IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
3 Adsorption Chillers
Tomas Núñez (Fraunhofer ISE)
3.1 General description of the technology
Adsorption systems are very similar to absorption systems. The main difference is that the
sorbent is a solid and not a liquid. The adsorption process is a physical process where the
molecules of the working fluid or refrigerant are bound to the surface of the adsorbent by
Van-der-Waals forces. Adsorbents used in adsorption chillers are very porous technical
adsorbents like silica-gel, zeolites and activated carbons. The high porosity and thus
extremely large internal surface of the adsorbents (several hundreds of m² per gram of
material) allows the adsorption of a significant amount of refrigerant.
The main characteristic of the adsorbent-refrigerant pair is the amount of adsorbed
refrigerant per unit of dry adsorbent.
m
x =
m
refrierant
adsorbent
= x( T,
p)
The loading x is a function of refrigerant pressure and temperature and is represented in
isosteric diagrams.
Figure 9 shows a typical isosteric diagram of the pair silica-gel / water.
The chilling cycle is carried out through four processes (Figure ):
1. isosteric heating: in section 1, the loaded adsorbent is heated at constant maximum
loading x max . The equilibrium pressure in the system increases until it reaches the
condensation pressure of the refrigerant at condensation temperature T cond .
2. isobaric desorption: the adsorbent is further heated up und thus desorbed at constant
pressure. The desorption ends as soon as the maximum temperature T max provided
by the external heat source is reached. This process is endothermic taking up the
desorption heat. The released refrigerant is condensed in the condenser.
3. isosteric cooling: the desorbed material is cooled down until the equilibrium pressure
reaches the evaporation pressure of the refrigerant in the evaporator.
4. isobaric adsorption: further cooling down of the adsorbent results in the adsorption
process; refrigerant is evaporated in the evaporator, thus producing the cooling effect,
and taken up by the adsorbent. The process ends as soon as the temperature of the
adsorbent reaches the heat rejection temperature and closing the cycle.
The choice of the adsorbent depends on the affinity to the refrigerant and the envisioned
application. The cooling energy Q cold in one cycle is proportional to the evaporation enthalpy
h evap of the refrigerant and the loading difference x max – x min . These loading limits are given by
the operation conditions: the minimum loading is given by the maximum desorption
temperature T Des and the condensation temperature T cond , the maximum loading by the
evaporation temperature T evap and the heat rejection temperature which is identical to the
condenser temperature T cond . The driving heat is given by the heat necessary for the isosteric
heating Q1 and isobaric desorption Qdes. Thus, the thermal COP of an adsorption chiller is
calculated as a first approximation through:
COP
th
Q
=
Q
cold
drive
h
=
evap
⋅ ( x − x
Q + Q
1
max min
)
Des
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