EXPERIMENTS ON SOLAR ADSORPTION REFRIGERATION USING ZEOLITE AND WATER

Siegfried Kreussler and Detlef BolzLaboratory for Solar Energy, University of Applied Sciences Luebeck, Stephensonstr. 3, D-23562 Luebeck, Germany

Phone +49 0451 500 5167, Fax +49 0451 500 5235, e-mail kreussler@fh-luebeck.de

Abstract - In developing countries there is a growing interest in refrigeration for food preservation. Especially in ruralareas, simple solar refrigerators working independently, i.e. not being provided with electrical energy, would be veryvaluable. Mechanical refrigerators powered by solar cells are available, but are too expensive. In the last years,adsorption refrigerators using water as refrigerant and zeolite as adsorber have been successfully developed. Theobjective of our project is the use of solar thermal power to regenerate the adsorbent. First, a laboratory apparatus wasconstructed. The water vapor in the evaporator is adsorbed by zeolite and the water cools down to 0 oC. The zeolite islocated in a vacuum tube solar collector which can be heated up to 180 oC by artificial sunlight. In doing so, the water isdesorbed and condenses at ambient temperature. After the collector cools down the cooling process can start again.Thisprocess was investigated systematically and quantitatively with different kinds of zeolite and silica gel. Typical resultsare: At 150 oC heating temperature there is a cooling energy of 250 kJ per kg of zeolite. These experimental resultsconstitute the basis for the simulation of a prototype. A storage volume of 125 l could be cooled down by the solarpower gained from a collector area of 3 m². Such a refrigerator is simply made of glass and metal, and apart from that,only contains the "natural“ materials water and zeolite. Therefore it could be built inexpensively in developing countriesand produce cooling power without pollution.

1. INTRODUCTION

In southern developing countries up to 30 % of food is spoiltdue to lack of refrigeration during transport and storage becauseconventional refrigeration systems are not affordable.Furthermore, electric power often is not available in thevillages.

In areas with abundant sunshine, solar radiation is the mosteasily accessible energy source. Solar refrigerators can workindependently of the electrical network. In Africa about 1800solar refrigerators are used to store vaccines (WHO). Usually,cold is produced by a vapor compression cycle, which is drivenby electric power gained by solar cells. However, theinvestment of about US $ 2000 is high and the populationcannot afford such systems. In addition, the high-techproduction of solar cells and the service of these refrigeratorsseems to be difficult in developing countries.

Therefore the solar refrigerator must be extremely simple andreliable; the local industries should be able to produce andrepair it. In this respect adsorption refrigerators look promising.In the simplest case they consist of two vessels connected by atube. They need no mechanical or electrical power, and aredriven by low temperature thermal energy, which is easilygained by solar collectors. The technology of solar collectors issimpler than that of solar cells and collectors can be built "in agarage“.

Especially simple is an adsorption cycle using the solidadsorbent zeolite and the refrigerant water. Both materials arecheap and neutral to the environment.Early experiments on this were done in 1981 by Guilleminot. Inthe last years such adsorption refrigerators have beensuccessfully developed by Maier-Laxhuber and Schwarz. Theserefrigerators are driven e. g. by the thermal waste energy ofcombustion engines.

The objective of our project is the use of solar thermal power toregenerate the adsorbent. In order to build this system simply,the adsorbent material shall be contained in the solar collector.This way additional heat exchangers, tubes, valves and so onare avoided.

An experimental rig was built to determine the desorptiontemperatures which can be reached with such a solar collector.Are these temperatures sufficient? How is the coefficient ofperformance influenced by the ambient temperature and thecooling temperature? Which cooling power can be achieved andwhich kinds of adsorbents are favorable? Could air in thesystem cause problems? All these experimental results shouldenable us to simulate and construct a prototype which is bestsuited for use in developing countries with great amounts ofsunlight.

2. PRINCIPLES

2.1 AdsorbentZeolite is a mineral consisting of SiO2-, AlO2- groups andalkali-ions. It is capable of adsorbing water vapor and other gasmolecules in the cavities of its complex crystal structure. Awater content up to 25 % (kg water/kg zeolite) can be adsorbedand then the zeolite is heated up by the adsorption enthalpy.

These processes are reversible. By heating up the zeolite to 150-250 oC the water can be desorbed. Zeolite is producedsynthetically and the crystalline powder is pressed to pellets ofabout 0.5 mm diameter. Huge quantities of zeolite are producedby chemical industries and used as molecular sieves or forwashing detergents.

Name ofproduct:

Y-zeolite Bayer-3A

Grace-4A AF 25

Material: zeolite zeolite zeolite silica gelCrystalstructure:

Y A A ---

Alkali ions: Na+ K+ Na+ ---Width ofpores in nm:

0.7-0.9 0.3 0.4 0.25

Producer: SlovnaftCzechRepublic

Bayer Grace Engel-hardGer-many

At minimum, the price is US $ 0.5 per kg. Zeolite does notharm the environment. Obviously, the refrigerant (water) isneutral to the environment and is cheap.

2.2 Adsorption cycleIn principle, the refrigerator may be constructed out of twovessels, the evaporator and the adsorber, which are connectedby a tube and a valve (Fig.1). The whole system is made devoidof air. In the evaporator only the vapor pressure of the water ispresent. When the valve is opened, the water vapor in theevaporator is adsorbed by the zeolite and thus the pressure isreduced. The water starts boiling and cools down toapproximately 0 oC. Even undercooled water and ice can beproduced. When the adsorbent is saturated, the refrigerationprocess stops. Now the adsorber vessel is heated up to 100-200oC. The desorbed water vapor condenses in the connecting tubeor in the evaporator at ambient temperature. After several hoursthe valve is closed and the adsorber cools down to ambienttemperature. Then by opening the valve the refrigerationprocess can start again. In practice the zeolite can beregenerated during the day using solar energy and at night therefrigeration process can be started. The thermal insulation andthe thermal capacity of the refrigeration chamber must besufficient enough to avoid a big increase in temperature duringthe day. By means of two zeolite vessels a continousrefrigeration process may be realized: one is regenerated, theother is used as adsorber.

Heat of condensation(30°)

Heat ofevaporation (Cooling energy)(0°)

Vapor

Desorption

Solar regeneration

Condensation

Zeolite

Adsorber

Vapor

Water

Refrigeration

Evaporation

Vapor

Solar thermal energy (150°)

Heat ofadsorption

(50°)

Fig. 1: Principle of adsorption cycle

3. THEORY

In a closed vessel filled with zeolite the pressure of water vaporpw at equilibrium is a function of the temperature Tz[K] and thewater content c of the zeolite (c[%]= adsorbed mass of water[kg]/ mass of adsorbent [kg]). If c is constant, then ln (pw, Tz)becomes a nearly linear function of the form (-1/Tz). Thesefunctions have been measured for different water contents anddifferent adsorbents by Henning and by Maier-Laxhuber andthey constitute the basis of our calculation. When the desorptionstarts the zeolite is nearly saturated with water: "high“ load ch.The desorbed water condenses in the evaporator or in acondenser. There the vapor pressure is equal to the saturationvapor pressure of water ps at ambient temperature Ta. Theprocess stops when equilibrium is reached. Then the "low“ loadcl of the zeolite causes a vapor pressure in the adsorber which isequal to the pressure in the condenser.

pw(cl,Tz)=ps(Ta) (1)

Both functions are known, Tz and Ta are measured, so theequation can be solved for cl . During refrigeration the watervapor is adsorbed by the zeolite while at ambient temperatureTa. The adsorption stops when the zeolite is saturatedcontaining the "high“ water content ch. At equilibrium thepressure in the adsorber is equal to the saturation vapor pressureof the refrigerant at the temperature Tc ("cold“.):

ps(Tc)=pw(ch,Ta) (2)

