cooling of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. The latent functional thermal fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Solar energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Smart textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Biomaterials and biomedical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7. Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8. Automotive industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9. Space applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.10. Food industry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Fire retardation of PCM-treated construction materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Long term stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69

100 100 101 102 103 104 106 106 106 107 108 109 111 112 112 112

1. Introduction Using phase change materials (PCMs) for thermal energy storage (TES) that can be released as sensible heat (SH) and latent heat (LH) became an important aspect for energy management following the 1973–1974 energy crisis. Today, the limited reserves of fossil fuels and concerns over greenhouse gas emissions make the effective utilization of energy a key issue. Using PCMs for TES provides an elegant and realistic solution to increase the efficiency of the storage and use of energy in many domestic and industrial sectors [1–7]. The application of PCMs for energy storage reduces the mismatch between supply and demand, improves the performance and reliability of energy distribution networks and plays an important general role in conserving energy [2,8–11]. PCMs exhibit a high enthalpy of fusion with the ability, in a relatively small volume, to store or release large amounts of energy as latent heat during melting and solidification. Additionally, practical PCMs require their upper and lower phase transition temperatures to be within the operational temperature range for a given application and possess high thermal conductivity for efficient heat transfer with congruent phase-change behaviour to avoid irreversible separation of their constituents [12,13]. During the development of PCMs, many different groups of materials have been studied, including inorganic compounds (salt and salt hydrates), organic compounds such as paraffins, fatty acids and even polymeric materials such as PEG. The relationship between the fundamental structure and the energy storage properties of these PCMs has been critically examined to determine the heat accumulation/emission mechanisms with reference to their ultimate energy storage characteristics. This paper reviews the present state of the art of phase change materials for thermal energy storage applications and provides a deep insight into recent efforts to develop new PCMs showing enhanced performance and safety. Specific attention is given to the improvement of thermal conductivity, encapsulation methods and shape stabilization procedures, as well as flame retarding properties. The broad range of PCM applications in the biomedical, electronic, textile, construction and automotive industries is presented and future research directions are outlined. 2. Thermal energy storage (TES) methods Thermal energy storage (TES) can be achieved by cooling, heating, melting, solidifying, or vaporizing a material with the energy becoming available as heat when the process is reversed. TES methods are classified as sensible heat thermal energy storage (SHTES) or latent heat thermal energy storage (LHTES). SHTES occurs when a material is driven to increase or decrease its temperature. The effectiveness of the method depends on the specific heat capacity of the material and, if volume is an important consideration, on the material’s density [5,14]. LHTES depends of a material’s ability to accumulate

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energy densities at almost isothermal conditions and over a narrow temperature range. Such phase change thermal energy storage systems offer a number of advantages over other systems (e.g. chemical storage systems), particularly the small temperature difference between the storage and retrieval cycles, small unit sizes and low weight per unit of storage capacity [15]. PCMs absorb energy as the phase change occurs during the heating process and then can release this energy during cooling [16]. 2.1. Sensible TES Sensible heat storage (SHS) involves storing thermal energy by raising the temperature of a solid or liquid. The principle is based on the material’s change of heat capacity and temperature during the process of charging and discharging. The amount of heat stored is a function of the specific heat of the medium, the temperature change and the mass of storage medium – Eq. (1) [4].

Q¼

Z

Tf

mC p dT ¼ mC ap ðT f  T i Þ

ð1Þ

Ti

where Q – quantity of heat stored (J), Ti – initial temperature (°C), Tf – final temperature (°C), m – mass of heat storage medium (kg), Cp – specific heat (J/kg K), Cap – average specific heat between Ti and Tf (J/ kg K).The storage materials absorb heat by the conventional heat transfer mechanisms of radiation, conduction and convection. As the materials cool at night or on cloudy days, the stored heat is released by the same modes. Active space heating systems commonly use tanks of water or rock bins as TESM. Water, stored in plastic, fibreglass, or glass-lined steel containers, is the typical thermal SHS medium for solar heating systems – as it absorbs heat, its temperature increases and the systems become warm to the touch. Water is the best SHS liquid available because of its low cost and high specific heat. [14]. 2.2. Latent TES Latent heat storage is a most efficient method of storing thermal energy. Latent heat storage (LHS) relies on the storage material absorbing or releasing heat as it undergoes a solid to solid, solid to liquid or liquid to gas phase change or vice versa. The storage capacity of a LHS system with a PCM [4] is given by:

Q¼

Z

Tm

mC p dT þ mam DHm þ

Ti

Z

Tf

mC p dT

ð2Þ

Tm

where am – fraction melted, DHm – heat of melting per unit mass (J/kg) [8]. LHS offers a much higher storage density with a narrower temperature range between storing and releasing heat than SHS [6,17,18]. The ideal PCM should meet a number of criteria related to the desired thermophysical, kinetic and chemical properties [8,17,19]: Thermal properties:    

a melting temperature in the desired operating range, a high phase transition latent heat per unit volume, a high specific heat, to provide significant additional SHS, high thermal conductivity of both phases. Physical properties:

    

a small volume change on phase transformation, a low vapour pressure at the operating temperature, favourable phase equilibrium, congruent melting of the PCM, a high density.

K. Pielichowska, K. Pielichowski / Progress in Materials Science 65 (2014) 67–123

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Kinetic properties:  no supercooling,  a high nucleation rate,  an adequate rate of crystallization. Chemical properties:     

long-term chemical stability, a completely reversible freeze/melt cycle, compatibility with the construction materials, no corrosion influence on the construction materials, it should be non-toxic, non-flammable and non-explosive to ensure safety.

The PCM should be readily available in large quantities at low cost [8,17,19]. In practice, those criteria are not fully met by most PCMs. However, recent progress in the design and characterization of novel materials for energy storage, including nanomaterials, has opened new possibilities for enhanced performance with extended lifetimes [8]. 3. Classification of PCMs Over the last 40 years different classes of materials, including hydrated salts, paraffin waxes, fatty acids, the eutectics of organic and non-organic compounds and polymers have been considered as potential PCMs. PCMs can be divided into three main groups – based on the temperature ranges over which the TES phase transition occurs: (i) low temperature PCMs – with phase transition temperatures below 15 °C, usually used in air conditioning applications and the food industry; (ii) mid temperature PCMs, the most popular – with phase transition temperatures in the range 15–90 °C with solar, medical, textile, electronic and energy-saving applications in building design; (iii) high temperature PCMs with a phase transition above 90 °C developed mainly for industrial and aerospace applications [6,20]. PCMs can be classified by their mode of phase transition: gas–liquid, solid–gas, solid–liquid and solid–solid systems, see Fig. 1. The applications of PCMs with a solid–gas or liquid–gas phase transition are limited in TES systems because of the large volume changes associated with the transition – even if they possess a high phase transition latent heat [12]. Significantly smaller volume changes occur, usually ca. 10% or less, with solid–solid and solid–liquid transformations. This makes them economically and practically attractive as materials for TES systems despite their smaller heat of phase transition [8]. Solid–solid PCMs employ the heat associated with the phase transition one to another crystalline form and can be considered as an alternative to solid–liquid PCMs [21]. Generally, the heat of phase transition for solid–solid PCMs is lower than that of solid–liquid PCMs. However, employing the former group of materials can avoid the problems of PCM leakage at temperatures above the phase transition temperature, a significant technical problem with solid–liquid PCMs [8,22,23]. 3.1. Solid–liquid PCMs Many different types of solid–liquid PCMs are employed for thermal storage applications, such as water, salt hydrates, paraffins, selected hydrocarbons, polymers and metal alloys. In the following subsections, the various classes of solid–liquid PCMs will be described. 3.1.1. Inorganic PCMs The various inorganic substances, eutectics and mixtures for low and high temperature applications that have been considered as potential PCMs are given in Tables 1–3. A number of the thermophysical properties, such as melting point, heat of fusion, thermal conductivity and density, is presented.

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Fig. 1. Classification of energy storage materials.

Table 1 Inorganic compounds with potential use as PCMs. Compound

Melting temperature (°C)

Heat of fusion (J/g)

Ref. No.

AlCl3 LiNO3 NaNO3 KNO3 Na2O2 KOH KClO4 LiH MgCl2 NaCl Na2CO3 KF LiF K2CO3 NaF MgF2

192 250 307 333 360 380 527 699 714 800 854 857 868 897 993 1271

280 370 172 266 314 150 1253 2678 452 492 276 452 932 235 750 936

[24] [24] [25,26] [26] [24] [26] [24] [24] [25] [25,26] [26] [25] [24] [26] [24] [24]

3.1.1.1. Salt hydrates. Salt hydrates with the general formula ABnH2O, are inorganic salts containing water of crystallization. During phase transformation dehydration of the salt occurs, forming either a salt hydrate that contains fewer water molecules:

ABn  nH2 O ! AB  mH2 O þ ðn  mÞH2 O or the anhydrous form of the salt

AB  nH2 O ! AB þ nH2 O

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K. Pielichowska, K. Pielichowski / Progress in Materials Science 65 (2014) 67–123 Table 2 Salt hydrates with potential use as PCM. Compound

Melting temperature (°C)

Heat of fusion (J/g)

LiClO33H2O KF4H2O Mn(NO3)26H2O CaCl26H2O LiNO33H2O Na2SO410H2O Na2CO310H2O NaCH3COO3H2O CaBr26H2O Na2HPO412H2O Zn(NO3)26H2O Zn(NO3)24H2O Zn(NO3)22H2O Na2S2O35H2O Na(CH3COO)3H2O Cd(NO3)24H2O Na2B4O710H2O Na3PO412H2O Na2P2O710H2O Ba(OH]28H2O (NH4)Al(SO4)212H2O MgCl26H2O Mg(NO3)26H2O

8 18.5–19 25.3 28.0–30.0 30 34 33 55.6–56.5 34 35–45 36 45.5 54 48–55 58 59.5 68.1 69.0 70 78 95 117 89.3

253 231 125.9 190–200 256 256 247 237–243 115.5 279.6 146.9

Thermal conductivity (W/mK) Liquid

Density (solid) (103 kg/m3)

Ref. No.

Solid [27] [17,28] [29] [17,28–31] [29] [29] [27] [32] [28] [28] [5] [33] [33] [28] [34] [27] [27] [27] [34] [5,27] [34] [5,34] [27]

1.45 0.540

1.088

1.80

0.476 0.464

0.514

2.19 1.52

201 226

1.75 1.45

184 266 269 169 150

Table 3 Eutectic and non-eutectic mixtures with potential use as PCM. Mixture

Melting temp. (°C)

Heat of melting (J/g)

Eutectic mixtures 45% CaCl26H2O + 55% CaBr26H2O 66.6% CaCl26H2O + 33.3% MgCl26H2O 50% CaCl2 + 50% MgCl26H2O 48% CaCl2 + 4.3% NaCl + 0.4% KCl + 47.3% H2O 47% Ca(NO3)24H2O + 53% Mg(NO3)26H2O 40% CH3COONa3H2O + 60% NH2CONH2 50% Na2SO410H2O + 50% NaCl 61.5% Mg(NO3)26H2O + 38.5% NH4NO3 58.7% Mg(NO3)26H2O + 41.3% MgCl26H2O 53% Mg(NO3)26H2O + 47% Al(NO3)29H2O 59% Mg(NO3)26H2O + 41% MgBr26H2O 14% LiNO3 + 86% Mg(NO3)26H2O 66.6% urea + 33.4% NH4Br 11.8% NaF + 54.3% KF + 26.6% LiF + 7.3% MgF2 35.1% LiF + 38.4% NaF + 26.5% CaF2 32.5% LiF + 50.5% NaF + 17.0% MgF2 51.8% NaF + 34.0% CaF2 + 14.2% MgF2 48.1% LiF + 51.9% NaF 63.8% KF + 27.9% NaF + 8.3% MgF2 45.8% LiF + 54.2% MgF2 53.6% NaF + 28.6% MgF2 + 17.8% KF 66.9% NaF + 33.1% MgF2

14.7 25 25 27 30 30 18 52 59 61 66 72 76 449 615 632 645 652 685 746 809 832

140 127 95 188 136 200.5

Non-eutectic mixtures H2O + polyacrylamide 50% Na(CH3COO)3H2O + 50% HCONH2

0 40

292 255

Density (kg/m3)

Ref. No.

2160 2225 2105 2370 1930 2090 2305 2110 2190

[8] [34] [8] [34] [8] [8] [29] [8] [8] [8] [8] [34] [13] [35] [35] [35] [35] [35] [35] [35] [35] [35]

125 132 148 168 180 161 (liquid) (liquid) (liquid) (liquid) (liquid) (liquid) (liquid) (liquid) (liquid)

[36] [37]

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Depending on the melting behaviour, the salt hydrates can be classified as:  salt hydrates with congruent melting behaviour – at the melting temperature the salt is soluble in the hydration water;  salt hydrates with incongruent melting – at the melting temperature salt is only partially soluble in the hydration water;  salt hydrates with semi-congruent melting – during the melting process the solid and liquid phases that are in equilibrium have different compositions because of the transformation of the salt hydrate to a salt hydrate with a lesser amount of hydration water [8]. Unfortunately, a large number of salt hydrates which could be considered as potential PCMs as they possess a large phase transition latent heat and suitable melting temperature melt incongruently. Consequently, the amount of released water is insufficient to dissolve the crystalline salt formed during the dehydration process. This leads to density differences, phase separation and sedimentation in containers causing serious technical problems in practical applications. To avoid or minimize this situation, Cabeza et al. postulated that segregation and sedimentation of the heavier phase could be avoided by adding gelling or thickening agents. The addition of a gelling (e.g. polymeric) material to the salt leads to the formation of a three-dimensional network to hinder salt sedimentation, while the addition of a thickening agent increases the viscosity of the salt hydrate and helps to hold the salt hydrate molecules together [32]. Another disadvantage of salt hydrates is their poor nucleating ability, which causes significant supercooling. To avoid this problem, nucleating agents are added, or small amounts of crystals are retained in the system to act as nucleation sites [8]. Additionally, there are reports of corrosion problems of metallic components in energy storage installations [38,39]. However, despite these disadvantages, salt hydrates are generally considered as suitable materials for TES applications because they possess large latent heat of fusion, appropriate phase transition temperature and they are very competitive in terms of economy and profitability. Within the large family of salt hydrates, the most frequently used is the low cost material calcium chloride hexahydrate (CaCl26H2O). However, its significant supercooling and high sensitivity to moisture are serious limitations for its long-term use [40]. CaCl26H2O was studied by Marinkovic et al. [40] in a binary mixture with calcium nitrate 4-hydrate (0.925 Ca(NO3)24.06 H20 + 0.075 CaCl26.11 H2O) and cobalt (II) chloride was introduced to control the light intensity in solar heated greenhouses. The absorption spectra exhibited a low absorbance in the visible spectral range at the melting temperature and a pronounced increase in the absorbance and change of colour occurred with an increase of temperature. The combined LHS and the change of the optical properties with temperature are desired features for agricultural applications – the increased absorbance with temperature increase is an auto-regulated shading protection from overheating [40]. Bilen and co-workers [30] investigated the parameters influencing the melting and the solidification time of CaCl26H2O as a PCM. They found that the effect of fins placed inside the PCM on the melting and solidification time was much more pronounced than the effects of the flow rate of water used as the heat transfer medium. Wu et al. [41] evaluated Al2O3/H2O nanofluid as a new PCM for the TES of cooling systems and showed that the addition of a 0.2 wt.% of Al2O3 nanoparticles significantly reduced the degree of water supercooling and reduced the total freezing time by ca. 20%. One solution proposed to avoid the problem of sedimentation during the melting and crystallization of salt hydrates is thickening the salt hydrate melts. Cabeza et al. studied the influence of bentonite, starch and cellulose (CEL) on the thermal behaviour of sodium acetate trihydrate. It was found that CEL exerts the best thickening effect on the PCM, but at temperatures higher than 65 °C phase separation occurred. The thermal behaviour of the mixtures was similar to that of the salt hydrate – the same melting point and an enthalpy decrease between 20% and 35%, depending on the type and amount of the thickening material used [32,38]. Nagano et al. [29] focused on Mn(NO3)26H2O as a new PCM for TES designed for cooling systems. Partial dissolving to yield MnCl24H2O made it possible to control the melting temperature and reduce the supercooling effect. In their later works [29,42] it was found that the addition of small amounts of MnCl24H2O to the Mn(NO3)26H2O shifted the melting point to more usable level. The TES in supercooled liquids, where the latent heat of fusion is released by triggering the crystallization of the supercooled substance, was investigated by Sandnes et al. [43]. More recently, Liu and coworkers [44]

