68-solar waste heat driven two-stage adsorption

Renewable Energy 23 (2001) 93101www.elsevier.nl/locate/renene

Solar/waste heat driven two-stage adsorptionchiller: the prototype

B.B. Saha *, A. Akisawa, T. KashiwagiDepartment of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology,

2-24-16 Naka-machi, Koganei-shi, Tokyo 184-8588, Japan

Received 20 October 1999; accepted 2 May 2000

Abstract

Nowadays, adsorption heat pumps receive considerable attention as they are energy saversand environmentally benign. In this study silica gelwater is taken as the adsorbent refrigerantpair. To exploit solar/waste heat of temperatures below 70 C, staged regeneration is necessary.A new two-stage non-regenerative adsorption chiller design and experimental prototype isproposed. Experimental temperature profiles of heat transfer fluid inlets and outlets arepresented. The two-stage cycle can be operated effectively with 55 C solar/waste heat in com-bination with a 30 C coolant temperature. In this paper the physical adsorption of silica gel,working principle and features of a two-stage chiller are described. 2000 Elsevier ScienceLtd. All rights reserved.

1. Introduction

The severity of the ozone layer destruction problem, due partly to CFCs andHCFCs, has been calling for rapid developments in freon-free air conditioning tech-nologies. With regard to energy use, global warming prevention requires a thoroughrevision of energy utilization practices towards greater efficiency. From this perspec-tive, interest in adsorption systems has been increased as they do not use ozonedepleting substances as refrigerants nor do they need electricity or fossil fuels asdriving sources. Several heat-pumping and refrigeration applications have been stud-ied using various adsorbent and adsorbate pairs. Some representative examples are

* Corresponding author. Tel. and fax: +81-42-388-7076.E-mail address: [email protected] (B.B. Saha).

0960-1481/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S 09 60 -1481( 00 )0 0107-5

94 B.B. Saha et al. / Renewable Energy 23 (2001) 93101

given in Table 1. Most of the cycles mentioned in Table 1 require medium and/orhigh temperature heat sources to act as the driving sources. However, silica gelwater and active carbonmethanol adsorption cycles have a distinct advantage overother systems in their ability to be driven by heat of relatively low, near-ambienttemperatures, so that waste heat below 100 C can be recovered, which is highlydesirable. In this study, silica gelwater has been chosen as the adsorbentrefrigerantpair because the regeneration temperature of silica gel is lower than that of activecarbon; and water has a large latent heat of vaporization.

In order to utilize near environment temperature solar heat/waste heat sourcesbetween 50 and 70 C with a cooling source of 30 C, a new two-stage, four-bed,non-regenerative adsorption cycle is introduced and its features are described.

2. Physical adsorption of silica gel

Silica gel is a partially dehydrated form of polymeric colloidal silicic acid [13].The chemical composition may be expressed as SiO2 nH2O. The adsorption desorp-tion equation for silica gel can be expressed as

Table 1Developments in adsorption heat pump systems (typical achievements)

Adsorbent/refrigerant System type Source Remarks

Activated Regenerative system Jones and 4 bed systemcarbon/ammonia Christophilos [1]Activated Intermittent system Pons and Guilleminot Solar driven ice makercarbon/methanol [2]Calcium Intermittent adsorption Lai et al. [3] Chemical heat pumpchloride/methanol systemComplex Intermittent adsorption Beijer and Horsman Promising uses: vehicles andcompounds/salts system [4] residential air conditioningActivated Regenerative system Miles and Shelton [5] Thermal wave system;carbon/ammonia Tregeneration is very highMonolithic Intermittent adsorption R.E. Critoph [6] Power density: 1 kW/kg ofcarbon/ammonia system carbonSilica gel/water Intermittent adsorption Saha et al. [7] and Waste heat driven cycle; heat

system, single stage Boelman et al. [8] of ads, Qst=2800 kJ/kgSilica gel/water Intermittent adsorption Saha et al. [9] Waste heat driven cycle;

system, three stage Tregeneration is very lowZeolite/ammonia Intermittent system Critoph and Turner Tregeneration is very high

[10]Zeolite/water Cascaded adsorption Douss and Meunier Application: heating; Heat of

system [11] adsorption Qst=3700 kJ/kgZeolite Intermittent adsorption Guilleminot et al. [12] Composites: (a) 65%composites/water system zeolite+35% metallic foam

and (b) 70% zeolite+30%natural expanded graphite

95B.B. Saha et al. / Renewable Energy 23 (2001) 93101

SiO23nH2O(s)5SiO23(n21)H20(s)1H2O(g) (1)where s and g denote respectively, solid phase and vapor phase. The adsorptiveaction of silica gel for vapors is a purely physical effect. When the particles becomesaturated, they do not suffer any change in size or shape, and even when completelysaturated the particles seem to be perfectly dry.

