Design and Analysis of an Ozone Generator System Operating ...

23 _________________________________________________________________________________________________©Revista Ciência e Tecnologia, v.15, n.27, p.23-35, jul./dez. 2012- ISSN:1677-9649

Design and Analysis of an Ozone Generator System Operating at High Frequency with Digital Signal Controller

1Gilson Junior Schiavon, 2Cid Marcos Gonçalves Andrade, 2Luiz Mario Matos Jorge, 2Paulo Roberto Paraíso

1Dr. Professor, Federal Technological University of Paraná, Coordination of Electronic Engineering, Paraná, Brazil, 2Dr. Professor, State University of Maringá, Department of Chemical Engineering, Paraná, Brazil.

[email protected]; [email protected]; [email protected]; [email protected]

Abstract – This paper presents the design and development of an ozone generator module with an output power maximum of 55 W, using an automotive coil in the flyback converter and digital control performed by a digital signal controller. The generator is designed based on the principle of corona discharge and can be applied in oxidative processes in general, by simply combining the necessary number of modules to achieve the power and concentration of ozone required for the application. The converters were designed and implemented in closed loop with proportional-integral action. The power stage consists of two high-frequency switching converters. Keywords – Ozone, Corona discharge, DC/DC converter, digital control, ozone generator, high frequency.

I. INTRODUCTION

Ozone has been currently studied for various applications, such as water treatment, domestic and industrial effluent treatment, medicine, dentistry, veterinary medicine, agriculture, disinfection of environments, and food preservation. As the use of ozone is growing fast, improvements in its generation by ozone generator systems is fundamental.

Ozone is known as the second most powerful oxidizing agent that can be used in large scale for water treatment and is being used by several countries for thousands of treatment systems. Ozone can be produced in three ways: by electrolysis, UV (ultraviolet), and corona effect. In commercial generators, ozone is mainly produced by UV radiation and corona effect. UV radiation, however, does not meet the large scale production required by industry. This way, corona electrical discharge is the most commonly used method to obtain significant amounts of ozone. In the corona discharge process, ozone is generated by an electrical discharge in a stainless steel tube called ozone reactor.

Traditionally, low-frequency sources are used in ozone generators. These systems require a very high output voltage, as they must operate at about 20 kV for a 1 mm gap in order to reach the required discharge power density. This may limit ozone production due to restrictions imposed by the dielectric strength of the dielectric material. Low-frequency systems also present high volume, low efficiency, and difficulty in controlling ozone production (Alonso et al, 2005).

Alonso et al (2003) analyzed and compared the performance of two ozone generators. The first operated

at high frequency, optimizing the power supply converters to produce the corona discharge and making the equipment lighter and more compact, while the second operated at low frequency. Experimental results were presented for validation of the system.

In high-frequency systems, the higher the frequency, the higher the power density and the lower the voltage applied to the reactor, thus allowing for an increase in the efficiency of the ozonizer. In addition, the equipment volume is decreased and ozone production can easily be controlled (Alonso et al, 2005). Alonso et al (2004) proposed a low-power high-frequency power supply for ozone generation. Satisfactory results were obtained in terms of yield and ozone production, using a 1 watt prototype.

To establish a constant concentration of ozone at the outlet of corona effect generators, some care is required to keep the output voltage (of the order of kV) constant, as ozone concentration depends on the voltage applied to the reactor (Alonso et al, 2005), to keep the secondary current and the air/oxygen flow rate constant, and to control the temperature. Electrostatic discharge usually occurs with low current between two electrodes separated by the oxygen (Bonaldo et al, 2010).

The ozone reactor used in the present work consists of two stainless steel electrodes and a glass dielectric in an arrangement of coaxial cylinders, the outer electrode being exposed to ground potential, followed by a gap through which air/oxygen flows and the dielectric medium in contact with the second electrode exposed to high voltage. Fig. 1 represents ozone generation from corona discharge.

Fig. 1. Ozone generation from corona discharge (Ozone Solutions, 2012).


In this context, this study deals with the design and development of a modular ozone generator system operating at high frequency. The standard module has maximum power of 55 W and can be used individually or combined with other modules according to the necessary concentration of O3. Technologies related to power electronics, digital control, and renewable energy are integrated in the search for a stable, compact, automatic, and high-yield system. This work focuses on the details of the electric model instead of the mathematical model, as the converter topologies that were used are already established in the literature, such as buck and flyback (Mohan et al, 2003).

