Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures

Solar Energy Materials and Solar Cells 167 (2017) 109–120

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Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures Samer Kahwajia, Michel B. Johnsonb, Ali C. Kheirabadic, Dominic Groulxb,c, Mary Anne Whitea,b, a b c

MARK

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Department of Chemistry, Dalhousie University, PO Box 15000, Halifax, Nova Scotia B3H 4R2, Canada Institute for Research in Materials, Dalhousie University, PO Box 15000, Halifax, Nova Scotia B3H 4R2, Canada Department of Mechanical Engineering, Dalhousie University, PO Box 15000, Halifax, Nova Scotia B3H 4R2, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords: Solar thermal energy storage Phase change material (PCM) Thermal properties Melt-freeze cycling studies Chemical stability

Organic phase change materials (PCMs) have many properties that make them desirable for integration in latentheat solar thermal energy storage (TES) systems operating in the ambient-to-moderate temperature range, but there are significant gaps concerning their material properties, and their reliability. We present a comprehensive study of thermal and related properties (transition temperature, transition enthalpy change, heat capacity, thermal conductivity, thermal diffusivity, density), thermal stability (on cycling through the transition up to 3000 times) and chemical stability (when in contact with 16 common containment materials at 75 °C) for six organic phase change materials: decanoic acid (aka capric acid, [CH3(CH2)8COOH]), dodecanoic acid (aka lauric acid, [CH3(CH2)10COOH]), tetradecanoic acid (aka myristic acid, [CH3(CH2)12COOH]), hexadecanoic acid (aka palmitic acid, [CH3(CH2)14COOH]), octadecanoic acid (aka stearic acid, [CH3(CH2)16COOH]) and 1-octadecanol ([CH3(CH2)17OH]). All show melting transitions in the temperature range 30–70 °C which is suitable for solar thermal applications, with substantial enthalpy changes (> 145 J g-1), and all are thermally stable over 3000 cycles. These materials all have significant potential for solar thermal energy storage applications.

1. Introduction Thermal energy storage, including from solar thermal capture, can be greatly enhanced with the use of phase change materials (PCMs). Ideal PCMs have large latent heats, a phase change at a temperature appropriate to the application, and are inexpensive, safe and reliable. Organic PCMs meet many of these criteria, making them desirable for integration in latent-heat thermal energy storage (TES) systems operating in the ambient-to-moderate temperature range (20–100 °C) [1–5]. In particular, non-paraffin organic PCMs, including fatty acids, esters and alcohols [3], can be a sustainable choice given that these materials are derived from renewable natural sources, and are non-toxic and abundant [6–11]. Also, fatty acid PCMs and their derivatives are especially advantageous for use in TES applications because life-cycle assessment has shown that the embodied energy associated with their production can be recouped in solar thermal applications in a very short time [12]. However, to achieve optimal energy storage requires reliable data, as the design of a latent heat TES system critically requires the PCM to have optimal thermophysical properties [13,14]. To be useful, the PCM must be thermally and chemically stable after many melt-freeze cycles

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and unreactive with materials in which it is in contact. Yet such information usually is not known. PCMs with melting points at moderate temperatures, between 30 and 70 °C, are particularly useful for solar thermal energy storage and building applications [3,4,15–18]. Storage tanks filled with PCMs can be added to solar hot-water systems to increase their heat storage capacity and improve their performance [19–21]. Although most investigations focus on paraffin PCMs for these systems [19,21,22], fatty acids and their mixtures have advantages when integrated in solar heating systems [23–28]. Tests involving tetradecanoic and dodecanoic acids in a trapezoidal solar water heater [27] showed that tetradecanoic acid helps to maintain the temperature of hot water during the night, whereas dodecanoic acid can stabilize the water temperature during the heating period. Recently, real-time experiments showed that a standard flat-plate solar thermal collector with a tank filled with 60 kg dodecanoic acid absorbs 20% more thermal energy than a conventional storage system with an equivalent mass of water [29]. A solar domestic hot water system with an 80:20 mixture of paraffin and octadecanoic acid increases the temperature of the water by 3–4 °C and has a high recovery efficiency of 74% [20]. Another interesting application for PCMs with phase transitions

