Solar/waste heat driven two-stage adsorption chiller: the prototype

Renewable Energy 23 (2001) 93 101 www.elsevier.nl/locate/renene Solar/waste heat driven two-stage adsorption chiller: the prototype B.B. Saha *, A. Akisawa, T. Kashiwagi Department 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 savers and environmentally benign. In this study silica gel water is taken as the adsorbent refrigerant pair. 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 is proposed. Experimental temperature profiles of heat transfer fluid inlets and outlets are presented. The two-stage cycle can be operated effectively with 55 C solar/waste heat in combination 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 Science Ltd. All rights reserved. 1. Introduction The severity of the ozone layer destruction problem, due partly to CFCs and HCFCs, has been calling for rapid developments in freon-free air conditioning technologies. With regard to energy use, global warming prevention requires a thorough revision of energy utilization practices towards greater efficiency. From this perspective, interest in adsorption systems has been increased as they do not use ozone depleting substances as refrigerants nor do they need electricity or fossil fuels as driving sources. Several heat-pumping and refrigeration applications have been studied using various adsorbent and adsorbate pairs. Some representative examples are * Corresponding author. Tel. and fax: +81-42-388-7076. E-mail address: bidyut@cc.tuat.ac.jp (B.B. Saha). 0960-1481/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 09 60-1481(00)00107-5

94 B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 given in Table 1. Most of the cycles mentioned in Table 1 require medium and/or high temperature heat sources to act as the driving sources. However, silica gel water and active carbon methanol adsorption cycles have a distinct advantage over other systems in their ability to be driven by heat of relatively low, near-ambient temperatures, so that waste heat below 100 C can be recovered, which is highly desirable. In this study, silica gel water has been chosen as the adsorbent refrigerant pair because the regeneration temperature of silica gel is lower than that of active carbon; and water has a large latent heat of vaporization. In order to utilize near environment temperature solar heat/waste heat sources between 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 SiO 2 nh 2 O. The adsorption desorption equation for silica gel can be expressed as Table 1 Developments in adsorption heat pump systems (typical achievements) Adsorbent/refrigerant System type Source Remarks Activated Regenerative system Jones and 4 bed system carbon/ammonia Christophilos [1] Activated Intermittent system Pons and Guilleminot Solar driven ice maker carbon/methanol [2] Calcium Intermittent adsorption Lai et al. [3] Chemical heat pump chloride/methanol system Complex Intermittent adsorption Beijer and Horsman Promising uses: vehicles and compounds/salts system [4] residential air conditioning Activated Regenerative system Miles and Shelton [5] Thermal wave system; carbon/ammonia T regeneration is very high Monolithic Intermittent adsorption R.E. Critoph [6] Power density: 1 kw/kg of carbon/ammonia system carbon Silica gel/water Intermittent adsorption Saha et al. [7] and Waste heat driven cycle; heat system, single stage Boelman et al. [8] of ads, Q st =2800 kj/kg Silica gel/water Intermittent adsorption Saha et al. [9] Waste heat driven cycle; system, three stage T regeneration is very low Zeolite/ammonia Intermittent system Critoph and Turner T regeneration is very high [10] Zeolite/water Cascaded adsorption Douss and Meunier Application: heating; Heat of system [11] adsorption Q st =3700 kj/kg Zeolite Intermittent adsorption Guilleminot et al. [12] Composites: (a) 65% composites/water system zeolite+35% metallic foam and (b) 70% zeolite+30% natural expanded graphite