The adsorptive property of silica gel arises from its tremendous porosity; it hasbeen estimated that 1 m3 gel contains pores having a surface of about 2.8 107 m2.The dimensions of the pores are sub-microscopic (20200 A ). Silica gel adsorbsvapor from a gas mixture until the pores of the gel are filled. The amount of condens-able vapor adsorbed in silica gel at any temperature increases as the partial pressureof the vapor in the surrounding gas approaches the partial pressure of vapor, whichwould exist if the gas were saturated at the gel temperature. Silica gel at 27 C incontact with air saturated at this temperature can adsorb up to 0.4 kg of water perkg of gel [14]. When vapor is adsorbed in silica gel, the heat liberated is equivalentto the latent heat of evaporation of the adsorbed liquid plus the additional heat ofwetting. The sum of the latent heat plus the heat of wetting is the heat of adsorption.During adsorption, the vapor latent heat is transformed into sensible heat, which isdissipated into the adsorbent, the metal of the adsorbent container and the surround-ing atmosphere. Hence, there is a need for cooling the adsorbent if an excessivetemperature rise of the gel is to be avoided. The amount of heat required to regeneratesilica gel varies with the design of the equipment. In addition to supplying the heatnecessary to release adsorbed refrigerant from the gel (heat of adsorption), heat mustbe added to raise the temperatures of the adsorbent bed and adsorber and also toovercome radiation losses. The action of silica gel is practically instantaneous underdynamic adsorption conditions, the length of the adsorption period may be arbitrarilyestablished. If automatic operation is desired, the cycle time may be only a fewminutes; as there is a trade off between time duration and cooling capacity. Forexample, during the first 5 minutes gel particles are close to saturation point in acommercial adsorption chiller [15] resulting in optimal cooling capacity. Followingthis period cooling capacity drops.

3. Working principle of the advanced two-stage adsorption cycle

The adsorption system can be compared to that of a conventional air conditioneror refrigerator, with the electric powered mechanical compressor replaced by a ther-mally driven adsorption compressor. The ability to be driven by heat, which is usedfor desorption, makes adsorption cycles attractive for electric energy savers. Also,since fixed adsorbent beds are usually employed, these cycles can be operationalwithout moving parts other than magnetic valves. This results in low vibration, mech-anically simple, high reliability and very long life time. The aforementioned charac-teristics make them well suited for space applications. The uses of fixed beds alsoresults in intermittent cycle operation, with adsorbent beds changing between adsorp-tion and desorption stages. Hence, if a constant flow of vapor from the evaporator


is required, two or more adsorbent beds must be operated out of phase as describedin the following paragraph.

As can be seen from the conceptual Duhring diagram of Fig. 1, the conventional(single stage) silica gelwater cycle will not be operational with a 50 C driving heatsource if the cooling source is at 30 C or higher, which would likely be the case ofan air-cooled cooling tower in summer, in Tokyo. For practical utilization of thesetemperatures to adsorption chiller operation, an advanced (two-stage) cycle isdesigned. As can be seen from Fig. 1, these cycles allow to reduce DTregen of theadsorbent (Tdes2Tcond) by dividing the evaporating temperature lift (Tcond2Teva) intotwo smaller lifts. Refrigerant (water vapor) pressure thus rises into two progressivesteps from evaporation to condensation level. In order to attain this objective, theintroduction of two additional sorption elements is necessary, as shown in Fig. 2.An advanced, two-stage cycle comprises of six heat exchangers, namely, a condenser,an evaporator and two pairs of sorption elements. In the cycle, valves 1, 3, 5 areopen to allow refrigerant flow between heat exchangers. The sorption elements 1and 4 (HX1 and HX4 in Fig. 2) are heated by hot water while the sorption elements2 and 3 (HX2 and HX3 in Fig. 2) are cooled by cooling water. The silica gel ineach sorption element is fixed inside the container, i.e. packed around the finnedheat transfer tubes which cannot be rotated or moved. Hence an uninterrupted supplyof cooling energy requires operating as a pseudo-continuous cycle, where adsorptionand desorption occur concomitantly and sorption elements repeatedly switch betweenadsorption and desorption modes. The thermophysical properties of silica gel usedin this experimental chiller are shown in Table 2.

