Water adsorption characteristics of novel materials for heat transformation applications

Applied Thermal Engineering 30 (2010) 1692e1702

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Water adsorption characteristics of novel materials for heat transformation applications S.K. Henninger a, *, F.P. Schmidt b, a, H.-M. Henning a a b

Fraunhofer Institute for Solar Energy Systems ISE, Dept. of Thermal Systems and Buildings, Heidenhofstr. 2, 79110 Freiburg, Germany Karlsruhe Institute of Technology (KIT), Dept. of Fluid Machinery (FSM), Kaiserstr. 12, 76131 Karlsruhe, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2009 Accepted 23 March 2010 Available online 28 March 2010

Within this article we illustrate recent development of sorption materials for heat transformation applications. A broad overview on the possible performance of currently available and recently developed materials ranging from zeolites across aluminophosphates and silicoaluminophosphates to the novel class of metal organic framework materials is given. Materials are evaluated with respect to the use in thermal driven adsorptive heat pumping and cooling applications with water as refrigerant. Therefore a new fingerprinting method is used to evaluate samples under two typical cycle conditions with driving temperatures of 95
C and 140
C. A unique aspect is that results can be used for closed as well as for open adsorptive systems. The highest water uptake for driving temperatures of 95
C was found for an AlPO-18 with 0.253 g/g, which is more than six times higher than the reference silica gel in our comparison. For driving temperatures of 140
C the highest water uptake was found for the metal organic framework Cu-BTC with 0.324 g/g. Furthermore we give first results on the integral heat of adsorption in the cycle and results of hydrothermal treatment of most promising materials. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Water adsorption Heat pumps Adsorption chiller Sorption materials

1. Introduction 1.1. Motivation and background In recent years, there has been significant research progress on micro- and mesoporous adsorbent materials suitable for open and closed system heat transformation applications [1e5]. Thermally driven chillers and heat pumps based on adsorption/ desorption of water in micro- and mesoporous materials provide a promising approach towards a rational use of energy, a sustainable energy policy as well as an effective climate protection through the reduction of the environmental impact of conventional heating and cooling devices [6]. Regardless current efforts on energy-efficiency almost 54% of the total final energy consumption in Germany in the year 2007 are used for the allocation of heat [7]. A large potential for energy saving and therefore CO2 reduction is seen in heating of buildings, which accounts for about 26.1% of the final energy consumption excluding hot water preparation. Furthermore sorption heating and cooling systems allow the use of low temperature heat sources (e.g. ground heat exchanger or * Corresponding author. Tel.: þ49 761 4588 5104; fax: þ49 761 4599 9000. E-mail address: [email protected] (S.K. Henninger). 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.03.028

boreholes) as well as industrial waste heat which improves the overall CO2 balance. State of the art with water as refrigerant are absorption chillers based on water/lithium-bromide and adsorption chillers and heat pumps based on zeolites, silica gels or recently developed new molecular sieves like UOP-DDZ70 or Mitsubishi AQSOA-Z02 [8e10]. Commercially available heat pumps or chillers still use adsorbents that have originally been developed for applications like gas separation or catalysis [11e14]. The present generation with water as working fluid mostly use silica gels as adsorbents. The development of improved silica gels for heat pumping and cooling applications has been a research priority for many years [3,15,16]. A principle problem with silica gel type adsorbents for these applications has been that most of the water adsorption occurs at very high relative pressure and therefore the usable temperature lift is small. Using these well known adsorbents current developments were focused on the intensification of heat and mass transfer [17e19]. 1.2. Outline of the publication This paper reports the results of an interdisciplinary networkproject with focus on the identification of new adsorbents for heat storage and heat transformation applications [20] with water as

S.K. Henninger et al. / Applied Thermal Engineering 30 (2010) 1692e1702

working fluid. The limitation is due to the fact that water has the highest evaporation enthalpy. Furthermore compared to other possible working fluids like methanol or ammonia [21], water is non-toxic and environmentally benign. As the adsorption characteristics strongly depend on the cycle conditions, we first define two different cycle conditions corresponding to two possible application areas. Each set of conditions is defined by the highest desorption temperature (driving temperature), the minimum adsorption temperature, the condenser and evaporator pressure (see Section 2.4). Regarding the adsorption equilibria, a key development target for heat transformation applications is to increase the amount of adsorbed working fluid within the cycle conditions. Therefore as a result of the first screening, we present the water uptake according to the defined cycles of different adsorbents in Section 3.1. Comparing the result with the reference material silica gel 127 B, the most promising materials are evaluated by further measurements. A second important parameter to evaluate adsorption materials is the adsorption enthalpy. This parameter has a direct influence on the cycled heat and thus the energy density for storage applications. It also influences the COP which strongly depends on the ratio between total sorptive and total sensible heat [22]. For a first indication on the cycled heat, measurements of the integral heat of adsorption (desorption) and the average molar adsorption enthalpy are given in Section 3.2. In addition stability considerations of the most promising materials are completing this section followed by the final conclusions. 2. Experimental

