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    nineAstakhov equation.
    3. Properties of the adsorbent and the adsorbate
    For cooling purposes, the adsorbent should have high adsorptive
    capacity at ambient temperature and low pressures, maintaining
    small capacity of adsorption at high temperatures and pressures. The
    required temperature at the evaporator depends on the adsorptive
    capacity under small pressures; it is the property that allows the
    adsorbent, at a given temperature, to retain vapors from a fluid at
    lower temperature.Onthe otherhand, themore intense this property
    is, the higher the level temperature for regenerating the adsorbent.- it can evaporate at a temperature largely below 0 
    C (its
    melting point is e 94 
    C);
    - its enthalpy of vaporization is high (w1200 kJ/kg, at e 5 
    C);
    - itsmolecule is small enogh (4 104
    mm) to be easily adsorbed
    into micropores, with a diameter smaller than 2  103
    mm;
    - its normal boiling point (w65 
    C) is much higher the room
    temperatures;
    - its working pressure is always lower than the atmospheric one,
    which means a safety factor in case of leakage.
    The activated carbon has been selected as the adsorbent
    material because of its affinity with methanol; the thermo physical
    properties are those of the AC-35, produced by CECA (France).
    These data and those concerning the thermodynamic equilibrium
    e the isosters e of the AC-35/methanol pair are well known [20].
    The activated carbon AC-35 is obtained from pinewood by heating
    it to 950 C in the presence of water vapor. It has a significant
    micropores volume, the empty spaces of which correspond to 78%
    of the total volume. The adsorbent bed is made up of cylindrical
    grains of 2 mm diameter and is 3 mm long in average. The
    maximum adsorption capacity of the activated carbonemethanol
    pair is about 0.3 kg of absorbate/kg of absorbent. According to
    experimental studies, the regenerating temperature of the acti-
    vated carbon AC-35 at this temperature ranges from 65 Cto
    100 C [21].
    4. Functioning principle and the ideal thermodynamic cycle
    The adsorption refrigeration cycle consists of two well-defined
    stages: one is described as the adsorber cooling, with its conse-
    quent adsorption process, when the evaporation of the working
    fluid (the adsorbate) takes place. The other stage consists of the
    solid medium (the adsorbent) regeneration, when the adsorbate is
    desorbed and condensed (Fig. 1a). The ideal thermodynamic cycle
    can be represented by two isosters (isolineswith constant adsorbed
    phase concentration, a) and two intercalated isobars, as shown in
    Fig. 1b.
    The adsorber cooling corresponds to the isosteric process 1e2,
    depending on the ambient conditions. This process continues until
    the adsorber pressure reaches itsminimumvalue (point 2), when it
    becomes equal to the evaporator pressure. At this point, the
    adsorption process starts and prolongs until its temperature rea-
    ches the minimum value (point 3). Then, the adsorber is heated,
    corresponding to another isosteric process (3e4), until its pressure
    reaches a maximumvalue (point 4). Desorption starts at this point,
    and goes on until the adsorber temperature reaches its maximum
    value (point 1), completing, in this way, the cycle.
    5. Energy equations
    The energy equations related to the adsorber, which will be
    given next, correspond to a multi-tubular system, whose outer
    surface exchanges heat with the water coming fromthe hot storage
    tank or from the water supply network, depending on the stage of
    the cycle. The adsorbent occupies an annular space delimited by the
    wall of the tube and a metal screen through which the adsorbate
    flows.
    For the heat transfer in the adsorbent medium, the following
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