The warm, humid air leaving the regenerator can also be used in an air to air heat exchanger to preheat the air that enters the regenerator.Three basic ways to regenerate the desiccant system is with a thermal source: solar, water heater, and a double effect. The water heater or boiler can be fueled by many sources, including natural gas, Combined Heat and Power , or even bio-fuels. Hot water can also be produced by recovered heat from an engine or fuel cell, or other energy source flows within the plates. Thermally driven single stage regenerators are typically supplied by hot water above 70oC, but preferably 82oC. The higher fluid temperatures increase the water removal capacity of regenerator significantly . Integration of LDAC with a dewpoint indirect evaporative cooler can efficiently deliver both latent and sensible cooling with minimal electric demands. This integration has been developed and patented by NREL and been named Desiccant enhanced Evaporative air-conditioning. As shown in Figure 14 in DEVap concept a portion of warm indoor air is mixed with ventilation air. This mixed air goes through the heat exchanger and comes into contact with the liquid desiccant through a permeable channel. The desiccant absorbs the water vapor in the air. Then,bato bucket air is cooled and supplied to the building space. A portion of supplied air which has its dew point reduced is recirculated as the secondary air stream. This secondary air comes into contact with the water layer through a permeable membrane.

Water evaporates into the air through the membrane. The two air steams are separated through the plastic sheets thermal energy including the heat of absorption flows. The use of membrane technology prevents the desiccant carry over into the supply air stream and it provides a large contact surface area for the heat and mass transfer between the liquid and air. As a result, membrane-based contactors can effectively improve the performance of a liquid desiccant-based air conditioning processes. The introduction of a membrane provides additional resistance to heat and mass transfer, therefore much work is focused on improving this with novel supported liquid membranes. The waterside membrane is not necessary and omitting it can reduce the cost. However, the semipermeable membrane is necessary for desiccant side to prevent desiccant leak to the air. The breakthrough pressure should be 20psi or lower to avoid desiccant leakage. The membrane should be 25μm and have pore size of 0.1μm. Its open area should exceed 70% to promote vapor transport. Polypropylene membrane from Celgard is one possible candidate for DEVap application. The deep cooling of the indirect evaporative cooler section requires a dry cooling sink; thus, some dry supply air is siphoned off to provide this exhaust air stream. This section is placed in a counter flow arrangement to maximize the use of this air stream. This is essential because it has been dried with desiccant, and thus has a higher embodied energy than unconditioned air. The result is that the temperature of supply air is limited by its dew point and will come out between 55°– 75°F depending on how much is siphoned off.

Combined with the desiccant’s variable drying ability, the DEVap Air Conditioning system controls sensible and latent cooling independently. DEVap does not require a cooling tower, which reduces its maintenance requirements. Dehumidification can be controlled by the desiccant concentration that is supplied to the device. The outlet humidity level can be controlled by controlling the supplied desiccant concentration or decreasing the flow of highly concentrated desiccant. The latter allows the highly concentrated desiccant to quickly be diluted and thus act as a weaker desiccant solution in the device. From inspection, the peak electricity draw of the DEVap A/C is considerably less than the standard A/C. This is primarily because compressor power is eliminated and replaced with only fan power to push air through the DEVap cooling core. NREL have designed a 1-ton DEVap system for a typical Gulf Coast condition with no water side membrane and LiCl as the desiccant as shown in Figure 15. Figure 16 shows the airstream process of DEVap on psychrometric chart. The design criteria were to supply cooling to the building at 7Btu/lb and a Sensible Heat Ratio of 0.6 while maintaining 55% indoor RH. NREL models shows that in all residential cases, the DEVap A/C provided more than necessary humidity control. Allowing indoor humidity to rise above 50% RH would have significant energy improvement. In the summertime, when sensible loads are high , the DEVap A/C continuously maintained the space at less than 50% RH. This level of humidity control can be reduced to create higher energy savings. In general, DEVap perform better in new construction building with added ventilation and tighter envelope because sensible heat ratio decreases.