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isotherms were performed on a SETARAM TG/DSC 111 coupled with the controlled humidity generator Wetsys. The assembly is sketched in Fig. 1. A dry gas flow, which is used as carrier gas, is split into two gas flows. In order to produce a gaseous flow with a controlled and stable humidity rate, the dry gas flow and the saturated gas flow are mixed within a mixing chamber. The humidity rate is measured with a Rotronic humidity sensor (precision 1.5% RH)) in the mixing chamber and controlled with the help of two mass flow controllers (MFC) for each flow. In this work, Argon (Argon 5.0, purity 99.999%, H2O  3 ppm) in combination with a moisture trap Agilent 400 has been used as carrier gas. The humidified carrier gas is pushed through a heated transfer line into the TG/DSC measurement cell. In order to detect mass and heat flow change simultaneous, the TG/DSC is put into vertical operation. Again, the gas flow is split into two parts which supplies the reference cell and the sample cell with identical humidified carrier gas. The reference crucible and the sample crucible are linked to a beam balance articulated on a torsion band. The microbalance has a measuring range of 200 mg and a resolution of 0.03 mg. To compensate any different flow rates within these two cells, a needle valve can be used to adjust the flow rates in both cells. In order to prevent condensation at the split-point and in the needle valve, these parts are placed within a thermostatically controlled box. The carrier gas flow rate in all measurements was set to 30 ml/ h, bath temperature in the Wetsys was set to 40
C, the humidity rate has been varied between 16.6% RH (corresponding to 1.23 kPa pure water vapour pressure) and 76.6% RH (corresponding to 5.6 kPa pure water vapour pressure) according to the cycle conditions described below.

2.1. Experimental setup e Setaram TG/DSC 111 2.2. Experimental setup e Rubotherm TG The water adsorption capacity and resulting heat of adsorption of different microporous materials like cation exchanged zeolites, aluminophosphates (AlPOs), silicoaluminophosphates (SAPOs) and the new class of metal organic framework materials were investigated by combined thermogravimetry and differential scanning calorimetry (TG/DSC) [23,24]. Water sorption isobars and

In addition to measurements performed on the Setaram apparatus, water adsorption isobars and isotherms have been measured on a Rubotherm thermobalance [25]. The results have been used for comparison (see Section 2.5) and for the calibration of the Setaram experimental setup (see next section) to allow the results to be

Fig. 1. Simultaneous thermogravimetric and differential scanning calorimetry SETARAM TG/DSC 111 coupled to the controlled humidity generator Wetsys.

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Fig. 2. Rubotherm magnetic suspension thermobalance for measurements under pure water vapour atmosphere.

used either for closed adsorption and open adsorption systems. The assembly of the Rubotherm thermobalance is sketched in Fig. 2. In contrast to the Setaram setup, the sample is linked to a suspension magnet which allows the measuring force to be transmitted contactless from the measuring chamber to the microbalance. As the microbalance is located outside the chamber this setup allows a precise measurement of the mass change under controlled atmosphere conditions (vacuum, pure adsorptive). Therefore the measuring cell can be connected to a water vapour reservoir or to the vacuum pump. Furthermore the measuring cell, the coupling housing and the water vapour reservoir can be temperature controlled by a fluid loop to achieve controlled thermal conditions. To obtain different sample temperatures or for higher measuring temperatures the cell can be heated electrically. The temperature of the measuring cell, the thermostat and the water reservoir is measured with Pt100 temperature sensors. The resulting water vapour pressure is measured with a capacitance pressure manometer type Baratron 128 from MKS instruments. All valves and connection as well as the pressure transducer are located within a thermostated containment. The used balance is a modified Mettler balance with a resolution of 10 mg and a reproducibility of 30 mg.

with DIN 66138 drying over a continuous dry gas flow is another possibility [26]. Unfortunately using the above described setup with a dry gas flow can lead to a different dry or reference mass. This is especially the case if a hydrophilic sample is characterised as illustrated in Fig. 3. Here a Y-zeolite sample has been measured within different instruments. All measurements have been performed with a commercial available Y-zeolite (Y-Sit V 15, Slovnaft Vurup) under different conditions for determination of the reference mass. The reference mass for the black line (see cubic symbols in Fig. 3) has been determined under continuous evacuation with a turbomelecular pump (model Turbo Cube TSH 071, Pfeiffer

2.3. Experimental setup e calibration for water load determination One major problem in the experimental investigation of the water loading curve is the determination of the dry adsorbent mass. As a recommended procedure to determine the reference mass the samples are heated up to 150
C and dried over several hours under continuous evacuation. This procedure is not appropriate for all kinds of sorption materials, as some samples are not stable under these conditions while others have to be treated for more than 16 h in order to achieve equilibrium. The measurement of a reference mass under atmospheric pressure is even more difficult. As the Setaram TG/DSC 111 setup used in this work is an open system with a carrier gas, it is not possible to determine the dry mass under continuous evacuation. Therefore in accordance

Fig. 3. Influence of drying method for determination of the reference mass. Illustrated are isobaric measurements at 1.2 kPa partial water vapour pressure of a commercial available Y-zeolite. Water uptake is calculated according to different reference mass conditions. Reference mass for black line has been determined under continuous evacuation and 140
C for 6 h. Reference mass for the grey isobar has been determined under argon carrier gas flow for approx. 6 h and 140
C.

