Using solid-liquid phase change materials (PCMs) in thermal energy storage systems

Using solid-liquid phase change materials (PCMs) in thermal energy storage systems

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F. Bruno, M. Belusko, M Liu, N.H.S. Tay University of South Australia, Australia

9.1 Introduction A phase change material (PCM) is a material that changes phase at a certain temperature. During the phase change process, a PCM absorbs or releases a large amount of heat in order to carry out the transformation. This action is known as the latent heat of fusion or vaporisation, and through this process energy is stored.

9.2 Principles of solid-liquid phase change materials (PCMs) 9.2.1 Classification of PCMs The two main prerequisites of a PCM are a suitable phase change temperature and a large melting enthalpy. However, for most applications, more requirements are needed, and these can be grouped into physical, technical and economic. Physical refers to repeatable phase change, minimal supercooling and good thermal conductivity. As for technical, low vapour pressure, small volume change, chemical stability and compatibility of the PCM with other materials are required. Finally, economic refers to the cost of the raw materials of the PCM and the ability to recycle or easily dispose of the material at the end of its life (Mehling and Cabeza, 2008). PCMs have been utilized for various heat storage systems since the 1800s (He and Setterwall, 2002). The classification of PCMs (Cárdenas and León, 2013) is shown in Figure 9.1. When a PCM is used as the storage material, the heat is stored when the material changes state, defined by latent energy of the material. The four types of phase change are solid to liquid, liquid to gas, solid to gas and solid to solid. PCMs that convert from solid to liquid and back to the solid state are the most commonly used latent heat storage materials (Mondal, 2008). The phase change between solid to liquid and vice versa by melting and solidification can store large amounts of cooling or heating. The best example is water-ice where solidification and melting occurs at a constant phase change temperature of 0°C. The stored energy can be calculated through the enthalpy difference (Mehling and Cabeza, 2008; Pincemin et al., 2008). Advances in Thermal Energy Storage Systems. http://dx.doi.org/10.1533/9781782420965.2.201 Copyright © 2015 Elsevier Ltd. All rights reserved.

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Storage material

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Water

Metals

Oils

Paraffins

Non paraffins

Molten salts

Fatty acids

Esters

Solid-liquid

Liquid gas

Inorganics

Organics

Alcohols

Figure 9.1 Classifications of phase change materials (Cárdenas and León, 2013).

Salt compositions

Metallic alloys

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Salt hydrates

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The PCMs used can be organic and inorganic materials. Organic materials include paraffins and non-paraffins such as fatty acids, while inorganic materials comprise salt hydrates, saline composites and metallic alloys. Figure 9.2 illustrates both sensible and latent thermal energy storage. Relative to sensible energy storage, the main advantages of such storage systems are the large storage capacity and the potential recovery of thermal energy at almost constant temperature (Choi and Kim, 1995; Agyenim et al., 2010a). Another advantage of using PCMs for thermal energy storage (TES) compared to sensible storage media, is the ability to store large amounts of energy where the temperature difference between the heat source and sink is low. This property makes them useful for thermal comfort in buildings (Riffat et al., 2013; Pomianowski et al., 2013; Whiffen and Riffat, 2013a, 2013b; Cabeza et al., 2011), solar heater systems (Halawa et al., 2005), stationary refrigeration (Tay et al., 2012a; Oró et al., 2012), mobile refrigeration (Liu et al., 2012a), low energy cooling (Helm et al., 2009; Waqas and Ud Din, 2013), concentrating solar power (CSP) plants (Liu et al., 2012b; Gil et al., 2010) and many other applications. In recent years, research on high temperature PCMs with melting temperatures above 300°C has been increasing due to its potential application for CSP plants (Gil et al., 2010). The majority of CSP plants installed today require PCMs with a melting range of 300–550°C. However, with the investigation of heat transfer fluids such as supercritical CO2 and other molten salts, PCMs with a temperature above 550°C will be required in the near future (Liu et al., 2012b). High temperature PCMs include inorganic salts, salt eutectic compounds, metal alloys and metallic eutectics. Among these, inorganic salts are of great interest and have been investigated by several researchers. Tables 9.2–9.4 show the inorganic salts that have been investigated, together with their thermophysical properties. Metals and metal alloys have not been as popular as inorganic salts due to the difference in weight. However, when volume is an important factor, metallic materials are good candidates. Metallic materials are also a good candidate for temperatures above 550°C. Table 9.4 presents metallic materials with its thermophysical properties. Temperature

Temperature of phase change

Sensible

Latent

Sensible

Sensible Stored heat

Figure 9.2 Latent heat storage for the case of solid-liquid phase change (Mehling and Cabeza, 2008).

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9.2.2 Advantages and disadvantages of organic and inorganic PCMs Generally, PCMs can be categorised as inorganic and organic compounds. Most organic PCMs such as paraffin waxes are chemically stable and non-corrosive. They also display little or no supercooling properties, high latent heat and are recyclable, making them well suited to most building materials. However, they have disadvantages such as low thermal conductivity, large volumetric change during the phase change process and flammability (Bruno, 2005). As a consequence, most organic PCMs need to be encapsulated in a container before being used in a thermal energy storage system. This additional requirement not only increases cost, but also reduces the performance of the system because of the increased thermal resistance caused by the encapsulation (Chen et al., 2008). Compared to organic compounds, inorganic compounds such as salt hydrates and salt composites have much higher latent heat per unit volume, higher thermal conductivity, are lower in cost, recyclable and are non-flammable (Bruno, 2005). However, one major disadvantage is that they are corrosive to most metals, which results in their short life span as well as the high cost in packing and maintenance (Chen et al., 2008). Unlike organic compounds, inorganic compounds can also undergo phase separation and supercooling, which will greatly affect their phase change properties (Bruno, 2005; Liu and Awbi, 2009). Metallic materials, however, do not possess the disadvantages of the inorganic salts, making them a potential solution for high temperature PCMs.

9.3 Shortcomings of PCMs in thermal energy storage systems PCMs have many advantages compared with sensible storage substances. However, some shortcomings still remain in the development of reliable and practical storage systems, such as incongruent melting, supercooling, low thermal conductivity and insufficient long-term stability. These shortcomings limit their applications. However, research and development of new materials is overcoming many of the shortcomings. It is not possible to find a PCM with all the requirements mentioned in Section 9.2.1. However, some methods have been formulated to resolve or avoid potential problems of the PCM. Some of these strategies are discussed in the following sections.

9.3.1 Incongruent melting/phase separation Most hydrated salts melt with decomposition as temperature increases, forming water and a lower hydrated salt. This process is called incongruent melting as presented in Figure 9.3. The lower hydrated salt usually sinks to the bottom as its density is higher than that of water. Consequently, only a thin layer on the top of the salt will recrystallise in the freezing process. Incongruent melting is an irreversible process

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Figure 9.3 Incongruent melting in hydrated salt (Streicher, 2006).

and will considerably reduce the storage efficiency (Farid et al., 2004). It was found from experimentation that the thickness of the PCM in the vertical direction affects the incongruent melting when developing inorganic salt-water solutions as PCMs. When the thickness is very small (e.g. <40 mm), no incongruent melting was observed during the experiment. However, when the inorganic salt-water solutions were tested at a thickness of over around 70 mm, salt deposited at the bottom of the container. For example, a salt-water solution with a composition of 10 wt.% of salt and 90 wt.% of water will remain a homogeneous liquid above –4°C. When this substance is cooled below – 4°C, water freezes out of the substance and the remaining substance has a higher salt concentration. As a result, with repeated freezing and melting, the substance will separate back into its two constituent components. Four methods to prevent incongruent melting are: the addition of suspension media or thickening agents, the extra water principle, chemical modification and dynamic melting (Abhat, 1983; Tay et al., 2013). Suspension media or thickening agents are used to keep the lower hydrated salt in suspension and thus assist in recrystallisation. Nevertheless, thickening agents displace a part of the PCM in the system. As a result, the volumetric heat storage capacity and the melting point of the PCM are lower than those of the pure substance. The extra water principle involves adding extra water to the hydrated salt, making the PCM a saturated salt solution at the melting point. Therefore, the saturated solution and the hydrated salt are in the system at temperatures below the melting point. This method again reduces the volumetric heat storage capacity of the system. Also, the melting range becomes broader (Mehling and Cabeza, 2008). A chemical modification technique has been

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suggested by Carlsson et al. (1979) and Carlsson (2009) to prevent incongruent melting in calcium chloride hexahydrate. In this method, a small amount of an isomorphous salt is added in the hydrated salt and they will form a solid solution. Consequently, the melting point of the modified PCM is altered and thus the formation of lower hydrated salt is avoided. Dynamic melting involves mixing the PCM when it is in the liquid state, keeping the solution homogeneous (Tay et al., 2013). While this method does not reduce the volumetric storage capacity nor alter the melting point of the PCM, it does require a means of mixing the PCM.

