Proceedings of the 9 th Biennial ASME Conference on Engineering Systems Design and Analysis ESDA2008 July 7-9, 2008, Haifa, Israel

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1 Proceedings of the 9 th Biennial ASME Conference on Engineering Systems Design and Analysis ESDA2008 July 7-9, 2008, Haifa, Israel ESDA HYBRID LIQUID DESICCANT / VAPOUR COMPRESSION AIR-CONDITIONING SYSTEMS: A CRITICAL REVIEW Roza I. Christodoulaki* Thermal Engineering Section Emmanuil D. Rogdakis Thermal Engineering Section Irene P. Koronaki Thermal Engineering Section School of Mechanical Engineering, National Technical University of Athens, 15780, Zografou, Athens, Greece. ABSTRACT Hybrid Liquid Desiccant * Cooling / Vapour Compression Systems is an environmentally friendly technology used to condition the internal environment of buildings. In contrast to conventional vapor compression air conditioning systems, in which the electrical energy drives the cooling cycle, desiccant cooling is heat driven; therefore, hybrid LDC/VCS have the potential to utilise cleaner energy sources such as gas, hot water, waste heat or solar thermal energy. In hybrid LDC/VCS, the latent cooling load is handled by the desiccant dehumidifier, while the sensible is handled by a conventional VCS. Hybrid systems combining liquid desiccant cooling with Vapor Compression Systems, Vapor Absorption Systems and Solar Collectors use less electrical energy compared to conventional air-conditioning alone, while these savings rise as the latent load increases. Unlike other surveys on desiccant cooling, this review focuses on a detailed coverage of the hybrid LDC/VC systems. Commonly used liquid desiccants are compared towards their physical properties. Hybrid LDC/VCS employing various components and features are summarized, while different system configurations are schematically presented. Key factors for the hybrid system performance are the desiccant material, the design variables and the conduction of experiments prior to operation. 1 NOMENCLATURE ARI: Air-conditioning and Refrigeration Institute COP: Coefficient of Performance DCHE: Direct Contact Heat Exchanger HVAC: Heating, Ventilation and Air-Conditioning LD: Liquid Desiccant LDCS: Liquid Desiccant Cooling System MEG: Monoethylene Glycol NTU: Number of Transfer Units PPG: Poly-Propylene Glycol RH: Relative Humidity * PhD candidate and author of correspondence TEG: Triethylene Glycol VAS: Vapor Absorption System VCS: Vapor Compression System 2 INTRODUCTION In recent years, ozone layer depletion has created considerable public concern. The excessive use of halogenated chlorofluorocarbons in refrigeration and airconditioning installations for about 60 years, provide the major source of stratospheric chlorine and are a major threat to the ozone layer [1,2]. These global environmental concerns along with improving air quality standards have all contributed to a change in design thinking. Overheating is now the predominant design consideration for new buildings, mainly because of the computer equipment increase, the inefficient lighting installations and often, architectural fashion. If development strategies compatible with economic and environmental sustainability are sought, then alternative refrigeration methods, such as hybrid Liquid Desiccant Cooling with Vapor Compression Systems LDC/VCS are the objective. The air-conditioning load is the sum of the sensible and latent load [3,4] and represents the 20-40% of the overall energy consumption in a building [5]. Dehumidification handles the latent load, while sensible cooling handles the sensible load [3,4,6]. Traditional VCS overcools the airstream to provide cooling and dehumidification [3,4,7,8,9,10,11,12]. Air-conditioning operates at a temperature colder than the supply air dew-point temperature [3,9,10,11,13,14,15], since for a typical humidity ratio of this occurs at 9 0 C [16], so the air needs reheating before entering indoors [3,6,7,8,11,16,17]. Additionally, the ratio between refrigeration capacity and electrical energy required is not higher than unity [18]. Unlike conventional evaporative cooling systems, hybrid LDC/VCS is an open heat driven cycle that provides indoor comfort even in hot and humid weather [5,13,18,19,20]. All studies have shown that, since the air does not have to be cooled below its dew point, utilization of waste heat or solar 1 Copyright 2008 by ASME

2 energy can be realized and that reduction in humidity, corrosion and microbiologic activity is achieved. The entire operation takes place at atmospheric pressure, eliminating the need for capital-intensive, pressure-sealed units [8]. This makes it a very environmentally friendly technology choice if properly designed, sized and managed in use. The desiccant cooling cycle was first developed in Sweden and has been used successfully in Scandinavia [21,22] and recently in the United Kingdom [22]. Desiccant systems are mainly applied where large latent loads are present or low dew point is required (supermarkets, museums, ice rinks, indoor pools, buildings ventilation systems), where high humidity can damage properties and where high air quality is necessary (hospitals, laboratories, archives, food, pharmaceutical industries) [2,6,7,23,24,25,26]. These systems generally require about 20% more plant room space, compared to a traditional system (3m height for 10m 3 /s air supply, or 4m for 14m 3 /s) [19]. During summer, the system efficiency is reduced, with a cooling output of kw per kw of heat input, and considerably more energy is consumed. However, if the summertime supply air temperature is set to 23 0 C, a supplementary cooling coil is not necessary [22]. Li et al. [5] add that energy efficiency ratio is during summer and during winter. During spring and autumn, there are significant energy savings, as evaporative cooling can be realized without the need to dry the air. Particularly, 30% running cost savings and 20% total investment savings have been observed, compared to a traditional chiller plant [19]. During winter, humidification can be conveniently realized by contacting the air with a LD previously heated from the condenser [5] and recovered heat may be used [19]. 