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Vaccum, end pressure <10e8 kPa) at 140
C for about 6 h in the Rubotherm TG. The continuous vacuum leads to the lowest adsorbent mass (“most dry sample”) and therefore to the highest relative loading. The reference mass of the second isobars has been determined under a continuous argon gas flow for approximately 6 h in the Setaram TG/DSC at 140
C. The determined reference mass for the dry gas flow is higher and therefore the loading curve is significantly lower than in the case of the measurement performed under vacuum. The reason is probably due to two effects. First, the drying process in the TG/DSC is most likely limited by diffusion processes, especially as the contact area between the dry gas flow and the sample in the Setaram setup is restricted to the upper opening of the platinum crucible. Second, the dry gas flow has to pass the mixing chamber which may still contain water molecules. However, if a suitable reference mass is determined under well defined conditions, more precisely conditions which can be controlled and set in both apparatuses, the relative difference in water uptake between the two measurements is within the error bars as can be seen in the following section. These results show the significance of a well defined reference and strengthen the decision to use a different reference mass. Furthermore as our focus is on the relative comparison of sorption materials with regard to open or closed system applications, there is no need for determination of the absolute dry mass. Both in closed and open adsorption cycles, the adsorbent never reaches a dry state but keeps a minimum water loading determined by the evaporator pressure (closed system) or by the partial pressure of water (open system) at maximum adsorbent temperature. Furthermore the employed procedure serves as a first fingerprint to identify the most promising materials.

2.4. Error analysis Each equilibrium data point has been averaged for at least 20 measurement points and the standard deviation has been calculated. The resulting error represents the statistical error of the weighing and is in the range of 0.1e1 mg. With regard to the calculation of the loading spread, this is defined as

Dm ¼

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m  mref mref

the error propagation according to Gauss yields to the following statistical error

sDm

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 ! u u 1 m t 2 $s2m ¼ smref þ mref m2ref

In the case of the Setaram TG/DSC, the main error arises from the determination of the reference mass. This is due to the fact, that the balance is designed to measure mass differences and not absolute mass. With regard to the measurement time (e.g. up to 5 days) a possible source for an additional error is the balance drift. Therefore the calculated statistical error according to the resolution of 0.03 mg (noise RMS 0.03 mg) and the systematic error due to vertical operation and gas flow of 0.2 mg/ml underestimate the real error. In order to assign a more realistic error several tests including resolution, reproducibility, balancing drift and mounting test were performed. As an upper estimation of the systematic error the following expression may be used

sDm

     1   m      ¼  þ $s $s mref  mref m2  m ref

which exceeds the calculated statistical error by far. Therefore the overall error has been calculated as an upper estimation. The error for the reference or absolute mass smref has been determined to 20 mg, the error for the weighing difference sm has been determined to 6 mg which is six times the measured weighing error but is consistent with the specific noise under a flow of 30 ml/h. All errors given in Table 1 are calculated as upper estimation according to these values. Likewise the error analysis for the heat flow measurements has to be discussed. The adsorption enthalpy is determined by the resulting heat flow and the mass change according to

DH ¼

Q

Dm

Table 1 Water loading spreads for the different cycle conditions. Sample

Silica gel Na e A Li e A Li-LSX sa1 Li-LSX sa3 Li-LSX sa2 Pb-Y Ni-Y Na-Y Li-Y LaNa-Y SAPO-34 sa1 SAPO-34 sa2 SAPO-34 sa3 AlPO-18 sa3 AlPO-18 sa2 AlPO-18 sa1 Cu-BTC sa1 Cu-BTC sa2 Cu-BTC sa3

Spread 1 [g/kg]

Spread 2 [g/kg]

Spread 3 e Spread 2 [g/kg]

desorp. 95
C/5.6 kPa adsorp. 40
C/1.2 kPa

desorp. 140
C/5.6 kPa adsorp. 30
C/1.2 kPa

adsorp. 140
C/5.6 kPa des. ref. point 140
C/1.2 kP

41  3 18  7 95 10  6 18  4 24  1 81 13  1 30  9 43  5 83  3 110  1 147  2 200  3 198  9 244  3 254  9 95  7 120  6 106  1

112  3 90  7 100  5 117  6 116  4 119  1 153  1 153  1 150  9 192  5 192  4 240  1 204  2 276  3 256  9 296  3 304  9 237  7 271  6 324  1

7 58 52 72 74 82 44 47 48 72 24 5 5 8 3 2 3 10 8 13

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The error consists of the above determined error for the mass difference sm and the error of the heat flow signal. The latter is dominated by the calibration method during which the sensor is operated in a horizontal position, whereas all measurements in this paper were performed in vertical operation. Error propagation by Gauss leads to

sDH ¼ DH

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  sQ s 2 þ Dm Dm Q

Unfortunately the vertical operation in combination with a gas flow is also a source for a systematic error. Contrary to the original setup and calibration of the calorimeter the released heat is partially not detected due to dissipation by the gas flow. Hence dehydration of CuSO4 and determination of the melting point and heat of fusion for indium and gallium at different heating and flow rates have been performed as a calibration test. All tests revealed an underestimation of the heat flow of about 5e7% compared to the literature value. Therefore statistical and systematic errors have been assigned to the adsorption enthalpy measurements given in Table 2.

