Available online at www.sciencedirect.com
Energy Procedia 30 (2012) 279 – 288
SHC 2012
Novel sorption materials for solar heating and cooling
Stefan K. Henninger*, Felix Jeremias, Harry Kummer, Peter Schossig,
Hans-Martin Henning
Div. Thermal Systems and Buildings Fraunhofer-Institute for Solar Energy Systems,Heidenhofstr. 2, 79110 Freiburg, Germany
Abstract
Heat transformation based on adsorption/desorption of water in microporous adsorbents has been considered for the
application as adsorption chiller (ACS), adsorption heat pump (AHP) or thermochemical storage (TCS) since the
1980s. Unfortunately, most of the available adsorbents like zeolites were not optimized for the use in these processes
as they originally had been developed for gas separation or catalysis processes.
Within the last decade, intensive research on adsorbents yielded in improved and very promising new sorption
materials with an enhanced adsorption capacity. This work gives a broad overview on current developments on
materials including the new class of metal-organic frameworks for the use in adsorption processes for heat storage
and transformation.
© 2012 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of PSE AG
Selection and/or peer-review under responsibility of PSE AG
Keywords: Adsorption chiller/heat pump; sorption materials; MOFs; SAPO; AlPO; zeolites; heat storage
1. Introduction
Heat transformation processes based on the adsorption of gases in microporous adsorbents have been
considered for the application as adsorption chiller (ACS), adsorption heat pump (AHP) or
thermochemical storage (TCS) since many decades. The discovery of the underlying principle, the
adsorption processes itself, dates back to the 18th century [1]. Although the technology can be seen as
mature, industrial products are still addressing niche markets compared to compression chillers or heat
pumps. Recently, the situation is slightly changing, as more and more thermally driven systems are
*
Corresponding author. Tel.: +49 761 4588 5104; fax: +49 761 4588 9000.
E-mail address: [email protected]
1876-6102 © 2012 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.
Selection and/or peer-review under responsibility of PSE AG
doi:10.1016/j.egypro.2012.11.033
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Stefan K. Henninger et al. / Energy Procedia 30 (2012) 279 – 288
entering the market [2, 3]. In addition to that, gas driven heat pumps are gaining more attention, a fact that
is also documented by an increasing number of industrial initiatives with the subject of accelerated
development and deployment of gas driven heat pumps.
Nomenclature
A
adsorption potential (J/g)
AlPO
aluminophosphate
LTA
Linde Type A framework type
FAU
faujasite framework type
Hads
heat of adsorption (kJ/mol)
MOF
Metalorganic framework
Qint
integral heat of adsorption (kJ/mol)
R
gas constant
SAPO
silica-aluminophosphate
T
temperature (°C, K)
W
adsorbed volume (cm³/g)
1.1. The adsorption process
The underlying process of any adsorption cycle consists basically of two phases, the regeneration
phase (a) and the adsorption phase (b) (see fig. 1). During regeneration, the working fluid is removed
(desorbed) from the adsorbent using high temperature heat, typically in a temperature range of up to
140°C. The working fluid returns into liquid phase, releasing heat at medium temperature. In case of a
cooling application this heat is simply rejected to the environment. Typical condenser temperatures are
29°-35°C. In the adsorption phase, the working fluid is evaporated and adsorbed in the material, releasing
heat at a medium temperature, thereby lifting (“pumping”) heat from the low temperature level. In case of
a chiller application, the useful cold is produced in the evaporator.
As heat storage is based on adsorption/desorption of the working fluid, the storage is in principle lossfree as long as the material can be stored without exposition to the working fluid or any other gas which
can be adsorbed. However, it should be mentioned that this process is not a true storage method compared
to e.g. hot water storage, as for discharging a low temperature heat source is required for evaporation.
From a thermodynamic point of view, adsorption heat storage is a heat pump process. Different
working pairs, i.e. combinations of the working fluid (adsorptive) and the porous material (adsorbent)
have been suggested and evaluated [4-6]. Research has been focused on water as working fluid, as it has
the highest evaporation enthalpy and – compared to other possible working fluids – is non-toxic and
environmentally benign. Furthermore, the adsorption of alcohols [7, 8] and ammonia on activated carbon
[9] plays an important role and intensive research on these working pairs has also been conducted.
Stefan K. Henninger et al. / Energy Procedia 30 (2012) 279 – 288
Fig. 1. The two phases of an adsorption process. In the regeneration phase (a), high
temperature driving heat is used to dry the material with the working fluid condensing at
ambient temperature. In the adsorption phase, a low temperature heat source is used to
evaporate the working fluid which is adsorbed by the dry material with release of heat
and a medium temperature useful for heating.