3 LIQUID DESICCANT MATERIALS 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 [2,13,20,25,27,28,29,30,31]. The moisture content depends on the desiccant and temperature at the same relative humidity [27,32]. 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 [27]. The desiccant selection is decisive in the overall performance of the system and depends on the boiling point elevation, energy storage density, regeneration temperature, thermo-physical properties, availability and cost [11]. In desiccant wheels where solid adsorbents (zeolite, silica gel, activated alumina, molecular sieves) are used, the absorption is adiabatic [28,33] and there is no chemical change [29,31]. Solids need C regeneration temperature to achieve sufficient dehumidification [28,33,34]. Because of the continuous regeneration, storage of dehumidification enthalpy within the desiccant wheel is not possible [28,33]. Packed beds with solid desiccants induce high pressure drops which require excessive fan power [29,34]. Liquid desiccants (triethylene glycol, CaCl 2 -H 2 O, LiBr- H 2 O, LiCl-H 2 O, KCOOH-H 2 O) change chemically when they absorb moisture and despite their less drying capability [26,35], they have still distinct advantages over solids [23,26,29,31,36,37]. They are potentially more efficient than solid desiccant ones: set point COP of 2 compared to 1 [38]. They can store energy [11,23,24,33,35,37,39] with 1000MJ/m 3 capacity [36,39,40], which is about 4-6 times of the conventional ice storage tank [39] and up to 3.5 times higher compared to solid desiccants [24,36]. This is achieved with a high air-to-solution mass ratio corresponding to a small desiccant flow [24,29,31,33,35] to achieve great differences between salt inlet and outlet concentrations [24,33]. A maximum storage capacity of 1540MJ/m 3 (or 1354 MJ/m 3 with 40% LiCl) is theoretically achievable [24]. Also, LDCS can theoretically cool the air to temperatures as low as C [41]. All researchers state that liquids enable more efficient dehumidification even at low regeneration temperatures, C, allowing the use of solar energy. Other advantages over solids include smaller pressure drop and easy transportation to the source of regeneration heat [3,26,28,34,35,38], easier integration within HVAC plants, more efficient removal of bacteria and dust [7,26,29,31,37,38,39,42] and less cost [7,37]. However, aging of the liquid desiccant reduces the cooling performance of the system [26]. Additionally, energy storage is performed at the expense of energy and of COP [11]. The energy storage capacity is limited because the moisture removed from the desiccant during regeneration equals to the moisture condensed from the air during dehumidification and because of the crystallization risk [11]. The crystallization line defines the conditions at which salt hydrates or anhydrous salt crystallizes from the solution [10,17]. In absorption chillers, the crystallization line is usually very close to the working concentrations [43] and is triggered by high ambient temperature, low ambient temperature with full load, air leak or non-absorbable gases produced during corrosion, excessive heat input to the desorber, failed dilution after shutdown, too low chilled water supply temperature with too high ambient and/or exhaust temperature [13,31,43]. Therefore, if the solution concentration is increased [8,31,43] or the solution temperature is reduced, crystallization may occur, interrupting machine operation [31,43]. Therefore, different desiccants have different operating concentration levels (45-60% LiBr, 30-45% LiCl, 35-45% CaCl 2, 90-98% TEG) [13,31]. Crystallization mostly occurs in the strong solution entering the dehumidifier, at its lowest temperature [43]. Crystallization in the piping network immediately blocks the flow, so the concentrated solution temperature needs to be raised above its saturation point, to dissolve the salt crystals. No need to mention that the recovery of the dehumidifier operation after crystallization is labour and time intensive [43]. 3.1 LITHIUM CHLORIDE LiCl, CALCIUM CHLORIDE CaCl 2, LITHIUM BROMIDE LiBr These salts require corrosion resistant materials (plastic, stainless steel), thereby increasing the equipment cost [2,11,28,39]. They have higher surface tension than glycols, 2 Copyright 2008 by ASME

3 so adequate surface wetting is more difficult [28,44]. At about 55 0 C all the three desiccants have the same mass transfer coefficient [45]. LiCl has been commercially available since 1937 [7,11,46,], as it is the most stable, has large dehydration concentration (30-40%) and can reduce RH by 60%, though it is expensive (8-16$/kg in 1998) [11,25]. LiCl and LiBr present better dehumidification than the KCOOH [7] and the cheapest CaCl 2 (0.4$/kg in 1998) [11,25,42,46,47,48]. The poor performance of CaCl 2 is attributed to its high vapor pressure and its dependence on the air inlet conditions [11,25]. The partial pressure of CaCl 2 increases as its concentration is decreased [48]. An increase in the CaCl 2 mass flow rate decreases the air humidity ratio and increases the primary air temperature [48]. 3.2 KCOOH KCOOH performs better than LiCl and LiBr in regeneration tests, it is less corrosive and expensive, fully compatible with the environment and allows significant humidity reductions [7]. 