thermostat for the control of wet and dry bulb temperatures respectively. This system is a suitable alternative to mechanical vapour compression systems and can be efficiently used for air-conditioning applications with fewer power requirements (5). Sultan et al. (2) review of the soliddesiccantcoolingsystem showed that at this stage, the actual large size of the system is an obstacle in the way of its acceptance as a technological advantage over alternative vapour compression (VC) system. For example, the Energy Efficiency Ratio (EER) for split and multi-split air conditioners is 2.5 (6) even though this value is still high compared to the coefficient of performance (COP) of DEC system. Dezfouli et al.(7) have reported a COP of 1.06 for two-stage solar desiccantcoolingsystem and Jani et al. (8) have achieved a COP of up to 2.0 for soliddesiccant-vapour compression hybrid air-conditioning system, some of the researchers also reported that DEC can be more acceptable for central cooling. At the same time, there is a lack of knowledge and familiarity with desiccant technology amongst designers, developers, architects and end users. As a result, it will be difficult for DEC to expand into the market unless highly efficient desiccant systems are designed and there is a wide acceptance by the end user due to government incentives. For this to happen, there is need for more research into the design of compact and efficient DEC and its performance assessed and quantified. The design should then be validated by the appropriate regulatory body.
The dehumidification of air by a desiccant is used both in air-conditioning and industrial applications, particularly when the latent load is large as compared to the sensible load. Desiccantcooling and air dehumidification is a good alternative for the traditional air conditioning system. In the desiccantcooling process, fresh air is dehumidified and then sensibly and evaporatively cooled before being sent to the conditioned space. Since this system works without conventional refrigerants, such as fluorocarbons, and it allows the use of low temperature heat to drive the cooling cycle, it attracted increased attention especially in humid air climate countries. The heat required for regenerating the weak desiccants can be supplied by fossil fuel, waste heat, or any form of low-grade thermal energy, and most of the time, solar energy is used in such processes. Several studies have been conducted on the operation of the desiccantdehumidification employing evaporativecooling processes [1-4].
energy. El-Dessouky, et al. 9 proposed a novel air conditioning system integrating a membrane dehumidification unit with an indirect/direct evaporative cooler, and found that 86.2% energy saving can be achieved compared with a stand-alone vapour compression system. Jradi and Riffat 10 developed a hybrid dehumidificationcoolingsystem, with which the supply air temperature and humidity reduce from 33.8 ℃ to 22.3 ℃ and 68.6% to 35.5% respectively. Regarding to the membrane-based liquid desiccantdehumidificationcoolingsystem, few researches have been carried out for feasibility study and performance evaluation through experimental work. In this study, a membrane-based hybrid dehumidificationcoolingsystem is built for experimental investigation. The system feasibility and performance are evaluated under various operation conditions; the influences of inlet air condition, desiccant concentration and regeneration temperature on the systemperformance are investigated based on the experimental results.
Desiccantcooling technology is an emerging technology in the field of air conditioning system which controls humidity levels for conditioned air spaces to great extent. Desiccant is a material which has strong affinity for moisture. These materials absorb moisture due to difference in vapor pressure. Desiccants are of two types namely soliddesiccant and liquid desiccant. Some of the solid desiccants are silica gel, calcium chloride. A liquid desiccant is a hygroscopic liquid used to remove water. Some liquid desiccants are glycols (diethylene, tri ethylene, tetra ethylene). The liquid desiccants can be regenerated. Regeneration means that the water absorbed by these substances can be separated from them and again they are used for dehumidification of air. The selection of the desiccant depends on their ability to hold large quantities of water, their ability to be reactivated and cost. Desiccantsystem works in conjunction with conventional air conditioning system to dehumidify the air. Liquid desiccant has several advantages over soliddesiccant. The pressure drop through the liquid desiccant is lower than that through a soliddesiccantsystem and can be stored for regeneration by some inexpensive energy. As the desiccant vapor pressure increases due to the presence of moisture that it has attracted, the desiccant material is transferred to a reactivation/regeneration process. In regeneration process, hot air passed over the desiccant. The vapor pressure of hot air is lower than the desiccant surface and this difference in vapor pressure forces the moisture to transfer from the desiccant surface into hot air stream.
