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Solar Powered Dehumidification Systems Using Desert Evaporative Coolers: Review
Abstract
This paper gives a detailed account of the general features of the major desiccant regeneration techniques and configurations of the related systems; meanwhile, attention has been paid to both technological development of solar powered regenerator, which is a key component of the liquid-desiccant dehumidification system. Studies to improve the system performance have been discussed. Benefits and conditions of the use of liquid desiccant for dehumidification purposes have been stated. It is clear from the survey that the desiccant dehumidification is more energy-efficient compared with the conventional vapor compression system. Moreover, new configurations of the solar regenerator, to improve the system performance, have been demonstrated. Some new hybrid systems that greatly expand the desiccant in residential applications, as well as effectively promoting the single system's performance, are also introduced.
... Liquid desiccant dehumidification technology has been shown to be very effective in controlling the humidity level, but it is often more complex to operate and control compared to conventional systems [108,113,115,116]. The evaluation of the installation cost is also still under debate. ...
... The evaluation of the installation cost is also still under debate. Some researchers have concluded that the vapor compression installation cost is higher than that of desiccant systems [117,118], and others have found that vapor compression systems have the lowest installation costs [112,116]. Thus, reducing the initial costs should be considered in order to achieve the economic benefits of desiccant cooling applications in hot climate regions. Ghoulem et al. [97] were motivated to use a solar regenerator combined with a desiccant to dehumidify and cool the climate inside the greenhouse to ensure crop growth for the humid and hot regions by dehumidifying and cooling (Figure 8). ...
... Comparison between desiccant and conventional systems[113,116,119]. ...
This work is motivated by the difficulty of cultivating crops in horticulture greenhouses under hot and arid climate conditions. The main challenge is to provide a suitable greenhouse indoor environment, with sufficiently low costs and low environmental impacts. The climate control inside the greenhouse constitutes an efficient methodology for maintaining a satisfactory environment that fulfills the requirements of high-yield crops and reduced energy and water resource consumption. In hot climates, the cooling systems, which are assisted by an effective control technique, constitute a suitable path for maintaining an appropriate climate inside the greenhouse, where the required temperature and humidity distribution is maintained. Nevertheless, most of the commonly used systems are either highly energy or water consuming. Hence, the main objective of this work is to provide a detailed review of the research studies that have been carried out during the last few years, with a specific focus on the technologies that allow for the enhancement of the system effectiveness under hot and arid conditions, and that decrease the energy and water consumption. Climate control processes in the greenhouse by means of manual and smart control systems are investigated first. Subsequently, the different cooling technologies that provide the required ranges of temperature and humidity inside the greenhouse are detailed, namely, the systems using heat exchangers, ventilation, evaporation, and desiccants. Finally, the recommended energy-efficient approaches of the desiccant dehumidification systems for greenhouse farming are pointed out, and the future trends in cooling systems, which include water recovery using the method of combined evaporation–condensation, as well as the opportunities for further research and development, are identified as a contribution to future research work.
... Liquid desiccant consist of an absorber and a regenerator (Al-Alili et al., 2014). One drawback of liquid desiccants is that a portion of absorbent might be carried away by the air stream through the absorption and regeneration processes (Kassem et al., 2013). On the other hand, liquid desiccants are better dehumidifiers and require a lower regeneration temperature (Kassem et al., 2013). ...
... One drawback of liquid desiccants is that a portion of absorbent might be carried away by the air stream through the absorption and regeneration processes (Kassem et al., 2013). On the other hand, liquid desiccants are better dehumidifiers and require a lower regeneration temperature (Kassem et al., 2013). Solid desiccant air conditioning systems are a compelling alternative to vapor compression systems (Panaras et al., 2011). ...
Hot summer days coincide with high solar insolation periods and high peak demands in the UAE. Solid desiccant air conditioning systems are capable of utilizing low-grade sources of energy such as solar energy. In this paper, two new air conditioning systems combining solid desiccant and Maisotsenko coolers are examined for use in humid climates such as the UAE. Solar energy in the form of a solar PV/T module is selected as the primary source of energy to operate the desiccant air cooling systems while an auxiliary heater is devised for supplementary air heating. A detailed thermodynamic analysis is performed to illustrate and evaluate the proposed air conditioning systems’ performance in the UAE under different operating conditions. A comparative analysis is carried out to signify the advantages and disadvantages of integrating the Maisotsenko cooler with conventional desiccant air conditioning units. In addition, a seasonal analysis is conducted to examine the performance of the proposed systems throughout a summer day in Abu Dhabi. Average COP for two proposed configurations during the investigated period are 0.2495 and 0.2713. Furthermore, their corresponding solar shares are 32.2% and 36.5%, respectively. Finally, with PV/T module collectors’ area of 681.0 m2 and 656.3 m2, annual electricity generation of 145.0 MWh and 139.7 MWh are reported, respectively.
... The main disadvantage of the liquid desiccants is that they are carried away by the air stream during dehumidification and regeneration. Nonetheless, their moisture removal capacity is greater than solid desiccants [13]. Additionally, liquid desiccants have lower regeneration temperature mostly in range of 40e70 C [13]. ...
... Nonetheless, their moisture removal capacity is greater than solid desiccants [13]. Additionally, liquid desiccants have lower regeneration temperature mostly in range of 40e70 C [13]. ...
Gas turbine thermal efficiency has significant dependency on climatic conditions. Evaporative cooling is commonly utilized as an inlet air cooling technique in hot and dry climates though an increase in atmospheric humidity will considerably diminish its performance. Implementing desiccants to dehumidify air will enhance evaporative inlet cooling effectiveness. Furthermore, a recently commercialized cooler named Maisotsenko cooler can be integrated in gas turbine inlet air cooling to replace conventional evaporative coolers. In this paper, four different inlet air cooling systems employing turbine waste heat are proposed for gas turbine power augmentation in hot and humid climates such as UAE. Detailed sensitivity analysis is performed to investigate the impact of ambient air conditions and regeneration temperature on the inlet air cooling systems' effectiveness. Recommended inlet air cooling techniques are evaluated against more commonly used inlet air cooling systems under UAE climatic conditions. Finally, economic and transient analysis are accomplished to signify the most economical inlet air cooling system that is most suitable for UAE gas turbine power augmentation. Maisotsenko evaporative desiccant inlet air cooling with life savings of 31.882 MUS$ is the most economically justified inlet cooling technique for a 50 MWe gas turbine power plant in UAE with life span of 25 years.
... Lithium chloride and calcium chloride are the most commonly used adsorbents in Liquid desiccants. One drawback for liquid desiccant is that a portion of absorbent might be carried away by the air stream through absorption and regeneration processes [2]. On the other hand, liquid desiccants are better dehumidifiers and require lower regeneration temperature [2]. ...
... One drawback for liquid desiccant is that a portion of absorbent might be carried away by the air stream through absorption and regeneration processes [2]. On the other hand, liquid desiccants are better dehumidifiers and require lower regeneration temperature [2]. ...