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developed nanofluid PCMs by suspending small amount of TiO2 nanoparticles in a saturated BaCl2 aqueous solution. The nanofluid PCMs show remarkably high thermal conductivity and good phase change performance. Moreover, an increase of the cool storage/supply rate and capacity compared to a pristine BaCl2 aqueous solution was observed. 3.1.1.2. Alloys. Metallic alloys are used as high-temperature PCMs as they offer high thermal reliability and repeatability [20]. The largest phase transition heat, on a mass or volume basis, has been found for binary and ternary alloys of the relatively plentiful elements Al, Cu, Mg and Zn, but not all of the potential materials are suitable for use in TES systems [45]. Compared to other latent heat energy storage materials eutectic aluminium alloys were principally investigated for use as PCMs in high temperature TES systems because of their suitable phase change temperature, high latent heat density and good thermal stability [46]. Huang et al. [47] determined the specific heat of the liquid and solid forms and the latent heat of fusion of Al–Si, Al–Si–Mg and Al–Si–Cu alloys. The thermal reliability and corrosion behaviour of Al–Mg–Zn alloy with respect to the number of thermal cycles for TES systems were studied by Sun et al. [48]. They found that the melting temperature changed by 3–5 K and the latent heat of fusion decreased after 1000 thermal cycles. 3.1.2. Organic PCMs Organic PCMs are constituted by a wide range of materials including paraffins, fatty acids and their eutectic mixtures, esters and other organic compounds. The various compounds and a selection of their thermophysical properties are shown in Tables 4–7. Table 4 Paraffins with potential for use as a PCM. Paraffin

Number of carbon atoms in molecule

Melting temp. (°C)

Heat of fusion (J/g)

n-Tetradecane n-Pentadecane n-Hexadecane n-Heptadecane n-Oktadecane n-Nonadecane n-Eicozane n-Heneicozane n-Docozane n-Trikozane n-Tetracozane n-Pentacozane n-Hexacozane n-Heptacozane n-Oktacozane n-Nonacozane n-Triacontane

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

5.8–6.0 9.9–10.0 18.0–20.0 22–22.6 28.0–28.4 32.0 36.6 40.2 44.0 47.5 50.6 53.5 56.3 58.8 41.2 63.4 65.4

227–229 206 216–236 164–214 200–244 222 247 213 249 234 255 238 256 235 254 239 252

Density (g/cm3)

0.773 0.778 0.776 0.785 0.788 0.791 0.794 0.796 0.799 0.801 0.803 0.779 0.806 0.808 0.775

Ref. No. [29,49] [29,49] [28,29,49–51] [28,29,50,51] [28,29,50,51] [28] [28] [28] [28] [28] [28] [28] [28] [28] [28] [28] [28]

Table 5 Fatty acids with potential for use as a PCM. Fatty acid

Number of carbon atoms in molecule

Melting temp. (°C)

Heat of fusion (J/g)

Density (g/cm3)

Ref. No.

Caprylic acid CA LA MA PA SA Arachidic acid Undecylenic acid

8 10 12 14 16 18 20 22

16.3 31.3–31.6 41–44 51.5–53.6 61–63 70.0 74.0 24.6

148 163 183–212 190–204.5 203.4–212 222 227 141

– – 0.87 0.86 0.942 0.94 –

[52] [52,53] [54,55] [52,55–60] [53,58,60,61] [53,58,60] [52] [50,51]

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Table 6 Fatty acid eutectic mixtures with potential for use as a PCM. Fatty acid eutectic mixtures

Content (wt.%)

Melting temp. (°C)

Heat of fusion (J/g)

Ref. No.

CA/LA CA/PA LA/MA LA/PA LA/SA MA/PA MA/SA MA/SA PA/SA

45/55 76.5/23.5 66.0/34.0 69.0/31.0 75.5/24.5 58.0/42.0 64.0/36.0 65.7/34.3 64.2/35.8

17–21 21.8 34.2 35.2 37 42.6 44.1 50–52 52.3

143 171.2 166.8 166.3 182.7 169.7 182.4 162.0 181.7

[17] [62] [55] [63] [64] [57] [59,63] [58] [65]

Table 7 Other organic compounds with potential for use as a PCM. Class of compounds

Compound

Melting temp. (°C)

Heat of fusion (J/g)

Ref. No.

Monohydroxy alcohols

1-Dodecanol 1-Tetradecanol

17.5–23.3 39.3

184.0–188.8 221.23

[17,66] [67]

Ketones Ethers Esters of fatty acids

Phorone Diphenyl ether Methyl palmitate Allyl palmitate Propyl palmitate Methyl stearate Isopropyl palmitate Isopropyl stearate Butyl stearate Ethylene glycol distearate Dimethyl sebacate Methyl-12-hydroxystearate Vinyl stearate

27 27.2 27 23 16–20 38–39 11 14 17–23 63.2 21 43 27

123.5 97 163.2 173 186–190 160.7 100 142 140–200 215.80 135 126 122

[50] [50] [66] [51,66] [17] [66] [68] [68] [17,68] [69] [68] [68] [68]

Halogen derivative

1-Iodehexadecane Chlorobenzothiazole 3-Iodoaniline

22.2 18.6 23

131 65 64

[50,51] [50] [50]

Sulphur compounds

Octadecyl 3-mercapto-propionate Octadecyl thioglycolate Dilauryl thiopropionate

21 26 39

141 91 159

[68] [68] [68]

Amides

Acetamide

82

241

[8]

Sugars

Erythritol Mannitol Xylitol Sorbitol Granulated sugar White superior soft sugar

117 165 93 97 179 172

344 341 280 110 179 110

[70] [70] [70] [70] [70] [70]

Dienes

Dicyclopentadiene

28.7

11.7

[29]

Oleochemical carbonates

Decyl carbonate Dodecyl carbonate Tetradecyl carbonate Hexadecyl carbonate Octadecyl carbonate

2.2 19.3 33.7 44.9 51.6

144 200 227 219 223

[71] [71] [71] [71] [71]

The main groups of organic PCMs are described in the next subchapters.

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3.1.2.1. Low-molecular PCMs. 3.1.2.1.1. Paraffins. Paraffins (saturated hydrocarbons with CnH2n+2 formula), which constitute the broadly used solid–liquid PCMs, possess a high latent heat storage capacities over a narrow temperature range and are considered as non-toxic and ecologically harmless. Paraffin waxes exhibit moderate TES density, but a large surface area is needed as they have low thermal conductivity. This reduces their rate of heat charging and discharging during the melting and solidification cycles [72–75]. The latent heat of paraffins is molar mass-based and their various phase change temperatures give the flexibility to select an appropriate PCM for a specific LHTES application. They are economically viable and repeated cycling across the solid–liquid transition does not induce phase separation [76]. Paraffins between C5 and C15 are liquids with the higher analogs being waxy solids with melting temperatures ranging from 23 to 67 °C [23]. Commercial grade paraffin wax, which is a mixture of different hydrocarbons, is produced by the distillation of crude oil. In general, the longer the average length of the hydrocarbon chain, the higher the melting temperature and heat of fusion [77,78]. This relationship can be employed to design the PCM properties by mixing physically different paraffins. In fact, most paraffin PCMs are mixtures of saturated hydrocarbons with different numbers of carbon atoms in the molecules. The literature data show that after 1000–2000 cycles commercial grade paraffin waxes and other pure paraffins have stable properties and good thermal reliability. Paraffin waxes are safe, non-reactive and are compatible with metal containers as they do not promote corrosion. However, care must be taken when using plastic containers as paraffins’ chemical similarity and affinity can lead to infiltrations and softening of some polymers, especially polyolefins [78]. Recently, many research programmes have studied the thermal characteristics of paraffins during their melting and solidification processes [49,76–92]. He et al. investigated laboratory- and technical-grade n-paraffin waxes based on hexadecane, tetradecane and their mixtures. The authors revealed that the mixtures investigated exhibit relatively high heats of fusion and show congruent melting behaviour with little or no supercooling. Good stability and repeatability over a large number of heating/cooling cycles was observed, with less than 10% volume contraction during phase transition [49,84]. It was revealed that, except for the minimum-melting point mixture, all mixtures melt and freeze over a temperature range and not at a fixed temperature, as it has often been assumed. They concluded that the storage density in any temperature range is a function of mixture’s composition [82,85]. Kaygusuz and Sari [93] established that good performance, in terms of the thermophysical and thermal characteristics, was obtained with technical grade paraffin wax encapsulated in the annulus of two vertical concentric pipes forming a LHTES system. Paraffin-based molecular alloys PCMs (MAPCMs) were studied by Ventola et al. [87,94]. They suggested that it is possible to select the appropriate composition for a binary or multicomponent mixture of organic compounds to provide the best option for energy storage having a tunable operating temperature. Alkan studied PCMs based on docosane and hexacosane sulfonated in three different mole percentages [76]. DSC thermal analysis revealed that the sulfonated paraffins absorbed and dissipated more energy than the pure paraffins used in this study during melting and solidification. A composite PCM comprised of a blend of paraffin and an organically-modified montmorillonite (Org-MMT) was studied by Fang et al. [95]. The thermal characteristics of the blended composite PCM were close to those of paraffin alone and exhibited good stability over 1500 heating–cooling cycles with a higher rate of heat transfer rate than pure paraffin [95]. Recently, Sarier et al. [96] obtained an organoclay composite with good heat storage and release capacity by intercalating n-hexadecane with Na–montmorillonite (Na–MMT) in a surfactant-assisted medium as indicated in Fig. 2. This composite material showed higher thermal conductivity than pure n-hexadecane. The major problem with paraffins as PCM materials is that their thermal conductivity is too low to provide the required rate of heat exchange. Generally, an improvement of the thermal conductivity of a PCM by incorporating conductive particles results in a reduction in the energy storage capacity. It is important that new designs of paraffin-based TES systems increase the thermal conductivity but avoid a decrease in the ability to store energy [97].

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Fig. 2. The suggested scheme for surfactant-mediated intercalation of n-hexadecane to galleries of Na–MMT. Reprinted from [96] with permission from Elsevier.

3.1.2.1.2. Alcohols. Alcohols have been tested for energy storage applications for four decades, but recent advances in such PCMs are associated with nanomaterials or special composites with unique properties, such as the introduction of electrical conductivity. Zeng et al. [98] investigated 1-tetradecanol (TD)/nano-Ag composite materials, synthesized in situ in aqueous solution, for their ability to store energy storage. It was established that an increased quantity of Ag nanoparticles led to enhanced thermal conductivity of TD/nano-Ag composites. The same group investigated TD/polyaniline (PANI) composites prepared by in situ polymerization which can simultaneously conduct electricity and store thermal energy [67]. An important feature is that they possess the ability to endure a degree of thermal shock when used as conductive materials. 3.1.2.1.3. Fatty acids. The interest in fatty acids (CH3(CH2)2n-COOH) as PCMs for energy storage has increased recently as they possess desirable thermodynamic and kinetic characteristics for low temperature LHS. They exhibit a high latent heat of fusion, compared to that of paraffins, and reproducible melting and freezing behaviour, with little or no supercooling. However, fatty acids are more expensive than technical grade paraffins, are mildly corrosive and possess a disagreeable odour [8]. With an increasing number of carbon atoms in the fatty acids molecule, the melting and freezing points, the heat of melting and the degree of crystallization gradually increase. Carboxylic acids with an even number of carbon atoms in the structure possess higher values of thermal parameters than those with odd numbers of C-atoms. The former show a tendency for more regular alignment and a more dense crystalline lattice [99] arising from hydrogen bonding between the carboxylic acid molecules. The melting and boiling points of fatty acids are relatively high and the saturated fatty acids exhibit low phase transition volume changes with very little or no supercooling when freezing [100]. Dimaano et al. [101] studied a mixture of CA and LA as possible PCMs for low-temperature TES systems. In subsequent work the authors made an initial assessment of the thermal performance of CA–LA acid/pentadecane (PD) mixtures and found that the 50:50 CA–LA:PD mixture provided the highest change of enthalpy of all the combinations studied [102]. The authors also evaluated the thermophysical and heat transfer characteristics of the CA–LA blend with some organic additives using DSC [103]. The most effective additive was methyl salicylate, having the narrowest melting range and the highest heat of fusion of the investigated additives. Buddhi and Sharma [104] found that the transmissivity of the liquid phase of SA is relatively high. Because of its high transmittance and low thermal conductivity it can be used as a transparent insulation and can be employed for PCM Trombe walls, solar windows or PCM window shutters. Sari and Kaygusuz [105] studied the phase change stability of SA experimentally and found that its melting stability is better in the radial direction than in the axial direction. The authors established that the PCM was more effective with

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steady phase change characteristics when the heat exchanger tube was horizontal rather than vertical. The same results were found for LA [54], MA [56] and PA [61]. The research group [64] also determined the thermal characteristics of an eutectic mixture of LA and SA, during the melting and solidification processes in a vertical two concentric pipe-energy storage system. It was established that the LA–SA binary system had good thermal and heat transfer properties during the melting and solidification processes. In a parallel study, the thermal characteristics of eutectic mixtures of PA–SA [65], MA–PA [57], LA–MA [55,63], LA–PA, MA–SA [59,106], CA–PA, CA–SA [62,107] were investigated. Based on the results, the authors concluded that these PCMs have good thermal properties and thermal reliability. 3.1.2.1.4. Esters. Fatty acid esters show a solid–liquid transition over a narrow temperature range and their mixtures can form eutectics, similar to numerous inorganic salt mixtures, with little or no supercooling. Most of the fatty acid esters are commercially available as large quantities are produced for the polymer, cosmetics, textiles industries and other applications [108–111]. Eutectic mixtures of methyl stearate–methyl palmitate, methyl stearate–cetyl palmitate and methyl stearate–cetyl stearate with a phase transition temperature close to room temperature, a high enthalpy of transition and low hysteresis were studied by Nikolic et al. [110]. Interestingly, commercial building materials, gypsum and bricks were impregnated with the selected molten esters by immersion. DSC results revealed that up to 30 wt.% of the esters could be absorbed so forming composite inorganic–organic PCMs. The thermal properties of ethylene glycol distearate (EGDS), synthesized from ethylene glycol and SA by direct esterification reaction, were investigated by Alkan et al. [69]. Compared to paraffin-based and fatty acids-based PCMs, EGDS has the advantages of greater energy storage capacity per unit mass, is less odorous and, since there are no acidic functional groups, less corrosive. Suppes et al. [112] studied triglyceride and alkyl esters of stearic, palmitic and oleic acids, with latent heats comparable to those of the commonly used paraffins, to identify candidate PCMs for HVAC (heating, ventilation, air conditioning) applications in buildings. However, as a PCM, triglycerides will encounter performance problems related to polymorphism – a problem not found with methyl stearate and methyl palmitate. 3.1.2.1.5. Others. In the search for novel PCMs, a binary mixture of urea–sodium acetate trihydrate (U-SAT) was modified by the addition of acetate trihydrate to raise its melting point to a practical level in order to store solar energy. Experimental results for the storage system showed that the melting point was increased to 44.5 °C from 32 °C for the system without the additive [113]. Kaizawa et al. [70] studied the thermophysical properties of various organic compounds and SAT as PCMs and the heat transfer between the PCM and an oil in PCM bath. Of the compounds investigated, erythritol was found to be the most suitable material for TES applications. Canik and Alkan [114,115] prepared hexamethylene dilauroyl, dimyristoyl, and dipalmytoyl amides as solid–liquid PCMs by the condensation of hexamethylene diamine with the respective acyl chlorides – lauroyl chloride, myristoyl chloride, and palmytoyl chloride. The highest phase change enthalpy was found for hexamethylene dipalmytoyl amide. Recently, Kenar [71] has proposed new organic PCMs based on oleochemical carbonates. These substances are biobased materials which may be readily prepared using a carbonate interchange reaction between renewable C10–C18 fatty alcohols and dimethyl or diethyl carbonate in the presence of a catalyst as illustrated in Fig. 3.