Around the world, increase of the energy consumption and desire to prevent further increased global warming has set a ma- jor focus on developing energy efficient and environmentally friendly system solutions. In the summer season especially, air conditioning systems represents a grooving market in commercial and residential buildings. Two of the main reasons behind this are that the demands for acceptable living standards are increasing as well as the comfort demands of the occupants. The air conditioning unit covers both temperature and humidity control, which leads to traditional vapor compression refrigeration sys- tems requires hug amount of power as well as exhausting a lot of usable waste heat. Traditional vapor compression air- conditioning systems usually decrease the temperature of the air to below dew point temperature to be capable to deal with both latent & temperature and humidity requirement of conventional room. And, it uses CFC and HCFC which are harmful for our environment. Utilization of innovative and clean energy sources has head technology research in new directions. One attrac- tive to traditio1ial vapor compression air-conditioning are a desiccantcoolingsystem where soliddesiccant wheels are used to dehumidified the air. Usually evaporative coo1ing ensures that the air temperature is decreased to acceptable indoor standards.
An integrated coolingsystem of desiccant dehumidifier and PDEC was evaluated for a typical dwelling in a hot humid climate in India after being combined with an earthtube ventilation system. A process for enabling the simulation of the proposed system has been reported. The system’s performance was investigated with a parametric analysis and it was found that by using the EAT+DW+PDEC system as opposed to using natural ventilation the peak indoor summer temperatures were reduced by about 8 °C while indoor relative humidity remained below 75%. The proposed system could provide space cooling in hot-humid climates and could be an alternative to high energy consuming conventional vapour compression AC units.This study analysed a worst-case scenario of a building without any shading and by assuming typical materials that are not of high thermal standards. With improvements however in building designs, the proposed system could ensure good levels of indoor thermal comfort.
Abstract: Membrane-based liquid desiccantdehumidification has attracted increasing interests with elimination of solution droplets carryover problem. A membrane-based hybrid liquid desiccantdehumidificationcoolingsystem is developed in this study, which has the ability to remove latent load by a liquid desiccantdehumidification unit and simultaneously to handle sensible load by an evaporativecooling unit. The hybrid system mainly consists of a dehumidifier, a regenerator and an evaporative cooler, calcium chloride is used as liquid desiccant in the system. This paper presents a performance evaluation study of the hybrid system based on experimental data. Series of tests have been conducted to clarify the influences of operating variables and conditions (i.e. desiccant solution concentration ratio, regeneration temperature, inlet air condition, etc.) on the systemperformance. The experimental results indicate that the system is viable for dehumidificationcooling purpose, with which the supply air is provided at temperature of 20.4°C for the inlet air condition at temperature of 34°C and relative humidity of 73%. At desiccant solution concentration ratio of 36%, the thermal COP th of 0.70 and electrical COP el of 2.62 are achieved respectively under
was found that maximum effectiveness between 0.55 and 0.706 with concurrently cooling and without cooling it was 0.38 to 0.55, i.e effectiveness of internally cooled packed bed dehumidifier is larger than adiabatic ones. Yonggao Yin et al.  by using LiCl as desiccant studied internally cooled plate fin heat exchanger for dehumidification. The results suggested that plate fin heat exchanger had good coolingperformance for the desiccant also low desiccant temperature cause more mass transfer. Improved dehumidificationperformance can be obtained using simultaneous internal cooling dehumidifier. Yonggao Yin et al. , by using LiCl as liquid desiccant investigated internally cooled dehumidifier and heated regenerator. The study shows that internally heated regenerator will avoid dehumidification risk which might happen in adiabatic one, and additionally it might provide higher regeneration potency that the adiabatic one. The internally cooled dehumidifier may also give higher dehumidificationperformance examination with adiabatic one, but its profit would be not nearly as good as internally heated regenerator. The commonly used liquid desiccants are inorganic salts like lithium chloride (LiCl), lithium bromide (LiBr) and calcium chloride (CaCl2). LiCl and LiBr are strong desiccants than CaCl2. A therotical model was proposed by I.P. Koronaki et al.  under the same operating condition for a small counter flow adiabatic dehumidifier. LiCl has the best efficiency, can reduce humidity ratio by 10% compared to LiBr 8% and CaCl2 6%. . For cross flow liquid desiccant dehumidifier C. G. Moon et al. , developed new empirical correlation by experimenting dehumidifier with CaCl2 as liquid desiccant. It was found that air flow rate, air humidity ratio, air temperature, solution
Other kinds of performance enhancement have also included the adoption of fin heat exchangers in the implementation of internally cooled film dehumidification, taking the form of plate-fin-heat exchanger (PFHE) [56, 67] or fin-tube heat exchanger .
