Solar air conditioning – An efficient adsorption cooling process

Research Associate: Dr.-Ing. Ana Marković, Dipl.-Ing. P. Günther, Dipl.-Ing. (FH) H. Aschenbrenner

Duration: 10/2009 – 08/2011

Cooperation partner: BEHR GmbH & Co. KG, Stuttgart

Solar air conditioning is a technology which converts heat collected from a solar thermal collector into cooling. This natural synergy between cooling demand and heat resource availability is ideal for air conditioning applications. The solar heat serves as a fuel saver by replacing conventional fuels that emit CO2 and other pollutants, whenever sun shines. Using such a technology electricity demand for cooling is reduced and environment is safer [1, 2].

How does solar air conditioning work?

Summer sun, which heats up offices, also delivers energy to cool them. Solar thermal collectors integrated on the roofs of the buildings provide thermal energy by concentrating the sun’s energy on plate collectors and heating the recirculated heat transfer fluid within the system. The generated heat is then used in conjunction with e.g. absorption or adsorption air conditioner to provide a renewable source. Both ab- and adsorption processes are thermally driven processes, the only difference is that, in absorption process vapor (refrigerant) is taken by liquid while in adsorption process, vapor is attached on the porous solid material. Currently, most of solar cooling systems are driven by absorption conditioners. However, due to simple vapor-solid operation, adsorption conditioners seem to provide a promising alternative.

The solar air conditioning process is illustrated in the figure below.


Figure 1: Working principle of solar air conditioning process

Project description

This project is based on development of new adsorption air conditioner in the collaborative work with the industry partner Behr, specialist for automotive air conditioning and engine cooling systems. Behr proposed new design of a solar air conditioning plant based on adsorption, which has to be optimized for commercial application. Focus of the research activities at the institute is to determine complex mass and heat- transfer in the adsorption air conditioner. These results will help to optimize suggested design and operation conditions which will gain in development of new energy efficient cooling plant.

The main role in the adsorption air conditioning plant has adsorbent/adsorbate working pair. Adsorption characteristics of the solid-vapor pair are one of the essential parameters that effect the system performance. It is thus indispensable to realize and understand the adsorption isotherms, adsorption kinetics and heat of adsorption of the employed adsorption pair. Extensive research [3, 4] propose active carbon as good adsorbent due to high adsorption capacity, high surface area and large pore volume affinity and methanol as adsorbate due to high vapor pressure. This system seems to be a promising adsorption pair for cooling applications.


The focus of this project is determination of rele vant thermo-physical properties of adsorbent and the adsorbent-adsorbate pairs: adsorption equilibria, adsorption kinetics, diffusion properties, heat conductivity and specific heat capacities. Afterwards these results can help to obtain different design aspects of adsorption cooling plant, which can enhance the operation performance and to eliminate the bottle-neck effects.

The present study is dealing with experimental investigation of adsorption isotherms and adsorption kinetics of active carbon-methanol pair using standard volumetric method. The simplified illustration of the adsorption apparatus is presented in Fig. 2 [5].


Figure 2: Experimental set-up for adsorption equilibrium measurements [5]

Important part of this project is also modelling of the physical processes (adsorption, heat transfer, mass transfer, phase change). Through the combined theoretical and experimental results it is possible to estimate all required parameters needed to understand process better and to identify the process limitation.

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  1. R.Z. Wang, R.G. Oliveira
    Adsorption refrigeration – An efficient way to make good use of waste heat and solar energy
    Prog. Energy Combust. 32 (4), 2006, 424-458

  2. T. Núñez
    Charakterisierung und Bewertung von Adsorbentien für Wärmetransformationsanwendungen
    Dissertation, Fakultät für Physik der Albert-Ludwigs-Universität in Freiburg (Breisgau), 2001

  3. O.StC. Headley, A.F. Kothdiwala, I.A. McDoom
    Charcoal-methanol adsorption refrigerator powered by a compound parabolic concentrating solar collector
    Sol. Energy 53 (2), 1994, 191-197

  4. H. Jing, R.H.B. Exell
    Adsorptive properties of active charcoal/methanol combinations
    Renewable Energy 3 (6-7), 1993, 567-575

  5. A. Gorbach, M. Stegmaier, G. Eigenberger
    Measurements and modelling of water vapor adsorption on zeolite 4A-equilibria and kinetics
    Adsorption 10 (1), 2004, 29-46

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