Sorption and thermal characterization of composite materials based on chlorides for thermal energy storage

Applied Energy xxx (2015) xxx–xxx

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Sorption and thermal characterization of composite materials based on chlorides for thermal energy storage q Kathrin Korhammer ⇑, Mona-Maria Druske, Armand Fopah-Lele, Holger Urs Rammelberg, Nina Wegscheider, Oliver Opel, Thomas Osterland, Wolfgang Ruck Innovation Incubator ‘‘Competence Tandem Thermal Battery”, Leuphana University Lüneburg, Building 16 – Scharnhorststraße 1, 21335 Lüneburg, Germany

h i g h l i g h t s
Composite materials design based on CaCl2 for thermochemical energy storage is presented.
Thermal and sorption characterizations are performed.
Hydration levels along with stoichiometry of composites are determined.
Heat transfer and water sorption enhancement is highlighted.

a r t i c l e

i n f o

Article history: Received 13 November 2014 Received in revised form 25 May 2015 Accepted 16 August 2015 Available online xxxx Keywords: Thermochemical heat storage Composites Mixtures Impregnation Sorption Thermal conductivity

a b s t r a c t Thermochemical heat storage is a promising technology towards efficient use of renewable energy resources. Materials based on salts and their hydrates have a high potential for a good energy storage density and the benefit of long-term storage ability. However, the process has not yet been successfully implemented due to limitations in mass and heat transfer. This paper investigates how to improve the less desirable properties of CaCl2 and its hydrates such as low melting points, agglomeration, low cycle stability and low sorption rates. The optimization of CaCl2 properties was achieved by mixing with KCl and impregnation in carrier materials to obtain a composite material. The tests show at first that, with the admixtures of KCl, water uptake during hydration is 2 times higher than that of CaCl2. Water release during dehydration is 1.3 times higher than that of CaCl2. Secondly, the use of compacted expanded natural graphite (ENG) or activated carbon foam (ACF) increases the cycle stability, thermal conductivity and the water sorption performance. Due to their hydrophobic nature those matrices have no influence on the reaction scheme, thus the total amount of water molecules sorbed by the salt-in-matrix is close to the value of CaCl2. The degree of impregnation varies from 31 to 90 wt% depending on the host matrix and the impregnating medium used. The water vapour uptake is up to 0.61 g g1 and the water released ranges from 0.12 to 0.72 g g1. The thermal conductivity of CaCl2-in-matrixis is 3 times higher than that of sole CaCl2. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Thermal energy storage is one of the key technologies towards an efficient use of renewable energy resources. Storing thermal energy and then releasing it to higher extent helps solving

q This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J Yan, Prof. DJ Lee, Prof. SK Chou, and Prof. U Desideri. ⇑ Corresponding author. Tel.: +49 4131 677 1768; fax: +49 4131 677 2822. E-mail address: [email protected] (K. Korhammer).

intermittent availability of energy and increases the energy efficiency. Although there are different ways to store thermal energy, the thermal performance of the system strongly depends on the thermodynamic properties of the used energetic media. Among other properties, high energy storage density and reversibility are required for the used materials to store and release thermal energy. Hereby thermochemical heat storage based on reversible chemical reactions, promises higher storage density than sensible or phase change heat storage for which researches and prototypes are well developed [1–3]. Based on reversible gas–solid reactions, long-term storage becomes possible. Many challenges are associated with potential thermochemical heat storage materials. Inorganic salts are used as the storage materials and generally water

http://dx.doi.org/10.1016/j.apenergy.2015.08.037 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

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K. Korhammer et al. / Applied Energy xxx (2015) xxx–xxx

Nomenclature ACF DSC dPore ENG ENGP k MSH M mC mM mS N nCaCl2 nH2 O p p0 S

activated carbon foam differential scanning calorimetry pore size (nm) compacted expanded natural graphite expanded natural graphite powder sorption rate (mg min1) molten salt hydrate molar mass (kg mol1) mass of dried composite material (g) mass of host matrix (g) mass of active substance (g) number of H2O molecules sorbed by one salt molecule x (–) moles of anhydrous CaCl2 (mol) moles of water (mol) actual pressure (mbar) standard pressure (mbar) soaking

as the reactive gas for environmental reason. Calcium chloride is chosen as the reference substance as it has been extensively studied and its properties are well known. Metal chlorides in combination with carbonaceous materials exhibit very high thermal conductivity [4]. In the past several salts have been tested in closed-cycle heat storage systems [5–9]. In those first tests with pure and powdered samples, heat and mass transfer problems such as agglomeration, swelling, expansion and phase change have been observed. These problems cause low matrix permeability and high pressure drop [10]. For example exposing the salt to water vapour at high relative pressure during hydration often results in the formation of a saturated solution instead of a hydrated form. Small deliquescence can be noticed; in fact a skin of the hydrated salt is sometimes formed on the surface of the bulk powder and strongly limits the diffusion of water vapour within the unreacted part of the sorbent [11]. Calcium chloride has a high deliquescence, a relative humidity of 29% at 30 °C, leads to the formation of a liquid salt solution [12]. Due to high kinetic water uptake hygroscopic salt hydrates agglomerate easily, reducing the water uptake rate and cycle stability of the storage material. Since the performance of the system depends on heat and mass transfer, this has to be improved within the reactive bed. In order to improve the water sorption kinetic performance of hygroscopic salts, composite materials have been developed over the past two decades [13–17]. This hybrid character of composites provides to the material some physical and chemical properties between the salt and the host. Composite materials are usually synthesized by impregnating salt into the pore structure of porous matrices. These matrices exhibit a high specific surface area and thus favour water–salt sorption interactions and are able to hold the salt and its solutions within their internal pore system. The main drawback of using composites is the reduction of the system’s coefficient of performance, which is still preferred over the aforementioned issues [12]. In order to overcome this disadvantage, many studies have been performed within the project ‘‘Thermal Battery” (Institute of Sustainable and Environmental Chemistry) [18,19]. The impregnation of salt hydrates into porous matrices prevented agglomeration (respectively deliquescence) and improved the diffusion of water vapour into the material [20]. Among the carbonaceous materials activated carbon and graphite were chosen for their high thermal conductivity and porosity. The addition of such host matrices enhances the heat transfer and water diffusion by creating a