To further enhance performance, some designs adopt evaporativecooling in the internal cooling channels of the absorber. Here, a standard plate heat exchanger, PHE, with several flow passages separated from each other by thin plastic plates, is employed such that dehumidification is carried out on one side of the plate, while evaporativecooling is concurrently effected on the other side, in a cross flow manner. Each thin plate, besides separating the water-air passage from the solution-air passage, also provides a contact area for heat and mass transfer between the fluids flowing in each passage. Saman and Alizadeh  conducted numerical analysis and indicated that the dehumidification efficiency of the PHE increases as the solution mass flow rate increases and up to 85% efficiency could be achieved at an optimal primary air velocity of about 0.7 m/s (0.3 kg/s) however; the performance achieved in an experimental study by Alizadeh  was only about 60%. Furthermore, an experimental study by Saman and Alizadeh  indicated the performance of the system is also dependent on the angle of inclination of the PHE, with an optimal angle about 25 – 45 o .
compression system. In food processing industry, applications require RH and temperature within the specific limit but by using cold coil dehumidification we can’t achieve required RH and temperature economically. The investigated method includes dehumidification with application of cooling, liquid desiccantdehumidification, dual cooling sources dehumidification and dehumidification wheel. By analysing following methods we understand that soliddesiccantdehumidification which is suited for food processing industry. Required RH and temperature can be achieved by using soliddehumidification wheel. The applications of desiccantsystem are also expanding widely and showed higher potential as compared to VAC system. The desiccant air-conditioning (DAC) involves desiccantdehumidification for humidity control and evaporativecooling for temperature control. The DAC is an attractive technology because it is free from CFCs/ HCFCs/HFCs and can be regenerated by low grade thermal energy.
3.2. System COP
Figure 2 shows the system COP for both the SDEC system and the VAV system. For the referenced VAV system, it is found that in Darwin, the system COP is almost constant at the value of 3, indicating that the VAV system is operating under the cooling mode all year round. While for other cities, especially for Hobart, Canberra and Melbourne, the COP deceases to zero in winter period, which implies that no cooling needed for those periods. For the SDEC system, Darwin has the highest monthly system COP, reaching up to about 7 in the summer and 2 in the winter. Hobart and Canberra have the lowest COP compared with other cities, just below 1 in the summer. The system COP diagram implies that for Darwin, the desiccantsystem can achieve much higher dehumidifying effect with less backup energy consumption due to substantial solar energy gain. Brisbane has a relatively high COP of 2.6 in the summer and 0.55 in the winter. It is also a cooling dominated climate which requires plenty of cooling demand all year round. However, the solar energy gain in Brisbane is not as sufficiently abundant as Darwin for regeneration purpose and thus requires more backup energy, leading to a lower COP value than Darwin. Adelaide, Perth and Sydney have similar system COP of just below 2 in the summer and nearly zero in the winter, indicating no dehumidification is needed in winter periods in these cities.
It has been seen that the standalone DAC can be operated by solar thermal energy for moderate humid climates for substantial reduction in operating cost of the same. The systemperformance increases with higher regeneration temperature. A dedicated DAC system for a given operating condition can obtain a certain level of performance and the design needs to be modified to achieve higher performance and economic feasibility. The present study reviewed on many kind of modifications namely VAC assisted hybrid system and multi-stage dehumidification. It has been found that multi-stage DAC system can be operated at lower regeneration temperature as compared to single-stage cycle delivering the same dehumidification amount. The moisture removal capacity in the two-stage dehumidification is found to be higher than single-stage with constant flow rate which helps to improve the system COP. Feasibility studies show that the potential of DAC in dry conditions is limited but there the evaporativecooling will become an optimum solution rather than conventional vapor compression system. Most of the earlier studies show that payback period of less than five years can be obtained by utilizing the DAC system intelligently and thus significantly contribute in ameliorating energy and cost saving and environmental protection.
