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Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper

CURRENT TECHNOLOGIES AND FUTURE PERSPECTIVES IN SOLAR POWERED ADSORPTION SYSTEMS Ahmed M. Qenawy1 and Abdulmajeed A. Mohamad1 1 Mechanical and Manufacturing Department, Schulich School of Engineering, University of Calgary, Calgary, Canada

ABSTRACT About 10 to 20% of the electric power produced world wide is consumed in cooling applications including air-conditioning and refrigeration applications. This highlights the fact that an energy efficient cooling is very important. Many adsorption cycles have been proposed and investigated by researchers. A review of these cycles is presented in this paper. The integration of solar energy to power these cycles is also reviewed. It is concluded that solar adsorption cooling systems are the most promising technology in solar cooling applications with respect to low cost, moderate coefficient of performance, ease of manufacture and low maintenance. The major challenge facing the researchers now is better enhancement of heat and mass transfer in the system in favor of higher performance. In general, solar adsorption systems are not yet in the stage of world-wide commercialization but it is expected it will have a potential market with further development.

INTRODUCTION Adsorption phenomenon was discovered and employed a long time ago. Historically, Egyptians were the pioneers to explore and to use this phenomenon. Around 3750 BC Egyptians used charcoal for reduction of copper, zinc and tin ores for manufacture of bronze. Later 1550 BC they used charcoal for medicinal purposes (Da browski 2001). Adsorption phenomenon has been used for a wide variety of applications since then. These applications include drying of gases, desiccant in packing, dewpoint control of natural gas, water purification, separation processes, pollution control and refining of mineral oils (Thomas 1998), as well as refrigeration and heat pumping applications. Adsorption refrigeration and heat pumping has recently received more attention (Sumathy, Yeung et al. 2003). In addition to their simple configuration, no moving parts, environmentally friendly, noiseless and simple operation, they can be powered with low grade energy such as waste heat and solar energy.

The use of solar energy as an energy source to power cooling systems is an attractive goal that is of growing interest among both researchers and energy planners (Henning 2004). Solar radiation is a free natural resource, the running costs of developed solar cooling systems can be expected to be low once the initial costs for their construction and installation have been met. Moreover, cooling load is generally high when solar radiation is high. Solar cooling potentially offers an excellent model of a clean, sustainable technology, which is consistent with the international commitment to sustainable development. Many solar cooling systems have been researched such as solar absorption, adsorption, vapor compression, thermoelectric and ejector systems. Sorption solar cooling has proven to be technically feasible (Meunier 1994). Adsorption refrigeration has received much attention in recent years (both for ice making and heat pump); various types of adsorption refrigerators and heat pumps were developed (Saha, Koyama et al. 2003; Alam, Akahira et al. 2004; Luo, Dai et al. 2006), mostly of activated carbon-methanol, zeolite-water, silica gel-water and calciumchloride-ammonia pairs. Due to the poor performance of the basic intermittent adsorption cycle, many modifications were suggested and analyzed in literature. These modifications include implementing a multi-bed system with heat recovery, mass recovery, thermal wave, convective thermal wave and cascade system. Those systems are reviewed regarding recent development trends and their integration with solar energy.

SOLAR POWERED BASIC CYCLE Generally, a solar operated adsorption unit consists of an adsorber connected to heat source (solar collector), a condenser, and an evaporator as shown in Fig. 1. The adsorber exchanges heat with a heat source at high temperature (Tmax) during the daytime and a cooling system at intermediate temperature (Tint1) during the night time. While the system consisting of the condenser plus evaporator exchanges heat with another heat sink at intermediate temperature (Tint2), and a heat source at low

Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper temperature (Tmin). Refrigerant vapor is transported between the adsorber, condenser, and evaporator. The cycle consists of four periods. The mentioned processes are shown on Clapeyron diagram (ln(P) vs. –1/T) of Fig. 2. The solar adsorption refrigeration cycle is comprised of four processes. These processes are: 1) Heating and pressurization (A-B): During this process, the adsorber receives heat from the solar collector while being closed i.e. constant concentration process, often called isosteric heating. The adsorbent temperature increases from (T1) to (T2), inducing a pressure increase from the evaporation pressure (Pe) up to the condensation pressure (Pc), Fig. 2. The evaporation pressure (Pe) and condensation pressure (Pc), are the saturation pressures of the adsorbate at the evaporator temperature (Te) and condenser temperature (Tc), respectively. 2) Heating with desorption and condensation (B-C): During this process, the adsorber continues receiving heat from the solar collector while being connected to the condenser, which now superimposes its pressure (Pc), i.e. constant pressure process or isobaric desorption. The concentration (x1) of adsorbate decreases to (x3). The adsorbent temperature continues increasing from (T2) to (T3) inducing desorption of vapor. This desorbed vapor (∆x= x1-x3) is liquefied in the condenser. The condensation heat (Qc) is released to a heat sink at an intermediate temperature. 3) Cooling and depressurization (C-D): At night, the adsorber releases heat to the solar collector while being closed, i.e. isosteric cooling. The adsorbent temperature decreases from (T3) to (T4) inducing a pressure decrease from the condensation pressure (Pc) down to the evaporation pressure (Pe). 4) Cooling with adsorption and evaporation (D-A): During this process, the adsorber continues releasing heat to the solar collector, while being connected to the evaporator, which now superimposes its pressure (Pe), i.e. isobaric adsorption. The adsorbent temperature continues decreasing from (T4) to (T1), inducing adsorption of vapor. This adsorbed vapor is vaporized in the evaporator by heat transfer from the surroundings, causing a cooling effect (Qe). The selection of the condenser and evaporator pressures as well as the adsorbate concentrations can be controlled by adsorbate/adsorbent type. Most solar powered adsorption systems have a physisorption mechanism (Keller and Staudt 2005) due to the reversible nature of the adsorption/desorption processes which makes it suitable for application of low grade energy such as solar energy. Most widely used adsorbents are Activated-carbons, Zeolites and

Silica Gels. Refrigerants commonly used are Ammonia, Water and Methanol. Other refrigerants such as Butane and R134a where studied and found to lead to very low COP (Critoph and Zhong 2005). Silica gel is used in most industries for water removal due to its strong hydrophilic property (DO 1998). Activated-carbon is the most widely used adsorbent reported in literature due to its extremely high surface area and micro pore volume. Activatedcarbon adsorbent is reported to fit the maximum temperature limit set by the solar energy, (Leite 2000). Moreover, Activated Carbon is recommended by many authors for use for solar energy applications with methanol or ammonia as refrigerant (Critoph 1988). Zeolite can be used with water, ammonia and methanol as refrigerants. (Leite 2000), evaluated several refrigerants with both Zeolite and ActivatedCarbon and concluded that for solar cooling Activated-Carbon gives a better COP. It is found that using Zeolite adsorbents require high collector temperature and therefore lower collector efficiency which makes them more suitable for gas-fired systems. Good refrigerants should have high latent heat, good thermal stability, small molecular dimension to allow easy adsorption and high working pressure at the evaporator temperature. Water has been widely used with Zeolites and Silica Gels. It has a high latent heat and it is environmentally friendly which makes it an ideal refrigerant. But, it has relatively low vapor pressure causes some technical difficulties because the system should be designed for air-tightness considerations. Also, due to this low vapor pressure the resistance to mass transfer will be higher which in turn will cause a significant reduction in cycle performance. The evaporating temperature is limited to about 3:5 oC, hence, its application is limited to air-conditioning and high temperature refrigeration. Ammonia has a high vapor pressure that removes the need for air scavenging systems. Mass transfer resistance may not be that important due to the high vapor pressure. It may be used for deep freezing applications because the evaporator temperature could reach -40 oC. On the other hand, it is toxic and corrosive. Methanol has a higher vapor pressure than water. It can be used for ice-making applications. It is unstable for temperatures above 120 oC. For those reasons, it can be considered as the best refrigerant to be used in adsorption systems. Adsorbent/adsorbate pairs selection criteria is well discussed in literature (Anyanwu 2004). Adsorption cycles performance parameters are usually measured in terms of the cycle coefficient of performance (COP) and its specific cooling power (SCP). COP is defined as the ratio of the useful thermal energy moved in or out of the cycle (Qev) to

Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper that of the high temperature thermal energy used (Qd), it can be expressed as: Qev (1) Qd The performance for solar powered adsorption cycles can be measured by its solar coefficient of performance (SCOP) which is the ratio between the useful energy output to the total solar energy insolation on the collector surface (I). COP =

COP =

Qev I

(2)

Another term that is useful in showing the cycle performance is specific cooling power (SCP) which is expressed as: SCP =

Qev M e .τ cycle

(3)

Me is the total mass of adsorbent and τcycle is the cycle time. The previously explained solar powered basic cycle is usually intermittent. Several attempts have been made to investigate the performance of integration of solar energy with the basic adsorption cycle discussed in the previous section. Ice-maker based on adsorption cycle using solar energy was performed by Boubakri, Arsalane et al. 1992. The solar collector area was l m2 charged with 20 kg of activated carbon. For condenser, evaporator and generator temperatures of 20 oC, 0oC, 95 oC respectively, the system produced more than 4 kg/m2 of ice for more than 60% of the days investigated with net SCOP between 0.08 and 0.12. Leite, 2000, theoretically studied the performance of this cycle powered with solar collector covered with polycarbonate honeycomb transparent insulating material (TIM). In this study, activated carbonmethanol pair was selected. The average six month (October–March) SCOP was estimated to be 0.13 with solar irradiations ranging from 20 to 23 MJ/m2, which corresponds to 7-10 kg/day of ice production per square meter of the collector. However, these results need to be proved experimentally. Hildbrand, Dind et al. 2004, experimentally tested an intermittent adsorptive refrigerator. Twelve tubes of Silica gel were used as adsorbent material to form the adsorber. Water used as refrigerant. The solar collector was double-glazed with a 2 m2 area. The system average SCOP was 0.13. In this study, the effect of the meteorological conditions of external temperature and irradiation on the performance of the system was discussed.

Recently, Li, Huang et al. 2005, investigated analytically and experimentally the effect of sky cloud cover on the performance of activated carbonmethanol intermittent solar adsorption refrigeration. The effective solar collector area was 0.94 m2. The results indicated that no ice would be obtained if cloudy conditions prevailed for intervals exceeding 3 h. A Year round performance tests of the solar ice maker were performed in Kunming, Yunnan Province, China (Luo, Dai et al. 2005). The test results show that the daily average SCOP of the solar ice maker is between 0.083 – 0.127 for daily solar radiation on the surface of the adsorbent bed being about 15–23 MJ/m2. The daily ice production varies within the range of 3.2–6.5 kg/m2. A plate adsorbent bed of activated carbon-methanol was chosen as the adsorbent-adsorbate pair. The adsorbent bed was made of a flat plate stainless steel box.

HEAT RECOVERY (REGENERATION) CYCLE In solar operated basic cycle, the cooling effect is intermittent; one of the attempts to obtain a more continuous cycle is semi-continuous cycle. Semicontinuous heat recovery cycle is usually operated with two adsorption beds. The adsorber to be cooled will transfer its heat to the adsorber to be heated. This arrangement will lead to a higher COP, but the operation of a practical system will be complicated (Sumathy, Yeung et al. 2003). The experimental work of Wang, 2001, showed that the heat recovery operation between two adsorption beds will increase the COP by about 25% if compared with one adsorber basic cycle system. It was also proved that mass recovery is very effective for heat recovery adsorption cooling operation, which may help to obtain a COP increase of more than 10%. Critoph, 2001, suggested a new continuous adsorption refrigeration system. The proposed system consists of 32 steel tubes with monolithic carbon bonded to the tube inner surface, Fig. 3. The estimated COP was 0.6 with SCP of 291 W/kg. Yet the system was not optimized but the effect of some parameters such as the number of the modules, generator temperature and others on the COP and SCP was shown. Chahbani, Labidi et al., 2004, investigated the effect of mass and heat transfer limitation on both the COP and SCP of adsorption regenerative heat pump. They showed that for a slow diffusing refrigerant, the COP and SCP are reduced due to the reduction of the cycled refrigerant.

Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper A solar/gas powered adsorption machine is constructed by Vasiliev, Mishkinis et al., 2001, employing three different arrangements. The first and second arrangements were a single stage system with active carbon fiber “Busofit” adsorbent and combined “Busofit” and CaCl2 adsorbent respectively. The SCOP was about 0.43 for the single stage system with “Busofit” and CaCl2 adsorbent. The third arrangement was a two-stage system with two sorbent beds with heat pipe recovery system, for which the SCOP was more than 0.44 for condenser, evaporator and generator temperatures of 50 oC, -18oC, 120 oC respectively.

Recently, Luo, Dai et al., 2006 built and tested a solar adsorption chiller. The solar adsorption chiller consists of a solar water heating system of 49.4 m2 collecting area, two silica gel-water adsorption chillers operating out of phase with mass recovery, a cooling tower and a fan coil unit. The performance of the adsorption chiller is tested with and without mass recovery. The SCOP and cooling power is found to increase by about 14.6% and 11.2%, respectively, with mass recovery more than that without mass recovery. The solar adsorption chiller daily SCOP was about 0.1-0.13 for daily solar radiation being about 16–21 MJ/m2.

Modeling and simulation of a solar powered two bed adsorption air-conditioning system with heat recovery was investigated by Yong and Sumathy, 2004. In this study a lumped parameter model was developed to investigate the performance of solar powered double bed continuous adsorption system.

Solar powered adsorption systems implementing mass recovery concept could not be found in literature except in the work of (Luo, Dai et al. 2006).

MASS RECOVERY Further improvement suggested in literature is the Mass recovery concept. Mass recovery can be done only in systems of more than one bed. The pressure difference between the adsoreber and the desorber in double bed adsorption systems could be used to enhance the system performance. Because of the pressure difference in the beds, adsorption/desorption processes will occur automatically without any heating or cooling application when adsorber and desorber connected together. The first law and second law analyses of vapor recovery is investigated for both heat recovery and thermal wave cycles as well as different possible presentations in the entropic diagram of the cycles are presented in the work of Pons and Poyelle, 1999. A four-bed mass recovery adsorption refrigeration cycle is proposed by Alam, Akahira et al., 2004. This study deals with an advanced four-bed mass recovery adsorption refrigeration cycle driven by low temperature heat source. The proposed cycle consists of two basic adsorption refrigeration cycles. The heat source rejected by one cycle is used to power the second cycle. The proposed cycle utilize different pressure levels to enhance the refrigeration mass circulation (mass recovery) that leads the system to perform better performances; The performances of the system evaluated numerically in equilibrium conditions and compared with those of the conventional and two-stage system. It is found that The COP of the system is higher than that of the conventional two-bed system if the heat source temperature is below 70 oC. However, higher desorption temperature than 70 oC will lead to lower COP of the conventional system.

HEAT AND MASS RECOVERY In most multi-bed adsorption cycles both heat and mass recovery concepts should be integrated to benefit the advantages of both of them. A study on heat and mass recovery done by Wang, Qu et al., 2002, showed that, the mass recovery process will enhance the cooling capacity per kg of adsorbent significantly to about 20%. The effect of mass recovery on the cycle performance strongly depends on the system operating conditions. It is found that, the mass recovery process will accelerate the adsorption cycle and increase the cycle cooling/heating power. Wang, 2001, indicated that a COP of 0.5 can be reached for adsorption chiller with maximum generation temperature of 100 oC and evaporator temperature of 5 oC. Alam, Akahira et al., 2004, investigated the performance for assisted mass recovery silica-gel with heating and cooling. The idea was to assist the refrigerant flow from the desorber to the adsorber due to the pressure difference, by further heating of the desorber and further cooling of the adsorber. The predicted COP of the proposed mass recovery cycle will be lower than that of the traditional mass recovery cycle. However, the cooling capacity of the proposed cycle is higher than that of conventional mass recovery cycle. Akahira, Alam et al., 2005, investigated the performance of silica gel–water adsorption refrigeration cycle with mass recovery process by experimental prototype machine. In this study, the effect of the operating conditions on the machine’s performance was investigated. It was found that mass recovery can significantly enhance the SCP rather than that of conventional adsorption systems. Recently, Wang, 2006, compared the performance of compound adsorbent with and without mass recovery. The compound adsorbent, consolidated

Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper with cement and water, was composed of 2:1 volume ratio of CaCl2 to activated carbon. It was shown that mass recovery improved the adsorption/desorption rate for the cycle by 47.6% compared the cycle without mass recovery. Also, SCP and SCOP have been improved by 48.6% and 54.5%, respectively.

THERMAL WAVE The concept of thermal wave could be outlined as discussed by Sward, LeVan et al., 2000. As shown in Fig. 4, the cycle consists of two adsorption beds, condenser, evaporator, heater and cooler. The heat transfer fluid (HTF) is heated before it is admitted to the bed. The HTF then generate a heating thermal wave in the bed (in this case works as desorber) and causing the temperature front to move through the bed and at the exit the temperature of the HTF approaches the bed temperature. The HTF is then further cooled before being admitted to the second bed. The cold HTF generates a cold thermal wave in the bed (adsorber in this case) causing bed cooling. The HTF flow is reversed when the temperature fronts are about to break through the beds. This configuration makes the cycle to be continuous. Heat regeneration occurs when the flow of the HTF is reversed allowing heat to be removed from one bed to be used to heat the other bed. For a heat source temperature of 393 K, 303 K condenser temperature and an evaporator temperature of 278 K, (Sward, LeVan et al. 2000) predicted a COP of 1.24. Sward, LeVan et al., 2000, mathematically modeled a thermal wave water-NaX Zeolite system in order to examine the effect of heat source and condenser temperatures, inlet and outlet valves placement, portioning the bed on the cycle performance. In their mathematical model they ignored the resistance to heat and mass transfer encountered in the system, and they used the local equilibrium theory model to build more realistic and less mathematically complex model. In this model, temperature and pressure are assumed constant within each bed at any given time. While they got some unexpected results, but the most important results show that division of the adsorbent bed into sections avoiding mass transfer between those sections has a little effect on the performance. Also, movement of inlet and outlet valves towards the center of the bed tends to enlarge the cycle time. Numerical analysis of the system that takes account of the heat and mass transfer consideration in the system showed that a COP greater than 1.0 could be obtained with cold production of about 200 W/kg, for condenser, evaporator and generator temperatures of 40 oC, 5oC, 220 oC respectively,(Ben Amar, Sun et al. 1996). Mass transfer resistance in the adsorbers is reported as the major problem in the system.

FORCED CONVECTION (CONVECTIVE THERMAL WAVE) Forced convection (convective) thermal wave cycle is similar to thermal wave adsorption cycles (Critoph 1996). Forced convection is employed between the adsorbent bed and the refrigerant. The refrigerant is heated externally before being admitted through the bed, as shown in Fig. 5. In the desorption phase, the hot refrigerant is used to heat the adsorbent bed causing generation of more refrigerant. This makes advantage of the characteristic heat transfer area of adsorbent beds. Although adsorbent beds have a poor thermal conductivity but also they have a large surface area could be employed for convective heat transfer. Additionally, a thermal wave is generated in the bed similar to that of the thermal wave system. Then, Lai, 2000, improved the system with periodic reversal forced convection. It has been shown that a COP of 0.9 and a SCP of 125 W/kg could be obtained for condenser, evaporator and generator temperatures of 40 oC, 3oC, 240 oC respectively,.