3.3 TRIETHYLENE GLYCOL (TEG), MONOETHYLENE GLYCOL (MEG), POLYPROPYLENE GLYCOL (PPG) The earliest liquid desiccant system used TEG and solar heated air for regeneration purposes [2,11,32]. TEG is preferred to MEG [35,49], since MEG has yielded inaccurate results and has higher vapour pressure [35]. 95% TEG has similar effectiveness with 40% LiCl [31]. Glycols work well as desiccants and are less corrosive than salts, but their high viscosity increases the pumping power of the system [2,11,28]. They have very low surface vapour pressure and their evaporation contaminates the air, whereas salts have essentially zero vapour pressure and cannot evaporate [2,7,11,28,29,35,50]. Though TEG is not particularly toxic, it may cause respiratory irritation [28]. These evaporation losses are unacceptable in an occupied building and increase the initial cost; therefore salts dominate LDCS [7,29,35]. However, adding 10% by weight of polystyrene sulfonic acid salts to TEG improves its moisture absorption capacity by 15%, lowers its vapor pressure, thus reducing evaporation [28]. Finally, TEG regeneration requires a higher solution temperature in humid climates compared to dry climates [50]. 3.4 MIXTURES Desiccant mixtures combine the properties of individual materials and improve their characteristics. Mixing LiCl with CaCl 2 produces a desiccant solution less costly than LiCl and more stable than CaCl 2 [11,25,28,31]. Saman et al. [48] and Al-Farayedhi et al. [45] found that CELD, a mixture of 50% CaCl 2 and 50% LiCl, has higher mass transfer coefficient and performance than CaCl 2 alone. 4 HYBRID LDC / VC PRINCIPLES AND OPERATION 4.1 ABSORPTION / DEHUMIDIFICATION The Hybrid LDC/VCS avoids the conventional problem of re-heating to compensate for the over-cooling, as it does not rely on cooling to produce dehumidification [51]. The LDCS dehumidification in a counter-flow packed bed is shown in Fig. 1 [52]. The strong, cool desiccant solution, after passing through a heat exchanger where it is pre-cooled by water, is sprayed at the top of the dehumidifier. The ambient humid air enters the dehumidifier at the bottom, transfers its moisture to the desiccant [28,42,52] and heat is liberated [28,52], rising the solution temperature and hence, the solution vapour pressure [52]. The moisture of the air is controlled by controlling the desiccant temperature and concentration [44,52]. Moisture is reduced by maintaining a low desiccant temperature or high concentration at the dehumidifier s inlet [52]. The dehumidified air exits at the top and the warm, diluted solution leaves the bottom of the dehumidifier and it is pumped for regeneration [52]. Figure 1: Air dehumidification with liquid desiccant [52]. Figure 2: More detailed air dehumidification with liquid desiccant [31]. Theoretically, the most efficient desiccant dehumidifier has an infinitely large effective desiccant surface combined with an infinitely low mass [3,31,34]. Indeed, increasing solution flow rate or increasing surface wetting (by about 15%) increases dehumidification; though, depending on the wetting percentage, a dehumidification threshold value is reached [2,11,42,48,52,53]. Better wetting of the plates with desiccant can be achieved with low surface tension [45]. As a result, the surface wetting with the concentrated LiCl is 3 Copyright 2008 by ASME

4 worse than that with CaCl 2, since LiCl has higher surface tension. The main challenge for high surface wetting and the distribution of the liquid desiccants is the intended low desiccant flow rate, which can be overcome by special spray nozzles or thin membrane tubes [42]. Dehumidification is increased by increasing the contact time allowed for the reaction [3,52], decreasing air flow rate [11,42,53], maintaining a low vapour pressure in the desiccant material [42] and increasing the desiccant temperature [2,54]. Liu et al. [55,56] agree with the latter, but they add that the increase in desiccant temperature reduces regeneration. Dehumidification is also enhanced by increasing the solution concentration [2,33,47,57] and the latter is valid for solar hybrid LDCS [20] as well. Another method is to increase the heat exchanger effectiveness or to decrease the water temperature [20,33,52]. The thermal performance of a dehumidifier depends also on the angle of the heat exchanger plates [42,53]. Particularly, the highest dehumidification (60%) [42], has been observed at the highest angle (45 0 ) [42,53]. The change in air inlet temperature slightly affects the dehumidifier performance [55,56]. Structured packed type TEG dehumidifier simulation showed that the dehumidification increases with air flow rate and temperature decrease [2,13]. The moisture removal increases with air and desiccant flow rate increase, air and desiccant temperature decrease and desiccant concentration increase [2,13] for both aluminium and wood packings [13]. Khan [57] also determined that the performance strongly depends on the physical size, NTU and the cooling-toprocess air mass flow rate. The performance is also related to the amount of water condensed; for each gram of water condensed from the air to the desiccant in the dehumidifier, one gram of water must be evaporated from the desiccant in the regenerator [52]. One potential problem of the dehumidifier may be the flooding of the channels, which usually happens at high air velocities (>8m/s) [48,53]. Thus, the maximum air velocity should be 0.5m 3 /s ( 1.5m/s) [48,53]. Additionally, the construction of dehumidifier units requires non-corrosive hydrophobic materials (polyethylene, polypropylene) [42]. Attention should also be paid to flow channel cross-section, which tends to narrow in the presence