The dehumidified air temperature is increased due to the heat of adsorption (process 1–2) as shown in Fig. 3 which depends on the type of adsorbent as discussed earlier. The dehumidified air is then cooled at constant humidity using heat exchanger (process 2–3) and meanwhile the recovered heat is used for the desiccant regeneration (process 6–7) which increases the system COP. Then the following cooling techniques are used to push the process air (stage 3) into the thermal comfort zone (stage 4 or 5) that includes: (i) direct/indirect evaporativecooling, (ii) cold coil, (iii) M-cycle cooling, and (iv) evaporator of the VAC. In case of DEC cooling the water vapours are added as an isenthalpic process as shown in process 3–4 whereas the rest of cooling is performed at constant humidity that can be noticed from process 3–5. M-cycle cooling technique is a kind of IEC technique which can cool the air near to the dew point rather than wet bulb temperature which is shown in Fig. 4. The process 8–9 in Fig. 3 shows the desiccant regeneration which can be made by heating the desiccant or reducing its pressure. However, heating is preferred in commercial applications whereas pressure reduction is adopted in industrial processes. A typical DAC system consists of a desiccant unit, heat recovery component, a cooling unit and a regeneration unit. The available solid DAC systems can be categorized as: (i) adsorbent bed or rotating wheel type, (ii) single-stage or multi-stage dehumidification type, (iii) single-rotor or multi-rotor type, (iv) hybrid system assisted by VAC, and (v) electricity/gas/solar/waste energy operated DAC system [17-22].
Consequently, the purpose of this study is to investigate empirically the performance of an ICLD using evaporativecooling for use of internal cooling sources. The ICLD used in this study was designed for a cross-flow type between the process and working air streams. The impact of the variation in working air stream on the dehumidificationcoolingperformance of an ICLD is a key factor of this study. Therefore, the dehumidificationcoolingperformance of an ICLD was examined under various working airflows in an environmental climate chamber. The airflow ratio (AFR) between the working and process air was adjusted from 0.25 to 1.0 while increasing the AFR 0.25 for each experimental step. The measured data were estimated using systemperformance indicators such as the cooling capacity, wet-bulb effectiveness, volume mass transfer coefficient, and coefficient of performance (COP) of the ICLD.
Vapour compression based air conditioners are being used for comfort cooling in residences, offices and commercial buildings in many countries throughout the world. Several issues arise seriously challenge the conventional technology which include harmful impact of CFCs and HCFCs on environment, energy and cost problems etc. However use of low energy and eco friendly techniques may be helpful to overcome these problems up to some extent. The evaporativecooling may be considered as one of such eco friendly methods of achieving comfortable conditions in buildings during summer. These systems have great potential to provide thermal comfort in places where ambient air humidity is low. Such systems alone are not efficient for humid climatic conditions. However such systems can be used if combined with desiccant based dehumidifier.
The selection of an evaporativecooling unit for an LDDC system depends on climate condition, supply air demand, cost and etc. The feasibility of an LDDC system with an indirect evaporativecooling unit is evaluated by experimental work in , in which the indoor air temperature reduces from 33.8°C to 22.3°C and the relative humidity decreases from 68.6% to 35.5%. With the similar design concept, a drop of 7.5°C in the indoor air temperature is achieved in . In response to the climate condition and air conditioning requirement in Hong Kong, a hybrid liquid desiccant air conditioning system is developed by integrating both direct and indirect evaporativecooling means , whose performance is investigated by theoretical modelling. The LDDC system with an evaporativecooling unit is proved with remarkable energy and cost saving potentials . By installing an evaporative-cooling assisted LDDC system for an open office building in Seoul, South Korea, 12% saving of the annual primary energy consumption could be achieved compared with the conventional air conditioning system . More than 50% of the lifetime operating cost can be saved with an LDDC system compared to the conventional air conditioning system in Singapore . By utilizing renewable energy in the regeneration process, the LDDC system can be more sustainable and economical, for example the abundant solar energy. Not only solar thermal but also photovoltaic technologies can be integrated into a solar-assisted LDDC system . The feasibility of a membrane-based liquid desiccantdehumidificationcooling (MLDDC) system powered by solar thermal energy is proved in with significant economic and environmental benefits.