CASCADING SYSTEMS In a recent study on a two stage cycle (Chua, Ng et al. 2001; Khan, Alam et al. 2006), an analytic investigation of a two-stage adsorption refrigeration chiller using re-heat was performed to determine the influence of the overall thermal conductance of sorption elements and evaporator as well as the adsorbent mass on the chiller performance. Analysis showed that the cycle performance is strongly influenced by the overall thermal conductance of sorption elements due to the sensible heating and cooling requirements resulting from batched cycle operation Another cascading configuration was proposed by (Douss and Meunier 1989; Liu and Leong 2006), based on three adsorbent bed configuration. In the work of Douss and Meunier, 1989, the cascading cycle consisted of a two zeolite-water high temperature stages and an activated carbon-methanol bottoming stage. The experimental COP was found to be 1.06. Despite the discontinuous operation of the system, it is reported to have a nearly continuous cooling capacity of 37 W/kg of adsorbent. This configuration was slightly modified by Liu, 2006, with two zeolite and one silica gel beds and the refrigerant for the three adsorbers is water, Fig. 6. The zeoilte is used as the topping stage with both mass and heat recovery employed in the system design. The reported COP was 1.35, which is larger than the COP found by Meunier, 1989, mainly due to the usage of heat and mass recovery. The configuration is reported to be produce a smaller specific cooling power than both the basic and the heat and mass recovery cycles.

Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper Chua, Ng et al., 1999, presented a transient model for a two-bed, silica gel-water adsorption chiller. Compared with experimental data, the model is found to successfully predict the performance of the system. Saha et al., 1995, investigated analytically the performance of the thermally driven, three-stage adsorption chiller utilizing low-grade waste heat of 50°C and lower temperatures as the driving heat source, in combination with a heat sink (cooling water) of 30°C. The cycle utilized the silica-gelwater adsorption system. A cycle-simulation program was constructed to analyze the influence of operating conditions (temperatures, flow rates and adsorptiondesorption cycle times) on the performance parameters. It is reported that the main advantage of the chiller is that, it is operational with smaller regenerating temperature lifts (ΔTregen = heat sourceheat sink temperature) than other heat-driven chillers. By cycle simulation, it was shown that the threestage chiller can be operated with heat sources of 50 and 40°C in combination with cooling sources of 39 and 30°C, respectively. The simulation results also show that for the chiller to operate effectively, heat sources of 50°C require cooling sources between 35 and 20°C (ΔTregen = 15 to about 30K), while heat sources of 40°C need cooling sources in the range of 28-20°C (ΔTregen = 12 to about 20K).

CONCLUSIONS Many conclusions can be gleaned from this study, these are: Adsorption solar cooling is a good alternative for traditional cooling and refrigeration systems from both environment and energy conservation perspectives. This paper presents an overview of the development of adsorption refrigeration systems. The most researched system is that based on the intermittent basic cycle concept. Several improvements such as heat recovery, mass recovery, thermal wave, convective thermal wave and cascading have been proposed in literatures. Those improvements show that adsorption refrigeration systems are promising and they are continuously under development. Although significant research and development of adsorption refrigeration cycles, a few work is found to power those cycles with solar energy. The use of solar energy to power systems based on mass recovery, thermal wave and convective thermal wave has not been found in literature. It is recommended for future work to investigate the performance of advanced adsorption cycles such as cascade and convective thermal wave, powered by solar energy. It is believed that a better system performance could be obtained if those advanced cycles are coupled with advanced adsorbent materials such as consolidated adsorbents.

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Canadian Solar Buildings Conference Montreal, August 20-24, 2004 Refereed Paper

Figure 1. Layout of simple solar powered adsorption refrigeration cycle.

Figure 2. Adsorption refrigeration cycle

Figure 3. Sorption tube (Critoph 2001)

Figure 4. Thermal wave sorption cycle (Abarzhi 2000)

Figure 5. Convective thermal wave adsorption cycle (Critoph 1999)

Figure 6. The heat and mass flows between the three adsorbers in the cascading cycle (Y. Liu 2006)