| Summary: | 博士 === 國立臺灣大學 === 職業醫學與工業衛生研究所 === 100 === An air conditioner is used to increase the occupants’ comfort by adjusting the indoor air temperature, but it is a major part of energy consumption. We can achieve equivalent or superior results by using general ventilation but drawbacks may include the incursion of outdoor air pollutants and an increase in radiant energy from sunlight into the home. This study designs a low-cost, practical, louver-window-type, electrostatic precipitator that can reduce pollutants entering the indoor space and shade the indoor area, while permitting increased ventilation in a home or small office environment. We also demonstrated that an electrostatic precipitator, originally designed for dust collection, could become an efficient nanoparticle generator under specific operating conditions. This unique feature was utilized in the present study to develop a nanoparticle generator based on corona discharge.
A lab-scale, adjustable, wire-plate positive louver ESP and a wire-plate corona discharger were built for measuring particle penetration, energy quality, ozone concentration and particle emission. Environmental contaminants were removed by HEPA filter, active charcoal and silica gel. Gold, tungsten, molybdenum, and stainless steel were used as the electrode to study the material dependency. Gas temperature was controlled by a feedback heater. A positive direct current power supply was employed to energize the corona discharger. A scanning mobility particle sizer with a nano differential mobility analyzer was employed to measure the aerosol number concentration and size distribution. Ozone concentration was monitored by using an ozone analyzer. The sampling locations of SMPS and ozone analyzer were 20 and 15 cm downstream the corona discharger, respectively. The major operating parameters included louver angle, electrode diameter, electrode spacing, electrode material, air velocity, air temperature, applied voltage and current.
The results showed that the louver adjustment significantly affected the ESP performance. The discharge wire should be positioned in the middle to provide optimal ESP performance, although moving around the electrode did not significantly change the energy consumption and ozone generation. The collection plates with excessive length were proven to be ineffective. The wire-to-plate distance decreased with increasing louver angle. The louver adjustments resulted in changes of the effective collection area, electric field strength and air velocity. The field strength should be as low as possible to obtain a high energy quality index. For a given energy consumption, the energy quality index was not significantly affected by the louver angle. This phenomenon was due to a trade-off between the electric field strength and the effective collection area. Therefore, all aerosol penetration curves showed within a narrow band.
The air temperature appeared to have a strong effect on ESP nanoparticle generation. At temperature above 37°C and flow rate below 9 L/min, the nanoparticle penetration of ESP exceeded 100%, indicating that the ESP was generating aerosol particles. Sputtering on the corona discharger appeared to be the key mechanism of aerosol generation. Particles were generated as soon as the ESP was on set. The ozone concentration increased with increasing corona current. The ESP reached a maximum number concentration at the electric field strength of 4.8 kv/cm when the air flow and temperature were fixed at 6 L/min and 40°C, respectively. The particle size ranged from 5 to 40 nm. Elementary components of the discharge wire were detected on the filter samples collected downstream the ESP and ground plates, indicating that nanoparticles were generated from the discharge wire. The ESP transit to a nanoparticle generator when it could not efficiently capture the particles generated from itself.
The maximum aerosol concentration occurred when the electric field strength was around 8.2, 9.8, and 11.2 kV for electrode diameter of 0.1, 0.2 and 0.3 mm, respectively. The smaller discharge electrode diameter generated more aerosol particles, but lower ozone concentration when compared to larger electrode diameter. The differences in aerosol concentration due to the change of electric field strength decreased with increasing electrode diameter, because the mean kinetic energy was more uniform in the larger electrode. Electrode materials did not affect the I-V curve but the aerosol generation rate and the ozone concentration were clearly material-dependent. Gold was chosen as the discharge electrode because of stable and high sputtering yield.
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