Air conditioning systems are responsible for a gigantic portion of energy consumption in the UAE. Specially, UAE’s temperature might reach to extremely high temperatures of 50oC during summer seasons. Fortunately, hot summers are coincided with high solar insolation periods and high peak demands. Therefore, it is wise to utilize solar energy for air conditioning systems to reduce the electricity consumption associated with the needs of air conditioning operation. Solid desiccant air conditioning systems are capable of exploiting low grade sources of energy such as solar for their operation.
Direct and indirect evaporative cooling are the most commonly used cooling methods implemented in solid desiccant air conditioning systems. The cooling best performance of these types of cooling methods is limited by the air wet bulb temperature and relative humidity. On the other hand, Maisotsenko cooling cycle is a recently commercialized cooling system capable of cooling air to its dew point temperature. Coupling Maisotsenko cooler with a solid desiccant enables the suggested cooling systems to be employed in humid conditions such as in UAE.
In this paper, two new air conditioning systems combining solid desiccant and Maisotsenko cooler are proposed and investigated for humid climates such as in UAE. Solar energy is selected as the primary source of energy to operate the desiccant air cooling systems. Detailed thermodynamic analysis is performed to illustrate and evaluate the proposed air conditioning systems performance in the UAE under different operating conditions. A comparative analysis is carried out to signify the advantages and disadvantages of integrating the Maisotsenko cooler with conventional air condition systems. Furthermore, a transient analysis is executed to examine the performance of the proposed systems throughout a day in summer for Abu Dhabi weather data. The results demonstrate the benefits of integrating Maisotsenko coolers with solid desiccants as a considerably efficient air conditioning system. Finally, Transient analysis illustrates that both MDVC and MDRC are capable of efficient operation in Abu Dhabi worst weather conditions without any difficulties.
... It is due to a lack of waterproo¯ng and design. 14,15 Hence the design should be in such a way that, the moisture must escape the system and it must remain dry for a long duration/period. 16,17 Insulation is a key factor in the moisture balance. ...
Heating ventilation air conditioning (HVAC) design mainly deals with moisture and its control. The moisture may be present inside ducts, conditioned spaces, or outdoors. The process of humidification and dehumidification requires equipment for mass and heat transfer, where the transfer of energy and mass takes place at varying concentrations and temperatures. The exchange of mass or heat depends on the type of flow and is conceivably in the form of gas to liquid or liquid–vapor. This paper aims to review the effect of moisture in the buildings and modulate its effect with several humidifying and dehumidifying techniques as sustainable techniques depending upon the external weather conditions to maintain thermal comfort. Various humidification and dehumidification techniques have been discussed with both their merits, limitations, applications and future scope to meet sustainable energy demands.
... The water vapour absorption process performed by droplets of the liquid desiccant solution in the dehumidifier is somewhat similar but opposed to the droplet evaporation process described in Section 3.3, where the low vapour pressure of the desiccant solution enables the mass transfer of moisture from the air to the solution [151]. The liquid desiccant technology is usually driven by low-grade heat sources, such as industrial waste heat [152], solar [153] and geothermal energy [154], to energy-efficiently control moisture and temperature, particularly for processes where moisture control and moisture removal is required. It has therefore found application in buildings with high latent loads (such as gyms and swimming pools [155]), in buildings where moisture control is essential for the conservation of goods (such as art galleries, libraries, museums and archives [156]), in hospitals and healthcare facilities [157], in refrigeration and cold rooms [158], in greenhouses [159], etc. ...
The world is facing on-going challenges due to the spread of the coronavirus SARS-CoV-2, which is affecting the health of people worldwide and the economy of countries. Social distancing, lockdown and quarantine measures have been implemented globally to limit the spread of the virus with a profound impact on people’s lives. These are interventions which are not considered to be permanent and reproducible in the long-term. As more evidence is growing around the airborne transmission routes of the virus, as previously identified for other viruses such as tuberculosis, measles, influenza and coronaviruses, the role of heating, ventilation and air-conditioning (HVAC) systems in buildings, enclosed spaces and public transport in limiting the transmission of airborne pathogens has become a topic of significant relevance. Although the HVAC strategies recommended by professional engineering associations are capable of minimising the transmission of airborne pathogens, they are also responsible for an increase in energy consumption and possibly in a reduction of thermal comfort for occupants. The objective of the study is to review the role of HVAC in airborne viral transmission, to estimate the energy penalty associated with the implementation of the main HVAC strategies for transmission reduction and understand the potential of liquid desiccant technology as an air scrubber. That is capable to a) energy-efficiently control temperature and humidity in buildings, enclosed spaces and public transport; b) increase the indoor air quality by offering the conditions of temperature and humidity less favourable to the growth, proliferation and infectivity of microorganisms; and c) inactivate pathogens. The main factors involved in the process of the inactivation of viruses or pathogens by liquid desiccant solutions are also described together with possible modifications to the solutions to increase their heat and mass transfer and sanitising characteristics. The study is ended by an economic evaluation of the potential energy benefits resulting from the use of liquid desiccant technology. It is concluded that the technology could be particularly favourable in those buildings where humidity control and/or moisture removal is required or in buildings where viruses are more likely to be present, such as in healthcare facilities/operating rooms, or in the event of an airborne viral outbreak.
... At the same time, in a regenerative section, a waste heat from the combustion process or solar energy is used to heat the desiccant and, consequently, dries the desiccant [65]. The main advantages of using solid desiccant can be summarized in the following points [66][67][68][69]: (i) high drying capability compared to liquid desiccant; (ii) more efficient when the latent heat load is larger than the sensible load; which leads to air dehumidification for 40-60% of the cooling load for air conditioning in hot and humid weather [70]; (iii) utilizing different energy sources especially low grade energy; (iv) clean technology, which operates without the use of harmful refrigerants; (v) control of humidity is better than conventional cooling system; (vi) cost of energy to regenerate the desiccant is less than the cost of energy to dehumidify the air by cooling it below its dew point; (vii) improves indoor air quality; (viii) capable of removing airborne pollutants and finally (ix) compact construction, resistance to corrosion, and ability to work continuously. The main disadvantages it requires a higher regeneration temperature of more than 70°C and a high pressure drop in the air stream, also requires a high energy system. ...
The energy demand for cooling and air conditioning systems is increasing worldwide, especially in regions with high solar radiation intensity. One of the reasons for this is the increase of comfort demands worldwide. The most cooling and air conditioning systems are the conventional electrically driven one type such as compression refrigeration machines and air conditioning systems.
Through the huge electricity consumption for cooling and air conditioning, the environmental problems get bigger and bigger, because of carbon dioxide (CO2) and other pollutant emissions.
One of the possibilities to reduce the primary energy consumption is through the use of solar energy for driving the thermal driven absorption or adsorption refrigeration systems, or desiccant cooling. Another possibility is using solar energy to produce electrical energy and this can be used to drive the conventional refrigeration systems.
Many research and developmental efforts in the last years have been done to enforce the spreading of solar-driven cooling systems.
This paper will illustrate the state of the art about the energy consumption for cooling and air conditioning systems, available solar-driven cooling systems and the potential of the utilization of such systems in comparison to the conventional ones. Moreover, this paper highlights some different methods of optimization, which used to maximize the performance and minimize the cost.