of film [48,53]. The film may also become wavy if the shear stress at the air/water interface becomes large or if the film thickness becomes large (>20% of the cross-section) to cause turbulence [48,53]. Dehumidification is accomplished using finned-tube surfaces in a column, coil-type absorbers, spray towers, packed towers [3,52] and solar stills [3]. Two types of dehumidifiers can be distinguished; the adiabatic and the internally cooled dehumidifier [11,33]. Both of them have similar COPs when operating in cross and counter flow arrangements [28]. Figure 3: Open cycle adiabatic and internally cooled dehumidifier [33]. Adiabatic packed bed dehumidifiers have the highest performance (COP=0.79) [11,28], they have been applied in industrial and residential units and they have large airdesiccant contacting area with simple geometry sides [11]. Potential drawbacks are the cost [28], the large pressure drop on process air and the increase in the desiccant s temperature during the moisture removal [11]. Figure 4: Adiabatic dehumidifier [44]. An internally-cooled dehumidifier, compared to an adiabatic one, operates at an order of magnitude lower ratio of desiccant-to-air flow rates [28]. However, the cooling coils into the packing materials make its installation difficult and an insulation layer is needed to avoid the adverse heat transfer from ambient air [11]. Only enough desiccant to wet the coil surface or the combination of a heat exchanger with a high coolant mass flow rate in the top portion of the dehumidifier is suggested for efficient operation [18]. Khan [18] observed poor load-removal performance in the top 25-30% of the internally cooled dehumidifier, since there is carried-over heat brought in by the regenerated solution. As with all dehumidifiers, its performance depends on water-to-air mass flow rate ratio, carried over regeneration heat, water inlet temperature, NTU and desiccant concentration [18]. 4 Copyright 2008 by ASME

5 Figure 5: Internally cooled dehumidifier [11]. There are three flow patterns namely, cross flow, parallel flow and counter flow as shown in Fig. 6, with counter flow being the most widely used [11]. This happens because parallel and cross flow dehumidifiers produce a less favourable mass transfer gradient between the air and the desiccant, resulting in a higher air exit humidity ratio [55,56,57]. Though, due to the non-uniform water flow, a performance limitation exists for a particular nozzle manifold and packing arrangement [58]. Figure 6: Flow configurations of dehumidifiers [11]. As opposed to counter flow, the performance of a parallel flow dehumidifier, does not affect the thermal performance. Thus, Khan et al. [57], Saman et al [48] and Liu et al. [55] state that parallel flow has less pressure drop and increased sensible heat transfer than the counter flow and, in contrast with Pietruscha et al. [42], they suggest that the parallel configuration is more desirable than counter flow [57]. According now to Pietruscha et al [42], cross flow dehumidifier offers the best opportunities, as the ducts arrangement are more practical [55,56]. The addition of Cuultra fine particles stabilizes the solution and enhances cross flow dehumidifiers effectiveness [11,31]. Simulation results showed that the average droplet radius strongly affects the thermal efficiency of a cross-flow cooling tower [59]. The moisture removal process increases the desiccant temperature. Single-stage dehumidification causes an adverse temperature increase, which weakens the mass transfer between desiccant and air. In multi-stage dehumidification, several single dehumidifiers are connected in series and the desiccant is separately cooled in every module, resulting in better dehumidification [11] and higher recovery efficiency [27]. This configuration would be especially beneficial in large buildings [37,28,31,41]. Figure 7: Multi-stage dehumidifier [11]. 4.2 REGENERATION The regenerator can have the same configuration and packing materials with the dehumidifier, except that the latter has an additional insulating and filter layer to remove the droplet of the desiccant [8,11]; however, the processes occurring are just opposite [11]. Both the air dehumidification and the desiccant regeneration involve simultaneous heat and mass transfer with large heat effects [28], but the main energy consumption occurs during regeneration [11,54]. The warm, diluted solution that leaves the dehumidifier enters the regenerator [52]. There, the desiccant is further heated, though its vapor pressure remains high so that it cannot absorb moisture. The hot, concentrated desiccant that leaves the regenerator passes through a heat exchanger where it is first cooled by the diluted desiccant leaving the dehumidifier and further cooled by a desiccant-to-water heat exchanger [52]. This cooling reduces the desiccant s vapor pressure, so that it can absorb moisture once again. This is referred to as regeneration [3,7,11,27,28,41,42,47,54]. Regenerating a strong desiccant solution requires high temperatures [44], high solution flow rates [44], as large as possible surface area exposure [54], extended contact time between the desiccant and the air [54] and low air flow rate [55,56]. Indeed, Hamed et al. [34] constructed a new rotating LiCl absorption disk that reduces the air humidity ratio by 13% (T regeneration <85 0 C, RH=50%, T ambient =30 0 C) and found that at lower speeds, the regeneration temperature is increased; thus, the amount of water absorbed is increased [34]. If the low-grade heat source cannot afford the regeneration, an ancillary heater may be needed [11]. The liquid-to-liquid heat exchanger improves the dehumidifier performance, reduces the heat input to regenerator by 10-15% and improves the system s thermal efficiency [31]. Instead of using thermal energy, mechanical energy could be used for regeneration [60]. Similarly to seawater desalination, reverse osmosis can concentrate the weak desiccants by removing the water from the solution. The osmotic pressure required for CaCl 2 is much less than that of LiCl [60]. 5 Copyright 2008 by ASME