9.3.2 Sub-cooling When some molten PCMs are cooled, they solidify at a temperature below their melting points. Figure 9.4 shows the sub-cooling during a freezing experiment with a eutectic solution made from an inorganic salt. The reason for sub-cooling is either the rate of nucleation or the rate of growth of the nuclei (or both) is slow (Dincer and Rosen, 2002). Sub-cooling will reduce the usability of PCMs, and can also completely prevent heat recovery if too severe (Lane, 1992). Sub-cooling only occurs during solidification. During sub-cooling, latent heat will not be released when the phase change temperature is reached. Instead, the temperature of the material will gradually decrease until such a point is reached that crystallisation begins. If crystallisation fails to happen, latent heat will be trapped in the material and thus the material only stores sensible heat. Therefore, sub-cooling could pose a significant challenge in PCM storage applications (Mehling and Cabeza, 2008). With sub-cooling, the efficiency of the cooling system will be reduced because lower temperatures are required to initiate freezing (Wang et al., 2002). Sub-cooling can be overcome by adding an appropriate nucleating agent with a crystal structure similar to that of the PCM (Lane, 1992). Nucleating agents, also named ‘seed-crystals’, can be utilised as nuclei for the PCM crystals to grow on them during the freezing process. Another method to prevent supercooling is the Temperature

Temperature

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Sensible

Melting temperature

Latent

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Nucleation

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Sensible

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Figure 9.4 Effect of sub-cooling on heat storage: (a) with little sub-cooling and nucleation, (b) severe sub-cooling without nucleation (Mehling and Cabeza, 2008).

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cold finger technique (Telkes, 1980). A nucleating device (tubular or other external attachments) is maintained cooler than the maximum supercooling temperature, and hence promotes nucleation by the formation of crystals. Sub-cooling can be reduced by the application of a surface roughness such as electrolytically polished surface or the porous surface on the system (Saito et al., 1990). Sub-cooling often occurs in encapsulated PCM systems due to the small volume of PCM presence in the nodules (Bédécarrats et al., 2009). When bulk PCM is used in a large PCM system, sub-cooling is unlikely to occur (Tay et al., 2012b).

9.3.3 Low thermal conductivity and heat transfer rate Most PCMs with high density have an unacceptably low thermal conductivity (Chow et al., 1996; Farid et al., 2004). This calls for the use of appropriate heat transfer enhancement techniques in latent heat thermal storage. During a phase change process for freezing, phase change starts at the heat transfer surface, causing the solid/liquid boundary of the PCM to move away from the heat transfer surface. This phase changed portion of PCM acts as an insulator, reducing the heat transfer to the HTF, thus increasing the thermal resistance. The heat transfer through the solid PCM is solely by conduction and due to its low thermal conductivity; the heat transfer rate within the PCM is low (Tay et al., 2012b). PCMs are known to have a low thermal conductivity. A low thermal conductivity reduces the transfer of the energy in and out of the PCM (Agyenim et al., 2010a). Generally, the melting process of the PCM is much faster than the solidification process. This is due to the effect of buoyancy during the melting process assisting the heat transfer process (Najjar and Hasan, 2008). To resolve the problem of low heat transfer rate, several heat transfer enhancement techniques are available.

9.3.3.1 Larger heat exchanger surface The main heat transfer resistance in a PCM storage system which utilises a heat transfer fluid is on the PCM side. Therefore, in order to reduce the thermal resistance of the PCM, extended surfaces on the PCM can be used. Fins embedded in the radial and axial directions on a tube surrounded by PCM have been investigated. It has been found that using annular fins is the most effective method (Lacroix, 1993). Choi et al. (1996) found that there was no difference between thin finned and unfinned tubes. The heat transfer coefficient of the thick finned tube system, however, was found to be two times higher than the unfinned tube system. An experimental validation of a mathematical model of a finned flat wall in contact with paraffin wax under solidification was conducted by Ito and Miura (1991). It was found that the model showed good agreement with experimental results. Finned tubes with different configurations have been proposed by various researchers (Morcos, 1990; Costa et al., 1998; Padmanabhan and Krishna Murthy, 1986; Velraj et al., 1997, 1999; Ismail et al., 2001; Ismail and de Jesus, 2001). Fins embedded in the PCM have been researched extensively because of their simplicity, low cost

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and ease of manufacture. Impregnation of metal matrix into the PCM is the next commonly researched technique for heat transfer enhancement. Some of the metal matrix investigated was carbon fibre and carbon brushes (Agyenim et al., 2010b). Agyenim et al. (2009) compared the results of circular and longitudinal finned tube systems with an unfinned tube system (see Figure 9.5). It was found that the radial temperature difference in the finned tube systems was consistently larger than in the unfinned tube system. The axial temperature difference for the finned tube systems was, however, smaller than that for the unfinned tube system in the liquid phase (Choi and Kim, 1995; Horbaniuc et al., 1999). Also, when carbon fibre was compared with carbon brushes, it was found that the carbon fibre achieved better heat transfer. However, in terms of overall heat transfer, the carbon brushes were better than the carbon fibre (Hamada et al., 2003). An experimental investigation was conducted by Castell et al. (2008) for vertical fins attached to vertical tubes. It was found that the fins dramatically increased the heat transfer from the PCM modules to the surrounding fluid. The time taken to freeze the PCM without fins was 17 min. Using a PCM module with 20 mm fins, the time taken was 13 min (23.53% reduction). With the PCM module with 40 mm fins, the time taken was 7 min (58.82% reduction). Fan and Khodadadi (2011) carried out a comprehensive review of the thermal conductivity enhancement of PCM. Humphries and Griggs (1977) made use of metallic fillers such as metallic wool, foam and honeycomb to enhance the thermal conductivity of the PCM. It was found that the honeycomb option achieved better heat transfer compared to the other two options. Hoover et al. (1971) did further work with aluminium honeycomb metal filler and found that it resulted in the largest increase in the thermal diffusivity of approximately 80%. De Jong and Hoogendoorn (1981) presented two enhancement techniques to improve the heat transfer in latent heat storage systems. The two techniques used were finned copper tubes as well as using a metal matrix structure in the PCM. The finned copper tubes had shown a great decrease in the solidification times of the PCM, while the metal matrices were found to reduce the solidification times by a factor of 7. Abhat et al. (1981) presented an experimental investigation of a heat of fusion storage system for solar energy using a finned-annulus heat exchanger, in which aluminium metal fins were placed radially in the container filled with PCM. It was also found that depending on the geometry of the system and the distance between the fins, the effect of buoyancy was greatly suppressed with the existence of fins in the system. This was further PCM

PCM

HTF

HTF

(a)

(b)

PCM

HTF Circular fins Longitudinal fins

(c)

Figure 9.5 Cross-sectional views of the (a) control, (b) circular finned and (c) longitudinal finned PCM systems (Agyenim et al., 2009).

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verified by Epstein and Cheung (1983), who found that the presence of metal fins significantly suppressed the effect of buoyancy; however, overall the heat transfer process was still greatly enhanced by conduction. Other researchers also found similar dramatic improvements in heat transfer by embedding a metal matrix within PCMs (Hoogendoorn and Bart, 1992; Trelles and Dufly, 2003; Tong et al., 1996). Chow et al. (1996) conducted a study on the encapsulation of various shapes of PCMs and a technique involving a metal/PCM composite. It was found that the composite provided a better enhancement in the thermal conductivity of the PCM. Velraj et al. (1999) performed three types of heat enhancement techniques on an aluminium cylindrical tube filled with paraffin wax. The three heat enhancement techniques were internal longitudinal aluminium fins with a cross-shaped cross-section, lessing rings of 1 cm diameter distributed in the tube and water/vapour bubbles that randomly appeared in the tube. It was found that the utilisation of lessing rings had the best enhancement results of the reduction of the solidification times by a factor of 9, while the internal fins was the next better enhancement technique. However, the third technique did not have any remarkable results. It was also found that with the addition of lessing rings in the paraffin, the thermal conductivity was increased by 10 times. The Lessing rings (Figure 9.6) are made of steel and have a thin-walled hollow cylindrical structure with a partition. Without partition, these rings are known as Raschig rings. Research has been conducted to improve the thermal conductivity of the PCM at the particle level. Siegel (1977) conducted a study on the improvement of the solidification rate in molten salt where high conductivity particles were dispersed in the PCM. A 17% improvement in the heat transfer rate was achieved with 20% volume of the particles. Considerable research has been conducted on developing

Figure 9.6 A photographic view of Lessing rings (Velraj et al., 1999).