B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 95 SiO 2 nh 2 O(s) SiO 2 (n 1)H 2 0(s) H 2 O(g) (1) where s and g denote respectively, solid phase and vapor phase. The adsorptive action of silica gel for vapors is a purely physical effect. When the particles become saturated, they do not suffer any change in size or shape, and even when completely saturated the particles seem to be perfectly dry. The adsorptive property of silica gel arises from its tremendous porosity; it has been estimated that 1 m 3 gel contains pores having a surface of about 2.8 10 7 m 2. The dimensions of the pores are sub-microscopic (20 200 Å). Silica gel adsorbs vapor from a gas mixture until the pores of the gel are filled. The amount of condensable vapor adsorbed in silica gel at any temperature increases as the partial pressure of the vapor in the surrounding gas approaches the partial pressure of vapor, which would exist if the gas were saturated at the gel temperature. Silica gel at 27 C in contact with air saturated at this temperature can adsorb up to 0.4 kg of water per kg of gel [14]. When vapor is adsorbed in silica gel, the heat liberated is equivalent to the latent heat of evaporation of the adsorbed liquid plus the additional heat of wetting. 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 is dissipated into the adsorbent, the metal of the adsorbent container and the surrounding atmosphere. Hence, there is a need for cooling the adsorbent if an excessive temperature rise of the gel is to be avoided. The amount of heat required to regenerate silica gel varies with the design of the equipment. In addition to supplying the heat necessary to release adsorbed refrigerant from the gel (heat of adsorption), heat must be added to raise the temperatures of the adsorbent bed and adsorber and also to overcome radiation losses. The action of silica gel is practically instantaneous under dynamic adsorption conditions, the length of the adsorption period may be arbitrarily established. If automatic operation is desired, the cycle time may be only a few minutes; as there is a trade off between time duration and cooling capacity. For example, during the first 5 minutes gel particles are close to saturation point in a commercial adsorption chiller [15] resulting in optimal cooling capacity. Following this 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 conditioner or refrigerator, with the electric powered mechanical compressor replaced by a thermally driven adsorption compressor. The ability to be driven by heat, which is used for desorption, makes adsorption cycles attractive for electric energy savers. Also, since fixed adsorbent beds are usually employed, these cycles can be operational without moving parts other than magnetic valves. This results in low vibration, mechanically simple, high reliability and very long life time. The aforementioned characteristics make them well suited for space applications. The uses of fixed beds also results in intermittent cycle operation, with adsorbent beds changing between adsorption and desorption stages. Hence, if a constant flow of vapor from the evaporator

96 B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 is required, two or more adsorbent beds must be operated out of phase as described in the following paragraph. As can be seen from the conceptual Dühring diagram of Fig. 1, the conventional (single stage) silica gel water cycle will not be operational with a 50 C driving heat source if the cooling source is at 30 C or higher, which would likely be the case of an air-cooled cooling tower in summer, in Tokyo. For practical utilization of these temperatures to adsorption chiller operation, an advanced (two-stage) cycle is designed. As can be seen from Fig. 1, these cycles allow to reduce T regen of the adsorbent (T des T cond ) by dividing the evaporating temperature lift (T cond T eva ) into two smaller lifts. Refrigerant (water vapor) pressure thus rises into two progressive steps from evaporation to condensation level. In order to attain this objective, the introduction 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 are open to allow refrigerant flow between heat exchangers. The sorption elements 1 and 4 (HX1 and HX4 in Fig. 2) are heated by hot water while the sorption elements 2 and 3 (HX2 and HX3 in Fig. 2) are cooled by cooling water. The silica gel in each sorption element is fixed inside the container, i.e. packed around the finned heat transfer tubes which cannot be rotated or moved. Hence an uninterrupted supply of cooling energy requires operating as a pseudo-continuous cycle, where adsorption and desorption occur concomitantly and sorption elements repeatedly switch between adsorption and desorption modes. The thermophysical properties of silica gel used in this experimental chiller are shown in Table 2. Refrigerant (water), evaporates inside the evaporator, picking up evaporation heat from the chilled water, is adsorbed by adsorber 2 via valve 3. Sorption element 3 Fig. 1. Conceptual Dühring diagram for both the conventional and two-stage cycles.

B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 97 Fig. 2. Schematic of the two-stage adsorption chiller. Table 2 Thermophysical properties of silica gel used in the two-stage chiller Type of Surface Porous Average Heat Thermal Density Volume gel area volume diameter capacity conductivity (kg/m 3 ) fraction (kg/m 3 ) (m 2 /g) (cm 3 /g) (mm) (kj/kg K) (W/m K) ( ) 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 to the condenser via valve 1. The desorbed refrigerant vapor is condensed in the condenser at temperature T cond ; cooling water removes the condensation heat Q cond. This condensed refrigerant comes back to the evaporator via the tube connecting condenser and evaporator to complete the cycle. The tube is bent to achieve a pressure drop resulting in the refrigerant being in liquid phase in the evaporator. The use of parallel cooling water circuits for the condenser and adsorbers 2 and 3 results in similar temperature levels at the condenser (T cond ) and those adsorbers (T ads ). When refrigerant concentrations in the adsorbers and desorbers are at or near their equilibrium level, the flows of hot and cooling water are redirected by switching the valves so that the desorbers switch into adsorption modes and the adsorbers change into desorption operations. During a short intermediate process (mode B or mode D) no adsorption/desorption occurs. This time is needed to preheat the adsorbers and precool the desorbers. The resulting low-pressure refrigerant is again adsorbed by