6 Figure 8: LDCS counter flow packed bed regenerator [54]. 5 SYSTEM COMPONENTS The three basic configurations for dehumidifiers and regenerators are the Packed bed (or tower), Spray bed (or tower) and Falling film (or wetted wall). These liquid-gas interaction equipments should be designed to handle large process air and low desiccant flow rates [31,35]. 5.1 PACKED BED / TOWER A packed tower is the most common column and it is filled with packing, preferably plastic or ceramic [61], providing large interfacial surface between air and liquid [3,16,23,28,31,32,35,47,52,54,61]. Packed towers may have high pressure drop and initial cost [31,32,35], but have high residence time, while crystallisation and dirt are of minor importance [35]. Dehumidification is particularly effective within the first sections of the packing, when the air has the highest humidity ratio [47]. Maximum humidity reduction is achieved when the partial vapour pressure of the air at the outlet of the tower is equal to the saturation pressure of the solution at the inlet of the tower, namely the water vapour pressure in air which has come into equilibrium with solution [47]. The performance of a packed bed absorption tower is influenced by the packing type (shape, size and material), desiccant distribution over the packing, flow configuration (cross or counter flow), tower height [3,11,13,34,52,55,61], desiccant temperature and concentration [3,13,52,55,56,58], air temperature and humidity [13,52,55,56], desiccant viscosity, density and surface tension [2,13,28,35,52]. The air velocity should be 50-80% of the flooding velocity to avoid flooding [61]. As the bed length increases, the absorption capacity is increased and the percentage of water removed is decreased [34]. Packing media other than LiBr and CaCl 2 are found to perform more efficiently [52]. Although in few studies [23,52] higher desiccant flow rates seem to reduce the performance, higher temperatures and flow rates of both air and solution enhance the regeneration [3,35,52]. Pre-cooling the air entering the dehumidifier and preheating the air entering the regenerator enhances the beds performance [3,35]. The finite difference model, the effectiveness-ntu model and the model based on fitted algebraic equations have been developed for the analysis of packed beds [11,52,62]. Gandhidasan [52] and Mei et al [11] suggest that the finite difference model gives more accurate performance predictions, while other studies [3,12,28] shows that the effectiveness-ntu model needs less computational time and agrees with the finite difference model Random Packing Random packings (ceramic saddles, Raschig rings, Berl saddles, Intalox saddles, plastic or polypropylene Pall rings and polypropylene Rauschert Hilflow rings) have been commonly used [11,31,50,55]. Intalox saddles have the lowest pressure drop, but Raschig rings are the cheapest [61]. In contrast to Mei et al. [11], random packings give better performance than structured packings [28,61] and their evaporation rate is % greater than in the structured packed bed [50]. Random packed towers facilitate more mass transfer by providing a larger contact area [35,45,49,52], but the air pressure drop is generally high [45,49,50,52]. Longo et al. [7] found that the dehumidification / regeneration efficiency in random packings is independent of the desiccant material. It has been found though that LiCl and LiBr have higher dehumidification (6-7g/kg) than the KCOOH (5g/kg); however, KCOOH shows better regeneration (0.45%) than LiCl and LiBr (0.25%) [7]. The random packed tower computer model of Lazzarin et al. [47] using LiBr and CaCl 2, matched closely with the experimental values. H 2 O/LiBr and H 2 O/LiCl dehumidify in temperatures up to 40 0 C, but need higher temperatures for regeneration (50 0 C) [7]. Therefore, it is possible to move from dehumidification to regeneration by simply increasing the solution inlet temperature, by C [7]. The transition concentration between regeneration and dehumidification in LiCl is 25%, in LiBr is 42% and KCOOH is 57% [7]. Changing the flow rate ratio L/G in the range 0.5-2, controls the dehumidification / regeneration [7]. An increase in solution concentration enhances the dehumidification up to the crystallisation conditions [7] Structured Packing The superiority of structured packings (cellulose rigid media pads, wood grids, expanded metal lash packing, double spiral rings) is evident; they redistribute the liquid flow, have low pressure drop, but they are expensive [11,45,49,52,58]. The fixed orientation of the mass transfer surfaces in structured packings provides improves the mass transfer effectiveness [11,55,58]. The closely spaced nozzles have better uniform flow and thermal performance [58]. The performance of the nozzles depends on the flow rate, drop sizes generated, drop velocity and operating pressures; the swirl-jet nozzles provide the best absorption than any other nozzles [63]. The most suitable spacing interval between the plates is 6-8mm [11]. They can be gauze-type or sheet-type [61]. In the gauze-type structured packing, the heat and mass transfer coefficients depend on the diameter of the flow channel in the packing [45]. The liquid-phase mass transfer coefficient increases by increasing the effective liquid velocity and the 6 Copyright 2008 by ASME