Corresponding author at: Department of Chemistry, Dalhousie University, PO Box 15000, Halifax, Nova Scotia B3H 4R2, Canada. E-mail address: [email protected] (M.A. White).

http://dx.doi.org/10.1016/j.solmat.2017.03.038 Received 25 January 2017; Accepted 30 March 2017 Available online 10 April 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

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Table 1 PCMs studied in this work with information about their suppliers and purity. PCM

Type

Molecular weight (g/mol)

Supplier

Purity

Abbreviation

Decanoic acid Dodecanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid 1-Octadecanol

Fatty acid Fatty acid Fatty acid Fatty acid Fatty acid Alcohol

172.26 200.31 228.37 256.42 284.47 270.49

Alfa-Aesar Sigma-Aldrich Alfa-Aesar Alfa-Aesar Acros Organics Alfa-Aesar

99% 98% 98% 95% 97% 97%

C10 C12 C14 C16 C18 octadecanol

between 30 and 70 °C is in passive cooling of electronic devices [30,31]. Several groups have found that electronic devices incorporating PCMbased heat sinks are effective at reducing the peak temperature of electronic components [32–41]. A thermally efficient smart phone containing PCM is now available [42]. Thermal stability is critically important for a PCM, to give reliable TES. For fatty acids, studies of dodecanoic acid, tetradecanoic acid, hexadecanoic acid and octadecanoic acid demonstrated that latent heats of fusion and melting temperatures do not change significantly after few hundred cycles [25,43–45], but the long-term stability of some fatty acids showed changes that might be attributed to low purity [46,47]. Long-term thermal stability is particularly important as PCMs are increasingly used in small-scale TES units, such as in portable electronics [36,42], textiles [48,49] and other consumer products [4], where the PCM will undergo many melt-freeze cycles per day. Equally important is the chemical compatibility of the PCM with containment materials. Corrosion immersion tests for PCMs have been largely focussed on salt hydrates [50–54], and usually limited to common metals, such as aluminum, copper, steel and brass. Although a few studies investigated the corrosion effects of fatty acids [51,55], they are not very thorough. The latent heat of fusion (or solidification) and the phase transition temperature of many organic, non-paraffin PCMs are known and tabulated in many review papers [1,3,8,56], but the literature shows significant discrepancies between the values reported by different groups for the same PCM. Other important thermal properties, such as the heat capacity and the thermal conductivity are scarce, yet important when numerical approaches are used to optimize the TES system [57,58]. Furthermore, although organic PCMs have been investigated and applied in many TES applications, the few reports on thermal cycling beyond the recommended 1000 thermal cycles [14] showed inconclusive results [46,47]. To have widespread use of reliable PCMs and more efficient thermal energy storage also requires information concerning chemical compatibility with a wide range of materials. Therefore, we were motivated to carry out a comprehensive study of thermophysical properties, thermal stability and chemical compatibility of inexpensive, safe organic PCMs, using a thorough and systematic methodology, where the same experimental procedures and PCM samples from the same batch were used for each area of the study. The PCMs investigated are: five straight-chain fatty acids (decanoic acid [aka capric acid, CH3(CH2)8COOH]; dodecanoic acid [aka lauric acid, CH3(CH2)10COOH]; tetradecanoic acid [aka myristic acid, CH3(CH2)12COOH]; hexadecanoic [aka palmitic acid, CH3(CH2)14COOH]; and octadecanoic acid [aka stearic acid, [CH3(CH2)16COOH]; and one fatty alcohol, 1-octadecanol ([CH3(CH2)17OH]). These materials were selected based on their transition temperatures, Tmpt, and high latent heats of fusion (ΔfusH≥150 J g-1), and potential for a wide range of applications, from solar thermal energy storage in the built environment to thermal buffering in buildings and electronic devices. Furthermore, knowledge of the properties of these fatty acids is useful to advance understanding of mixtures of fatty acids. The latter can produce eutectic PCMs with melting temperatures in the ambient temperature range (20–28 °C). Eutectic mixtures of fatty acids are potentially useful PCMs for thermal energy storage in comfort control applications in buildings [16,59–61]. Octadecanol was included here because it can be combined with a fatty acid to give a fatty acid ester PCM with Tmpt

