High Frequency Testing and Modeling of Silent Discharge Ozone Generators

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OZONE SCIENCE & ENGINEERING 0191-9512/03 $3.00 + .00 Vol. 25, pp 363-376 International Ozone Association Printed in the U.S.A. Copyright 2003

J. M. Alonso1, M. Valdés1, A. J. Calleja1, J. Ribas1, J. Losada2

1Electric and Electronic Department, University of Oviedo Campus de Viesques, Edificio 3, E-33204 Gijón, Spain, E-Mail: [email protected]

2 Etronecology, S. L. Plaza Primo de Rivera, 1, E-33001 Oviedo, Spain, E-Mail: [email protected]

Received for review: 5 November 2002 Accepted for publication: 10 March 2003

Abstract

This paper deals with high frequency modeling of silent discharge ozone generators (OGs). The electrical characteristics of two simple silent discharge OGs operated at low and high frequency are analyzed and compared. An equivalent electric model is proposed for the operation of the OG at high frequency. This model can be used to optimize the electronic power converter used to supply silent discharge OGs at high frequency. Experimental results measured in the laboratory for two particular OGs are presented to validate the proposed model.

Keywords

Ozone; Ozone Generation; Silent Discharge; High-Frequency Ozone Generators; High-Frequency Model;

Introduction In order to facilitate the design of power converters capable of generating ozone at high frequency, the At the present time, electrical discharges provide the testing and modeling of OGs at high frequency is best way for ozone generation, although they still mandatory. The electrical models obtained for OGs present a low ozone yield (1Dascalescu, 1993; at low frequency (below 1kHz) are not very suitable 2Dimitriou, 1990). This technique has been at high frequency since their behavior changes at investigated for a long time and is largely frequencies over 1 kHz. This paper is related to the industrialized nowadays, but much progress still development of electrical models of OGs at needs to be done to increase the overall efficiency of frequencies around 15-20 kHz. The proposed models existing ozone generators (OGs). A possibility to are compared to those existing for OGs operating at increase the efficiency is the use of high frequency low frequency. converters to supply the OG, as opposed to the low The paper is organized as follows. frequency power supplies traditionally used. High frequency converters provide lower power losses, • Description of the configuration of the OGs lower size and weight and the possibility to control under study. the amount of ozone generated (3Masuda et al 1988, 4,5 6 • A revision of the low frequency behavior Wang et al 1998, Potivejkul 1998). and existing models of OGs is performed. 363

364 J. M Alonso et al. • The experimental workbench used to carry • Several experimental results obtained at out the laboratory tests is described. high frequency from the OGs under study are discussed. • The new model of OGs operating at high frequency is presented and discussed. An • Summary and conclusions easy procedure to obtain the model parameters from the Lissajous figures is also included in this section.

Ozonizer type A 0.3 mm

1 mm

Borosilicate Glass Stainless Steel Net 1 mm

23 mm

280 mm Steel foil (0.2 mm thickness) 300 mm

Ozonizer type B

250 mm

200 mm 5 mm 180 mm

4mm Borosilicate Glass Stainless Steel Foils (0.2 mm thickness)

230 mm

Figure 1. Configuration of the two types of ozone generators.

Materials and Methods well fixed to the glass cylinder, in order to attain a good facing each other. OG type B consists of a 200

mm x 250 mm planar borosilicate glass of 4 mm Description of Ozone Generators and Basic thickness. One of the electrodes is a 180mm x Operation 230mm stainless steel foil and the other one is a steel strip with 230 mm length and 5 mm width, both of Two types of OGs were used in this work. The them of 0.2 mm thickness, as shown in Figure 1. configurations of the OGs are shown in Figure 1. OG type A was made using a borosilicate glass The two OG presented are especially designed for cylinder with 1mm thickness, 23 mm outer diameter operation at high frequency (beyond 15kHz) and and 300 mm longitude. The glass is surrounded by ozone is generated basically by means of a silent an inner coaxial cylinder of stainless steel foil (0.2 discharge. In this type of discharge a high ac mm thickness) and an outer coaxial cylinder of voltage is applied between the two electrodes, one of stainless steel net (0.3 mm diameter and 1mm gap), which is covered with a thin dielectric layer, such as forming the two electrodes. The two electrodes are glass, ceramic material, etc. The dielectric barrier