II. PROPOSED MODEL

Two topologies of DC/DC (direct current/direct current) converters were used for the development of the ozone generator: buck and flyback. Pulse width modulation (PWM) used for the control of these converters was digitally generated by a digital signal controller (DSC) in closed loop with PI control action, aiming at constant voltage and current output.

The designed generator has some signs and protections to increase system reliability, such as a temperature sensor with two actions: the first is the activation of a ventilated cooling system and the second is the triggering of an alarm and complete blocking of generator operation, thereby protecting the generator components from overheating. It also monitors mains voltage, signaling abnormal AC (alternating current) voltage (low or high) through a LED (Light Emitting Diode). In addition, the generator has an inrush current limiter to limit the starting current of the equipment and a soft start drive.

Using a DSC made possible the implementation of all signs and protections, as well as the generation of the closed loop PWM pulses via programming, significantly reducing the amount of electronic components and consequently the size and complexity of the circuits. Fig. 2 shows the block diagram of the proposed ozone generator.

Fig. 2. Block diagram of the ozone generator.

The strategy presented in Fig. 2 is justified by the

choice of an automotive coil for the flyback converter, due to its ease of acquisition, quality assurance regarding electrical insulation, ease of installation, low cost, and high durability, thus avoiding production problems.

As ozone is a gas with a relatively short half-life (about 15 min at 25°C under atmospheric pressure), its storage is not feasible. For practical reasons, it needs to be generated at the application site. Ozone is highly soluble in water and has a high disinfecting and oxidizing power. For these reasons, the generator was designed to operate with the electric grid, with solar energy, or both, allowing for sustainable ozone generation for water disinfection, food storage, or use in remote areas, along with the environmental contribution by making use of a clean and renewable energy source.

Feeding the flyback converter with 36.6 V makes possible to feed the system through the electric grid (rectifier and buck converter) or by means of a photovoltaic system, without the use of a voltage boost converter. A digital timer was added to the system to operate automatically as scheduled. The input current of the flyback converter was digitally limited to 1.5 A, thus protecting the automotive coil.

III. RECTIFIER CIRCUIT

The rectifier circuit comprises a diode bridge, a line filter and an inrush current limiter. Table I presents the technical specifications of the rectifier circuit.

Table I. Specifications of the rectifier circuit

Inlet Outlet Fin = 50/60 Hz Iout_max = 500 mA V in_min = 90 Vrms Vout_min = 127 V V in_max = 240 Vrms Vout_max = 339 V

An RFI filter containing resistors, capacitors, and

inductors was used. The capacitors were of type X and Y, for interference suppression. At the initial condition (normally closed - NC) the inrush current limiter circuit inserts a 47Ω/5W resistor in the circuit to limit the current. The timed command from the relay to remove the resistor is digitally sent by the DSC after 2 s. The input is protected by a 0.5 A fuse. The complete rectifier circuit is shown in Fig. 3.

Fig. 3. Complete rectifier circuit.

IV. BUCK CONVERTER

The buck converter block has the function of reducing the voltage that was rectified and filtered by the rectifier block. The voltage level at the output is adjusted at 36.6 V to feed the flyback converter, enabling the use of a solar energy system in combination with a bank of batteries of 36 V (nominal). Table II presents the specifications for the buck converter.


Table II. Technical specifications for the buck converter V in_min = 127.0 V Vd1 = 0.5 V Vout = 36.6 V V in_max = 339.0 V Vds = 1.0 V Iout = 1.5 A Core: Type EE Fpwm = 24.0 kHz Pout = 55.0 W

The complete circuit of the buck converter is shown

in Fig. 4, while the pulse drive is shown in Fig. 8.

Fig. 4. Buck converter circuit.

The control strategy implemented for the buck

converter was of closed loop voltage control, with Proportional and Integral action (PI), where an analog input of the dsPIC monitors the level of the converter output voltage, compensating the PWM signal when necessary. The variable resistor RV1 provides the feedback signal to an analog input of the dsPIC.

