EVALUATION OF A ZEOLITE-WATER SOLAR ADSORPTION REFRIGERATOR

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Abstract.-
This paper presents some of the experimental evaluations of a prototype solar refrigerator, based on an
intermittent thermodynamic cycle of adsorption, using water as refrigerant and the mineral zeolite as
adsorber. This system uses a mobile adsorber, which is regenerated out of the refrigeration cycle and no
condenser is applied, because the solar regeneration is made in the ambient air For the regeneration, a
SK14 solar cooker is considered. The cold chamber, with a capacity of 44 liters, is aimed for food and
vaccine conservation. The objective is to analyze the advantages and disadvantages of the eventual use
of this refrigerator in rural regions of Peru, where no electricity is available. On the bases of the results
obtained, a new prototype of refrigerator for rural regions is designed, based on the same thermodynamic
cycle, but including changes in design and operation.
1. INTRODUCTION
The use of sorption processes to produce refrigeration
has been extensively studied in the last twenty years as a
technological alternative to vapor compression systems.
Several theoretical and experimental studies
demonstrated that sorption refrigeration systems,
especially those using solid-gas heat powered cycles, are
well adapted to simple technology applications. They can
operate without moving parts and with low-grade heat
from different sources such as residual heat or solar
energy. The two main technologies concerning the solidgas
sorption concept are the adsorption and the chemical
reaction, including metal hydrides. The similarities and
differences between these systems, as well as the
advantages and disadvantages of each one are extensively
described by Meunier (1998).
Refrigeration is an interesting application of solar
energy because the incident radiation and the need for
cold production both reach maximum levels in the same
period. In developing countries, solar refrigeration is an
increasingly acknowledged priority in view of the needs
for food and vaccine preservation and due to the fact that
solar energy is generally widely available in these
countries. Different solar refrigeration systems using
sorption processes have been proposed and tested with
success. In relation to the solar adsorptive refrigeration
systems, different types of solid-gas were considered. The
zeolite-water and silica gel-water pairs were chosen for
cold storage, while the activated carbon-methanol pair
was chosen for ice production (Leite, 1996). The
activated carbon-ammoniac pair was also employed for
different refrigeration applications using solar energy.
The adsorptive systems development is still limited by
the adsorber/solar collector component cost, and by the
intermittence of the incident solar radiation, which makes
it difficult to be competitive with conventional
compression systems.
In the present work the description and the operation of
a solar adsorptive prototype refrigerator using the zeolitewater
pair is shown. The system operates under an
intermittent cycle, without heat recovering, and is aimed
to regenerate the adsorber with solar energy, using a
SK14 solar cooker. The adsorber is mobile and is
regenerated out of the refrigerator. No condenser is
applied because the solar regeneration is made in the
ambient air.
The purpose of the refrigerator is food and vaccine
conservation in rural areas of Peru, where no electricity is
available.
2. CHARACTERISTICS OF THE ADSORBENT– ADSORBATE PAIR
The choice for the working fluid – the adsorbate –
depends on the evaporator temperature and must have
high latent heat of evaporation and small molecular
dimensions to allow an easy adsorption.
The prototype is aimed for cold storage, using water as
adsorbate., whose most important property is the high
enthalpy of vaporization (2438 kJ/kg at 25ºC). The
pressure necessary to obtain temperatures around 0 ºC is
about below 6 hPa.
With water as adsorbate, zeolite is a very suitable
adsorbent. This material is basically porous aluminum
silicate that can be found raw or synthesized, is
innocuous, well available and is cheap. Zeolite is widely
used in industrial applications, especially in hydration
processes. In our case, spherical pellets of synthesized
Zeolite of 4 mm average diameter are used (Fig. 1). For
30 ºC ambient temperature, the maximal adsorption
capacity of the zeolite-water pair is about 0.3 kg of
adsorbate/kg of adsorbent. The free energy for the
desorption of water is about 1800 kJ/kg (from X = 0,3 to
X = 0,05), (Hauer and Laevemann, 1998), resulting in a
C.O.P of about 0,25. To regenerate this adsorbent,
temperatures around 200 – 300ºC are necessary. These
temperatures can be reached with a SK14 solar cooker or
with a CPC solar collector with weak concentration.
Fig.1. Pellets of synthesized zeolite (from Zeo -Tech,
Germany)
3. FUNCTIONING PRINCIPLE
The refrigeration system is based on an intermittent
cycle, without heat recovering. This cycle consists of two
typical stages: the cooling stage, characterized by the
adsorption process, when the evaporation of the working
fluid (the adsorbate) takes place, and another consisting
of the regeneration of the solid medium (the adsorbent)
by solar energy, when the adsorbate is condensed.
The solar refrigerator for cold storage is basically
composed of an evaporator positioned at the top of a cold
chamber connected to an adsorber (a cylindrical steal
chamber containing zeolite) and a SK14 solar cooker for
regenerating the adsorbent. Its principle of functioning is
shown in figures 2 and 3.
The refrigeration process begins when the adsorber
chamber, with dry adsorbent, is connected with the
evaporator, containing the adsorbent, and the pressure in
the system is lowered below 2 hPa. At that moment, the
evaporation takes place in a very quick process, attaining
temperatures bellow 0 ºC, when the solid-gas equilibrium
is reached and remaining at these temperatures for the
whole evaporation period. When the adsorber is
saturated, it is disconnected from the evaporator (letting
in air at atmospheric pressure) and put in the solar cooker
for regenerating.
Fig.2. Scheme of solar cooler (from EG-Solar)
Fig.3. Scheme of functioning of the zeolite-water solar
refrigerator.
4. DESCRIPTION OF THE PROTOTYPE
The actual prototype of the solar refrigerator (Fig. 4)
was constructed by EG-Solar, in Germany, and donated
to the Renewable Energy Center of the National
Engineering University (CER-UNI), in Lima, Peru, to be
tested. The adsorber of this refrigerator is mobile, to
allow the regenerating process out of the system, by using
a solar SK14 cooker. Therefore, no condenser is used
because the desorbed water is transferred to the ambient
air. For each cycle, it is necessary to evacuate the system
with a vacuum pump and to restore the water. The
evaporator of stainless steel has the shape of a rectangular
box with a heat transfer area of 0.15 m2 and a capacity to
contain up to 1 liter of water. It is positioned inside the
cold storage chamber at the top, fixed to the lid. The
external dimensions of the refrigerating chamber are 0.71
x 0.56 x 0.49 m and its useful capacity is of 44 liters. The
adsorber is composed of a vacuum tight stainless steel
cylinder containing a removable vessel with 4.2 kg of
zeolite (Fig. 5) and can be connected with a vacuum
hosepipe to the evaporator.
Fig.4. Zeolite-water refrigerator. The adsorber
chamber, at right, can be opened, in order to remove the
vessel with zeolite
Fig.5. Metallic vessel containing the zeolite.
In figure 5 the whole refrigeration process is shown in a
isosteric diagram. At the end of the refrigeration cycle, at
point “B”, the system is opened, loosing the vacuum, in
order to extract the vessel with the zeolite and to
regenerate it (drying it at high temperature in a stove or
solar cooker). After regeneration of the zeolite, it is put
again in the adsorber chamber and with a mechanical
pump the vacuum is restored below 2 hPa.
Fig.6. Isosteric diagram of the EG-SOLAR prototype
refrigerator (squematic);; AB: isobaric adsorption, CD:
isobaric desorption, DA: restoring of vacuum (adapted
from Zanife T. (1991)).
5. EXPERIMENTAL RESULTS
5.1 Regeneration process
Figure 7 shows drying curves of the used zeolite (from
Zeo-Tech), obtained at atmospheric pressure with small
samples of zeolite put at different temperatures in an
electric stove. From these curves we conclude that it is
not recommendable to reduce the humidity from its initial
30-35 % (dry bases), at saturation, to below about 5%.
Further drying would result only in a small improvement
of the capacity of the zeolite to adsorb water, but at the
expense of a big amount of additional energy and time
required.
We conclude therefore that 250 °C is an appropriate
regeneration temperature. At this temperature one gets in
2,5 hours zeolite with 5 % humidity (with 84% of the
water extracted). At 200 °C, one would need 6 hours to
reduce the humidity to 10 % (extracting 66 % of the
water).
Fig. 7. Drying curves of zeolite at different temperatures
(humidity in dry bases)
This high regeneration temperature of zeolite is a
shortcoming of this adsorber, making it difficult to
regenerate it with simple solar equipments. Originally,
the EG-Solar refrigerator was thought to use the SK14
solar cooker, disseminated world wide through EG-Solar.
But all our trials in Lima to dry the zeolite in the adsorber
vessel putting it in the focus of the SK14 solar cooker
failed, mainly because even very slow air flows around
the vessel did not permit to heat the adsorber vessel up to
the required 250°C. Therefore, we used only an electric
furnace to regenerate the zeolite, postponing to find an
appropriate solution for this stage of the solar
refrigeration cycle.
5.2 Cooling process
First tests with the prototype EG-Solar refrigerator were
made using a mechanical hand pump, but experiences
showed that its use was unsuitable for obtaining the
required pressure level of 2 hPa. Therefore, a small
electrical vacuum pump, of 92 W and working at 12
Volt DC, was used. The operation time of this electrical
pump to obtain the low pressure in the system was of
about 10 minutes It was thought that the energy required
for this pump could be provided by a small photovoltaic
module (Ramos and Horn,2001).
Fig.8. Different temperatures during cooling cycle; initial
water amount: 0.5 kg
The results of the cooling experiments are shown in
figure 8 and 9. Figure 8 shows the temperatures at
different places in the system (cold chamber, evaporator,
adsorber, ambient) and figure 9 gives the temperature and
pressure in the evaporator for different initial water
amounts.
A temperature of 0°C was reached in the evaporator in
about 20 minutes, maintaining the temperature below 0°C
for about 24 hours (or less, if less water was initially in
the evaporator). The coldest temperature reached in the
evaporator was minus 8,7 °C, with a pressure of 1,8 hPa
(the saturation pressure of ice at this temperature is
3hPa). In the absorber, the temperature raised up to 45°C
in 90 minutes, falling then slowly to room temperature
(about 28°C), while, at the same time, in the evaporator
the temperature increases in 3,5 days to room temperature
and the pressure increases up to 6 hPa.
Fig. 8. Temperature and pressure in evaporator for
different initial water amounts: 0,3 kg, 0,4 kg, 0,5 kg.
After some hours, during the period when the
temperature and the pressure again rise in the evaporator,
one observes again a falling of these parameters during
some time. The moment this happens depends of the
initial amount of water in the evaporator, and does not
happen with initially 0,3 kg of water, as can be seen in
figure 8. We think that this phenomena is caused by an
initial partial obstruction with ice of the tube connecting
the evaporator with the adsorber, if an initial grand
amount of water in the evaporator violently evaporates,
and subsequently cools below freezing, even in the hose,
reducing so the possibility of the zeolite to adsorb water
vapor in the evaporator. After some time this obstruction
disappears and the adsorption process speeds again up,
resulting in a new increase of the cooling during some
time.