7 mass transfer coefficient increases by increasing the desiccant concentration [45]. Among all, the plastic sheet-type Mellapack-250Y presents the lowest pressure drop [61]. Honeycomb rotors, from porous paper or fiberglass sinusoidal flutes impregnated with LiCl or CaCl 2, have been used for many years [34,37]. The concentrated liquid desiccant is sprayed over the packing honeycomb paper and film is formed along the honeycomb wall [46,64]. When the hot/humid air is brought into contact with the film, its moisture is absorbed and heat is produced [64]. The honeycomb paper is porous and allows repeated wetting and drying [46], can acquire large exposed area and has good heat and mass transfer performance with simple geometry [46]. Honeycomb papers are adiabatic, come in different wave angles (45 0, 60 0 ) and are alternatively aligned to form the air channel [46]. Zurigat et al. [2,13] found that structured packings present enhanced moisture removal rate by increasing inlet TEG concentration, TEG flow rate and air flow rate (for aluminium and wood packings) and with increasing inlet air temperature (for aluminium only). Aluminum has high thermal conductivity, so the heating of the plates reduces the TEG s viscosity, causing the desiccant to spread more easily on the plates and providing more contact [13]. The effectiveness can be further increased by increasing the inlet TEG temperature [13]. Another study [49] proposed design guidelines for structured packing LDCS using TEG and developed correlations for the heat and mass transfer coefficients. The condensation rate has been found to increase with increasing the air flow rate [13,49], due to the good wetting of the packing when high liquid flow rates were employed, based on liquid-to-air flow rate ratios between 0.97 and 1.3 [13,49,52]. Abdul-Wahab et al. [2] also predicted the dehumidification effectiveness in a TEG structured packing column. In a LiCl structured packing column, Yin et al. [65] showed that the mass transfer coefficient increases with increasing heating temperature and decreases with increasing desiccant concentration. The average mass transfer coefficient is 4g/m 2 s and the maximum is 7.5g/m 2 s (20%LiCl, T heating = C) [65]. Under 40% desiccant concentration and 70% RH, the dehumidification rate increases with increasing air humidity, though there is maximal tower efficiency for specific tower dimensions and specific air humidity [65]. The condensation rate is decreased by increasing the equilibrium humidity ratio and by decreasing the inlet air humidity ratio or the packing height [49]. 5.2 SPRAY TOWER Although spray towers (sprayed coils, cross flow plate heat exchangers, heat pipes) are well known for their simplicity, their low pressure drop on airside, their low cost and their compact size, their absorption effectiveness is little and they enhance liquid carryover and large pressure drops [31,32]. Figure 9: A counter flow wet cooling tower [66]. The wet bulb and water temperatures are the most important input parameters, since second-law efficiency is increased with the increase in inlet wet-bulb temperature or the decrease in inlet water temperature [66]. 5.3 FALLING FILM A falling film (or wetted wall) column is a vertical tube or plate over which the desiccant solution flows by gravity. It has low pressure drop and low cost, it provides high contact area per unit volume, but the difficulty arises in achieving a thin film [31,32]. The problem of corrosion can be minimized by the addition of Li 2 CrO 4 [32]. The carryover of solution can be reduced by putting eliminators at the dehumidifier / regenerator outlet [32]. The crystallization of salt can be handled by decreasing the solution s concentration [32]. Better wetting of the absorber tubes and uniform spray of the solution over the regenerator plates increases the performance [32]. 5.4 INDIRECT/DIRECT EVAPORATIVE COOLER According to Dai et al. [64], a direct evaporative cooler is set before the heat exchanger and captures the droplet of the desiccant leaving the dehumidifier, when the air humidity ratio is very low. Dehumidification is followed by indirect evaporative cooling, which is the cooling of the air inside the heat exchanger, by an air mass that was previously cooled in the evaporative cooler [64]. Second-law efficiency is decreased as the inlet wet bulb temperature is decreased or as the refrigerant temperature is increased [66]. 5.5 HEAT EXCHANGER The liquid-to-liquid heat exchanger pre-heats the weak solution leaving the dehumidifier and recovers heat from the hot strong solution leaving the regenerator [8]. The heat exchanger between the weak solution flowing out of the dehumidification unit and strong solution flowing in it can effectively preheat the former and pre-cool the latter [11,28]. In order to maintain the low surface pressure of the desiccant solution, methods such as adding insulating layer to prevent the increase of solution temperature are taken [11]. A typical single-stage heat exchanger comprises of two Direct Contact Heat Exchangers [5]. In summer, the LD from 7 Copyright 2008 by ASME

