Hiroyuki

IEA Annex 10 PCMs and Chemical Reactions for Thermal Energy Storage2nd Workshop, November 11-13 1998, Sofia, Bulgaria

A Study of Erythritol as Phase Change Material

H.Kakiuchi, M.Yamazaki, M.Yabe, S.Chihara Y.Terunuma, Y.Sakata and T.Usami

Tsukuba Research Center, Mitsubishi Chemical Corporation8-3-1,Chuo, Ami, Inashiki, Ibaraki 300-0332, Japan

[email protected]

Abstract Erythritol has melting point 119 C and heat of fusion 339.8KJ/Kg. The possibility of Erythritol as heatstorage material was examined according to the standard such as thermodynamic criteria, kinetic criteriaand chemical criteria. The heat exchange performance of Erythritol was investigated with ice-on-coil typeheat exchanger vessel.

Introduction Recently, heat storage technology, especially latent heat storage technology , are paid attention from aviewpoint of the global environmental problems and the advanced utilization of energy in the world. InJapan, ice storage system is becoming popular due to peak sift of electrical demand. This system storescold heat in night time using surplus electricity, then uses stored cold heat for air-conditioning in daytime. These are many types of ice storage system such as ice-on-coil type, dynamic type and capsule type.Sagara (1998) reported that floor heating system came into wide use in Japan for public buildings, such asfacilities of the aged and residential buildings [1]. PCM provided by Japanese manufactures for floorheating are Sodium Sulphate Decahydrate, having melting point of about 30C, and Paraffin havingmelting point of 50C. Every manufactures use plate or tube capsules and install its into under floor. For space heating of office building, Sodium acetate trihydrate having melting point of 58C, SodiumAcetate Trihydrate salt mixture having melting point of 47C and Calcium Chloride Hexahydrate havingmelting point of 27C have been used by capsule type heat storage system [2]. For air-conditioning system, Sodium Sulphate Decahydrate salt mixture having melting point of about 10C has been developed(Telkes,1974) (Kakiuchi,1994) [3, 4].

However, these temperature of phase change of these latent heat storage materials are inappropriate forstoring solar energy or waste heat given in hot water supply or in a boiler. These are required to have arelatively high temperature of phase change of from 90 to 200C. Typical phase change material havingmelting point of from 90 to 200C are Polyethylene and Pentaerythritol. Polyethylenes melting point isabout 120 to 140C and its heat of fusion is 214KJ/Kg. Polyethylene have been used for small sizeelectrical stove as PCM in Japan. Pentaerythritol can store heat on going through phase transformations,but here the transformation is solid-solid phase transition. Takahashi et al (1988) reported thatPentaerythritol s thermal conductivity was relatively low [5]. So, they tested the pentaerythritol slurry forraising thermal conductivity.

Hoermansdoerfer (1989) proposed that sugar alcohol such as Erythritol, Mannitol and Galactitol wereable to use as a heat storage material which had a high temperature of phase change in his Patent [6].Besides his patent, Xylitol was also proposed as phase change material (Guex,1981) [7]. In this report, weinvestigated the possibility of Erythritol as phase change material, concerning with thermodynamicproperties, kinetic behaviour, thermal stability and heat exchanger experiment.


Sugar Alcohol Phase Change Material Sugar alcohol is extremely safe material used as sweetening agents. Erythritol, Xylitol, Sorbitol andother things originally exist in nature. Erythritol is used for drinks in order to sweeten without calorie.Xylitol is very famous sweetening agents to prevent tooth decay. A gum using Xylitol is very popular inJapan, too. Melting point, heat of fusion and cost of typical sugar alcohol are shown in Table 1. Latentheat of fusion were measured by using differential scanning calorimeter (DSC220C) of Seiko InstrumentsInc.. Reagent grade of sugar alcohol were used without purifying. The price of sugar alcohol in Japandepends on individual surveys of Mitsubishi Chemical Co., Ltd.

Every sugar alcohol showed in Table 1 has comparatively large heat of fusion except D-Sorbitol. Itis found that these sugar alcohol have possibility of good phase change material. These sugar alcoholsstructural formulas are shown in Table 2. It is found that configuration of Erythritol and Galactitol ismirror symmetric, configuration of Mannitol is axis symmetric, configuration of Xylitol and Sorbitol isntdivided into before mentioned both groups. Hoermansdoerfer (1989) proposed in his patent that if theirchain length was an even number and at the same time their configuration in reference to the chain centrewas symmetric, they not only showed a significant higher heat of fusion, and also possessed a noticeableless supercooling of the melt [6]. But we think that it needs to investigate relation between symmetricconfiguration and thermodynamic behaviour and between chain length and thermodynamic behaviour.

