Ozone Generation by Surface Dielectric Barrier Discharge

Abstract

Surface dielectric barrier discharge (SDBD) is used in a variety of different applications; however, the ozone generated in the discharge can be toxic to people in the vicinity. In this paper, we study the SDBD (using generators with smooth-edge, serrated and thin-wire high-voltage electrodes) in terms of ozone generation. The electrical measurements and the time-resolved plasma imaging revealed differences in the discharge current, dissipated power and plasma morphology for the different types of SDBD generators and showed significant suppression of the streamer formation from the thin-wire electrode. We determined the amount of ozone produced by each generator and found that despite the observed differences in discharge between the generators, the ozone production yield and the maximum volumetric concentration of ozone for all three generators is a linear function of only one parameter—the discharge active power. We also found that the ozone production efficiency of 9.66 g/kWh is constant for all three generators. Our results show that SDBD generators can be safely used in the enclosed space if the SDBD discharge operates with relatively low active power (the SDBD generator working with the active power of 1.7 W did not exceed the ozone concentration of 0.1 ppm in the 60 m³ room).

1. Introduction

As one of the strongest oxidants, ozone is widely used for sterilization, water treatment, food processing, and storage, in agriculture, textiles, and other branches of industry [1,2,3,4,5]. Of the many methods, the dielectric barrier discharge (DBD) has proven to be one of the most effective for ozone production and is commonly used at both commercial and industrial scales [6,7]. Although the vast majority of related research has studied ozone generation in the volumetric DBD (VDBD) geometry [8,9,10,11,12], there are also some attempts to use surface DBD (SDBD) discharge for ozone production. Kim et al. [13] compared the performance of small-scale VDBD and SDBD ozone generators in the planar configuration. They found that SDBD is capable of generating higher ozone density than a VDBD generator. A similar comparison was made by Nassour et al. [14], who studied cylindrical ozone generators. They noted that SDBD generators have the advantage of higher energy efficiency compared to VDBD generators. Muto and Hayashi [15] studied the sterilization process using a cylindrical SDBD plasma torch. They found that the nitrogen oxides generated in the discharge were the main species contributing to the sterilization process. Portugal et al. [16] and Choudhury et al. [17,18] studied the ozone flow generated by the fan-shaped SDBD reactor. They found that the fan-shaped electrode improves ozone distribution and the decontamination process compared to the traditional comb-shaped design. Pekarek et al. [19] studied the influence of the applied voltage and its frequency on ozone production with an SDBD reactor with the radial electrode configuration. They found that ozone production generally increases with the average discharge power. Xie et al. [20] studied ozone production with a planar multi-electrode SDBD reactor fed with pure oxygen. They found that they could significantly increase the ozone production yield by improving the uniformity of the feed gas in the reaction chamber. Abdelaziz et al. [21] studied the influence of the feed gas (oxygen) concentration and humidity on ozone production using a symmetric SDBD reactor. They found the optimal gas feed rate to maximize the ozone production yield. They also confirmed the negative influence of humidity on ozone production.

However, in recent years, SDBD has been finding more and more applications outside ozone generation, such as thrust generation [22,23,24,25,26,27], EHD (electrohydrodynamic) airflow generation [28,29,30] and flow modification [31,32,33], as well as air and water purification [34,35,36,37,38]. Yet, the toxicity of the ozone generated by the SDBD discharge may be an obstacle to the implementation of SDBD in some applications, especially if the discharge is to work in an enclosed room where people are present [39]. To the best of our knowledge, so far, only Hong et al. [40] have studied the process of ozone generation as a potential obstacle to controlling the detachment of the boundary layer from the surface of vehicles using SDBD actuators. They concluded that even though the amount of ozone generated in the SDBD discharge is lower than in the case of VDBD, in order to introduce this technology to the automotive industry, the amount of generated ozone would still have to be significantly reduced.

The aim of this work was to determine whether SDBD generators (in the form of flow actuators, ionizers and precipitators) can be safely used in close proximity to people (especially indoors). In this paper, we experimentally studied the performance of the three types of SDBD generators (varying in the geometry of the high-voltage electrode) in terms of the amount of ozone they produce. We extensively studied the electrical characteristics of the discharge by measuring the discharge current and the dissipated power. We used a time-resolved imaging technique to investigate the morphology of the discharge plasma. Finally, we measured the ozone production yield as a function of the applied voltage for all three generators and studied the ozone concentration in the closed room in which the SDBD generator was working. Our results show that SDBD generators can be successfully applied in enclosed spaces if the discharge is operating with relatively low power.

