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Solar Air Conditioning System Using Dessicants

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Solar Air Conditioning System Using Dessicants ISSN: 2348 9510 International Journal Of Core Engineering & Management(IJCEM) Volume 1, Issue 2, May 2014 SOLAR AIR CONDITONING SYSTEM USING DESSICANTS Arjit Sharma Student, Mechanical Engineering Department Medicaps Institute Of Technology & Management [email protected] ABSTRACT- A new technology for cooling and dehumidifying INTRODUCTION: buildings using desiccants is now entering the The basic principle behind (solar-) thermal market. This technology, which can efficiently driven cooling is the thermo-chemical process of serve large latent loads, will greatly improve sorption: a liquid or gaseous substance is either indoor air quality by both allowing more attached to a solid, porous material (adsorption) ventilation as well as more tightly controlling is taken in by a liquid or solid material humidity. Furthermore, since a desiccant air (absorption). The sorbent (i.e. silica gel, a conditioner can be packaged as a roof-top air- substance with a large inner surface area) is handler, it will compete directly with the most provided with heat (i.e. from a solar heater) and popular cooling system now .A desiccant is dehumidified. After this "drying", or dehumidifier in conjunction with evaporative desorption, the process can be repeated in the coolers can reduce air conditioning operating opposite direction. When providing water vapor costs significantly since the energy required to or steam, it is stored in the porous storage power a desiccant cooling system is small and medium (adsorption) and simultaneously heat is the source of this required energy (solar, waste released. heat, natural gas) can be diverse . Processes are differentiated between closed My paper focuses on the use of solar energy for refrigerant circulation systems (for producing this purpose. cold water) and open systems according to the 56 ISSN: 2348 9510 International Journal Of Core Engineering & Management(IJCEM) Volume 1, Issue 2, May 2014 way in which the process is carried out: that is, systems are already available in the markets whether or not the refrigerant comes into long time ago, mostly not solar driven except contact with the atmosphere. The latter is used few hundred units around the world (mainly in for dehumidification and evaporative cooling. Europe) with growing interest for solar powered Both processes can further be classified application. The most common ones are the according to either liquid or solid sorbents. In absorption and adsorption chillers. addition to the available refrigerating capacity, Open systems: The most common systems the relationship between drive heat and realized based on the principle of desiccant cooling and cold energy (coefficient of performance; COP) is using rotary dryers with solid sorbents, these also an essential performance figure of such systems allow a full treatment of the air that is systems. cooled and dehumidified to ensure the needs of In Solar thermal driven air conditioning systems, comfort. The refrigerant is water, in direct the heat generated by the solar is used to power contact with the ambient air. the cooling process. The Solar thermal air Desiccants are chemicals with great affinity to conditioning systems generally classified into moisture. They absorb (or release) moisture two types: because of the difference in vapour pressure between the surface of the desiccant and the surrounding air. Dehumidification is said to occur when the vapour pressure of the surface Closed systems: refrigeration equipment of the desiccant is less than that of the powered by thermal carriers (hot water or surrounding air. Dehumidification continues until the desiccant material reaches equilibrium steam) directly producing chilled water, which with the surrounding air. Regeneration of this can be used in the air conditioning systems air desiccant is said to occur when the vapour handling unit (cooling, dehumidification) or pressure of the desiccant is larger than that of the surrounding air, which is usually achieved by distributed through a network of pipes heating the desiccant to its regeneration decentralized terminal conditioning in several Temperature and exposing it to an airstream. rooms to be conditioned (e.g., fan coil). These Desiccants can be classifed as either adsorbents, 57 ISSN: 2348 9510 International Journal Of Core Engineering & Management(IJCEM) Volume 1, Issue 2, May 2014 which absorb moisture without accompanying used desiccant materials in desiccant cooling physicaland chemical changes, or absorbents, systems. which absorb moisture accompanied by physical or chemical changes. Desiccants can be solids or liquids and can hold moisture through adsorption or absorption as described earlier. LITERATURE