SBET t T Tm TGA VI VPore w

specific surface area (m2 g1) time (min) temperature (°C) melting point (°C) thermogravimetric analysis vacuum impregnation pore volume (cm3 g1) salt content (wt%)

Greek symbols Dmdes water release per dry substance (g g1) DHdes energy storage density of desorption (J g1) Dmsorp water uptake per dry substance (g g1) DHsorp energy storage density of sorption (J g1) k thermal conductivity (W m1 K1) qb bulk density (kg m3) qa area density (g m2)

large active interface [21,22]. They can achieve 40 times the thermal conductivity of untreated salts [23]. The control of the water vapour access into the material prevents the deliquescence at microscale. However, this procedure is not manageable at macroscale because of heterogeneous reactions occurring in the storage bulk. An alternative to the impregnation method is the synthesis of compounds by intermixing different salts. Several salt mixtures of metal chlorides have shown high cycle stability despite their deliquescence [19]. This paper focuses on the synthesis and characterization of saltporous-carrier composites and mixed salts for utilisation in thermal energy storage systems. The energy storage density, water uptake and thermal conductivity of the compounds are then compared to the properties of the untreated materials to monitor the achieved improvements or declines. 2. Materials and methods In this study the suitability of tailored two-component composites and mixed salts has been evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both hydration (discharging) and dehydration (charging) measurements were performed at atmospheric pressure to determine the water sorption rate and energy storage density of the untreated salts and designed materials. The hydration levels were calculated from the mass loss measured by TGA. The effect of the carbonaceous heat transfer enhancers on the thermal conductivity has been examined using the heat flux DSC method. 2.1. Sample preparation Anhydrous CaCl2 (AppliChem Company, reinst), CaCl2  6H2O (AppliChem Company, p.A.) and KCl (Roth, PhEur) were used for all experiments. Two different carbonaceous supports with precise pore size distributions were selected (Table 1). The activated carbon foam media ACF was purchased from Blücher GmbH, the compacted expanded natural graphite ENG and the expanded natural graphite powder ENGP were provided by the SGL Group. For the synthesis of the salt-porous-carrier composites (Table 2) two different procedures were applied: wet impregnation under vacuum (VI) and soaking (S). In the wet impregnation method a

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K. Korhammer et al. / Applied Energy xxx (2015) xxx–xxx Table 1 Thermo-physical properties of the matrices. Matrix

ID

Shape

dPore (nm)

VPore (cm3 g1)

qb (kg m3)

SBET (m2 g1)

qa (g m2)

k (W m1 K1)

Expanded natural graphite powder Expanded natural graphite compacted Activated carbon foam media

ENGP ENG ACF

Powder Pellet Foam

– – 0.70–3.00

– – 0.70–1.30

90–130 – -

20 – 1432–1946

– 750 4600

– – 0.25–0.30

Table 2 List of synthesized composite materials based on impregnation. Material

Matrix

Active substance

Synthesis method

Drying temperature

CaCl2-ACF-VI CaCl2-MSH-ACF-VI CaCl2-ENG-VI CaCl2-MSH-ENG-VI CaCl2-ENGP-S-4 CaCl2-ENGP-S-2 CaCl2-ENGP-S-1

ACF ACF ENG ENG ENGP ENGP ENGP

CaCl2 Molten hydrated CaCl2 CaCl2 Molten hydrated CaCl2 CaCl2 CaCl2 CaCl2

Wet impregnation Wet impregnation Wet impregnation Wet impregnation Soaking Soaking Soaking

200 °C RT 200 °C RT 200 °C 200 °C 200 °C

saturated aqueous salt solution of CaCl2, which volume exceeded the pore volume of the matrix, and molten salt hydrate (CaCl2-MSH) were used as impregnating media. Prior to impregnation the carriers (ACF and ENG) were dried in an oven at 200 °C and cooled down in a vacuum desiccator to remove any residual moisture. After drying they were immersed into the saturated aqueous solution or molten salt hydrate placed in a vacuum sealed vessel which was evacuated with a membrane vacuum pump to 10 mbar. The impregnation time was set to several minutes. The wet CaCl2-based composites were finally dried in an oven at 200 °C until the weight remained constant, whereas the composites made from CaCl2-MSH were dried over night at room temperature. All samples were stored in a sealed bottle in a dry area to protect them against moisture and exposure. In the latter method the ENGP was soaked in a saturated aqueous solution of CaCl2 with a CaCl2:ENGP ratio of 4:1, 2:1 and 1:1 while stirring. The excess solvent was removed by evaporation and drying at 200 °C. The mixed salts of CaCl2 and KCl were crystallized from a solution of their basic salt components in pure water. The first step is to prepare an under-saturated aqueous solution of CaCl2 and KCl. In the second step the two solutions were mixed together. The samples were either dried at 70–120 °C in the oven for quick use or stored in a desiccator for slow crystallization and to avoid melting and formation of amorphous phases in the mixture. The mixing ratios depend on the compound expected to form and were calculated by molecular weight of the involved salts. The bulk density of the materials was calculated by dividing the mass of this latter with the total occupied volume by the material. The void fraction and density of the bed in Table 6, are respectively determined using the following equation:

Void fractionð%Þ ¼ bed densityðqb Þ ¼

VS mS  100% ¼  100% Vo dC  V o

mS Vo

ð1Þ ð2Þ

where Vs is the volume of the sample, Vo the volume occupied by the sample in the cylinder, ms the sample weight (g), ds the theoretical density of the sample and qb the bulk density of the bed. 2.2. TGA/DSC: measurements of change in mass and enthalpy Thermogravimetric analysis and heat flux measurements were carried out using a simultaneous TGA/DSC 1 device from Mettler ToledoÓ. This device determines simultaneously the change in sample mass (precision of mass determination is ±0.1 lg) as a

function of time while the specimen is subjected to a temperature scanning programme in a controlled atmosphere and the heat flux (precision of heat power determination is ±1 mW) into the specimen compared to a reference. The experimental setup is shown in Fig. 1. A gas box, providing two different mass flow controllers, connected to the TGA/DSC controlled the gas flow rate of the protective purge gas. We used silica gel-dried nitrogen as purge gas at a flow rate of 50 ml min1. During hydration a reactive gas stream, which is humidified via a tempered gas bubbler, is used, in addition to the purge gas stream. By forcing this gas stream through a glass frit filled with glass wool, the containing water droplets are homogenized and the gas is adapted to the ambient temperature in the laboratory. Samples of 10–20 mg were tested in crucibles made of alumina with a volume of 70 lL under realistic conditions. All experiments were performed under realistic conditions of common closed-type sorption systems. The hydration measurements consisted of three consecutive steps varying in the operating pressure (10 mbar, 17 mbar and 20 mbar) in order to obtain equilibrium data on the pressure dependence of the water sorption behaviour of the samples. The hydration temperature was adjusted to 25 °C to avoid condensation. In dehydration-runs the salt mixtures and composite materials were gradually heated in two steps to 100 °C and 200 °C at a constant heating rate of 5 K min1. A low temperature-ramp rate was chosen to avoid overlapping stages. Previous experiments showed that the salts used can completely be dehydrated below 200 °C. The water amount sorbed and the integration of the hydration heat fluxes over time were done manually with the Mettler Toledo STARe Software 11.00a. 2.3. TGA/DSC: measurements of thermal conductivity The TGA/DSC 1 from Mettler Toledo was also used for the analysis of the heat fluxes and the temperature through a sample in order to evaluate the thermal resistance of the material (Fig. 2a and b). In the DSC cell, both temperature and heat flow are measured at the contact area between sample and sensor [24]. The temperature at the opposite side of the sample cannot be measured directly. By placing a pure metal with a known melting point on top of the sample, this temperature can be measured indirectly by observing its melting behaviour [25]. Thermal conductivity can be determined without modification of the DSC cell, using a method based on the assumption that the bottom side of the sample at the heat source follows an applied temperature modulation [26]. This implies that no thermal resistance between

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Fig. 1. Schematic illustration of the experimental setup.

Fig. 2. (a) The schematic description of the measurement, showing aluminium pans, the reference (R) and the sample (S) with the corresponding melting curves, of indium on top of the sample and the indium alone. (b) The physical model of DSC measurements.

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K. Korhammer et al. / Applied Energy xxx (2015) xxx–xxx

the sample and the furnace exists [27] which has a significant effect on the sample heat capacity, hence on the thermal conductivity. The porous and host materials were measured at different heating rates with nitrogen flow rate of 50 ml min1. The amount of materials used is around 0.1 g. For the calibration, two sensor materials (gallium and indium) with known thermal conductivities, which give clear different melting peaks at 29.7 °C and 156.6 °C respectively, were used with a standard deviation of 0.0006 W for heat flow and of 0.03 °C for temperature. However, thermal property measurements with high accuracy can be performed using DSC, if the calibration is carefully implemented [28]. In addition to the given resolution on DSC, the results can be meaningfully interpreted with up to four significant digits. For each sensor material, three calibration–measurements were done in order to find the accurate one to be used for the final measurements. Then the apparatus was adjusted to the calibration sample parameters, and finally a re-calibration was performed to validate the calibration process. As the first step of DSC measurements, sensor material pellets (diameter = 1 mm) – prepared under atmospheric pressure – were placed in an aluminium pan with an inner diameter of 5 mm and specific height of 5.1 mm before the melting curves were determined (Fig. 2a). During the second step, the sensor material was placed on top of the non-compacted specimen in the aluminium pan and subsequently measurements were carried out until the sensor material melted. However, the sample height in the pan is about 4.9 mm. The surface area (As = 7.8  107 m2) of the indium corresponds to the exact surface area of the sample (delimited region on sample pan on Fig. 2a) where the thermal resistance is measured. Using DSC measurement for thermal conductivity evaluation implies to find a good balance for the heating rate, in other terms, to choose the appropriate heating rate in the slope equation (Eq. (3)). Some hydrated salts may partially or completely melt within the actual temperature range, depending on the heating rate, since their melting points are typically lower than those of anhydrate phases. A slow heating rate allows for the crystal water to escape before the respective melting points of the hydrated salts are reached. As in the previous configuration heat loss was observed, the satisfying results for thermal resistance may still be inaccurate. In his research, Hakvoort et al. [25] obtained best results when the sensor totally covers the sample, having the same cylindrical diameter as the sample height with a sample height not more than 3 mm. Due to technical limitations the measurements were performed with a sensor covering only a fourth of the total material diameter. The purpose of this measurement is to determine the thermal conductivity values from an adaptation to Camirand’s method [29]. His method required only one sample and used a constraint equation determined by the slope of the increasing part of the heat flow curve. The method used here utilises the measurement of heat flow rate into a sensor material during its first order transition, obtaining the thermal resistance of a material placed between the sensor material and the heater in DSC. Starting at low temperature, when the indium is solid, sample and reference pans are heated with a constant heating rate (1, 5 and 10 K min1). Instead of using a fixed heating rate as Camirand, in this study the heating rate was selected individually for each specimen to avoid the melting of the respective sample. The sensor temperature and the DSC signal were recorded against time. During melting of the sensor (indium), the temperature of the sensor remains constant, so that the top of the sample remains at a constant temperature as well, while the temperature on the lower side of the sample continually increases at a constant rate. A scan is performed to measure the difference in heat flow during the melting of the sensor substance. The curve obtained tends to decrease linear before melting and decreases exponentially during melting (Fig. 2a).