... Desiccant contactors function based upon vapour pressure difference rather than dew point condensation due to air saturation. Therefore, desiccant systems offer the potential for humidity regulation in indoor air at ambient temperatures (Dean et al., 2012;Dijkink, Tomassen, Willemsen, & van Doorn, 2004;Hwang, Radermacher, Al Alili, & Kubo, 2008;Kassem, 2013;Khan, 1994 Zhang, Yin, & Zhang, 2016;Zhang, Zhang, & Xu, 2016). Desiccants may be attractive for humidity removal over refrigeration-based dew point systems in that they offer the potential for a lower grid-energy footprint. ...
Humidity control is an important factor affecting the overall sustainability, productivity, and energy efficiency of controlled environment agriculture. Liquid desiccants offer the potential for pinpoint control of humidity levels in controlled environments. In the present work, a dehumidification processes utilizing liquid desiccants pumped through the lumens of triple-bore PVDF hollow fibre membranes is implemented in a bench scale controlled environment agriculture system. Hydrophobic hollow fibre membranes were combined into an array and placed near the crops. Concentrated magnesium chloride liquid desiccant solution with a low vapour pressure was pumped through the hollow fibre lumens. The dehumidification permeance rate responded dynamically to the changing transpiration rate of the plants, as influenced by changes in environmental factors such as light, temperature, and vapour pressure. The dehumidification permeance rate increased from an average of 0.26–0.31 g m⁻² h⁻¹ Pa⁻¹ as the velocity of the liquid desiccant through the hollow fibres increased from 0.023 to 0.081 m s⁻¹. Humidity levels were targeted to be maintained within a range of 70–90% relative humidity at 23 °C. The membrane-based liquid desiccant system was demonstrated to successfully control humidity within a bench-scale controlled environment agricultural setup.
... Liquid desiccants also offer the opportunity to use renewable energy sources, like solar energy, for desalination and regeneration of the draw solution [5][6][7][8][9][10][11][12][13][14][15]. Desalination of the draw solution also offers a unique ...
This paper documents the testing of a vacuum membrane distillation system intended for use with liquid desiccants. Liquid desiccants offer the possibility for low-energy, ambient temperature dehumidification. Effective desalination and purification of diluted desiccants outputs two important products: a concentrated desiccant for reuse in dehumidification and fresh water. In this study, vacuum membrane distillation was used in the laboratory to purify diluted liquid desiccants. Calcium chloride and magnesium chloride were the desiccants selected for testing. Desiccant solutions were pumped through the lumens of poly(vinylidene fluoride) (PVDF) hollow fiber membranes at varying feed inlet temperatures, solution velocity rates and vacuum set points during membrane distillation. An average flux of 8 kg m⁻² h⁻¹ was obtained using 30 wt% magnesium chloride solution at a temperature of 50 °C while applying vacuum to achieve 25 mbar absolute pressure on the air side of the membrane. The results are promising for the development of a full-scale vacuum membrane distillation process for desiccant solution regeneration and fresh water recovery. In addition, the recovered condensate was of sufficient quality for use in agricultural irrigation or drinking water.
... higher initial installation cost, corrosion factor in heat exchanger and proper monitoring of saline concentration [4]. On the other hand, solid desiccants which also functions due to partial pressure difference between the air flow and its surface. ...
This paper presents the performance investigation of a solid desiccant dehumidifier integrated with Maisotsenko Cycle (M-Cycle) based cross flow heat and mass exchanger (MC-DAC). The experimental test rig consisting of a silica gel based desiccant wheel and a heat recovery wheel is coupled with M-Cycle indirect evaporative cooler. The effect of wide range of inlet air parameters such as ambient temperature, humidity ratio, and regeneration temperature on the performance of the integrated system was analyzed and compared with the conventional desiccant air conditioning (DAC) system. Set of experiments were carried out for both systems at constant process as well as return air mass flow rates under different operating conditions. MC-DAC system was observed to be around 60-65% more efficient than the other system in terms of COPth providing same supply air conditions at low regeneration temperatures.
... Because of this feature, the electrical demand is estimated as low as 25% that of vapor-compression air conditioning [10,15]. In the context of renewable energy sources, liquid desiccants offer an advantage over solid desiccants when using solar energy for regeneration [16][17][18][19][20][21][22][23][24][25][26]. ...
Dehumidification is responsible for a large part of the energy consumption in cooling systems in high humidity environments worldwide. Improving efficiency is therefore essential. Liquid desiccants offer a promising solution for dehumidification, as desired levels of humidity removal could be easily regulated. The use of membrane contactors in combination with liquid desiccant is attractive for dehumidification because they prevent direct contact between the humid air and the desiccant, removing both the potential for desiccant carryover to the air and the potential for contamination of the liquid desiccant by dust and other airborne materials, as well as minimizing corrosion. However, the expected additional mass transport barrier of the membrane surface can lower the expected desiccation rate per unit of desiccant surface area. In this context, hollow fiber membranes present an attractive option for membrane liquid desiccant contactors because of their high surface area per unit volume. We demonstrate in this work the performance of polyvinylidene fluoride (PVDF) based triple-bore hollow fiber membranes as liquid desiccant contactors, which are permeable to water vapor but impermeable to liquid water, for dehumidification of hot and humid air.
Hybrid Photovoltaic (PV) units are systems that produce both electrical and thermal energy in a single unit. The purpose of this
research is to investigate the feasibility of using PV solar waste heat to regenerate liquid desiccant in the solar air conditioning
system. A typical liquid desiccant regenerator requires a temperature range between 50˚C and 60˚C. Thus, the heat recovery system
is designed on a 50W solar PV that focuses on recovering, delivering, and transferring of heat from panel PV to heat the circulating
water. This paper also discusses the comparison of output power and efficiency between heat recovered and a standard solar
photovoltaic panel. The calculated maximum output power for heat recovered and standard solar PV system was 52.20W and
40.15W, with an efficiency of 11.72% and 9.65% respectively. The maximum temperature for the heat recovered system was
55.10˚C at 1.00 pm and the standard system was found at 62.30˚C. The experiment results show that stored water temperature in
the reservoir able to reach 55˚C, within the range of a preferred liquid desiccant regenerator. Thus, it is possible to utilise the solar
PV waste heat for desiccant solution regeneration and simultaneously benefit from the added benefit of improved efficiency.
The extensive effort has made to fabricate and analyze the evaporative cooler with liquid-desiccant regeneration process. A temperature drop of around 4 degrees and a humidity reduction of around 14-16 percent as compared to direct contact evaporative cooler achieved. The enhanced average cooling efficiency of the evaporative cooler is41.828 % achieved with Average dehumidification capacity obtained at different concentrations of desiccantare: At 20% concentration of desiccant = 4.3695 lph, At 30% concentration of desiccant = 5.4845 lph, At 40% concentration of desiccant = 6.391 lph
This paper presents the performance investigation of a solid desiccant dehumidifier integrated with
Maisotsenko cycle (M-Cycle) based cross flow heat and mass exchanger (MC-DAC). The experimental test
rig consisting of a silica gel based desiccant wheel and a heat recovery wheel is coupled with M-Cycle
indirect evaporative cooler. The effect of wide range of inlet air parameters such as ambient temperature,
humidity ratio, and regeneration temperature on the performance of integrated system was analyzed and
compared with the conventional desiccant air conditioning (DAC) system. Set of experiments were carried out for both systems at constant process as well as return air mass flow rates under different operating conditions. MC-DAC system was observed to be around 60–65% more efficient than the other
system in terms of COPth providing same supply air conditions at low regeneration temperatures.