Fig. 3. Overall synthetic scheme to prepare oleochemical carbonates from renewable fatty alcohols. Reprinted from [71] with permission from Elsevier.

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As yet, oleochemical carbonates have not been subjected to detailed study regarding their applicability as PCMs. It was revealed that they exhibited sharp phase transitions and good latent heat properties. As such they offer a novel class of renewable-based PCM materials that could supplement fatty acids, fatty alcohols, and fatty acid esters in energy storage applications. Additionally, they are potentially valuable biobased alternative to paraffin waxes and salt hydrates which currently dominate the PCMs market. 3.1.2.2. Polymeric PCMs. 3.1.2.2.1. Poly(ethylene glycol). PEG, an OH-terminated poly(ethylene oxide), is an important semi-crystalline polymer with a repeating unit of –CH2–CH2–O–. It is used in water paints, textile fibres, paper coatings, as a component of packaging materials and as a solubilising agent in drugs. A relatively new PEG application area is related to the TES – it can be used as a PCM as it has a large heat of fusion which is attributed to a high degree of crystallinity [116]. Molecular weight is the key issue for PEG’s application as a material for TES – see Table 8. The melting point of PEG is dependent on the molecular weight and may vary from ca. 4 to 70 °C, with the heat of fusion in the range of 117–174 J/g. An increase in the molecular weight of PEG causes an increase in the melting temperature and the heat of phase transition. The molecular weight also influences the degree of crystallinity which ranges from 83.8% to 96.4% [118]. Pielichowski and Flejtuch [100] investigated a series of binary blends of three polyethers: poly(ethylene glycol) (PEG), poly(propylene oxide) (PPO), and polytetrahydrofuran (PTHF), characterized by similar average molecular weights, with selected fatty acids (CA, LA, MA, PA and SA) as TESMs. A synergistic effect was found to occur for PEG/fatty acid blends, as evidenced by the values of the enthalpy of the phase Table 8 Temperature and heat of fusion for PEG with various molecular weights. Polymer

Melting temperature (°C)

Heat of fusion (J/g)

Ref. No.

PEG PEG PEG PEG PEG PEG PEG PEG PEG

4.2 12.5 40.0 63.4 65.9 67.7 68.7 67.0 70.0

117.6 129.1 168.6 166.8 171.6 160.2 166.9 175.8 174.0

[117] [29] [118] [99,118] [99,118] [99,118] [99,118] – –

400 600 1000 3400 10000 20000 35000 100000 1000000

Table 9 Melting temperature and heat of fusion of PEO/fatty acid blends. Mixture

Melting temperature (°C)

Heat of fusion (J/g)

Ref. No.

PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO PEO

32.2/42.0 47.1 50.7/54.9 51.2/58.0 54.2/60.3/68.9 31.3/55.1 33.3/50.1 33.3/43.9 46.8 46.4/52.3 44.4/56.5 57.1 57.3 58.7 58.1/63.3 63.2 61.6 59.4/72.2 60.4/68.9 63.8

169 188 207 209 205 162 174 174 195 203 184 190 185 191 206 205 192 252 211 203

[100] [100] [100] [100] [100] [117] [99] [117] [117] [99] [117] [117] [99] [117] [117] [99] [117] [117] [120] [120]

3400/CA 1/1 w/w 3400/LA 1/1 w/w 3400/MA 1/1 w/w 3400/PA 1/1 w/w 3400/SA 1/1 w/w 10000/CA 1:3 w/w 10000/CA 1:1 w/w 10000/CA 3:1 w/w 10000/LA 1:3 w/w 10000/LA 1:1 w/w 10000/LA 3:1 w/w 10000/MA 1:3 w/w 10000/MA 1:1 w/w 10000/MA 3:1 w/w 10000/PA 1:3 w/w 10000/PA 1:1 w/w 10000/PA 3:1 w/w 10000/SA 1:3 w/w 10000/SA 1:1 w/w 10000/SA 3:1 w/w

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transition [119]. In further work a series of blends of PEG with different molecular weights with CA, LA, MA, PA or SA as TES materials were investigated using DSC. The results are presented in Table 9 [99]. Detailed studies were made on PEG/LA and PEG/SA blends with different compositions which were characterized by DSC and IR [120–122]. Again, synergistic action of the components was found for PEG 10000/SA and for PEG/LA blends – the heats of transition were respectively ca. 15% and 35% higher than for pure fatty acid and PEG [99]. The origin of this effect probably arises from the formation and decay of hydrogen bonding during the phase transition process. FTIR spectra of the blends investigated, collected during melting and crystallization, show only one maximum band from m(C@O) in the solid state, indicating that C@O groups are engaged in the formation of hydrogen bonds. In the liquid state there are two maxima – the position of the first being characteristic of m(C@O) vibrations of C@O groups that do not participate in the formation of hydrogen bonds, whereas the position of the second maximum in the liquid state proves the presence of C@O groups being involved in formation of hydrogen bonds [119–122]. In recent work, carbon nano-nucleating agents; carbon black, graphite, carbon fibres, single walls carbon nanotubes (SWCNTs) and fullerenes, were incorporated into a PEG matrix to increase the nucleation rate, the enthalpy of phase transition and the thermal conductivity while reducing the degree of supercooling. It was found that in PEG/carbon-based systems, the temperature range and heat of phase transition depends on the composition of the blends and type of the carbon additive. Moreover, for all systems a higher heat of melting than the theoretical values was detected – the highest being found for the PEG/fullerene (90/10 w/w) and PEG/SWCNT (90/10 w/w) systems [123]. 3.2. Solid–solid PCMs Currently, it is mainly solid–liquid PCMs that are studied and used in energy storage applications because the solid–solid PCMs generally show smaller latent heat of phase transition. However, the solid–solid PCMs have the major advantages of a smaller volume change during the phase change than solid–liquid PCMS and they cannot leak [124]. 3.2.1. Polyalcohols Polyalcohols such as glycerine, pentaerythritol (PERT) [C(CH2OH)4], pentaglycerine (PG), [CH3–C(CH2OH)3], neopentylglycol NPG [(CH3)2C(CH2–OH)2], tris(hydroxymethyl)aminomethane (TRIS) [(NH2)C(CH2OH)3], 2-amino-2-methyl-1,3-propanediol (AMPL) [(NH2)(CH3)C(CH2OH)2] and others, are potential TES materials as they exhibit solid-state phase transformations. At low temperature, polyalcohols are heterogeneous, but they form a regular face-centred cubic crystalline phase able to absorb the hydrogen bond energy when the temperature rises to the solid–solid phase transition temperature – see Table 10. Experiments have revealed that the polyalcohols experience a first-order phase transition with zero change in their Gibbs energy [21,124]. The polyalcohols undergo a small change in volume and do not segregate or suffer phase separation; even more important is the possession of high enthalpies and low temperatures of phase transition. These advantages are of great interest for energy storage [127]. Feng et al. [127] studied the infrared spectra of PERT, PG, NPG and the mixture NPG/PG at various temperatures. The results of the FTIR experiments, the shifts of –OH absorption bands, revealed the mechanism at the solid–solid phase transition, which involves the reversible breaking of Table 10 Polyalcohols as SS-PCMs. PCM

Phase transition temperature (°C)

Heat of phase transition (J/g)

Ref. No.

Polyhydroxy alcohols Glycerine Pentaerythritol Pentaglycerine Neopentylglycol tris[Hydroxymethyl]aminomethane 2-Amino-2-methyl-1,3-propanediol

18.2 185–187 82 42–44 132.4–134.5

289.0–339.5 172.58 110.4–119.1 285.3–295.6

[29] [21,125,126] [21,127] [21,125–127] [21,125,126] [21]

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nearest-neighbour hydrogen bond in the molecular crystals. It was also shown that ageing has a substantial influence on the thermal properties of polyalcohol mixtures. Wang et al. studied the heat storage performance of binary systems – NPG/PERT and NPG/TRIS as solid–solid PCMs [124,126]. The effects of some important factors, such as the sealing of the container, adding silicone oil and graphite powder to the mixture and the preparation routes were also discussed. Chandra et al. [21] established phase diagrams for AMPL/NPG, AMPL/PERT and AMPL/TRIS binary systems and is of interest that two high temperature phases co-exist in all three systems. The phase change temperatures and heats of transition of single, binary and three-component systems consisting of NPG, PERT and TRIS with different compositions were studied experimentally by Quanying et al. [125]. They found that the phase change temperatures and heats of binary system NPG/PERT and NPG/TRIS were much lower than those of single polyalcohols and that the lowest phase change temperature and heat was displayed by the three-component NPG/PERT/TRIS system. 3.2.2. Others He et al. [128] synthesized and investigated bis-alkylammonium tetrachlorometallates (II) – (C14H29NH3)2CuCl4(s) and (C15H31NH3)2CuCl4(s) as new solid–solid PCMs. These are a type of layered perovskite compounds with an orderly magnetic transition at low temperatures which show fluorescence. It was found that in the temperature region from 78 to 395 K two solid to solid–solid phase transitions occur for each of the two compounds studied. 3.2.3. Polymers Polymeric materials are beginning to play a growing role as PCMs. They offer the scope for making significant chemical and physical modification which may facilitate the production of specific end products with ‘tailor made’ energy storage properties. Ding et al. prepared by chemical modification and physical blending PCMs of cellulose diacetate (CDA) and PEO. It was found that the PCMs exhibited different characteristics: the chemically linked materials were solid–solid PCMs, but the blended materials were solid–liquid PCMs. In the former category the PEO end groups were chemically linked with CDA. In the PEO/CDA blends, the PEO and CDA were integrated only by relatively weak specific interactions. Consequently, at or above the melting temperature of PEO, its amorphous phase in the blends became free and migrated within the CDA frame, even to the blends’ surface [129]. Guo et al. [130] studied PEO/CEL blends obtained from a solution in N,N-dimethylacetamide/lithium chloride (DMAC/LiCl) and dimethylsulfoxide/paraformaldehyde (DMSO/PF). The authors found that the phase change properties of PEO/CEL blends were related to their miscibility in the solvents. In DMAC/LiCl, the miscibility between CEL and PEG is limited; the obtained composites exhibited a solid–liquid phase transition. But in DMSO/PF the composites underwent a solid–solid phase change with a larger enthalpy of phase transition [130]. In other work, Guo et al. [131] investigated the phase change behaviour of PEG/CDA blends by mixing PEG with CDA in acetone. It was observed that the phase-change behaviour in the obtained solid composite could be changed from a solid–liquid to a solid–solid phase transition. If the PEG fraction within the composite was less than 85%, the composite exhibited solid–solid phase change, but with more PEG it possessed a solid–liquid transition. Jiang et al. [132] grafted PEG onto a CDA skeleton and obtained copolymers having typical solid–solid phase transition properties and a good ability to store energy. Recently, Li et al. [133] have synthesized CEL-graft-PEG copolymers (Fig. 4) that showed a solid–solid phase-transition which offered a high thermal storage density and high thermal stability. The phase transition was that between the crystalline and amorphous states of the PEG side chains in the CEL-graft-PEG copolymers. The phase transition temperature could be adjusted from room temperature to 50 °C by changing the content and molecular weight of the PEG side chains. In subsequent work by this group, a series of CEL-graft-PEG copolymers was synthesized in an ionic liquid by using 4,40 -diphenylmethane diisocyanate as a coupling reagent as shown in Fig. 5. The thermal conductivity of copolymers was significantly improved by the addition of up to 5 wt.% expanded graphite [134]. More recently, a series of PEG blends with CEL or CEL derivatives – carboxymethyl cellulose (CMC), cellulose acetate (CAC) and cellulose ether (CET), have been investigated as possible PCMs for TES. For PEO/CEL blends a solid–solid phase transition was observed over the whole concentration range, while for PEO/CMC and PEO/CET blends a solid–solid phase transition occurred only for a PEO

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Fig. 4. Synthesis of the CEL-graft-PEG copolymer. Reprinted from [133] with permission from Wiley.

Fig. 5. Synthesis of CEL-graft-copolymer. Reprinted from [134] with permission from Elsevier.

content of 25 or 50 wt.% in the former and 25 wt.% in the latter [135]. Chen and co-workers [136] obtained ultrafine composite fibres based on CAC and PEG with five molecular weight grades as a novel class of form–stable PCMs by electrospinning. Because of the shielding and supporting effect of the CAC matrix on the PEG dispersed crystalline regions, the electrospun composite fibres showed defect-free form–stable morphology and revealed better thermal properties than those of the corresponding cast films after repeated thermal cycles treatment – Fig. 6. Moreover, with the increase of the molecular weight of PEG, the tensile properties of the PEG/CA composite fibres increased from 6.0 to 7.8 MPa [136]. In subsequent work the same research group studied toluene diisocyanate (TDI) – crosslinked electrospun PEG/CAC composite fibres. They found that crosslinking enhanced the water-resistance and the thermal stability of the fibres, but reduced the enthalpy of the fibre’s phase transition. The morphology and average diameter of the composite fibres varied with the PEG content [137]. Ultrafine composite fibres consisting of LA, poly(ethylene terephthalate) (PET), and silica nanoparticles (nano-SiO2) were prepared by electrospinning [138]. SEM microphotographs revealed that the LA/PET/n-SiO2 composite fibres possessed the desired morphology with a smaller average fibre diameters than those of LA/PET fibres without the

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Fig. 6. SEM images of the electrospun PEG 10000/CA fibres before (a) and after (b) 100 heating–cooling cycles. Reprinted from [136] with permission from Wiley.

