5. Heat and mass exchangers for Liquid desiccant de-humidification
The Heat and mass exchanger of a desiccant dehumidification unit is where the liquid desiccant comes in direct contact with the process air.
The desirable characteristics for heat and Mass exchanger for high-performance liquid desiccant dehumidifiers
1.
High heat and mass transfer rates
2.
Low pressure drop in process air flow
3.
Small liquid-side resistance to moisture diffusion
4.
Large contact transfer surface area per unit volume
5.
Compatible desiccant/contact materials (non corrodible with high wetting coefficient)
6.
Zero carryover of liquid desiccant droplets into process air
7.
Use of common materials and inexpensive manufacturing techniques
8.
Classified various thermally activated Desiccant cooling technologies as shown in fig. 4
Figure 4. Heat and mass exchanger configurations for various desiccant cooling technologies
The packed-bed conditioner has been the focus of many R&D projects on Desiccant De-humidifiers. More recent R&D on packed-bed heat and mass
exchangers includes the work of [9] in which the performance of packed-bed heat and mass exchangers flooded with lithium chloride solutions were experimentally measured. The researchers first implemented their conditioner and regenerator as internally cooled units using either copper tubes or polypropylene tubes as the contact surface. However, the copper tubes were too easily corroded by the desiccant, and the polypropylene tubes were too difficult for wetting.
[10] modelled and experimentally measured the performance of packed-bed, lithium chloride heat, and mass exchangers that used a random, polypropylene packing with a volumetric surface area of 210 m2 per m3. They reported that the lithium chloride solution did not uniformly wet the packing because of its high surface tension.
High flooding rates are necessary to keep the desiccant cool and complete wetting. But High flooding rates may cause carryover of liquid desiccant droplets into air stream and also is responsible for pressure drop in air flow. Conditioners that are internally cooled do not have to operate at the high flooding rates of packed-bed units as the desiccant temperature is maintained close to coolant temperature [11]. A cross flow heat exchanger is shown inthe Fig. 5 which is internally cooled by coolant where the process air flow and desiccant flow contact at right angles. A coolant liquid provided from a cooling tower or chilled water enters through the pipe section throughout the heat exchanger and hence internally cooling desiccant.
Figure
5.Internally
cooled
Cross flow heat exchanger
At low liquid desiccant temperature the vapour pressure also remains low and thus allowing more moisture absorption into the liquid desiccant.
6. Solar Hybrid Desiccant Cooling System
[12] has investigated the solar hybrid desiccant
air cooler (SHDCS) shown in Fig. 7 for its applicability and performance in commercial premises with high latent cooling load in subtropical Hong Kong. Vapour compression chiller was used to provide chilled water to a supply air cooling coil. Desiccant wheel was adopted and its regenerating heat primarily came from the solar thermal gain of the evacuated tubes. The desiccant wheel dehumidified the fresh air to the required level and the supply air coil provided the sensible cooling. For commercial premises with high latent cooling load (60% RH) It is observed that SHDCS had more superior cooling and energy performances than the conventional centralized air-conditioning (AC) system in the subtropical Hong Kong. The annual primary energy consumption saving could be around 49.5% in comparison to conventional vapour compression systems.
Figure 7. Solar Hybrid Desiccant Cooling system with solar heating for desiccant regeneration, Desiccant Dehumidification and compression based cooling
[13] simulated a hybrid desiccant cooling system comprising the conventional vapour compression air conditioning system coupled with a liquid desiccant dehumidifier which was regenerated by solar energy. The study suggested that, when the latent load constitutes 90% of the total cooling load, the system can generate up to 80% of energy savings. [14] conducted a comparative study of a standalone VAC, the desiccant-associated VAC, and the desiccant and evaporative cooling associated VAC as shown in following figure. The authors found an increase of cold production by 38.8–76% and that of COP by 20–30%. [15] have studied the performance of three possible hybrid system configurations in supermarket applications and have compared their performance with traditional VAC system. As reported, a total air conditioning saving ranging from 56.5% to 66% could be achieved for specified design conditions (ambient conditions: 30 °C, 16 g/kg.da; indoor conditions: 24 °C, 10.4 g/kg.da; room sensible heat ratio: 0.35). [16] have modelled the performance of a desiccant integrated hybrid VAC system. The waste heat rejected from a VAC cycle is utilized to activate a solid desiccant dehumidification cycle directly. The performance sensitivity of a first generation prototype hybrid VAC system to variable outdoor conditions has been studied and compared to the performance of conventional VAC systems. Results showed that the performance improvement over VAC systems could be 60% at the same level of dehumidification under ARI summer conditions. [17] have simulated the transient performance of a hybrid desiccant VAC system for the ambient conditions of Beirut. The annual energy consumption of the hybrid system in comparison with the conventional VAC system has been studied for the entire cooling season. A payback period less than five years was achieved.
[18] has reviewed various thermally activated cooling technologies and has tabulated a summary of the main features of up-to-date thermally activated cooling technologies which is shown in table 1. It is observed that of all the technologies liquid desiccant cooling technologies has lowest regeneration temperatures and has better COP value compared to other technologies.
7. Conclusion
Liquid Desiccant dehumidification systems although limited to industrial applications, but could exhibit huge potential energy and economic savings for HVAC industry by
•
Reducing the peak electricity demand created by compressor – based AC’s
•
Improving the indoor air quality and reduce the indoor humidity levels that could be difficult to be controlled by conventional air conditioners.
Hybrid liquid desiccant cooling technology has demonstrated its superior performance for hot and humid climatic conditions and could save more that 50% operational energy saving compared to conventional vapour compression cooling technology. One of most important advantages of desiccant cooling systems undoubtedly lies in the possibility of using solar energy which can be effectively utilized to regenerate saturated liquid desiccants by using relatively low-cost solar thermal collectors. LDAC’s have low regeneration temperatures (60 - 90°C) and have high COP values (0.5 – 1.2) compared to other thermally activated cooling technologies.
Future research should include development of non corrosive desiccant materials having lesser regeneration temperatures, developing control strategies to prevent mixing of liquid desiccant droplets in the process air stream and design of small and compact systems for application in residential buildings.
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