Chapter 13

Phase Change Materials

John A. Noël
Samer Kahwaji
Louis Desgrosseilliers
Dominic Groulx
Mary Anne White    Dalhousie University, Halifax, Nova Scotia, Canada

Abstract

Phase change materials (PCMs) store thermal energy via the latent heat of phase transitions. PCMs can be used to provide district cooling (subambient transition temperatures), to buffer thermal swings in buildings (near ambient transition temperatures), and to store solar thermal energy for short-term or seasonal applications (higher transition temperatures). When thermal energy is from an intermittent source, such as solar radiation or waste heat, the ability to store energy in a compact form can provide a readily accessible source of heat. Issues concerning the thermal conductivity, stability, cyclability, phase segregation, supercooling, containment, cost, safety, and sustainability of PCMs are addressed, and research challenges that, when solved, would allow widespread use of PCMs are presented.

Keywords

phase change materials
waste heat
solar thermal energy
phase transitions
supercooling
thermal energy storage
sustainability

1. Introduction

1.1. Thermal Energy Storage

To reduce our dependence on fossil fuel energy sources, renewable energy sources must be developed to meet our power needs and to reliably provide energy. The difficulty with some renewable energy sources, such as the Sun or wind, is their intermittency. Wind turbines cannot operate when the wind is not strong enough or is too strong. Photovoltaic (PV) panels cannot generate electricity at night or on days with thick cloud cover. For direct electricity–generating technologies, the intermittency of the power source can be overcome by charging charge-storage devices, for example, batteries and fuel cells, during peak hours to store energy for later use. For applications using the Sun’s thermal energy, thermal energy storage (TES) materials can be used to store heat. TES can be broadly categorized into two classes: chemical storage and physical storage.
Chemical storage involves the breaking and formation of chemical bonds [1,2], employing a reversible chemical reaction with a large enthalpy change. Heat from the Sun, or other thermal source, drives the reaction to form high-energy products, charging the system. During discharge, the reaction proceeds in the opposite direction, releasing heat. Physical storage utilizes the thermal properties of the TES material to store heat and is the focus of the chapter.
Physical TES can be considered in two categories: sensible heat storage and latent heat storage [1–4]. Sensible heat storage materials store heat via their heat capacities. For a given material the amount of heat that can be stored, Q, depends on both the temperature range over which the material is heated and the mass of the material, m, such that [5]:

Q=∫T1T2mcpdT,

image(13.1)
where cp is the specific heat capacity at constant pressure; T1 is the initial temperature; and T2 is the final temperature. The best sensible heat storage materials are those with high heat capacity. In this regard, water is one of the best sensible heat storage materials, with a specific heat capacity of 4.2 J K–1 g–1 [5]. TES by water is exemplified by differences in climate: consider the moderated temperatures of a maritime region compared with the large temperature swings of dry continental climates. The range of temperatures encountered by the material is also important. The quantity of heat stored is related to the integral over the temperature range, so a wider temperature range leads to more heat stored. The other central term here is the mass. Dense materials such as concrete or stone present a large thermal mass per unit volume and can store a significant quantity of heat.
Over a small temperature range, latent heat storage materials can outperform even the best sensible heat storage materials. These materials store heat through the latent heat of a phase transition and are referred to as phase change materials (PCMs) [1–4,6–8]. When a PCM is heated to its transition temperature, it is converted from one phase to another. If the transition of a pure substance is first order, it occurs isothermally and requires an input of energy at the transition temperature [9]. This input of energy is the transition enthalpy change, also called latent heat. If the phase change is reversible the energy can be recovered through cooling. Furthermore, any input of energy required to heat the material to its transition temperature, and any energy input to raise the temperature of the material after the phase transition, is stored as sensible heat [4]. Therefore, the total quantity of heat that can be stored by a PCM over the temperature interval T1 to T2 is [5]:

Q=∫T1Ttrsmcp,1dT+m∆trsH+∫TtrsT2mcp,2dT,

image(13.2)
where Ttrs is the transition temperature; ∆trsH is the transition enthalpy change; cp,1 is the specific heat capacity of the low-temperature phase; and cp,2 is the specific heat capacity of the high-temperature phase. Fig. 13.1 compares the sensible heat storage of liquid water with heat storage in octadecane PCM.
In principle, PCMs can utilize any phase change transition, but solid–liquid transitions are most prevalent [1–4,6]. While solid–gas and liquid–gas phase transitions often have large transition enthalpy changes (>300 J g–1), they also involve a very large change in volume, making them impractical in most cases. Solid–solid PCMs are used in some applications [10–12]; these transitions usually have only a small volume change, and both phases are immobile and therefore easier to contain. However, solid–solid transitions generally have low transition enthalpy changes (typically <100 J g–1) [10,12]. Therefore, PCMs with solid–liquid transitions are by far the most applied and studied. These transitions have high enthalpy changes (∼200 J g–1), and the transition from solid to liquid phase only has a small volume increase (<10%). The liquid phase requires containment, but this is more easily achieved than the containment of a gaseous phase.
image
Figure 13.1 Comparison of energy stored by liquid water (sensible heat storage) with energy stored (sensible + latent) by octadecane (PCM with melting point 28 °C) over the temperature range (0–50) °C.