Traditional vapour compression systems (VCS) employing vapour compression cycle have been used for air-conditioning & refrigeration in both industrial and residential applications. Thus, these systems are quite popular and well known. But some limitations of the VCS technology are that the refrigerant gases used are harmful for the environment and also these systems have high energy consumption in the form of electricity. An alternative to these systems are the evaporativecooling systems (ECS). These systems achieve cooling effect through evaporation of water. The ECS can be categorized into direct contact and indirect contact devices. Since the working fluid is water, there are no harmful by-products of this system. Major advantage of ECS is that it blows fresh air into the desired location instead of recirculating same air like VCS. Only a fan or blower is required to circulate cool air. Also, pumping device is required to circulate water throughout the circuit. Hence, ECS have comparatively less electricity consumption than VCS. Despite these advantages, the ECS can be employed only in environments having less humidity. This is because ECS work on the principle of evaporation of water. As warm air comes in contact with cool water, the water at the contact evaporates.
 , for example, they represent around 39% and 40% of energy consumption and 38% and 36% of CO 2 emission in the US  and Europe  respectively. Within the building section,
humidity control is of vital importance for ensuring indoor thermal comfort  and product quality . As a matter of fact, 20-40% of energy consumption in heating, ventilation and air- conditioning (HVAC) systems is used for air dehumidification . The conventional vapour compression technology which uses cooling coil to dehumidify moist air has several problems. For instance, it has relatively weak ability of dealing with latent heat load, which leads to low coefficient of performance (COP) in humid area. It also has the problems of growths of mould and bacteria, surface corrosion . Thus the current trend is to make HVAC system more energy efficient and less dependent on electrical power from fossil fuels .
The most important concluding remarks in this study are: Some desiccantcooling cycles have been analyzed and suggested a most efficient desiccantcooling cycle for selected climatic conditions. Direct and indirect evaporativecooling methods can be used for different cycles of desiccantcoolingsystem. Many studies emphasized on the optimization of operating parameters and used exergetic manufacturing cost (EMC) method for deciding the minimum regeneration temperature and R/P ratio. The effect of different operating parameters on the performance of desiccantcoolingsystem analysed and presented minimum running cost. Optimum wheel speed of about 17.5 rpm for high moisture removal and maximum COP. The R/P ratio, regeneration temperature, desiccant material, rotation of desiccant wheel, outdoor conditions etc. are the important parameters which affect the performance of desiccant wheel. The operating cost of the desiccantsystem in summer Italian conditions, interesting saving up to 35% are obtained and reduced thermal cooling power up to 52% and pay back period obtained about 5-7 years. Among the Ventilation, Recirculation and Dunkle cycle, the Dunkle cycle is better for wide range of outdoor conditions. Many studies evaluated the performance of different desiccant material and found the material which has the higher moisture adsorption capacity. Hybrid desiccantcoolingsystem economies 37.5 % electricity power when the process air temperature and relative humidity are maintained at 30 ºC, and 55% respectively. Solar energy may be suitable option for regeneration of desiccantcoolingsystem and it saves the regeneration power. Some studies stressed the energy saving potential by solar assisted desiccantcoolingsystem. Mathematical modeling and simulation study of solar based desiccantcoolingsystem performed by some researchers. A composite wheel can be absorbs more moisture than conventional desiccant wheel with same operating conditions. Compounding of desiccant wheel obtained the greater moisture adsorption rate. Some researchers applied the system in different fields and proved the feasibility of the DCS.
64 Fig. 4: (a) Side view and (b) angle view of evaporativecooling box.
Two different materials namely activated carbon foam and luffa pad were tested one by one. The thickness of each type of pad is set to be 2 cm for each sheet of pad. Initially set up was run for about 10 min to ensure near steady state condition. Then dry bulb and wet bulb temperatures of air at inlet and dry bulb temperature at outlet were measured using thermocouples and recorded using data logger. The saturation effectiveness of cooling pads was calculated using the following relation as mentioned in equation (1) below: