Among the thermal processes of solar energy, solar refrigeration is one of the most suitable processes for storage, transport and marketing of energy. Among its numerous applications, the adsorption refrigerating machine seems to be an interesting alternative to conventional refrigeration systems in isolated regions, where conventional electrical power is unavailable. However, these machines are not fully automatic because of manual interventions needed for its operation.
The adsorption refrigeration system using an adsorbent /adsorbent working pair, is composed by different elements: a solar collector which contains the adsorbent /adsorbate working pair (in our case it is the activated carbon/methanol), a condenser where the adsorbate vapor condenses, an evaporator where water plates are laid out to be transformed into ice, in order to store cold for deferred use, and a refrigeration compartment.
The development of solar adsorption refrigeration systems appeared in the late 1970s, following the needs of non-oil countries, and several studies have been undertaken since that time. Marmottant have studied and manufactured in 1990 a solar ice maker based on adsorption/desorption phenomena which operates intermittently and uses the working pair activated carbon /methanol. A solar adsorption refrigerator was built and tested in 2000 by Hildbrand in Switzerland using the pair Silica gel-Water. The system does not contain any movable part, and the author has obtained a COP between 0.12 and 0.23. Mayor made an adsorption refrigerator working with the pair silica gel/water. This refrigerator is characterized by its compactness and its ability to be transported. The working volume of this refrigerator is 100 liters, the surface of the solar collector is 1m2 and its mass reaches 150 kg. This machine was built with materials to minimize the mass of the system. For better insulation of refrigeration compartment, vacuum panels (VIPs) were used, while a large storage volume capacity was maintained. An independent valve was developed to eliminate any human manipulation. Abu-Hamdeh investigated some work on solar adsorption refrigerator using parabolic trough collector and uses olive waste as adsorbent with methanol as adsorbate. The author showed, from the COP values, that the optimal adsorbent mass varied between 30 and 40 kg while the optimum tank volume varied between 0.2 and 0.3 m3. Wang developed a novel two-stage adsorption freezing machine, which is powered by the heat source with the temperature below 100°C. The composite adsorbents of CaCl2 and BaCl2 developed by the matrix of expanded natural graphite were chosen as adsorbents. The experimental results showed that the optimal coefficient of performance (COP) and specific cooling power (SCP) at 15 °C refrigeration are 0.127 and 100W.kg-1, respectively. COP and SCP increased with the increasing heat source temperature and decreased with the decreasing evaporating temperature.
The goal of this work is to develop an adsorption refrigeration system for cold production under Algeria’s climate. Experimental test was done on a prototype elaborated in laboratory in order to test the feasibility of the machine. Software was elaborated, giving an estimate of the activated carbon and methanol quantities in the adsorption refrigerator, the energy balance and the design of its various components, as well as performance coefficients of the machine.
In developed industrialised countries where there is grid electricity, there is no cost effective application for solar powered refrigeration of food, medicines etc. However, there may still be the possibility of costeffective solar air conditioning. The target cost per unit of cooling must be competitive with conventional systems, perhaps around £1 / Watt. None of the above mentioned systems are likely to achieve this figure. A few photovoltaic powered air conditioning systems have been built experimentally, but they are prohibitively expensive. Evacuated tube solar collectors have been used in conjunction with conventional Lithium Bromide - Water absorption air conditioners but are still far too costly and complex. The best possibility for cost effective solar powered air conditioning appears to be desiccant cooling. Desiccant cooling has been known and available for many years but has become popular in recent years due to two factors. The first is that the damage to the ozone layer by conventional chlorofluorocarbon refrigerants has necessitated the search for alternatives to vapour compression refrigeration. Secondly, the need to replace the peak load demand for electricity for air conditioning applications coupled with the desire of gas utilities to balance their heating loads with a summer alternative has lead to the development of heat powered refrigeration cycles. The result has been research into improved desiccant materials and cycles to both improve performance and reduce costs. Desiccants are substances that have a strong affinity for water and, because of this, can absorb moisture from an air stream. Desiccants can be solids such as lithium chloride, silica gel or molecular sieves, or liquids such