1.2. Properties of Phase Change Materials

Many variables must be taken into account when selecting a PCM. Transition temperature is of primary concern. If the transition temperature is not within the temperature range of the application, no phase change will occur, and the material will only store sensible heat. The predicted temperature difference between transition temperature and the charging (hotter) and the discharging (cooler) temperatures also plays a crucial role since it directly influences the overall energy transfer rate to the storage system. In the chapter, materials used in subambient, ambient, moderate, and high-temperature applications are described.
The next major consideration for a PCM is the latent heat (enthalpy of phase change) of the phase transition. To achieve high energy storage density, it is necessary to have as high a latent heat as possible to maximize the energy stored in the phase transition. If space is of concern, the volumetric transition enthalpy change is of more relevance than the gravimetric value and dense materials would be favored in this case.
There are many other important properties to consider for a viable PCM, including thermal conductivity, stability, cyclability, phase segregation, supercooling, containment, cost, and safety [1–4,6–8]. Note that care should be taken when searching the literature for PCM properties: for some compounds, widely varying values of physical properties have been reported for the same material, and some errors have been carried through multiple reviews. For this reason, it is best to consult the original literature sources for the physical properties of PCMs.
Thermal conductivity quantifies the conduction of heat in the material. If thermal conductivity is too low, it will be difficult to charge the PCM, or extract energy from it, in a reasonable amount of time.
For any long-term application the PCM must be stable. The material should not degrade over time, should not react with the ambient air or moisture, and should not degrade its containment vessel. The PCM and container must be stable over thousands of thermal cycles. Over its lifetime, it should not exhibit a significant decrease in latent heat or change in transition temperature. In addition, repeated melting and crystallization should not degrade or otherwise alter the material.
One of the biggest limits to the cyclability of PCMs is separation into different phases [1,2,6]. Such degradation is most common in multicomponent PCMs in which the components differ significantly in density. One component could separate from the other(s) by gravity, altering the melting point of the system. An example is incongruently melting salt hydrates, as discussed in Section 4.1. The segregation of phases can be enhanced over many cycles and lead to a gradual but significant decrease in performance [3,6,7].
A drawback to some otherwise promising PCMs is supercooling. Supercooling, sometimes termed “subcooling,” “undercooling,” or “supersaturation”, refers to persistence of the high-temperature phase below its transition temperature. Below the transition temperature, the high-temperature phase is metastable, but could supercool by 100 K or even more before finally undergoing transition to the stable phase. If the system supercools below the minimum temperature of the application, the latent heat that was stored will not be regained, and after the first heating the PCM would act only as a sensible heat storage material. Fig. 13.2 shows supercooling in the differential scanning calorimetry (DSC) thermogram of a material. In some applications, supercooling can be desirable as it provides a means for long-term storage of energy at ambient temperatures. In that case, nucleation of the solid phase can be artificially initiated when required.
image
Figure 13.2 Schematic representation of supercooling in a material in terms of its DSC thermogram.
The peaks indicate phase transitions and in this case crystallization does not occur until well below the melting point.
The PCM also needs to be compatible with its container. The PCM should not corrode, degrade, or soften/dissolve the material containing it. For melting phase changes the containment vessel must be able to hold the liquid phase without leaking its contents, and in all cases the containment vessel needs to accommodate any volume change associated with phase change and thermal expansion of the PCM over the application temperature range.
If latent heat storage is to achieve large-scale use in an economical way the PCM must be readily available and at low cost. A material with excellent thermal properties and stability might be unsuitable as a PCM if its cost is excessive. In most circumstances a desirable PCM should be safe to use on a domestic scale. This means that it should have low toxicity, should not be violently reactive, and should not pose an elevated fire risk.
In reality, it is difficult or impossible to find a material that is ideal for all criteria. Material selection must be carried out such that the needs of the specific application are met in an optimal way [3].

1.3. Sustainability

In many circumstances the environmental cost of the PCM also should be considered. For example, if the payback time for the embodied energy of the material is high, it might not be recouped over the lifetime of its use. In this regard, production of PCMs from biological, renewable sources is very attractive [13]. Many oily compounds produced by algae and by plants, including oil palm and rape seed, can be used for TES [13]. These sources reproduce quickly and use solar energy as the primary energy input; thus, there exists the potential to extract organic PCMs in a sustainable manner, at little energetic cost. Lifecycle analysis is a valuable tool to quantify the sustainability aspects of a PCM choice [13–15].