Fig. 7. Schematic representation of the hydrogen bonding interactions between nano-SiO2, LA and PET in the ultrafine composite fibres. Reprinted from [138] with permission from Elsevier.

nano-SiO2. This was probably due to the increased conductivity of the spin dopes and the strong hydrogen bonding between the (nano)components of the fibres – Fig. 7. DSC measurements indicated that the amount of nano-SiO2 in the fibres had an influence on the crystallization of LA and on the heat of phase transition but it had no appreciable effect on the phase change temperatures of the composite fibres. Novel biodegradable PEO/potato starch blends as form–stable PCMs were obtained by Pielichowska and Pielichowski [139]. DSC results and microscopic investigations showed that the presence of starch changed the PEO phase transition behaviour and that it was dependent on the ratio of the components. A solid–solid phase transition for a PEO/starch 1:3 and 1:1 w/w was observed but for a PEO/ starch blend 3:1 w/w a solid–solid phase transition with partial melting occurred. The heat of phase transition depended on the strength and the quantity of hydrogen bonds in the blends – the depression of the heat of phase transition and the crystallinity of the blends implied that there is a strong intermolecular interaction between starch and PEO. A similar approach was used by Alkan et al. [140], who studied PEG/CEL, PEG/agarose, and PEG/chitosan blends as form/stable PCMs. 3.2.3.1. Polyurethanes. Polyurethanes constitute a class of versatile macromolecular materials that derive most of their useful properties from the incompatibility of the hard segments, made from

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diisocyanate and a chain extender, and soft segments usually composed of long-chain diols. Specific interactions between the phases via hydrogen bonding, connected with phase intermixing and reorganization processes, influence the microdomain structure and properties of these promising macromolecular candidates for energy storage applications. Cao et al. [141] prepared a series of hyperbranched polyurethane copolymers (HB-PUPCM) using a branched polyester as a chain extender. The copolymers obtained show the typical solid–solid phase transition behaviour. The reversible phase transition between the amorphous and the crystalline phase of the soft PEO segments accounted for the good energy storage whilst the hard segments in HB-PUPCM served as a skeleton to restrict the free movement of the molecular chains of PEO at high temperature. When the temperature was raised to the melting point of PEO, the polyurethane did not melt into the liquid unlike the PEO, but changed to an amorphous solid state. In HB-PUPCM, the PEG molecules are probably tied to the hard segments of the chain by the chemical bonds and so cannot liquefy, but they do lose their crystallinity during the phase transition [142]. Su and Liu [143] synthesized a polyurethane solid–solid PCM (PUPCM) comprised of a high molecular weight PEG as the soft segment with 4,40 -diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO) together as the hard segment. DSC results indicated that the PUPCM showed the typical solid–solid phase transition where the hard segments acted as physical cross-links, restricting the molecular chain of the soft segment’s free movement at higher temperature. A solid–solid phase change heat storage material was also prepared via the condensation reaction of PEG 10000 with tetrafunctional pentaerythritol isocyanate by Li and Ding [144]. The results showed that PEG/MDI/PE tertiary cross-linked copolymer had a typical solid–solid phase transition property, retained its solid state when heated to 150 °C and had a suitable transition point. Meng and Hu [145] synthesized a shape memory thermoplastic polyurethane (PU) as a PCM by employing PEG as the soft segment by bulk polymerization. They reported that the PEG-based polyurethane (PEGPU) had a well-formed phase separation structure which accounted for most of the material’s phase change properties and shape memory effect. Polyurethanes containing PEG segments have also been synthesized as solid–solid PCMs for thermal energy storage using three Table 11 Thermal properties of polyurethane-based PCMs. Polyol

Isocyanate

Chain extender

Soft segments content (%)

Heat of phase Ref. Temperature transition (J/g) No. of phase transition (°C)

PEG 6000

MDI

BoltornÒ H20

Toluene-2,4diisocyanate (2,4-TDI) MDI MDI Isophorone diisocyanate (IPDI) Hexamethylene diisocyanate (HMDI) HMDI HMDI IPDI IPDI TDI TDI MDI MDI MDI MDI MDI MDI

BoltornÒ H30

67 66 62 57 67

138 118 103 91 125

[141]

PEG 6000

90 80 70 60 Stoichiometric

BDO Pentaerythritol BDO

Stoichiometric 65 Stoichiometric 59 Stoichiometric 47

139 153 114

[143] [144] [145]

–

Stoichiometric 19

109

[146]

60 58 58 59 57 57 40 41 50 57 59 56

176 171 166 169 161 162 118 109 145 132 145 137

PEG 10000 PEG 10000 PEG 3400 PEG 1000 PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG PEG

6000 10000 6000 10000 6000 10000 1000 2000 3000 6000 8000 6000

– – – – – – – – – – – Tetrahydroxy compound

Stoichiometric Stoichiometric Stoichiometric

Stoichiometric

[142]

[146] [146] [149]

[148]

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different kinds of diisocyanate by Alkan et al. [146]. The results showed that some PUs had typical solid–solid phase transition properties with high enthalpies reaching 179 J/g, and transition points between 19 and 60 °C. Pielichowska and Szatkowski [147] obtained polyurethane-based PCMs modified with graphene using a single step bulk polymerization method. The introduction of graphene increased the thermal stability of the PUPEG matrix up to 7 °C and the thermal conductivity. An interesting form–stable thermoplastic polyurethane solid–solid PCM was prepared using a tetrahydroxyl compound. The authors suggested that when PEG 4000 was chosen as the branch chain, TPUPCM showed outstanding processing and mechanical properties, and could be processed directly [148]. The thermal properties of polyurethane-based PCMs are presented in Table 11. 3.2.3.2. Others. Trans-1,4-polybutadiene (TPB) has also been studied as a potential PCM [150]. TPB, which has two stable crystalline structures, transforms reversibly between them on heating or cooling at a temperature 78.1 °C. The theoretical transition enthalpy at equilibrium phase transition is relatively large, 144 J/g, which is approximately twice as large as that of a melting endotherm. Elsewhere, Han et al. [151] suggested that cross-linked high-density polyethylene (HDPE) has a thermally stable form and it could be utilized as a TES material. The thermally stable form of HDPE does not require separate packaging which would inevitably increase the cost of the TES system. Recently, an environmentally friendly PHBV/PEG copolymer network to be employed as a PCM has been synthesized by free-radical solution polymerization with PHBVDA and PEGDA as macromers. This network had a high latent heat enthalpy (107 J/g), and had good thermal stability to above 300 °C [152]. 4. PCMs encapsulation (Micro)encapsulation techniques provide opportunities to fabricate advanced PCMs with a greater heat transfer area, reduced reactivity with the outside environment and controlled volume changes during the phase transition [6]. For these reasons, microencapsulated phase change materials (MEPCMs) have attracted considerable attention for over 20 years [153–156]. Microcapsules can be described as particles that contain core material surrounded by a coating or shell [157] and have diameters in the 1–1000 lm range. Microencapsulation is widely used in commercial applications including carbonless copying paper, functional textiles, adhesives, cosmetics, pharmaceuticals and other medical applications [158–165]. MEPCMs have also been used in solar energy installations and advanced building materials [166–168]. However, the potential use of microencapsulated PCMs in various thermal control applications is limited by their cost. For PCM encapsulation MEPCMs need the appropriate properties, such as desired morphology, proper diameter distribution, thermal stability, shell mechanical strength, and penetration abilities [166]. A literature survey on MEPCMs indicates that urea–formaldehyde (UF) resin, melamine–formaldehyde (MF) resin and polyurethanes (PU) are usually selected as the microcapsule shell materials for the protection of PCMs [169]. Different encapsulation techniques may be applied to prepare microcapsules with a polymer cover and a PCM core. The strategies employed involve complex coacervation, suspension, emulsion, condensation or polyaddition polymerization. The characteristics of different MEPCMs are given in Table 12. 4.1. Coacervation Coacervation is an encapsulation technique that involves the use of more than one colloid [83,170]. Coacervation results from the mutual neutralization of two or more oppositely charged colloids in an aqueous solution. As a result of the reduction, the coacervated particles separate out to form two new phases with rich and poor colloid concentrations [175]. Hawlader et al. [158] investigated the influence of different parameters on the characteristics and performance of a MEPCM in terms of the encapsulation efficiency, energy storage and release capacity. They revealed that complex coacervation and spray drying methods could both be used to prepare microcapsules of paraffin wax. Microcapsules of natural coco fatty acid mixture were prepared using coacervation methods by Ozonur et al. [159]. The microscopic results showed that microcapsules produced by the coacervation process attain a geometrically spherical shape and FTIR spectra revealed

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K. Pielichowska, K. Pielichowski / Progress in Materials Science 65 (2014) 67–123 Table 12 Characteristics of MEPCMs. Microcapsules size (lm)

Ref. Heat of No. phase transition (J/g)

Encapsulation method

PCM

Shell material

Coacervation Coacervation Coacervation Emulsion polymerization Emulsion polymerization Polyaddition

Paraffin Paraffin wax Coco fatty acid n-Octacosane

Gelatine and acacia Gelatin and acacia Melamine–formaldehyde 1–1000 PMMA 0.15–0.33

29–31 50.6

86

[170] [158] [159] [171]

Docosane

PMMA

0.14–0.47

41

55

[169]

Paraffin mixture

Epoxy resin

189

[172]

Polycondensation

Hexadecane

5–20

210

[160]

Polycondensation

Octadecane

5–20

28

150

[160]

Polycondensation

Lauryl alcohol

110

[173]

Polycondensation Polycondensation

Butyl stearate n-Dodecanol

Polycondensation Polycondensation

n-Tetradecane n-Octadecane

Melamine–formaldehyde resin Melamine–formaldehyde resin Melamine–formaldehyde resin Polyurea Melamine–formaldehyde resin Urea–formaldehyde resin Polyurea

45 ± 0.5 °C and 58 ± 0.5 °C 23

Temperature of phase transition (°C)

86 193–221

5–10 20–35 Mean diameter of 30.6 Ca. 100 nm 3–25

29 21.5

80 [162] 87–187.5 [163]

5–9 26–28

100–130 153–189

[164] [174]

that chemical stability of the mixture was not affected by microencapsulation. Bayes-Garcıa et al. [176] studied novel phase change microcapsules obtained by using two different bio-based coacervates: sterilized gelatin/arabic gum (SG/AG) and agar-agar/arabic gum (AA/AG). 4.2. Suspension polymerization Microcapsules with a polymer cover and a PCM core can be obtained by a process based on suspension polymerization. This process generally involves the dispersion of a monomer, mainly as small droplets of liquid, into an appropriate medium with the polymerization initiator being dissolved in the monomer. For PCM microencapsulation, the site for the generation of free radicals will be the interface between water and the oil droplet with the paraffin inside the forming polymer covering layer. This method can be regarded as an ‘‘inside-out’’ approach for PCM microencapsulation [175]. Sanchez et al. [175] microencapsulated various PCMs with a polymer shell of polystyrene by employing a suspension free radical polymerization process; it was possible to obtain particles where the PCM comprised almost 50% of the microcapsule mass. Recently, Li et al. [177] fabricated a series of MEPCMs by suspension-like polymerization with n-octadecane as the core, and styrene–1,4-butylene glycol diacrylate copolymer (PSB), styrene–divinylbenzene copolymer (PSD), styrene–divinylbenzene–1,4-butylene glycol diacrylate copolymer (PSDB), or polydivinylbenzene (PDVB) as the shell. 4.3. Emulsion polymerization PMMA microcapsules with a controlled narrow particle size distribution and containing docosane were successfully prepared by emulsion polymerization [169]. SEM analyses revealed that the MEPCMs had a compact surface and the average capsule diameter of 160 lm. The authors concluded that PMMA/docosane microcapsules were reliable as a TESM based on thermal cycling tests, and the high temperature resistance [169]. In other studies PMMA microcapsules containing n-octacosane or n-eicosane were investigated [171,178]. The best PCM properties were exhibited by the PMMA microcapsules containing 43 wt.% of n-octacosane as a core.

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4.4. Polycondensation An in situ polycondensation process was applied by Su et al. to prepare a series of melamine–formaldehyde (MF) microcapsules with a PCM core [166]. The study demonstrated that the shell structures of MEPCMs can be controlled by the formation process using an optimum dropping rate of shell material of 0.5 ml/min and a temperature increase of 2 °C/10 min. DSC revealed that the melting point of the PCM in shells remained virtually unchanged to that of an uncapsulated PCM [166]. In a subsequent development, double-shell-structured microcapsules with melamine–formaldehyde resin as the coating material of PCM were fabricated [173]. Compression tests on the single and double shell structured MEPCMs showed that the latter structure had greater mechanical stability than the former one [173]. Lee and co-workers [160] prepared MEPCMs by in situ polycondensation of melamine and formaldehyde for the shell with hexadecane or octadecane as the core. They found that particle size decreased and its uniformity was enhanced if the mixing intensity during emulsification was increased. Additionally, MEPCMs were incorporated in gypsum and it was established that the thermal fluctuations in such PCM-building materials were smoother and smaller than these in building materials without PCMs. To evaluate the heat storage characteristics of the material, gypsum wallboards containing MEPCMs were produced as shown in Fig. 8. More recently, Yu et al. [163] studied the effects of the polarity of the PCM and the types and amounts of emulsifier on the properties of microencapsulated PCMs. They synthesized microcapsules containing polar PCM (n-dodecanol) by in situ polycondensation using melamine–formaldehyde resin as the shell and styrene–maleic anhydride copolymer (SMA) as an emulsifier. The results showed that the type and amount of SMA emulsifier had a major influence on the properties and morphology of the MEPCMs. SMA was suitable for the encapsulation of n-dodecanol, and an increase in the amount of emulsifier initially caused the phase change latent heat and encapsulation efficiency to increase, but then to decrease [163]. Capsules containing paraffin as phase change core were synthesized in situ by Jin et al. [179] by the absorption and condensation polymerization of urea–formaldehyde pre-polymer onto the core using hydrolyzed styrene–maleic anhydride copolymer in an aqueous phase as the emulsifier, see Fig. 9. This approach can be extended to other paraffins having tunable melt/crystallization temperatures. Fang et al. [164] obtained PCM nanocapsules containing n-tetradecane by in situ polycondensation of urea and formaldehyde. SEM analysis indicated that the nanocapsules size was ca. 100 nm and that the core material was well encapsulated. Moreover, the thermal stability of the nanocapsules could be improved by a NaCl addition during the polymerization process. Peng et al. [172] encapsulated low melting temperature paraffin wax PCMs in cured epoxy resin or styrene–ethylene–butylene (SEB) terpolymer. It was found that the thermal conductivity of the composites increased when the

Fig. 8. Conceptual diagram of building materials containing MEPCM. Reprinted from [160] with permission from Springer.

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Fig. 9. Chemical structures of styrene–maleic anhydride (SMA) and its hydrolyzed form (HSMA) (a), chemical structures and reaction schemes of UF polymer (b) the zeta potential of the O/W emulsion with HSMA as the emulsifier at different pH values and illustrative emulsification and fabrication of PCM capsules (c). Reprinted from [179] with permission from Elsevier.

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PCM was liquid, partly because of better wetting of the epoxy by the liquid paraffin. Oxidized hard Fischer–Tropsch paraffin waxes as PCMs in an epoxy resin matrix were prepared by Luyt and Krupa by mechanically mixing wax powder and a liquid resin at room temperature, and then curing by UV [180]. The results indicated that the PCM distribution in the polymer matrix improved with an increasing wax content and its presence influences the mechanical properties of the PCMs, especially above the melting point of the wax. 4.5. Polyaddition Chen et al. [162] prepared polyurea microcapsules containing PCMs using toluene-2,4-diisocyanate (TDI) and ethylenediamine (EDA) as monomers, and butyl stearate as the core material. MEPCMs based on a core of n-octadecane and polyurea shells, synthesized by polyaddition of TDI as an oilsoluble monomer and various amines, e.g. EDA, diethylene triamine, or polyetheramine as the water-soluble monomers, were prepared by Zhang and Wang [174] as displayed in Fig. 10. It was revealed that the microcapsules synthesized by using polyetheramine had a smoother and more coherent surface and larger mean particle size with a narrow size distribution than those using EDA or diethylene triamine. Furthermore, the microcapsules synthesized with a core/shell weight ratio of 70/30 possessed the optimum properties for TES applications [174].

Fig. 10. Schematic formation of the microencapsulated n-octadecane with the polyurea shells containing different soft segments through interfacial polycondensation. Reprinted from [174] with permission from Elsevier.

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4.6. Other methods Microencapsulation of n-tetradecane with different shell materials such as acrylonitrile–styrene copolymer (AS), acrylonitrile–butadiene–styrene terpolymer (ABS) and polycarbonate (PC) was carried out by phase separation method – [181] see Fig. 11. AS was the most suitable shell material as it gave the highest encapsulation efficiency and the greatest transformation enthalpy. The molecular weight of the shell material also influenced the microencapsulation – a lower molecular weight gave greater encapsulation efficiency because of the higher mobility of the shell molecules, but it reduced the shell’s strength [181]. Recently, Jin et al. [182] have produced microcapsules of PCM using silica as the shell in a one-step procedure without surfactants or dispersants. This allowed fabrication of capsules with controlled size and dispersity incorporating various core materials. The benefits of the method include an easy scale-up and no necessity for a stabilizing agent as the amine groups self-stabilize. Macro-capsules containing SSPCM consisting of 50 wt.% of n-octadecane and 50 wt.% of HDPE were prepared using calcium alginate (CaAlg) as the shell material [183]. The MEPCM based on n-octadecane core and a silica shell was designed and synthesized via a sol–gel process by Zhang et al. targeting an enhancement of the thermal conductivity and phase-change performance [184]. The MEPCMs were spherical with a well-defined core–shell microstructure giving a single endothermic peak on the DSC profile, quite similar to that of the bulk material. The thermal conductivity of the studied MEPCMs was significantly superior to that of the non-structured materials. 5. Form–stable (shape-stabilized) PCMs (SS-PCMs) When the costs of encapsulation and those related to the increase of PCM conductivity are analyzed, the increasing attention being given to the development of form–stable, shape stabilized, composite materials, is clearly the rational approach. In a paraffin/HDPE composite PCM, paraffin as a solid–liquid PCM is dispersed in the polymer matrix. The latter prevents leakage of the molten paraffin from the composite at temperatures between the melting temperature of the paraffin and that of the HDPE. Thus, the technical problems for encapsulation are solved by using this type of form–stable PCMs [75,185,186]. Various polymers are applied as matrices in shape-stabilized PCMs and they are described in the following sub-chapters. 5.1. SS-PCMs with a polymer matrix 5.1.1. Polyethylene Polyethylene (PE), due to its properties and chemical affinity to paraffins, is widely used in form– stable PCMs as a supporting material [187]. Sari [90] prepared paraffin/HDPE composites as SS-PCMs by melt mixing. The maximum amount for two different types of paraffin in the PCM composites was

Fig. 11. SEM images of (a) AS/n-tetradecane, (b) ABS/n-tetradecane and (c) PC/n-tetradecane microcapsules. Reprinted from [181] with permission from Elsevier.