A thermally driven air conditioner that uses liquid desiccants as the working fluid may be an attractive alternative to the compressor-based technology that is now used in most HVAC applications. The operation of a liquid-desiccant air conditioner is first explained and several basic concepts are reviewed. This review focuses on the development of liquid-desiccant conditioners and regenerators that are better suited to comfort conditioning (as opposed to industrial dehumidification). This includes work on conditioners and regenerators that use low flow rates of desiccants and have internal heat exchange. These conditioners and regenerators will have lower pump and fan power than packed-bed units, and will be much less likely to introduce desiccant droplets into the process air. Work to identify and develop regenerators that have COPs over 1.0 and research on alternative liquid desiccants is also reviewed.
In this work a novel energy efficient air-conditioning system utilizing lithium chloride (LiCl) solution as liquid desiccant has been proposed and simulated. The simulation of this system is mainly formulated with two packed columns, one for regenerating the weak desiccant and the other for the dehumidification of ambient air. The air is first dehumidified in the dehumidifier and then sensibly cooled in the indirect and direct evaporative coolers. First and second laws of thermodynamics have been used to analyze the effect of five key variables on the performance of the system. High efficiency could be achieved if proper values of these variables are selected.
A nominal 3 ton (10.5 kW) open-cycle liquid desiccant cooling system has been designed, installed and operated at the Solar Energy Applications Laboratory, Colorado State University. Performance data for the regenerator, operated in a decoupled mode, were obtained for a variety of inlet conditions. Depending on the operating conditions, capacities of 1.4 – 3.0 refrigeration tons (4.9 –10.3 kW) were obtained in the regenerator. Liquid distribution in the regenerator was studied for two systems: a gravity tray liquid distributor and a spray nozzle manifold. Higher capacities (40–50% increase) in the regenerator were obtained with the spray nozzle system. Another advantage of the spray nozzle arrangement, as compared to the tray liquid distributor, was a reduction in pressure drop (by 30–40%) for the air flow across the regenerator. The independent variables affecting the capacity were correlated by statistical analysis of the data.
Dehumidified air is required in many industrial applications. In recent years attempts have been made to use liquid desiccants for dehumidifying air. Dehumidifying with a liquid desiccant usually scrubs the air stream, not only providing conditioning but also cleaning and disinfecting the air. Different liquid desiccants are available in the market, lithium chloride and calcium chloride are the most commonly used desiccants in industry. In this paper performance of these two desiccants in dehumidifying the air is reported in detail. The heat and mass transfer coefficients have been evaluated for calcium chloride-air and lithium chloride-air contact systems using ceramic Raschig rings as the packing material. The derived transfer coefficients are used to analyze the performance of the dehumidifier. The results clearly indicate that lithium chloride-air contact system performance is better than the performance of the calcium chloride-air contact system. The heat and mass transfer analysis is extended to study the effect of various parameters such as liquid inlet temperature, liquid inlet flow rate, air inlet temperature, etc. on the performance of the dehumidifier.
Dehumidifier is the core component in a liquid desiccant air-conditioning system, whose efficiency directly affects the whole performance. In this study a cross-flow dehumidifier was established in which LiCl solution and Celdek structured packing were used. Enthalpy efficiency and moisture efficiency were adopted to describe the heat and mass transfer performance of the dehumidifier. Influence of desiccant and air inlet parameters on the dehumidifier performance was experimentally investigated, and efficiency comparison in different air outlet regions was also carried out, as well as influence of packing structure size on dehumidifier performance. The results show that the outlet parameter values of air and solution can be easily gained with enthalpy efficiency and moisture efficiency which was obtained from experimental regression. Efficiency has obvious regional characteristics. By increasing the thickness, width or height simultaneously, better performance of Dehumidifier can be achieved without increasing the pressure loss.
In this paper, ideal dehumidification process is achieved by both condensing dehumidification and liquid desiccant dehumidification methods, and the minimum input work and ideal COP of the two dehumidification methods are derived. For liquid desiccant dehumidification, the humidity ratio of regeneration air is the most important influencing factor of COP. For condensing dehumidification, the temperature of heat sink is the most important influencing factor of COP. The essence of dehumidification is transportation of moisture between indoor and outdoor two different humidity ratios. Exhaust heat into heat sink in condensing dehumidification method and exhaust moisture into moisture sink in liquid desiccant dehumidification mode can be compared, due to the fact that heat and moisture are unified in saturated line. The two dehumidification modes will have the same performance, when heat sink of condensing dehumidification is dew point of outdoor air by indirect evaporation or outdoor air is saturated. Otherwise, the ideal COP of condensing dehumidification will be lower than liquid desiccant dehumidification process.
This paper presents the modeling and simulation of solar-powered desiccant regenerator used for open absorption cooling cycles. The input heat, which is the total radiation incident on an inclined surface, is evaluated via a solar radiation model in terms of the location, day of the year, and time of the day. Calcium Chloride (CaCl2) is applied as the working desiccant in this investigation. The solar radiation model is integrated with the desiccant regenerator model to produce a more realistic simulation. A finite difference method is used to simulate the combined heat and mass transfer processes that occur in the liquid desiccant regenerator. The system of equations is solved using the Matlab-Simulink platform. The effect of the important parameters, namely the regenerator length, desiccant solution flow rate and concentration, and air flow rate, on the performance of the system is investigated. It has been found that the vapor pressure difference has a maximum value for a given regenerator length. It is also shown that for specified operating conditions, a maximum value of the coefficient of performance occurs at a given range of air and solution flow rates. Therefore, it is essential to select the design parameters for each ambient condition to maximize the coefficient of performance of the system.
Feasibility for an open-cycle absorption solar cooling (air-conditioning) system operated in Kaohsiung, Taiwan is studied via a computer simulation program using previous obtained experimental correlations for the collector/regenerator (C/R) performance and the statistical meteorological data for the summer season at Kaohsiung, Taiwan. Comparisons of the system performance for two C/R models are discussed in this study. They are the systems with natural convection single-glazed C/R and with the forced convection double-glazed C/R. The effects of the C/R area, the C/R solution flow rate, the solution storage, the chilled water temperature and the daily cooling demand on the system performance in terms of seasonal solar fraction are studied and discussed. The results show the influence of the solar C/R performance on the total performance of the cooling system. It is shown that the double glazed forced convection C/R gives a better system performance.
A computer simulation study has been made for an open-cycle absorption solar-cooling system operated in a humid area. The system configuration is similar to the one proposed by Wood at Arizona State University. The system with a glazed solar collector/regenerator shows sufficient solar regeneration capacity for operation in Kaohsiung, Taiwan. Many parameters controlling the system performance have been studied, including the amount of solution storage, absorber effectiveness, absorber solution flow rate, and the thermal capacitance of the structure. System simulation, using average local weather data, shows that the solar system can provide 75 to 97% of the cooling load needed for the summer season (from May to October) under various operating conditions.