2. Heat storage at subambient temperatures

Several materials that qualify as good PCMs have transition temperatures below ambient temperature [2,16] and are most commonly used for cooling applications. Among these PCMs, ice is the most familiar and least expensive. With its high latent heat of fusion (334 J g–1 [5]), it takes about 11 kg of ice to store the equivalent of 1 kW h of energy at 0 °C. Although the ∼10% volume change that occurs when ice freezes puts high demands on its containment, ice has been integrated as a PCM in large cooling projects. For eons, natural ice and snow have been collected in a number of countries during the winter season and stored in underground pits or thermally insulated constructions, to be used to cool goods or buildings during the hot summer months. The heat from the surroundings is normally transferred to the ice by air, either from a fan or by natural convection, or by water circulating in pipes in direct contact with the ice. Projects based on cooling by natural ice and snow storage are discussed in Refs. [17] and [18].
In a similar concept, district cooling uses a central facility to produce and store ice for the purpose of cooling residential and commercial buildings. An example is the Minato Mirai 21 Central District in Yokohama (Japan) [19], which uses an ice storage system to transfer chilled water to cool buildings within the district (Fig. 13.3). In addition to ice, encapsulated PCMs based on fatty acids, paraffins, and salt hydrates with fusion temperatures between –33 and 27 °C are also used in many of the district cooling storage tanks installed by Cristopia Energy Systems around the world [20].
image
Figure 13.3 (a) Yokohama Landmark Tower at Minato Mirai 21, and (b) a simplified schematic of the PCM-based cooling system at Minato Mirai 21 Central District showing the circulation of fluid that removes heat from the buildings. The PCM is frozen at night using off-peak power, thereby shifting the cooling energy from the usual daytime to the night and leveling overall energy demand.
On a smaller scale, systems such as the IceBank designed by CALMAC [21] are installed to individually cool large commercial buildings. This system operates a chiller and an antifreeze solution (i.e., water-glycol solution) as a heat transfer fluid to freeze water stored in large tanks at night, when off-peak electricity rates are the lowest. As with the Minato Mirai 21 district cooling system the ice stored in the tanks is then used the next day to cool the water–glycol solution circulating in heat exchange coils. A fan blows air on the coils to deliver cold air to occupants throughout the building. This system has a smaller footprint on the building infrastructure than the traditional air-conditioning system, since up to 80% less piping [21] and smaller size chillers are required for IceBank installation.
The concept of cooling based on thermal energy storage is becoming increasingly popular worldwide and many energy-conscious corporations have already adopted it. Buildings cooled by ice storage systems include the tallest building in the world, Burj Khalifa in the United Arab Emirates, where the outside temperature reaches 50 °C [22], the Google Data Center in Taiwan, and several buildings in New York City, including the Rockefeller Center, Bank of America Tower, and Goldman Sachs headquarters. The shift of cold production to off-peak hours not only lowers the cooling cost, but also has a positive environmental impact. By easing the load on the power grid during peak hours, CO2 gas emissions from power plants operated by fossil fuels are significantly reduced. It is also good business: it is estimated that the ice storage cooling system saves Goldman Sachs $50 000 per month on their utility bill during the summer [23].
PCMs with a subambient transition temperature also can be found in many commercially available products. Paraffins, glycols, and salt hydrates have been integrated as PCMs in storage containers to maintain the temperature of perishable goods near the cold phase transition temperature of the PCM. For example, containers designed with built-in PCM packs also offer a simple approach to keep temperature-sensitive medical and pharmaceutical products at the desired temperature during transport [18].

