A dynamic model is presented for a chiller working with a composite adsorbent (silica activated carbon/CaCl2)–water pair in a solar-biomass cooling installation. The main objective is determining a link between two possible evaporator configurations and the refrigerator’s performances. The two considered evaporators work at different pressure levels. The related time evolution profiles of temperature, pressure and water content are studied. Moreover, the effects of hot water inlet temperature and cooling water inlet temperature on the specific cooling capacity (SCP) and coefficient of performance (COP) are predicted by means of numerical simulations. The results show that an increase in the temperature of hot water and a decrease in the temperature of the cooling water allow an increase in COP and SCP. In particular, for a hot water inlet temperature of 85°C and a cooling water inlet temperature of 40°C, the COP and Qev are 0.67 and 4.3 kW, respectively.
During the last decades, we are witnessing a large, generalized and even more accentuated energy deficit in developing countries. Noble energies are mainly produced by fossil sources which not only are gradually depleted but also have harmful environmental consequences: pollution of the atmosphere, destruction of the ozone layer, production of greenhouse gases contributing to the global warming. The current trend is therefore to find new forms of energy from renewable sources to overcome this problem. The ideal would be to produce all the energy used by renewable sources, but unfortunately this sector still faces many technological and commercial challenges.
The increasing demand of the air conditioning pulled a significant increase of the demand of the primary energy resources. Adsorption refrigeration systems are one of the processes which can be fed using renewable energies. Several works in the field of adsorption chiller have been successfully realized.
The purpose of which to survey the performance evaluation parameters of adsorption cooling systems such as coefficient of performance and specific cooling power. Optimization methods for improving the performance of a refrigeration system by adsorption. Among these factors, adsorption refrigeration pairs is the most important role for system performance, and the development of another type of adsorbent (composite adsorbent) to improve system performance [
Improving system performance involves improving mass and heat transfer by increasing the adsorption rate, subsequently improving COP and SCP, and since the adsorbent bed is the heart of the machine. adsorbent bed configuration and Adsorbent are the keys to improvement for this fact several adsorbent have been tested and several configuration are studied.
Many configuration of adsorption systems with different pairs have been studied in the literature for refrigeration application, a new adsorption pair AlPO4-34/water is synthesized by Kim et al. [
In order to provide maximum efficiency under specified operating conditions [
The objective of this paper is to improving the performance of a refrigeration system by solar-biomass adsorption by using an composite adsorbent, which presents a mixture of silica gel, activated carbon and of chloride of calcium with the percentage following 10% activated carbon, 30% CaCl2 and 60% silica gel in order to maximizing cooling power.
The collectors absorb solar energy to heat the heat transfer fluid, which is then pumped to a storage tank. The biomass boiler is located between the adsorption chiller and the hot water storage tank. The boiler is considered as an additional source; in case of lack of sun or if solar energy is insufficient. It is controlled by a controller and heats the water entering the refrigerator. The refrigerator consists of two beds and two evaporators operating with the pair (silica/activated carbon/CaCl2)–water. The heat required for desorption is drawn from hot water coming from the storage tank heated with solar energy or biomass. While the cooling circuits is supplied by the cooling tower. Chilled water produced by the refrigerator provides comfort conditions through the room fan coil.
The diagram of the refrigerator with double evaporator is drawn in
After the adsorbent bed is cooled by the cooling water to decrease the temperature levels and subsequently its pressure to connect with the low pressure evaporator in the next step. Then the adsorbent bed is linked to the low pressure evaporator allowing low pressure adsorption. The chilled water first enters the high pressure evaporator and then passes through the low pressure evaporator. The water in the evaporator evaporates and the vapor is adsorbed by the adsorbent, (
The rate of desorption or adsorption is calculated using the linear driving force kinetic equation. The coefficients of this equation for the silica activated carbon/CaCl2)–/water couple are determined by Chihara et al. [
The effective mass transfer coefficient inside the pores Ks is given by:
The effective diffusivity is defined as follows:
where:
Dso = 2.54 × 10−4 m2/s, RP = 1.7 × 10−4 m, Ea = 4.2 × 104 J/mol, F0 = 15, R = 8,314 J/mol K.