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PCM–graphite composites since graphite has a very high thermal conductivity. Cabeza et al. (2002) and Py et al. (2001) investigated PCM/graphite inside metal modules and submerged in water. It was found that there was a heat barrier from the metal modules to the water. A paraffin/expanded graphite composite PCM was developed by Yin et al. (2008). This composite was prepared by absorbing liquid paraffin into the pores of expanded graphite. No liquid leakage was experienced during the phase change process and an improved thermal conductivity of 4.676 W/m·K relative to pure paraffin of 0.2697 W/m·K was observed. However, Kim and Drzal (2009) found that with the addition of expanded graphite, the latent heat capacity of the composite decreased. To resolve this problem, a new composite of paraffin and exfoliated graphite nanoplatelets (xGnPTM) was investigated. The results showed that the thermal conductivity of the new composite improved with increasing xGnP loading contents and there was no significant difference in the latent heat between the paraffin and paraffin/xGnP composite. Chen et al. (2008) developed a composite of fatty acids/polyethylene terephthalate (PET) ultrafine by electrospinning. The fatty acids were long-chain fatty acids, lauric acid, myristic acid, palmitic acid and stearic acid. It was found that this composite demonstrated good form-stable morphology and good thermal properties were also found after the thermal cycle treatment. Pincemin et al. (2008) developed composites consisting of graphite and inorganic salts for solar applications. The experimental results revealed that the composite was able to attain a thermal conductivity of 8 W/m·K, with 40% by weight of natural graphite flakes in the composite. However, with this high amount of graphite in the composite, the effective thermal storage capacity was decreased drastically. There were also other PCM composites that were formulated that yielded good results. They were: high conductivity polyethylene glycol (PEG)/silica dioxide (SiO2) composites with b-aluminium nitrate as an addition (Wang et al., 2009b), a novel shape-stabilised composite of PEG and silicon dioxide (Wang et al., 2009a), an oxidised hard Fischer–Tropsch paraffin wax as PCM in a UV-cured epoxy resin matrix (Luyt and Krupa, 2009), manganese (II) nitrate hexahydrate as a PCM which showed a reduction in supercooling and quantity of the heat of fusion (Nagano et al., 2003). Heat transfer enhancement was also achieved when graphite was added in PCM slabs (Marín et al., 2005). The PCM/graphite systems are effective at improving heat transfer rates; however, they are costly. Heat pipes are very powerful at transferring heat in a very compact space, minimising any loss of storage volume. Heat pipes have been shown to transfer heat 500 times more than that of copper and can effectively shorten the charging and discharging duration of the phase change process of the PCMs (Wang et al., 2002). This passive heat transfer device also enabled the phase change process to be controllable which in turn improved its system efficiency (Wang et al., 2002). Mathematical models based on heat pipes in a latent heat storage system were presented by Horbaniuc et al. (1996, 1999). Performance enhancements were measured in terms of reduced phase change time, increased heat transfer, or an effectiveness measurement based on an ideal case (Shabgard et al., 2010; Robak et al., 2011). Two thermal conductivity enhancement techniques using carbon fibres were

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investigated in a tube-in-tank system. Carbon fibres have a higher thermal conductivity than copper and also minimise the space they occupy within the storage system. The fibres were randomly distributed in the PCMs in the first technique. The second technique employed a fibre brush such that the directions of the fibres coincided with the heat flow. The brush type technique significantly enhanced the effective thermal conductivity in the direction of the fibre orientation (Fukai et al., 2000). Carbon fibre brushes were further investigated by arranging the brush along with the tubes as shown in Figure 9.7 (Fukai et al., 2002). The result of this experiment showed that the transient thermal responses in the composite improved as the length of the brush increased.

9.3.3.2 Dynamic PCM system The heat enhancement techniques discussed so far improve the performance of the system; however, they also increase the cost of the storage system substantially. He and Setterwall (2002) first investigated the possibility of using a dynamic storage system to enhance the heat transfer of the PCM. This technique used direct contact between the storage material and the heat transfer medium which resulted in excellent heat transfer. Martin et al. (2010) explored this technique further with experimental validation of a numerical model. For the experimental setup, the PCM used was paraffin-based and water was used as the HTF. Both charging and discharging performance were evaluated. Figure 9.8 shows the charging process of the PCM-water cold storage. It was found that with increasing packing factor and temperature difference of the storage system, the capacity increases. Several limitations of this technique also surfaced, such as expansion of the PCM bed when the flow rate increased, liquid PCM trapped within the frozen porous bed which caused expected storage capacity loss and frozen PCM shells with enclosed water collapsed as the charging progressed. The conclusion was that more investigations were needed for this technique.

PCM Tube

Carbon brush Tank

(a)

(b)

Figure 9.7 Thermal energy storage units where brushes made of carbon fibre are inserted (Fukai et al., 2002).

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Figure 9.8 Charging of a direct contact PCM-water cold storage (Martin et al., 2010).

Tay et al. (2013) have also conducted research into dynamic PCM systems. A new heat transfer enhancement concept was developed using recirculation of the PCM during the melting process, known as dynamic melting. The thermal storage unit consisted of PCM placed in a tube-in-tank type storage system. The average effectiveness increased between 33% and 89% for the experiments using a high temperature gradient, and between 58% and 82% for the experiments using a low temperature gradient. The time taken for the melting process was also found to be shorter when a recirculating pump was utilised for dynamic melting. The investigation showed that the effective thermal conductivity could be doubled with dynamic melting, which is comparable to using fins on the tubes. Furthermore, this was achieved without a reduction in the compactness factor. It was concluded that this technique can effectively enhance the heat transfer during the melting process in a tube-in-tank thermal storage system, and dramatically increase the energy storage effectiveness. Further investigation is warranted to identify the full potential of this method. Overall, the addition of fins and pin type conductors can dramatically enhance the heat transfer in both melting and freezing of a PCM, and as yet there has been no optimisation of these options. Micro enhancement using graphite is also effective; however, it is an expensive option. Dynamic PCM systems have potential for high heat transfer enhancement that is cost-effective and so more investigations are needed. One other advantage of using this latter technique is that it can prevent inorganic PCM from phase segregation.

9.3.4 Insufficient long-term stability ‘Insufficient long term stability of the storage materials and containers is a problem that has limited widespread use of latent heat stores’ (Zalba et al., 2003). This is

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due to the poor stability of PCMs and the corrosion between PCMs and containers. Appropriate PCMs must be capable of undergoing a large number of cycles of melting and freezing without their properties degrading. This must be experimentally tested. At least 1000–2000 cycles are recommended in laboratory measurements (Abhat, 1983). Also, PCMs should be compatible with the materials of their containers.

9.4 Methods to determine the latent heat capacity of PCMs 9.4.1 Differential scanning calorimetry (DSC) DSC is a thermo-analytical technique where a material is heated and cooled isothermally and the transitions events are investigated as a function of time or temperature against a standard reference (Ford and Timmins, 1990). Perkin Elmer is one manufacturer of DSC machines. The DSC 8000 is an example of one of their models. The DSC 8000 is a power-compensated differential scanning calorimeter as shown in Figure 9.9(a) which is coupled to a dedicated chiller (Figure 9.9(b)) enabling it to test samples from –170°C to 750°C. The sample and reference material are placed in a separate, self-contained calorimeter in the DSC as shown in Figure 9.10. A computer containing Pyris Software for Windows is connected to the DSC 8000 as shown in Figure 9.9(c). This software controls the analyser via temperature control programs. Via this software, the DSC 8000 is able to analyse the melting, glass transitions, solid-state transitions and crystallisation of the sample material. The temperature range is scanned by changing the temperature at a linear rate. The sample material has to be prepared and encapsulated before placing it into the DSC 8000 sample holder. Before the sample is placed in the sample pan (Figure 9.11(b)), the weight of the empty sample pan needs to be determined. Then the sample to be analysed is placed in this pan. The sample pan containing the sample is weighed again so that the weight of the sample to be analysed can be determined. A sample lid (Figure 9.11(c)) is then placed on top of the sample pan containing the sample. The sample pan with the lid is then placed on the pan press (Figure 9.11(a) and (d)) and the pan encapsulation created (Figure 9.11(e)). After encapsulating the sample, the reference capsule has to be prepared. The best reference material is an empty sample pan and lid of the same type in which the sample material is encapsulated. The reference capsule is placed at the right side of the sample holder while the sample capsule is placed at the left side of the sample holder as shown in Figure 9.10. The latent heat of fusion is released or absorbed by a material when it is changing phase without varying its temperature. A sample in an encapsulated pan is heated or cooled from the initial temperature past its phase change temperature, remains isothermal for a short time before being heated or cooled back to its initial temperature. The heat of fusion can then be calculated using the DSC data analysis program. Figure 9.12 shows a typical latent heat of fusion chart using the DSC 8000.

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(b)

(c)

Figure 9.9 (a) DSC 8000, (b) chiller and (c) computer with Pyris Software.