1.2. Adsorption materials development
The development of new porous materials for the use in adsorption chiller/heat pump processes is still
a fundamental research topic. Within the last decade, several exciting improvements and numerous
publications appeared. A first figure of merit for the evaluation of the usability within an adsorption
process is the possible working fluid uptake or exchange within the corresponding thermodynamic cycle.
As illustrated in fig. 1, the thermodynamic cycle is defined by the four temperature levels (T high,
Tcondenser, Tevaporator and Theating).
The sensitivity of an adsorbent towards a temperature change, which then results in a corresponding
working fluid exchange, is strongly connected with the working pair properties. For example, while the
working pair H2O/zeolite is very sensitive to any change of the driving temperature level T high above
120°C, this dependency is much lower for methanol/activated carbons.
Since the working fluid loading of any adsorbent is depending on the temperature at all process phases,
namely regeneration, evaporation, adsorption and condensation, common boundary conditions must be
defined for materials characterization. These boundary conditions should correspond to typical conditions
in existing applications such as e.g. solar cooling, gas-driven heat pumps or storage applications for low
temperature heating. The characterization of adsorption materials has not been standardized so far,
although some common measurement procedures were proposed (e.g. [10]).
However, the limitations induced by high driving temperatures (e.g. high fluid circuit pressures)
affected the material development towards lower desorption temperatures.
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Silica gels are widely studied as very common sorbents for water as working fluid, [11-14] and still
represent the state of the art in commercially available sorption chillers like the ACS 08 from the
manufacturer Sortech AG [15, 16]. A disadvantage of the silica gel/H2O working pair is the low
hydrophilic characteristic, which leads to a low water exchange within a typical cycle, although the
maximum adsorption capacity is quite good.
This can be illustrated by the characteristic curve according to Dubinin’s theory of “the volume
mechanism of the filling of a limited adsorption space W” [17]. For existing adsorbents, the value W
represents the volume of the adsorbed molecules per unit mass of the adsorbent in
cm³(water)/g(adsorbent). The abscissa is the adsorption potential A=RT ln(p/p0) with the saturation
pressure p0 at the temperature T, which can be interpreted thermodynamically as the change of Gibbs free
energy upon adsorption (see fig. 2).
Within the working window of typical boundary conditions, like adsorption at 35°C and 1.2 kPa versus
desorption at 150°C and 5.6 kPa (represented as the grey area), the working fluid exchange for the chosen
silica gel is strongly limited. This is quite a typical behavior for many types of silica gel. Therefore
improvement of the adsorption characteristics by several approaches, like modification of the grain size or
pore sizes, were evaluated and reported in literature.
Beside these modifications, the development of hybrid materials where silica gel is used as a host
matrix for impregnation with hygroscopic salts were subsequently developed and still are in focus of
further research [18-21].
As another well-known class of sorbents, zeolites were intensively evaluated for the use in adsorption
processes. The most commonly studied zeolites for solar heating and cooling applications are faujasites
(FAU framework type) and zeolite A (LTA framework type). Generally speaking, zeolites are more
hydrophilic compared to silica gels, which is concurrently a major drawback for solar driven systems as
desorption temperatures above 200°C are required for extensive regeneration of the material (see fig. 2).
Therefore the reduction of the strongly hydrophilic character of zeolites by ion exchange, variation of the
Si/Al ratio or even dealumination was investigated. In case of faujasites, modifications by ion exchange
or dealumination were reported with promising results [22-25]. Especially for the Li- or rare earthexchanged faujasite types, an increased uptake compared to the standard sodium form was observed.
A newer adsorption material development led to aluminophosphates (AlPO) and silicaaluminophosphates. These materials are zeolite-like and often named “zeotype materials”, as they exhibit
similar frameworks and pore systems. Unlike the hydrophilic zeolites or the less hydrophilic silica gels,
these materials show quite a different type of adsorption characteristics, which is often classified as an Sshape type of the isotherm, emphasizing the fact that the isotherm has a steep increase within a narrow
relative pressure range. This behavior is advantageous for the use in adsorption heat transformation
applications, especially if the steep increase lies within the working window of the application. Various
promising AlPOs with different frameworks and water uptakes exist [26]. Unlike the common zeolites,
AlPOs feature a neutral framework without any charge balancing cations inside the framework which
leads to a different adsorption characteristic. Especially the compound AlPO-18 shows an interesting
behavior, starting from a more hydrophobic characteristic which changes suddenly by a steep increase of
the adsorption curve [25]. With regard to low temperature adsorption chillers, AlPO-18 has been
intensively evaluated with very promising results [24, 27, 28]. Unfortunately, the industrial production of
this material is comparatively expensive due to the template based synthesis of this compound.
