Performance Improvement of Mass Transfer Through Membrane Using Ultrasound for HVAC Applications

A. Gurubalan^1, 2, 3\*^, Patrick J Geoghegan^2^, M.P. Maiya^1^, Carey Simonson^3^

^1^Indian Institute of Technology Madras, Chennai, India - 600036

^2^ Oak Ridge National Laboratory, USA – 37831

^3^University of Saskatchewan, Saskatchewan, Canada, S7N0W5

^\*^Corresponding author email: gurumariner60@gmail.com

Air conditioning systems presently account for the maximum consumption of the total electricity usage in buildings. Its energy consumption in India and the United States is likely to be around 2200 TWh by 2050. This is due to increase in (i) cooling degree days (ii) economic growth (iii) affordability (iv) population (v) urbanization (vi) ageing and (vii) illnesses \[1\]. More than 90% of the conventional air conditioning systems (CACS) are of vapor compression refrigeration type, which becomes energy inefficient due to subsequent overcooling and reheating of air for the humidity control \[2\]. Thus, it becomes mandatory to meet the aforementioned energy demand using an alternative energy-efficient system.

The liquid desiccant system is a promising alternative to CACS for the humidity control. It is broadly classified as packed bed and membrane types. In the former, air carries tiny droplets of desiccant to the air conditioning room due to direct contact between the air and desiccant. This desiccant carryover not only corrodes the air handling equipment but also deteriorates the indoor air quality. On the other hand, the membrane between the air and desiccant in the membrane dehumidifier prevents direct contact and allows only the water vapor to transfer through its pores. Hence, the membrane dehumidifier is preferred to avoid the desiccant carryover. However, it's mass transfer performance is lesser than that of the packed bed dehumidifier. This is due to additional mass transfer resistance of the intermediate membrane. The resistance by the boundary layer formed at the membrane-air interface accounts for a significant portion of the overall mass transfer resistance. Breaking of such boundary layer using the ultrasound is an attractive technique to reduce the resistance. Therefore, the present study focuses on the potential of ultrasound to enhance the mass transfer performance of the membrane dehumidifier.

The present study experimentally investigates the influence of ultrasound on the mass transfer performance of a membrane humidifier. Subsequently, with the experimental results of the humidifier, the effect of ultrasound on the performance of the membrane dehumidifier is numerically studied. It is found that the ultrasound (70W, 30 kHz) increases moisture addition rate and latent effectiveness from 0.007 to 0.013 g/s and 33 to 55%, respectively. The improvement is due to the vibration of the ultrasound transducer, which creates micro-turbulence near the interface. This disturbs the concentration boundary layer and thereby increases the mass transfer performance of the membrane humidifier. Similarly, the numerical results found that the ultrasound enhances the performance (i.e., moisture removal rate and latent effectiveness) of the membrane dehumidifier by 1.5 times. These preliminary results show that great promise for the improvement of mass transfer in membrane devices using ultrasound. 

References

1.      The future of cooling opportunities for energy efficient air conditioning by International Energy Agency, Available at \< http://www.indiaenvironmentportal.org.in/files/file /The_Future_of_Cooling.pdf \> \[Last accessed 23^rd^ Nov 2019\].

2\. Alternatives to Vapor-Compression HVAC Technology. Available at \< https://www.ashrae.org/File%20Library/Communities/Committees/Standing%20Committees/Refrigeration%20Committee/Alternatives-to-Vapor-Compression-HVAC-William-Goetler.pdf \> \[Last accessed 23^rd^ Nov 2019\].

 


Performance Improvement of Mass Transfer Through Membrane Using Ultrasound for Hvac Applications

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