Refrigerant (water), evaporates inside the evaporator, picking up evaporation heatfrom the chilled water, is adsorbed by adsorber 2 via valve 3. Sorption element 3

Fig. 1. Conceptual Duhring diagram for both the conventional and two-stage cycles.


Fig. 2. Schematic of the two-stage adsorption chiller.

Table 2Thermophysical properties of silica gel used in the two-stage chiller

Type of Surface Porous Average Heat Thermal Density Volumegel area volume diameter capacity conductivity (kg/m3) fraction(kg/m3) (m2/g) (cm3/g) (mm) (kJ/kgK) (W/mK) (2)

A 650 0.36 0.7 0.92 0.175 2200 0.341

also adsorbs refrigerant from the desorber 4 via valve 5. Desorber 1 is connected tothe condenser via valve 1. The desorbed refrigerant vapor is condensed in the con-denser at temperature Tcond; cooling water removes the condensation heat Qcond. Thiscondensed refrigerant comes back to the evaporator via the tube connecting con-denser and evaporator to complete the cycle. The tube is bent to achieve a pressuredrop resulting in the refrigerant being in liquid phase in the evaporator. The use ofparallel cooling water circuits for the condenser and adsorbers 2 and 3 results insimilar temperature levels at the condenser (Tcond) and those adsorbers (Tads).

When refrigerant concentrations in the adsorbers and desorbers are at or near theirequilibrium level, the flows of hot and cooling water are redirected by switching thevalves so that the desorbers switch into adsorption modes and the adsorbers changeinto desorption operations. During a short intermediate process (mode B or modeD) no adsorption/desorption occurs. This time is needed to preheat the adsorbers andprecool the desorbers. The resulting low-pressure refrigerant is again adsorbed by


the adsorbent to continue the process. The time chart of the chiller operation is shownin Table 3.

4. Two-stage chiller specification

A prototype was built on the roof of our experimental building located at theTokyo University of Agriculture and Technology to test experimentally the perform-ance of the advanced, two-stage adsorption chiller. A side-view of the two-stagechiller prototype is shown in Fig. 3. As can be observed all heat exchangers aretotally covered with metallic enclosures and are thermally insulated to prevent heatloss to the external environment. Inside the metallic enclosure of eachadsorber/desorber heat exchanger, and in the condenser, there is a small passage forhot water flowing preventing capillary condensation on the surface of the wall. Allfour adsorber/desorber heat exchangers are removable to facilitate replacing any ofthe four heat exchangers by one of a higher performance eventually. The rated coo-ling capacity of the chiller is 1 RT (3.54 kW) and the COP is 0.34. External para-meters regarding the chiller operation are listed in Table 4.

5. Temperature profiles

Fig. 4 shows experimental temperature profiles of the heat transfer fluid inlets andoutlets obtained for the standard operating conditions listed in Table 4. After only420 s, the hot water outlet temperature approaches the inlet temperature; from thispoint there is practically no more consumption of driving heat. This led us to selectthe standard adsorption/desorption cycle time as 420 s. But the cooling water outlettemperature from the adsorber after 420 s is still 2 C higher than its respective inlettemperature. The reason for this is the increasing amount of refrigerant requiringcooling at the end of the adsorption cycle, in contrast to the desorption cycle wherelittle refrigerant remains to be heated. Cooling water outlet temperature graduallyreturns to its inlet at 30 C confirming that condensation takes place satisfactorily in

Table 3Chiller operation time charta

Cycle Adsorption/ Pre-heating/ Adsorption/ Pre-heating/desorption cycle pre-cooling cycle desorption cycle pre-cooling cyclemode A mode B mode C mode D

Time (s) 420 20 420 20Valve 1,3,5 s

2,4,6 s HX 1,4 Hw Cw Cw Hw

2,3 Cw Hw Hw Cw

a Vvalve; sopen; closed; Hwhot water; Cwcooling water; HXheat exchanger.


Fig. 3. Photograph of the experimental prototype.