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77% and it was observed that the paraffin was well dispersed in the solid HDPE matrix. In addition, to improve the thermal conductivity of the SS-PCMs, graphite, expanded and exfoliated by heat treatment, was added and increased the thermal conductivity by 14–24% [75,90]. Form–stable composites, based on low-density polyethylene (LDPE) mixed with soft and hard Fischer–Tropsch paraffin waxes were studied by Krupa et al. [86]. They found that the two waxes behaved totally differently. The Fischer–Tropsch paraffin wax co-crystallized with the LDPE crystals to form a more compact blend than the soft paraffin wax. The blends were found to be efficient SS-PCMs with the LDPE matrix keeping the material in a compact shape at the macroscopic level. In other work, Cai et al. [188] investigated shape stabilized phase change nanocomposite materials based on HDPE–ethylene–vinyl acetate (EVA) alloy, organically-modified montmorillonite (OMT), paraffin and intumescent flame retardant (IFR), processed in a twin-screw extruder. The results indicate that the HDPE–EVA/OMT nanocomposites acted as the supporting material and formed a three-dimensional network structure, while the paraffin PCM was dispersed in the network. SEM and DSC showed that the IFR additives hardly have effect on the network structure or on the latent heat of the shape stabilized nanocomposites. However, incorporating a suitable amount of OMT into the form–stable PCM increased its thermal stability as revealed by TGA data. 5.1.2. Acrylics Acrylic-based matrixes are easily processed, possess end-use properties and are cost effective. In this field, Alkan et al. [189] studied PEG blends with acrylic polymers such as PMMA, Eudragit S (Eud S – copolymer of methacrylic acid and methyl methacrylate) and Eudragit E (Eud E, copolymer based on dimethylaminoethyl methacrylate, butyl methacrylate, and methyl methacrylate) as form–stable PCMs. The PEG acted as a phase change LHTES material with the acrylic polymers as supporting materials. The maximum percentage of PEG without leakage was found to be ca. 80 wt.%. In their later work [190] blends of Eudragit S with SA, PA and MA were prepared and characterized. The maximum mass percentage of fatty acids in the blends to avoid their seepage in the molten state was found to be 70%. These results were confirmed by Kaygusuz et al. for blends of Eudragit E and fatty acids [191]. Later a series of fatty acid/poly(methyl methacrylate) (PMMA) blends such as SA/PMMA, PA/PMMA, MA/PMMA, and LA/PMMA was tested as SS-PCMs. The blends were prepared by a solution casting method with different mass fractions of fatty acids to determine the maximum blending ratio with no leakage above the melting temperature [192]. Recently, Zhang et al. [193] investigated PEG/ PMMA and PEG/PMMA/aluminium nitride (AlN) composites as form–stable PCMs. It was found that for a PEG mass fraction less than 70%, the SS-PCMs remained solid without leakage above the PEG melting point. With the increase in the mass fraction of PEG, the latent heat capacity of the composite PCMs increased accordingly, which effectively correspond to the theory of mixtures. Thermal analysis showed that the prepared PCMs possessed desirable latent heat capacities and thermal stability, and AlN additive effectively enhanced the heat transfer properties of the organic PCM. 5.1.3. Poly(vinyl chloride) The structure of poly(vinyl chloride) (PVC) gives rise to a relatively tough and rigid material able to accept a wide range of additives, including PCMs. In this context, blends of PVC/PA and PVA/PA were investigated by Sari et al. [194]. They established that there was no leakage of PA even in the molten state and that the maximum miscibility ratio of PA with both polymers was found at the level of 50% while maintaining the shape-stabilization effect. Blends of PVC, the supporting material, with fatty acids as PCMs, were studied. The maximum PVC/fatty acid ratio for which no leakage of fatty acid was observed above their melting temperatures for several heating cycles of the SSPCMs was 50 wt.% [195]. 5.1.4. Polyurethanes Polyurethane (PU) foams are light-weight materials with a high strength/weight ratio, superior insulating properties, and high-energy absorbancy. The mechanical properties of PU foams are an important consideration for structural and semi-structural applications, such as composite foam cores and mould patterns. Unlike thermoplastic foams, PU foams are formed by reactive processing in which polyaddition and foam blowing occur simultaneously – the network structure must build up rapidly to support the brittle foam, but not too rapidly to prevent bubble growth.

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For energy storage purposes, PU foams were modified by n-hexadecane or n-octadecane during polymerization process by Sarier and Onder [196]. An FTIR analysis showed that the chemical compositions of the PU samples were significantly different from that of the PU control sample, and it was verified that they displayed the functional groups which indicated the presence of linear chain hydrocarbons. SEM showed that the PCM micelles were located in the cells of a perfect honeycomb structure – see Fig. 12. Polyurethane foams containing PCMs can be assumed to be leakage-protected, and this gives the prospect for PU-PCM production on an industrial scale [196]. Composites obtained by impregnation of rigid PU foams with PEG have also been tested [197]. DSC analysis shows that PU–PEG materials yielded high enthalpies over certain temperature intervals suggesting that the heat absorption/release capacities of PU foams may be improved by the incorporation of PEG. You et al. [198] studied PU composite foams containing micro-encapsulated PCMs produced by in situ polycondensation process of melamine and formaldehyde, for the shell, and n-octadecane as the core. PU composites were fabricated by adding heat-treated micro-encapsulated PCMs during the synthesis which resulted in them being evenly distributed inside the foam. It was not possible to produce qualified MEPCMs/PU composite foams containing more than ca. 12 wt.% PCM. The limitation was probably caused by the existence of reactive hydroxymethyl groups in the polymerized melamine–formaldehyde shell. In a recent development, Ke et al. [199] prepared porous membranes based on PU and PEG. The results showed that the PU/PEG membranes had a highly porous structure and suitable transition temperatures and enthalpy changes. 5.1.5. Other polymers In an effort to search for novel shape-stabilized PCMs, blends of poly(vinyl alcohol) (PVA) with LA, MA, PA or SA were studied by Sari and Kaygusuz [200]. The maximum mixture ratio for all the fatty acids in the shape-stabilized form was found to be 50 wt.%. In their later work, fatty acids were introduced to a maleic anhydride copolymer matrix by the solution casting method. As much as 85 wt.% of

Fig. 12. PU containing n-hexadecane: (a) magnification 500, (b) magnification 20,000 and (c) magnification 30,000). Reprinted from [196] with permission from Elsevier.

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fatty acids were incorporated successfully and no leakage above the melting point was detected [201]. Mengjin et al. [202] prepared fibres of PVA and paraffin using a wet spinning technique. The thermal regulating fibre had an acceptable thermal stability when the content of paraffin in fibre was below 30%. An interesting approach was adopted by Chen et al. [203] who obtained ultrafine composite fibres of stearyl stearate/poly(ethylene terephthalate) (SS/PET) by electrospinning. Depending on the SS/PET mass ratio, the morphology and thermal properties of the composite fibres changed considerably. In other work they prepared ultrafine fibres based on the composites of PET and a series of fatty acids – LA, MA, PA and SA by electrospinning [204]. Cai et al. [138] also prepared by electrospinning ultrafine composite fibres consisting of LA/PET/nano-SiO2 as a form–stable PCM. DSC indicated that the amount of nano-SiO2 in the fibres had an influence on the crystallization of LA, and played a significant role on the heat enthalpies of the material. The morphology, thermal and thermo-mechanical properties of polyamide (PA) 12/maleic anhydride grafted wax blends and the possible interactions between PA12 and the functionalised wax were studied by Luyt et al. [205]. Results showed that it was practically impossible to prepare homogeneous blends containing more than 30 wt.% of wax and a low heat of phase transition was observed. At lower wax contents, the wax crystals were homogeneously distributed through the PA12 matrix, and there was no leakage of the wax from the matrix during processing. The novel form–stable phase change composite materials of PEG/epoxy resin (EP) were prepared by Fang et al. [206]. As the epoxy resin was the supporting material for the PEG PCM, the mechanical deformation of PEG/EP composites was very small, and the composites retained their shape even when the phase change from solid to liquid took place. It was demonstrated that the overall transition was a solid–solid phase change. Zhang et al. [207] produced a series of polyol acetal derivatives by condensation reactions of aromatic aldehyde with polyols. Three-dimensional structural PCMs were obtained using paraffin doped with different gelation agents, which proved to be thermally stable and showed no leakage of paraffin above the melting point of the saturated hydrocarbon. 5.2. SS-PCMs with expandable graphite matrix Paraffin (n-docosane)/expanded graphite (EG) composites prepared by absorbing liquid paraffin into the EG, as a form–stable PCM were studied by Sari et al. [91]. A composite PCM with a 10% mass fraction of EG was found to be a form–stable. Because of the capillary and surface tension forces of EG no leakage of liquid paraffin was observed during the phase transition. The enhancement of thermal conductivity and the latent heat capacities of the PCM materials were roughly equivalent to the theoretical values calculated based on the basis of the mass ratios of the paraffin and EG in the composites [91]. Fatty acid/expanded graphite (EG) composites prepared by a vacuum impregnation method were investigated, too. It was found that the maximum fatty acid absorption of EG was 80 wt.% without molten fatty acid oozing from the composites. DSC results indicated that the melting and solidification temperatures of the composite PCMs were almost identical to those of the fatty acids, but the latent heats of the composites were slightly lower than those of the pure fatty acids [208,209]. A paraffin/ expanded graphite phase change TES material was investigated by Zhang and Fang [92]. The composites were prepared by absorbing the paraffin into EG – a material which, because of its layered micro-porous morphology, has excellent absorbability – see Fig. 13. In such a composite, the expanded graphite acts as the supporting material. The capillary and surface tension forces prevent leakage of the molten paraffin from the porous structure [92]. Wang et al. [210] investigated the composite made by blending PEG with expandable graphite. The maximum mass percentage of PEG dispersed in PCM composites without any leaking of the polymer was found to be as high as 90 wt.%. It is of interest that the thermal conductivity was considerably increased because of the thermally conductive network formed by the EG’s porous structure. Zhang et al. [211] investigated EG/paraffin composite PCMs with an EG mass fraction varying 0–10 wt.%. Thermal characterization of the composite PCMs by DSC revealed shifts in the phase change temperatures. Initially the latent heat of the paraffin in the composite PCMs increased but then decreased with an increase in the fraction of EG. An SSPCM composed of PEG and mesoporous active carbon (AC) was prepared by a blending and impregnating method [212]. Lower phase change temperatures and enthalpies were observed, as the content and molecular weight of PEG were decreased. The authors concluded that

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Fig. 13. SEM photomicrographs of the expanded graphite and paraffin/expanded graphite composite PCM: (a) expanded graphite, (b) paraffin/expanded graphite composite PCM. Reprinted from [92] with permission from Elsevier.

the phase change properties of the PEG/AC PCMs were influenced by the adsorption confinement of the PEG segments in the porous structure of AC and by the role of the AC during PEG crystallization. 5.2.1. Other SS-PCMs Xing et al. [213] employed silica gel to encapsulate form–stable paraffin PCMs, in which the paraffin served as the LHS material and HDPE as the supporting material. It was found that keeping a high mass percentage of paraffin, it was possible to encapsulate form–stable paraffin with random ratios of the mass of the core materials to that of the coating materials. The results also indicate that there are more advantages to using PE form–stable paraffin as the core material rather than using paraffin directly because of the lower cost of producing the PCMs, the higher mass percentages of paraffin encapsulated, better hydrophilicity and better fire resistance [213]. PEG/SiO2 composites were investigated by Wang et al. [214] with PEG as the PCM and SiO2 as the supporting material. They were prepared by dissolving PEG in water and adding silicon dioxide which enable composites containing between 5 and 95 wt.% of SiO2 to be obtained. It was found that up to 85% PEG could be dispersed in the PCM composite without any leakage of the molten PEG. It was established that the thermal conductivity was improved because of the thermal conductive network formed by the porous structure of the SiO2. Recently, Tang et al. [215] obtained PEG/SiO2 hybrid form–stable PCM with improved thermal conductivity by in situ Cu doping via the chemical reduction of CuSO4 using an ultrasound-assisted sol–gel process. Zhang et al. [216] produced granular phase change composites for TES by means of

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a vacuum impregnation method, using organic PCMs, including fatty acids, their derivatives and paraffins, with inorganic porous materials, including expanded clay, expanded fly ash and expanded perlite. This method enabled a volume of up to 65% PCMs to be loaded into porous materials [216]. Vacuum impregnation was also used by Karaipekli and Sari [217–219] to incorporate an eutectic mixture of CA and MA into expanded perlite (EP), and pure CA or LA in EP. The maximum fatty acid absorption of EP was found to be 55–60 wt.% without molten PCM seepage from the composite and, therefore, this mixture was described as a form–stable composite. Thermal cycling tests of the form–stable composite PCM indicated good thermal reliability up to 5000 thermal cycles. Moreover, the addition of 10 wt.% of EG improved the thermal conductivity of the form–stable CA–MA/EP composite PCM by about 58%. Subsequently, CA–MA eutectic mixture/vermiculite (VMT) composites were obtained by vacuum impregnation [220]. The CA–MA eutectic mixture was restricted to a maximum percentage of 20 wt.% without seepage of molten PCM from the porous structure of the VMT. The introduction of 2 wt.% of EG into the composite increased the thermal conductivity of the form–stable CA–MA/VMT composite PCM by about 85%. Li et al. [221] prepared CA–PA binary blends impregnated into attapulgite. The pore structure of the CA–PA/attapulgite composite PCM was found to be an openended tubular capillary type, which was beneficial for the adsorption processes. The same group of researchers also investigated binary fatty acid/diatomite shape-stabilized PCMs. Taking account of the phase diagrams, a series of binary fatty acids composed of CA, LA, PA and SA was prepared. The binary fatty acids were absorbed into four types of diatomites having different specific areas, which then acted as the supporting material. The results showed that there is an optimum absorption ratio between the binary fatty acids and the diatomite [222]. Karaman et al. [223] incorporated PEG into the pores of diatomite and characterized the resulting PEG/diatomite composite as a novel form–stable composite PCM. It was found that up to 50 wt.% PEG could be retained in the pores of the diatomite without the leakage of molten PEG from the composite. The effects of the porosity and thermal properties of a porous medium infiltrated with PCM were investigated by Mesalhy et al. [224]. They employed carbon foam matrices with various porosities and different thermal properties as the porous medium and paraffin wax was introduced into the matrix pores as the PCM. The matrix composite was

Fig. 14. (a) Photographs of large porous alumina (PAO), graphene-coated porous alumina (G-PAO), SA-filled porous alumina (SA-PAO), and SA-G-PAO. (b) SEM image of G-PAO. c) DSC curves of SA and SA-G-PAO composite. (d–f) Thermal transport evolution of SA-PAO and SA-G-PAO. The thermal images visually illustrate the excellent thermal characteristics of the G-PAO. Reprinted from [228] with permission from Wiley.