3. Heat storage at ambient temperature

PCMs with a phase transition temperature falling within the ambient temperature range (21–28) °C are predominantly used to improve thermal comfort in a living space by reducing temperature fluctuations during the day [e.g., reducing the daily temperature range (20–27) °C, to a smaller range such as (22–25) °C)], or to provide thermally regulated clothing for use in hot environments [24]. Fig. 13.4 illustrates that PCMs reduce temperature fluctuations. The addition of a PCM achieves this leveling effect through the absorption of excess heat generated within the space and the storage of thermal energy at the temperature of the PCM’s melting point. The stored energy is later released during solidification when the freezing point of the PCM is reached, usually at night when the ambient temperature is lower. As in the case of ice storage systems discussed earlier, PCMs integrated in building materials can ease the load on the air-conditioning system during the peak hours of the day when the power rates are more expensive. A list of organic and inorganic PCMs used in building comfort applications along with some of their thermophysical properties has been compiled by Cabeza et al. [25]. These PCMs can be incorporated passively in several different parts of the building, including the wallboards, concrete, insulation, and the ventilation system [26]. Different methods for the incorporation of PCMs into construction materials have been investigated including [27]: direct incorporation, where the PCMs are directly mixed with the construction materials (e.g., gypsum, concrete, mortar); immersion, where the building materials are impregnated with liquid PCM; encapsulation, where either small particles (1–1000) μm of PCMs are enclosed in thin shells (microencapsulation) or several liters of PCMs are packed in containers (macroencapsulation) such as tubes, spheres, and panels before being introduced into building materials.
image
Figure 13.4 Temperature-moderating effect of PCMs in a room over 8 days in August in a building in Ljubljana (Slovenia).
Without PCM, the ambient temperature underwent large fluctuations, whereas a room with PCM only underwent small fluctuations, and achieved a lower maximum temperature. (Reproduced from Ref. [28] with permission from Elsevier.)
The behavior of PCMs incorporated in building materials has been widely studied in different climates, and several commercial products are readily available. Octadecane wax is one example of paraffin PCM used to impregnate wallboards and increase their thermal storage capacity [7]. Paraffin waxes are commonly found in commercial products such as Micronal (R) [29], which is used in wallboards and metal ceiling tiles, and Energain (R) thermal mass panels [30]. Mixtures of fatty acids such as decanoic (aka capric acid) and dodecanoic (aka lauric acid) acids, and esters such as butyl stearate and propyl palmitate, have also been intensively studied for integration in gypsum and concrete [27,31]. These nonparaffin organics are particularly attractive for building applications because they are derived from renewable sources and because they are nontoxic, biodegradable, and can be easily recycled. Impregnation of building materials with salt hydrate PCMs has also been shown to increase their thermal storage capacity. For example, encapsulated calcium chloride hexahydrate (CaCl2·6H2O) has been embedded in concrete slabs to develop a floor-heating system with improved heat storage [32].

4. Heat storage at moderate temperatures

4.1. Moderate-Temperature PCMs

Many PCMs, including many salt hydrates, have their phase transitions in the moderate-temperature range, 40 °C to just over 100 °C. Salt hydrate materials consist of an inorganic or organic salt with one or more waters of hydration. Salt hydrates were the first materials to be investigated for use as PCMs in the ground-breaking work of Telkes [33,34]. Salt hydrates are generally high in density and tend to have greater volumetric heats of fusion than other materials in the moderate-temperature regime. Sharma et al. [1], Pielichkowska and Pielichowska [4], and Farid et al. [6] all reviewed a number of salt hydrate phase change materials. Most salt hydrate PCMs have moderate to high latent heats of fusion, typically (100–300) J g–1 [1,2,4].
While salt hydrates are very attractive PCMs from the standpoint of energy storage density, they do have some shortcomings [1,2,6]. The first challenge is that some salt hydrates melt incongruently: upon heating, a portion of the salt hydrate dehydrates to a less hydrated phase before melting. This phase transition is marked by a peritectic point in the phase diagram. The less hydrated phase typically will not melt in the operational temperature range and will be denser than the solution formed with the water of dehydration. Thus, the less hydrated salt will settle to the bottom of the containment vessel and might not be available to rehydrate upon cooling [3,6]. The resulting phase segregation leads to a gradual irreversible loss in performance. However, there are ways by which this can be overcome. Mechanical agitation and mixing can keep the solid particles suspended, allowing them to be in contact with the fluid phase to recombine on cooling. Thickening agents can be added to increase the viscosity and thereby prevent the solid phase from segregating to the bottom of the containment vessel [35]. In addition, the size of the containment vessel can be modified to reduce the tendency for a salt hydrate to undergo irreparable phase segregation. A smaller container reduces the propensity for the dehydrated solid to settle far from the liquid, increasing the probability of rehydration upon cooling [3].
A second difficulty with salt hydrate PCMs is their tendency to supercool (Fig. 13.2). Even if many crystal nuclei form on cooling, crystallization does not proceed until a crystal nucleus of critical radius is formed. Formation of critical-sized nuclei is a balance between the energy benefit in forming the thermodynamically stable solid phase and the energy cost to create high-energy surfaces at the interface between the solid and liquid. The addition of nucleating agents can help remove the effects of supercooling. Nucleating agents provide surfaces on which crystal nuclei can form, reducing the requirement to form new solid surfaces [33,36]. The nucleating agent must remain solid over the entire temperature range of the application, and the most effective nucleating agents often have crystals similar to the PCM that they nucleate. Another technique used to overcome supercooling is the application of a “cold finger,” which is a piece of thermally conductive material extending into the PCM, attached to a cold source. The cold finger provides a locally cooled area in which critical nuclei can form more readily due to the larger temperature drop, and thereby initiate crystallization throughout the bulk of the material.
A third material property concern for salt hydrates is that in their molten state they are aqueous salt solutions and thus very corrosive to many metals. Care must be taken when selecting materials for their containment to avoid corrosion limiting the system’s lifetime.
Numerous organic materials also undergo phase transitions in the moderate-temperature range including: paraffins (H3C(CH2)nCH3) [37], fatty acids (H3C(CH2)nCOOH) [38–40], and sugar alcohols [41–43]. Fatty acids and sugar alcohols have the attractive feature of availability by extraction from renewable, plant-based sources [13]. Fatty acids have been shown to cycle well, maintaining their thermodynamic properties over hundreds of cycles as illustrated in Fig. 13.5 [38,39,41,42].
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Figure 13.5 Onset temperature for melting and latent heat of fusion for dodecanoic acid PCM over several hundred melt–freeze cycles, as determined by DSC.
The dashed lines indicate standard deviation in the values of onset temperature (i.e., melting point) and enthalpy change for the initial sample. Within 500 cycles, there was no significant change in either property. (Reproduced from Ref. [39] with permission from Elsevier.)
Organic paraffinic and fatty acid PCMs do not undergo the supercooling or phase segregation processes that plague salt hydrates. The melting points of paraffins and fatty acids are correlated with their alkyl chain length, with longer chain compounds melting at higher temperatures. Therefore, by selection of the appropriate chain length, these PCMs can be used in a variety of applications which require phase transition temperatures in the range −56 °C to 80 °C [1,2,4,16,37]. While the gravimetric latent heat of these materials is typically moderate to high (100–250) J g–1 [1,2,4], they tend to have low densities (<1 g cm–3) and thereby lower volumetric energy density than salt hydrates. The thermal conductivities of these organic materials are also quite low (∼0.2 W m–1 K–1 [2]), resulting in low inherent rates of charge/discharge. There are ways by which the effective thermal conductivity can be increased, such as the introduction of metallic particles or nanoparticles [44–46], insertion of metallic structures such as fins or rods [47–49], or impregnation of the PCM in graphite [50,51].
Sugar alcohols differ from paraffins and fatty acids in that they have higher densities, leading to higher volumetric latent heat, in some instances exceeding 400 J cm–3 [1,2,4]. However, sugar alcohols, unlike other moderate-temperature organic PCMs, undergo significant supercooling, as much as ca. 100 K below their melting point. This extreme supercooling is likely due to kinetic limitations in the formation of crystallites, a result of the complex hydrogen-bonding networks in sugar alcohol crystal structures [52,53].