5

The thermal resistance (Rth) to heat flow (qt) through a sample under temperature difference between the heater and the melted sensor can be expressed as follows:

Rth ¼

ðTðtÞ  T onset Þ qt

ð3Þ

Taking up the slopes of the DSC curves at melting the stage of the sensor material, the thermal resistance of the sample is determined by the difference between the measurement with the indium on top and without as follows:

Rs ¼ R  R0

ð4Þ

where R is the thermal resistance between calorimeter and sensor material, R0 is the thermal resistance between calorimeter and sensor material on top of the sample (see Fig. 2b). Measurement of the slope of the decreasing part of the curve allows determination of the thermal conductivity of the sample. Taking into account all the thermal resistances, the slope can be defined from the linear side of the melting peak [30]. The slope calculation is based on the principle of Fig. 3 with the aid of the STARe software 11.00a from Mettler Toledo. The first derivative of the heating curve (meaning the thermal power against the time) gives a specific slope which is further divided by the heating rate. Therefore we obtained the following expression:

Slope ¼ b 

qt ðtÞ  qt onset b ¼ Rth TðtÞ  T onset

ð5Þ

where qt_onset and Tonset are the heat flow and melting temperature of salt at the onset of melting. qt(t) is the thermal power at a given time, b the heating rate and Rth the thermal resistance (can be R or R0 depending of what is measured). This correlation of temperature in the material to the thermal resistance is in principle similar to the resistance temperature detector (RTD) where only the thermal resistance is measured and correlated to the temperature. It clearly shows the heating rate dependence. The obtained total (crucible, sample and sensor) thermal resistance in comparison with the thermal resistance of the sensor and the crucible yields directly to the bed sample thermal conductivity as follows:

ks ¼

Ls Ls ¼ A  Rs A  ðR  R0 Þ

ð6Þ

where Ls is the sample length, A is the contact area between sample and sensor material.

Fig. 3. Example of slope calculation on heat flow vs. temperature curve for salt sample [30].

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The used method requires an accurate measurement of the axial (middle central line of the crucible) temperature gradient to the sensor at the bottom. If not properly accounted for, some uncertainty may lead to inaccurate heat flow measurement. Some possible uncertainty source may come from thermal contact with the bottom of the pan, if it is not good enough. Samples are not thin enough and no lid is used to cover the sample in order to maintain the contact. 3. Results and discussion 3.1. Measurements of water sorption and sorption enthalpy 3.1.1. Composites based on mixture The untreated calcium chloride showed higher release of water than uptake, caused by the mentioned deliquescence and agglomeration behaviour (Table 3). The mixing of calcium chloride and potassium chloride gave a more heterogeneous picture. The salt mixtures showed complete rehydration depending on the ratio of calcium chloride to potassium chloride. As there is no literature about natural occurring KCl–CaCl2 minerals, it is unknown whether the sample contains a mixture or a compound. However, even if no new compound of the two educts is formed, replacing up to two thirds of the CaCl2 with KCl is supposed to increase cycle stability due to the KCl lacking deliquescence and therefore reducing agglomeration and thus improving the water transfer within the sample. Another potential effect is that with a significant higher KCl content the extreme low melting point of CaCl2  6H2O (Tm = 30 °C) can be raised (KCl: Tm = 770 °C), especially in case of an eutectic mixture. In Table 4, the water release and uptake at different temperature steps are calculated from weight loss/gain during the TGAanalysis.

N¼

nH2 O mH2 O Mx ¼  nx mx M H2 O

ð7Þ

The water sorption N is expressed as moles of water nH2 O sorbed per mole anhydrous salt x nx , where x is either CaCl2, KCl + CaCl2, KCl + 2CaCl2 or 2KCl + CaCl2 and mH2 O is the water loss, mx the mass of the dehydrated salt and M x the molar mass of the anyhdrous salt. In case of the composites the latter was calculated from the difference of the residual sample mass detected in the TGA/DSC measurements and the mass of the host matrix. In case of the slight weight increase of the KCl sample during the hydration step, water probably gathered only on the surface of the salt grains without undergoing chemical bonding with KCl. Temperature for the hydration was kept constant with only the influx of water vapour changing. 3.1.2. Composites based on impregnation The salt content is expressed as the percentage of salt based on the dry mass of the sample. It was calculated from the mass

difference obtained by weighing the support before and after impregnation as follows:

