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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
In order to increase the heat transfer coefficient between the substrate and the adsorbent
two techniques are in the focus of research. One option is to glue the adsorbent onto the
heat exchanger (see Zhu [58], and the patent of SorTech [76]. Moreover, several working
groups work on the principle to directly crystallize adsorbent onto the heat exchanger
substrate (Coronas [50] Beving [51], Yang [52], Scheffler [54], Tatlier [55] and SorTech [77]).
3.3.2 Working pairs
A wide variety of working pairs are possible solutions for application in adsorption chillers.
Wongsuwan [56], Srivastava [57] and Henninger [60] give an overview of possible working
pairs. Most working pairs use water as the refrigerant due to the highest evaporation
enthalpy of water compared to other possible working fluids. Furthermore, water is not toxic
and easy to handle. The disadvantage of water is its limitation to applications above 0°C. For
ice making or refrigeration below 0°C, methanol or ammonia as refrigerant is used with a
variety of solid adsorbents; the main focus is on activated carbons.
The following section gives an overview of classical and modern adsorption materials as well
as the latest developments. Here the first mentioned substance is the refrigerant, while the
second mentioned is the adsorbent. With regards to the application in heat transformation
processes, the main scope of research is the stability of materials – which means
hydrothermal and mechanical stability of the pure material and possible composites and new
synthesis routes, with regards to production costs. The focus is on template free syntheses
and general investigations on the water vapour adsorption characteristics.
3.3.3 Classical working pairs
Classical working pairs are water/zeolites, water/silica-gel and methanol/activated-carbon. A
comparison between water/zeolite, water/silica-gel and methanol/activated-carbon can be
found in San [72] or in the review of Dieng [71].
Zeolites are hydrated alumino-silicates which consist basically of SiO 4 and AlO 4 . The
coordination of these tethradrones form secondary building units which composes a 3-
dimensional framework. Within this framework water can be adsorbed. In the “Atlas of Zeolite
Framework Types” [62] 133 lattice types are known but the corresponding webpage now lists
179 different zeolite lattice types (June 2008). Hauer [61] and Henninger [60] give a good
overview of zeolites and their properties.
Silica gels are solid, porous silicic acids produced by the synthetic dehydratisation of a hydro
gel. Therefore, silica gel consists of more than 99% of SiO 2 . The porosity can be varied by
the control of temperature and ph-value. Material properties can e.g. be found in Núñez [63],
Ng [67] and Aristov [59].
Using methanol as the refrigerant, the solid adsorbent is in most cases activated carbon.
This offers the possibility of refrigeration below 0°C. Physical properties are given in Carrott
[68]. Wang [69] focuses on the heat and mass transfer in methanol/carbon working pairs.
Stoeckli [73] also gives data about water/activated carbon.
3.3.4 Modern working pairs
Modern working pairs are:
• water/selective-water-sorbents (SWS),
• water/aluminium phosphates (AlPOs),
• water/silica-aluminium phosphates (SAPOs) and
• water/metal-aluminium phosphates (MAPOs).
Selective-water-sorbents (SWS) and selective-water-sorbents-like materials are a
composition of an adsorbent and a salt, which means in principle that adsorption and
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