V. FLYBACK CONVERTER

The output voltage of the buck converter or from the battery bank (36.6 V) feeds the flyback converter block, which has the function of generating the high voltage (about 2.5 kVrms) to establish the corona effect in the ozone reactor, using a common automotive coil. Table III presents the technical specifications for the flyback converter.

Table III. Specifications for the flyback converter

V in = 36.6 V Fpwm = 1.0 kHz Vout = 2.5 kVrms Iin_max = 1.5 A Vds = 1.0 V Iout_max = 20.0 mA Coil: Bosch F 000 ZS0 105

For this converter, a control strategy was adopted to

limit the input current, whose signal extracted from the shunt resistor is adjusted and sent to an analog input of the dsPIC, which is responsible for reducing the pulse width of the PWM signal of the flyback converter, if the input current exceeds 1.5 A. The switching frequency of the flyback converter is 1 kHz. The complete circuit of the converter is shown in Fig. 5. The power board is fully presented in Annex 2.

Fig. 5. Complete circuit of the flyback converter.

VI. OZONE REACTOR

The ozone reactor block is responsible for the cleavage of the oxygen molecule and the consequent generation of ozone in the electrical discharge environment (corona effect).

A suitable geometry for non-uniform fields, which is often used in the construction of high voltage devices, is the arrangement of coaxial cylinders. By correctly choosing the radial dimensions of the cylinders, it is possible to optimize such a system to obtain a maximum, rupture-free corona discharge (Kuffel et al, 2000). In this configuration the field distribution is symmetrical with respect to the central axis of the cylinder. The field lines are radial and the field E is a function only of the distance x from the center of the cylinder. Fig. 6 (Kuffel et al, 2000) shows the configuration used for the construction of the ozone reactor.

Fig. 6. Coaxial cylinders configuration.

The surface of the cylinders is uniformly charged with a charge per unit length Q/l. If a voltage V is applied to the electrodes, according to Gauss' law the field intensity E(x) is given by Equation (1).

( ) ( ) xrr

V

x

lQxE

1

/ln

1

2 12

==πε

(1)

Where: - E = electric field [N/C] - Q = electric charge [C] - l = length [m] - V = voltage [V] - r1 = radius of the inner cylinder [m] - r2 = radius of outer cylinder [m] - x = radial distance [m]


When the voltage in the smaller cylinder reaches rupture level, stable corona discharge or complete rupture can occur (Kuffel et al, 2000). The optimal configuration must be defined in terms of security, avoiding complete rupture, and not in terms of maximum voltage for discharge.

The designed reactor has the following dimensions: Diameter of outer cylinder: 5 cm; Diameter of inner cylinder: 4.5 cm; Length of the cylinders: 30 cm.

VII. AC/DC CONVERTER

The AC/DC (alternating current/direct current) converter block is an auxiliary source switched at high frequency and low power (11 W), proposed in (Chip-Rail, 2009). This converter has the function of supplying the voltage to the auxiliary circuits and the digital PWM control, to start and maintain the operation of the converters. The auxiliary AC/DC converter provides a voltage of 12 V and a maximum current of 900 mA. Its technical specifications are presented in Table IV. The complete circuit of the AC/DC converter is shown in Fig. 7 and annex 5.

Table IV. Specifications for the AC/DC converter

Fin = 50/60 Hz Iout_max = 900 mA V in_min = 90 Vrms Vout_min = 12 V +/- 5 % V in_max = 240 Vrms Fpwm = 60 kHz Pout = 11 W η > 87 %

Fig. 7. Complete circuit of the AC/DC converter.

VIII. AUXILIARY BLOCKS

The Drive blocks are intended to provide the voltage and current necessary to put the mosfets into conduction and, in the case of the buck converter, isolate PWM pulses. Fig. 8 shows the complete circuit of the drive for the buck converter.

Fig. 8. Drive circuit for the buck converter.

The "Level adjust" block has the function of reducing

the level of the battery voltage from 36 V (nominal) to 12 V, acting only in the absence of mains power, with the system operating in conjunction with a battery bank, or in the case of using only the photovoltaic system. In this condition, the DSC block receives the signal of lack of mains power and disables the PWM switching signal of the buck converter.

In case of installation with solar energy, the "Photovoltaic Panel" block is responsible for maintaining the batteries charge, keeping them under load or fluctuation, depending on the type of batteries, during the entire period of sunshine.

