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2.3 CONCLUSIONS
The air‐conditioning load of a building is the sum of the sensible and latent load and represents the 20‐40% of the overall energy consumption in a building. Dehumidification handles the latent load, while cooling handles the sensible load. Traditional vapour compression systems need to overcool the air stream to provide, apart from cooling, dehumidification. This means that air conditioning operates at a temperature colder than the supply air dew‐point temperature, so the air needs reheating before entering indoors.
Vapour compression systems are therefore not so energy efficient for handling the latent load in buildings.
Open cycle evaporative cooling systems with liquid desiccant mediums, as known as liquid desiccant cooling systems, have been proven to be an energy efficient method for air dehumidification, compared to conventional air conditioning systems. In contrast to vapor compression air conditioning systems, in which the electrical energy drives the cooling cycle and the air is overcooled, desiccant cooling is heat driven and the air does not have to be cooled below its dew point. Desiccant cooling avoids the conventional problem of re‐heating to compensate for the over‐cooling, as it does not rely on cooling to produce dehumidification. Therefore, liquid desiccant systems have the potential to utilize cleaner energy sources such as waste heat or solar thermal energy. The entire operation takes place at atmospheric pressure, eliminating the need for capital intensive, pressure sealed components. This makes it a very environmentally friendly technology choice if properly designed, sized and managed in use.
Desiccants are hygroscopic materials that absorb or give off moisture to the surrounding air due to a difference between the water vapor pressure at their surface and that of the surrounding air. The moisture content depends on the desiccant and temperature at the same relative humidity. If the desiccant contains more moisture than the surrounding air, it releases moisture, absorbs heat and produces a cooling effect equal to that of evaporation.
If it contains less moisture, it absorbs moisture from the air and releases heat equal to the latent heat given off if a corresponding quality of water vapor were condensed.
The liquid desiccant cooling systems consist of two main components; the dehumidifier and the regenerator. Firstly, the strong desiccant solution is sprayed at the top of the dehumidifier. The ambient humid air enters the dehumidifier at the bottom, transfers its moisture to the desiccant and heat is liberated. At the same time, cold water, derived from
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solution temperature and hence, the solution vapour pressure at low levels. The dehumidified air exits at the top and the warm, now diluted solution leaves the bottom of the dehumidifier and it is pumped for regeneration. More efficient dehumidification of air is achieved at low desiccant inlet temperatures and high desiccant inlet concentration.
The regeneration is the process in which the diluted desiccant solution retrieves its initial concentration. This process has great impact to the energy efficiency of the liquid desiccant cooling systems, since this is where heat is required. The regenerator device has the same configuration with the dehumidifier; however, the process occurring is just the opposite. The diluted desiccant solution comes into contact with the ambient air. Since the vapor pressure of the desiccant is now higher than that of the air, moisture is evaporated from the solution and transferred to the exhaust air stream. At the same time, hot water, derived from a low temperature source, circulates in coils inside the regenerator, in order to maintain the solution temperature and hence, the solution vapour pressure at high levels. At the end of the process, the hot humid air is rejected to the ambient air and the concentrated solution is driven to the dehumidifier.
Liquid desiccant cooling systems are ideally applied where large latent loads are present, where a low dew point is required, where high humidity can damage properties and materials and where high air quality is necessary. Indicative examples are the supermarkets, museums, ice rinks, indoor pools, hospitals, laboratories, archive buildings, food industries and pharmaceutical industries.
This chapter provided a review on the available literature regarding the heat and mass transfer operations in liquid desiccant cooling systems. Plenty of research papers have been written on the absorption (dehumidification) and desorption (regeneration) principles, with the earlier tracing back to 1969. Since then, great strides have been made; insightful theoretical models on simultaneous heat and mass transfer process during dehumidification and regeneration have been established. Most of the studies reviewed concentrated more in counter flow configuration, and less in cross flow or co flow, due to its high heat and mass transfer effectiveness. Researchers have been experimenting with apparatuses, in order to produce the heat and mass transfer correlations, as well as to validate their theoretical models. Practical conceptions, such as comparisons between types of desiccant solutions, system configurations and flow patterns, have been proven very useful to following researchers. The effects however of some inlet parameters on the efficiency remain uncertain up to now; among others, the optimum air to desiccant flow ratio, the wetting ratio of the desiccant solution over the packing, the relationship between the heat transfer
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area and the mass transfer area and the distribution of the absorption heat between the desiccant film and the process air. Further analytical and experimental investigations are needed to improve the overall performance, to make systems more predictable and to promote their widespread application.
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