2003 Elsevier Ltd. All rights reserved.

The use of phase change materials (PCMs) in energy storage has the advantage of high energystorage vs. sensible heat storage, such as water. Another advantage of latent heat storage is itsisothermal operating characteristics.

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Applied Thermal Engineering 23 (2003) 16971704Corresponding author. Tel.: +34-973-702-742; fax: +34-973-702-702.

E-mail addresses: lcabeza@diei.udl.es (L.F. Cabeza), mehling@muc.zae-bayern.de (H. Mehling).Keywords: Phase change material; PCM; Thickening; Sodium acetate trihydrate; Thermal performance

1. Introduction

Thermal energy storage has recently become a major interest issue due to the interest in use ofrenewable energies, such as thermal solar energy, and waste heat. The mismatch between energydemand and energy availability can only be overcome by the use of an energy reservoir.Thermal performance of sodium acetate trihydratethickened with dierent materials as phase change

energy storage material

Luisa F. Cabeza a,*, Gustav Svensson b, Stefan Hiebler b, Harald Mehling b,1

a Departament dInformaatica i Eng. Industrial, Universitat de Lleida, Jaume II, 69, 25001 Lleida, Spainb ZAE Bayern, Abt. 1 Energy Conversion and Storage, Walther-Meissner-Str. 6, 85748 Garching, Germany

Abstract

The use of phase change materials (PCMs) in energy storage has the advantage of high energy density

and isothermal operation. Although the use of only non-segregating PCMs is a good commercial approach,

some desirable PCM melting points do not seem attainable with non-segregating salt hydrates at a rea-

sonable price. The addition of gellants and thickeners can avoid segregation of these materials. In this

paper, sodium acetate trihydrate is successfully thickened with bentonite and starch. Cellulose gives an even

better thickened PCM, but temperatures higher than 65 C give phase separation. The mixtures would showa similar thermal behavior as the salt hydrate, with the same melting point and an enthalpy decrease

between 20% and 35%, depending on the type and amount of thickening material used.

www.elsevier.com/locate/apthermeng1 Tel.: +49-89-32944222; fax: +49-89-32944212.

1359-4311/03/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S1359-4311(03)00107-8

material. Its melting temperature (about 58 C) makes it suitable for applications such as solar

polyacrylic amide, sodium polyacrylate, and polyvinyl alcohol was not successful, so the experi-ments carried out with sodium acetate trihydrate focused on thickening with starch, cellulose and

1698 L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 16971704bentonite.In this study, the thermal performance and storage parameters of sodium acetate trihydrate

thickened with four dierent materials were investigated experimentally.

2. Experimental method

2.1. Materials

All the experiments here presented were done using NaCH3COO 3H2O of ACS quality fromMerck. The nucleator used was Na2HPO4 7H2O from Merck.As thickeners, starch, cellulose and bentonite were used. The starch used in the experiments was

simple wheat our. There are a variety of dierent starches for industrial usage available on themarket, but the wheat our should work equally well and is conveniently available.Two dierent celluloses were tested: methylhydroxyethyl-cellulose (MHE-cellulose), and methyl-

cellulose (M-cellulose). These are commercial industrial products and manufactured and sold byWol Cellulosics, Wahlsrode Germany, under the product name Walocel.One type of bentonite was tested, manufactured and sold by Suud-Chemie, Krefeld Germany,

under the name Ceratosil CV, where the cation is calcium, and is activated.

2.2. Methodologyenergy storage for domestic hot water.One of the problems that some salt hydrates exhibit when used as PCMs is phase segregation.

This means that under continuously melting and thawing, the dierence in density between thewater and the salt components of the salt hydrate, results in segregation. The consequence is a badcrystallization of the compound, and a change in the thermophysical properties of the PCM.Segregation can be prevented changing the properties of the salt hydrate with the addition of

another material that can hinder the heavier phases to sink to the bottom. This can be achievedeither with gelling or with thickening materials. Gelling means adding a crosslinked material (e.g.polymer) to the salt to create a three dimensional network that holds the salt hydrate together.Thickening means the addition of a material to the salt hydrate that increases the viscosity andhereby holds the salt hydrate together [11].This is benecial not only for preventing gross segregation of the lower hydrates or anhy-

drous salts formed in peritectic reactions, but is also helpful in suspending insoluble nucleatingagents.In a previous investigation, Svensson [12] showed that gelling salt hydrates with materials likeSodium acetate trihydrate has been studied as PCM by several authors [110] because of itslarge latent heat of fusion (about 240 J/g) which makes it attractive as a latent heat storageThe procedure to do the experiments was:

100 g of sodium acetate trihydrate was mixed with 1 g of nucleator and melted in a water bathat 80 C.

Then the liquid was transferred to a beaker and the thickener was added carefully while mixedrapidly.

The thermal performance was tested with dierential scanning calorimetry (DSC) on smallsamples, and with cool down experiments on a larger sample. The cool down experiments werecarried out as follows:

The thickened PCM was transferred to 14 ml plastic tubes. The tubes were heated up to 80 C in a water bath and then let them cool down at ambienttemperature.

In a second experiment, the tubes were heated up and let cool down in a styrofoam containerfor isolation of the samples. With this experiment, the samples can be considered isothermal.

To make a comparison, a sample of sodium acetate trihydrate with 1 wt% nucleator was alsotested in the DSC.