98 B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 the adsorbent to continue the process. The time chart of the chiller operation is shown in Table 3. 4. Two-stage chiller specification A prototype was built on the roof of our experimental building located at the Tokyo University of Agriculture and Technology to test experimentally the performance of the advanced, two-stage adsorption chiller. A side-view of the two-stage chiller prototype is shown in Fig. 3. As can be observed all heat exchangers are totally covered with metallic enclosures and are thermally insulated to prevent heat loss to the external environment. Inside the metallic enclosure of each adsorber/desorber heat exchanger, and in the condenser, there is a small passage for hot water flowing preventing capillary condensation on the surface of the wall. All four adsorber/desorber heat exchangers are removable to facilitate replacing any of the four heat exchangers by one of a higher performance eventually. The rated cooling capacity of the chiller is 1 RT (3.54 kw) and the COP is 0.34. External parameters regarding the chiller operation are listed in Table 4. 5. Temperature profiles Fig. 4 shows experimental temperature profiles of the heat transfer fluid inlets and outlets obtained for the standard operating conditions listed in Table 4. After only 420 s, the hot water outlet temperature approaches the inlet temperature; from this point there is practically no more consumption of driving heat. This led us to select the standard adsorption/desorption cycle time as 420 s. But the cooling water outlet temperature from the adsorber after 420 s is still 2 C higher than its respective inlet temperature. The reason for this is the increasing amount of refrigerant requiring cooling at the end of the adsorption cycle, in contrast to the desorption cycle where little refrigerant remains to be heated. Cooling water outlet temperature gradually returns to its inlet at 30 C confirming that condensation takes place satisfactorily in Table 3 Chiller operation time chart a Cycle Adsorption/ Pre-heating/ Adsorption/ Pre-heating/ desorption cycle pre-cooling cycle desorption cycle pre-cooling cycle mode A mode B mode C mode D Time (s) 420 20 420 20 Valve 1,3,5 2,4,6 HX 1,4 Hw Cw Cw Hw 2,3 Cw Hw Hw Cw a V valve; open; closed; Hw hot water; Cw cooling water; HX heat exchanger.

B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 99 Fig. 3. Photograph of the experimental prototype. Table 4 Standard operating conditions Hot water inlet Cooling water inlet Chilled water inlet Temperature 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 0.17 Cycle time: 440 s Adsorption/desorption cycle 420 s Pre-heating/precooling cycle 20 s the condenser. The delivered chilled water temperature, however, continues below the inlet temperature in the whole cycle, showing that cooling energy production is steady which is highly desirable. For the standard operating condition, the experimental cooling capacity value is 3.2 kw and the coefficient of performance (COP) is 0.36.

100 B.B. Saha et al. / Renewable Energy 23 (2001) 93 101 Fig. 4. Experimental heat transfer fluid temperature profiles. 6. Conclusions There is an increasing need for energy efficiency and so thermally driven sorption systems in many world regions are essential. Regions with a warm climate and no steady electricity supply offer most potential. From this perspective, a new advanced two-stage adsorption chiller design and its features are presented in this paper. The prototype of the chiller is built to examine experimentally its performance. The main advantage of the two-stage adsorption chiller is its ability to utilize low temperature solar/waste heat (40 75 C) as the driving heat source in combination with a coolant at 30 C. With a 55 C driving source in combination with a heat sink at 30 C, the COP of the two-stage chiller is 0.36. Flat plate solar collectors in any tropical climate can effectively produce the required driving source energy of the chiller making it superior to any other commercially existing cooling technology. From the above perspectives, the use of unexploited low-temperature solar/waste heat may offer an attractive possibility for improving energy conservation and efficiency. References [1] Jones JA, Christophilos V. High-efficiency regenerative adsorption heat pump. ASHRAE Trans 1993;99(1):54 60. [2] Pons M, Guilleminot JJ. Design of an experimental solar-powered solid adsorption ice maker. Trans ASME J Solar Energy Engng 1986;108:332 7. [3] Lai H, Li C, Zheng D, Fu J. Modeling of cycling process of CaCl 2 /CH 3 OH solid vapor chemical heat pump. In: Radermacher et al, editors. Proceedings of the International Absorption Heat Pump Conference, ASME-AES, New Orleans, USA, vol. 31, 1994. pp. 419 24. [4] Beijer HA, Horsman JWK. S.W.E.A.T. thermochemical heat pump storage system. In: Radermacher et al. editors. Proceedings of the International Absorption Heat Pump Conference, ASME-AES, New Orleans, USA, vol 31, 1994. pp. 457 62.

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