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located in a cylindrical enclosure while it experienced heat from a heat source set on the top of the enclosure. The results showed that the porosity and thermal conductivity of the matrix composite played important roles in the thermal performance of the device. In another study, a form–stable PCM prepared by impregnating SA into a silica fume matrix using a solution impregnation technique was investigated [225]. The results show that the form–stable composite PCM has the optimal effect, preventing the leakage of SA from the composite, which emerged when the SA and silica fume mass ratio is 1:0.9. Experimental investigations on the thermal performance of paraffin/bentonite composite PCMs, prepared by a solution intercalation process, were conducted by Li et al. [226]. The results showed that the interlayer distance of bentonite was increased from 1.492 nm to 1.962 nm through organic modification – paraffin can be thus intercalated into the layers of bentonite to form SSPCM. The presence of bentonite enhanced the heat transfer rate of the composite material. Mei et al. [227] investigated the CA/halloysite nanotube (CA/HNT) composite as form–stable composite PCM. The composite can contain up to 60 wt.% CA without any leakage after 50 melt–freeze cycles. A graphite addition improved the performance of the composite with the thermal storage and release rates increased by 1.8 and 1.7 times, respectively. Recently, Zhou et al. [228] have prepared highly conductive 3D porous graphene/Al2O3 composites using ambient pressure chemical vapour deposition. The formation mechanism of graphene was attributed to the carbothermic reduction occurring at the Al2O3 surface to initiate the nucleation and growth of the graphene. It was shown that such a porous composite is attractive as a highly thermally conductive reservoir for PCMs (SA) for TES – see Fig. 14. 6. Applications of PCMs PCMs find applications in the building industry, textiles, the automotive sector and solar energy installations. In recent years an increasing number of applications, including those in electronics and medicine, has emerged. The traditional sectors, such as the construction industry, are being advanced by novel, more sophisticated TES materials for smart textiles and thermoregulated biomaterials, etc. [6,229]. 6.1. Thermal storage in buildings TES for space heating and cooling of buildings is becoming increasingly important because of the increasing cost of fossil fuels and environmental concerns. In extremely cold or hot areas, electrical energy consumption varies greatly during the day and the night due partly to the varying demand for domestic heating or cooling [9,230–239]. PCMs in passive or active storage systems are able to minimize these variations. 6.1.1. Passive storage systems Passive heating or cooling systems refer to the technologies or design features for heating or cooling buildings without active mechanical devices using a system which uses little no external energy [240]. Passive energy storage studies in Japan, US and Germany for heating greenhouses date back to the 1980s. The early installations were with CaC126H2O which were followed by Na2SO410H2O, PEG and paraffins. The quantity of PCMs per square meter of greenhouse covered area and the melting temperatures varied from ca. 5 kg/m2 to 85 kg/m2. Most applications were installed in either double-glazed greenhouses or those with one or more thermal screens. However, it was shown that PCMs could be used for both energy storage and humidity control in greenhouses giving efficient energy management provided the correct design was chosen for the system [241,242]. In a detailed study, Feldman et al. [109] impregnated gypsum wallboard with a mixture of methyl palmitate/methyl stearate having a melting-freezing interval between 23 and 26.5°C and a latent heat of phase transition of at least 180 kJ/kg. In later work, the authors determined the effects of the heating–cooling cycling on the thermal characteristics of PCM in wallboard and estimated its heat storage capacity [109,243]. Later, researchers evaluated full scale tests in a room lined with PCM wallboard having LHTES capacity. When the results were compared with those conducted in a similar room lined with ordinary wallboard it was concluded that the PCM wallboard could function efficiently as a

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thermal storage medium for peak load shifting [108,244]. Finally, the authors demonstrated that using appropriate combinations of PCM and concrete to produce blocks capable of storing the PCMs latent and sensible heats as well as the sensible heat of the concrete made if feasible to achieve variation of the storage and release of heat with time [245]. Zhang et al. [246] proposed a two-step procedure to produce thermal energy storage concrete (TESC). The first step involved making thermal energy storage aggregates (TESAs) by infiltrating porous aggregates with PCMs under vacuum. The TESC was then produced by mixing the TESAs in the conventional mixing process. It was found experimentally that the energy storage capacity of the TESC was comparable with that of a commercially available PCM. Facade panels with double glazing combined with PCM were studied by Weinlader et al. [247]. The latent heat storage effect of the PCM provided a more equalized energy balance in the course of day, when moderate heat gains with very low heat losses were an experience. This could make such systems a good choice for buildings of lightweight construction. It has been shown that on winter evenings, facade panels with PCM considerably improve thermal comfort. Darkwa and O’Callaghan [248] found that a laminated wallboard with a narrow phase change zone would be more effective in moderating night time temperature in a passively designed room. Shilei et al. used eutectic mixtures of CA and LA as the PCM to impregnate wallboards for building energy storage [249–251]. After 360 thermal cycles the melting temperature and latent heat of the PCM wallboards showed no obvious changes. Preparation of phase change gypsum wallboards incorporating the eutectic mixture of fatty acids was also studied by Sari et al. [252]. They found that a maximum of 25% of the total mass of CA/MA eutectic could be incorporated into the gypsum wallboard, but after 1000 cycles PCM leakage occurred in the range 20–50 °C. In subsequent work, a CA/SA eutectic mixture was used as a PCM for wallboards and the results were comparable to those of CA/MA system [253]. Cabeza et al. [254] constructed two full size concrete cubicles to study the effect of the inclusion of a PCM with a melting point of 26 °C. The results confirmed the benefits of the energy storage capabilities of concrete-based PCM materials compared to conventional concrete materials. Thermal analysis data of a building brick containing PCM were reported by Alawadhi [255]. The model consisted of bricks with cylindrical holes filled with PCM. The obtained results indicated that the heat gain was significantly reduced when the PCM was incorporated into the brick although increasing the quantity of the PCM had a positive effect. Pasupathy and Velraj [256] carried out a detailed analysis on the thermal performance of the roof of a building incorporating PCM and they recommended a double layer PCM in the roof for the purpose of narrowing the variation in indoor air temperature. The application of PCM to store ‘outdoors coolness’ during the night, and to provide indoor cooling during the day, was suggested by Zalba et al. [257]. The authors designed and constructed an installation to test the performance of PCMs. A new type of composite wall system incorporating PCMs was proposed by Diaconu and Cruder [258]. They proposed a new three-layer sandwich type wall system consisting of two PCM wallboards, impregnated with different PCMs as an outer layer, and a middle layer of conventional thermal insulation. The results of experiments using PCM in concrete floors, in which thermal energy provided by the sun is stored in a concrete/PCM composite, were presented by Entrop et al. [259]. Incorporating PCMs in concrete floors resulted in a reduction of the maximum floor temperatures by 16% and an increase in the minimum temperatures of up to 7%. In a work by Kuznik et al. [260], a renovated office building was monitored for about a year to assess the potential of PCM wallboards. A room was equipped with PCM wallboards in the lateral walls and in the ceiling. The results showed that the air temperature and radiative effects of the walls PCM wallboards enhanced the thermal comfort of occupants. Passive solar heating and cooling technologies were fully described in review papers prepared by Chan et al. [240], Tyagi et al. [261], and Kuznik et al. [262]. 6.1.2. Active storage systems Active storage systems are used mainly for off peak storage of thermal energy in buildings. Thus, the peak loads may be reduced and shifted to night time when electricity costs are generally lower [19,263]. PCMs are continuingly being investigated for active storage systems, including floor heating systems and photovoltaic devices [264].

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Hence, Lin et al. [265] developed a new structure of under-floor electric heating system with SSPCM plates. The SS-PCM consisted of 75 wt.% paraffin as a dispersed PCM and of 25 wt.% polyethylene as the supporting material. The system increased the indoor temperature of an experimental house without increasing the temperature difference, and the temperature of the PCM plates was maintained at the phase transition temperature for a long period after the heaters stopped working. More than 50% of the total electric heat energy was shifted from the peak period to the off peak hours [265,266]. In another development, Li et al. [267] prepared and studied a PCM, consisting of microencapsulated paraffin as the latent heat storage medium and a high density polyethylene/wood flour composite as the matrix. They also carried out simulated studies to determine the temperatureregulating and cost-reduction effects of the materials for electric under floor heating system. The results showed that the effects were dependent on the floor’s heating mode and the thickness of the PCM layer.

Fig. 15. Illustration showing panel construction from (a) a plan view and (b) a cross-sectional view, (c) illustration of a typical installation for an active closed-loop system appropriate for single dwelling. Reprinted from [272] with permission from Elsevier.

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PCMs were also used to thermally regulate building-integrated photovoltaics (PV) [268–270], because the efficiency of PV devices is reduced by elevated operating temperatures. Incorporating a solid–liquid PCM having a phase change temperature similar to the PV characteristic temperature of 25 °C for the thermal regulation of building integrated PV under cyclic time-dependent solar energy input, was a novel approach to PV temperature control. The authors concluded that a significant improvement in the thermal performance was achieved by using metal fins in the PCM container. The same group undertook an experimental evaluation of the effects of convection and crystalline segregation in a PCM as a function of the efficiency of heat transfer within the finned PV/PCM system [271]. Malvi et al. [272] presented an energy balance model for a combined PV solar thermal (ST) system incorporating a PCM – see Fig. 15. The authors suggested that this is a promising integration of different technologies as the PV converts visible and ultra-violet parts of the solar spectrum, the ST utilizes infra-red parts of the spectrum and the waste heat from the PV and the PCM reduces the temperature of the PV thereby increasing its efficiency. 6.2. Heating/cooling of water The behaviour of encapsulated salt hydrates, used for LHS in a heat transfer system with a domestic hot water tank, was analyzed by Barba and Spiga [273]. They found that the shortest time for complete solidification occurred with small spherical capsules, with a high Jakob number and high thermal conductivity. Cabeza et al. [274] studied a hot-water storage tank with stratification containing a PCM module consisting of several cylinders. A granular PCM–graphite composite was selected as the PCM for the experiments, which showed that immersing a PCM module in a water tank for domestic hot-water supply provided hot-water for a longer period of time even without an external supply of energy. Charging tests of a water and a PCM–water tank at different flow rates in a test installation were performed by Sole et al. [275]. As only a small amount of PCM was introduced to the PCM-tank the additional energy stored was only 3%. This group also investigated the use of PCM modules in a stratified domestic hot water tank. The authors tested three 80:20 wt.% mixtures of paraffin and different fatty acids (SA, PA and MA) as PCMs. During reheating experiments, it was found that 3 kg of PCM could increase the temperature of 14–36 l of water in the upper part of a domestic hot water tank by 3–4 °C. This effect occurred within 10–15 min, with the best results for thermal performance enhancement being obtained using SA as the PCM [276]. 6.3. The latent functional thermal fluids In recent years, research on latent functional thermal fluids (LFTFs), or two-phase heat transfer fluids, has increased because they exhibit greater apparent specific heat in the phase change temperature range than conventional single-phase heat transfer fluids. These LFTFs are composed of PCM particles and a heat transfer fluid, and may exist as a phase change microcapsule slurry or a phase change emulsion. Since the PCM will absorb or dissipate latent heat in its phase change temperature range, it will show a much greater apparent specific heat. In addition, it may significantly enhance the heat transfer rate between the fluid and the tube wall, reduce the mass flow rate and energy consumption of the pump being used. Therefore, LFTFs have many potentially important applications for heating, ventilating, air-conditioning, refrigeration and heat exchange [277–280]. In industrial practice three kinds of dispersion are currently employed (a) a phase change slurry where the phase change component is microencapsulated, or as pellets of SSPCM, suspended in the heat transfer fluid; (b) an emulsion of fusible components, where the PCM is dispersed in a carrier fluid and maintained in suspension by a surfactant; (c) an ice slurry [277,281,282]. Extensive research efforts in the area of LFTFs are focused on the production of stable dispersions of PCM and development of (micro)encapsulation techniques.

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Royon et al. [283] elaborated a stable dispersion of n-alkanes in water for low level heat storage and their experimental results were in quantitative agreement with those obtained theoretically by considering the properties of the individual components. Phase change microcapsule slurry with n-tetradecane encapsulated by PS and PMMA in a core–shell structure was investigated by Yang et al. [277]. They found that n-tetradecane encapsulated by PMMA showed better heat storage property than encapsulated by PS, and its supercooling was even better than pure n-tetradecane. Augood et al. [284] studied the use of slurries containing PCMs based on hydrated hydrophilic polymers for both the storage and transmission of thermal energy at temperatures below the ambient. At temperatures above the phase transition temperature, concentrations P10% of polymeric particles caused a significant increase in the measured heat transfer coefficient. Paraffin/water emulsion for cooling applications in the temperature range of 0–20°C was studied by Huang et al. [285]. An emulsion containing 30–50 wt.% of paraffin was deemed suitable for practical applications because its energy density is twice that of pure water with a relatively low viscosity. Diaconu et al. [286] investigated a microencapsulated slurry containing paraffin-based PCM (Rubitherm RT6) with a high concentration of 45% w/w. It was found that the heat transfer coefficient for the PCM slurry was higher than for water under identical temperature conditions within the phase change range. MEPCM slurry was also investigated by Zhang et al. [287]. A microencapsulated PCM was prepared by in situ polymerization method, with core materials composed of several kinds of n-paraffin waxes, principally n-nonadecane, and the membrane was a form of melamine resin. More information on PCM slurries can be found in a recent review paper [288]. 6.4. Solar energy storage Extensive efforts have been made in recent years to use the LHS materials in thermosolar energy systems, where it is required to store heat during the day for use at night. The studies focused on an examination of the key aspects of heat transfer in PCMs and their behaviour in full size heat storage units [6,289]. Along this area, Kurklu et al. [290] developed a solar collector combining a PCM and water which could be an alternative to the traditional hot water solar collectors, provided that the absorption and insulation characteristics of the collector are improved. Hammou and Lacroix [291] proposed a HTESS for simultaneously managing the storage of heat from solar and electric energy. Simulations carried out over a period of four consecutive winter months indicated that the system reduced energy consumption for space heating by nearly 32% [292]. The current revival of solar thermal electricity generating systems shows that economic TES for the temperature range from 250 °C to 500 °C are still needed. The TES-benchmark for parabolic trough power plants is direct two tank storage, as installed at the solar thermal electricity generating plant at Barstow (USA). Cascaded latent heat storages are marked by a minimum of necessary storage material. One of the early studies favoured cascaded latent heat storages and concrete regenerators [293,294] – see Fig. 16. Experimental and numerical verification of the positive effect of a CLHS application compared to conventional LHS has been provided [294]. In another development, Adinberg et al. [295] proposed

Fig. 16. Proposal of cascaded latent heat storage with five PCM according to Dinter et al. [293]. Reprinted from [294] with permission from Elsevier.

