|Fig. 1: Diagram that explains operating principle of electrostatic precipitator. (Source: Wikimedia Commons)|
ESP is an air filtering device that uses electrostatic charge to remove dust particles. ESP ionizes the air by applying high voltage to the electrodes. Dust particles are charged by the ionized air and collected on the oppositely charged collecting plates. Since ESP actively removes dust and smoke from the gas, the system works well for wide range of biomass including wood, feces and low-quality coal which produce a lot of smoke. Moreover, ESPs boast collection efficiency (the ratio of particle counts entering and leaving the filter) that is usually higher than 99 %.  Provided that the right design approach is taken, it is possible to implement a versatile low- power ESP air cleaner.
Fig. 1 illustrates the operation of an electrostatic precipitator. The airflow (represented as two blue arrows pointing right) with dust particles (green circles) passes between charger electrodes (blue vertical rods) at negative electrical potential ranging from a few kilovolts to tens of kilovolts. Particles are charged negatively (blue circles) and are subsequently attached to collector plates (red square sheets) which is at positive electrical potential of a few to tens of kilovolts.
Fig. 2 shows an implementation of a practical industrial ESP in which charger electrodes are located at the center of hexagonal collector plates.
|Fig. 2: Electrodes and hexagonal collector plates inside electrostatic precipitator. (Source: Wikimedia Commons)|
ESP technologies are widely used for industry applications such as coal- and oil-fired electricity-generating utilities, chemical industries including oil refineries, pulp mills, and waste incineration facilities as shown in Fig. 3. In some cases, ESPs are also used to sample biological particles such as fungi, bacteria, or viruses for environmental monitoring and research purposes.  Consumer air filtration is another application of ESP where the product is designed for portability, ease of maintenance, and is capable of operating on 110 V to 240 V mains (household AC) electricity.
|Fig. 3: Electrostatic precipitator for waste incinerator facility in Gdańsk, Poland. (Source: Wikimedia Commons)|
ESP potentially represents a viable solution to indoor air pollution problem which kills 4.3 million people every year.  However, existing designs occupy large space and/or require access to reliable and plentiful source of electric power because they have been developed with focus mainly on large-scale industry applications for factories and power plants. 
The necessity of high input voltage and power is due in part to the use of relatively antiquated high voltage generator designs such as a Cockcroft-Walton multiplier or a Marx generator that date back to as early as 1900s. The resulting system size, cost, safety, and electrical efficiency have rendered ESPs difficult to be used in rural areas of developing countries where electricity supply is unreliable or even non-existent.
Attempts are being made for miniaturization and input power reduction of ESPs by using recently developed resonant circuit topologies and wide bandgap power semiconductor devices. High frequency, high voltage resonant dc-dc converters can achieve an order of magnitude improvement in power density and a reduction in weight while maintaining high electrical efficiency, lifting a major obstacle to utilization of ESPs in rural settings. For example, a proof-of-concept small-scale ESP filter that requires only 2 W power supply was recently demonstrated using a 36 V-to-3.7 kV 7 MHz dc-dc converter. 
© Sanghyeon Park. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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