Sample

Reference

Figure 9.10 DSC 8000 sample holder.

From an experimental point of view, PCMs have characteristics that make it difficult to determine their properties. Some of these include sub-cooling, hysteresis, crystallisation problems due to sample size and wide melting range (Lázaro et al., 2013). Furthermore, the DSC result can be influenced by the sample mass, the heating/

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Figure 9.11 (a) Pan press, (b) sample pan, (c) sample lid, (d) sample pan and lid in press and (e) encapsulated pan. 80

Heat flow endo down (mW)

70 60 50 40 30

Delta H = 243.3449 J/g Area = 4039.525 mJ

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Figure 9.12 A typical latent heat of fusion curve using DSC 8000.

cooling rate, the DSC operation mode and the DSC itself. He et al. (2004) concluded that a DSC with a high heating rate fails to provide correct information because of the lack of phase equilibrium within the sample including thermal equilibrium and chemical equilibrium. The two common operation methods for a DSC are the constant heating rate mode (dynamic) and variable heating rate mode (step, also known as isothermal step or step-scan mode). Castellón et al. (2008) investigated different measurement procedures for a DSC to determine the enthalpy–temperature relationship of one

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particular paraffin sample. Using the dynamic method, large uncertainties in the temperature usually observed can be reduced only at the expense of increasing uncertainty of the enthalpy. It is therefore not possible to obtain results with sufficient accuracy as required for the design of many applications. Using the step method, the accuracy can be increased to a satisfactory level. Additionally, the step method is far less sensitive to a variation in the measurement parameters. Günther et al. (2009) concluded that a DSC, using the isothermal step mode, offers sufficient precision for typical PCM applications when a temperature uncertainty of less than 1 K is required for the determination of enthalpy as a function of temperature. Both Castellón et al. (2008) and Günther et al. (2009) showed that measurements of a PCM with a DSC needed a slow heating and cooling rate, usually lower than 1 K per minute. After investigating the melting temperature and the latent heat of fusion of paraffin and salt hydrates using the two DSC modes, Barreneche et al. (2013c) concluded that a slow dynamic mode is the most suitable mode when evaluating the result for salt hydrates and both DSC modes are appropriate to obtain proper results for paraffin. Lázaro et al. (2013) compared results for a reference PCM using four different DSCs to determine their latent heat of fusion as well as their melting and solidification behaviour. The DSCs used were manufactured by TA Instruments (DSC Q200), Perkin Elmer (Jade DSC), Mettler Toledo (DSC 1 Starte System) and Netzsch (DSC 204 F1 Phoenix). The first comparison of results obtained for the round robin test showed a high deviation in enthalpy and temperature. The causes encountered were the measurement procedure, the DSC itself, the DSC calibration, the sample preparation and sample crucibles used, and the data evaluation. From this study, a procedure is recommended that should be followed for calibration and measurement using a DSC for PCM applications. Generally, the sample in the DSC analysis is homogeneous; however, many samples are mixtures of different substances. Barreneche et al. (2012) designed and tested a new experimental methodology to analyse polymeric matrices incorporating PCMs using a DSC, involving the use of different blanks instead of an empty reference crucible. Improvements in the signal and accurate curve evaluation were achieved, compared to that obtained using an empty reference crucible. In many applications for PCMs the exact temperature range is not known or fixed. In such cases, the evaluation of the storage density and the comparison of different PCMs are difficult and the standard approaches do not give accurate and easy-toread results. Mehling et al. (2010) present a new method that is simple, accurate, and allows a visual evaluation of the heat storage density for arbitrary temperature ranges. This is done by plotting the enthalpy difference in a two-dimensional contour plot with the upper and lower storage temperatures as the two dimensions. In a second step, the temperature differences used for heat transfer, for example at a heat exchanger, can be included. This way, the new method can be used as an aid in the design of a PCM storage system and for its technical and economical optimization.

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9.4.2 T-history method The T-history method represents a relatively new technique for assessing the thermophysical properties of PCMs. Developed in 1998, the method investigates the temperature history of a sample in relation to a reference material, and as such it assesses the latent heat of fusion as well as specific heat capacities in both solid and liquid phases. Contrary to DSC and differential thermal analysis (DTA), the T-history method can be used to measure the thermophysical properties of several different PCMs simultaneously, and has the ability to assess larger sample sizes than is possible with DSC and DTA. The temperature range over which the method can be applied enables one to assess the suitability of PCMs to many applications. However, the method is relatively new, and as such is still subject to inaccuracies stemming from invalid physical assumptions and the numerical analysis techniques employed. The method has accordingly been subject to development over the last ten years, with many research papers building on its reliability through refined practices and methodologies. Solé et al. (2013) have reviewed the different techniques used to measure the thermophysical properties of PCMs available in the literature focusing on the original T-history method. They present previous methods designed for the same purpose and/or based on the same principle. Other scientific publications focused on improving the method are also mentioned and their contribution is discussed. Zhang et al. (1999) describe a method for the calculation of thermophysical properties of PCMs based upon recording the temperature history (hence T-history) of the material as it undergoes a phase transition. The T-history of a reference material with well-known thermophysical properties (such as water) is also recorded simultaneously and under the same experimental conditions as that of the PCM. The temperature at which the phase change occurs is contained within the temperature interval over which the temperature difference operates. Such an example would be heating a PCM (of melting temperature 30°C) to 40°C and exposing it to an ambient temperature of 20°C, and monitoring the temperature of the PCM as it decreases and undergoes phase change. Typical T-history curves of a PCM undergoing cooling with and without sub-cooling are shown in Figures 9.13 and 9.14, respectively. During this process, the PCM is subjected to heat transfer through natural convection to the surrounding air. The rate at which the natural convective heat transfer occurs is a function of the area over which the heat transfer operates (area being the dimensions of the tube in which the PCM is contained) and the difference in temperature. Given that there will be a slight difference in the rate of heat transfer between the tube and the PCM (owing to the difference in mass and thermal conductivity), a method is adopted through which the temperature distribution throughout the sample can be considered uniform. That uniformity is achieved by satisfying the condition that the Biot number, Bi, is less than 0.1 (the Biot number represents the ratio of heat transfer through convection to that of conduction). Utilising the lumped capacitance method, we assume a uniform temperature distribution throughout the PCM in the tube, and that the convection acting on the tube is large compared with that of internal conduction (Holman, 2010). The T-history curve of a reference material

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Tm DTm

Ts A1

A2

Tr

A3

Ta,• or Tw, • 0

t 1

t 2

t 3

t(s)

Figure 9.13 Typical T-history curve of a PCM undergoing cooling, with supercooling present (Zhang et al., 1999). T(°C) T0

Tm,1 Tm,2 A1

A2

Tr Ta,• or Tw, •

A3 0

t 1

t 2

t 3

t(s)

Figure 9.14 Typical T-history curve of a PCM undergoing cooling, with no supercooling present (Zhang et al., 1999).

(in this case water), subjected to the same experimental conditions as the PCM, is presented in Figure 9.15. The accuracy of the original method as described by Zhang et al. (1999) is constrained by several unjustified assumptions in the analytical methodology adopted. The set of equations used to determine the specific heat capacities of the PCM in its solid and liquid state, as well as the latent heat of fusion is subject to the defining of the beginning and end of the phase change period. The accurate selection of these points on such a T-history as that presented in Figure 9.13 is of critical importance as the integral (area under the curve) over each phase period is utilised in the respective equations of specific heat and enthalpy. In the case that the PCM exhibits sub-cooling, the original method prescribes the use of the release temperature of sub-cooling to signify the beginning of the phase change period (t1) as displayed in Figure 9.13.

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T(°C) T0

Ts A1¢

A2¢

Tr Ta,• or Tw, • 0

t 1

t 2

t (s)

Figure 9.15 Typical T-history curve for water subject to cooling (Zhang et al., 1999).