REVIEW: Most absorbents are liquids and most adsorbents are solids. Several types of solid The use of sorption air dehumidification – desiccants are widely used in desiccant cooling whether with the help of solid desiccant systems; silica gels, lithium chloride and molecular sieves. material or liquid desiccants – opens new Silica gels are solid desiccants and adsorbents possibilities in air-conditioning technology. and contain numerous pores and capillaries in This can offer an alternative to classic which water is condensed and contained. Silica compression refrigeration equipment. gel has a high capacity to absorb moisture and Alternatively, if it is combined with standard then release it at a higher temperature. It is low vapour compression technology, it leads to in cost and available in sizes from 3/16 inch higher efficiency by an increase of the beads to powder-like grains. Lithium chloride required evaporator temperature of the (LiCl) is an absorbent and is found in dry form Compression cycle. Desiccant systems are when each LiCl molecule holds two water used to produce conditioned fresh air directly. molecules. If each LiCl molecule holds more than They are not intended to be used as systems two water molecules, for former becomes liquid where a cold liquid medium such as chilled and continues to absorb moisture. water is used for heat removal, e.g., as for Lithium chloride has a high capacity to absorb thermally driven chiller based systems. and hold moisture and is widely used in rotary Therefore, they can be used only if the air- wheel dehumidifiers. Among the above, silica conditioning system includes some equipment gels and lithium chloride are the most widely to remove the surplus internal loads by supplying conditioned ventilation air to the 58 ISSN: 2348 9510 International Journal Of Core Engineering & Management(IJCEM) Volume 1, Issue 2, May 2014 building. This air-flow consists of ambient air, for evaporative cooling. The evaporative which needs to be cooled and dehumidified in cooling process uses the evaporation of liquid order to meet the required supply air water to cool an air stream. The evaporation conditions. Desiccant cooling machines are heat that is necessary to transform liquid designed to carry out these tasks. water into vapour is partially taken from the air. When water comes into contact with a primary warm air stream it evaporates and Economic advantages arise for desiccant absorbs heat from the air, thus reducing the cooling equipment when it is coupled with air temperature; at the same time, the water district heating or heat supplied from a vapour content of the air increases. In this combined heat and power (CHP) plant. Of case, the supply air is cooled directly by particular interest is the coupling with thernal humidfication and the process is referred to as solar energy. The components of such systems direct evaporative cooling. are generally installed in an air-handling unit Indirect evaporative cooling involves the heat and are activated according to the operation exchange with another air stream (usually mode of the air-conditioning system. These exhaust air), which has been previously operation modes implement different physical humidified and thus cooled. In this case, the processes for air treatment, depending on the water vapour content of the primary air load and the outdoor air conditions. These stream is not influenced. These two systems are based on the physical principle of techniques of evaporative cooling can also be evaporative and desiccant cooling. combined in a process that is known as Unsaturated air is able to take up water until a combined evaporative cooling. state of equilibrium, namely saturation has Complementing combined evaporative cooling been achieved. The lower the relative with desiccant dehumidification enhances the humidity of the air, the higher is the potential cooling capacity of the cycle and thus it is 59 ISSN: 2348 9510 International Journal Of Core Engineering & Management(IJCEM) Volume 1, Issue 2, May 2014 possible to reach even lower temperatures. form of packed towers or the like. This combined cooling process is referred to as Regeneration heat must be supplied in order desiccant cooling. Using evaporative cooling, to remove the adsorbed (absorbed) water either direct, indirect or in a combined from the desiccant material. The required heat process, it is not possible to reduce the vapour is at a relatively low temperature, in the range content of the ventilation air. But, using a of 50°C to 100°C, depending on the desiccant desiccant cycle, in principle lowering of the material and the degree of dehumidification. temperature and the humidity ratio of Moreover, the solar desiccant cooling system, ventilation air is possible. Fresh air conditions depending on the cooling loads and have a considerable
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