2.5. Cycle conditions in the fingerprinting method In order to allow for a fast assessment of sorption material samples a fingerprinting method has been developed. This method gives a first indication on the suitability of a material for different possible technical applications with a small number of measurement points. We believe that this method could be used as a basis of a measurement standard for characterization of adsorption materials in heat transformation applications. For the purposes of the fingerprinting method we decided to redefine the reference mass conditions to enable a direct comparison between gas flow measurements and pure water atmosphere measurements with a minimal number of data points per sample. As the common reference point for both gas flow adsorption measurements (TG/DSC) and pure water atmosphere adsorption measurements we chose the following conditions (cf. Fig. 4): temperature 140
C, pressure 1.23 kPa water vapour. All measurements of adsorption equilibria are related to the reference mass of the sample obtained at this point. With respect to adsorption cycles, isobaric measurements of adsorption equilibria are more useful than the usual isothermal measurements, since the cycle (ideally) consists of two isobaric phases of desorption and adsorption at condenser and evaporator pressure, respectively. The following two different possible application areas and associated adsorption cycle conditions are subject of this paper. Table 2 Results of simultaneous TG/DSC measurements. Given are the relative amount adsorbed (mads, at 40
C/5.6 kPa according to reference mass 150
C/1.2 kPa), the integral heat of adsorption Qint including sensible heat, the average molar adsorption enthalpy and the range of water loading. (* ¼ the results for the Cu-BTC sample have to be treated carefully, as the material shows a degradation.) Values are given with statistical and systematic errors. Sample

mads [g/g]

Qint [kJ/mol]

Hads [kJ/mol]

Range [g/g]

Li-Y SAPO-34 sa 3 LaNa-Y AlPO-18 SAPO-34 sa 1 Cu-BTC*

0.297 0.228 0.237 0.383 0.313 0.371*

67.0  0.8 65.8  1.0 63.5  0.7 62.3  1.6 61.7  0.4 48.1  1.0*

51.0  2.5 57.3  2.9 51.9  2.6 55.1  2.8 55.5  2.8 50.7  2.9*

0.272e0.298 0.194e0.228 0.210e0.238 0.284e0.381 0.267e0.312 0.199e0.361

(þ3.4) (þ3.3) (þ3.2) (þ3.1) (þ3.1) (þ2.4)

(þ2.6) (þ2.9) (þ2.6) (þ2.8) (þ2.8) (þ2.5)

First cycle conditions (CC 1) are corresponding to adsorption heat pumps and adsorption chillers operating at temperatures up to 95
C. This limit is motivated through the use of pressureless water systems, connections to, e.g. heating networks or possible mobile applications like automotive air conditioning. The minimum adsorption temperature has been set to 40
C. This temperature is motivated by current available low temperature heating systems operating in the temperature range of 45
C (low temperature) and 35
C (very low temperature) [27]. Second cycle conditions (CC 2) are corresponding to applications with higher driving temperatures up to 140
C, e.g. stationary applications. In combination with a condenser pressure of 5.6 kPa and an evaporator pressure of 1.2 kPa possible applications are heating of low energy buildings or stationary solar cooling applications [14]. The minimum adsorption temperature has been set to 30
C. This value corresponds for example to a heat rejection at 27/ 29
C (wet/dry cooling tower) in the cooling case. The measurement cycle is displayed in Fig. 4. Both cycle conditions have the same evaporator and condenser pressure levels but different adsorption temperatures as described above. 2.6. Open and closed system e comparison of measurements An important point for the rating of the measured data for different systems is the specification under which conditions these data can be used. As mentioned before, almost all measurements have been performed under atmospheric pressure with a humidified carrier gas. Therefore, these data can be used with regard to open sorptive systems for air conditioning and dehumidification. As closed pure adsorptive systems like adsorption chillers are also within the focus of our research, we compared the results of some samples with measurements in a pure water atmosphere Rubotherm thermogravimetry. In Fig. 5 measurements performed in pure water vapour atmosphere and under a continuous humidified carrier gas flow of the same zeolites LSX sample as shown in Fig. 4 are compared. Adsorption data points were taken at 1.2 kPa, desorption data points at 5.6 kPa. Illustrated is the water uptake against the relative pressure p/p0. In the case of isobaric measurements p0 is the water saturation vapour pressure corresponding to sample temperature. The results under pure water atmosphere are in good agreement with the water adsorption data in a humidified carrier gas flow if the water uptake is calculated with respect to the reference mass at 140
C and 1.2 kPa determined with each experimental setup. The error bars in x as well as in y direction are in the range of symbol size. In the case of the TG/DSC measurements the dominating error in x-direction is the accuracy of the Rotronic humidity sensor (w1.5% RH) which leads to an error in the relative pressure up to 0.004 p/p0. Hence it is possible to use the open atmosphere measurements performed with the Setaram TG/DSC in the same way to determine approximately the loading differences within closed (pure water vapour atmosphere systems. The vapour pressure in the case of the closed system can be easily calculated by the relative humidity and the temperature of the bath and water vapour temperature respectively. 3. Results and discussion 3.1. First screening and comparison vs. silica gel 127 B as reference The criteria for evaluation of adsorbents for heat transformation applications has been discussed in detail by Núnez [28]. A key quantity given by the application conditions is the minimal usable temperature lift DTlift between evaporator and adsorber. In a typical solar cooling application, the minimal DTlift is about 20 K. The