High Frequency Testing and Modeling of Silent Discharge Ozone Generators 365 prevents electric arc generation between the two charge generates an electric field opposed to the electrodes. Therefore, the plasma that takes place in primary one and after a short period of time (in the a silent discharge self-extinguishes when the charge range of nanoseconds) the current flow is interrupted built up on the surface of the dielectric layer reduces at this spot. However as long as the external voltage the local field. exists, another microdischarge takes place in a different location. Therefore, the dielectric barrier In these type of OGs the current flows through the performs two important roles: limits the amount of discharge gap by means of millions of charge and energy in each individual microdischarge microdischarges, each one consisting of a thin and distributes the microdischarges over the whole plasma cylinder of nanoseconds duration. Due to the electrode surface. current flowing through the microdischarge, a charge is stored on the dielectric surface. This stored

Vs B

VOG

VPEAK

C tA tB t A VTH

IOG

D Microdischarges

(a)

V B

VTH C A Q

(b) Figure 2. Low frequency behavior of the OGs: (a) Operating waveforms (b) V-Q Characteristic

Review of Low Frequency Behavior

366 J. M Alonso et al. The voltage and current waveforms of this type of OG operating at low frequency are shown in Figure T1 2a. The current flows through the discharge gap in a great number of discrete microdischarges, as stated previously. These microdischarges present a Cd different nature for negative and positive corona. For Vdis the negative corona the microdischarges are known Cg as Trichel pulses, and they are very regular in their magnitude, shape and repetition rate. The frequency of the Trichel pulses increases gradually with the T2 applied voltage from a few kHz initially to several MHz just before attain the glow discharge. For the positive corona the microdischarge is explained by the streamer mechanism. As oppose to Trichel pulses, the streamers present random and irregular occurrence. Figure 3. Equivalent circuit of the OG at low frequency Since the OG behaves mainly as a capacitor, the current through the discharge (IOG) is in advance with respect to the voltage across the OG (VOG). At Although the OG presents a discrete behavior, its point A, the voltage across the discharge gap is high operation can be described adequately by means of enough to initiate the microdischarges; this takes average quantities. The power delivered to the OG place for a threshold value of the input voltage VTH, can be obtained as follows: as illustrated in Figure 2a. During the active phases 1 T (A-B and C-D intervals), microdischarges take place POG = v OG (t)i OG (t)dt [1] ∫0 and the voltage across the discharge gap is nearly T constant. This voltage is usually named as discharge Assuming that during both active phases A-B and C- voltage (VDIS). D, the voltage across the discharge gap is nearly Figure 2b illustrates the typical voltage-charge (V- constant and equal to VDIS, then

Q) characteristic of the OG operating at low t VDIS B frequencies. The V-Q characteristic is very useful to POG = 2 i OG (t)dt [2] ∫t describe the behavior of the OG. Lines A-B and C-D T A represent the active phases where microdischarges Since during the active phase the voltage across the take place. Lines B-C and D-A represent the passive discharge gap is fixed at VDIS , du/dt = 0 and there is phases where the input voltage is under the threshold no current through the discharge gap capacitor. In voltage VTH. The charge stored in the OG is usually these intervals the voltage across the dielectric measured by means of the voltage across an external capacitance is equal to vS – VDIS, vS being the input capacitor placed in series with the OG. voltage. Therefore, the power in the OG can be Figure 3 illustrates the equivalent circuit used for an expressed from [2] as follows: OG operating at low frequencies (7Chang et al, t VDIS B d 1995). Capacitors Cd and Cg represent the equivalent POG= 2 CvVdt d() S−= DIS capacitance of the dielectric and discharge gap Tdt∫ tA [3] existing in the OG respectively. The voltage source t VDIS B VDIS represents the discharge sustaining voltage. The 22CdvCV=−f []VV dSdDISBA∫ t diode rectifier is used to implement the discharge T A

condition, that is, the minimum voltage across the At instant tA, the input voltage is equal to VA= -VTH, discharge gap necessary to initiate the and at instant the tB the voltage VB is equal to the microdischarges. peak input voltage, then the final expression of the power in the OG is the following:

High Frequency Testing and Modeling of Silent Discharge Ozone Generators 367 and (b) V-Q characteristic (5 kV/DIV, 10 µC/DIV, 50Hz). POG = 2C d VDISf []VPEAK + VTH [4]