The "Charge Controller" block monitors and controls the charge level of the batteries, disconnecting the panels in case of full charge and disconnecting the batteries from the system in case of discharge to a certain level, thus avoiding a deep discharge and ensuring their useful life.

The "Battery Bank" block consists of three 12 V batteries connected in series, totaling 36 V (nominal). The current supply capacity is designed in accordance with the consumption of the system and autonomy required.

The ozone generator system has also a timer, with which the user performs the scheduling as needed, automatically starting and stopping the system without human intervention. This is very useful, for example, in the treatment and maintenance of swimming pools.

IX. DIGITAL CONTROL

The block "digital control, protections and signs (DSC)" is the intelligent block of the ozone generator and has the function of controlling and monitoring the entire system, generating the PWM pulses for the DC/DC converters, closing the loop with PI action for the buck converter, limiting the current to the flyback converter, monitoring mains voltage, monitoring and controlling temperature levels, timing the inrush current relay, performing the soft start of the system, and providing audible and visual signals of the current condition of the ozone generator.

A DSC was chosen because it combines the characteristics of microcontrollers and DSPs in a single chip, eliminating the use of DSP platforms for digital control, protections and signs, and thus considerably reducing the final cost of the equipment. The board


must be designed according to the needs of the equipment.

A routine provided by Microchip, called pid.s, was used for the correction of the PWM signal through PI action for the buck converter, as did in Treviso, 2011. Fig. 9 shows the block diagram of the implemented digital control board, with the respective signs, protections, and power feed.

Fig. 9. Block diagram of the digital control board.

As shown in the block diagram of Fig. 9, the DSC

used was Microchip’s dsPIC30F2010, which is a DSC of 28 pins that operates at 30 Mips. The DSC circuit is shown in Fig. 10. The control board is fully presented in Annex 3.

Fig. 10. DSC Circuit.

The block "local signs" is responsible for signaling

the conditions of mains voltage and blocking the PWM pulses for high temperature sounding the alarm at this condition, as shown in Fig. 10.

Temperature signal comes from a temperature sensor NTC10k mounted on the power board. After adjustment the temperature signal (EA_TEMP) is responsible for two actions – activate a cooler on the power board and block PWM pulses of the DC/DC converters turning off the equipment.

The block "DC/DC pulse converters interface" receives PWM signals sent by the DSC at 5 V (Pcon_1 and Pcon_2) and amplifies these pulses to 12 V (PWM_BUCK and PWM_FLYBACK) using operational amplifiers.

The output signal "SD_RELAY" is a signal with

amplitude of +5 V from a digital DSC output. Two seconds after the system starts, DSC releases this signal, which is responsible for limiting the inrush current via a 47Ω/5W resistor shown in Fig. 3.

The signal "+12 V" passes through the "regulator" block, which is responsible for generating +5 V power for the DSC and reference for comparisons, using a voltage regulator circuit 7805.

The signal "SD_ALARM", with amplitude of +5 V, is sent to the power board, where it activates a buzzer responsible for high temperature lockout indication.

The block "shunt interface" receives a signal of the order of mV (V_SHUNT) from the shunt and amplifies it to levels from 0 to 5 V for the analog input (EA_SHUNT) responsible for limiting the input current of the flyback converter, reducing the width of the PWM pulse from the flyback in case of overcurrent.

VCA1 and VCA2 signals correspond to a sample of the grid for monitoring via DSC. To that end, a circuit was designed, annex 4 (“rectification and adjust of mains voltage” block), which generates from 0 to 5 V (EA_MAINS), when proportionately supplied with a voltage from 0 to 300 VAC.

AC low and AC high signs are adjusted at 90 Vac and 240 Vac, respectively.

The output signal "SD_COOLER" comes from a digital output of the DSC and has the function of activating a cooler via bipolar transistor, in the occurrence of high temperature.

The input signal "EA_OUT_BUCK" is a sampling signal of the output voltage of the buck converter, used to close the voltage loop and consequently to automatically control the width of the PWM pulses of the buck converter, as shown in Fig. 4.

Finally there is the input signal "EA_BATTERY" that monitors the voltage level of the batteries, and a connector for in-circuit recording of the DSC. Annex 1 shows the flowchart of the implemented program, where you can observe the sequence of operation of the program with its checks and control actions.