in the range 42–65 °C, and quite high latent heat of fusion (ΔfusH > 200 J g-1) [11]. The results presented herein provide all data necessary for feasibility studies on the integration of the selected PCM in TES units and information to facilitate the selection of a reliable PCM for a given application based on its chemical compatibility. 2. Experimental methods 2.1. Phase change materials The PCMs studied here are listed in Table 1. Decanoic (abbreviated C10 hereafter), dodecanoic (C12), tetradecanoic (C14), hexadecanoic (C16) and octadecanoic (C18) fatty acids and 1-octadecanol with purities between 95% and 99% were purchased from different suppliers. The thermal properties of the PCMs were measured without any additional purification. 2.2. Materials for compatibility studies To investigate the chemical compatibility of the PCMs, 16 materials that could potentially be used for containment materials of PCMs in solar heating systems and in portable electronic devices were chosen. The selected materials (Table 2) include 9 metals/metal alloys, and 7 plastics. The wide range of materials, including different alloys, covers typical materials used for encapsulation, housing and gaskets, each with unique mechanical and electrical properties and densities. 2.3. Determination of thermophysical properties Heats of fusion, ΔfusH, and the melting temperatures, Tmpt, for each PCM were determined using a TA Instruments Q200 differential scanning calorimeter (DSC). The DSC was calibrated prior to each set of experiments by measuring ΔfusH and Tmpt of a high-purity indium standard (ΔfusH=28.71 J g-1 and Tmpt=156.6 °C [62]). For each DSC measurement, a PCM sample of mass between 5 and 10 mg was hermetically sealed in a standard DSC aluminum pan and the He flow rate in the cell was 25 mL min-1. Following the recommendations of Ref. [63]. ΔfusH was determined from measurements performed at heating (and cooling) rates of 10 K min-1 whereas Tmpt was determined from the onset temperature, at rates of 2 K min-1. For each ΔfusH measurement, five melt-freeze cycles were performed and averages are reported here. The melting point temperatures were generally determined after the five melting cycles. The typical uncertainties of this DSC system are ± 10% for ΔfusH and ± 1.5 °C for Tmpt. The DSC was also used to measure the heat capacity at constant pressure as a function of temperature of each PCM, in both the solid (Cp,s) and liquid (Cp,l) phase. The procedures for Cp(T) measurement by DSC were as outlined in ASTM Standard E1269-11 [64]. The method is comparative relative to a high-purity reference material with known Cp(T), here high-purity sapphire [65]. We used different Al pans for all three heat-flow measurements (empty Al pan; reference material; actual sample), and applied a correction to account for the small (< 5%) difference in the masses of the pans, as recommended by the ASTM Standard [64]. We estimate that the Cp(T) measured with this DSC method is accurate to within 10%. 110

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Table 2 List of materials tested for chemical compatibility with the PCMs. The names and thicknesses are listed as received from the suppliers.

The densities of solid PCMs, ρs, were determined at room temperature (23–24 °C) using a 10 mL glass pycnometer and Archimedes’ principle: the volume of a solid PCM sample is found from the volume of the liquid displaced when the pycnometer containing the sample is filled with water. Masses were determined using a high-accuracy balance ( ± 0.02 mg). The liquid was distilled water with a very small amount of detergent to reduce surface tension and prevent formation of air bubbles.