As example, Figure 4 illustrates the laboratory measurements at low frequency for the OG type A. Description of the Experimental Workbench The voltage and current in the OG are shown in Figure 4a and the V-Q characteristic is shown in for High Frequency Testing Figure 4b. The two OGs have been tested at the laboratory using the arrangement shown in Figure 5. The OG under test is supplied from a radio-frequency Power amplifier by means of a step-up transformer. The transformer has a turn ratio of 1:120 and can be operated at frequencies up to 20 kHz (model Current Voltage HFF106, Plasma Technics, USA). Voltage, charge and current waveforms in the OG are continuously measured by means of a digital oscilloscope equipped with a Hall current probe and a high voltage probe. In order to measure the charge in the OG a capacitor C is placed in series with the OG. Therefore, the instantaneous charge in the OG is equal to C times the voltage across the series capacitor. If a capacitor of 1 µF is used, 1 V across C

is equivalent to a charge of 1 µC in the OG. (a) The OG is placed in a sealed chamber (300 mm x 300 mm x 120 mm) and fed with a gas of 95% Voltage oxygen. An oxygen generator (WorkHorse 15, SeQual Technologies, USA) is used to supply the chamber. This generator operates following a pressure swing adsorption cycle using a zeolite molecular sieve. It is able to deliver up to 7 Nl/m of gas with a minimum of 90% of oxygen purity. The Charge chamber output is connected to an UV Ozone Monitor (model H1, InUSA, USA). It can be used to measure ozone concentrations up to 50 g/Nm3. The oxygen flow is also used to cool down the test chamber. The pipes used are made of PTFE of 10 mm inner diameter for the chamber inlet and outlet and 5 mm inner diameter for the input of the ozone monitor. An additional valve is employed to regulate the input to the ozone monitor to the necessary value (b) of 0.5 Nl/min as required for the correct measurement, as shown in Figure 5. Figure 4. OG operating at 50 Hz: (a) Voltage and current waveforms (5 kV/DIV, 2 mA/DIV, 20 ms/DIV)

368 J. M Alonso et al.

Oxygen PTFE Generator ( 10 mm)

O2 Inlet RF Amplifier 1 : 120 Voltage OG

C PTFE O3 Outlet ( 10 mm)

Reg. valve GND 0.5 l/m Test Chamber PTFE Charge Current ( 5 mm) Ozone Monitor

Exhaust Scope

Figure 5. OG test bench

High Frequency Modeling of Silent frequency. However, this model is highly non-linear and it is also difficult to use, especially for purposes Discharge Ozone Generators of designing high frequency power supplies for OGs. The parameters of the equivalent circuit, At line frequencies (50-60 Hz) the electrons and ions discharge voltage V and capacitances Cd and Cg in the discharge gap have much time to cross the DIS are very difficult to measure accurately at high discharge gap, and the discharge is developed as it frequency. Therefore, the idea proposed in this paper does under a static field of the same magnitude. is to simplify the model and propose a new model of When the supply voltage is a sinusoidal waveform, the OGs especially developed for the high frequency as shown in Figure 3a, the voltage across the operation range. discharge gap changes within the line period and the different discharge modes take place. Basically, in Figure 6 illustrates the derivation of the proposed an OG positive corona and negative corona high frequency model of the OG. First, rectifier and discharges materialize within a line cycle. discharge voltage source are substituted by an equivalent resistance Rp. The power dissipated in When the OG is supplied at high frequency the this resistance represents the power supplied to the current that flows due to corona discharge presents a OG in the form of ozone creation, heat and light, higher magnitude than that at low frequencies. Also, which are the three basic phenomena consuming the brightness generated in the discharge is more active power in the OG. intense. These phenomena are usually explained due to the capture of electrons and ions inside the In a second step, the two capacitances Cd and Cg are discharge gap. The mean life of these particles inside integrated in a single one, raising the capacitance Cp the discharge gap increases due to the highly in parallel with resistance Rp. The complete changing electric field. At low frequencies the proposed model for the OG is then shown in Figure particles have ample time to drift to the electrodes 6c. This model is a linear one in which parameters and leave the discharge gap. The higher density of Rp and Cp are very easy to evaluate. Besides, the ions and electrons in the discharge gap highly use of this model in the design of power converters decreases the mean free path of electrons, increasing for OG is very advantageous since it is a linear and the number of collisions. This is one of the reasons simple model. The proposed model can be also for the higher efficiency of OGs operating at high easily implemented in computer simulation frequencies. programs as SPICE based programs or mathematics programs. On the other hand, the use of this model The model presented in Figure 3 can also be used to allows engineers to design the power converter as a represent the behavior of the OG operating at high resonant inverter instead of a dc-to-dc converter, as