From this equation ch can be calculated. In the adsorption cyclethe mass of water mw is adsorbed by the mass of zeolite mz:

mw=mz (ch-cl) (3)

If mw is evaporated, the refrigerant is cooled down by thecooling energy Qc:

Qc= r mw (4)r: enthalpy of vaporization

Using these equations the specific cooling energy per mass ofadsorbent qc was calculated as a function of the heatingtemperature Tz. In this calculation the cooling temperature andthe ambient temperature were set constant: Ta= 30 oC and Tc= 0oC. Fig. 5 and fig. 6 show the results for two kinds of zeoliteand silica gel. However, the resulting Qc is not exactly equal tothe maximum cooling power of the refrigerator, because thedesorbed refrigerant and the evaporator vessel must also becooled down by Qc.

4. EXPERIMENT

4.1 Experimental apparatusThe experimental setup is shown schematically in fig.2 and as aphoto in fig. 3. The sun simulator consists of three lamps with1200 W electric power ( bulb type Osram HGI-T400/DH).The spectrum is similar to the solar spectrum and the incidentirradiation in the plane of aperture is 300 W/m². This radiationis focused onto a vacuum tube collector by a parabolic mirror.

The cylindrical solar absorber is black in the visible light buthas low emissivity for infrared radiation. The solar absorber isfilled with zeolite pellets. In the axis of the cylinder a channel isheld free by a metallic grid for the streaming of vapor. Duringexperiments the radiation and the temperatures of the glass, ofthe solar absorber and of the zeolite are measured with thermalresistors. The evaporator vessel is made of glass and the boilingor freezing of the water can be seen. The water temperature ismeasured by a resistor. The water level in the evaporator can beread with the help of a scale. Because the total mass of water inthe apparatus is known, the adsorbed mass of water can then becalculated. The pressure is measured by a piezo sensor. Alltemperatures, radiation intensity and pressure data are storedevery 30 seconds by a datalogger. Only the water level has to beread by the operator.

4 .2 Starting procedure500 g of dry adsorbent pellets are soaked by a known mass ofwater and placed into the solar absorber. Then the vacuumpump is started and the zeolite is heated up by the solarsimulator. The desorbed water condenses in the condenserwhich is held at 10 oC and the water drops fall into theevaporator. One part of the water vapor is pumped out and thiswater condenses in a cooling trap and can be measured. Aftersix hours the valves are closed and the "sun“ is switched off.

Condenser

Solar collector containing zeolite

Water

Evaporator

V1

Coolingwater

Solar radiation

V2

Pressure gauge

Vacuum pump

Fig. 2: Scheme of experimental setup

In the following course of the experiment the vacuum pump isnot used again. When the collector has cooled down to ambienttemperature, the valve between collector (adsorber) andevaporator is opened and the first refrigeration process isstarted. By this procedure the desired status of the experimenthas been reached: The air is pumped out and the water mass inthe apparatus is known.

4.3 Experimental cycleThe experimental cycle corresponds to the proposed adsorptioncycle of the solar refrigerator working without a vacuum pump.As an example, the measured temperatures of one experimentare shown as a function of time in fig. 4. In the beginning, thezeolite was saturated with water (ch= 20 %). The solar collectorwas heated up by the solar simulator and after six hours thetemperature of the zeolite reached 175 oC. The water vaporpressure was 50 mbar. The condenser was not cooled and thewater vapor condensed at approximately 30 oC. During thisprocess the water content decreased to cl= 8 %. At t= 6 h thevalve was closed and the "sun“ was switched off.

At t=8 h the collector had cooled down and by opening thevalve the refrigeration process was started. Due to theadsorption enthalpy the temperature of the zeolite increased upto 70 oC, at which point the rate of adsorption decreased and theadsorber temperature slowly tended to ambient temperature.The refrigerant was quickly cooled down to - 3 oC (undercooledwater). Over time the cooling power decreased and due to theheat flow from the surroundings the temperature of therefrigerant slowly increased to 10 oC after 12 hours.