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Fig. 4. Fingerprinting method. Data points taken per measurement cycle according to the cycle conditions CC 1 and CC 2 for different possible applications (“fingerprinting method”).

typical problem with silica gels as adsorbents for these applications has been that most of the water adsorption occurs at high values of relative pressure and therefore the usable temperature lift is small. If a temperature lift of 20 K is required, microporous silica gels with enhanced hydrophilicity such as Engelhard type N or Grace 127B enable a higher loading spread than mesoporous silica gels. Since the silica gel Grace 127B has already been applied in a commercial adsorption chiller [12,29] it has been used as the reference material here. The loading spread of adsorbed water for different materials measured in the Setaram TG/DSC 111 under the above mentioned cycle conditions is shown in Fig. 6. The corresponding numerical data including errors calculated according to Section 2.4 are given in Table 1. Spread 1 describes the water loading spread between desorption at 95
C/5.6 kPa and adsorption at 40
C/1.2 kPa. This corresponds to a possible water uptake within applications conform to

Fig. 5. Comparison between measurements on zeolite Li-LSX in pure water atmosphere and humidified carrier gas flow. Error bars both in x and y directions are in the range of symbol size, lines are guides for the eye.

CC 1. The condenser pressure of 5.6 kPa correlates to a release of the condensation enthalpy at 35
C in the desorption process. Spread 2 describes the water loading spread between desorption at 140
C/5.6 kPa and adsorption at 30
C/1.2 kPa. These temperature values comply with CC 2. As the desorption temperature is higher and the adsorption temperature lower this spread always exceeds the spread under condition 1 and is plotted incrementally in the following figures. Finally Spread 3 describes the water loading spread between desorption at 140
C/1.2 kPa (reference point) and adsorption at 30
C/1.2 kPa. Within the fingerprinting method, this spread gives a lower boundary for the water sorption capacity of the material. Furthermore, the difference between spread 2 and 3 is a lower boundary for the residual water loading, outlined in the last column of Table 1, which is never desorbed within the cycle conditions given above. The reference silica gel (Grace 127 B) shows a water loading of about 43 g/kg (spread 1) and 112 g/kg loading (spread 2) as shown in Fig. 6. The residual water loading between 140
C at 5.6 kPa and 1.2 kPa (diff. spread 3 e spread 2) is 7 g/kg and hence remains quite small. The next samples within the first group are two zeolite A samples with different cations. Although zeolite A is known as strongly hydrophilic, we were interested in the influence of the charge balancing cation. Both samples show a lower spread 1 compared to the reference material. Even with higher desorption temperatures under cycle conditions 2 these zeolite types show a lower loading spread 2 than the silica gel. Comparing both samples it must be stated that the influence of the cation is not significant. Although the maximum water loading at higher temperatures (spread 3) is larger compared to the reference, this spread cannot be used with regard to desorption temperatures up to 140
C. Hence the residual water loading indicated by the difference between spread 3 e spread 2 of about 54 g/kg is quite large. Zeolites of the faujasite framework type (X and Y zeolites) have been considered for the use as adsorptive storage material or in adsorptive heat pump and cooling applications since the 1980s [30e33]. In the case of the hydrophilic zeolite X, amongst different possible cations, the lithium exchanged form has been selected as

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Fig. 6. Comparison of water loading spread for three cycle conditions for different materials with respect to the reference adsorbent mass at 140
C and 1.2 kPa water vapour pressure.