A computer simulation of a regenerating-type solar collector is presented. The collector is used for the regeneration of a liquid desiccant and forms part of an open-cycle solar air-conditioning system. Correlations for heat and mass transfer coefficients and air pressure drop in the collector, derived from earlier experiments, were employed in the computer model. Optimum ranges of design parameters and performance trends are indicated.
A solar desiccant cooling system was operated at the Solar Energy Applications Laboratory, Colordado State University, throughout the 1986 summer. The system comprises an American Solar King fresh air heating/desiccant evaporative cooling unit, a Sunmaster evacuated tube solar collector, hot water solar storage tank, auxiliary electric boiler, controls, and accessories. The cooling unit is operated in the ventilation mode, fresh air being dried in a rotating desiccant matrix, and cooled by heat exchange and evaporative cooling. Return air is used as a cooling medium in a rotating heat exchange matrix, heated by solar energy in a heat exchange coil, and discarded through the rotating desiccant bed. The solar-driven system provided over 90 percent of the seasonal cooling requirements in an experimental, residence type building at average COP levels of 1.0 and solar collection efficiencies of 50 percent when supplied with solar heated water at temperatures of 50 to 65°C. Detailed operating results, including total and average solar cooling provided, coefficients of performance, and overall solar cooling performance ratios are presented.
The system efficiency of desiccant air dehumidification is usually evaluated by a coefficient of performance (COP) that is defined as the ratio of the cooling capacity and the sum of net electrical power input. The common efficiency evaluation method has many weaknesses especially if applied on liquid type of desiccant, low humidity different between inlet and outlet, and also on the system that has automatic operating control. This paper offers a solution for weaknesses of common method with a proposed novel method to evaluate the performance of liquid desiccant air dehumidification system.
This paper investigated a desiccant solution regenerator using hot air to concentrate diluted desiccant solution, aiming to utilize the waste heat of hot air, such as the hot air from the condensers of vapor compression refrigeration systems. To verify the feasibility of the utilization of the hot air for the desiccant solution regeneration and disclose the performance of such kind of regenerators, performance analysis was conducted numerically by a validated mathematical model and parametric distribution of the air in a typical case was explored. The results showed that it was possible to use hot air for the desiccant solution regeneration when the requirement of the lowest inlet solution temperature was met and a typical case showed that the suggested hot air temperature was around 65°C. Effects of main operation parameters on the regeneration thermal efficiency and regeneration rate were discussed and the result showed the regeneration thermal efficiency could achieve the maximum (ηreg, max) when the R was around 8 and lower flow rate of the desiccant could achieve higher ηreg, max. In addition, effects of dimensions of the regenerator on the regeneration performance were disclosed and some suggestions of design of the regenerators were introduced based on the study.
Theoretical and experimental investigation on the application of flat plate solar water heater coupled with air humidifier for regeneration of liquid desiccant has been presented in this work. The heated water from the storage tank of the solar heating system is circulated in a finned tube air heater. Hot air from the air heater is blown through a packing of a honeycomb type for the purpose of regeneration of calcium chloride (CaCl2) solution. An experimental system has been designed and installed for this purpose. The system comprises a solar water heater with a storage tank connected to an air/water heat exchanger. Hot air from the heat exchanger is blown to the air humidifier, which functions in this study as a regenerator. Calcium chloride solution is applied as the working desiccant in this study. Solution concentration is determined at the end of regeneration process and the mass of evaporated water is evaluated. It is observed that the heating temperature varies, at day time, in a range of about 5 °C. This limited variation in hot water temperature demonstrates the importance of the storage tank to attain a nearly steady state operation of the system. Experimental results show that solution with 30% concentration can be regenerated up to 50% using solar energy. In the theoretical part of this study, a multiple-layer artificial neural network (ANN) model has been applied to study the performance of a solar liquid-desiccant dehumidification/regeneration system when calcium chloride solution is applied as the working desiccant. The experimental results of the present study are used to construct and test the ANN model. Then the model has been utilized to describe and analyze the effect of the inlet conditions of air on the regeneration process. Good agreement between the outputs from the ANN model and the corresponding results from the experimental data has been found. The proposed model can work well as a predictive tool to complement the experiments.
This paper studies the feasibility of using a solar-powered liquid desiccant system to meet both building cooling and fresh water needs in Beirut humid climate using parabolic solar concentrators as a heat source for regenerating the liquid desiccant. The water condensate is captured from the air leaving the regenerator. An integrated model of solar-powered calcium chloride liquid desiccant system for air dehumidification/humidification is developed. The LDS model predicted the amount of condensate obtained from the humid air leaving the regenerator bed when directed through a coil submerged in cold sea water. An optimization problem is formulated for selection and operation of a LDS to meet fresh water requirement and air conditioning load at minimal energy cost for a typical residential space in the Lebanon coastal climate with conditioned area of 80m2 with the objective of producing 15l of fresh drinking water a day and meet air conditioning need of residence at minimum energy cost. The optimal regeneration temperature increases with decreased heat sink temperature with values of 50.5°C and 52°C corresponding to sink temperatures of 19°C and 16°C.
Desiccant cooling systems have the ability to provide efficient humidity and temperature control while reducing the electrical energy requirement for air conditioning as compared to a conventional system. Naturally, the desiccant air dehumidification process greatly influences the overall performance of the desiccant system. Therefore, the effects of variables such as air and desiccant flow rates, air temperature and humidity, desiccant temperature and concentration, and the area available for heat and mass transfer are of great interest. Due to the complexity of the dehumidification process, theoretical modeling relies heavily upon experimental studies. However, a limited number of experimental studies are reported in the literature. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (triethylene glycol) and air in a packed bed absorption tower using high liquid flow rates. A high performance packing that combines good heat and mass transfer characteristics with low pressure drop is used. The rate of dehumidification, as well as the effectiveness of the dehumidification process are assessed based on the variables listed above. Good agreement is shown to exist between the experimental findings and predictions from finite difference modeling. In addition, a comparison between the findings in the present study and findings previously reported in the literature is made. The results obtained from this study make it possible to characterize the important variables which impact the system design.
The regenerator is one of the key components in liquid desiccant air-conditioning systems, in which desiccant is concentrated and can be reused in the system. The regeneration heat is supplied into the regenerator by either hot air or hot desiccant. The heat and mass transfer performances of these two regeneration modes are analyzed and compared in detail. In the hot air driven regenerator, the parallel-flow regenerator has the best mass transfer performance and the counter-flow performs poorest under the same conditions, because the heat transfer process is the governing process and the mass transfer performance depends on the promotion of the heat transfer to the mass transfer process. In the hot desiccant driven regenerator, counter-flow configuration has the best mass transfer performance and parallel-flow is the poorest at the same conditions, since mass transfer is the governing process. Regeneration heat should be chosen to heat the desiccant instead of the air in the packed bed regenerator, since the hot desiccant driven regenerator has apparent better mass transfer performance. The proposed regeneration mode and flow pattern will be helpful in the design and optimization of the regenerators.