8 the lower DCHE, flows into the upper DCHE and soaks the padding. The desiccant s moisture content is transferred to the air. Then the concentrated LD flows by gravity to the lower DCHE, where it is heated and diluted, while the fresh air is cooled and dehumidified. The LD returns back to the lower DCHE and completes the cycle [5]. Figure 10: Single stage liquid desiccant heat exchanger [5]. Figure 11: Single channel exchanger design [33]. In contrast to his older study [48], Saman et al. recently [53] proved that heat exchanger effectiveness is increased by increasing the air inlet temperature [53]. Heat exchanger effectiveness is also increased by the increase in air velocity [48], the decrease in air humidity ratio [53] or by the increase of the contact area via padding [5]. A high solution to air mass flow rare ratio increases the performance of the dehumidifier, but an upper limit of is imposed [48]. Kessling s calculations [24] show that for high energy storage capacity a very small specific solution flow (0.2 l/hm 2 ) has to be distributed uniformly over the exchanger surface. This surface should sufficiently cool the desiccant, withstand the desiccant s corrosive forces and consist of inexpensive materials, such as polypropylene [24]. According to Pietruschka et al. [42], a cross-flow type plate heat and mass exchanger used as direct contactor for liquid desiccants offers the best opportunities. Saman et al. [48] studied a similar direct contact, cross-flow plate heat exchanger used as a dehumidifier and indirect evaporative cooler and found that the performance depends on the heat and mass transfer area, solution concentration and ratio of secondary to primary air mass flow rates. Figure 12: Cross flow heat exchanger absorption unit [42]. A more recent study of Saman et al. [53] investigated the same heat exchanger with 40%w/w CaCl 2. The advantage here was the simultaneous absorption of the heat removed from the air on one side of the plate by the evaporatively cooled return air on the other side; thus each chamber of the dehumidifier operated efficiently and nearly isothermally [53]. 6 HYBRID LDC/ VC SYSTEMS CONFIGURATION In hybrid systems, the desiccant dehumidifier handles the latent cooling load and a conventional vapour compression system handles the sensible load [9,16,28,37,39]. Hybrid systems offer reduced energy costs, reduced equipment size and independent humidity and temperature [16,31]. The performance may be further improved if waste heat from the condenser in a VCS is used for regeneration [9,16,28,31,39,64]. The evaporator temperature need not be at the dew point of the air delivered; thus, the COP of the VCS is increased [16,28]. The overall COP of the hybrid desiccant systems is [16] or even [31]. 6.1 LDC / VC SYSTEMS Although conventional VCS have high heat transfer effectiveness and compact size, they involve large electricity consumption and dependence on deep freezing to remove the latent heat [64]. The hybrid LDCS/VCS eliminates the process of reheating occurring in VCS and the deep dehumidification occurring in LDCS [64]. Hybrid LDC/VCS systems use 40-80% less electrical energy compared to a VCS alone and these savings increase as the latent load is increased [28,64]. Other studies calculated this figure as two thirds [28] or 35% [9,11,16,40], while under ARI conditions, the COP is 0.71 [36]. Kinsara et al. [3] showed that the energy consumption is 1/3 of a conventional a/c (T ambient = 40 C, air humidity = 0.015kg/kg). Further benefits include less flow rate of condensation air and reduced size of VCS by 34% [64]. The simulation of a hybrid system showed that the lower the latent load, the higher the COP and the lower the VCS equipment size is [64]. The cooling production is 20-30% higher than a VCS [64]. 8 Copyright 2008 by ASME

9 Figure 13: Hybrid LDCS / VCS psychrometric analysis. A/c is abcd, LDCS is aefgd, hybrid LDC/VCS is aed [64]. Figure 14: Hybrid LDC / VCS [64]. Kinsara et al. [3] proposed an energy-efficient CaCl 2 LDCS, in which waste heat from the VCS is used for regeneration. They later showed [4] that the COP is increased by increasing desiccant temperature, increasing heat exchanger effectiveness and decreasing the space sensible heat ratio [4]. Figure 15: LDCS/VCS in a supermarket. 1) Inlet of recirculated air, 2) filtration-dehumidification-cooling, 3) airflow into the room, 4) inlet of outside air, 5) heat pump, 6) VCS heating, 7) regeneration, 8) outlet of regenerated air [6]. Lazzarin et al [6] tested a LiBr LDC/VCS in a supermarket in Italy and found that the primary energy consumption for an average July day can be lowered up to 63%. 6.2 LDC / VC / SOLAR COLLECTORS SYSTEMS These systems have many advantages over other solar a/c systems [8,54], mainly in terms of energy consumption [3,8,20,23,28,42,54,65,67,68]. Low operating costs and six years payback time have been observed [28], though electrical energy consumption remains unaffected from the utilization of solar energy [21]. The collector area for LDCS is 1.5 m 2 /kw, corresponding to 10m 2 per 1000m 3 /h of nominal air flow rate [69]. The use of solar energy can reduce annual gas consumption by 70% in northern Europe [21]. Indeed, Halliday et al. [22] simulated a solar LDCS under United Kingdom s meteorological conditions and showed substantial energy savings, comparing to LDCS alone. Mavroudaki et al. [21] demonstrated that solar LDCS can be feasible in south Europe as well, provided that the latent heat gains are not excessive. However, if RH is too high, then gas consumption rises [21]. Up to 2006, 6% of 70 installed solar a/c systems in Europe use LD [69]. In representative cities of different climatic conditions the gas energy savings are % and in Oslo (lowest latent gain) are 93% [21]. Gas energy savings in summertime in England are 39-45% with fewer savings in Scotland and in winter, no additional gas energy is required [22]. LDC/VC/Solar collector systems can reduce gas consumption by 70% annually [22] and reduce power consumption by 24-48%, compared to a conventional VCS [28]. Considering that cooling loads from solar gains obviously occur when solar radiation is high, plenty of free heat is available for regeneration [3,7]. Although Gandhidasan [20] proved that the heat removal is increased with insolation, Mei et al. [11] state that in overcast sky, the solar collectors are not sufficient for the regeneration energy. By storing the concentrated desiccant, the system continues to operate when there is little solar energy available, thereby improving its performance [28,38]. The combination of solar energy and recovered heat supplies most of the energy required, especially in the heating mode [3,22,64]. In this case, the components are reduced by 9 Copyright 2008 by ASME