4.2. Applications of Moderate-Temperature Phase Change Materials

4.2.1. Solar Thermal Hot Water

A major application of PCMs in the moderate-temperature range is in domestic solar hot-water applications. A heat transfer fluid, such as water or glycol, circulated through a flat plate or an evacuated tube solar collector, collects solar radiation as heat to be used to heat water for household use. This type of system can replace or supplement electric- or gas-powered hot-water heaters, for substantial energy savings and reduction in CO2 emissions. Solar hot-water systems require energy storage, sensible or latent. Water could be heated and stored in a large tank to be used directly or by heat exchange with cold water, when there is insufficient solar gain to heat cool water. This type of sensible storage is, however, necessarily quite large and massive, possibly ruling out its use in some domestic or commercial locations, especially for retrofitting where space is problematic. If, instead, latent heat storage with a PCM is used, the volume and mass requirements are much lower [54], in some instances allowing use of solar thermal hot water when otherwise only nonrenewable sources would be feasible. In this case, solar energy can be used to charge a much smaller volume of a PCM, and cool water can then be heated by circulating through a heat exchanger in the PCM tank.
Choice of PCM is important for this application. It is desirable to have a high melting point PCM to provide water at a higher temperature. However, if the melting point of the material is chosen based on the maximum achievable temperature in the heat transfer fluid from the collector on a day with high solar gain, on days with less ideal conditions the PCM would only melt partially, if at all. In the latter case the bulk of the PCM would only provide sensible heat storage. Transfer of heat to the PCM also is important. The time during the day when there would be sufficient solar gain on the collector to melt the PCM is limited, so heat must be transferred efficiently. Most moderate-temperature PCMs have low thermal conductivity, so the heat exchange system must be designed to promote melting across the entire PCM. Numerous solar water heater designs employing PCMs have been explored [55–59].

