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ABSTRACT
In this paper the basic principles and theories of solid adsorption solar refrigeration is presented. The criteria for selection of adsorbent/adsorbent pairs are also reported. This is a hybrid system that combines the solar refrigeration and heating cycle and uses working pair as activated carbon and methanol. The hybrid system has been designed for heating 50 liters of water from 25oC to 90oC as well as cooling 10 liters water from 25oC to 10oC within 1 hour at a coefficient of performance of 0.85. A programming code has also been developed for designing the concentrating collector.
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. INTRODUCTION
In recent years, the global increasing demand for refrigeration (e.g. air-conditioning, food preservation, vaccine storages, medical services and cooling of electronic devices) led to production of more electricity and consequently more release of CO2 all over the world. The accelerated consumption of fossil fuels now has been recognized to cause serious environmental and energy problems such as global warming, ozone depletion, atmospheric pollution, and worldwide acute shortage of energy. The International Institute of Refrigeration (IIR) in Parris has estimated that approximately 15% of all electricity in the world is used for various kinds of refrigeration and air conditioning. The energy consumption for air-conditioning systems has recently been estimated to 45% of the whole households and commercial buildings [1]. Solar assisted solid adsorption system is thought to be an attractive way to serve the needs for refrigeration, Air-conditioning applications. Solar cooling systems have been most commonly employed as the means of extending the vaccine cold chain to rural areas without conventional energy supplies. They may also be used for making ice. Fig. 1 illustrates a possible classification of solar refrigerators and refrigeration cycles [2, 9].
2. SELECTION OF ADSORBENT/ADSORBATE
2.1 Materials of adsorbate
The adsorbate or refrigerant for the application being investigated should have the following properties:
• Evaporation temperature below 0°C.
• Small molecular size to enable it to be absorbed into the adsorbent.
• High latent heat of vaporization and low specific volume.
• Thermally stable with the adsorbent at the cycle operating temperature ranges.
• Non-toxic, non-corrosive and non-flammable.
• Low saturation pressures (slightly above atmospheric) at normal operating temperature.
The fact that methanol and water operate at sub-atmospheric saturation pressures at the operating temperatures makes them favorable choices. Ammonia does not have this problem because its outward leak could be tolerated for some time, but its saturation pressure of 13 bars at the condensing temperature of 35°C is quite high. In the case of methanol, with a normal boiling point of 65°C, the low saturation pressures could be exploited advantageously to detect leakages, as it must result necessarily in abnormal increases in system pressure and poor operational performance [4, 5].
Ammonia, methanol and water, all have relatively high latent heats of 1368, 1160 and 2258 kJ/kg respectively and their specific volumes are low on the order of about 10−3 m3/kg. Ammonia is toxic and corrosive whereas water and methanol are not. The problem with alcohols is that they are flammable.
Water is the most thermally stable in the presence of suitable adsorbents, closely followed by
methanol and ammonia. However, water cannot be used for freezing purposes because its freezing temperature is 0°C. This makes methanol a favored adsorbate for pairing with a stable adsorbent.
2.2. Material of adsorbent
The important considerations influencing the choice of a suitable adsorbent are
1. adsorption of large amount of the adsorbate under low temperature conditions to yield good COP
2. desorption of most of the adsorbate when exposed to thermal energy
3. possession of high latent heat of adsorption compared to sensible heat
4. no deterioration with age or use
5. non-toxic and non-corrosive; and low cost and widely available
Properties (1) and (2) tend to be mutually exclusive, although activated carbon strikes the best compromise of the adsorbents discussed in the literature.
Natural zeolites also have possibilities, but significantly larger
quantities would be required for water as the adsorbate since only a small amount of adsorbate is desorbed in going from room temperature to flat plate solar collector temperatures.
However, zeolites have another unique property that their adsorption isotherms have extremely nonlinear pressure dependence, which is of importance in solar refrigeration applications. Activated carbon and silica gel have almost linear pressure isotherms. Silica gel satisfies properties (1)–(5) above but is expensive and may not be available in most developing countries t creating the environmental pollution.
3. BASIC PRINCIPLES OF ADSORPTION COOLING
3.1. Solid adsorption principle
Adsorption is the fixation of gas molecules on the surface of a solid, according to Vander Waals interaction. Adsorption is reversible and exothermic. It neither induces change of solid volume nor involves any chemical reaction [6].
The operation principle of the solid adsorption refrigeration system utilizing solar heat is shown in Fig. 2. The system is composed of a container of adsorbents, which serves as a solar collector, a condenser and an evaporator which acts as a refrigerator. A combination of adsorbent and adsorbate is confined in a closed system where no carrier gas exists. The collector is supplied with activated carbon (A.C) which is adsorbed with methanol
During the day-time the activated carbon along with the methanol is heated in the collector. Methanol evaporates from the activated carbon and then is cooled by the condenser and stored in the evaporator.
During the night-time, the collector is cooled by ambient air and the temperature of the activated carbon reaches a minimum. In this period, methanol begins to evaporate by absorbing heat from the water to be frozen and is adsorbed by the activated carbon. As the evaporation of the methanol continues, the water temperature decreases until it reaches 00C, where ice starts to be formed.
From the Clapeyron diagram, the total energy gained by the system during the heating period QT will be the sum of the energy QAB used to raise the temperature of the A.C+ methanol from point A to B and the energy QBD used for progressive heating of the A.C to point D and desorption of methanol.
Coefficient of Performance
4. EXPERIMENTAL SET UP
The hybrid system consists of a parabolic solar concentrator, water tank, adsorbent bed, condenser, expansion device, receiver and evaporator. The schematic design of a hybrid solar powered water heater and adsorption refrigerator is shown in Fig.4.
The heating of water in the tank starts in the morning through the solar concentrator by natural circulation. With the increase of the water temperature, the temperature in the adsorbent bed rises, which causes the vapour pressure of the desorbed refrigerant reach up to the condensing pressure, desorption at constant pressure is initiated. The desorbed vapour is condensed in the condenser and collected in the receiver.
The high pressure liquid refrigerant expands in the capillary tube up to evaporator pressure. The low pressure liquid refrigerant enters in to evaporator
The hot water from the tank could be drained out and movies into another tank. With the refilling of the water tank with cold water, the temperature of the adsorbent bed reduced rapidly and the pressure in the adsorber drops to a value below the evaporator pressure. Evaporation will happen, if connecting vale is open and cooling water will made in the evaporator box.
5. DESIGN DETAILS
Design of Solar Concentrator
The solar concentrator has been calculating for heating of 50 liters water from 250C to 900C. The design details are given according to our requirement, shown in table.1. As well as condenser, adsorbent bed details are also given,
6. CONCLUSION
According to the literature survey the adsorbent refrigerant pair, Activated carbon-methanol has better performance than other pairs and discussed their properties. The design of hybrid solar assisted adsorption system has been done. The hybrid system heats 50 litres of water from 25oC to 90oC as well as cools 10 litres of water from 25oC to 10oC within 1 hour. The theoretical COP of the system is found to be 0.85.Comparing with the PV powered refrigerator this system is economical and utilizes the maximum available solar energy