High Frequency Testing and Modeling of Silent Discharge Ozone Generators 369

it would be necessary using the previous model. 1 2 2 −1 X P [8] X P = ZP = R p + X P ϕ P = tan Therefore, the efforts of this work were focused to ωCp R P test and evaluate the proposed model for two Using [6] and [7] for canceling the time variable, the different high frequency OGs. voltage-current characteristic can be obtained: In order to obtain the model of a particular OG, the 1 1 2 2 [9] two parameters Cp and Rp should be evaluated from i OG (t) = v OG (t) ± Vg − v OG (t) some laboratory measurements. It will be shown in R p X p the next paragraphs that the two model parameters Figure 7 illustrates the Lissajous figure can be easily obtained from the Lissajous figures of corresponding to equation [9]. Using v = 0 in [9], voltage-current and voltage-charge in the OG. OG the value of the current can be calculated as follows In the proposed model (Figure 6c) the voltage and (see also Figure 7): current in the OG are related by means of a simple V first-order differential equation as follows: I = ± g [10] O X dv (t) 1 P OG [5] i OG (t) = C p + v OG (t) Therefore, by measuring the value IO from the dt R p Lissajous figure of the OG, the equivalent parallel where iOG and vOG are the current and voltage of the capacitance CP can be calculated using [8] and [10], OG respectively. resulting the following expression: The typical voltage applied to the OG at high I I C = O = O [11] frequency is a pure sine waveform of amplitude Vg P ωVg 2πf Vg and angular frequency ω, this is: For example, from the Lissajous diagram shown in [6] v OG (t) = Vg sinω t Figure 7 a value of 35 pF can be calculated. This value corresponds to the OG type B operating at Therefore, the current through the OG can be nominal power, as will be shown shortly. Hence, the obtained from [5] as follows: measuring of the equivalent capacitance of an OG V V V results very simple from the V-I characteristic. g g g [7] iOG (t) = cosω t + sinω t = sin()ωt +ϕ P X p R p Zp where

T1 Cd Vdis iOG Cd T1 Cg + vOG Cp Cg Rp T2 Rp - T2 T2

(a) (b) (c)

Figure 6. Derivation of the proposed model for high frequency operation

370 J. M Alonso et al.

0.04 Q = ±C V [16] Current O R g (A) 0.02 Then, the equivalent parallel resistance RP of an OG Io using the proposed model in this paper can be obtained from the V-Q characteristic by means of 0 the value QO, as follows:

0.02 Vg Vg R P = = [17] ωQO 2πf QO 0.04 8000 6000 4000 2000 0 2000 4000 6000 8000 As an example, from the Lissajous diagram shown in Voltage (V) Figure 8a value of the equivalent resistance RP equal to 2 MΩ is obtained. Therefore, the equivalent Figure 7. Voltage – current characteristic of an OG with parallel resistance of the proposed high frequency Rp=2 x 106 Ω, Cp= 35 pF supplied at 20 kHz. model can be very easily obtained from the V-Q characteristic of the OG.

7 4 10 On the other hand, the charge in the OG of the Charge proposed model can be easily obtained taking into (C) 7 account the relationship between current and charge: 2 10 Qo dq OG (t) i (t) = [12] 0 OG dt

7 Then, by integrating equation [5], the charge in the 2 10 OG is obtained:

7 1 4 10 q (t) = C v (t) + v (t) dt [13] 8000 6000 4000 2000 0 2000 4000 6000 8000 OG P OG ∫ OG R P Voltage (V)

For the typical case of a sinusoidal supply voltage across the OG given by [6], the charge in the OG can Figure 8. Charge – current characteristic of an OG with be calculated as: Rp=2 106 Ω, Cp= 35 pF supplied at 20 kHz.