X. EXPERIMENTAL RESULTS

This section will present the results obtained experimentally for the buck converter, flyback converter, digital control by DSC, and ozone concentration at the outlet of the reactor, measured by iodometric titration.

A. Buck Converter

The developed buck converter presented 89% yield under the conditions shown in Table V.

Table V. Buck converter yield

Inlet Outlet Yield V in = 175.3 V Vout = 36.65 V η = 0.89 Iin = 336 mA Iout = 1.425 A Pin = 58.90 W Pout = 52.23 W


Fig. 11 presents the behavior of the PWM signal for the buck converter without load and supplied with 220 Vrms. The output signal of the buck converter is shown in Fig. 12.

Fig. 11. PWM signal for the buck converter without load, supplied with 220 Vrms.

Fig. 12. Output signal from the buck converter without load, supplied with 220 Vrms.

The signal obtained at the output of the buck

converter was stable and had low noise level. When subjected to load steps of 50% and 100% it retained its characteristics. Figs 13 and 14 present the buck converter output at startup, supplied with 220 Vrms, respectively without and with PI action. Comparing these figures it is clear the efficacy of the PI control implemented for the buck converter, obtaining rapid stabilization of the output signal.

Fig. 13. Buck converter output at startup without PI action, supplied with 220 Vrms.

Fig. 14. Buck converter output at startup with PI action, supplied with 220 Vrms.

Figs 15 and 16 show the application of 0-50% and 50-100% load steps, respectively, with load application and removal. According to Fig. 15, a variation of 2.4 V for a period of 140 ms was observed at the application of the 50% load, while at load removal the variation was of 2.4 V for a period of 220 ms. Fig. 16 shows that when load was changed from 50 to 100% a variation of 1.6 V was observed for a period of 90 ms, while a variation of 2.0 V was observed for a period of 140 ms when the load was decreased from 100% to 50%.

Fig. 15. Load step from 0% to 50% with load application and removal.


Fig. 16. Load step from 50% to 100% with load application and removal.

B. Flyback Converter

Fig. 17 shows the relationship between input and output voltages for the flyback converter. The increase of output voltage showed a nearly linear behavior, being very close to the results obtained in (Alonso et al, 2005).

Fig. 17. Vout x Vin plot for the flyback converter.

The relationship between input power and output

voltage for the flyback converter is presented in Fig. 18. It can be seen that the voltage increase followed a non-linear behavior, especially at the beginning of the curve.

Fig. 18. Vout x Pin plot for the flyback converter.

The corona discharge effect may be seen in the output signal, especially in the positive half cycle of Fig. 20. These microdischarges are the basis for ozone generation. Therefore, the ozonizer is also an EMI generator by nature. A filter stage must be added at the input of the converter to avoid conducted interference. Moreover, the ozone reactor must be grounded both for safety and for avoiding radiated interference (Alonso et al, 2005).

Fig. 19 shows the PWM signal of the flyback converter in normal operation, that is, being supplied with a voltage of 36.65 V and an input current of 704 mA, resulting in input power of 25.8 W.

Fig. 19. PWM signal of the flyback converter

Fig. 20 shows a sample of the output signal of the flyback converter, which evolves according to the graph presented in Fig. 17 when the voltage is applied to the ozone reactor. Vrms is approximately 2.5 kV. The growth of the output signal of the flyback converter from the moment the equipment is energized under the influence of the soft starter system implemented in the DSC can be observed in Fig. 21.

Fig. 20. Sample of the output signal of the flyback converter


Fig. 21. Output voltage growth with soft-start action

C. Ozone Production

A photograph of the prototype implemented in the tests is shown in Fig. 22. Fig. 23 presents the ozone production results. During the ozone production tests, the generator presented an output effective voltage of approximately 2.5 kV while the flyback converter was supplied with a voltage of 36.65 V and consumed a current of 704 mA, giving an input power of 25.8 W. It can be observed in Fig. 23 that the ozone generator achieved the best production/flow rate relation when fed with 9 L/min of air.

Fig. 22. Performance tests of the ozone generator.

Fig. 23. Ozone production at the generator outlet.