The thermal conductivities (κ) of all PCMs were determined using the thermal transport option (TTO) of a Quantum Design Physical Property Measurement System (PPMS). The PPMs determined the thermal conductance of the sample, KPCM, and the thermal conductivity was calculated from KPCM and the known dimensions of the sample. All measurements were carried out in vacuum in steady-state conditions in which a 1-D heat pulse was applied to the sample and the temperature response across the sample was monitored. The measured thermal conductance was corrected for radiative losses and conductance of the thermometer assembly [63]. A custom-designed cell similar to that described in Ref. [61]. was used as a sample holder with a sample-space volume of about 0.1 mL. The thermal conductivities in both the solid phase, κs, and the liquid phase, κl, were measured using this cell in the temperature range 0–100 °C, i.e., through the solid-liquid phase transition of the PCMs. Since the thermal conductance of the cell plus sample setup follows the system of conductances in series, as verified by measurement of standards and by traditional steady-state measurements of the solid PCMs without containment, the determination of κs and κl required measurement of the thermal conductance of the empty cell, Kcell, followed by measurement of the filled cell, Ktotal, with the thermal conductance of the PCM, KPCM, given by KPCM=Ktotal-Kcell. The uncertainties in the measured values of the thermal conductivities are estimated to be within 15% at room temperature.

2.4. Thermal stability To determine the long-term thermal stability of these PCMs, 3000 melt-freeze cycles were carried out, using a custom-built thermal cycler described elsewhere [61]. The cycler was programmed to repeatedly melt and solidify a sample (ca. 2 g) from 10 to 50 °C or 10–90 °C, depending on the melting point, with ca. ten minutes at the lowest temperature to ensure complete solidification. Aliquots were taken for thermal measurements (Tmpt and ΔfusH) as a function of numbers of cycles. 2.5. Chemical compatibility Chemical compatibility of the PCMs was determined by immersion corrosion tests in which potential contact materials were immersed in 111

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cut into coupons (length ~2 cm, width ~0.5 cm, thickness between 0.1 and 1.6 mm [as acquired; see Table 2] as shown in Fig. 1). The aluminum and stainless steel samples were polished before cleaning. All coupons were first cleaned in a soap and distilled water solution then rinsed with distilled water and dried. The metal coupons were then soaked in a 1:1 mixture of acetone and toluene for few minutes, and the plastic coupons were soaked in isopropyl alcohol. After solvent cleaning, the samples were rinsed in distilled water and dried again. The copper samples were further cleaned in a ~19% solution of hydrochloric acid (HCl) to remove surface oxide. Dimensions of each coupon were measured with a Vernier caliper (uncertainty of ± 0.02 mm) and the total surface area was calculated. The coupon's mass was determined with a high-accuracy balance ( ± 0.02 mg). Each coupon was placed in a screw-cap vial containing about 8 mL of PCM (Fig. 1), capped and placed in an oven at 75 °C. At this temperature, all PCMs were liquid and the coupons were completely immersed in the PCM during the tests. Corrosion tests in liquid PCMs are a standard practice [51–53]. Considering the relatively low melting temperatures of the PCMs, this test provides corrosion rates for more extreme conditions

Fig. 1. (a) An example of coupon (silicone rubber) used in chemical compatibility studies. (b) The coupon inside a capped vial containing liquid PCM at 75 °C.

liquid PCMs and their corrosion rates were evaluated gravimetrically, according to the procedures described in ASTM G1 Standard [66]. The indicator of compatibility is the corrosion rate, CR, calculated from the change in mass (resulting from a chemical reaction of the specimen with the PCM) over a given period of time. The materials selected for the chemical compatibility studies were

-1

0 -1

Exo

-2

20

30

40

50

-1 -2 40

50

T / °C

T / °C

Tetradecanoic acid

Hexadecanoic acid -1

0 -1 -2 30

0

30

Heat flow / W g

-1

10

Heat flow / W g

Dodecanoic acid

Heat flow / W g

Heat flow / W g

-1

Decanoic acid

40

50

60

0 -1 -2 50

70

60

60

T / °C

70

80

T / °C

Octadecanoic acid

Octadecanol Heat flow / W g

Heat flow / W g

-1

-1

2 0

-1

-2 50

60

70

80

1 0 -1 -2 20

90

T / °C

30

40

50

60

70

80

T / °C

Fig. 2. DSC thermograms from which Tmpt values were determined. All thermograms were measured on heating, at a rate of 2 °C/min. Exothermic direction is up. For octadecanol, the cooling thermogram also is shown (upper curve).