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the concept of reflux heat transfer storage for producing high-temperature superheated steam in the range of 350–400 °C with a Zinc–Tin alloy, acting as the PCM. More information on PCMs for solar cooling applications may be found in a recent review paper [296]. 6.5. Smart textiles In the early 1980s NASA developed the technology to incorporate PCM microcapsules in fibres to improve their performance for managing the thermal barrier properties of garment fabrics, particularly space suits. PCMs, e.g. nonadecane, were encapsulated in textiles with the hope of reducing the impact of extreme temperature variations encountered by astronauts during space missions [16,297]. Still in the 1980s, Vigo and Frost [298–302] prepared various types of thermo-regulating fibres with different temperature ranges. This was achieved by immersing hollow fibres into a PCM solution, such as an aqueous solution of hydrated inorganic salt and low molecular weight PEG. Although the temperature-adaptable fabrics offered desirable thermal storage and release properties, they were unreliable and exhibited poor thermal behaviour after repeated thermal cycling. Therefore, other approaches were developed, such as micro-encapsulation, to enhance the performance of thermo-regulating fibres [303]. PCM capsules are now used commercially for textile fibres and fabric coatings (Fig. 17), [304,305]. They are used for outdoor wear and in blankets, duvets, mattresses and pillowcases. As well as being designed for warmth, textiles containing PCMs can also help to prevent overheating, and so their effect can be described as thermoregulation [297]. Thermoregulating characteristics can be imparted to a fibre by adding MEPCMs to a polymer solution before fibre extrusion. Coating, lamination, finishing, melt spinning, bi-component synthetic fibre extrusion, injection molding and foaming techniques are other convenient processes for incorporating PCMs’ into a textile matrix [16,306]. Shin et al. [307] prepared melamine–formaldehyde microcapsules containing eicosane by in situ polymerization. These were then added to polyester knit fabrics by a conventional pad–dry–cure process to develop thermoregulating textile materials. After five launderings, the fabrics retained 40% of their heat storage capacity [308]. Wang et al. [309] reported on the impact of PCMs on intelligent thermal-protective clothing. They revealed that under the same conditions, the electrical energy consumed by the clothing assembly with PCM is about 31% less than that consumed by the clothing assembly without PCM. Chen et al. [310] obtained ultrafine fibres of PEG/cellulose acetate composite in which PEG acts as a PCM via electrospinning. It was found that PEG was distributed on the surface and within the core of the fibres. DSC revealed that the thermal properties were reproducible after 100 heating–cooling cycles. Sarier and Onder [196] aimed at establishing a manufacturing technique based on in situ polymerization and complex coacervation [311] to micro-encapsulate PCMs. They suggested that a far superior method to enhance the thermal capacities of fabrics, or to enlarge their phase transition intervals, would be to use a combination of microcapsules containing different types of PCMs or paraffin waxes

Fig. 17. PCM microcapsules coated on the surface of fabric (a) and embedded within fibre (b). Reprinted from [297] with permission from Elsevier.

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rather than those including a mixture. Another approach based on a three-layer composite assembled from two waterproof, but water vapour-permeable membranes, interleaved with hydrophilic fabric, was presented by Rothmaier et al. [312]. Water absorbed in the fabric will be evaporated by the body temperature resulting in cooling. Koo et al. [313] investigated the thermal and physical properties of waterproof nylon fabric coatings containing temperature stabilizing MEPCMs. The thermal release capacity and thermal insulation of the coated fabrics increased with an increasing content of PCM. The dual-coated fabrics are expected to perform better than those coated by the dry or wet coating method, because of their enhanced water entry pressure, rate of water vapour transmission and thermal performance. Polystyrene microcapsules containing paraffin wax were synthesized by a suspension-like polymerization process, and their suitability for textile applications was studied by Sánchez et al. [314]. Technical problems with an efficient fixation of the PCM microcapsules to textile material to ensure resistance to washing, rubbing and ironing treatments have been described. Shin et al. [315] studied natural dyed fabrics with thermo-regulating properties. Microcapsules containing n-octadecane and n-eicosane were applied to natural indigo-dyed cotton fabrics using a dot-screen printing method. After 20 laundering cycles, about 94% of the latent heat capacity was retained. 70–89% of the latent heat capacity was retained after rubbing tests and 92–96% after ironing tests using a damp covering fabric. Thermoregulating textile fabrics based on polyurethane with melamine–formaldehyde microcapsules containing a mixture of n-alkanes were developed in a padding process by Salaün et al. [316]. The thermoregulating response was found to be dependent on the surface deposited weight and the mass ratio of binder to microcapsules. Poly(methyl methacrylate) (PMMA)/n-hexadecane microcapsules were prepared using an emulsion polymerization by Alkan et al. [317]. The PMMA/ n-hexadecane microcapsules were incorporated into several fabrics by a pad-cure method. It was established that the fabrics constitution affected the heat storage capacity of the microcapsules which was related to the chemical compatibility of the fabrics and the microcapsules shell material. Porous phase change membranes with PEG 1000/2000 and PU possessing a smooth surface and porous sub-layer structure of the membrane were prepared by Ke et al. [199]. FT-IR and WAXD analyses revealed that the chemical composition and crystalline structure were not changed by the fabrication procedure and measurements of the phase change behaviour indicated that the compound membrane had suitable transition temperature and high transition enthalpy. Izzo Renzi et al. [318] coated natural leathers with a polymeric binder containing microencapsulated PCMs to provide the thermoregulating properties required by some sport-oriented consumers. The use of microcapsules enhanced the thermal response of the leather during heating or cooling with the degree of thermal sensitivity being dependent on the percentage of added microcapsules. Interesting results were presented by Oliveira et al. [319] who examined the influence of a dielectric barrier discharge plasma treatment on the adhesion of PCM microcapsules to woollen fabrics. It was shown that the plasma treatment greatly increased the hydrophilicity, surface energy, and adhesion of the wool fabric; this proved that more microcapsules were adsorbed on the treated fabric and more microcapsules remained on its surface after the washing procedures. In a study by Zhang et al. [320], poly(N-hydroxymethyl acrylamide)/PEG (PNHMA/PEG) interpenetrating polymer network gels and PP/PNHMA/PEG phase change fibres were prepared as novel form– stable composite PCMs. They indicated that the melting temperatures of PEG in the composite PCMs and PP/PNHMA/PEG phase change fibres showed little fluctuation, and the relevant data indicated that the latent heat enthalpy of the composite PCMs and fibres were slightly below the theoretical value. 6.6. Biomaterials and biomedical applications PCMs are currently considered as promising materials for different biomedical applications which require thermal protection, such as special bandages or dressings for burn wounds. PEG-treated fabrics may be useful for biomedical applications where both liquid transport and antibacterial properties, such as surgical gauzes, nappies and incontinence products, are desirable. Thermo regulated textiles can maintain skin temperature within a desired range which enables them to be used as bandages and for burns and heat/cool therapies [321]. Mondieig et al. [322] presented examples of the application of molecular alloys for the thermal protection of biomedical products during transport or storage, especially for the thermal protection of blood elements. The thermo-regulating potential of

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Fig. 18. Sketch of thermal protection by microencapsulated PCM nanoparticles in biological tissue embedded with one tumour during cryosurgery. Reprinted from [325] with permission from Elsevier.

PCMs to reduce peak temperatures generated during in situ polymerization of PMMA for bone cement composites was investigated by De Santis [81]. The thermal regulating capability of the absorption or release of heat during the phase change of PMMA/PCL is of primary importance for injected bone cements and their in situ polymerization [323]. The exothermic reaction yielding significant temperature rise in the injected material may cause necrosis of the surrounding host tissue. Micro-encapsulated paraffin-based PCMs have been incorporated into a PMMA matrix to improve the thermo-mechanical properties. The results indicated that the PCM phase had a negligible effect on the glass transition temperature of the PMMA matrix, and the thermal regulating capability spanned around body temperature by absorbing or releasing thermal energy up to 30 J/g. Incorporation of the PCM into the cement caused a decrease in the peak temperature developed during the exothermal reaction [81]. Application of PEG-based PCMs to decrease the peak polymerization temperature in acrylic bone cements was _ described by Pielichowska and Błazewicz [324] and it was revealed that the use of PEG-based PCMs could decrease the maximum cement polymerization temperature to from about 67 °C to 45 °C without any significant deterioration of the cement’s mechanical properties. A novel and interesting concept was presented by Lv et al. [325], who proposed healthy tissue around a cancerous tumour could be thermally protected during cryo-surgery by the use of micro-encapsulated PCMs. These PCMs with large latent heat and low thermal conductivity could absorb energy by latent heat and thus protect the healthy tissue as indicated in Fig. 18. The theoretical results proved that the proposed method could maximize the destruction of cancer cells within a defined spatial domain while minimizing cryoinjury of the surrounding healthy tissue. It was suggested that not embedding the PCMs directly adjacent to the cancerous tissue would help to improve protection efficiency. In another biomedical-oriented study Wang and co-workers described an innovative thermal bio-sensing technique for the highly sensitive and selective detection of thrombin using RNA aptamer-functionalized phase change nanoparticles as thermal probes [326]. 6.7. Electronics The technological enhancements of electronic devices have resulted in increased functionality and reduced form factors, and squeezed ever more power into ever smaller packages. As a consequence, thermal management has become more critical for the successful design of electronic devices such

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as cellular phones, notebooks, tablets and digital cameras. Such devices are not normally operated continuously for long periods, so that a PCM based cooling system has great potential. As an alternative to a passive cooling technique, PCM-based heat sinks could maintain the temperature of electronic devices below the critical level. Generally, the global maximum allowable temperatures of various chips to avoid damage due to overheating ranges from 85° to 120° [327,328]. Tan and Tso [329] conducted an experimental study on the cooling of mobile electronic devices such as personal digital assistants (PDAs) and wearable computers by using an internally fitted heat storage unit filled with n-eicosane PCM. They established that the high latent heat of n-eicosane in the unit absorbed the heat dissipation from the chips and could maintain the chip temperature in a PDA below the permitted service temperature of 50 °C for 2 h of transient operation. In another development, Krishnan et al. [330] proposed a hybrid heat sink which combined an active plate fin heat sink with its tip immersed in a passive PCM. Composite heat sinks (CHS) using a vertical array of fins made of PCM and highly conductive base material (BM) were constructed by Akhilesh et al. [331]. The effect of the orientation of PCM based heat sinks for transient thermal management of electronic components was studied by Wang et al. [332] and their computed results showed that orientation had a limited effect on the thermal performance of the hybrid cooling system. Kandasamy et al. [333] experimentally applied a novel PCM package for the thermal management of portable electronic devices. The results showed that the thermal resistance of the device and the power level applied to the PCM package were of critical importance for the design of a passive thermal control system. In another study, a PCM based heat sink for transient thermal management of plastic quad flat package (QFP) electronic devices was investigated as shown in Fig. 19 [328]. The inclusion of PCM in the cavities of the heat sinks enhanced the cooling performance compared to the heat sinks without PCM when the input power level was relatively high [328]. In another work, a rapid thermal response composite PCM was prepared by the incorporation of paraffin into expanded graphite by Yin et al. [334]. Applying the composite PCM to an electronic device’s heat sink effectively improved the performance in resisting the shock of a high heat flux and ensured greater reliability and operating stability. The experimental results showed that the apparent heat transfer coefficients of the experimental heat sink with the PCM were up to three times better than those of the heat sink without the PCM. Paraffin and porous expanded graphite composites were also applied as supporting material in electronic cooling systems and the experimental results showed that the apparent heat transfer coefficients of the composite systems were 1.25–1.30 times higher than those of the traditional cooling system. It was found that the dosage of composite material has a positive impact on the performance of electronic cooling [335]. Numerical studies of the heat convection in a rectangular enclosure heated by three discreet protruding electronic chips were carried out by Faraji et al. [336]. It was shown that PCMs can be used to absorb the heat emitted by electronic devices and the use of a PCM can reduce the size of the cooling system. Fok et al. [337,338] experimentally studied the cooling of portable hand-held electronic devices using n-eicosane as the PCM located in heat sinks with and without internal fins. The experimental findings indicated that the use of n-eicosane in an

Fig. 19. Cross-sectional schematic of PCM filled heat sink with QFP package. Reprinted from [328] with permission from Elsevier.

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aluminium heat sink helps to stabilize the system temperature and extends the time of use. In another development, Weng et al. [339] found that a heat pipe module with tricosane as the PCM can reduce the fan’s power consumption by up to 46%, and lower the average heater temperature by 12.3 °C compared to a device with no thermal storage material. 6.8. Automotive industry In the automotive industry, PCMs are used for pre-heating catalytic converters [340,341], engine cooling [342], enhancing the thermal comfort of passengers [343] and in internal combustion engines [344–347]. Gumus [348] employed a TESS to reduce the cold-start emissions from an internal combustion engines. The CO and hydrocarbons emissions decreased by about 64% and 15%, respectively, through the pre-heating engine at the cold start and in the warming-up period. In the subsequent work an evaporator and pressure regulator (EPR) using a PCM to store thermal energy for overcoming cold start problems in LPG fueled vehicles was described. It was found that the EPR with PCM could partially solve the cold start problem of the LPG powered engines after waiting duration [349]. In addition the technical feasibility of using a heat accumulator containing an advanced PCM for an automotive engine cooling was explored [342]. The novel concept is to store a larger quantity of heat in the accumulator, which could contribute to a downsizing of the engine cooling system. 6.9. Space applications In space applications, energy storage is a critical requirement. An analytical model was developed by Yimer and Adami [350] to study TES systems that utilize lithium hydride (LiH) for space applications. The model evaluates the influence of various geometric and thermal parameters on the performance of the energy storage systems. Elsewhere, a numerical study was proposed for investigating and predicting the thermal performance of graphite foams infiltrated with PCMs, for space and terrestrial energy storage systems. For space applications, the average output power of the new energy storage systems has been increased by more than eight fold, while for terrestrial applications, the average output power using carbon foam with 97% porosity is about five times greater than that for using pure PCM [351]. Solar heat receivers with solid–liquid PCMs are very important components of space power management schemes. For space applications phase-change salts, such as a LiF–CaF2 eutectic mixture, are favourable candidates for LHTES in the harsh environment of space [352]. Cui et al. [353] proposed a solar receiver with thermal storage module consisting of a triple-PCM unit. It was concluded that the use of multiple PCMs rather than a single PCM could enhance the energy rate and greatly decrease the fluctuation of the exit temperature of the gas. SSPCMs have been proposed for the effective absorption of heat to prevent faults in the thermal control system when the spacecraft’s outer heat flux changes dramatically [354]. 6.10. Food industry PCM applications currently being investigated include those applicable to the food industry. Devahastin and Pitaksuriyarat [355] studied the feasibility of using LHS to conserve energy during the drying of sweet potatoes for use in the food industry. It was found that the drying rate increased with a reduction in the inlet ambient air velocity. Lu et al. [356] designed a cabinet shelf containing a PCM which can reduce the food temperature rise by 1.5 °C during a defrosting period and improve the uniformity of the food’s temperature. Johnston et al. [357] described nano-structured calcium silicate (NCS) impregnated with paraffin as a composite PCM to provide effective thermal buffering for paperboard packages during the transportation and temporary storage of chilled perishable food. It was established that the NCS–PCM composite provided sufficient thermal buffering capacity to maintain the temperature inside the container at 10 °C for some 5 h after the outside temperature increased to about 23 °C. PCMs are also used in solar dryers for agricultural food products. In a solar dryer with a PCM storage unit, food can be dried even in the late evening which is not possible in traditional solar

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dryers. More information on solar dryers using PCM technology for food applications has been given in a recent review article [358].