a promising one. Therefore, three different samples (sa1, sa2 and sa3) of a Li-LSX (low silica X, Si/Al ratio ¼ 1) zeolite, ion-exchanged to contain >95% Lithium cations, have been investigated. Very similar to the results of the zeolite A samples, the water loading spread for lower desorption temperatures are fewer than the reference. The different loading spreads amongst the samples are probably due to different ion exchange or slightly different Si/Al ratio but not known. However, the loading spreads for higher desorption temperatures are slightly larger compared to the reference varying between 116 and 119 g/kg being equal within the calculated error range. With regard to the residual water loading, these samples show the highest values ranging from 72 up to 82 g/kg. In addition to the X-type zeolite, five samples of zeolite Y with different cations have been tested. As can be seen in Fig. 6, the samples show a continuous increasing water loading spread for conditions 1 in the sample-sequence Pb-Y < Ni-Y < Na-Y < LiY < LaNa-Y (see also ref. [34]). Whereas the loading spread 1 for PbY, Ni-Y and Na-Y is still smaller than the reference, the sample Li-Y (43 g/kg) and especially LaNa-Y (83 g/kg) show a significant increase in the water loading spreads. This is a combined effect of different cations and slightly different Si/Al ratio. As in the case of Pb-Y, Ni-Y and Na-Y the Si/Al ratio is not exactly known, there is no absolute certainty of the cation influence. However, in the case of Li-Y and LaNa-Y the Si/Al ratio has been determined to approx. 2.5 (Li-Y) and 2.7 (LaNa-Y) with a quite large difference in the loading spread 1. This is most likely an effect of the rare-earth cation. With regard to the Si/Al ratio it should be mentioned, that a stronger dealumination of the framework leads to lower hydrophilicity and therefore less residual water content. At the same time the dealumination procedure leads to a loss of porosity or partial destruction of the pore system and therefore lower water loading spreads (see ref. [35]).

Different results were found with regard to higher desorption temperatures (loading spread 2). Here samples Pb-Y, Ni-Y and Na-Y show the same loading spread in the calculated error range with about 153 g/kg. This is an increase of about 36% compared to the reference and about 30% compared to the zeolite X samples. The samples Li-Y and LaNa-Y show an even higher spread 2 of 192 g/kg. This is an increase of about 71% compared to the reference. In terms of maximum water loading capacity, the zeolite X and the zeolite Y samples show quite the same loading spread 3 (roughly 200 g/kg) except the Li-Y and LaNa-Y samples. Especially with 264 g/kg the Li-Y sample exhibit an even larger water capacity than the LaNa-Y sample. This effect can most likely be attributed to the small Li cation. The next group in Fig. 6 are three different samples of a SAPO34. Silica-aluminophosphates (SAPOs) and aluminophosphates (AlPOs) often show a regular pore system and three-dimensional networks similar to zeolites. Unlike zeolites, AlPOs show a neutral framework whereas the SiO4 sites in the SAPO are involving a charged framework [2]. Sample 1 has a grained consistency, whereas sample 3 has a powder consistency and grains smaller than 5 mm. All samples display a large loading spread for low desorption temperatures. Especially sample 3 shows an impressive spread 1 of about 200 g/kg. Sample 2 is also a grained SAPO-34 which shows a spread 1 of 147 g/kg. The loading spread for conditions 2 of this sample is however fewer than the other samples, which is possible due to a higher fraction of binding agent. The largest spread under condition 1 has been obtained with the AlPO-18 samples, which are shown next to the SAPO-34 results. While the first sample reveals a spread of 198 g/kg, which is quite the same as the best SAPO sample, the next two samples of the aluminophosphate are exceeding this value by far with 244 g/kg and 254 g/kg. The superior sorption behaviour depending on their

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about 324 g/kg. As can be seen in Fig. 6 and Table 1, the different CuBTC samples show a large variety of loading spreads exceeding the calculated error range. This is due to different solvents used and resulting crystal sizes. It should be noted here that this could also be an effect of the pre-treatment and different stability of the samples which is discussed in the next section.

3.2. Specific measurements and stability considerations of most promising materials

morphology has been reported before [10,17]. Same results are obtained for conditions 2. While sample 3 shows a lower loading compared to the best SAPO sample, the samples 2 and 1 show a loading spread of 296 g/kg and 304 g/kg respectively. With regard to the residual water loading, these two samples show a very low value of about 2e3 g/kg. These materials are therefore quite well optimized for the applications in the focus of this paper, as the biggest load share can be reached with driving temperatures below 140
C. As a quite novel class of materials for sorption processes some samples of metal organic framework materials (MOF) have been tested. Although these materials have been originally designed for hydrogen storage [36] the application for water adsorption shows to be promising too [37,38]. The copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC) metal organic framework has been identified as the most promising candidate out of this class so far. The loading spread under conditions 1 account for 95 e120 g/kg which is fewer than the AlPO and SAPO samples, exceeding the performance of the zeolite samples. With regard to higher desorption temperatures, the conditions are changing. Under conditions 2 the best MOF sample even outperforms the best AlPO sample with an impressive loading spread 2 of