Theoretical and experimental investigation on the desorption characteristics of a packed porous bed is presented in this study. The granules of burned clay are applied as a desiccant carrier. Calcium chloride is used as the working desiccant. The theoretical model defines the transient gradient of air stream parameters (humidity and temperature) as well as desiccant concentration in the bed. In the experimental study, transient concentration gradient in the bed is evaluated by weight method. The bed is divided into seven separate layers. Air stream at low temperature and nearly constant inlet parameters are used for desorption purposes. Concentration gradient in the bed is found highly dependent on the mass transfer rate. For the specified operating conditions and stated assumptions, experimental measurements shows acceptable agreement with the analytical solution.
Calcium chloride solution is a cheap desiccant. It is unstable at certain solution concentrations and dehumidified air temperatures. The aim of this research is to stabilize it by mixing with calcium nitrate in different weight combinations. The physical properties of a proposed liquid desiccant such as density, viscosity, and vapor pressure were obtained. Heat and mass transfer analysis between a thin liquid layer of the proposed desiccant and the air flowing through rectangular channel has been studied. The different factors affecting the dehumidification process of air were studied.
In open cycle liquid desiccant air conditioning, the solar collector regenerator is one of the effective ways of regenerating liquid solution. In this work, the regeneration of liquid solution using cross flow of air stream with flowing film of desiccant on the surface of a solar collector/regenerator has been investigated. To evaluate the effect of cross flow of air stream on the performance of the unit, two identical units are constructed and tested in the same conditions of operation. One of the two units was augmented with air blower. The absorber plate is a black cloth layer. The forced air stream, which flows across the absorber removes the moisture from the liquid solution. The regeneration in the other collector/regenerator unit is free. The results show enhancement of regeneration efficiency for the forced cross flow compared with the free regeneration. The effect of concentration and flow rate on the performance is discussed. Two relations for regeneration efficiency as a function of concentration for the two units are introduced.
This paper presents a theoretical and experimental study of the transient adsorption characteristics of vertical packed porous bed. The theoretical model describes the effect of independent parameters (time and vertical distance through the bed) on the vertical gradient of adsorbable fluid in the bed. A simplified analytical solution, for specific operating conditions, is also presented. In the experimental study, porous granules of burned clay are applied as a desiccant carrier in the fixed bed. The granules of the packed bed are impregnated with liquid calcium chloride solution to form the porous adsorbing surface. The isothermal adsorption of water vapour from atmospheric air using the prepared bed is experimentally studied. Transient values of the mass of adsorbed water vapour, solution concentration and vapour pressure through the bed layers are evaluated from the experimental measurements. The model output, which show the effect of dimensionless relative time (Tr) on the potential ratio (C−C∗)/(C−C∗0), is compared with the experimental results and good agreement is found.
The processes occurring in a packed bed dehumidifier, which is part of a liquid desiccant solar cooling system, are mathematically simulated. The air flows in a counter flow direction to the liquid desiccant (CaCl2). The effect of varying the air, liquid flow rates and bed geometry are studied in addition to studying the effect due to varying the air and liquid desiccant inlet coditions. The inlet temperature of the liquid desiccant during the air dehumidification process has a strong effect on the other parameters, while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and liquid desiccant enhance the liquid desiccant regeneration processes but by different ratios. The study showed that both the air and liquid desiccant flow rates have negligible effect on the bed exit humidity ratio of air whereas the liquid flow rate has a strong effect on the bed exit moisture content of the liquid. It is also observed that increasing the air flow rate enhances the liquid desiccant regeneration (air humidification) process. Regardless of the inlet moisture content of the liquid desiccant, it is found that as the product (LAs) gets larger, the exit air humidity gets less.
The regeneration system represents a vital part of any desiccant air conditioning system. The need of a solar assisted desiccant regeneration system is more important today. In this paper, an experimental study of a novel regeneration system modified from solar tilted still is carried out. A corrugated blackened surface is used to heat the desiccant and an air flow is used to regenerate calcium chloride solution. The effect of the liquid to air flow rate ratio; the desiccant temperature; the desiccant concentration and the inlet air humidity ratio on the evaporation rate has been studied experimentally. A wide range of liquid to air flow rate ratios are employed. The optimum value of the liquid to air flow rate ratio for higher evaporation rate is reported.
Liquid desiccants are widely used in many solar applications. In order to analyze the performance of the system using desiccant technology, the thermophysical properties of desiccants are essential. In particular, the vapor pressure of the liquid desiccant is one of the important properties in air dehumidification. In this paper, an attempt is made to predict this property based on a classical thermodynamics approach and it is found that the predicted values for lithium chloride agree very well with the experimental results. The desired sorption properties can also be obtained by mixing the desiccants, which is another method of developing a new cost-effective liquid desiccant. In this paper, simple mixing rules are used to predict the vapor pressure, density, and viscosity of the desiccant mixture, namely calcium chloride and lithium chloride. It is found that the interaction parameter need not be included in calculating the density and vapor pressure of the above mixture but must be included in predicting the viscosity.
Mass-transfer characteristics of a structured packing dehumidifier/regenerator were studied using a lithium chloride solution as the desiccant. Experiments included air dehumidification and desiccant regeneration in typical operating ranges of air-conditioning applications. When the air velocity increased from 0.5 to 1.5m/s, the overall mass-transfer coefficient in the structured packing dehumidifier and regenerator varied from 4.0 to 8.5g/m2s and from 2.0 to 4.5g/m2s, respectively. Higher solution temperature resulted in lower overall mass-transfer coefficients. Dimensionless overall mass-transfer coefficient correlations were developed for the dehumidifier and regenerator. Deviations between predicted values and experimental values were within ±20%.
This paper presents an experimental study of the performance of a forced convection glazed solar collector/regenerator (C/R) operated under the meteorological conditions of Kaohsiung, Taiwan. The C/R is the key component of an open-cycle absorption solar cooling system because its performance directly indicates the system performance. The present results show that the counterflow case can improve the C/R efficiency when inlet solution concentration is low. The effects of individual parameters on the C/R performance are analyzed. Dimensionless heat and mass transfer coefficients are given.
Energy consumption due to conventional air-conditioning is very high, especially in hot, humid areas. The total air-conditioning cooling load can be divided into sensible and latent parts. Liquid desiccant solutions have the property of holding water vapor, therefore they can be used to overcome the latent part of the air-conditioning cooling load. In this work, an energy-efficient system is proposed to reduce energy consumption in air-conditioning (AC) systems using CaCl2 solution as a liquid desiccant. The proposed system consists mainly of two packed beds, a heat pump, air washers and a cooling coil. Mathematical modeling of various components and fluid properties of the proposed system was undertaken. A computer program was prepared to study the system performance under different conditions. Over most of the conditions at which the program was tested, the proposed system was found to be more energy-efficient than a conventional AC system.