2. Experimental Setup and Methods

The construction of the SDBD generators used in this study is typical of those used in the literature for EHD generation [41,42,43,44]. It is composed of two electrodes placed asymmetrically on the opposite sides of a ceramic plate (1 mm thick) which acts as a dielectric barrier (Figure 1A). We studied three types of SDBD generators, which differ in the geometry of the high-voltage electrode (smooth-edge, serrated and thin-wire electrode). The smooth-edge and serrated high-voltage electrodes and the grounded electrode of the generators were made of 50 µm thick copper foil, while the thin-wire electrode was made of thin steel wire with a diameter of 50 µm. The active length of the high-voltage electrodes for all three generators was 75 mm.

The experimental setup used to measure the electrical properties of the SDBD discharge, the time-resolved imaging of the discharge plasma and the ozone production yield is shown in Figure 1B. To power the SDBD generators, a high-voltage sine wave (with a voltage (peak-to-peak) ranging from 8 kV to 21.6 kV and a frequency of 1 kHz) was generated using a function generator (Tektronix AFG3101, Tektronix Inc., Beaverton, OR, USA) and a high-voltage amplifier (Trek 40/15). The usable voltage was limited to this range because, at lower voltages, the discharge was not uniform along the entire length of the electrode, and at higher voltages, we observed distortion of the voltage function due to the limitations of the high-voltage amplifier. The voltage function was recorded on an oscilloscope with a Tektronix P6015A (Tektronix Inc., Beaverton, OR, USA) high-voltage probe, and the SDBD discharge current was determined from the voltage drop across an R = 100 Ω resistor mounted on a grounding wire. To measure the power dissipated in the SDBD discharge, we used a measuring capacitor method (sometimes also called the Lissajous figures method) [45,46,47]. In this method, a high-voltage capacitor (C = 0.8 nF) is placed between the grounded electrode and the electrical ground (interchangeably with the measuring resistor) to measure the transported charge. The Lissajous curves were recorded on the oscilloscope by plotting the transported charge on the vertical axis and the applied voltage on the horizontal axis. The power dissipated in the discharge in one voltage cycle was calculated as the area limited by the Lissajous figure multiplied by the frequency of the voltage wave function (1 kHz). The investigation of the discharge plasma morphology was performed using an Andor iStar DH734 (Andor Technology Ltd., Belfast, UK) fast-gated intensified ICCD camera equipped with a 10× microscope lens, which we used to capture time-resolved images of the discharge [44,48]. The observation area was a 6 mm × 6 mm square focused on the discharge edge of the electrode. The ICCD camera was triggered by the signal generated by the function generator, which allowed us to synchronize the camera with the voltage waveform and capture sets of images at various time delays with respect to the voltage waveform. A set of 200 images was recorded in each measurement.

To measure the ozone yield, the SDBD generator was placed inside a chamber (120 mm × 130 mm × 700 mm) that was open on one side and with an exhaust air system mounted on the opposite side. Clean air was first forced into the chamber (by the small fan mounted on the exhaust tube), then through the Delta Ohm flow sensor with an AP472-S2 vane probe and an ozone meter (Aeroqual 300—Aeroqual Inc., Auckland, New Zeland, with an electrochemical probe, model EOZ 0–10 ppm), and, finally, out of the room. The ozone yield was calculated based on the ozone concentration and the total airflow rate in the exhaust tube (60 mm in diameter) at the exit of the measurement chamber. We also measured the ozone concentration in the ambient air in the room (volume of 60 m³, which is approximately the size of a typical 2-person office room in Europe [49]) in which the SDBD generator was working, as a function of time. To do this, we activated the SDBD generator in the room (without the measurement chamber and exhaust system) and registered the ozone concentration every 30 s using an Aeroqual ozone meter with an OZL 0–0.5 ppm probe. The ozone meter was approximately 2 m from the SDBD generator, and to ensure a uniform ozone concentration, an office fan (not shown in Figure 1B) was activated which forced the airflow inside the room. All measurements were taken in the air at a temperature of 23 °C and a humidity of 27%.

3. Results and Discussion

We studied the electrical properties of the SDBD discharge by measuring the voltage and discharge current waveforms, as well as the power dissipated in the discharge. Next, we investigated the SDBD plasma morphology using a time-resolved imaging method. Finally, we measured the ozone production yield as a function of the applied voltage for all three generator types and the variation in the ozone concentration in the room in which the SDBD generator was working. The results of these studies are presented and discussed in this section.