This experimental procedure was repeated for differentdesorption temperatures and different kinds of zeolite as well asone kind of silica gel.

5. RESULTS

During experiments, some temperatures were nearly constant.The cool temperature Tc of the refrigerant depends on the rate ofevaporation and the heat flow from the surroundings. Tc variedbetween 0-10 oC. According to our calculations, its influence onthe cooling energy is negligible. These values of Tc aresufficient for the cooling of food.

The condenser is cooled by the surrounding air and thecondenser temperature was always approximately Ta= 30 oC,slightly higher than room temperature (25oC).

The temperature of the zeolite increases in the beginning of theadsorption process, but later on it tends to Ta= 30 oC.

The maximum water content ch depends on Tc and Ta, both ofwhich were nearly constant in all experiments. The measured ch

lies between 16-20 % for all kinds of zeolite. The maximumwater content of silica gel was lower: ch= 16 %.

The low water content cl depends on the heating temperature Tz

and the constant condenser temperature. For all kinds ofexamined adsorbents Tz was varied between 90 and 180 oC.

S

V

P

C

Va

E D

P Parabolicconcentrator

S Solar collectortube

V Vacuum pump

C Condenser

Va Valve

E Evaporator

D Datalogger

Fig. 3: Photo of experimental setup.

There is nearly no desorption from zeolite at Tz=90 oC. AtTz=170 oC cl descends to 6-8%. Silica gel can be regenerated atlower temperatures: cl= 7 % at tz= 90 oC and cl= 2.6 % at Tz=170 oC. The most important result is the cooling energy Qc,which is equal to the evaporation enthalpy of the adsorbed massof water. In fig. 5 and fig.6 the experimental results for qc

(cooling energy/mass) are shown as a function of heatingtemperature.

The total pressure p in the apparatus was registered by thedatalogger. Generally when the adsorber is heated, p isapproximately 70 mbar, while the saturation pressure in thecondenser is 40 mbar at 30 oC. During adsorption p is lowerthan 2 mbar while the saturation pressure in the evaporator is i.e. 10 mbar at 7 oC.

The total pressure may increase due to leaks or desorption of airfrom inner surfaces. The air molecules are then adsorbed by thezeolite and the adsorption of water molecules is reduced.Furthermore, the boiling temperature of the refrigerant increasesand the rate of evaporation decreases. In order to test thisinfluence air was let in the apparatus by means of a valve. Apartial pressure of air pL= 10 mbar reduces the cooling powersignificantly and at pL= 20 mbar the cooling power is nearlyzero.

In addition, the efficiency of the whole system depends on theefficiency of the solar collector. The thermal capacity of thecollector was calculated according to the masses of metal,zeolite and glass. When the solar simulator was switched off,the solar collector cooled down and the temperature wasmeasured as a function of time.

With the known incident radiation as well as all the previousresults, the parameters of our solar collector could becalculated. The optical efficiency is η0=0.85 and the heat losscoefficient for the cylindrical surface of the collector is k=2.6W/(m²K) .

Fig. 4: Typical results of experimental cycle

Fig. 5: Theoretical and experimental results with zeolite

6. DISCUSSION

The cooling energy per mass, which has been calculatedtheoretically, is shown in fig. 5 and fig. 6 as a function ofheating temperature. In the same figures the experimentalresults are also shown. In experiment and in the theoreticalcalculation the cool temperature Tc is approximately 0-5 oC.The condensation temperature and the temperature of the zeoliteduring adsorption are Ta= 30 oC.

The theoretical cooling energy of the Y-zeolite is based on theexperiments of Maier-Laxhuber. The experimental results withthe Y-zeolite should fit to these results.

We did other experiments with Bayer 3A and Grace 4A. Thesekinds of zeolite have a width of the pores of 3 nm and 4 nmrespectively. Our theoretical result for Na-5A zeolite is basedon experiments of Henning. This kind of zeolite has a width ofthe pores of 5 nm and therefore the results should becomparable to our experiments with Bayer 3A and Grace 4A.