An investigation on the operation of packed tower for the regeneration of liquid desiccant is presented. A theoretical model demonstrating the effect of the system parameters is described. The experimental results are plotted to illustrate the effect of air and liquid parameters on the output variables. The regeneration process is shown to be highly dependent on the air inlet conditions, namely, temperature, humidity and flow rate. Also, the effects of the liquid temperature, concentration and flow rate is discussed. Data obtained were correlated to estimate the rate of water evaporation (regeneration rate) from values of variables that influence the rate of mass transfer in the column.
A forced flow solar collector/regenerator is one of the effective ways of regenerating the weak liquid solution in an open cycle liquid desiccant air conditioner using solar energy. In this system, the weak solution flows over the absorber plate of a tilted collector/regenerator as a thin liquid film. The forced air stream, which flows parallel or counter to the solution film, removes the moisture which is evaporated from the liquid solution due to absorption of solar energy. The absorber plate of the collector/regenerator is blackened and glazed to enhance the solar energy absorption and protect it from the environment. To evaluate the thermal performance of the solar collector/regenerator, a computer model has been developed using calcium chloride as the desiccant. A parametric analysis of the system has been performed to calculate the rate of evaporation of water from the solution as a function of the system variables and the climatic conditions.
Solar assisted air conditioning systems using liquid desiccants represent a promising option to decrease the high summer energy demand caused by electrically driven vapor compression machines. However, for these systems high efficient and reliable components need to be developed and the design has to be adjusted to each respective building design, location and user demand. The paper presents the design of a prototype for one of the main components of liquid desiccant systems and reports on its experimental performance. A numerical model has been developed for the simulation environment TRNSYS and validated using the experimental data. With this model parametric studies of the component performance have been carried out.
Desiccant material has been used in drying applications because of its low energy consumption, among other advantages. Desiccant material can produce hot and dry air that is beneficial for the drying process. The advantages of using desiccant material in a drying system include continuous drying even during off- sunshine hours, increased drying rate due to hot and dry air, more uniform drying, and increased product quality especially for heat-sensitive products. Some problems in desiccant system such as pressure drop in solid desiccant, carries over of liquid desiccant by air stream and low moisture adsorption capacity may be improved by optimize the design of desiccant system. Numerous researchers have studied the low cost and low regeneration temperature of desiccant material, and the optimization of desiccant application to produce more competitive energy. The use of heat to regenerate desiccant material in a drying system has limitations in energy saving. However the use of low energy or free available energy such as solar energy and waste heat from industrial processes for regeneration of desiccant material will make the system more cost-effective. This paper presents several works on the regenerative method of the desiccant system and its application in the drying system for both solid and liquid desiccant materials.
In the present work, a novel configuration of solar-powered desiccant dehumidification system is demonstrated. A rotating wick made of double layered cotton-cloth has been applied as a desiccant solar regenerator. The blackened wick-surface, which is impregnated with calcium chloride solution, moves between two rotating pulleys at an inclination angle of 20 degrees. Solar radiation incident on the wick surface regenerates the liquid desiccant in the wick. In the experimental study, instantaneous values of the desiccant solution concentration is evaluated and recorded with time. Apparent values of system coefficient of performance around 2 could be attained in the dry climate of Taif city. Instantaneous as well as average values of mass transfer coefficient are evaluated from the experimental measurements. System operational problems are discussed and highlighted.
Description du modele de simulation d'un systeme hybride de refrigeration combinant le refroidissement indirect par evaporation avec un deshumidificateur et une unite de refrigeration par compression. Etude des performances du systeme dans 2 types de climat et 2 charges representatives de bâtiments commerciaux
A solar liquid regenerator that embodies energy saving effect is a key part in solar liquid cooling air-conditioning system. Solar air pretreatment liquid collector/regenerator as a novel solar C/R (collector/regenerator) can achieve liquid regeneration in lower temperature, which is suitable to be employed in the high humidity area. The heat and mass transfer process was simulated in the novel liquid regenerator and the conclusions show that the increment of solution outlet concentration increases 70%, regeneration efficiency ηz augments 45.7% and storage capacity SC increases 44% as effective solution proportion ESP falls from 100% to 62%. For higher solution outlet concentration needed in the dehumidifier, both lower solution mass flow rate and higher solution inlet concentration all can be adopted in the novel C/R, in which the decrease of effective solution proportion ESP can increase the rate of evaporation G significantly. Along with the augment of air mass flow rate, the rate of evaporation G rises fast firstly and then falls slowly. The simulated results show that there is huge potential of improving and regulating solution regeneration performance by employing the novel C/R.
Theoretical investigation on the performance of lithium chloride (LiCl) absorption cooling system using an artificial neural network (ANN) model is presented. Tabulated data from the literature are used to construct the ANN model. Solar collector desiccant/regenerator is applied to re-concentrate the working solution. Using the proposed model, the effect of system design parameters; namely regenerator length, and air flow rate on the performance of the system is demonstrated. The variation of the thermo-physical parameters along the regenerator length is highlighted.
In this paper, high performance packing, namely, structured packing that has good heat and mass transfer characteristics, is proposed for dehumidification of air using liquid desiccants and for regeneration of liquid desiccants. In order to design a structured packing tower for liquid desiccant - air contacting operations, heat and mass transfer coefficients for each phase are required. This paper is concerned with the interface transfer of heat and mass when air-is brought into contact with the liquid desiccant solution. A theoretical study of evaluating heat and mass coefficients in an air-desiccant contact system employing three liquid desiccants, namely calcium chloride, lithium chloride, and a mixture of 50% calcium chloride and 50% lithium chloride (called cost effective liquid desiccant, CELD) is investigated. Moreover, air phase transfer coefficients are correlated with flow rates of air and liquid and the temperature of air, whereas liquid phase coefficients are correlated with rates of air and liquid flow, and the temperature and concentration of the liquid. The findings for the three liquid desiccants are compared and discussed.
In this paper, a solid desiccant cooling system with a backup vapour compression system is simulated using TRNSYS and the performance of the system is evaluated in four cities in the United States with different climates. Economic analysis is performed in order to assess the feasibility of these systems and to determine the relevant economic parameters such as life cycle costs, life cycle savings and payback periods. Results show that the system has higher COP values for the locations with more latent loads. The air conditioner was able to meet the cooling demand in all four regions, but it needed more auxiliary energy in the Eastern and Mountain regions than in the Central region, because of the higher solar fraction in the Central region. The simulation also showed that the desiccant cooling system by itself was capable of meeting the cooling demand and hence the requirement of a backup system may be eliminated. Thermal and economic parameters were analysed for varying solar subsystem sizes which proved helpful in optimizing the design of the solar system. Recommendations to minimize the auxiliary energy costs using different methods for supplying the thermal energy for desiccant regeneration are described. Copyright © 1999 John Wiley & Sons, Ltd.