w¼

mS mC  mM  100 ¼  100ðwt%Þ mC mC

ð8Þ

where mS (g) is the mass of the CaCl2 or molten hydrated CaCl2, mM (g) is the mass of the host matrix and mC (g) is the mass of the dried composite material. As shown in Table 6 the degree of impregnation varied from 31 to 90 wt% depending on the host matrix and the impregnating medium used. Samples with ENG contain about 87 wt% CaCl2 and 90 wt% molten hydrated CaCl2, the ENG-based composite with molten salt hydrate has a slightly higher salt content than those impregnated with aqueous salt solutions. Composite materials with ACF and either CaCl2 or molten hydrated CaCl2 could only incorporate about 31 wt% and 58 wt% of salt. The compacted expanded natural graphite serves as a good carrier for the salt particles due to its honeycomb structure with stacks of parallel planes of carbon atoms between which the salt can be inserted. ACF has a well-defined foam and ordered microporous structure but due to voids in the framework between single ACF beads the salt can easily be washed out during impregnation, drying and in particular while reacting with water vapour. Moreover, the number of salt molecules that can access the micropores is limited. A homogenous and uniform distribution of the CaCl2 particles inside the pores and internal surface could not be achieved as seen on microscopic images. Formation of salt clusters on the external surface could be observed, in particular in composites with ACF. The preparation method strongly influences the dispersion and fixation of the active substance within the supporting structure. Thus, the effect of varied synthesis parameters on the sorption characteristics of the composite sorbents will be investigated in detail in future studies. The thermogram of the designed materials is shown in Fig. 4. The TGA curves were blank curve corrected in order to exclude buoyancy terms. The percentage of the mass loss is plotted versus the reaction time. Calcium chloride hydrate undergoes dehydration in three distinct stages. Major mass loss occurs below 100 °C. A similar multi-stage mass loss can be observed in composites containing ENG. The steep steps indicate a large mass loss rate. The curve of composites with ACF is more continuous in shape revealing that water is released more gradually. Desorption is completed slightly above 100 °C. The amount of water desorbed is small compared to the mass loss in the untreated salt and the composites with ENG as support. Samples based on ENG show good sorption behaviour. The addition of ENG, which possesses a high thermal conductivity, seems to enhance the heat transfer inside the sample so that the heat can be transported faster onto the internal surface. Moreover, the water vapour can easily be released through the diffusion channels of the host. ACF is also a good heat transfer promoter but due to the lower CaCl2 loading the rate of sorption is likely to be lower in

Table 3 Water release and uptake – mass and enthalpy (ACF and ENG showed no sorption properties). Material was first dehydrated at 100 °C and then at 200 °C, hydration enthalpies and weight loss were measured after both dehydrations. The water release and the enthalpy are calculated on the basis of the residual sample mass at the end of the 200 °C dehydration process.

a

Material

Water release (100 °C) (g g1)

Enthalpy of hydration stage (100 °C) (J g1)

Water release (200 °C) (g g1)

Enthalpy of hydration stage (200 °C) (J g1)

CaCl2  nH2O CaCl2  6H2O [31] KCl  nH2O (KCl + 2 CaCl2)  nH2O (KCl + CaCl2)  nH2O (2 KCl + CaCl2)  nH2O

0.96 CaCl2  6H2O ? CaCl2  2H2O + 4H2O 0.00 0.36 0.29 0.14

1190 1295 –a 1092 433 564

0.70 CaCl2  2H2O ? CaCl2 + 2H2O 0.02 0.58 0.42 0.39

1369 1584 –a 1294 536 758

The enthalpy could not be calculated as the detected DSC signals were too low.

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Table 4 Water release and uptake at different temperature steps. As the total water uptake depends on the number of cations in the mixture, a theoretical total is calculated for each compound from the measurements of the untreated salts. Each sample was tested in three dehydration cycles with two hydrations in between. Material

CaCl2

Stage 1

Dehyd1 Dehyd2 Hyd1 Hyd2

Stage 2

Stage 3

Stage 4

nH2 O calc total

nH2 O total

– – – –

– – – – –

5.9 4.3 3.3 3.8

T (°C)

N

T (°C)

N

T (°C)

N

T (°C)

N

61.07 47.67 – –

3.8 0.1 0.2 0.3

96.28 97.28 – –

2.1 1.9 2.2 1.5

– 151.83 – –

– 2.2 1 2

– – – -

KCl

None

KCl + 2CaCl2

Dehyd1 Dehyd2 Dehyd3 Hyd1 Hyd2

58.28 74.89 72.55 – –

5.9 3.2 2.8 1 1.6

– 142.79 144.96 – –

– 5 4 4 5

– 169.02 168.48 – –

– 1.3 1.6 3.9 5.2

– 190.19 193.29 – –

– 0.8 0.7 – –

11.8 8.6 8.6 6.6 7.6

5.9 10.3 9.2 8.9 11.8

KCl + CaCl2

Dehyd1 Dehyd2 Dehyd3 Hyd1 Hyd2

64.61 79.83 69.06 – –

4.4 1.7 0.8 0.2 0.4

– 141.05 142.44 – –

– 2.4 3.3 1.4 1.9

– 148.65 – – –

– 2.1 – 1.5 2.3

– – – – –

– – – – –

5.9 4.3 4.3 3.3 3.8

5 6.2 4.1 3 4.6

2KCl + CaCl2

Dehyd1 Dehyd2 Dehyd3 Hyd1 Hyd2

44.46 62.11 62 – –

2 1.3 1.1 0.6 0.7

– 147.18 122.98 – –

– 4.3 0.1 2.5 3.3

– – 141.06 – –

– – 1.5 2.2 3.4

– – 147.29 – –

– – 2.2 – –

5.9 4.3 4.3 3.3 3.8

2 5.6 5 5.3 7.4

Fig. 4. Mass loss in untreated CaCl2 and CaCl2-ENG/ACF-composites as a function of time.

comparison to ENG-based sorbents. According to Fujioka et al. [32] the average sorption rate in composites made from expanded graphite could indeed be increased with increased effective thermal conductivity. Therefore, the effect of ENG and ACF as heat and mass transfer enhancers on the water sorption has been investigated. According to Zhu et al. [33] the absolute sorption rate k can be obtained from the differential of the sorption amount (dm) to time (dt).

k¼

dm  mg  dt min

Fig. 5. Conversion in untreated CaCl2 and CaCl2-ENG/ACF-composites as a function of time.