In all experiments the cooling energy only amounts toapproximately 70 % of the theoretical value. The theoreticalcalculations are based on the equilibrium water vapor pressureof zeolite which is reached after an "infinite“ time. However,the adsorption and desorption processes in this experiment werestopped after several hours. Therefore the adsorbed mass ofwater and the resulting cooling energy are lower than predictedby theory. Taking this into account, experiment and theorycorrespond satisfyingly. The dependence of cooling energy onheating temperature is similar in theory and in experiment.

Fig. 6: Theoretical and experimental results with silica gel

These results may be summarized as follows. If zeolite is usedas adsorber, a heating temperature of 100 oC is too low todesorb water, whereas temperatures higher than 200 oC do notincrease the cooling power any more. Hence desorptiontemperatures of 120-180 oC are necessary. The comparison ofthe results of the different kinds of zeolite shows that theexamined zeolite with Y-structure gives higher cooling powerthan the A-zeolite. That may be caused by the different size ofthe pores in the cystal structure.

In fig. 6 the theoretical and experimental results for silica gelare shown. The theoretical calculation is based on theexperiments of Henning. Our experiment was done with adifferent kind of silica gel and therefore the comparisonbetween theory and experiment can only be approximate. Here,temperatures of 90 - 110 oC are sufficient for desorption and themaximum of cooling power is reached at 150 oC desorptiontemperature. Thus, silica gel is to preferred as adsorber if thetemperatures are in the range of 100-150 oC. However, in ourexperiments the adsorption rate of water vapor by silica gel wassignificantly lower than the adsorption by zeolite. The resultingcooling power was lower and the temperature of the refrigerantonly went down to 10 oC in contrast to 0oC and below in thecase of zeolite.In contrast, higher cooling power is achieved with zeolite iftemperatures greater than 150oC can be reached.

7. PROTOTYPE

In the course of our work a lot of experimental data wasobtained which could only be described partially in thispublication. This experiment has shown us what pitfalls toavoid. The data together with our experience constitute the basisfor the following approximate simulation of a prototype. Thecooling chamber is a cube of volume 0.125 m³. It is thermallyinsulated by a layer of polystyrol which is 20 cm thick. Thetemperature in the cooling chamber is assumed to be 0 oC, theambient temperature 30 oC. By heat conduction 0.3 kWhthermal energy flow through the walls per day. In addition, 20kg of meat, having ambient temperature, are put into therefrigerator constituting a cooling load of 0.7 kWh. In total, thecooling load is 1 kWh per day.

The adsorption refrigerator is characterized by the followingparameters: The solar collector consists of four vacuum tubes of20 cm diameter, each 2 m long. The tubes together contain 16kg of zeolite. The area of aperture is 3 m² and parabolicreflectors behind the tubes focus the whole incident radiationonto the tubes. These parabolic trough collectors need not beadjusted during the day. The solar collector used in ourexperiment has an efficiency of 30 % at 150 oC. If we assume amediocre solar irradiance of 4 kWh/(m²d), we gain 4 kWhthermal energy per day. 2 kWh of which are used to heat thecollectors and the contained zeolite mass up to 150 oC. Thedesorption energy is higher than the evaporation enthalpy andthe remaining thermal energy of 2 kWh is able to desorb 1.5 kgof water. The vapor streams through a tube and an open valveinto the condenser, which is cooled by the surrounding air ofapproximately 30 oC. Due to the condensation enthalpy, thetemperature in the interior of the condenser is somewhat higher,i. e. 35 oC. The heat exchanging surfaces must be large enoughto minimize this difference, because the condensationtemperature has a great influence on the efficiency of theadsorption cycle.

The desorbed water is accumulated in the condenser. In the lateafternoon the valve between condenser and solar collector isclosed and the collector cools down to ambient temperature.