The end of the phase change period is then taken when the temperature drops below that of supercooling, requiring a relatively steady decline in temperature over this transition. This methodology hence exposes the resultant properties of the PCM to possible inaccuracies. Should the degree of sub-cooling not be obvious, or not exist (such as in Figure 9.14), the boundaries of the phase change period are subject to uncertainty and loss of validity of the method ensues. The validity of sub-cooling as the signifying of a phase transition is also questioned by Hong et al. (2004). Furthermore, the variations in thermal conductivities and volumetric changes with temperature change are not taken into account. Owing to the above shortcomings, a refined methodology of the T-history method was proposed by Hong et al. (2004), who attempt to better define the phase change period. This is achieved by taking the first derivative of the T-history curve and identifying subsequent inflection points. The underlying principle advocated by Hong et al. is based on a recognition that the rate of temperature decrease is at a minimum during the release of latent heat (this constitutes a minimum point of inflection), and that the rate then decreases exponentially during the loss of heat through sensible action. While theoretically sound, the time at which the phase change is complete is difficult to ascertain, and as such the transition interval is still very much subject to interpretation. Here, it is suggested by the authors that the end of the latent heat period, and therefore the beginning of sensible heat period, can be taken as arbitrary temperatures below that of the inflection point. This could have adverse effects in the form of an overestimation of, for example, the specific heat capacity of the solid phase under a cooling programme. Problems in this procedure also arise when supercooling is not obvious (a problem shared with the original method), where the significance and advantages of the inflection point approach as compared with the T-history become redundant. Further problems with the original method arise with sub-cooling. Hong et al. (2004) identify the first crucial problem in the original method as being to adopt the release point of sub-cooling as the end of the phase changing period. A degree of sub-cooling varies with conditions such

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as cooling speed, purity and vibration and is completely independent of the end of phase change. Marín et al. (2003) in their study question the necessity to consider property variance (such as specific heat capacity) in the materials as a result of temperature change. These variations can be accounted for through the calculation of enthalpy vs. temperature curves, where the cumulative enthalpy of the system with respect to temperature change is presented. As noted by Marín et al. (2003), this is achieved through ‘varying the temperature over very small intervals, DTi, corresponding to time intervals DTi = ti+1 – ti and Dt i¢ = t¢i+1 – t i¢ for the PCM and reference respectively.’ They also note that temperature–enthalpy curves are better suited to the study of impure materials with unclear phase change boundaries, as is used for analysing results obtained through DSC. Furthermore, the specific heats of the materials can be calculated at each temperature as desired through the determination of the slope of the enthalpy curve at any desired temperature. The energy released (or absorbed) can also be easily determined by graphical examination. This method presents a more comprehensive evaluation of T-history data, and possesses the capability to account for material property variation, as well as the ability to easily evaluate the PCM properties through the generation of the enthalpy vs. temperature curve. With regard to experimental procedure, the authors also suggested that a metallic coating (low emitting and highly reflecting) should be applied on the external walls of the tubes in an effort to minimise heat loss through radiation. The advantages of temperature–enthalpy curves were elaborated upon by Lázaro et al. (2006). They state that single data points of enthalpy at phase change temperature are insufficient in describing, with any appreciable precision, the properties of PCMs. This is attributed to the fact that the phase change of most PCMs (being impure substances), takes place over a temperature range and not at a given temperature, as is confirmed by Günther et al. (2009). As a result, the temperature–enthalpy curve should be treated with a degree of uncertainty smaller than that which accompanies the useable temperature range. With regard to error analysis, attention was also drawn by Lázaro et al. (2006) to the importance of attaining both heating and cooling curves of the PCM and sample, in order to negate the effects of hysteresis during data analysis. This enables the determination of the properties of a PCM with a higher degree of certainty than is possible with just one of these curves. These properties should be evaluated with no bias of analysis towards the intended application. Peck et al. (2006) investigated the effects of the internal thermal gradient of the test tubes imposed by a vertical setup as a result of the strength of natural convection being proportional to the length of the tube. They experimented with a horizontal setup, thereby reducing the temperature difference in the longitudinal direction of the tube. This affects the natural heat transfer coefficient, as the characteristic dimension is indicative of the vertical dimension of the setup (the length of the tube or the diameter in a vertical and horizontal setup, respectively). However, this leads to complications owing to the volumetric variation of the PCM as a result of phase change, and additional heat exchange through the vapour layer of the test tube. Consequently, an increase in the complexity of the heat transfer analysis

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ensues; this is mitigated through an attachment to the test tube (referred to as an ‘E’ tube) capable of controlling volume. While this results in an increase in accuracy, obtaining cyclic heating and cooling curves is near impossible. The authors conclude that implementing a horizontal setup without the use of an E tube and assuming the heat transfer through the vapour layer (in the case of distilled water) to be negligible resulted in a higher degree of accuracy. While there are benefits of using a horizontal setup in order to minimise temperature gradients within the tube, the problem of volumetric variation over the phase change process necessitates an overly complicated methodology. While assumptions can be made to mitigate these complications, the viability of the method will be subject to the volumetric properties of each individual PCM, and increase the difficulty of cyclic testing. A method using differential formulation was proposed by Moreno-Alvarez et al. (2010), whereby the inclusion of temperature change values nearest transition regions are considered in property evaluation. This necessity is attributed to the thermal properties of all materials varying with the rate of cooling or heating. Thermodynamic consistency is hence sought after, and can be achieved, through the rearrangement of the equations from the original method into a differential formulation. The refinement of the T-history proposed by Moreno-Alvarez et al. (2010) centres on the generation of dT/dt vs. temperature curves. These curves are nevertheless more susceptible to rapid changes in temperature gradients between the sample and the ambient temperature.

9.5 Methods to determine other physical and technical properties of PCMs 9.5.1 Thermal conductivity measurement Thermal conductivity can be determined by the measurement of the heat flux and the temperature differences of the PCM samples. There are many methods and instruments available to measure the thermal conductivity of a fluid (Czichos et al., 2006). Table 9.1 presents an overview of several methods used for the measurement of thermal conductivity and thermal diffusivity. The thermal conductivity of the PCM samples can also be measured using the T-history method described by Zhang et al. (1999). In order to measure the thermal conductivity of a PCM using this method, the PCM sample is placed in a long tube with its length at least 15 times larger than the diameter; therefore, it can be assumed that the heat transfer is one dimensional. Figure 9.16 shows the phase change of the PCM in a long tube. The temperature of the PCM in the tube is uniform, and slightly higher than the phase change temperature (TPCM) of the PCM. The PCM in the tube is suddenly dipped into a cool bath whose temperature (T∞) is lower than the phase change temperature. The time taken for the phase change to take place (tf) and the thermal conductivity of the PCM in the solid state (ks) can be calculated from the following equation:

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Table 9.1 Comparison of measurement methods for the determination of thermal conductivity and thermal diffusivity Method

Temperature Uncertainty Materials range

Advantages

Disadvantages

Guarded hot plate

80–800 K

2%

Insulation materials, plastics, glasses

High accuracy Long measurement time, large specimen size, low conductivity materials

Cylinder

4–1000 K

2%

Metals

Temperature range, simultaneous determination of electrical conductivity and Seebeckcoefficient possible

Long measurement time

Heat flow meter

–100–200°C

3–10%

Insulation materials, plastics, glasses, ceramics

Simple construction and operation

Measurement uncertainty, relative measurement

Comparative

20–1300°C

10–20%

Metals, ceramics, plastics

Simple construction and operation

Measurement uncertainty, relative measurement

Direct heating (Kohlrausch)

400–3000 K

2–10%

Metals

Simple and fast measurements, simultaneous determination of electrical conductivity

Only electrically conducting materials

Pipe method

20–2500°C

3–20%

Solids

Temperature range

Specimen preparation, long measurement time

Hot wire, hot 20–2000°C strip

1–10%

Temperature Liquids, gases, low range, fast, conductivity accurate solids

Limited to low conductivity materials

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Table 9.1

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Continued

Method

Temperature Uncertainty Materials range

Advantages

Laser flash

–100–3000°C 3–5%

Solids, liquids

Temperature Expensive, not for insulation range, most solids, liquids materials and powders, small specimen, fast accuracy at high temperatures

Solids, liquids, gases, thin films

Usable for thin films, liquids and gases

Photothermal 30–1500 K photoacoustic

Not known

Disadvantages

Non-standard, knowledge about accuracy

Source: Czichos et al. (2006)

Liquid PCM Solid

2RPCM 2Rc 2RPCM 2Rc

Figure 9.16 An illustration of two phases of a PCM in a long tube (Zhang et al. 1999).

È c p (TPCM – T• )˘ ks = Í1 + ˙ Hl Î ˚

Ê t f (TPCM – T• ) ˆ – 1 ˜ 4Á 2 hw Rc ¯ Ë r pcm R H l

(9.1)

where rpcm is the density of the PCM and Hl is the latent heat of the PCM. Since

9.5.2

t f (TPCM – T• ) >> 1 in most casses es, thereforem, 1 can be neglected. r pcmm R 2 H l hw Rc hw Rc

Viscosity measurement

This section describes the viscosity measurement of PCM samples. There are two categories for viscosity measurement. Viscometers are normally used for fluids whose

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viscosity remains constant under varying flow conditions while rheometers are used for fluid whose viscosity varies as the flow condition varies. There are many types of viscometers available, most of which are used in the laboratory. They are falling sphere and piston viscometers, rotational viscometers, bubble viscometers and orifice viscometers. Falling sphere viscometers make use of Stokes’ Law. A sphere with known properties and dimensions is allowed to fall through a fluid and the time is recorded for travel between two points. Viscosity can then be calculated. Falling piston viscometers make use of the same principle as the falling sphere viscometer. The only difference is that they calculate the viscosity by measuring the resistance of the piston that is falling through the fluid. As for rotational viscometers, the resistance of the fluid against the rotating motion is used to find the viscosity of the fluid. For bubble viscometers, viscosity can be calculated by measuring the time taken for bubbles to rise through a fluid. Another method of measuring the viscosity of a fluid is by using an orifice viscometer, which is popular due to its simplicity and ease of operation. An example is a Redwood viscometer as shown in Figure 9.17. The Redwood viscometer is one of the first orifice viscometers ever developed (Nash and Bowen, 1937). It measures the viscosity of the fluid as a time of flow in seconds. The apparatus of the Redwood viscometer is illustrated in Figure 9.18. The Redwood viscometer consists of a reservoir and orifice with a temperature control

Figure 9.17 Redwood Viscometer.