as glycol, sulphuric acid or lithium bromide solution. There is a partial pressure of water vapour than can exist in equilibrium with a desiccant at a particular temperature. If the actual vapour pressure is above the equilibium value, moisture will be absorbed, but if it is lower then moisture will evaporate from the desiccant. The process is therefore reversible. The most common arrangement of desiccant system is the desiccant rotor. A desiccant rotor consists of a honeycomb support which has been impregnated with a finely divided desiccant. As air flows axially through the narrow honeycomb channels, moisture is absorbed by the desiccant. The design of the rotor gives a large surface area of contact between air and desiccant. As the air stream passes through the rotor, moisture is absorbed and the heat of absorption, almost equal to the latent heat of condensation, is released. The resulting air stream is therefore warmer but drier. The latent enthalpy contained in the moisture vapour is effectively exchanged for sensible enthalpy in the temperature of the resulting air. It is arranged that the rotor rotates slowly so that desiccant that has been exposed to moist process air moves into a separate sector. Here warm air, in which the vapour pressure is less than the equilibrium vapour pressure, carries away moisture that evaporates from the desiccant on the rotor A schematic of a desiccant cooling system is shown in Figure 6. Latent heat contained in the fresh air (1) drawn into the building is exchanged on the desiccant rotor for sensible and the air temperature rises. This warm dry air (2) is then passed through a heat wheel. The heat sink for the heat wheel is extract air (5) from the building that has been cooled by evaporative cooling (6). The resulting air stream (3) is therefore cool and dry. If moisture is added to this air stream, evaporative cooling takes place and cool air (4) is supplied to the building. The warm moist extract air (6) after the heat wheel is heated further and the hot gas (7) passed through the desiccant rotor. Moisture leaves the rotor and so a warm moist air stream (8) is discharged to outside the building. 30°C
WORKING PRINCIPLE
The following figure shows the prototype of the machine adsorption in semi pilot scale manufactured. The realization and test of a prototype at this scale allows evaluating the feasibility of the pilot scale adsorption refrigerator and its operating parameters. The prototype has the following components: a thermally insulated refrigeration compartment, an evaporator, a condenser and an adsorption tube collector. The operation principle of the machine consists in heating by solar radiation the adsorbent contained in the adsorption collector, which is disposed horizontally. This energy should be sufficient to desorb the molecules of the adsorbate (methanol) and to be transformed from its liquid phase into vapour. Then, the methanol vapours are condensed in a condenser and collected in a tank then evacuated towards the evaporator in a liquid phase. The adsorbent starts to cool gradually when solar radiation begins de decrease in the evening to reach the ambient temperature. This decrease in temperature involves the adsorption phenomenon of the activated carbon with the methanol. Cold production is the result of the energy needed to evaporate the methanol in the evaporator, which willbe adsorbed by the activated carbon. This phenomenon will cease when the adsorbent is completely saturated with methanol for a temperature slightly higher than the environmental temperature and the initial vacuum pressure.
The adsorption refrigerator Prototype with semi pilot scales.
Adsorption solar collector scheme
SIMULATION
Refrigerated cabinet containing evaporator and ice for cold storage
Temperature evolution of ice used for cold thermal storage (time simulation: 4 hours 30 min).
THE ADSORPTION CYCLE FOR REFRIGERATION
DEVELOPMENT OF A SOLAR POWERED ADSORPTION CHILLER
CONCLUSION
The goal of this work is to develop an adsorption refrigeration system for cold production able to answer the socioeconomic requirements, in particular in term of total low costs (solar collector, equipment, maintenance) and technological simplicity (system without valve and self-adapting in the external conditions). A prototype on a semi pilot scale was elaborated, and the experimental tests were carried out in a laboratory. Cold thermal storage is used in order to store cooling energy use while shifting. Simulation of the phase change phenomena is undertaken in order to determine the quantity of PCM (ice) required to counteract the heat losses at the walls during its melting cycle (night period). Computation programme was elaborated, giving an estimate of the activated carbon and methanol quantities in the adsorption refrigerator, the energy balance and the design of its various components, as well as the thermal and solar performance coefficients of the system. A manufacture and optimization work is being done for an adsorption refrigeration machine on a pilot scale, for a refrigeration compartment volume of 100 L.
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