q OG (t) = C P Vg sinω t − C R Vg cosω t = C E Vg sin(ω t − ϕ q ) Using the laboratory setup presented in previous section the two types of OGs were tested at high [14] frequency. As an example, Figure 9 illustrates the where: voltage and current waveforms for the OG type B. It can be seen from this figure that the current through 1 2 2 −1 C R the OG is highly in advance with respect the voltage C R = C E = C P + C R ϕ q = tan ωR P C P across the OG. Therefore, the behavior of the OG is near to a pure capacitance. This also means that the equivalent parallel resistance Rp is very high, Using [5] and [12] for canceling the time variable, typically in the range of MΩ. This can be inferred the voltage-charge characteristic is obtained as also from the voltage current characteristic presented follows: in Figure 10a. The current through the OG presents also high frequency spikes due to the q (t) = C v (t) ± C V 2 − v2 (t) [15] OG P OG R g OG microdischarges within the interval in which corona discharge materializes. Besides, microdischarges are Figure 8 illustrates the Q-V Lissajous diagram of an more intense during the positive period of the input OG at high frequency, corresponding to the voltage. expression in equation [15]. In this case, using vOG = 0 the following value of the charge in the OG is obtained (see also Figure 8):

High Frequency Testing and Modeling of Silent Discharge Ozone Generators 371 Figure 10b shows the characteristic of the OG in 11a illustrates the Lissajous figure measured for the comparison to the approximation using the proposed OG type B. The approximated voltage-charge model. As can be seen, very good correlation is characteristic using the proposed model is shown in obtained with the proposed model. Regarding the Figure 11b. Again, a very good correlation is characteristic of voltage-charge in the OG, Figure obtained.

CH1=5000V CH2=100mV Power

Voltage

Current

Figure 9. OG type B tested at high frequency: Voltage and Current waveforms (5 kV/DIV, 100 mA/DIV, 10 µs/DIV)

(a) (b) Figure 10. OG type B tested at high frequency: (a) Measured Voltage-Current characteristic and (b) approximation using the proposed model (X: 2 kV/DIV, Y: 50 mA/DIV)

372 J. M Alonso et al.

(a) (b) Figure 11. OG type B tested at high frequency, (a) Measured voltage-charge characteristic and (b) approximation using the proposed model (X: 2 kV/DIV, Y: 20 µC/DIV)

In the proposed model, the power delivered to the of the proposed model Rp and Cp were measured at OG can be easily calculated from the power handled different power levels. Figure 12 illustrates the by the equivalent resistance Rp: values measured in the OG type A. As can be seen,

2 in this type of OG the two parameters are nearly Vg independent of the power applied to the OG. POG = [18] 2R p Figure 13 illustrates the values measured for the OG type B. For this OG the equivalent capacitance is Using [17] in [18], the power in the OG can be also also nearly independent of the applied power. expressed as a function of the charge Q , as follows: o However, the equivalent resistance decreases when

POG = π Vg f QO [19] increasing the power handled by the OG. The luminosity emitted by this type of OG is higher than Therefore, knowing the supply voltage and that of the OG type A. This could mean that the frequency the power consumed by the OG can be silent discharge is closer to the arc discharge obtained from only one laboratory measurement: the characteristic than in the OG type A, what derives in value of the charge Qo at the instant when input the negative resistance characteristic shown in voltage is equal to zero. This is another important Figure 13a. advantage of the proposed model. Figure 14 illustrates the voltage applied to the OG as a function of the power handled by the OG. As can Experimental Results be seen, OG type A exhibits an increasing voltage with the power whereas OG type B shows a voltage The two types of OGs have been tested at the nearly independent of the applied power. This is in laboratory using the arrangement shown in section 4. agreement with the previous comment regarding the Some results are presented in this section. operation of the OG type B near the arcing region. First, the equivalent parameters of the proposed Nevertheless, the proposed model can be used to model were measured. The power applied to the OG represent the behavior of the two OG independently was varied by changing the gain of the of its operation region. radiofrequency amplifier. Therefore, the parameters

High Frequency Testing and Modeling of Silent Discharge Ozone Generators 373

1,2E+05 1,0E+05 8,0E+04 6,0E+04 4,0E+04 Rp(ohm) 2,0E+04 0,0E+00 0 5 10 15 20 25 30 35 40 P(W)

(a)

5,0E-10 4,0E-10 3,0E-10

Cp(F) 2,0E-10 1,0E-10 0,0E+00 0 10203040 P(W)

(b)

Figure 12. (a) Equivalent resistance (Rp) and (b) equivalent capacitance (Cp) of the ozone generator type A measured at 15 kHz

1,5E+07

1,0E+07

5,0E+06 Rp(ohm)