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T1

225

T

K

S

B H

C

V A

J D E

A. Oil cup

D. Tap

J. Agate jet

S. Thermometer clip

B. Levelling wire

E. Heating tube

K. Stirrer handle

T & T1. Thermometers

C. Copper bath

H. Stirrer

V. Ball valve

Figure 9.18 Schematic of Redwood viscometer (adapted from Nash and Bowen, 1937).

jacket, and a receiving flask. The sample under test is first poured into the oil-cup (A) of the viscometer. The level of the sample in the cup is adjusted to a definite height. When the desired temperature is attained, the valve (V) at the base of the cup (J) is opened (Nash and Bowen, 1937). The time required for a specified volume of sample to discharge through the orifice into a measuring vessel placed below is measured. The measured time in seconds is a purely arbitrary expression of viscosity and is usually designated as Redwood seconds. The Redwood seconds can then be converted into centipoise by using a conversion table. As mentioned earlier, for fluid whose viscosity can be affected by varying flow conditions, rheometers are used. Several commercial rheometers are available in the market. They include, ThermoFisher’s CaBER (force below 10 Pa), Fiser (force between 1 and 1000 Pa), Gottfert Rheotens (force above 100Pa) and Xpansion Instruments Sentmenat (force over 10kPa).

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9.5.3 PCM volumetric expansion measurement Volumetric expansion of the PCM can be measured using a scaled measurement vessel. Different sample sizes of the liquid PCM are measured and their mass weighed at room temperature. The measurement samples are then placed in a freezer at a temperature below the phase change temperature. The volume of the samples is measured after the samples have been frozen. The solid and liquid densities can then be calculated.

9.5.4 Thermophysical properties of building materials incorporating PCMs The use of PCMs in building envelopes can significantly reduce the energy consumption of the building. The thermophysical characterisation of building materials with PCMs is an important factor when deciding on the construction systems and materials during the design of a building (Barreneche et al., 2013a). Commercial devices such as a DSC only allow the measurement of the properties using small sample sizes and can give distorted results. For this reason, researchers have developed equipment to test different thermophysical properties of materials using macroscale samples. De Gracia et al. (2011) have developed a new device designed to measure the thermal response and heat capacity of composite walls of different materials simulating real building envelopes (Figure 9.19). The equipment was used to test the improvement in the thermal response of a building envelope due to the incorporation of PCM. This study was focused on wood structural panels attached to a gypsum board, which

Wooden envelope

Water flow Bath A Insulation

Sample Pt-100

Copper coiler

Water flow Bath B

Figure 9.19 Device designed to measure the thermal response and heat capacity of composite walls of different materials simulating real building envelopes (De Gracia et al., 2011).

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was impregnated with and without PCM. The four edges of the composite sample were properly insulated to ensure one-dimensional heat flow. The two faces of the sample were exposed to controlled environments which were heated and cooled by heat exchangers supplied by temperature controlled water baths. The measured surface heat fluxes at both surfaces of the sample and temperature distribution in the sample provide an accurate assessment of thermal mass and dynamic response of the composite wall, while the steady state measurements provided an accurate estimate of its effective thermal transmittance. The new equipment provided highly accurate and repeatable measurements, presenting good agreement between the measured and the calculated U-value in the experiment. Barreneche et al. (2013b) have developed two devices capable of characterising the effective thermal conductivity of construction materials at the macroscale level and recording the temperature–time response curves produced by the inclusion of PCM in the construction system for thermal inertia increase. These are the Conductimeter and the T–t curves device. The Conductimeter is a device designed to characterise the thermal conductivity of different construction systems. Figure 9.20 shows a schematic cross-section view of the Conductimeter. Surrounding insulation made of refractory brick gives the equipment a total dimension of 825 ¥ 825 mm so that a z-axis thermal gradient can be obtained. The sample is placed between a hot plate and a cold plate, and both plates are insulated with polyurethane foam to minimise the effect of ambient temperature. Type-T thermocouples are used to measure the temperature of each sample surface and the room temperature. The guarded plate area surrounding the measurement area ensures one-dimensional heat flow and this is controlled by differential thermocouples. The thermal conductivity of the samples is calculated through the sample thermal gradient obtained due to the power supplied to the hot plate and the cold temperature regulated by a thermostatic bath connected to the cold plate. By adjusting the hot plate and cold plate, the Conductimeter is able to characterise different materials incorporating different phase change temperatures, from nearly 100°C to subzero temperatures. When the system is at steady-state conditions, the effective thermal conductivity can be calculated. The Conductimeter device is able to measure the thermal properties of products with low to medium resistance, with accuracy close to ±2%.

Insulation

Sample

Hot plate Cold coiler

Figure 9.20 Sketch of the Conductimeter (Barreneche et al., 2013b).

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The T–t curve device registers the evolution of the temperature over time during a cooling process of a sample using temperature–time responses (T–t curve) at the macroscale level. This device was developed based on the T-history method. This equipment has a sandwich configuration as shown in Figure 9.21. Temperature regulation is achieved by controlling power to a heating plate. The sample under study is placed on one side of the heating plate and the reference on the other side (the reference is the same material without PCM). The sample dimensions are 300 ¥ 300 ¥ 30 mm. During the T–t curve experiments, the sample and the reference are thermally compared. There are six RTD temperature sensors (Pt-100) located in the middle of both surfaces of the sample and reference and in the centre of the sample. The temperature used to analyse the results is the temperature in the centre of the sample/reference. When the temperature is at steady-state conditions, power to the heated plate is disconnected and the thermal evolution during the cooling process is recorded. The temperature difference produced by the addition of the PCM is related to the thermal inertia. Laboratory-scale construction systems have been tested using three different devices for comparison purposes (Barreneche et al., 2013a). The materials tested were gypsum blocks containing PCMs. The effective thermal conductivity, the total amount of heat accumulated and the specific heat were measured using these devices. The devices included that developed by De Gracia et al. (2011) and the Conductimeter, both described above. The third device (called the C.R. device) provides controlled changes in the external temperature of the samples using a thermostatic bath. This equipment is shown in Figure 9.22. With this device, tests were carried out using a thermostatic bath with a set-point step change from 18 to 42 ± 0.1°C while the Insulation casing Heating device

Test plates

Figure 9.21 T–t curve device (Barreneche et al., 2013b).

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Computer

Heat flux sensors

Thermocouples Peristaltic pump Rotameter

USB ports

229

T

Tup

Tcentre

PV 38.4 Status SV 37 Run

RUN

Gypsum block Aluminium cell Cork insulating structure

Thermostatic bath

Figure 9.22 Sketch of the C.R. device (Barreneche et al., 2013a).

external block temperatures and inlet–outlet heat fluxes were recorded. The locations of sensors are also shown in this figure. The size of the samples analysed in these experiments was 6 ¥ 10 ¥ 2 cm. As shown in Figure 9.22, the sample is placed on the upper surface of the aluminium cell and further insulated with foam boards of 3.9 cm thickness. Seven thermocouples were used to measure temperatures: two were placed on the external sample surface, two were placed at the cell, two in the middle of the sample, and the last one was placed in the upper foam surface. Four heat flux sensors were used to measure heat fluxes: two were installed at the top and bottom of the sample, one at the front and the last one in the wallboard. The thermal conductivity can be obtained by applying the heat conduction equation in one dimension. The comparison study concluded that the thermal properties of real materials used for building applications can properly be measured by the three different devices since the results obtained were consistent and in the range of those values found in the literature.

