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gas, have been introduced in markets all over the developed worlde in the US, Japan, France, Italy, Spain and more recently in
Portugal. As to adsorption systems e in spite of several studies that
demonstrate their technical feasibilities e, there exist only exper-
imental units, though stand considerable chances of becoming
economically viable, especiallywhere thermal comfort applications
are concerned [11].
This paper presents some of the adsorption fundamentals and
also the energy equations for adsorbers. Then it puts forwards the
characterization and the pre-dimensioning of a 20 kW air
conditioning unit based on both the adsorption process and solar
energy. Cold water is produced during night-time in an activated
carbonemethanol adsorption chiller and then stored into a tank
in order to supply cooling air during day-time. Solar energy is
used for regenerating the adsorbent medium by means of highly
efficient flat-plate collectors. Thermal energy is stored in a hot-
water tank so as to feed the chiller at night. The central unit was
created to provide thermal comfort to four adjoined laboratory
rooms, whose total area is 110 m2
.
2. Adsorption fundamentals
Adsorption constitutes a solid sorption process by which the
binding forces between fluid molecules (adsorbate) and the solid
medium (adsorbent) derive from an electrostatic origin or from
dispersionerepulsion forces. It is an exothermic process as a result
of the gaseliquid phase change. The energy liberated during
adsorption is called isosteric heat, and it depends on the nature of
the adsorbenteadsorbate pair.
Anumber of state equationseknownas isostherms of adsorptione
are proposed. These functions correlate the temperature T, the pres-
sure P, and the concentration of the adsorbed phase a,sothat f(T, P,
a) ¼ 0. The main isotherms of adsorption are: (a) Henry’s law, valid
only for weak concentrations; (b) Langmuir’s approach, which takes
up adsorption on monomolecular layers where there is a dynamic
equilibriumbetween the phases; (c) Gibbs’ theory,which is based on
the perfect gas equation, where the adsorbate is treated as a micro-
scopic and bi-dimensional form; and (d) Eucken and Polany’stheory,
the so-called potential adsorption theory, in which it is assumed that
within the space around each solid, it is possible to find some iso-
potential surfaces restricting the adsorbate that is adsorbed at pres-
sures (or concentrations) lower than those corresponding to the
potential value, being such isopotential surfaces specificforagiven
solid surface [12].
Dubinin and Radushkevich have proposed the micropore volume
filling theory, which is related to Eucken-Polanyi potential theory.
The DubinineRudushkevich isotherm describes adsorption as
a single type of uniform pores that is similar to Langmuir-like local
isotherms in adsorption on energetically heterogeneous solids. This
theory was later expanded by Stoeckli, allowing it to describe
adsorption on energetically heterogeneous solids with a contin-
uous distribution of pore sizes. Leite [13] puts forward a detailed
analysis of the thermodynamics of adsorption and its different
isotherms.
In general, all microporous materials are adsorbent and are
characterized by their high porosity. Their structures have pores
with diameters smaller than 20 A. The most common absorbents
are silica gel, activated carbon, zeolite and alumines. Zeoliteewater
and activated carbonemethanol are one of the most adsor-
benteadsorbate pairs utilized in refrigeration systems. Methanol is
easily desorbed from activated carbon when heated, whereas in
zeolite, the water is sustained for much longer. Thus, the activated-