Out of the class of silica-aluminophosphates, the compound SAPO-34 was identified as most
promising for the use in thermal driven sorption chillers and heat pumps. This compound was intensively
evaluated and is already commercially available e.g. as AQSOA-Z 02 by Mitsubishi Plastics. Further
developments using this material are heat exchangers which are coated directly or with the help of a
binder [29, 30].
Stefan K. Henninger et al. / Energy Procedia 30 (2012) 279 – 288
In addition, the influence of the elemental composition and framework structure on the sorption
characteristics for AlPO-18, SAPO-34 and the bridging material APO-Tric was investigated in [31],
showing possibilities for modifying the sorption characteristics by changing the Al-coordination. This
underlines the potential of this material class for low temperature heat transformation processes.
Fig. 2. Characteristic curves of three different sorption materials, Grace Silica Gel 127 B, SAPO-34 and
Zeolite 4 ABF supplied by CWK. The grey area represents a possible working window for adsorption
chillers or heat pumps with adsorption at 35°C/1.2 kPa and desorption at 150°C/5.6 kPa. Measurements
were carried out at the thermo-analysis laboratory at Fraunhofer ISE.
Beside the well-known inorganic materials like zeolites and the newer class of aluminophosphates and
silica aluminophosphates, a novel class of microporous materials namely the Metal-Organic Frameworks
(MOFs) has emerged [32, 33]. Out of first measurements it can be concluded, that this class is by far the
most capable class of microporous materials in terms of internal surface area, micropore volume and
water uptake capacity [34-40]. With regard to the similarity of the metal-linker bond to concepts in
coordination chemistry, these solids can also be labeled as coordination polymers.
Due to unique features such as huge surface areas and pore volumes accompanied by tunable pore
sizes and compositions, MOFs have attracted a still increasing attention over the past years with various
applications ranging from ion exchange, separation processes up to drug delivery, catalysis and sensor
technology [41]. Like zeolites and aluminophosphates, metal organic frameworks are crystalline open
porous materials with a one, two or three-dimensional framework. In contrast to zeolites, MOFs are not
purely inorganic but inorganic-organic hybrid materials based on metal ions or metal ion clusters as
nodes, which are linked by organic, at least bidentic ligands.
Compared to traditional adsorbents used in heat pump applications, MOFs exhibit a much richer
variety in composition, pore structure and topology. The cluster/linker concept allows for the tuning of
pore structures and chemical functionality over a wide range.
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Another advantage is the mechanism of synthesis, in which the solvent itself acts as the main template
in contrast to the template-based synthesis of e.g. SAPO-34, where a high temperature activation process
is needed. With regard to the industrial production, several MOFs are already commercially available and
larger production has been launched e.g. by the chemical company BASF.
Out of this large and still fast growing class of porous materials, two compounds showing a huge
potential for water adsorption processes were identified: First, the chromium(III) terephthalate MIL-101
with a similar framework as the MZN zeolite showing a maximum water uptake of up to 1.4 g/g [34, 38,
42]. A working fluid exchange of up to 939 g/kg was reported for a typical adsorption cycle. Second, the
compound MIL-100 (iron or aluminum trimesate) has attracted attention as promising adsorbent for water
adsorption [36, 37, 40]
2. Experimental results
Independent of the use in a chiller or heat pump application, the first figure of merit remains the
working fluid exchange within the cycle. This fluid exchange is the loading spread between the adsorption
and desorption path of the cycle as defined in fig. 1. This cycle is in principle characterised by the
temperatures of the evaporator Te, condenser (and adsorber) Tc and external heat source Tdrive. With regard
to solar applications, the driving temperature is limited by the collector performance, therefore typical
possible temperatures for solar cooling applications are (Te=10°C, Tc=35°C, Tdrive=95°C). The choice of
the driving temperature of 95°C is motivated by the possible use of standard (high-efficient) flat plate
collectors. In addition to that, this is a typical temperature level with water as heat transfer medium, like in
heating networks or in environments where waste heat is provided and cannot be used further. The
possible water loading spread for different materials under these conditions is given in table 1. Starting
from classical materials, the improvements are clearly visible for the different new material developments.
Although the working fluid exchange can be enhanced even for zeolites under these low temperature
conditions, a maximum exchange of 83 g/kg is comparatively low. The newer class of AlPOs and SAPOs
shows a very good exchange of up to 314 g/kg, although at higher adsorption pressure. This holds true for
the MOFs. The huge working fluid exchange of up to 939 g/kg, is only possible for very high adsorption
pressure of 5.6 kPa as marked in Table 1. However, a tremendous capacity and therefore great potential of
MOFs for these applications can be clearly stated.
With regard to the possible heat storage capacity, the integral heat of adsorption is the next important
value. The integral heat includes the heat of adsorption which is released during the adsorption process
and the sensible heat, i.e. the heat capacity of the material itself. These values are determined by a
simultaneous thermogravimetry/differential scanning calorimetry (TG-DSC) isobaric measurement.