9.6 Comparison of physical and technical properties of key PCMs Figure 9.23 shows the classes of candidate materials that can be used as PCMs and their typical range of melting temperature and melting enthalpy. So far, thousands of

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Carbonates

Fluorides

900 800 Melting enthalpy (kJ/l)

700

Chlorides

600

Hydroxides

Salt hydrates

500

Nitrates

400

Water 300 Water-salt Sugar alcohols solutions 200 Parafines 100

Polyethylene glycol 0 +100

0 –100

0.1 kWh/l

Clathrates Fatty acids

+200 +300 +400 +500 Melting temperature (°C)

+600 +700 +800

Figure 9.23 Melting temperature and melting enthalpy of common PCM candidates.

materials which are either single or mixtures of two or more substances have been investigated for their use as PCMs. This book aims to provide typical examples of different classes of PCMs and more comprehensive lists of PCMs can be found in the literature. Earlier papers which list potential PCMs include Abhat (1983), Lane (1980, 1983, 1986), Zalba et al. (2003), Farid et al. (2004) and Schröder (1985). Over the last few years there has been a number of review papers published which have a list of PCMs. A comprehensive list suitable for building applications can be found in Cabeza et al. (2011) and for free cooling of buildings in Waqas and Ud Din (2013). Kenisarin and Mahkamov (2007) list PCMs suitable for solar thermal storage systems. Liu et al. (2012b), Gil et al. (2010) and Kenisarin (2010) have published papers which list PCMs applicable for concentrating solar power plants. Oró et al. (2012) have conducted an extensive review of PCMs for cold thermal storage applications and Li et al. (2013) for subzero applications. Jankowski and McCluskey (2014) have compiled a comprehensive list of PCMs for vehicle component thermal buffering. Finally, Youssef et al. (2013) have put together a list of PCM slurries. Eutectic salt-water solutions usually have melting temperatures below 0°C and some examples are listed in Table 9.2. The principle of this is that when inorganic salt is added to water, it will lower its freezing point. When the salt and water are mixed at eutectic compositions, they will solidify simultaneously out of the liquid at a lower temperature than any other compositions. The temperature at which it solidifies is known as the eutectic temperature. Generally, sub-cooling occurs in the solidification process and it can be reduced or prevented by the techniques mentioned in Section 9.2.3. Eutectic salt-water solutions have similar thermal conductivity to that of water. Also like water, they can show considerable volume change during melting

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Table 9.2 Examples of eutectic salt-water solutions and some of their thermophysical properties Composition

Melting temperature (°C)

Melting enthalpy (kJ/kg)

Thermal conductivity (W/m·K)

Density (kg/m3)

30.5 wt.% Al(NO3)3/H2O

–30.6

131

–

1283 (l) 1251 (s)

22.4 wt.% NaCl/H2O

–21.2

222

–

1165 (l) 1108 (s)

19.5 wt.% KCl/H2O

–10.7

283

–

1126 (l) 1105 (s)

H 2O

0

333

0.6 (l) 2.2 (s)

998 (l) 917 (s)

Source: Schröder (1985)

and solidification (5–10 vol.%) (Mehling and Cabeza, 2008). They are chemically quite stable but can cause corrosion to some materials like metals (Mehling and Cabeza, 2008). Several material classes cover the medium temperature range from 0°C to 200°C, including organic substances (e.g., paraffins, fatty acids, sugar alcohols) and inorganic substances (e.g., salt hydrates, some nitrates and hydroxides and their eutectic mixtures). Table 9.3 shows examples of different substances and eutectics that have been studied for their potential use as PCMs. Most organic PCMs have a density less than 1000 kg/m3, which is less than the density of most inorganic substances. In most cases, organic PCMs, except for sugar alcohols, have a lower melting enthalpy per unit mass and per unit volume than inorganic PCMs. The volume change from solid to liquid is in the order of 10 vol.%, which is similar to that of many inorganic substances (Mehling and Cabeza, 2008). Organic PCMs have good chemical and thermal stability and they show little or no sub-cooling. In most cases, they are not corrosive to the containment materials. However, the thermal conductivity of the organic substance is comparatively low, resulting in poor heat transfer between the PCM and the heat transfer fluid (HTF). Inorganic substances usually have a large storage density per unit mass and also per unit volume due to their high density. Their thermal conductivity is much higher than that of organic substances. However, salt hydrates, consisting of two or more components (water and salts), can potentially separate into different phases after a number of melt-freeze cycles. Also, inorganic salts show sub-cooling when solidifying and many of them are corrosive to metals. The temperature range over 200°C is covered by salts, salt eutectics, metals and metal alloys. Some examples are presented in Table 9.4. Kenisarin (2010) and Zalba et al. (2003) comprehensively reviewed potential PCMs with melting temperatures up to 1000°C. The salt PCMs are on the basis of fluorides, chlorides, bromides, hydroxides, nitrates, carbonates and other salts. High temperature PCMs often have

Melting temperature (°C)

Melting enthalpy (kJ/kg)

Thermal conductivity (W/m·K)

Density (kg/m3)

Reference

Paraffin C14

4.5

165

–

–

(Abhat, 1983)

Paraffin C18

28

200, 245

0.148 (l @ 40°C) 0.358 (s @ 25°C)

774 (l @ 70°C) 814 (s @ 20°C)

(Mehling and Cabeza, 2008)

Paraffin C16–C28

42–44

189

0.21 (s)

765 (l @ 70°C) 910 (s @ 20°C)

(Abhat, 1983)

Paraffin wax

64

173.6

0.167 (l @ 63.5°C) 0.346 (s @ 33.6°C)

790 (l @ 65°C) 916 (s @ 24°C)

(Lane, 1980)

Paraffin C21–C50

66–68

189

0.21 (s)

830 (l @ 70°C) 930 (s @ 20°C)

(Abhat, 1983)

High density polyethylene

100–150

200

–

–

(Zalba et al., 2003)

Caprylic acid CH3(CH2)6COOH

16

148.5

0.149 (l @38.6°C)

901 (l @ 30°C) 981 (s @ 13°C)

(Dincer and Rosen, 2002; Lane, 1980)

65 mol.% Carpic acid+lauric acid

18

140.8

0.139 (l) 0.143 (s)

894.9 (l) 900.0 (s)

(Dimaano and Watanabe, 2002)

Capric acid CH3(CH2)8COOH

32

152.7

0.153 (l @38.5°C)

878 (l @ 45°C) 1004 (s @ 24°C)

(Lane, 1980)

75.5 wt.% Lauric acid+stearic acid

37

182.7

–

–

(Sari and Kaygusuz, 2002)

Composition

232

Table 9.3 Examples of PCMs with medium temperature range of 0–200°C and some of their thermophysical properties

Paraffins

Advances in Thermal Energy Storage Systems

Fatty Acids

44

177.4

0.147 (l @50°C)

862 (l @ 60°C) 1007 (s @ 24°C)

(Abhat, 1983; Lane, 1980)

Stearic acid CH3(CH2)16COOH

69

202.5

0.172 (l @70°C)

848 (l @ 70°C) 965 (s @ 24°C)

(Abhat, 1983; Lane, 1980)

Xylitol C5H7(OH)5

94

263

–

1500 (s @ 20°C)

(Mehling and Cabeza, 2008)

Erythritol C4H6(OH)4

120

340

0.326 (l @ 140°C) 0.733 (s @ 20°C)

1300 (l @ 140°C) 1480 (s @ 20°C)

(Mehling and Cabeza, 2008)

Galactitol C6H8(OH)6

188

351

–

1520 (s @ 20°C)

(Mehling and Cabeza, 2008)

CaCl2·6H2O

29

190.8

0.540 (l @ 38.7°C) 1.088 (s @ 23°C)

1562 (l @ 32°C) 1802 (s @ 24°C)

(Lane, 1980)

Na2HPO4·12H2O

36

265

–

1522

(Telkes, 1975)

Ba(OH)2·8H2O

48

265.7

0.653 (l @ 85.7°C) 1.225 (s @ 23°C)

1937 (l @ 84°C) 2070 (s @ 24°C)

(Lane, 1980)

61.5 wt.% Mg(NO3)2·6H2O +NH4NO3

52

125.5

0.494 (l @ 65°C) 0.552 (s @ 36°C)

1515 (l @ 65°C) 1596 (s @ 20°C)

(Lane, 1980)

58.7 wt.% Mg(NO3)2·6H2O +MgCl2·6H2O

59

132.2

0.510 (l @ 65°C) 0.678 (s @ 38°C)

1550 (l @ 50°C) 1630 (s @ 24°C)

(Lane, 1980)

Mg(NO3)2·6H2O

89

162.8

0.490 (l @ 95°C) 0.611 (s @ 37°C)

1550 (l @ 94°C) 1636 (s @ 25°C)

(Lane, 1980)

Sugar alcohols

Salt hydrates

233

(Continued)

Using solid-liquid phase change materials (PCMs) in thermal energy storage systems

Lauric acid CH3(CH2)8COOH

Continued

234

Table 9.3

Composition

Melting temperature (°C)

Melting enthalpy (kJ/kg)

Thermal conductivity (W/m·K)

Density (kg/m3)

Reference

MgCl2·6H2O

117

168.6

0.570 (l @ 120°C) 0.694 (s @ 90°C)

1450 (l @ 120°C) 1569 (s @ 20°C)

(Lane, 1980)