Latent heat of fusion per unit volume of these sugar alcohol and selected phase change materials areshown in Figure 1. It is found that Erythritol, Mannitol and Galactitol are able to store high energy parvolume than other materials. We investigate Erythritol thermal properties and examine the behaviour inheat exchanger in this report. Heat of fusion of Barium Hydroxide Octahydrate is very large and thetemperature of phase change is suitable for supplying hot water. But, Barium Hydroxide Octahydrate isclassified in highly toxic material in Japan and has corrosiveness. So, we think Barium HydroxideOctahydrate is not suitable for PCM.

Table 1 Melting point, Heat of Fusion and Cost of typical sugar alcohol

Melting Point(C)

Heat of Fusion(KJ/Kg)

Density(at 20C)

Cost(US$/Kg)

ErythritolD-MannitolGalactitolXylitolD-Sorbitol

120 [8] 166-168 [9] 188-189 [9] 93.0-94.5 [9] 96.7-97.7 [8]

339.8316.4351.8263.3185.0

1.45 [8]1.52 [9]1.47 [9]1.52 [9]1.5 [8]

5.06.7-7.5

-

6.7-8.31.1

*1US$ = 120YEN

CCCCCH2OH

CH2OHHOHO

HH

OH

CCCCCH2OH

CH2OHH

HO H

OH

OHCCCH2OH

CH2OH

H OH H

Mannitol GaractitolErythritol

Table.2

H H

H

OH

CCCCH2OH

CH2OH

HOH

H

CCCCCH2OH

CH2OH

HO HOHOHOH

Xylitol Sorbitol

H

OH

H OH

HOHH

HO

H

Table 2 Representative sugar alcohols structural formulas [8]


Figure 1 Latent heat of fusion per unit volume of selected phase change materials [8][9][10]*Both data of Pentaerythritol and Pentaglycerol are solid-solid phase transition point .

*Density value is used of solid except H2O. H2O value is liquid density.

Figure 2 Enthalpy Curve of Erythritol


Material Investigation of Erythritol

Thermodynamic Investigation Thermal properties of Erythritol (food additive grade ; Mitsubishi Kagaku Foods Co. Ltd. ) are shownin Table 3. Specific heat of Erythritol were measured by using heat insulation type specific heatmeasurement apparatus (SH-3000) of Vacuum Science and Engineering Company. Specific heat curve isshown in Figure. 2. It is found that Erythritol melts at 119C from this curve. Erythritol is a single elementlike ice, because it melts congruently. It has huge heat of fusion 320KJ/Kg almost equal ices. Erythritolis able to store 1.4 times thermal energy than ice in a heat storage system, because Erythritols liquiddensity is larger than Ices solid density. Thermal conductivity is higher than Polythene. On the otherhand, specific heat is less than inorganic salt hydrate, because Erythritol doesnt have water molecule. InErythritol, the biggest problem is volume change in phase change of solid to liquid. Volume of Erythritolchanges about 10% during solid to liquid phase transition. So, heat exchanger is required tough structureor special method reducing volume change.

Table 3 Thermal Properties of Erythritol (Food additive grade ; Mitsubishi Kagaku Foods Co. Ltd.)Chemical Structure C4H10O4Molecular Weight 122.2Melting Point (C) 118.0Heat of Fusion (KJ/Kg) 339.8Specific Heat (KJ/Kg]C) at 20C at 140C

1.3832.765

Density (Kg/dm3) at20C at 140C

1.481.30

Heat Conductivity (KJ/m]h]C) at 20C at 140C

2.6401.173

Melting point and Heat of fusion were mesured by DSC (Seiko Instruments Inc. ) Specific heat were measured by heat insulation type specific heat measurement apparatus

(Vacuum Science and Engineering Company) Density were measured by Archimedes method. Heat conductivity were measured by hot wire method.

Kinetic Investigation

A cylinder container made of stainless steel of volume 6.0*10-5m3 was produced, to evaluatesolidification and fusion behavior of Erythritol. We poured Erythritol ( Mitsubishi Kagaku Foods Co. Ltd.) 0.05Kg into the container and measured temperature change of it at the height of 0.01m from the bottomof container and recorded it in a recorder. Thermocouple of copper-constantan were used for measurment.The container containing Erythritol was sunk to the temperature controled oil bath and heated to meltErythritol from 30 to 140 in 3 hours. After that, holding 140 for 6 hours, erythritol was completelymelted. Then, heating was quitted to solidify Erythritol, the container in the oil bath was cooled bynatural. A temperature change of the Erythritol in this time is shown in Fig. 3.