As a result of the first fingerprinting of the different materials the most promising candidates have been identified. With regard to the performance for the different application field Li-Y, SAPO-34, AlPO-18 and the Cu-BTC compound have been further investigated. In a second step, isobaric and isothermal TG measurements of the chosen samples have been performed for a deeper insight into the different adsorption behaviour. Fig. 7 gives the results of the thermogravimetric adsorption and desorption measurements of the Li-Y sample. Shown are isothermal and isobaric measurements of the water adsorption compared to the reference mass as function of the relative pressure. The relative pressure in the case of isobaric measurements is defined as p/p0, where p0 is the water saturation vapour pressure corresponding to the sample temperature. Therefore isothermal and isobaric measurements show a similar curve. As can be seen, this sample shows a steep increase at low relative pressure values. Although this shape is well known for hydrophilic zeolites, this is not the preferred water load curve for the applications in focus as most of the water is already adsorbed at low relative pressures which therefore implies high driving temperatures. However, with regard to the hydrothermal stability this zeolite showed no degradation over approx. 20 cycles between 140
C and 20
C at 1.2 kPa water vapour pressure. Furthermore there is no significant hysteresis between the adsorption and desorption path visible. As can be seen in Fig. 8 the thermogravimetric investigations on the adsorption and desorption characteristics of the SAPO-34 compound shows a smooth slope which is in addition shifted towards higher relative pressure compared to the measurements of the Li-Y sample. With regard to lower driving temperatures for solar thermal applications this is a more convenient adsorption characteristic than in the Li-Y case. Unlike the faujasite framework of the Li-Y compound the stability of the SAPO-34 framework

Fig. 8. Isobaric and isothermal measurement of the SAPO-34 sample 1 vs. reference mass at 140
C/1.2 kPa.

Fig. 9. Degradation of SAPO-34 through continuous cycling between 150
C and 30
C under a water vapour pressure of 1.2 kPa.

Fig. 7. Isobar and isothermal measurement of the Li-Y sample vs. reference mass at 140
C/1.2 kPa.

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Fig. 10. Isobar and isothermal measurements on SAPO-34 sample 3 vs. reference mass at 140
C/1.2 kPa.

depends crucially on the synthesis route. The so called “morpholine synthesis” recipe proposed by the IZA synthesis commission [39,40] leads to a framework which is unstable in the presence of water vapour below 100
C. As illustrated in Fig. 9, this results in a continuous and irreversible loss of water loading capacity within a few cycles between 20
C and 140
C at water vapour pressure of 1.2 kPa. The use of another template like tetraethylammonium hydroxide or diethylamine on the other hand leads to a water vapour stable framework at low temperatures [41]. However this SAPO shows a reversible modification of the framework during water uptake but changes the slope of the adsorption and desorption path respectively [42]. In Fig. 10 isothermal and isobaric measurements on the SAPO-34 sample 3 are shown. Unlike the sample out of the morpholine synthesis, first stability tests show no significant degradation within a few (<20) cycles. Furthermore sample 3 shows a higher loading with a maximum water uptake of approx. 0.310 g/g for high relative pressure. In contrast to the above mentioned materials, the AlPO-18 shows an unique adsorption behaviour as can be seen in Fig. 11. The isothermal and isobaric measurements show almost no water uptake up to a relative pressure of about 0.1e0.15 p/p0. This hydrophobic characteristic is different to the adsorption

Fig. 11. Isobaric and isothermal measurement of the AlPO-18 sample 1 vs. reference mass at 140
C/1.2 kPa.

characteristic of common zeolites and similar materials. Following, the adsorption isotherms show a steep step within a narrow range of relative pressure. Consequently it is possible to desorb the main part of the water at a relative pressure of 0.1 p/p0 which means a desorption temperature of 87
C for a condensation pressure of 5.6 kPa. The resulting uptake is approx. 0.25 g/g and yields to the large load for low desorption temperatures (cycle conditions 1). Unlike the zeolites this material exhibits a sorption hysteresis between the adsorption and desorption path. The hysteresis can be seen in Fig. 11 as a shift between desorption and the adsorption path by about 0.1 p/p0. In the case of desorption temperatures lower than 90
C this hysteresis effect strongly reduces the loading spread. As a new material class one type of a metal organic framework has been evaluated for the use in heat transformation applications. The Cu-BTC samples showed the highest load of all samples. With regard to the given cycle conditions the three samples investigated showed quite different results as illustrated in Fig. 6. This is not only because of differences in the synthesis route. Further investigations have been carried out and showed a fast degradation of the material under hydrothermal conditions. As can be seen in Fig. 12 the large water uptake of approx. 0.35 g/g (relative to the reference mass defined above) decreases within 30 cycles to 0.22 g/g, which is a dramatic loss of capacity of about 37 percent. However there are new synthesis routes for water vapour stable MOFs available as reported recently [38]. Therefore this new class of materials is very promising for the use in sorption processes. With regard to the possible application in sorption heat pump systems, the integral heat of adsorption and the average molar adsorption enthalpy of the different materials have been measured within the Setaram TG/DSC 111. For this the samples have been prepared at 150
C and 1.2 kPa water vapour pressure. Subsequently, the sample temperature has been lowered to 40
C and the water vapour pressure has been increased to 5.6 kPa. The resulting heat flow released has been measured as integral adsorption heat Qintad which includes the sensible heat for the temperature step. This procedure has been repeated in the reverse direction leading to the integral desorption heat Qintdes. As the resolution of the DSC sensor is reduced due to the vertical operation of the TG and the continuous flow of the humidified carrier gas the two heats had been summarized to an integral heat of adsorption for the cycle Qint ¼ ½(Qintad þ Qintdes). The resulting average water uptake and heat flow of this measurement are shown in the second and third column of Table 2.