Their analysis shows regional water use for the DEVap system is 2.0–3.0gal/ton.hr, is similar to the regional impact that DX A/C imposes . The DEVap A/C does increase site water use, but in general, the regional impact is small. Implementation of the DEVap A/C in commercial cases shows a higher energy savings level than the residential cases, primarily because of the higher cooling loads of commercial buildings and their increased ventilation requirements. DEVap regional water use is expected to be 2.0–3.0gal/ton.hr for commercial buildings. Similar to the residential case, the DEVap A/C has minimal impact on regional water use compared to DX A/C. The integration of LDCS with VCS can provide subcooling effect from the condenser outlet to the compressor inlet which leads to improved system performance. She et al. proposed a new energy-efficient refrigeration system subcooled by liquid desiccant dehumidification and evaporation. In the proposed system, liquid desiccant system produces very dry air for an indirect evaporative cooler, which sub-cools the vapor compression refrigeration system to get higher COP than conventional refrigeration system. The desiccant cooling system uses the condensation heat for the desiccant regeneration. The results show great improvement of COP for the proposed system, with maximum COPs about 18.8% higher than that of conventional VCS. Khalil experimentally investigate a multipurpose desiccant integrated with vapor-compression air conditioning. The system is designed to meet the cooling load of spaces with large latent heat and at the same time to extract water from atmospheric air. Te integrated system total cooling capacity is up to 6.2kW. The effect of regeneration temperature, condenser and evaporator temperature on the system COP and specific moisture recovery were analyzed under different liquid desiccant flow rates. The authors concluded that the COP of the proposed system was as high as 3.8 , with further 53% more annual energy saving. Based on the similar testing, Bassuoni has applied CaCl2 as the desiccant for the experimental testing. The results showed that about 54% COP improvement was achieved. Many studies have proposed hybrid LDAC- VCS where the liquid desiccant has been used to dehumidify the supply air. Mansuriya et al. analyzed a modified liquid desiccant dehumidification incorporated vapor compression refrigeration system based on thermoeconomic approach. The system is optimized based on the co-efficient of performance and the total annual cost of the system. The results show that 51.3% of the total cooling load can be handled by the desiccant dehumidifier alone. This significant sharing of heat duty by addition of liquid desiccant dehumidification to conventional vapor compression refrigeration system justifies this hybridization for humid and hotter climates. COP of optimized system shows 68.4% improve compared to coefficient of performance of standalone vapor compression refrigeration system for handling the same cooling load. Dai et al. studies the integration of desiccant dehumidification,dutch bucket hydroponic evaporative cooling and vapor compression air conditioning. In this system, latent cooling of the air is carried out in the dehumidifier, and air sensible cooling via the VCS cooling coil. Experimental investigation demonstrates significant increase in cooling production and COP of the new hybrid system compared with vapor compression system alone. Capozzoli et al. has economically analyzed a case based on the climatic conditions of three cities in Italy.

Their results show considerable reduction of electric energy demand as well as better control of thermal-hygrometric conditions were noted. A simple payback of about 1 year has been obtained. Researchers have also studied integration of solar driven hybrid liquid desiccant and VCS. Li et al.analyzed this system for Hong Kong climate condition using EnergyPlus. The results showed that an annual operation energy saving of 6760kWh compared to conventional VCS and a payback period of 7 years for a 19kW cooling capacity design system. Su et al. looks at driving vapor compression chiller by generated electricity from solar for cooling the desiccant solution for a two-stage dehumidification, and the released heat from the collector is used for the desiccant regeneration. Simulation results show the proposed system has a superior power saving ability of 55.65% comparing with the conventional one, besides the equivalent power generation efficiency reaches 8.7% in the base design condition. A comparative driven force analysis showed the two-stage dehumidification has a better match of driven force compared with the single-stage liquid desiccant dehumidification, thus leading to a reduced irreversible loss of 65.43%. Sensitivity analysis indicated that the dehumidification temperature has a decisive effect on the system performance. In heap pump driven systems, the cooling capacity from the evaporator is used to cool the liquid desiccant and exhaust heat from the condenser is used to regenerate the liquid desiccant. In this process, there are two air streams, one outdoor fresh air that will be cooled and dehumidifier, and the second one , a stream air used to regenerate the liquid desiccant . In this configuration there are three liquid desiccant circulation flow as shown in Figure 17: one in dehumidifier , one in regenerator , and one circulating between the heat exchanger . Many studies have been conducted on combined heat pump desiccant dehumidifiers for different case studies, different configurations, and different desiccant solutions. All these studies have theoretically and experimentally shown improvement in COP and energy savings. Sanaye et al.modeled and optimized a hybrid liquid desiccant-heat pump system containing dehumidification and cooling section. The system was analyzed in four energy, exergy, economic and environmental aspects. Then, the system was optimized for total annual cost and exergy efficiency. Results for our case study showed that the proposed optimized system decreased the electricity consumption for 33.2% in comparison with that for an electrical HP system during seven months of operation in a year . This amount of lower electricity consumption also provided 1.85e5kgCO2/year lower CO2 production in comparison with that for a conventional HP system. The COP of the system at the optimum point was also about 4.83 . Jradi and Riffat experimentally investigated a micro-scale tri-generation consisting of an organic Rankine-cycle, dehumidification and cooling unit. A scroll expander was used in the Rankine unit for heat and power generation. Cooling unit was a dew point evaporative cooler to provide the cooling capacity through air cooling. An experimental setup was built and the micro-scale tri-generation system was tested under different operational conditions using a wood pellet biomass boiler as a heating source. It is shown that the proposed system is capable of providing about 9.6kW heating power, 6.5kW cooling power and 500W electric power. The overall efficiency of the tri-generation system is about 85%. The dehumidification-cooling system has a thermal COP of 0.86 and electrical COP of 7.7. Solar has been used as the heating source for solution regeneration. The solution temperature is usually below 80℃ which fit well with solar provided temperature and solar radiation availability matches with cooling demand. During recent years many researches have been investigating integration of LDAC with solar theoretically and experimentally. Katejanekam and Kumar simulated a solar regenerated liquid desiccant ventilation pre-conditioning system for Thailand climate. In their proposed system, the solar energy is used for regeneration process and cooling water from a cooling tower for precooling. The effects of various parameters on the moisture removal rate and evaporation rate were analyzed theoretically. The results suggest that the solar radiation, ventilation rate, and desiccant solution concentration have the most influence on the system. They also suggested that 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.