4.2.2. Seasonal Heat Storage

Long-term “seasonal” heat storage is characterized by multi-month to multi-annual heat storage retention. Two principal mechanisms contribute to the ability of a PCM heat storage system to accomplish seasonal heat storage: insulated thermal mass and stable supercooling. The first mechanism is common to all heat storage media and not unique to PCMs, whereas stable supercooling is a unique property of PCM storage [60].
Insulation minimizes the rate of heat loss from a heat storage mass relative to its storage capacity. Improved insulation around smaller heat storage vessels can be practical but limited by cost and maximum feasible volume, while the inherently low surface-area-to-volume ratio of larger heat stores requires less effort to insulate adequately [60]. Buried and bermed (semiburied) heat storage vessels (Fig. 13.6) are especially effective to insulate large heat stores when the cost of above-ground insulation is prohibitive [60]. Higher temperature PCM storage requires greater efforts to combat self-discharge by heat loss than lower temperature storage.
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Figure 13.6 Buried and bermed tanks[60]. (Reproduced with permission from Elsevier.)
Stable supercooling PCMs are materials that readily supercool and can remain supercooled at ambient temperatures for seasonal durations (e.g., some salt hydrates and sugar alcohols) [61–65]. Supercooling of the PCM to ambient temperature has the advantage of long-term storage without heat loss (i.e., no self-discharge).
Practical supercooling is limited by the volume of the PCM and the degree of supercooling (difference between transition temperature and storage temperature), where larger volumes and greater supercooling increase the probability of autonucleation [3]. There is no available theory to predict the maximum practical contiguous size of a supercooling PCM [3], and limits need to be defined empirically by incremental scaleup.
As a consequence of stable supercooling, these PCMs require a solidification triggering mechanism to initiate heat discharge on demand. Compression springs and other prestressed materials that generate hard surface contacts are useful internal mechanical triggering devices [66,67], while the addition of seed crystals and cascading solidification through a capillary channel require external material input [3,68]. Although PCMs can be chilled to their autonucleation point [69,70], it is rarely practical to use this mechanism to initiate crystallization. Ultrasonic vibration and electric fields do not appear to reliably initiate solidification in most PCMs [68,71].
Heat released during the initial solidification of a supercooled PCM elevates the temperature of the PCM to its corresponding two-phase equilibrium state (isenthalpic process, Fig. 13.7 [61,63,70]). The degree of supercooling influences the quantity of retained thermal energy available for discharge and thus the storage efficiency and, for incongruent PCMs, the maximum discharge temperature achieved at the onset of solidification (Fig. 13.7 [61,63]). These factors limit the effectiveness of higher transition temperature supercooling PCMs when supercooled to room temperature.
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Figure 13.7 (a) Enthalpy–temperature profile of diluted NaCH3COO·3H2O including supercooling and solidification [72], and (b) relative enthalpy profile for a complete charge–discharge cycle for diluted NaCH3COO·3H2O [63]. (Part a: reproduced with permission from Elsevier.)

5. Heat storage at high temperatures

5.1. High-Temperature PCMs

For high-temperature heat storage, in the range of several hundred to well over 1000 °C, the PCMs used are metals and anhydrous salts. For the latter, various carbonates, chlorides, sulfates, nitrates, and nitrites are commonly used, as well as their eutectic mixtures [73]. Nitrates and nitrites typically melt in the range (300–550) °C, but have relatively low latent heat, ∼(100–175) J g–1 [74]. Carbonates and chlorides do not melt until temperatures above 700 °C, but have higher latent heat, generally above 200 J g–1 [74]. Fluoride salts provide very high latent heat, 790 J g–1 for LiF/CaF2 eutectic, but are typically avoided due to cost and material compatibility [74]. Containment of molten salts at high temperatures introduces material-related challenges, and molten salts also have high safety risks. Molten salts are corrosive to many steels and their vapors are often reactive, and therefore expensive alloys and coatings are required to achieve an acceptable lifetime for the system [73]. Salts also have low thermal conductivity, so metal or graphite rods or fins are required to transfer heat within the bulk of the PCM [74]. Although metals and metal eutectics are more expensive than most salts and typically have lower latent heat, molten metals are less corrosive than molten salts, and they have high thermal conductivity, such that in some cases the use of metals is competitive with salts. Liu et al. [74] have reviewed many salt and metal PCMs and their eutectics.

5.2. High-Temperature Applications

5.2.1. Concentrated Solar Power: Andasol

Solar thermal power stations use large volumes of molten salt for thermal energy storage. The use of thermal storage in concentrated solar power (CSP) plants decouples power production from solar radiation availability [74,75]. Other solar power systems, such as PVs, require direct solar radiation to produce electricity, but thermal energy storage in CSP allows for night-time power production and mitigates the drop in production on days with nonideal solar conditions. In most thermal storage systems for solar thermal power plants, only the sensible heat of the molten salt is used. The Andasol Solar Power Station in Andalusia (Spain) is one such example. Andasol consists of three 50 MW projects, each producing about 165 GWh of energy annually [76]. Andasol is a parabolic trough-type solar thermal station and operates just under 400 °C [75,76]. In this configuration, parabolic mirrors focus solar radiation onto a system of pipes containing biphenyl–diphenyl oxide eutectic as a heat transfer fluid [77], which carries heat to a steam generator and turbine, and to tanks of molten salt for storage. Andasol 1 has two tanks for thermal energy storage, holding 28 500 t of salt, a mixture of 60% NaNO3 and 40% KNO3 [75,76]. The 28 500 t of salt used in Andasol 1 gives 1000 MW h of stored energy when heated from 300 to 400 °C and can power the station for 7.5 h [75,76], allowing the station to produce electricity through the night or on cloudy/rainy days.