3. Results

90air freezing

L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 16971704 16990

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0:00:00 1:00:00 2:00:00 3:00:00 4:00:00

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ture

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styrofoam freezingFig. 1 shows the thermal behavior of the cool down experiments in two dierent samples ofsodium acetate trihydrate with 1 wt% nucleator thickened with 20 wt% starch. It should beFig. 1. Thermal performance of thickened PCM: sodium acetate trihydrate + 20 wt% starch.

pointed out that when starch is added to the salt hydrate, it turns out to be a material very dicultto handle, because it becomes very sticky. In the same way, the mixture is a material which be-havior is dicult to measure. As it can be seen in Fig. 1, starch seems to change the melting andfreezing temperatures of the sodium acetate trihydrate (melting point now is 70 C, and freezingpoint is 4550 C with 510 C of subcooling), but this behavior was not corroborated in DSCmeasurements (Table 1).When using bentonite as thickening material, the viscosity does not increase slowly, but there is

a sudden change in the sample when a certain amount of bentonite is added. With sodium acetatetrihydrate, no change was found with the addition of 40 wt% of bentonite, so 50 wt% addition wasnecessary. Fig. 2 shows the thermal performance of the thickened material in two dierentsamples. It can be seen that the melting and freezing temperature remains to be around 58 C, aswith pure sodium acetate trihydrate, but a subcooling of about 10 C occurs.

60

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C)

air freezing styrofoam freezing

Table 1

DSC results of sodium acetate trihydrate with 1 wt% Na2HPO4 7H2O as nucleator, with dierent thickening materialsCompound Melting temperature

(C)Measured enthalpy (J/g) Extrapolated enthalpy of

NaCH3COO 3H2O (J/g)NaCH3COO 3H2O 55.656.5 237243 +20 wt% starch 54.7 180189 225236

+50 wt% bentonite 56.4 153161 306322

+30 wt% MHE-cellulose 55.1 165 234

1700 L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 169717040

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(Fig. 2. Thermal performance of thickened PCM: sodium acetate trihydrate + 50 wt% bentonite.

The use of cellulose seemed to be one of the best chances for thickening of salt hydrates. Theviscosity of the sample increases with increasing amounts of cellulose, so dierent concentrationscan be used depending on the application. Fig. 3 shows the thermal performance of sodium ac-etate trihydrate thickened with 30 wt% of MHE-cellulose in two dierent samples, on cool downexperiments. As can be seen, the melting temperature decreases by a small amount, to about 5556 C, but the subcooling is not as severe as with the other materials.Fig. 4 shows the thermal performance of sodium acetate trihydrate thickened with 15 wt% of

MHE-cellulose in two dierent samples. The curves are very similar to the one of Fig. 3. Theproblem with these samples appeared after a few cycles of heating/cooling, when a separationbetween the salt hydrate and the cellulose occurred.To check if this was due to the cellulose used, a new mixture with methyl cellulose (M-cellulose)

was prepared and tested (Fig. 5). Again, after a few cycles the salt hydrate and the celluloseseparated.Finally, one fresh sample of sodium acetate trihydrate thickened with 30 wt% of MHE-cellulose

and one thickened with 30 wt% of M-cellulose were cycled using a maximum heating temperatureof 65 C. The thermal performance is presented in Fig. 6. These samples did not separate afterrepeated cycling, and the thermal behavior of both samples was very similar.Table 1 presents the results of the DSC measurements done to the dierent materials used in

these experiments. As can be seen, the melting temperature of the samples has not changeddramatically with any of the thickening materials used. The entalphy of the mixtures decreasesfrom 240 J/g of the sodium acetate trihydrate to between 155 and 190 J/g in dierent samples.

L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 16971704 1701When the enthalpy of the mixtures is extrapolated to see that due to the sodium acetate trihydrate

0

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air freezing styrofoam freezingFig. 3. Thermal performance of thickened PCM: sodium acetate trihydrate + 30 wt% MHE-cellulose.

1702 L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 1697170490in the samples, all of them turn to be close to that of sodium acetate, except in the bentonitesample.

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Fig. 4. Thermal performance of thickened PCM: sodium acetate trihydrate + 15 wt% MHE-cellulose.

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air freezing styrofoam freezing

Fig. 5. Thermal performance of thickened PCM: sodium acetate trihydrate + 30 wt% M-cellulose.

L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 16971704 17034. Conclusions

The experiments here carried out showed that thickening of salt hydrates is possible. Sodiumacetate can be successfully thickened with starch and bentonite. Cellulose gives a better material,but then the mixture can only be heated up to 65 C to avoid separation of the materials. Themixtures would show a similar thermal behavior as the salt hydrate, with the same melting pointand an enthalpy decrease between 20% and 35%, depending on the type and amount of thickeningmaterial used.

Acknowledgement

This project was partially funded by the Vicerectorate of Research of the University of Lleida.

References

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materials, J. Thermal Anal. 35 (1989) 10091031.

[3] F. Lindner, Waarmespeicherung mit Salzen und Salzhydraten, Ki Luft- und Kaaltetechnik 10 (1996) 462467.

Fig. 6. Thermal performance of thickened PCM: sodium acetate trihydrate + 30 wt% cellulose.

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[8] T. Wada, F. Yokotani, Y. Matsuo, Equilibria in the aqueous ternary system containing Na, CH3CO2 , and P2O47

between 38 and 85 C, Bull. Chem. Soc. Jpn. 57 (1984) 16711672.[9] T. Wada, F. Yokotani, Y. Matsuo, Equilibria in the aqueous ternary system containing Na, CH3CO2 , and

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1704 L.F. Cabeza et al. / Applied Thermal Engineering 23 (2003) 16971704

Thermal performance of sodium acetate trihydrate thickened with different materials as phase change energy storage materialIntroductionExperimental methodMaterialsMethodology

ResultsConclusionsAcknowledgementsReferences