Fig. 12. Cycle stability test of Cu-BTC samples through continuous cycling between 150
C and 30
C under 1.2 kPa water vapour pressure.

S.K. Henninger et al. / Applied Thermal Engineering 30 (2010) 1692e1702

Additionally, an isothermal TG/DSC measurement at 40
C has been carried out to determine the average molar adsorption enthalpy Hads, obtained by dividing the integral heat by the integral amount adsorbed for a pressure step between 1.2 and 5.6 kPa. The resulting heat flow and the water loading step are given in the fourth and fifth column of Table 2. The table also gives an indication of the maximum possible amount adsorbed using a higher desorption temperature of 150
C and an adsorption temperature of 40
C. As the water vapour pressure has been set to 5.6 kPa this implies a very high p/p0 value of 0.76 which is quite close to the dew point. The Li-Y sample shows the highest integral heat but the lowest adsorption enthalpy except for the Cu-BTC sample. As the integral heat is measured starting from a high temperature and therefore low residual water content some of the high energy adsorption sites are not occupied. In contrast, the isothermal measurement started at a relative water content of 0.272 g/g ending at 0.298 g/g. In this case all high energy sites are occupied and the interaction of the water molecules with the zeolites framework is small and the heat of adsorption is dominated by fluidefluid interactions. Following are the SAPO-34 sample 3 and the LaNa-Y sample with an integral heat of 65.8 and 63.5 kJ/mol respectively. While the integral heats are in the same range, the adsorption enthalpy of the SAPO-34 sample 3 is significantly higher compared to all other samples. The AlPO-18 and the SAPO-34 sample 1 with quite similar adsorption enthalpies show the lowest integral heat. Again, the AlPO-18 shows an impressive load of about 0.383 g/g closely followed by the Cu-BTC sample with 0.371 g/g. Unfortunately these results have to be treated carefully as the tested samples were not hydrothermal stable. 4. Conclusion Screening measurements of the water adsorption of 20 different samples out of seven different classes of materials have been performed. Based on two typical cycle conditions for sorption heat pumps or chillers a fingerprinting method has been developed which allows fast assessment of the suitability of an adsorption material for heat transformation application. Most promising candidates have been identified using this method. The first fingerprint showed that several materials are available with significantly increased loading spread in comparison to the reference material silica gel. The results of the first measurements have been backed up by detailed characterisation of the most promising samples. In the case of the classical zeolites, samples of the Linde type A and the faujasite framework type with different cations have been evaluated. Amongst different possible cations, the lithium exchanged form has been selected as the most promising one. Whereas the different A- and X-zeolites showed no increase or even a smaller loading capacity for low driving temperatures (<95
C) in comparison to the reference material, the lithium exchanged form of a Y-zeolite showed a slightly better performance. In case of driving temperatures of 140
C the water uptake has been increased by a factor of 1.7. Within further measurements this sample showed the highest integral adsorption heat for the testing cycle. The measurements of the newer materials like microporous aluminophosphate AlPO-18 and silica-aluminophosphate SAPO-34 revealed a high optimization potential. The water uptake in case of low driving temperatures can be improved considerably compared to the reference material by the use of SAPO-34 and AlPO-18. The best SAPO-34 samples showed a water uptake of 200 g/kg which is a factor of 4.9 larger compared to the reference silica gel. This result is only exceeded by the best AlPO-18 sample with a measured

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water uptake of 254 g/kg for low driving temperatures. This equals an improvement by a factor of 6.2. The reason lies within the slope of the adsorption isotherm. All AlPO-18 samples showed a steep water uptake within a narrow relative pressure range. Nevertheless the samples showed a desorption hysteresis which will reduce the possible water loading spread at desorption temperatures lower than 90
C. With regard to the hydrothermal stress in heat pumps, the SAPO-34 samples synthesized with morpholine as template are not stable. Preliminary cycling tests on samples synthesized by the use of other templates showed a strongly improved stability. The cycling of the AlPO-18 samples showed a small degradation. Further tests are ongoing. With the Cu-BTC a first candidate out of the new class of metal organic framework materials has been examined. With regard to the water adsorption properties, this sample showed the highest water loading capacity of about 324 g/kg for higher desorption temperatures. This corresponds to a 2.9 times higher water uptake compared to the reference. Although the cycle stability analysis showed a fast degradation, this is a new promising material class for the use in sorption processes. These results demonstrate the need of an appropriate selection of the adsorbent for the application in focus. By adaptation to the specific conditions like heat source and sink temperature levels, the thermodynamic cycle can be operated at a maximum efficiency.

Acknowledgements This work has been carried out within the joint network “Neue Hochporöse Materialien und Systeme für Wärmespeicherung und etransformation” (New highly porous materials and systems for energy storage and heat transformation). Funding by the German Ministry of Education and Research (BMBF) under grant 01SF0303 is gratefully acknowledged. We also like to thank Prof. Schwieger (Erlangen), Prof. Bein (TU München), Dr. A Hahn and Dr. B. Marler (Bochum) for provision of samples.

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