When heating was started, temperature of Erythritol rose with oil and became with 119 degreeconstant. It was understand that it was melting. In the cooling process, the change of temperature wasntobserbed even if it passed 119 degree that was a melting point in a cooling process. After that, it wascrystallized at 82 degree and a temperature rose up to the melting point. It is found that Erythritolcrystalizes between 60 to 100 degree from liquid phase by our experiments. We think that a nucleatingagent becomes necessary to inhibitate supercooling if it is used in a capsule type heat storage system.


Figure 3 Melting and Solidification curve of Erythritol

Chemical Investigation (Thermal stability of Erythritol) Erythritol originally exists in nature and is extremely safe material used as sweetening agents.

Although erythritol is combustibles, we are not thinking so danger. Because, the flash point of erythritolis very high which is 245 C in comparrison with normally used temperature. Corrosiveness to containermaterial is evaluating now.

The heat degradation of Erythritol becomes a problem most in the case that Erythritol is used as aheat storage material. It was found that Erythritane was formed when Erythritol was heated. Erythritane isformed by erythritols dehydration of 1,4 position. It is well known that cyclic ther is synthesized fromsugar alcohol by heating in the presence of an acid catalyst. Even this reaction is thought with the kind ofits. We evaluated thermal stability of erythritol in the sealing container made of stainless steel. Inside

volume of the stainless steel container was about 1.4*10-5m3. Erythritol was filled up to 50vol %, 90vol %of the container and was carried out evaluation. It was found that the degradation reaction from erythritolto erythritane that was suggested with Table4 was comparatively restrained in a stainless steel sealingcontainer. Also, it was found that the degradation reaction depended on oxygen concentration, becausethe degradation speed of filling quantity 50vol % was faster than the degradation speed of 90vol %. Itwas suggested that a acid catalyst didnt form sufficiently because of low oxygen concentration in thesealing container. Next, we carried out the degradation rate constant of erythritol in stainless steal sealing containerabout each temperature (140,150,160,180 ) and Arrhenius plot. From above results, the degradation rateof erythritol was able to suppose a first order reaction style. The result is shown to Table. 5.

A heat storage material is repeated solidification and fusion for a long time. Accordingly, theprediction of heat of fusion quantity versus using time is very important. Lifetime prediction of erythritolwas carried out on the basis of the experiment result in the sealing container made of stainless steel. Theterminating point was determined when purity of erythritol reached to 90% of initial. The purity oferythritol decreases to 90% about 70,000 hours later when it continuously is heated to 140 degree in thesealing container. It is understood to have sufficient lifetime as a heat storage material, becausedegradation is 10% in initial even if it is used for about 8 years with continuation heating. Although weare doing a repetition test of solidification and fusion at present, the acceleration of the degradation bysolidification and fusion has not been observed during evaluation. We think that lifetime of erythritolhigly depends on temperature than repeat of solidification and fusion.


Table 5 E and A value of Erythritol in stainless steal container

E(Kcal/K) A(1/hr)50vol% 90vol% 50vol% 90vol%

Erythritol 31.5 32.9 1.24E11 4.21E11* E : Activation Energy ; A : Frequency Factor

O

HO OH

O

HO OH

O

HO OH

Acid

+ Acid + H2O

Erythritol Erythritane

+ O2

C C C COHOH

HHH

OH

HH

OHH

C C C COHOH

HHH

OH

HH

OHH

Table 4 Mechanism of Erythritol Degradation

Figure 4 Lifetime of Erythritol as PCM


Heat Exchanger Investigation using Erythritol

Experimental Latent heat storage material is used actually by heat exchange with heat transfer medium. Therefore, itis very important to understand theproperties of heat exchanger. The heat exchange vessel shown inPhoto1 was produced to evaluate heat exchange characteristics of erythritol. A heat exchange method wasadopted the simplest ice-on-coil type. Inside volume of the heat storage vessel used to evaluation is210dm3 and erythritol is included 240 . This system consists of electricity heater unit (3kW), feed waterunit, bypass line and hot water supply line other than it. Discharging performance were measured swinging flow rate with 3,5,8,10 dm3/min. in condition ofroom temperature 20 22 , heat storage temperature 140 , Tin10 within 3 . Experiments enforced8 times a total, by twice every each flow rate. Yet, bypass valve was controlled, for the purpose of doingnot discharge steam from exaust gate, while Tout had being over 100 degree.