Table 5 Comparison of the mass loss rates k50 and k90 of untreated and composite sorbents. Material

Dm (g g1)

t50 (min)

k50 (mg min1)

t90 (min)

k90 (mg min1)

CaCl2 hydrate CaCl2-ACF-VI CaCl2-MSH-ACF-VI CaCl2-ENG-VI CaCl2-MSH-ENG-VI

0.54 0.10 0.08 0.61 0.52

57 32 34 37 36

0.05 0.07 0.05 0.08 0.07

74 47 53 69 56

0.25 0.005 0.003 0.10 0.04

ð9Þ

The sorption amount Dm is defined as the amount of water in gram sorbed per gram dry composite for the composite materials and per gram anhydrous CaCl2 for the untreated CaCl2, respectively. In order to compare the dehydration behaviour of the different materials, the overall degree of conversion has been normalized and is presented in Fig. 5. The reaction time required for desorbing 50% (t50) and 90% (t90) of the maximum water

content of the initial sample and the obtained sorption rates have been compared in Fig. 6. The results summarized in Table 5 confirm the assumption that among the used supports, ENG has a sorption promoting effect. By using ENG impregnated with CaCl2 the water desorption can be accelerated over the whole temperature range on account of the increased sorption rate, particularly at lower temperatures (Fig. 6).

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Fig. 6. Sorption rate in untreated CaCl2 and CaCl2-ENG/ACF-composite as a function of time.

Towards the end of the dehydration phase, when more than 90% of the water is desorbed, the water sorption is slowed down. Composites with a higher degree of CaCl2 impregnation have a higher total mass loss rate compared to the other samples. The water sorption in the ACF-supported CaCl2 and molten hydrated CaCl2 samples are lower than 0.2 mg min1 according to the mass loss rate values. Table 6 presents the total water uptake Dmsorp and water release Dmdes of the samples tested over a temperature range of 25–200 °C. The results are related to the dry mass of the material. Samples with ENG as host matrix with high salt content demonstrate the highest water sorption ability comparable to untreated CaCl2. More than 70% of the water sorbed can be released during the dehydration process. The composite with ENG and molten salt hydrate (CaCl2-MSH-ENG-VI) appears to have a higher water uptake than the CaCl2-ENG-VI composite, but less of the water sorbed can be desorbed. The shape of the TGA curves is similar, except in the last part in which a higher mass loss is detected for CaCl2-ENG-VI. The more salt is confined to the matrix surface and into the matrix pores the lower the diffusivity of the water vapour during hydration and dehydration due to partial pore blocking and thus the maximum water amount sorbed is decreased. In addition to the pore blocking effect, the formation of a dry layer on the surface is also considered to lead to a diffusion barrier for the water vapour from inside the material. The water sorption capacity of ACF-supported composites is less than 0.20 g g1. This finding indicates that the sorption behaviour is not only affected by the amount of salt dispersed within the ACF but also its morphology. The steric hindrance of the nonpolar functional groups in the ACF is likely to limit the effective water uptake.

The hydrophobicity of the untreated carbonaceous supports could be confirmed by TGA/DSC measurements. Hence, the contribution of those materials to the sorption is negligible. In addition to possessing a high thermal conductivity, carbonaceous supports do not tend to uncontrolled swelling or variation in volume making them advantageous over other common supporting materials. The beneficial effect of expanded graphite addition has also been reported in other studies. Okada et al. [34] used the dry impregnation technique to prepare CaCl2-impregnated activated carbon composites from solutions with different CaCl2 concentrations. The variation in the solution salt concentration resulted in different salt contents (0, 30, 40, 50 and 70 wt%) and water vapour uptake performances ranging from 0.04 to 0.52 g g1, respectively. The higher the salt content of the composite the higher the water amount sorbed. Only composite sorbents with active supports such as zeolite or silica gel proved to have higher water sorption capacities as given in Refs. [33,35,36], for instance, combined a microporous zeolite and mesoporous silica gel with CaCl2 and obtained superior results in terms of the latter. The impregnated silica gel containing 33 wt% of CaCl2 adsorbed around 0.85 g g1 at 27 °C at 33.3 bar water vapour pressure. Tokarev et al. [35] presented a novel CaCl2-confined to MCM-41 material that demonstrates a sorption capacity of 0.75 g g1. When comparing those values the different testing conditions must be considered. Zhu et al. [33] obtained similar values (0.73 g g1 at 30 °C and p p1 0 = 0.8) in their experimental study on composite silica gel supported CaCl2 sorbent. Silica gel and zeolite possess a very high hydrophilic surface and can therefore contribute to the overall water absorptivity. The associated sorption and desorption enthalpies are also displayed in Table 6. In general, the higher the water sorption capacity of the materials, the higher are both dehydration DHdes and hydration enthalpy DHsorp and the higher the total energy storage density. Two melting events occurred in the DSC diagram which contribute to the high desorption enthalpy of the materials. The satisfying dehydration achieved below 100 °C and the very high water uptake and energy storage density make composites consisting of ENG and either CaCl2 or molten hydrated CaCl2 attractive for thermal energy storage in applications that use low-grade heat sources. Examples of water sorption/desorption cycles are illustrated in Fig. 7 for untreated CaCl2, CaCl2-MSH-ENG-VI and CaCl2-ENG-VI composite. A hysteresis loop common for gas–solid reactions is observed. The confinement of CaCl2 to the compacted expanded graphite leads to a widening of the loop due to larger sorption

Table 6 Sorption properties of the synthesized composite materials. Material

CaCl2-ACF-VI CaCl2-MSH-ACF-VI CaCl2-ENG-VI CaCl2-MSH-ENG-VI CaCl2-ENGP-S-4 CaCl2-ENGP-S-2 CaCl2-ENGP-S-1 a

Dehydration

Hydration

w (wt%)

Dmdes (g g1)

DHdes (J g1)

Dmsorp (g g1)

DHsorp (J g1)

31 58 87 90 87 66 51

0.10 0.08 0.61 0.52 –a –a –a

235 298 1642 1905 –a –a –a

0.12 0.20 0.67 0.72 –a –a –a

198 471 1451 1310 –a –a –a

Not analysed via TGA/DSC.

