Layout of simple solar powered adsorption refrigeration cycle. 

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... 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. 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, dew- point 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. 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 (T max ) during the daytime and a cooling system at intermediate temperature (T int1 ) during the night time. While the system consisting of the condenser plus evaporator exchanges heat with another heat sink at intermediate temperature (T ), and a heat source at low temperature (T min ). 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 (T 1 ) to (T 2 ), inducing a pressure increase from the evaporation pressure (P e ) up to the condensation pressure (P c ), Fig. 2. The evaporation pressure (P e ) and condensation pressure (P c ), are the saturation pressures of the adsorbate at the evaporator temperature (T e ) and condenser temperature (T c ), 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 (P c ), i.e. constant pressure process or isobaric desorption. The concentration (x 1 ) of adsorbate decreases to (x 3 ). The adsorbent temperature continues increasing from (T 2 ) to (T 3 ) inducing desorption of vapor. This desorbed vapor ( ∆ x= x 1 -x 3 ) is liquefied in the condenser. The condensation heat (Q c ) 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 (T 3 ) to (T 4 ) inducing a pressure decrease from the condensation pressure (P c ) down to the evaporation pressure (P e ). 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 (P e ), i.e. isobaric adsorption. The adsorbent temperature continues decreasing from (T 4 ) to (T 1 ), inducing adsorption of vapor. This adsorbed vapor is vaporized in the evaporator by heat transfer from the surroundings, causing a cooling effect (Q e ). 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. Activated- carbon 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 Activated- Carbon 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 o C, 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 o C. 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 o C. 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 ( Q ) to that of the high temperature thermal energy used ( Q ), it can be expressed ...