112

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than in most applications. To determine the corrosion rate, samples were removed from the vials, cleaned, dried and weighed on the microbalance. The mass loss was determined for the same coupon at three time intervals: at 1, 6 and 12 weeks after initial immersion. In addition to measuring the mass loss, each sample was carefully inspected for visible changes such as corrosion, pitting, surface oxidation or coloration, bubbling and swelling. The corrosion rate, CR, was calculated after each time period, Δt, from the measured change in mass, Δm, and the measured total surface area of the sample, A, assumed to be smooth, using the equation [66]:

Table 3 Measured values of Tmpt and ΔfusH and range of values (shown in parentheses) reported in Ref. [8] and references therein. The densities of the solid PCMs, ρs, measured at 23–24 °C are also included. Values for the liquid density, ρl, are from Ref. [69]. PCM

Tmpt (°C)

ΔfusH (J g-1)

ρs (g cm-3)

ρl [69] (g cm-3)

Decanoic acid

32.0 ± 1.5 (31–36)

145 ± 15 (152–163)

0.85 ± 0.08

0.886 (at 40 °C)

Dodecanoic acid

43.6 ± 1.5 (41–49)

176 ± 18 (177–212)

0.82 ± 0.08

0.868 (at 50 °C)

Tetradecanoic acid

54.7 ± 1.5 (49–58)

186 ± 19 (183–205)

0.86 ± 0.09

0.858 (at 60 °C)

Hexadecanoic acid

61.7 ± 1.5 (55–64)

206 ± 21 (163–203)

0.90 ± 0.09

0.848 (at 70 °C)

Octadecanoic acid

68.4 ± 1.5 (60–70)

211 ± 21 (187–212)

0.84 ± 0.08

0.902 (at 75 °C)

Octadecanol

58.0 ± 1.5 (55–56)

218 ± 22a (145–246)

0.91 ± 0.09

0.808 (at 65 °C)

a

CR =

A

∆m . ∆t

(1)

where CR is reported in units of mg cm-2 yr-1. The values of the corrosion rates reported here are calculated from the change in mass over the time period since the prior mass measurement.

includes transition enthalpy change from solid-solid transition at ~56 °C.

Decanoic acid

Dodecanoic acid 4

-1

Cp / J K g

-1

Cp / J K g

-1

-1

4

3

liquid

solid

3

2

2 10

20

30

40

50

1

60

10

20

T / °C

40

50

60

T / °C

Tetradecanoic acid

Hexadecanoic acid

4

-1

3

3

-1

Cp / J K g

-1

Cp / J K g

-1

4

30

2 30

40

50

60

70

2

80

30 40 50 60 70 80 90 100

T / °C Octadecanoic acid

Octadecanol

-1

4

3

-1

Cp / J K g

3

-1

Cp / J K g

-1

4

T / °C

2 30 40 50 60 70 80 90 100

T / °C

2 30

40

50

60

70

80

T / °C

Fig. 3. Temperature-dependence of the specific heat capacities at constant pressure, Cp (T), of all studied PCMs. The heat capacities of all these materials approach infinity in the region of the transitions.

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Fig. 4. Thermal conductivity, κ, as a function of temperature for the solid (

) and liquid (

) PCMs. The arrow indicates the melting temperature of the PCM.

3. Results and discussion Table 4 Thermal diffusivities of the solid, αs, and liquid, αl, PCMs calculated from the measured values of heat capacities, thermal conductivities and solid density. PCM

αs (mm2 s-1)

αl (mm2 s-1)