50 mol.% NaOH+KOH 169–171

202–213

–

–

(Takahashi et al., 1987)

33 wt.% LiNO3+KNO3 133

170

–

–

(Kenisarin, 1993)

55.4 wt.% LiNO3+4.5 wt.% NaNO3+KCl

160

266

–

–

(Gasanaliev and Gamataeva, 2000)

49 wt.% LiNO3+NaNO3

194

265

–

–

(Kenisarin, 1993)

Hydroxides and nitrates

Advances in Thermal Energy Storage Systems

Melting temperature (°C)

Melting enthalpy (kJ/kg)

Thermal conductivity (W/m·K)

Density (kg/m3)

Reference

NaNO3

306

178

0.514 (@ 306°C)

1908 (l @ 306°C) 2260 (s @ 25°C)

(Bauer et al., 2009)

NaOH

318

165

0.92

2100

(Steinmann and Tamme, 2008)

KNO3

335

95

0.425 (l) 0.5 (s)

2109 (s)

(Michels and Pitz-Paal, 2007; Shabgard et al., 2010)

Ca(NO3)2

560

145

–

–

(Kenisarin, 2010)

NaCl

802

420

5.0

2160

(Pilkington Solar International GmbH, 2000)

K2CO3

897

236

2.0

2290

(Pilkington Solar International GmbH, 2000; Zalba et al., 2003)

Composition

Pure salts

Salt eutectics 54% KNO3 + NaNO3

222

100

–

–

(Kenisarin, 2010)

7.8% NaCl + 6.4% Na2CO3 + NaOH

282

316

–

1550

(Kenisarin, 2010)

60% MgCl2 + 20.4% KCl + NaCl

380

400

–

1800 (s)

(Michels and Pitz-Paal, 2007)

32.1% Li2CO3 + 34.5% K2CO3 + Na2CO3

397

276

–

–

(Shin et al., 1990)

504

279

1.00 (l)

2150 (s)

(Kenisarin, 2010)

46% LiF + 44%NaF2 + MgF2

632

858

1.2 (s)

2240

(Kenisarin, 2010) (Continued)

235

29% NaCl + 5% KCl + CaCl2

Using solid-liquid phase change materials (PCMs) in thermal energy storage systems

Table 9.4 Examples of PCMs with high melting temperatures over 200°C and some of their thermophysical properties

Continued

236

Table 9.4

Composition

Melting temperature (°C)

Melting enthalpy (kJ/kg)

Thermal conductivity (W/m·K)

Density (kg/m3)

Reference

67% NaF + MgF2

832

616

4.65 (l)

2140

(Kenisarin, 2010)

Lead (Pb)

328

23

–

–

(Akiyama et al., 1992)

Aluminium (Al)

660

397

–

–

(Akiyama et al., 1992)

Copper (Cu)

1083

193.4

350

8930

(Maruoka et al., 2002)

59% Al + 33% Mg + Zn

443

310

–

2380 (s)

(Farkas and Birchenall, 1985)

52% Mg + 25% Cu + Ca

453

184

–

2000

(Farkas and Birchenall, 1985)

68.5% Al + 5% Si + Cu

525

364

–

2938

(Gasanealiev and Gamataeva, 2000)

56% Cu + 27% Si + Mg

770

420

–

4150

(Farkas and Birchenall, 1985)

56% Si + Mg

946

757

–

1900

(Farkas and Birchenall, 1985)

Metals

Metal alloys

Advances in Thermal Energy Storage Systems

% in weight

Using solid-liquid phase change materials (PCMs) in thermal energy storage systems

237

a large melting enthalpy which rises approximately proportional to the melting temperature given in K (Mehling and Cabeza, 2008). One critical issue with high temperature applications is corrosion, which decreases the life cycle of the system and also reduces the thermal performance. In the literature, there is limited information on high temperature salt and metal corrosion with repeated cycles of melting and solidification. Commercially available PCMs have been used frequently for low temperature and medium temperature ranges. The manufacturer/suppliers are summarised by Waqas and Ud Din (2013). The information is updated in this book and shown in Table 9.5.

9.7 Future trends This chapter has discussed the types and methods associated with the development of PCMs. PCMs are chemicals in which thermal energy is stored during a non-gaseous phase change. The development of PCMs has traditionally been conducted through the investigation of the phase change properties in isolation of the application being considered. Over the last decade this trend has changed with PCM development conducted in conjunction with the application. In order for a thermal storage system with PCM to operate successfully, the cycling stability, sub-cooling effects, corrosiveness, thermal conductivity and thermal stability must be considered. During cycling multi-component PCMs can segregate due to differences in density of each component (Farid et al., 2004). Depending on the volume of PCM, sub-cooling can be dramatic, preventing successful freezing of the PCM in the storage system. The thermal conductivity of the PCM affects the thermal resistance between the PCM and the heat exchange fluid, and therefore must be sufficiently high to enable effective heat transfer. Finally, the PCM must not corrode the containment material or decompose within the temperature range of operation of the storage system. Active storage systems incorporate a defined chamber in which the PCM is stored and a heat transfer fluid is pumped from the chamber to the thermal load. PCM encapsulated in spheres has been the traditional method to successfully overcome any instability within PCMs as well as preventing corrosion. A more recent example of a PCM being considered with the storage facility is presented by Helm et al. (2009), in which a tube-in-tank thermal storage system with a bulk hydrated salt was used. The thermal storage system with the PCM was cycled through melting and freezing over 280 times with no degradation in the PCM observed. Other issues, such as the thermal conductivity, sub-cooling, chemical stability and corrosiveness were all considered, making this PCM viable for the application considered. The typical example of a modern active system is graphite/PCM systems (Wang et al., 2002). In these systems, graphite is mixed with the PCM to deliver a high conducting bulk material which does not separate during cycling. Although expensive, this technology has been developed for use in tube-in-tank arrangements where the heat transfer fluid exchanges heat with the PCM through a tube. More recently,

238

Table 9.5

Advances in Thermal Energy Storage Systems

PCM manufactures/suppliers around the world

Company

Country Products of origin

Product ID

Melting temperature range (°C)

BASF (www.basf. com)

Germany –

Micronal® DS

–

Climator AB (www.climator.com)

Sweden

Salt hydrates

ClimSel C

–21 – +71

Cristopia Energy Systems (www.cristopia.com)

France

–

AC

+27

AN

–15 – –3

SN

–33 – –18

Environmental Process Systems Ltd (www.epsltd.co.uk)

UK

E

–62 – –2

Eutectic organics

E

–100 – –22

Organics

A

0 – +167

Salt hydrates

S

+7 – +117

Solid-solid

X

+25 – +180

Inorganic salts/ eutectics

H

+90 – +885 –

Phase Change Australia Salt hydrates Products Pty Ltd (pcpaustralia.com.au)

PC

–21 – +58

Rubitherm GmbH (www.rubitherm.de)

Germany Salt hydrates/ Blend

SP

–21 – +90

Paraffins

RT

–10 – +82

Paraffins (60%) + power base

PX

–10 – +100

Granules

GR

–10 – +100

Compound

PK

+6, +42 – +80

Salt hydrates/ eutectics

LATEST™

–50 – +89

Teappcm (www. teappcm.com)

India

Salt hydrates

PlusICE

STL

Mitsubishi Chemical (www.m-kagaku. co.jp)

Japan

Eutectic salt-water solutions

Tay et al. (2013) has applied the method of mixing the PCM in a tube-in-tank application to overcome instability issues as well as improve the effective conductivity. Martin et al. (2010) has investigated when the PCM is in direct contact with the heat transfer fluid, improving heat transfer and potentially overcoming stability issues.

Using solid-liquid phase change materials (PCMs) in thermal energy storage systems

239

PCM solutions for high temperature storage have also been developed in context. DLR (Laing et al., 2013) have developed and tested a fin arrangement with molten salts investigating heat transfer, stability and corrosiveness of the PCM. Zheng et al. (2013) investigated encapsulated molten PCM in specially designed spheres to prevent corrosion. PCM slurries have also proved to be an effective method of enabling successful development of PCM thermal storage. Slurries can be two-phase mixtures of a PCM, or PCM microencapsulated within a compatible host liquid (Delgado et al., 2012). These systems overcome PCM instability, corrosiveness and poor thermal conductivity issues. Passive storage systems involve a PCM storage element in direct contact with the thermal load, with the typical example being wall panels used for internal cladding of buildings. These PCMs predominantly involve paraffins (Cheng et al., 2010). These systems contain a host material which has the PCM bound within, preventing leakage when molten. They are typically stable with cycling, have sufficiently high conductivity and are compatible with the environment in which they are used. Overall, these examples demonstrate the importance that PCM development is fundamentally connected with the application being considered. It is likely that this future research direction will only consolidate as more and more successful applications of PCM are developed.

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