5.2.2. Industrial Heat Scavenging

Excess process heat in continuous processes is routinely and economically captured in heat exchangers called economizers and using combined or cogeneration cycles [78,79]. Continuous capture is not feasible in batch and semibatch processes due to their inherent intermittency, but they present opportunities for heat recovery via heat storage.
High-temperature (>250 °C) batch and semibatch processes are common in metal foundries, pulp mills, and the cement industry [78,80]. Sensible heat storage masses (e.g., bricks) are presently used to store excess heat from exhaust gases in large furnaces, then used to preheat inlet gases to active furnaces [78]. Suitably encapsulated PCMs could be readily deployed to substitute bricks for thermal mass and improve the heat storage densities in these processes since operating temperatures are well defined and storage durations are short.
Similar opportunities could also exist with the secondary heat exchange fluids used in batch or semibatch processes (e.g., heat transfer oil or process steam). PCM heat storage could be implemented in a reservoir serving a closed loop for added thermal mass, and used either to preheat the heat exchange fluid at the beginning of the successive batch operation, or to preheat the equivalent fluid in the alternating batch. The former scenario is analogous to using a PCM to store excess automotive engine heat accumulated by the engine oil; this stored heat is used to mitigate engine cold-start inefficiencies by preheating the engine oil/engine block [81].

6. Heat transfer in PCM-based thermal storage systems

Heat transfer is a major issue for PCMs and their storage systems, for both charging (PCM melting) and discharging (PCM solidifying) modes.
When energy is added to a fully solid PCM, either through the flow of a heat transfer fluid (HTF) on the outside wall of the PCM encapsulation or from heat generated internally (e.g., Joule heating from an electrical element), heat is carried to the PCM first by conduction, resulting in the phase transition (melting) of the first layers of PCM. Most of the energy added is stored as latent heat in the PCM with the remaining energy increasing the temperature of the PCM. Once enough PCM has melted, the main heat transfer mode changes from conduction to natural convection resulting in higher temperature liquid PCM moving upward in the system, melting the upper portion of the PCM faster (Fig. 13.8). From this point, natural convection dominates the melting process until the entire PCM is melted, providing higher heat transfer rates than for conduction alone. Higher temperature differences between the heat source and the PCM result in faster energy storage and melting. To a lesser extent the increased flow rate of the HTF, which increases the strength of forced convection on the outside surface of the PCM, also results in faster storage rates [82].
image
Figure 13.8 Melting of dodecanoic acid in a rectangular cavity with the right wall at 60 °C.
(a) After 10 min of heating, conduction is the dominant heat transfer mode. However, the influence of convection is clearly apparent at (b) 90 min and (c) 170 min. (Reproduced from Ref. [83] with permission from Elsevier.)
When the PCM is fully liquid and energy is removed from the system through a cold-temperature source (e.g., circulation of a cold HTF), the first layers of PCM contacting the outside surface cool and start solidifying on the surface. (Note that the solidification temperature might be different from the melting temperature due to supercooling.) The solidification process is completely controlled by conduction and is therefore typically slower than melting for the same system geometry [84]. The PCM solidifies in successive layers, increasing in thickness and thereby imparting an ever increasing additional thermal resistance between the cold surface and the remaining liquid PCM. Changing the HTF flow rate has little effect on the energy extraction rate, although reducing the HTF cold-temperature results in an increase in the discharge rate.
The major thermal problem in PCM storage systems could be quantified as a “rate problem,” that is, the theoretical quantity of energy that can be stored can be directly calculated based on system volume and PCM properties, but the heat transfer rates for energy storage or discharge are inherently small for most PCMs due to their usual small thermal conductivities (∼0.2 W m–1 K–1 [2]). This rate problem requires significant research and design to have a system store the right amount of energy in the right amount of time. Various methods are used to increase the overall transfer rates during both charging and discharging, although discharging is often the more limiting factor. Most solutions are geometrical: adding fins to the PCM-side of a system, and tailoring the encapsulation shape to increase the available surface area for heat transfer [85], or using a specifically designed heat exchanger to enhance heat transfer rates [86]. Fig. 13.9 shows several possibilities for increasing heat transfer in a cylindrical system.
image
Figure 13.9 Various examples of heat transfer enhancement methods used for PCM energy storage and recovery (a–p). (Reproduced from reference [49] with permission from Elsevier.)
A third mode of operation is theoretically possible for PCM storage systems: allowing the heat source and cold source to operate at the same time, leading to simultaneous charging and discharging of the system. Very little research has been reported for this mode of operation in which the system is designed for heat transfer both through the PCM and the enhancement features (geometry, fins) of the system [87].
An additional mode of heat transfer is sometimes present during melting of a PCM: close contact melting (CCM). CCM is present when a solid PCM is in close contact with a warmer solid surface below it [88], such that heat is transferred from the surface to the PCM through a thin melted PCM layer, providing higher heat transfer rates since the heat conduction resistance of this thin layer is extremely small. Spherical encapsulation in a packed bed storage system is an example of a system using CCM [89].