C10 C12 C14 C16 C18 Octadecanol

0.12 ± 0.02 0.14 ± 0.02 0.20 ± 0.03 0.17 ± 0.03 0.17 ± 0.03 0.19 ± 0.03

0.09 ± 0.01 0.09 ± 0.01 0.11 ± 0.02 0.11 ± 0.02 0.09 ± 0.01 0.11 ± 0.02

3.1. Thermophysical properties Melting temperatures determined from the onset in temperature in DSC thermograms as in Fig. 2, and latent heats of fusion, ΔfusH, are presented in Table 3. For comparison, the range of literature values is also included. As expected, Tmpt increases as the length of the carbon chain increases. Latent heats of fusion (Table 3) of the fatty acids are generally high. The thermogram of octadecanol (Fig. 2) shows two phase transitions. On heating, the lower-temperature transition appears as a shoulder peak on the main endothermic peak. The two transitions are more evident on cooling (Fig. 2). This finding aligns with the report by Tanaka et al. [67] that long-chain alcohols generally exhibit two phase 114

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Fig. 5. Thermal stability of the PCMs after 3000 melt-freeze. The values of ΔfusH (

) and Tmpt (

) are shown as a function of thermal cycle number. The solid lines correspond to ΔfusH

and Tmpt of a fresh sample, and the dashed lines show the range of their uncertainties: ± 10% for ΔfusH and ± 1.5 °C for Tmpt.

transitions, first a solid-solid transformation [67,68], followed by melting. We determined that the solid-solid transition is at (55.8 ± 1.5) °C whereas Tmpt is (58.0 ± 1.5) °C. It was not possible to objectively separate the contributions of each transition to ΔH, and for the purposes of energy storage, the total ΔH is more important. The measured densities of solid PCMs, ρs, are also reported in Table 3, along with densities of liquid PCMs, ρl, obtained from the equations of Ref. [69]. The latter values are required later to calculate the thermal diffusivities of the liquid PCMs. The temperature-dependences of the heat capacities, Cp(T), are shown in Fig. 3. Because the DSC experiment is non-equilibrium, Cp(T) curves show singularities near the region of the phase transition and data points in this region are omitted. In general, the heat capacities of the six PCMs measured vary between 1.8 and 2.2 J K-1 g-1 for the solid phase, and between 2.1 and 2.5 J K-1 g-1 for the liquid phase. Measured values of the heat capacities at selected temperatures for each PCM are

presented in Table S1 of Supplementary Materials. The thermal conductivities of the solid, κs, and liquid, κl, PCMs are shown in Fig. 4 in the temperature range 0–100 °C. As expected for organic materials, the values of the thermal conductivities are low, typically around 0.3 W K-1 m-1 for κs and dropping to around 0.2 W K1 m-1 for κl. There are no significant variations with temperature for κs and κl within the temperature range considered and the values obtained here are within uncertainty of values reported elsewhere [70–72]. The low thermal conductivity of the selected PCMs can be a challenging problem for large-scale TES systems, as stated before. To improve the heat transfer and achieve effective charging and discharging rates of the PCMs, methods including the design of a TES system with extended contact surfaces or the incorporation of high-conductivity fillers with the PCMs have been suggested [72–74]. Thermal diffusivity is important to determine the extent of phase change when a PCM absorbs or releases heat. From the measured values 115

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Fig. 6. Corrosion rates, CR, determined for the metallic samples in each PCM. Materials with CR values between −10 and 10 mg cm-2 yr-1 (shown as solid green boundary lines) are recommended for long-term use whereas for CR > 50 mg cm-2 yr-1, the materials are not recommended for use with the PCM. The small negative CR values for these metallic specimens are due to experimental uncertainty.

3.2. Thermal stability

of the heat capacities and thermal conductivities, we calculated the thermal diffusivities of the solid (αs) and liquid (αl) phases of the PCMs (α=κ/Cp×ρ). For αl, values of ρl from Table 3 were used. The values obtained for αl are well within the range of values determined from [75] and from the “best path to prediction” method of Ref. [76]. Thermal diffusivities of the solid PCMs were between αs=0.12 mm2 s-1 for C10 and αs=0.20 mm2 s-1 for C14. The thermal diffusivity generally decreases when the PCM melts, to a value around 0.10 mm2 s-1 for the PCMs measured here, as shown in Table 4.