Experiment result is shown in Figure 5 and fine reproducibility was confirmed in each flow rate.Also, discharged heat quantity of experiment is biggest 142MJ and agreed for the most part withtheoretical discharged heat quantity 147MJ. It was found that erythritol acted as heat storage materialwith ice-on-coil type heat exchanger vessel from this result.

Photo. 1 Experimental Apparatus of Heat Storage Vessel using Erythritol


Simulation analysis

A simulation model was designed in accordance with the following principle.

1. Heat storage behavior of Erythritol is able to expresse with all specific heat at constant volume ( Melting point is 119 120 ).2. There is no supercooling.3. There is no convection heat transfer of a liquid phase.4. Heat transfer tube is a straight tube.5. Heat storage material exists only around the tube. ( Namely, heat storage material grows to radius 25 mm thickness.)6 .Water does not boil.7. Outside of system makes a complete heat insulation condition except for entrance of water.8. The stream makes turbulent flow that progressed.

The comparative result between actual hot water supply experiment and caliculation in a flow rate0.312m3/h., average Tin 7 by simulation are shown in Figure6. In this figure, the result of simulationare showed thin line, experimental data with thick line. A difference is observed in the diachargingtemperature curve of hot water supply in part, but accumlation curve of discharging heat quantity fitsexperimental discharging curve. A calculation experiment of erythritol with heat storage vessel of ice-on-coil type became possible by using this simulation from this result. We think that it is needed to evaluateheatdischarging properpies changing plumbing pitch and plumbing diameter etc. according to a purpose,from now on.

Figure 5 Evaluation of Heat discharging property


ConclusionIt was found that erythritol was able to use as heat storage material from this investigation. The possibilityof erythritol as heat storage material was examined according to the standard such as thermodynamiccriteria, kinetic criteria and chemical criteria. Erythritol was investigated heat exchange performamncewith ice-on-coil type heat exchanger vessel.Details are shown as follows:

Thermal properties of erythritol necessary as a heat storage material was measured. It was found thaterythritol was an excellent heat storage material and heat of fusion per unit volume was 1.4 times ofice. However, it was suggested that an attention was necessary to the design of heat storage apparatusbecause volume change about 10% on going through phase transformation.

Erythritol melted congruently and phase separation was not observed. Erythritol melted sharply at themelting point. Supercooling phenomenon was observed. It was found that erythritol crystallised60 100 degree in a range.

It was found that Erythritane was formed by erythritols dehydration of 1,4 position when erythritolwas heated. Also it was found that degradation of erythritol was retarded throughout a sealingcontainer.

It was found that the purity of erythritol decreased to 90% about 70,000 hours later when itcontinuously was heated to 140 degree in the sealing container filled up to 90% of volume.

We carried out a discharging experiment by using erythritol as a heat storage material with ice-on-coiltype heat storage vessel. It was obtained 97% of theoretical discharging heat quantity in a dischargingprocess. Although a discharging experiment was enforced 8 times a total changing a flow rate,reproducibility was admitted. It was understood that erythritol was able to use with ice-on-coil heatstorage vessel.

A discharging simulation program of erythritol in ice-on-coil type heat storage vessel was prepared.The calculated discharging heat quantity was reproducing observed data well.

Figure 6 Comparison of calculation result using Erythritol simulation model with experimental result


Reference1. Sagara, K., 1998, Present Situation of PCM Thermal Energy Storage in Japan, IEA Annex-10,

Phase Change Materials and Chemical Reactions for Thermal Energy Storage, First Workshop, 16-17 April, Adana, Turky.

2. Ono, K., and Sue, N., STL Heat Storage System , Air-conditioning and Refrigerator, vol.26, No.6.(in Japanese)

3. Telkes, M., 1974, Solar Energy Storage , ASHRAE.J., 16, 38.

4. Kakiuchi, H., 1994, Efficient Use of Electric Energy and Latent Heat Thermal Energy Storage,MIFS94, Mie, Japan.

5. Takahashi, Y., Kamimoto, M. and Abe, Y., 1988, Heat Capacity, Heat of Transition and ThermalConductivity of Pentaerythritol and its slurry , Netsu Bussei, 2[1], 53-58.

6. Hoermansdoerfer, G., 1989, US Patent 4,795,580.

7. Guex, W. Riehen, K. and Fullinsdorf, P., 1981, US Patent $,295,517.8. Monick, J. A., 1968, Alcohols , Reinhold Book Corp. USA, 416-417.

9. The Merck Index 11th Edition, 1989, Merck & Co. Inc.

10. Seki, N., 1995, Chikunetsu Kougaku , vol.1, Morikitasyuppan. (in Japanese)

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