The equilibrium uptake of adsorbent pair is estimated using the equation developed by Boelman et al.[
where Ps is the corresponding saturation vapor pressure of the refrigerant. Ps for water vapor is determined by the following equation:
The adsorption energy balance is described by:
The outlet temperature of cooling water can be expressed as:
The desorption energy balance is described by:
The outlet temperature of hot water can be expressed as:
The condenser energy balance equation can be written as:
The outlet temperature of cooling water can be expressed as:
The energy balance in the evaporator is expressed as:
The outlet temperature of chilled water can be written as:
The mass balance for the refrigerant can be expressed by neglecting the gas phase is:
where,
System performance equation
The COP value is defined by the following equation:
The cooling capacity of the system is expressed by:
where:
Specific Cooling Power
where:
ma = 50 Kg,
Ca = 0.930 kJ/kg.K, Cpr = 4.18 kJ/kg.k,
The chilled water (heat transfer medium at the evaporator) was cooled first from 18°C to about. 14°C–15°C by the higher pressure evaporator, then it is cooled again to approx. 11°C–12°C by the lower pressure evaporator.
Cold water outlet temperature peaks were observed around 450, 870, 1290, and 1710 s, during which the adsorbent bed (the adsorbent) was in the pre-cooling phase.
There is no evaporation of the refrigerant in the evaporator during the pre-cooling which leads to the increase of the cold water temperature. The serial flow of cold water causes a small lag between the peak outlet temperatures of the two evaporators.
The water content and the temperature of the adsorbent beds under the operating conditions mentioned above are shown in
The water content of the adsorbent has been reduced, During the first 420 s, to approximately 0.12 kg/kg during the desorption phase, Once the desorption is complete, the temperature of the adsorbent bed approaches thathot water inlet temperature. the adsorbent bed is pre-cooled for a period of 40 s, and its temperature decreases to approximately 55°C. Then the low pressure adsorption starts and the adsorbent bed is connected to the low pressure evaporator. The adsorbent bed 2 was cooled with the cooling tower, and at the end of the low pressure adsorption process, the water content of the adsorbent bed 2 increased by about 0.118 kg/kg. At 700 s, to start the low pressure adsorption process and the adsorbent bed 2 is connected to the low pressure evaporator which leads to a water content of the adsorbent at around 0.125 kg/kg.
In this paper, we have studied a two element adsorption system with double evaporator. Good performance is obtained with this design. The effect of the operating conditions (hot water temperature, cooling water temperature) on the performance of the system is made in order to improve them. The results of the simulations show that the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption cooler are highly dependent on the generation temperature. It is observed that an increase in the temperature of hot water and a decrease in the temperature of the cooling water allow an increase in COP and SCP, has a best performance at large temperature difference between inlet and outlet chilled water temperature, also, The biomass boiler makes it possible to keep a constant desorber heating temperature, which facilitates the sizing of the installation for a well-defined energy requirement.
Heat transfer area, m2
Specific heat, kJ/kg.K
Coefficient, m2/s
Heat exchanger efficiency
Latent heat of vaporization, kJ/kg
Masse, kg
Mass flow rate, kg/s
Isosteric heat of adsorption, kJ/kg
Pressure, Pa
Heat, kJ
Specific cooling power, kW/kg
Time, s
Temperatures, °C
Overall conductance, W/m2.K
Instantaneous Uptake, Equilibrium uptake, kg de réfrigérant/kg d’adsorbant
Coefficient of performance of the machine
Adsorbent (silica gel)
Adsorber
Adsorption
Condenser
Cycle
Evaporator
Desorber
Coolant
Inlet
Coolant indice
Maximum
Minimum
Numerical
Outlet
Refrigerant
Refrigerant vapor
Vapor