Fig. 7. Water sorption/desorption cycle of CaCl2-MSH-ENG-VI, CaCl2-ENG-VI and untreated CaCl2.

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[8]. As a consequence the crystal structure might be changed. A small melting peak could also be seen in the DSC of the CaCl2-MSH-ENG-VI sample. According to the calculations in Table 7 this sample took up only 3.6 moles of water which is lower than N = 4. It is assumed that the composites comprise of a mixture of salt hydrates of different hydration states. In case of samples of salt hydrates with a number of water molecules equal or higher than four moles per mole of salt anhydrate, melting occurred but did apparently not affect the reaction reversibility of the material. Long-term thermal stability is an important factor in heat storage applications: fast degradation of the salt limits the operating period of the storage unit. In order to reliably assess the overall lifetime of such a system a cycle stability test consisting of more than twenty cycles is recommended and will be performed in future experiments.

and desorption rates. During hydration water is not only chemically attached but also physisorbed due to over-hydration (overstoichiometric uptake of water molecules) resulting in a moisture loss after the run was finished. The slight gain in mass before the dehydration measurement shows that despite impregnation of the salt into a matrix, its deliquescent behaviour cannot fully be prevented. The influence of the host matrices on the dehydration process and reaction temperature has been evaluated by calculating the degree of hydration in each dehydration step. The water sorption N is defined in Eq. (7). The results are listed in Table 7. As the salt content of the composite sorbents analysed via TGA/DSC differs from the total salt content determined by weighing due to inhomogeneous dispersion of the salt within the porous structure, the accuracy of the given values are affected with considerable uncertainties. As shown in Table 7 the use of heat promoting additives results in a shift in the reaction temperature to lower temperatures of up to 18 °C in the first reaction stage and up to 52 °C in the second reaction stage, respectively. The composites based on ENG showed a sorption behaviour similar to untreated CaCl2, as a result the total amount of moles sorbed per mole on CaCl2-ENGB-VI and CaCl2-MSH-ENGB-VI is with 4.4 and 3.6 close to the value for untreated CaCl2. In the first stage around 2 moles of water are released, except for CaCl2-MSH-ACF-VI and CaCl2-MSH-ENGB-VI which desorb only 1 mol of water. ACF-based composites could only hold 2 moles of water maximum. It is well-known that the morphology and the distribution of salt within the sample exert a strong influence on the position of the peak temperature in the DSC signal. The findings are in accordance with the results for the different water sorption rates mentioned above (see Figs. 4 and 6). It was observed that samples with N P 4 undergo melting between 40 and 48 °C. This temperature corresponds to the melting point of the CaCl2 tetrahydrate

3.2. Measurements of thermal conductivity In order to validate the experimental and developed methods, the thermal conductivity of anhydrous glass is determined and compared with the literature and supplier data. The glass studied here is commercially known as ‘‘glass beads – GP”, produced by KuhmichelÒ. The chosen glass presented as spheroidal beads is in a homogeneous spherical form with the following properties in Table 8. The thermal conductivity of the glass beads measured under transient heat flow is calculated by using Eq. (6). The results are shown in Table 9. The bulk (effective) thermal conductivity of the bed, as a function of parameters such as the thermal conductivity of the beads, the bed porosity and the dimension of the grains, is not determined here, since it requires some model to be designed or adapted. The average thermal conductivity of glass beads under dry air at 100 kPa is ks = 1.16 ± 0.02 W m1 K1 (mglass  139 mg). The evaluated errors and uncertainties in this

Table 7 Calculation of the hydration level of the composite materials based on impregnation. Material

Stage 1

CaCl2 CaCl2-ACF-VI CaCl2-MSH-ACF-VI CaCl2-ENG-VI CaCl2-MSH-ENG-VI

Stage 2

Stage 3

Total

nH2 O (mmol)

N1

T (°C)

nH2 O (mmol)

N2

T (°C)

nH2 O (mmol)

N3

T (°C)

Ntotal

0.12 0.05 0.04 0.12 0.06

2.0 1.8 0.8 2.0 1.3

98 76 93 80 88

0.09 <0.00 <0.00 0.10 0.09

1.4 0.2 0.1 1.8 2.0

145 111 – 98 93

– – 0.03 0.01

– – 0.6 0.3

– – 128 125

3.4 2.0 0.9 4.4 3.6

Table 8 Glass beads physico-chemical properties [37]. The data on thermal conductivity is taken from [38]. Grain shape Melting point Bulk density (depending on granular size) Average grain size Thermal conductivity according to the chemical analysis and temperature range

Spherical Approx. 730 °C Approx. 1.5–1.6 g cm3 200–300 lm 0.8–1.2 W m1 K1 for % SiO2 < 96 and % Fe2O3 < 1 at T > 20 °C

Chemical analysis of the used glass beads

SiO2 (70–75%) MgO (max. 5%) Na2O (12–15%) Al2O3 (max. 2.5%) CaO (7–12%) K2O (max. 1.5%) Fe2O3 (max. 0.5%) Others (max. 2%)

Table 9 Experimental results for effective thermal conductivity of anhydrous glass beads using DSC. Material

Specific energy (J g1)

Bed length (m)

Void fraction (%)

Temperature range (°C)

Thermal resistance (°C W1)

Effective thermal conductivity (W m1 K1)

Uncertainty of effective thermal conductivity (W m1 K1)

Glass beads

2.21 2.62 2.9

0.0051 0.0051 0.0051

89 89 89

100–200 100–200 100–200

5586 5154 5968

1.29 1.12 1.07

0.019 0.020 0.016

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Table 10 Effective thermal conductivity measurement results. Material

Mass (mg)

Bulk density (kg m3)

Specific energy (J g1)

Effective thermal conductivity (W m1 K1)

Uncertainty on effective thermal conductivity (W m1 K1)

Literature values of thermal conductivity (W m1 K1)