Thermal stability of a PCM was evaluated from changes in ΔfusH and Tmpt with respect to the values for a fresh (uncycled) PCM. One meltfreeze thermal cycle here consisted of a temperature excursion from 10 to 50 to 10 °C for decanoic acid, and from 10 to 90 to 10 °C for all the other PCMs. Fig. 5 shows that all the PCMs are thermally stable: there are no systematic variations in ΔfusH and Tmpt as the number of thermal cycles 116

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As for Cu, the Mg alloy AZ91D did not show good compatibility with the fatty acid PCMs. After one week in decanoic and dodecanoic acids, CR values were already significantly higher than the recommended level. The reaction of Mg AZ91D was slightly slower with tetradecanoic and hexadecanoic acids and slower still in octadecanoic acid. However, overall, Mg AZ91D had the highest corrosion rates in fatty acids among all the tested metal alloys. The nickel sample, Ni C7521, only reacted significantly with decanoic acid and, to a lesser extent, with dodecanoic and tetradecanoic acids (Fig. 6). The corrosion rate in decanoic acid at week 12 was ~70 mg cm-2 yr-1. Unlike the metals, most of the plastic materials did not show good compatibility with the PCMs. In most cases, the plastic specimens deformed and swelled from absorption of the PCM, as is evident from the negative CR values shown in Fig. 8. There is also a contribution to the degradation of the plastic coupons from the high testing temperature, 75 °C. The impact-resistant polycarbonate film initially seemed to be compatible with all PCMs, but after 6 weeks in decanoic acid the opacity of the polycarbonate specimen increased and CR ~−9.5 mg cm2 yr-1, which is at the upper limit of the recommended range. After 12 weeks, the specimen became white and the originally colourless decanoic acid turned light yellow. In octadecanol, the polycarbonate film softened over time and became brittle. Nevertheless, the corrosion rates in octadecanol for all 3 time periods were below 1 mg cm-2 yr-1 (Fig. 8). The ABS plastic does not have good chemical compatibility with most fatty acid PCMs: highly negative CR values were measured in decanoic, dodecanoic and tetradecanoic acids (Fig. 8). The chemical reaction with this plastic and absorption of the PCM was evident at week 1. By week 12, the thickness of ABS specimen was ~60% greater than the initial value, and its mass increased by approximately 90%. As shown in Fig. 9, in addition to swelling, bubbling at the surface of the specimen and discoloration of both the ABS sample and the PCMs were evident. The corrosion rates of the other plastic materials show that polypropylene cannot be recommended for use with any of these PCMs. Similarly, silicone rubber is not recommended for use with any of the tested PCMs due to the high rate of PCM absorption. The chemical-resistant Type I PVC is compatible with octadecanol and octadecanoic acid, but not with decanoic acid. Caution is recommended when using Type I PVC with the other fatty acid PCMs. Likewise, cast acrylic is not recommended for use with decanoic and dodecanoic acids. The CR values of wear-resistant nylon measured at week 1 were generally close to 50 mg cm-2 yr-1 for all PCMs. Therefore, nylon should be avoided for contact with the selected PCMs in long-term applications. The full results of these chemical compatibility studies are summarized in Fig. 10.

Fig. 7. Changes in decanoic acid and Cu 101 as a function of immersion time.

increases and all measured values remain within the uncertainty range of the initial values ( ± 1.5 °C for Tmpt and ± 10% for ΔfusH). The influence of purity on the melting temperature was shown to be insignificant when two substantially different chemical grades of dodecanoic acid (98% and < 80% purity) were compared and thermally cycled 500 times [45]. The present cycling results confirm that each PCM is stable for 3000 cycles, which is equivalent to eight years of daily thermal cycling, so long as they do not react significantly with their environment.

3.3. Chemical compatibility The chemical compatibility of the materials tested by immersion in the PCMs was determined from the values of the corrosion rate, CR, calculated after 1, 6 and 12 weeks of immersion in the liquid PCM at 75 °C. The recommendations for materials usage are adapted from reference [77] and summarized in Table S3 in Supplementary Materials. A material is recommended for long-term use with a PCM if its CR is positive but less than 10 mg cm-2 yr-1. We added two grades of negative values to Table S3 for cases where the mass of the coupon has increased, as observed here for some plastic materials. For CR < −50 mg cm-2 yr-1 the material is not recommended for use due to major deformation or swelling. The corrosion rates calculated for the metallic samples for each PCM are shown in Fig. 6. Both stainless steel alloys, SS 304 and SS 316, and the three aluminum alloys, Al 5052, Al 6061 and Al 6063, show good compatibility with all tested PCMs with their CR < 10 mg cm-2 yr-1 for all PCMs tested. Moreover, there was no noticeable reaction at the surface of the coupons after 12 weeks of immersion. However, the two copper alloys, Cu 101 and Cu 110, are not compatible with fatty acid PCMs, due to their high CR values: decanoic, hexadecanoic and octadecanoic acids have CR > 50 mg cm-2 yr-1 and dodecanoic and tetradecanoic acids have CR between 10 and 40 mg cm-2 yr, which necessitates caution when using Cu 101 and Cu 110 with these PCMs for long-term applications. Fig. 7 shows the changes in appearance over time of a Cu 101 sample in decanoic acid. Similar observations were reported by Ferrer et al. [51] for Cu immersed in eutectic mixture of decanoic/tetradecanoic acids and in eutectic mixture of decanoic/ hexadecanoic acids at 38 °C. Their CR values were lower, likely because our temperature was higher (75 °C). For octadecanol, we did not observe any significant reaction with Cu; CR < 1 mg cm-2 yr-1 (Fig. 6).

4. Conclusions This work presents a thorough determination of the thermophysical properties, thermal stability and chemical compatibility of six organic PCMs with melting points between 30 and 70 °C. Our measurements followed the same procedures using the same experimental techniques and instruments for all the PCMs to provide a direct comparison. The PCMs were chosen based on their potential as latent heat thermal energy storage media in solar heating applications and passive cooling of portable electronics. Using DSC and PPMS, Tmpt, ΔfusH, and the temperature dependence of Cp and κ of decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, octadecanoic acid and octadecanol were measured. As well, the density of the solid PCMs was measured with a pycnometer. The experimental determinations of the temperature dependence of Cp and κ, and the calculation of the thermal diffusivities, now allow full consideration of these PCMs for energyrelated applications. In addition to the thermophysical characterization, the thermal stability of the PCMs and their chemical compatibility with sixteen 117

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Fig. 8. Corrosion rates, CR, determined for plastic materials in each PCM. Materials with CR values between −10 and 10 mg cm-2 yr-1 (shown as solid green boundary lines) are recommended for long-term use whereas for CR < −50 or CR > 50 mg cm-2 yr-1, the materials are not recommended for use with the PCM. Negative CR values are due to PCM absorption.

different materials was determined. Long-term cycling results showed that all six PCMs examined are thermally stable over 3000 melt-freeze cycles. Even for a lower-purity sample of hexadecanoic acid, there were no noticeable changes for Tmpt and ΔfusH. Therefore, these are reliable PCMs for thermal energy storage applications. The comprehensive information presented herein concerning their thermophysical properties will facilitate future applications. Furthermore, the chemical compatibilities of the PCMs with nine metal alloys and seven plastic materials were determined from corrosion immersion tests performed at 75 °C. The chemical interactions of a PCM with a specific material were based on the value of the corrosion rates, CR, calculated from mass loss. In general, two copper alloys, Cu 101 and Cu 110, and the magnesium alloy, Mg AZ91D, are not compatible with the fatty acids, whereas Ni C7521 was only compatible with hexadecanoic and octadecanoic acids. In contrast, octadecanol showed good compatibility with all tested metallic alloys. Amongst the plastic materials, only polycarbonate did not significantly react with

Fig. 9. (a) ABS plastic sample before immersion in the PCMs and after 12 weeks in (b) decanoic acid and (c) dodecanoic acid. The PCMs also changed from colourless to amber over the 12-week period.

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