7. Fire retardation of PCM-treated construction materials It is relatively rare to find reports on the thermal stability and flame resistance of the form–stable PCMs. The lower thermal stability and poor flame resistance have severely restricted broader applications, especially in building and aerospace fields. It follows that research to improve the thermal and flame resistance properties of form–stable PCMs are very important. Various technological approaches have been studied by Salyer and Sircar to improve the fire retardance of PCM filled plasterboard [359], including (i) sequential treatment system for plasterboard, firstly a PCM and then an insoluble liquid fire retardant, (ii) application of halogenated (mainly brominated) n-alkanes in combination with antimony oxide to yield synergistic effects, and (iii) use of fire retardant surface coatings forming a physical barrier to heat transfer. Banu et al. [360] conducted flammability tests on gypsum wallboards impregnated with approximately 24% of an organic PCM and evaluated the surface burning characteristics. Comparison of the test results with similar data for other building materials indicated the possibility of reducing the flammability of energy storing wallboards by the incorporation of a flame retardant [17]. More recently, Cai et al. [80] investigated halogen-free flame retardants based on an intumescent flame retardant system using expandable graphite (EG) and different synergistic additives, such as ammonium polyphosphate (APP) and zinc borate (ZB). TGA indicated that the halogen-free flame retardant form–stable PCM composites produced a greater amount of charred residue, contributing to the improved thermal stability properties, whereby DSC revealed that the flame retardant additives had little effect on the TES capacity of the paraffin [80]. The possible interaction mechanism is mainly a condensed-phase action through the formations of a charred residue with partial gas phase flame retardancy [361]. A paraffin/high density polyethylene (HDPE)/intumescent flame retardant system (IFR), containing APP, pentaerythritol (PERT), melamine and iron was prepared as a flame retardant shape-stabilized phase change material (FSPCMs) by Zhang et al. [362]. Results showed that the introduction of iron into the paraffin/HDPE/IFR system, improved the flame retardancy of FSPCMs because of the synergistic effect between iron and the IFR. The latent heats of the FSPCMs were found to be dependent on the mass fraction of paraffin, but they were lower than the theoretical values. In subsequent work the flame retardant shape-stabilized PCMs based on paraffin, HDPE, IFR and EG were studied. The latent heats of SSPCMs were lower than the theoretical values calculated by multiplying the mass percentage of paraffin owing to lower molecular heat mobility of paraffin confined by HDPE and EG. The efficiency of the IFR was improved by simultaneous application of EG, because the EG formed the first char layer at the onset of combustion and then an interaction between EG and intumescent char layer formed by paraffin/HDPE/IFR occurred. These two char layers were efficient barriers preventing (i) heat and oxygen from transferring into the matrix interior, and (ii) flammable volatiles into flame zone [363] – see Fig. 20. Sittisart and Farid [364] prepared form–stable PCM consisting of paraffin (or propyl ester), HDPE and fire retardants such as magnesium hydroxide, aluminium hydroxide, EG, APP, PERT and modified MMT. The results from a vertical burning test have shown that form–stable PCMs which contained APP + PERT + MMT or APP + EG displayed the best improvement in fire resistance as they can selfextinguish by forming a large residue. DSC showed that adding fire retardants to PCM did not cause any significant changes to its thermal properties. PA/SiO2 composites with melamine as the flame retardant were investigated by Fang et al. [365]. The addition of melamine in the composites improved the thermal stability and flame retardance. The microstructure of the charred residue after combustion indicated that the homogeneous and dense char residue decreased the flammability of the composites. Song et al. [366] studied form–stable PCMs based on EPDM (supported material), paraffin (dispersed PCM), nano-structured magnesium hydroxide (nano-Mg(OH)2) and red phosphorus (RP) with various compositions. TGA results indicated that the introduction of nano-Mg(OH)2 into the form–stable PCM blends increased the amount of char residue at 700 °C, thereby improving the flame retardance.

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Fig. 20. The possible combustion mechanism for paraffin/HDPE/IFR/EG as a shape stabilized PCM. Reprinted from [363] with permission from Elsevier.

8. Long term stability The economic feasibility of employing a LHS material depends on its service life, during which there should be no major changes in the melting temperature or latent heat of fusion caused by the operational thermal cycling of the storage material [367]. Two factors are responsible for storage materials’ inadequate long term stability: the poor stability of the materials themselves and/or corrosion between the PCM and the container [6]. Sharma et al. [368] conducted accelerated laboratory experiments to study the change in latent heat of fusion, melting temperature and specific heat of commercial grade SA, acetamide and paraffin wax subjected to 300 and 1500 melt/freeze cycles. They found that no degradation of the material was caused [369]. The long term stability and geometrical profile of encapsulated paraffin were investigated by Hawlader et al. [170]. It was established that encapsulated paraffin retained its geometrical profile and that the energy storage capacity was virtually unchanged after 1000 thermal cycles. The

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thermal reliability of SA, PA, MA and LA as PCM after 120, 560, 850 and 1200 melt/freeze cycles was studied by Sari [370]. In another study industrial grade fatty acid (SA, PA, MA and LA) with melting temperatures between 40 and 63 °C were subjected to thermal cycling to assess their thermal stability and corrosive influence of metallic materials [371]. Stainless steel and aluminium alloys, which have naturally occurring protective oxide coatings, are both suitable as storage containers as they are compatible with these fatty acids. Karaipekli et al. [372] carried out accelerated thermal cycling tests of CA/LA and CA/MA eutectic mixtures and the results showed that the PCMs have good long-term reliability in terms of the limited changes in their phase change temperature and latent heat capacities. It was established that accelerated melting/freezing cycles do not cause degradation of the fatty acids chemical structure.

9. Thermal conductivity All the conventional PCMs – both organic and inorganic – possess very low thermal conductivity ranging from 0.1 to 0.6 W/m K which is unacceptably low as it leads to slow charging and discharging rates. Despite the conventional PCMs offering high energy density the slow rates of melting and solidification limit the applications of LHTES systems. However, the thermal conductivity can be enhanced by various techniques involving the addition of high conductivity materials [18,72,373–382]. Other methods include (i) the use of finned tubes with various configurations, (ii) insertion of PCM into a metal matrix, (iii) micro-encapsulation, and, (iv) PCM impregnation of porous materials [383,384]. For instance, the low thermal conductivity of paraffin wax could be enhanced by embedding a metal matrix or fins [24]. Bugaje [74] introduced methods of enhancing the thermal response of paraffin wax by incorporating aluminium thermal conductivity promoters with various designs. It was found that the time for the phase change was reduced significantly during the heating and cooling processes. Erk and Dudukovic [385] utilized paraffin wax incorporated into a porous silica catalyst [224]. Carbon fibres with a high thermal conductivity to enhance the thermal conductivities of PCMs were employed by Fukai et al. [386]. Carbon fibres are resistant to corrosion and chemical decomposition and are to be compatible with most PCMs. Carbon fibre has a thermal conductivity similar to those of aluminium and copper and the density of carbon fibre is theoretically less than 2260 kg/m3, which is lower than that of the metals that are usually used to enhance conductivity. In subsequent work, brushes made of carbon fibres were used to improve the thermal conductivities of PCMs packed around heat transfer tubes [97,387]. The experimental results showed that the brushes essentially improved the heat exchange rate during the charge and discharge processes even with a 1% volume fractions of fibres [388,389]. Elgafy et al. [390] showed that dispersing carbon nanofibres (CNF) into paraffin wax significantly improved the thermal properties of the PCM. Increasing the mass ratio of the CNFs raised the cooling rate during the solidification process of the new PCM nanocomposites. Blends of PA with multi-walled carbon nanotubes (CNT) were obtained via mechano-chemical treatment by the ball milling of the mixture of potassium hydroxide and pristine CNTs by Wang et al. [391]. The CNT addition substantially enhanced the thermal conductivity of the composite PCM which increased with the amount of CNT loading. Interestingly, the thermal conductivity of the composite PCMs suddenly increased near the melting point in the solid state and then decreased when the PCMs transformed to their liquid phase. In another study a LHS nanocomposite was prepared using SA as the PCM and multi-walled carbon nanotubes as the additive; it was revealed that the addition of MWCNT improved the thermal conductivity of SA but weakened the natural convection of the liquid SA [392]. The thermal properties of carbon nanofibre (CNF) and CNT filled PCMs of soy wax and paraffin wax were studied by Cui et al. [393]. The CNF/PCMs and CNT/PCMs mixtures had enhanced thermal conductivity compared to that of the pure wax, with the degree of enhancement increasing with the mass fraction of CNF and CNT. A composite material containing PCM embedded inside a graphite matrix was developed by Mehling et al. [383] and Py et al. [89]. The main advantage of this type of material is the increased thermal conductivity of the PCM with little reduction in the energy storage capacity, but additional advantages could be the decrease in subcooling of the salt hydrates and the decreased volume change of the paraffin [88]. Py et al. produced and characterized paraffin/compressed expanded natural graphite (CENG)

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composites. Paraffin contents from 65% to 95% weight were obtained depending on the density of the bulk graphite matrix. The thermal conductivities of PCM/CENG composites were found to be similar to those of the graphite matrix [89]. The influence of expanded graphite (EG) and carbon fibres (CF) as heat diffusion promoters on the thermal conductivity of SA was evaluated by Karaipekli et al. [394]. More recently, Xiang and Drzal [395] have investigated composite PCMs made by mixing two types of exfoliated graphite nanoplates into paraffin wax. They revealed that a higher thermal conductivity of a composite PCM can be obtained with nanofillers having a larger aspect ratio, better orientation and lower interface density. Sanusiu et al. [396] experimentally studied the effect of graphite nanofibres (GNFs), aspect ratio and power density for the thermal storage capacity and the solidification time of a PCM embedded between two sets of aluminium fins. This research indicates that the impregnation of GNF into PCMs is an effective method to improve the TES capacity and the solidification rate of paraffin-based PCMs. In another study, the enhancement of the heat transfer rate between a PCM (PA–LA (80:20) (PA–LA) and SA–MA (80:20)) and the heat transfer fluid, by using high thermal conductivity additives like stainless steel, copper and graphite, was investigated. The best results for heat transfer enhancement were found for the PCM–graphite composite. However, changing the flow rate did not affect the rate of heat transfer when graphite was used as the additive [397]. Additionally, nanographite was added to organic PCMs to improve their thermal conductivity. Nano-layers of NG were dispersed in paraffin in a random orientation and it was found that the distributed NG enhanced the rate of heat transmission and improved the energy storage technologies in term of efficiency in a PCM. The main advantage of using nanographite is its low cost with the ability to give significant improvement effect in the thermal conductivity [398]. Zheng et al. [399] obtained a composite PCM consisting of a three-dimensional graphene aerogel (GA) and octadecanoic acid (OA). The thermal conductivity of the composite PCM was about 14 times that of pure octadecanoic acid, while the high heat storage capacity was very close to the capacity of the acid. Form–stable composite PCMs prepared by vacuum impregnation of paraffin into graphene oxide (GO) sheets showed no leakage, and greatly enhanced thermal conductivity from 0.305 to 0.985 W/mK [400]. More recently, Xuan et al. [401] have prepared and characterized a functional fluid, magnetic microencapsulated PCM (MMPCM). The structure and surface morphology of MMPCM are illustrated in Fig. 21. They found that the volume fraction of MMPCM particles, the content of iron nanoparticles contained inside the shell of MMPCM particles, and the external magnetic field are the three most important factors influencing the thermal conductivity. Iron nanoparticles can enhance the thermal conductivity of the suspension and make the particles magnetic. The thermal conductivity of MMPCM suspensions can be enhanced by the presence of an applied magnetic field and such enhancement is related with the matching synergic relationship between the orientation of the applied magnetic field and the temperature gradient inside the suspension [401]. Nanofluid PCMs, developed by suspending a small amount of TiO2 nanoparticles in a saturated BaCl2 aqueous solution were studied by Liu et al. [44]. The resulting nanofluid PCM possessed a remarkably high thermal conductivity compared to the

Fig. 21. Structure and surface morphology of MMPCM: (a) structure schematic, and (b) SEM photograph. Reprinted from [401] with permission from Elsevier.

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base material. Recently, Zeng et al. [402] have found that 1-tetradecanol/Ag nanowire composite PCMs showed remarkably high thermal conductivity and reasonably high phase change enthalpy. This behaviour was attributed to the high aspect ratio of the Ag nanowires and their high interface thermal conductivity in the composite PCM. A review paper dealing with the thermal conductivity enhancement of PCMs for thermal energy storage applications was recently published by Fan and Khodadadi [403].

10. Future trends Thermal energy storage using PCMs is a dynamically growing research area and the interest in this research field can be illustrated by the number of research papers published in the last two decades – Fig. 22. It is clear that there has been a significant growth of interest in PCMs for TES since 2002. A possible explanation for this trend is the growing need, particularly in the last decade, to minimize and mitigate the environmental impact of energy consumption in the focus for sustainable development. Thermal energy storage is a necessary component of a modern concerted strategy for energy sustainably and intelligent next generation materials design. Effective thermal energy storage methods are required for the wider application of solar energy – the most abundant renewably energy source, and for novel materials development and application in e.g. the medical and electronics fields. Future trends in the area of PCMs will most likely be related to the use of nano- and bio-based materials. In fact, nanomaterials offer vast possibilities in the design of novel advanced composites with far superior properties to those of traditional materials. If properly designed and manufactured they may exhibit remarkable advantages and provide a new stimulus for the development of more efficient PCMs. However, before biomedical or textile applications are widely used, the safety aspects of nanomaterials, such as nanotoxicity, must be fully investigated. Such studies are currently underway but the long-term effects are still not well understood. Bio-based materials made, at least partially, of renewably resources are an alternative to petroleum-based chemicals, and are already used for cosmetic, biomedical and polymer technology applications. A good example of recent developments is the use of solid–solid PCMs based on polyurethane elastomers made of polyol from rapeseed-oil that have been developed for biomedical applications in implants with temperature control.

Fig. 22. The number of articles dedicated to PCMs for thermal energy storage for the period of 1994–2013. Source: Science Direct, ‘‘phase change materials’’ and ‘‘thermal energy storage’’.

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11. Conclusions It is generally agreed that the intensive investigations undertaken in the last three decades have given latent PCMs significant advantages over sensible systems including the lower mass and volume of the system, higher storage density and lower energy losses to the surroundings. Although the physical properties of most PCMs necessitates the application of additional techniques such as microencapsulation to increase the heat transfer area, reduce reactivity with the outside environment and control volume changes during the phase transition, these materials have found numerous applications in various industrial sectors. Inorganic compounds for energy storage have a higher latent heat per unit volume and a higher thermal conductivity than organic compounds. However, they are more corrosive to metallic materials and supercooling effects may adversely influence their phase change properties although the use of nucleating and thickening agents may mitigate these disadvantages. Organic compounds are promising as low temperature PCMs because they display high phase transition heat capacity, are chemically stable, noncorrosive and exhibit reproducible melting and crystallization behaviour after a high number of thermal cycles. Currently attention is being given to form–stable organic composites such as HDPE-supported composites with paraffins as they are showing evidence of good long term performance and high compatibility with heat transfer agents. An example being a paraffin/expanded graphite composite phase change material prepared by absorbing the paraffin into a micro-porous structure of expanded graphite when the capillary and surface tension forces prevent leakage of the molten paraffin from the porous carbon structure. Highly efficient PCMs which are stable over a large number of cycles are being increasingly applied in various sectors, such as the construction, textile or the rapidly developing electronic industries. The technological enhancement of electronic devices with increased functionalities and reduced form factors requires efficient thermal management for which PCMs offer a viable alternative. However, the economic feasibility of employing latent heat storage materials depends on their service life during which there should be no major changes in the phase change temperature and enthalpy caused by the operational thermal cycling. One of the obstacles that hinder the development of PCM applied technologies is a critical lack of international technical standards for testing PCMs. This has led to research groups employing their own methods for characterizing energy storage materials. Consequently it is difficult to directly compare thermophysical properties such as latent heat of fusion and thermal conductivity and this necessitates the repetition of experiments rather than focusing on the development of novel advanced materials for energy storage. Acknowledgment Authors are grateful to the Polish National Science Centre for financial support of projects under the Contract Nos. DEC-2011/03/B/ST8/05255 (Kinga Pielichowska), and UMO-2011/01/M/ST8/06834 and UMO-2011/02/A/ST8/00409 (Krzysztof Pielichowski). References [1] Kenisarin M, Mahkamov K. Solar energy storage using phase change materials. Renew Sust Energy Rev 2007;11:1913–65. [2] Garg HP, Mullick SC, Bhargava AK. Solar thermal energy storage. Dordrecht, Holland: Reidel Publishing Company; 1985. [3] Lane GA. Solar heat storage: latent heat materials. Background and scientific principles, vol. 1. Boca Raton, USA: CRC Press; 1983. [4] Lane GA. Solar heat storage: latent heat materials. Technology, vol. 2. Boca Raton, USA: CRC Press; 1985. [5] Dincer I, Rosen MA. Thermal energy storage: systems and applications. Chichester, England: Wiley; 2002. [6] Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Convers Manage 2004:1597–615. [7] Fernandes D, Pitié F, Cáceres G, Baeyens J. Thermal energy storage: how previous findings determine current research priorities. Energy 2012;39:246–57. [8] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sust Energy Rev 2009;13:318–45. [9] Zhang Y, Zhou G, Lin K, Zhang Q, Di H. Application of latent heat thermal energy storage in buildings: state-of-the-art and outlook. Build Environ 2007;42:2197–209.

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