This paper presents the modeling and simulation of solar-powered desiccant regenerator used for open absorption cooling system. The input heat, which is used to re-concentrate the desiccant solution, is estimated via a real-time solar radiation model in terms of the location, day of the year and time of the day. Lithium chloride (LiCl) and calcium chloride (CaCl2) solutions are applied as the working desiccants in this investigation. To compute the thermo-physical properties, a state equation is used for the calcium chloride desiccant while tabulated data along with an artificial neural network (ANN) model is used for the lithium chloride desiccant. A finite difference method is used to simulate the combined heat and mass transfer processes that occur in the liquid desiccant regenerator using the Matlab–Simulink platform. Using the proposed model, the effect of the important parameters, namely the regenerator length, desiccant solution flow rate and concentration, and air flow rate, on the performance of the system is investigated. It has been found that the vapor pressure difference has a maximum value for a given regenerator length with higher values obtained with the calcium chloride desiccant than those obtained with the lithium chloride desiccant. The proposed model can be successfully used for investigating the effect of different operating parameters under different ambient conditions and for predicting the overall performance of the system.Highlights► We present simulations of solar-powered open absorption cooling systems. ► CaCl2 and LiCl solutions are used. ► A real-time solar radiation model is used. ► Vapor pressure difference has a maximum value for a given regenerator length. ► Higher values of mass transfer are obtained with the CaCl2 desiccant.
This paper presents the review of the development and application of the thermally activated desiccant cooling technologies. The paper first introduces the global problem of energy and the environment related to the consumption of the carbon-based energy sources (gas, oil and coal). The contribution of the building sector to the above problem so as to maintain indoor environment providing human thermal comfort is shown. In this paper, the alternative methods for the provision of the human thermal comfort through thermally activated desiccant cooling technologies are discussed--solid desiccant, liquid desiccant and hybrid desiccant cooling systems. These technologies are potential alternatives to the mechanical vapor compression cooling technologies in the provision of human thermal comfort conditions. However, the development and application is mostly in developed and advanced developing countries. For a global scale solution to the problem of energy and environment contributed by the building sector for maintenance of comfortable conditions, dissemination of ideas and technologies to the developing world (Africa, South and South East Asia, South America) enhances the applicability and practicability of these technologies.
In this paper, the principles underlying the operation of desiccant cooling systems are recalled and their actual technological applications are discussed. Through a literature review, the feasibility of the desiccant cooling in different climates is proven and the advantages it can offer in terms energy and cost savings are underscored. Some commented examples are presented to illustrate how the desiccant cooling can be a perfective supplement to other cooling systems such as traditional vapour compression air conditioning system, the evaporative cooling, and the chilled-ceiling radiant cooling. It is notably shown that the desiccant materials, when associated with evaporative cooling or chilled-ceiling radiant cooling, can render them applicable under a diversity of climatic conditions.
The performance of an air dehumidifier using triethylene glycol (TEG) as desiccant under hot and humid conditions was investigated. The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate and the dehumidifier effectiveness. A packed bed column (dehumidifier) was employed, with low packing density (77 m2/m3), to provide direct contact between the air and the TEG. Two different structured packings were used, wood and aluminum. The experiments covered a wide range of parameter space that included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and inlet TEG concentration. The liquid flow rate investigated is much less than that covered in previous studies (<1 kg/m2 s). The trend of the dehumidifier performance was similar to that reported in the literature using high density and random packing. The results were compared to the Chung and Luo correlation, which over predicted the effectiveness. The Martin and Goswami correlation failed to predict the effectiveness under the conditions of this study. In the present study, it was found that the moisture removal rate increased with increasing inlet TEG concentration, TEG flow rate and air flow rate. This was seen for both the wood and the aluminum packings. In addition, the moisture removal rate is increased with increasing the inlet air temperature for the aluminum packing only. The effectiveness of the column was increased by increasing the TEG flow rate and inlet TEG temperature for the two packings.
A solar-regenerated liquid desiccant ventilation pre-conditioning system has been proposed for use in hot and humid climates. The system aims to dehumidify the ventilation air which is the major source of latent load. A heat exchanger is used to cool the dehumidified air instead of typical evaporative cooling to maintain the dryness of the air. The use of solar energy at the regeneration process and cooling water from a cooling tower makes the system more passive. The simulation procedure for the proposed system has been presented. By inputting the climatic data and the physical parameters of all equipments, the operating parameters at each equipment and the performance parameters of the system can be evaluated. The simulation procedure is demonstrated by showing the daily profiles of the operating and performance parameters on a typical day as well as investigating the influence of the selected operating parameters on the system performance. The results suggest that the most influential parameters are solar radiation, ventilation rate, and desiccant solution concentration. The balance between the water removed at the dehumidifier and that evaporated at the regenerator needs to be considered to maintain uniform performance during continuous operation.
This paper describes current trends in solar-powered air conditioning, which has seen renewed interest in recent years due to the growing awareness of global warming and other environmental problems. Closed-cycle heat-powered cooling devices are based mainly on absorption chillers, a proven technology employing LiBr–water as the working fluid pair. Recent developments in gas-fired systems of this type make available double- and triple-effect chillers with considerably higher COP than their single-effect counterparts, which makes it possible to reduce the amount of solar heat required per kW of cooling. These systems require, however, high-temperature solar collectors. The principles of multi-staging absorption systems are described. An economic comparison is provided which shows the total system cost to be dominated by the solar part of the system. At current prices, the high COP, high temperature alternative is still more costly than the low temperature one. Open-cycle desiccant systems employing either solid or liquid sorbents are described. While the main thrust in research on novel closed-cycle absorption systems has been toward increasing the operating temperature in order to improve efficiency through multi-staging, open-cycle absorption and desiccant systems have been developed for use with low temperature heat sources such as flat plate solar collectors. A novel open-cycle (DER) system is described, which makes it possible to use the solar heat at relatively low temperatures, for producing both chilled water and cold, dehumidified air in variable quantities, as required by the load.
One of the main components of a liquid desiccant cooling system is the regenerator. In a liquid desiccant air conditioner, outside air is dehumidified by liquid desiccant and cooled within the absorber. The diluted desiccant solution thus obtained has to be concentrated for reuse, by passing through the regenerator and the cycle is, consequently, repeated. The regenerator used in this application is a forced parallel flow type solar collector/regenerator. The regenerator has been designed and optimized and the prototype of the solar collector/regenerator has been built and tested. Calcium chloride has been used as the absorbent solution. The results of the tests conducted as a parametric analysis indicate that the air and solution mass flow-rates and the climatic conditions affect the regenerator performance. Furthermore, a comparison between the experimental data obtained and a previously developed model for a forced parallel flow solar collector/regenerator reveals that the experiments are in good agreement with the model predictions. Finally, it was concluded that the proposed solar collector/regenerator performs satisfactorily under the summer conditions of Adelaide, Australia.
Liquid-desiccant dehumidification has been proved to be an effective method to extract the moisture of air with relatively less energy consumption, especially compared with conventional vapour compression system. To date, the conventional dehumidification mode with desiccant solution has been improved or replaced by newly emerged energy-saving systems with better performance. This paper gives a detailed account of the general features of the major desiccant dehumidification techniques and configurations of the related systems; meanwhile, attention has been paid to both technological and theoretical development of regenerator, which is an indispensable component of the liquid-desiccant dehumidification system. Moreover, a summary of the experimental and analytical studies to optimize the system performance has been made. Some new hybrid systems that greatly expand the desiccant dehumidification technique in industrial and residential applications, as well as effectively promoting the single system's performance, are also introduced. Finally, future study and application for liquid-desiccant dehumidification techniques are concluded.