10 one by transferring condensation of the refrigerant from a condenser to the environment [8], though this requires great design effort [69]. Thermodynamic modelling of a hybrid LiBr VCS/Solar collector system showed that 45% energy savings are achieved over standard VCS [9]. The proposed system has minimal hardware requirements, low evaporator operating temperature, small condenser and high dehumidification efficiency. It is recommended for hot/humid climates or for high latent heat loads, since it saves 80% energy at 90% latent heat load [9]. Dehumidification is increased by decreasing the air inlet temperature or by increasing the air enthalpy difference between the dry air and the humid air [20]. The condensation rate increases with airflow rate, inlet air temperature or desiccant mass flow rate; and it decreases when the inlet desiccant temperature is increased [15]. The outlet air temperature increases with airflow rate, inlet air temperature or inlet desiccant temperature and decreases when the desiccant mass flow rate is increased [15]. An efficient solar desiccant wheel performance requires 1-5kg/min of airflow rate; it approaches 0.65 during dehumidification and 0.92 during regeneration at 1.5kg/min [37]. Regeneration of desiccants using solar energy can be brought about by different methods. The direct solar regenerators, where the desiccant is itself the heat collecting fluid, have high effectiveness though they face corrosion, dust and dirt problems [54]. The indirect solar regenerators use solar air or water heater that provides hot water for the desiccant regeneration [54]. The higher water temperature or heat exchanger effectiveness, the greater the rate of evaporation [54]. The multi-stage regenerator uses lowtemperature heat to treat the weak solution, achieving a highenergy utilization ratio [11]. Among the available solar collectors, namely open-type, closed-type, natural convection and forced convection, the forced convection collectors are the most effective and widely used [11]. Figure 17: Solar LDCS with CaCl 2 and honeycomb desiccant wheel [37]. Figure 18: Absorber & VCS of a LDC/VCS/Solar collector system [14]. Figure 16: Regeneration with solar collector [11]. Figure 19: Regenerator & Solar collector of a LDC/VCS/Solar collector system [14]. One drawback is the desiccant concentration increase along the collector plate, which reduces the vapor pressure difference between the solution and the air, making the regeneration difficult [11]. The only study suggesting that solar collectors are practically infeasible for LDCS assumed regeneration by condensing steam, while solar collectors were used only to preheat the steam [28]. 10 Copyright 2008 by ASME

11 6.3 LDC / VC / VAS SYSTEMS Liu et al. [39] established a LiBr LDC/VCS/VAS system, which had 40% less CO 2 emissions, compared to a conventional HVAC, two years payback time and mean COP 1.0. In the summer, the carryover of solution into the air is reduced by re-designing the liquid spray and by positioning filters at the outlet of the dehumidifier/regenerator. In winter, heat recovery is utilized [39]. 7 CONCLUDING REMARKS The undisputed fact of global warming has ushered a/c not only in hot and humid climates, but also in countries with no a/c tradition [8]. Uncertainties regarding safe fluids, electricity tension during peak demand, inefficient moisture and bacteria control are pressing for alternative solutions, but without any penalty in the perceived life quality [10,28,65]. Such alternatives are the hybrid LDC/VCS since by decoupling the latent cooling and sensible cooling process may yield not only energy savings but may also improve indoor air quality by more efficient humidity control [70]. Their applications are favourable especially in hot/ humid areas [4,13,29,46], since they remove latent heat and utilize low-grade energy [4,27,28,32,46,68,69]; thereby improving indoor comfort and productivity [32]. From this brief review, it appears that significant work has been done on hybrid LDC/VCS [30] and that computer simulation is necessary to identify the best energy-cost performance [11,65,69]. However, the mass and heat transfer processes, still remain uncertain [11]. Almost no standardized design guidelines exist and there is still a lack regarding common construction practices [69]. Till now, people are still seeking an optimal mixed desiccant with relatively lower surface vapor pressure and cost effectiveness [11]. Their potential drawback of big dimension, unstable operation and climaterelated performance hinder their widespread applications [11,28,46,69]. The emergence of these hybrid systems that outweigh conventional evaporation coolers in handling the sensible cooling load, can lessen the LDCS s instability [11,28,31,46,69]. As the newcomer must overdo the incumbent by a large advantage to be seriously considered, additional work is needed to obtain further improvements in the current state-ofthe-art [10,30]. Above all, further analytical and experimental investigations should be made to deepen the understanding of mass and heat transfer so as to improve the system s overall performance [11]. 8 ACKNOWLEDGEMENTS This literature review was supported by Prof. E. Rogdakis and supervised by Dr. I. Koronaki. 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