Obviously, as shown in table 1, the integral heat depends on the overall water uptake, as with a higher
water uptake, the heat of adsorption converges to the limit of the evaporation enthalpy of water (43.3
kJ/mol). Additionally, the heat of adsorption determined either by direct isothermal measurements in the
TG-DSC or by calculation out of two isotherms is given.
The hydrophilic zeolites show both a high integral heat and a high heat of adsorption. This is due to the
strong interactions with the charged framework and the charge balancing cations, as can be seen for the
Ni-Y zeolite showing the highest values with an integral heat of 72.6 kJ/mol and an adsorption enthalpy
of 58.4 kJ/mol. As these are averaged values, there are only slight differences between the various forms
except for the Li- and rare-earth-exchanged forms. The heat of adsorption is significantly lower for Li-Y
and LaNa-Y zeolites, which is due to lower Al-content and the subsequentlyreduced number of cations.
Additionally, the interaction with the small Li-cation is not so strong as investigated by e.g. molecular
simulations [25].
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Stefan K. Henninger et al. / Energy Procedia 30 (2012) 279 – 288
The aluminophosphates and silica-aluminophosphates have quite similar heats of adsorption, with
APO-Tric showing the lowest value.
MOFs show the lowest values for the integral heat as well as the heat of adsorption. This is a sound
result, as due to the high working fluid uptake, fluid-fluid interactions are more dominant than fluidframework interactions. In addition to that, these materials are highly porous, therefore showing a lower
heat capacity which manifests in a very small difference between the integral heat and the heat of
adsorption.
However, in view of the huge water capacity of the novel materials even their application as long term
storage materials becomes visible. This is supported by their synthesis routes that turn out quite energysaving in comparison to the template based synthesis of other, highly promising materials such as AlPOs
and SAPOs. Thus, the embodied energy needed for production of the storage materials is much lower for
this new class of adsorbents.
Table 1. Overview on sorbent materials with possible working fluid exchange, integral heat of adsorption including the sensible heat
and the average heat of adsorption. (*= possible degradation effects included; ** = higher adsorption pressure in order to show the
possible uptake)
Material
desorption/adsorption
temperature / condenser
(°C)
pressures
(kPa)
realized working
fluid
Qint
(kJ/mol)
Hads
(kJ/mol)
Reference
uptake (g/kg)
Silica Gel
95/40
5.6/1.2
41
--
--
[24]
Na-A
95/40
5.6/1.2
18
66,1
56,7
--
Li-LSX
95/40
5.6/1.2
24
66.9
58,0
[24]
Ni-Y
95/40
5.6/1.2
14
72,6
58,4
--
Li-Y
95/40
5.6/1.2
43
67.0
51.0
[24]
LaNa-Y
95/40
5.6/1.2
83
63.5
51.9
[24]
SAPO-34
95/40
5.6/1.2
200
65.8
55.5
[24]
AlPO-18
95/40
5.6/1.2
253
62.3
55.1
[24]
APO-Tric
95/40
5.6/5.6**
314
60.6
53.6
[31]
HKUST-1
95/40
5.6/1.2
106
48.1*
50.7*
[24]
MIL101(Cr)
90/40
5.6/5.6**
939
46.1-47.2
45.13
[34]
MIL100(Fe)
90/40
5.6/5.6**
400
--
43.3-44.4
[35, 37]
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Stefan K. Henninger et al. / Energy Procedia 30 (2012) 279 – 288
3. Conclusions and outlook
A broad variety of adsorption materials, which can be used for heat transformation (heat pump,
cooling) and heat storage applications, exists. In particular, advanced materials such as inorganic
crystalline frameworks (AlPOs and SAPOs) have been developed in the last decade and are now
implemented in components and systems that are starting market deployment in heat pumps and chillers.
Novel materials based on metal-organic frameworks are still investigated on a laboratory scale, but show a
very promising potential for the application in the heating/cooling and storage sector. The many degrees
of freedom in the composition and molecular geometry of these materials open a huge range of adjusting
possibilities regarding their properties to particular boundary conditions needed in different applications.
However, further R&D work is still needed in order to develop materials which are long-term durable and
survive hundred thousands of cycles without significant ageing, which is required for the application in
periodically working heat pumps and chillers. These new materials are also promising candidates for long
term thermal storage applications due to their extremely high water uptake and low embodied energy,
which is crucial for applications with a low number of cycles within their lifetime, like e.g. seasonal heat
storage.
Acknowledgements
Funding by the Federal German Ministry of Economics and Technology (BMWi) under grant 0327851
is gratefully acknowledged.
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