ACF ENG CaCl2 CaCl2-ACF-VI CaCl2-ENGP-S-1 CaCl2-ENGP-S-2

53.7 2.3 49.7 64.9 8.5 4.6

537 23 497 649 85 46

3.8 93.8 2.5 2.7 21.1 35.7

0.52 4.78 0.54 1.03 1.64 0.74

0.17 1.30 0.15 0.30 0.47 0.20

0.15–0.5 [40]; 0.36 [41] 3–10 [42] 0.1–0.5 [24,43,44] n.a. 1.7 (d = 550 kg m3); 1.66 (d = 450 kg m3) [41] 1.51 (d = 550 kg m3); 1.23 (d = 450 kg m3) [41]

work are purely systematics and based on the guide to the expression of uncertainty in measurement (GUM) [39] (Appendix A). For DSC measurements, it is calculated from the standard deviation on heat flow and temperature, mentioned in GUM (Section 2.3). However, Gomez et al. [28] affirmed that a DSC can perform thermal property measurements with low uncertainty, if the calibration is carefully performed. In addition to the given resolution of the DSC, the results can be interpreted with up to four significant digits. The salt inside the composites will partially or completely melt in the applied temperature range, depending on the heating rate. After calibration experiments were completed and validated with glass beads, the calculations showed consistent results for a heating rate of 10 K min1, so it has been adopted for the following measurements. Additionally a blank was included in the method developed for DSC measurements, in order to correct systematic errors from external influences on the possible dehydration or hydration reactions. The results obtained by the DSC are within the range of literature values (Table 8) with a relative uncertainty of less than 10%, as shown in Table 9, which ensures the validation of our measurement procedure. As a measurement procedure, each sample is run three times under the same conditions and the mean values of the results are used. Table 10 shows the thermal conductivity of matrices and composites based on chlorides with different proportions when the temperature ranges from 100 to 200 °C. Results show that the measurements are in good agreement with the data from literature. It indicates that thermal conductivity increases when the mass ratio of the salt decreases. However, accuracy of obtaining this thermophysical property is around 29% according to the uncertainties in Table 10. The thermal conductivity of composites made of CaCl2 and ENG at an equal mass ratio is 3 times higher than that of CaCl2, highlighting the heat transfer enhancement of the untreated salt.

4. Conclusion To overcome the disadvantages of sole calcium chloride in thermal energy storage, the material has been mixed with potassium chloride and impregnated into matrices such as ACF or ENG. The synthesized salt mixtures and ENG-supported CaCl2 and molten hydrated CaCl2 composites showed remarkable dehydration and hydration behaviour in microscale sorption experiments and enhanced thermal conductivity in comparison to untreated CaCl2. Of the KCl–CaCl2 mixtures the ratio of 2 KCl to 1 CaCl2 yielded the best results concerning water uptake, with an increase to 5.3 H2O molecules into the salt structure after a dehydration at T = 100 °C and to 7.4 H2O molecules after a dehydration at T = 200 °C. Compared to the expected amounts, calculated from the measurements of untreated CaCl2 and KCl this is an improvement to 160% (100 °C) and 195% (200 °C) efficiency. Due to the higher water uptake, the salt releases an increased amount of water during the next dehydration cycles as well, which

keeps the energy storage density at a stable level. However, concerning the energy storage density, the mixture with the ratio of 1 KCl to 2 CaCl2 is more effective as the loss of released energy compared to untreated CaCl2 after dehydration at T = 100 °C is only 8.2% and after dehydration at T = 200 °C is only 5.5%, while the energy release of the mixture with the 2 KCl to 1 CaCl2 ratio decreases by 52.6% (T = 100 °C) and respectively 44.6% (T = 200 °C). Composites consisting of ENG and either CaCl2 or molten hydrated CaCl2 possess good water sorption properties below 100 °C. With a water uptake of 0.67 g g1 and 0.72 g g1, respectively, and an energy storage density of 1451 J g1 and 1310 J g1 these materials are very attractive for thermal energy storage applications driven by low-grade heat sources. The high thermal conductivity of the host ENG improves the heat transfer in the composite materials based on CaCl2. The overall thermal conductivity was tripled compared to untreated CaCl2 samples. In future studies, we will use the modified salt mixtures to impregnate porous materials such as ENG, silica gels and zeolites to obtain a stabile thermochemical storage material with high energy storage density and improved mass and heat transfer performance. Moreover, we will focus on the development of better mixing methods and impregnation techniques to create composite sorbents comprising of other salt hydrates. Acknowledgements We thank Dagmar Schuchardt, who has helped us with mixture preparation, and Melanie Böhme, who has assisted in the data evaluation. Rammelberg, H.U. thanks the Friedrich-Ebert-Stiftung for financial support. We gratefully acknowledge the financial support of this project by the European Regional Development Fund (EFRE) in the framework ‘‘Thermische Batterie – Project” (application number: CCI N° 2007DE161PR001) of the Innovation Incubator Lüneburg. Appendix A. Uncertainty calculation for the DSC The determination of the thermal conductivity is based on the measurement of the heat flux through the sample and the corresponding temperature difference with the reference pan. That heat flux or thermal power has a standard deviation or systematic uncertainty of 0.0006 W. The temperature measurement has a standard deviation of 0.03 °C. The height and diameter of the sample is measured using a digital calliper (Powerfix) with the uncertainty of ±0.02 mm. The error calculation of thermal conductivity under transient state measurement is obtained via Eqs. (5) and (6):

dslope dqt dT dRs dqt dT ¼ ¼  þ  slope q T Rs q t T t

ðA-1Þ

dk dLs dA dRs dLs dA dRs ¼  þ þ   k L A Rs Ls A Rs s

ðA-2Þ

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Please cite this article in press as: Korhammer K et al. Sorption and thermal characterization of composite materials based on chlorides for thermal energy storage. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.08.037