A system with six reactors was mounted on the prototype level, using six flyback converters with automotive coils, each one feeding a reactor. For these tests, flyback converters were fed with 36.6 V by means of a more powerful bench power supply instead of the buck converter. A strong odor of ozone was detected at the output, producing approximately 7.5 gO3/h, fed with ambient air.

XI. CONCLUSION

The use of high frequency power allowed for an increase in power density applied to the reactor and an increase in ozone production, while at the same time the voltage required for ozone production was decreased.

According to the results, the digital control showed good performance in the control steps, as well as in those of protections and signs, giving greater reliability to the use of the equipment due to the reduction of components.

The digital control strategy adopted met the power needs of the designed converters and also provided a very small response time in the PWM signal correction, due to real-time processing performed by DSC.

At the end of the ozone production tests, it was found that the best performance of the reactor was achieved with ambient air feed flow rate of 9 L/min, which yielded about 20.8 mgO3/min (1.25 gO3/h). For this production the ozone generator consumed 25.8 W. Alonso et al (2005) reached with their proposed topology, a maximum ozone production of 8 gO3/h, with a 50 W prototype fed with pure oxygen. According to the literature (Rice et al, 1986), ozone production can be increased if the ozone generator is fed with pure oxygen. Thus, it is expected that the production of ozone to this generator, approximates the results obtained in (Alonso et al, 2005).

The potential difference generated by the flyback converter in the reactors was approximately 2.5 kV (effective).

The modular system made possible to obtain higher ozone concentrations. According to the results, the proposed topology is an excellent choice for corona discharge supply.

REFERENCES ALONSO, J. M.; VALDES, M.; CALLEJA, A. J.; RIBAS, J.; LOSADA, J. (2003). High Frequency Testing and Modeling of Silent Discharge Ozone Generators. Ozone: Science & Engineering Journal, Vol. 25, No. 5, pp. 363-376. ALONSO, J. M.; CARDESIN, J.; COROMINAS, E. L.; RICO-SECADES, M.; GARCIA, J. (2004). Low-Power High-Voltage High-Frequency Power Supply for Ozone Generation. IEEE Transactions on Industry Applications, vol. 40, No. 2, pp. 414-421, March/April. ALONSO, J. M.; GARCIA, J.; CALLEJA, A. J.; RIBAS, J.; CARDESIN, J. (2005). Analysis, design, and experimentation of a high-voltage power supply for ozone generation based on current-fed parallel-resonant push-pull inverter. IEEE Transactions on Industry Applications, vol. 41, No. 5, pp. 1364-1372, Sept/Oct.


BONALDO, J. P.; POMILIO, J. A. (2010). Control strategies for high frequency voltage source converter for ozone generation. 2010 IEEE International Symposium on Industrial Electronics, pp. 754-760, July. CHIP-RAIL, 2009. Datasheet CR6238T. Available in<http://www.szmsx.com/uploadfiles/filetr/2009121922594429656.pdf> (Access in 27/09/11). KUFFEL, E.; ZAENGL, W. S.; KUFFEL, J.; High Voltage Engineering: Fundamentals. 2nd ed. Newnes 2000. MOHAN, N.; UNDELAND, T. M.; ROBBINS, W. P.; Power Electronics. 3rd ed. John Wiley & Sons, Inc, 2003. OZONE SOLUTIONS. Available in < http://www.ozonesolutions.com/journal/category/ozone-generators/how-is-ozone-made-ozone-generators/> (Access in 19/05/12). RICE, R. G.; BOLLYKY, L. J.; LACY, W. J.; Analytical aspects of ozone treatment of water and wastewater. Chelsea: Lewis, 1986. SCHIAVON, G. J.; TREVISO, C. H. G. (2011). Complete Design for a 1.2 kVA UPS with Sinusoidal Output Stabilized, Operating with Digital Control for DSC. COBEP-11, Natal, RN, Brazil, September.

ANNEX 1 - FLOWCHART OF THE PROGRAM. MAIN:

INTERRUPTION_A/D:

INTERRUPTION_TIMER 1:


ANNEX 2 - POWER BOARD.


ANNEX 3 - CONTROL BOARD.


ANNEX 4 - SENSOR AC.


ANNEX 5 - AC/DC CONVERTER.

Design and Analysis of an Ozone Generator System Operating ...

Documents