Table 4Standard operating conditions

Hot water inlet Cooling water inlet Chilled water inletTemperature Flow rate (kg/s) Temperature Flow rate Temperature Flow rate (kg/s)( C) ( C) (ads+cond) ( C)

(kg/s)

55 1.2 30 1.8 (1.2+0.6) 14 0.17Cycle time: 440 sAdsorption/desorption cycle 420 s Pre-heating/precooling cycle 20 s

the condenser. The delivered chilled water temperature, however, continues belowthe inlet temperature in the whole cycle, showing that cooling energy production issteady which is highly desirable. For the standard operating condition, the experi-mental cooling capacity value is 3.2 kW and the coefficient of performance (COP)is 0.36.


Fig. 4. Experimental heat transfer fluid temperature profiles.

6. Conclusions

There is an increasing need for energy efficiency and so thermally driven sorptionsystems in many world regions are essential. Regions with a warm climate and nosteady electricity supply offer most potential. From this perspective, a new advancedtwo-stage adsorption chiller design and its features are presented in this paper. Theprototype of the chiller is built to examine experimentally its performance. The mainadvantage of the two-stage adsorption chiller is its ability to utilize low temperaturesolar/waste heat (4075 C) as the driving heat source in combination with a coolantat 30 C. With a 55 C driving source in combination with a heat sink at 30 C, theCOP of the two-stage chiller is 0.36. Flat plate solar collectors in any tropical climatecan effectively produce the required driving source energy of the chiller making itsuperior to any other commercially existing cooling technology. From the aboveperspectives, the use of unexploited low-temperature solar/waste heat may offer anattractive possibility for improving energy conservation and efficiency.

References

[1] Jones JA, Christophilos V. High-efficiency regenerative adsorption heat pump. ASHRAE Trans1993;99(1):5460.

[2] Pons M, Guilleminot JJ. Design of an experimental solar-powered solid adsorption ice maker. TransASME J Solar Energy Engng 1986;108:3327.

[3] Lai H, Li C, Zheng D, Fu J. Modeling of cycling process of CaCl2/CH3OH solid vapor chemicalheat pump. In: Radermacher et al, editors. Proceedings of the International Absorption Heat PumpConference, ASME-AES, New Orleans, USA, vol. 31, 1994. pp. 41924.

[4] Beijer HA, Horsman JWK. S.W.E.A.T. thermochemical heat pump storage system. In: Radermacheret al. editors. Proceedings of the International Absorption Heat Pump Conference, ASME-AES, NewOrleans, USA, vol 31, 1994. pp. 45762.


[5] Miles DJ, Shelton SV. Design and testing of a solid-sorption heat-pump system. Appl Therm Engng1996;16(5):38994.

[6] Critoph RE. Rapid cycling solar/biomass powered adsorption refrigeration system. Renew Energy1999;16:6738.

[7] Saha BB, Boelman EC, Kashiwagi T. Computer simulation of a silica gelwater adsorption refriger-ation cyclethe influence of operating conditions on cooling output and COP. ASHRAE Trans1995;101(1):34857.

[8] Boelman EC, Saha BB, Kashiwagi T. Experimental Investigation of a silica gelwater adsorptionrefrigeration cyclethe influence of operating conditions on cooling output and COP. ASHRAETrans 1995;101(1):35866.

[9] Saha BB, Kashiwagi T. Experimental investigation of an advanced adsorption refrigeration cycle.ASHRAE Trans 1997;103(2):508.

[10] Critoph RE, Turner HL. Performance of ammonia-activated carbon and ammoniazeolite heat pumpadsorption cycle. In: Proceedings of the International Conference on Pompes a Chaleur Chimiquesde Hautes Performances, Perpignan, France, 1988. pp. 20211.

[11] Douss N, Meunier F. Experimental study of cascading adsorption cycles. Chem Engng Sci1989;44(2):22535.

[12] Guilleminot JJ, Chalfen JB, Choisier A. Heat and mass transfer characteristics of composites foradsorption heat pumps. In: Radermacher et al., editors. Proceedings of the International AbsorptionHeat Pump Conference, ASME-AES, New Orleans, USA, vol. 31, 1994. pp. 4016.

[13] Ruthven DM. Principles of adsorption and adsorption processes. New York: John Wiley andSons, 1984.

[14] Dehler FC. Silica and gel adsorption. Chem Metall Engng 1940;37:30710.[15] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Modeling the performance of two-

bed, silica gelwater adsorption chillers. Int J Refrig 1999;22:194204.

68-solar waste heat driven two-stage adsorption

Documents