7. Gaps in knowledge

In both the materials science and applied science aspects of PCM science, there is much yet to be learned to have widespread implementation of PCMs in active and passive energy systems. Disparate efforts by researchers have indeed advanced knowledge of PCMs, but a comprehensive and unified effort in key areas could reduce barriers, uncertainty, and risk for future deployment of PCM heat storage technologies.
In fundamental evaluations of PCM thermophysical properties, liquid-phase properties are known much better than solid-phase properties [39]. Even then, liquid PCM viscosity, essential for proper modeling, and understanding of natural convection during melting, is not always determined or reported. Thorough investigations of both liquid and solid phases of potential PCMs would better equip scientists and engineers to determine the suitability of candidate materials, as well as help identify errors in property determinations that are committed in the absence of meaningful comparisons; for an example, see Ref. [39]. A reliable database of PCM properties would be very useful in this regard.
Conventional PCM groupings oversimplify the nature of phase change itself and provide no additional insight for new material discoveries with respect to Ttrs and ∆trsH [90–92]. Details of the atomic and molecular bonding in each of the phases reveals commonalities in the entropy changes associated with the transitions, thus providing an atomic-level basis for PCM classification [90–92] and a rational way to search for candidate PCMs.
The fundamental limitations to practical supercooling of PCMs remain undiscovered. Simple models have been used to approximate intensive limits on the maximum degree of supercooling [3], but the degree and duration of supercooling is fundamentally an extensive property [3], depending largely on the amount of PCM as the determinant of the probability of autonucleation. In the absence of accurate models, supercooling PCM heat storage devices are designed conservatively to insure the desired supercooling behavior and considerable experimental effort would be required to relax these conservative limits.
Validated methodologies to predict PCM heat exchanger sizing and performance, analogous to conventional heat exchanger design, are another gap [93]. The absence of standard heat exchange methodologies for PCM energy storage systems leads engineers to rely heavily on capital-intensive incremental scaleup and computationally expensive computer simulations to predict PCM heat exchange and heat storage performance. A normalized understanding of heat exchanger performance for PCM heat storage would bridge these gaps in scaleup more easily [93], ultimately allowing greater effort to be allocated to optimization.
It has been noted recently that the results of numerical modeling of phase change heat transfer, including melting and solidification, are increasingly published without any experimental validation [94]. Much of this numerical work relies on built-in features in commercial software, for which some parameters, such as the Carman–Koseny or mushy zone constant, are not properly validated through experimental studies [95]. For such studies the limit of applicability of the model is difficult to determine, and in some cases it is apparent that the results are unphysical. With the ever increasing computing power available for numerical studies, it is time to gain a better understanding of the models through validation with well-designed experiments.

8. Outlook

PCM-based thermal storage might be the oldest form of energy storage known to humanity, as our ancestors valued ice for exactly that purpose. Today, what could be termed “cold storage” is well developed and finds applications in food preservation and peak shifting of energy loads from day to night. All this is possible through the use of the best and most abundant PCM: water.
The use of other PCMs, especially for storage at higher temperatures, has seen slower development. With the worldwide need for renewable energy sources and their intermittent nature, along with improved PCM characterization, PCM thermal storage is now becoming an important player in energy technologies. Numerous pilot projects are under way in which PCMs are incorporated in buildings as part of the building structure. Research, backed by various industries, is also under way looking at incorporation of PCMs in electronic components, PV panels, clothing, and packaging for temperature control, as well as in applications for waste heat harvesting.
The largest challenges in the further development of PCM thermal storage are design and integration of PCMs into particular applications, with the rate problem at the forefront of those considerations. From an engineering point of view, most experimental and modeling research concerning PCMs concentrates on accurate understanding of the overall thermal and energy behavior of PCMs and PCM systems, and mechanisms to address the rate problem. Industry is adding to this mix by looking at economical ways to achieve the desired storage.
Encapsulation of PCMs is still an important issue to be considered, with solutions optimized for heat transfer, cost, and ease of manufacture yet to come. Breakthroughs in this area will increase the use of PCM thermal storage. Finally, some chemists, materials scientists, and engineers in the field are investigating another critical matter: development of novel PCMs with enhanced physical properties. Such materials could help solve the rate problem, reducing the required size of storage systems, while facilitating manufacturing and encapsulation processes.

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