0,0E+00 0 5 10 15 20 25 30 P(W)

(a)

4,0E-11 3,0E-11 2,0E-11 Cp(F) 1,0E-11 0,0E+00 0 5 10 15 20 25 30 P(W)

(b) Figure 13. (a) Equivalent resistance (Rp) and (b) equivalent capacitance (Cp) of the ozone generator type B measured at 20 kHz

374 J. M Alonso et al.

3000 2500 2000 1500 1000 V(Vrms) 500 0 0 10203040 P(W)

(a)

8000

6000 4000

V(Vrms) 2000

0 0 5 10 15 20 25 30 P(W)

(b) Figure 14. Voltage versus power measured in the OGs: (a) type A and (b) type B

Finally, Figure 15 shows several results obtained converters used to supply silent discharge OGs at from the two OGs. The ozone concentration in high frequency. The model results are very simple g/Nm3 were measured together with the power and easy to obtain from typical laboratory delivered to the OG. The oxygen flow was 3 Nl/m measurements. Two types of OGs have been tested for OG type A and 5 Nl/m for the OG type B. From and modeled, showing a quite different behavior, but data measured at the laboratory, the values of ozone both of them can be well described at electrical level production in g/h and efficiency in g/kWh were using the proposed model. Some experimental calculated. As can be seen, the behavior of the two results measured at the laboratory for the two OGs is quite different. The best characteristics are particular OGs have been also included in order to obtained with OG type B, reaching efficiencies validate the proposed model. higher than 100 g/kWh and concentrations up to 11 g/Nm3. Acknowledgement Conclusions In this paper a new model for the high frequency This work has been supported by Ministerio de operation of silent discharge OGs has been Ciencia y Tecnología (MCYT) under research grant proposed. The model can be particularly used to number TIC99-0884. Authors would like to thank describe the electrical behavior of the OG at high them for the help received during the course of this frequency. This model can also be employed to work. optimize the operation of the electronic power

High Frequency Testing and Modeling of Silent Discharge Ozone Generators 375

4,0 25 g/h g/hour g/kWh 3,5 g/Nm3 20 g/Nm3 g/kWh 3,0

15 2,5

2,0 10

1,5 5

1,0

0 0,5

0,0 -5 0 5 10 15 20 25 30 35 P(W)

(a)

12 450 g/h g/kWh g/Nm3 g/hour 400 10 g/Nm3 g/kWh 350

8 300

250 6 200

4 150

100 2 50

0 0 0 5 10 15 20 25 30 P(W)

(b) Figure 15. Characteristics measured in the two types of OGs: (a) type A tested with a 3 Nl/m of oxygen flow and (b) type B tested with 5 Nl/m of oxygen flow

References

376 J. M Alonso et al. 1. L. Dascalescu; An Introduction to Ionized Gases. Theory and Applications. Published by Toyohashi University of Technology. 1993. 2. M. A. Dimitriou (Ed.); Design Guidance Manual for Ozone Systems. International Ozone Association (IOA). Pan American Committee. 1990. 3. S. Masuda, K. Akutsu, M. Kuroda, Y. Awatsu and Y. Shibuya; “A ceramic-based ozonizer using high- frequency discharge,” IEEE Trans. on Industry Applications, Vol. 24, No. 2, March/April 1988, pp. 223-231. 4. S. Wang, Y. Konishi, M. Ishitobi, S. Shirakawa and M. Nakaoka; “Current-source type parallel- compensated load resonant inverter with PDM control scheme for efficient ozonizer,” Proc. of IEEE International Power Electronics Conference (CIEP), Morelia, Mexico, 1998, pp. 103-110. 5. S. Wang, M. Nakaoka and Y. Konishi; “DSP-based PDM&PWM type voltage-fed load-resonant inverter with high-voltage transformer for silent discharge ozonizer,” Proc. of IEEE Power Electronics Specialists Conference (PESC), Fukuoka, Japan, 1998, pp. 159-164. 6. S. Potivejkul, V. Kinnares, P. Rattanavichien; “Design of ozone generator using solar energy,” Proc. of IEEE Asia-Pacific Conference on Circuit and Systems (APCCAS), 1998, pp. 217-220. 7. J.-S. Chang, A. J. Kelly and J. M. Crowley (Eds.); Handbook of Electrostatic Processes. Marcel Dekker, Inc. 1995.