At night, a second valve at the bottom of the condenser isopened and by force of gravitation the water flows through atube into the cold evaporator. Then the valve betweencondenser and solar collector is opened as well. The watervapor streams from the evaporator to the solar collector and isadsorbed in the zeolite.

In the course of the night, the former desorbed water mass of1.5 kg is now re-adsorbed and thus the needed cooling energyof 1 kWh is produced.

The evaporator has to be constructed in such a way that thesurface of the refrigerant is approximately 40cm x 40 cm. Bycooling down to 0 oC, the water is frozen and a thickness of iceof 4 cm may easily be achieved. Thus, approximately 5 kg of icehave been produced. The next day, the solar collector is heatedup again and during the day no cooling power by adsorption isavailable. By means of the stored mass of ice a cooling energyof about 0.5 kWh is provided and a temperature increase of thecooling chamber is avoided.

By means of thermostatic valves the operation of the wholeadsorption cycle may be done automatically.

In case of vacuum problems the refrigerator could be equippedwith a manually operated vacuum pump to restore the vacuumevery few days.

8. CONCLUSIONS

It has been proven, that cold can be produced by a very simple

[ Unit of energy flow: kWh/d ]

Refrigerator volume: 125 l

ThermalInsulation:20 cm Polystyrol

Cooling Energy

1

Thermal losses of the collector

0°C

0,70,3Heatconduction

4

Desorptionenergy

Heat capacityof collector

12

30°CAmbient temperature

2

Evaporation Energy

Cooling load (20 kg meat)

3 m² collector area

8

12 kWh/dInsolation4 kWh/(m²d)

Fig. 7: Energy flow diagram of prototype

adsorption cycle using zeolite as adsorbent and water asrefrigerant. The adsorbent material is contained in a solar tubecollector developed by the authors. A solar irradiation of 300W/m² is sufficient to reach desorption temperatures of 170 oC.The resulting cooling energy density is 350 kJ/kg zeolite andthe COP is 8 % (cooling energy/ solar energy). However, apartial air pressure of about 10 mbar may cause problems. Silicagel can be regenerated at lower temperatures but the coolingpower is lower. The experimental results and the theoreticallycalculated cooling power correspond satisfactorily. On thisbasis a prototype was simulated: In a sunny region a solarcollector area of 3 m² is sufficient to power a small householdrefrigerator. Our next step will be the actual construction of theprototype we have simulated.

REFERENCES

Journal articles:Guilleminot J.J., Meunier F. and Mischler B. (1981). Etude decycles intermittants a adsorption solide pour la refrigerationsolaire. Revue Phys. Appl. 15, 441-452

Schwarz J. (1990) Die Adsorptionskältemaschine mit demStoffpaar Zeolith/Wasser. Die Kälte und Klimatechnik. 9,492-505

Report:World Health Organisation (WHO) (1993). Solar Energy andHealth. SC.93/Conf. 003/8

Thesis:Maier-Laxhuber P.K. (1983). Sorptionswärmepumpen undSorptionsspeicher mit dem Stoffpaar Zeolith-H2O . TechnicalUniversity MunichThesis:Henning, H.-M. (1994). Regenerierung von Adsorbentien mitsolar erzeugter Prozeßwärme. VDI- Verlag, Düsseldorf, Reihe 3(Verfahrenstechnik) Nr. 350

Private Communication:Henning, H.-M. (1998). Experimental results of adsorptioncycle zeolite/water. Fraunhofer Institute for Solar EnergySystems, Freiburg, Germany

ACKNOWLEDGEMENTS:

The following students have done much of the experimentalwork in the course of their diploma thesis:Beuck J. (1991), Borg S. (1992), Rickert J.(1993),Matuszewska M. (1994), Wruck J. (1996), Jensen K. (1997),Wolf M. (1999)

The members of our staff, A.Knebelau and P.Labedzki, havecontributed to the construction of the experiment and havegiven technical assistance to the students.