3.1. Voltage and Discharge Current Measurements

Figure 2 shows the typical voltage and discharge current waveforms recorded over one voltage cycle for all three types of SDBD generators (with smooth-edge, serrated, and thin-wire electrodes). Each generator was powered with the same voltage waveform, and the power supply was not equipped with the impedance matching circuit [50,51]. In each case, the voltage was a sine wave with an 18.5 kV peak-to-peak voltage and a frequency of 1 kHz. In general, the current–voltage waveforms are typical of the SDBD actuators used for flow control [52,53,54,55]. For all three cases, the current waveforms consisted of two batches of pulses appearing in each cycle: a batch of positive pulses occurring at the positive-going voltage half cycle, and a batch of negative pulses at the negative-going half cycle. The batch of positive pulses is characterized by relatively high discharge currents reaching up to about 100 mA and is associated with the formation of positive streamers. On the other hand, the batch of negative pulses is associated with so-called micro-glow discharge and is characterized by less frequent current pulses and smaller currents [56,57,58]. The waveforms for the smooth-edge and serrated electrode generators are similar; they consist of dense batches of both positive and negative pulses, which indicates an intense discharge taking place from those electrodes. On the other hand, with the thin-wire electrode, the current pulses in both batches are scarcer and the batch duration itself is noticeably shorter. This suggests that the geometry of the thin-wire electrode inhibits the SDBD discharge.

3.2. Discharge Power Measurements

Typical Lissajous curves representing the relation between the transported charge and the instantaneous voltage in one full voltage cycle, for seven values of applied peak-to-peak voltage, are shown in Figure 3 (since the Lissajous curves for all three generators look similar, in Figure 3 we showed only the case for the smooth-edge generator). The area of a quasi-parallelogram limited by the Lissajous curve represents the energy dissipated in the SDBD discharge in each voltage cycle. As expected, this area increases with the applied voltage.

We numerically calculated the area of each Lissajous figure by multiplying it by the frequency of the voltage waveform to obtain the active discharge power as a function of the applied voltage, which increases with the voltage for all three types of generators, as can be seen in Figure 4. This is consistent with the research by Joussot et al. [59], who experimentally studied SDBD airflow actuators and observed that the active power of the discharge behaves like a power function of an input voltage. The data points in Figure 4 almost overlap for the smooth-edge and serrated generators (with a difference smaller than 5%), while the active power for the thin-wire generator is significantly lower (on average about 25% smaller than the other two). These results are consistent with the results of the current measurements discussed in Section 3.1, where we observed a reduction of the number of current pulses for the thin-wire electrode in comparison to the other two electrode types. In our studies, a maximal active power of 4.82 W was recorded for the smooth-edge electrode generator, 4.67 W for the serrated electrode generator and 4.08 W for the thin-wire-electrode generator for the applied voltage of 21.6 kV, which are typical power levels for SDBD actuators of this size. A similar active power of about 4 W was recently registered by Papadimas et al. [60] for their SDBD plasma actuator with a 70 mm smooth-edge electrode, with a thin dielectric barrier made of polyamide and an applied voltage of 14 kV (peak-to-peak).

3.3. SDBD Plasma Imaging

To observe the differences in the plasma morphology for three types of SDBD generators, we applied a time-resolved imagining technique. To this end, three characteristic 40 μs time windows (A, B and C) were selected from the full cycle of the sinusoidal waveform and a series of fast-gated images was taken in each of those time windows. Time windows A and B were recorded at different times on the rising slope of the voltage waveform, during the streamer formation (the time delay between time windows A and B was t_AB = 100 μs), and time window C (t_AC = 650 μs) was recorded on the falling slope, during the regime of micro-glow discharge. The time position of those windows with respect to the voltage waveform is shown in Figure 2 (rectangle A). Figure 5 shows a set of typical plasma images captured in each time window for all three generators and a fixed applied voltage of V_pp = 18.5 kV. In each image, the high-voltage electrode is located at the bottom, and the position of the smooth-edge and the serrated electrode is marked in the images with the dashed line (the position of the thin-wire electrode can be easily seen without marking it in the image). All the images are presented in the same pseudo-color scale representing the intensity of the plasma radiation.

Plasma generated from the edge of the smooth-edge electrode at the positive-going half cycle of the voltage (time windows A and B) is characterized by the dense formation of positive streamers emerging at random locations along the entire length of the electrode and extending up to about 4 mm from its edge. Similarly, during the negative-going half cycle (C), micro-glow discharges are generated at random locations at the edge of the electrode. Those micro-glow discharges are of a lower intensity than the streamers and extend only about 2 mm from the electrode’s edge. In the case of the serrated electrode, both the streamers and the micro-glow discharges develop primarily from the tips of the serrations, where the intensity of the electric field is the greatest. The length of the streamers emitted from the serrated electrode is similar to the ones generated from the smooth-edge electrode. However, while the micro-glow discharge emits more radiation at the tips of the serrated electrodes, the total plasma length is significantly shorter. In the case of the thin-wire electrode, streamers appear mainly during the first time window (A), while in time window B, only residual streamer radiation can be observed. Further, in the regime of micro-glow discharges (C), uniform radiation is emitted from around the wire, caused by the strong electric field forming around the wire due to its high curvature. The time-resolved images again confirmed that the thin-wire electrode inhibits the discharge formation. Debien et al. [61] suggested that this behavior can be linked to the absence of irregularities at the surface of the thin wire, which suppresses the onset of the plasma streamers despite the high electric field.

3.4. Ozone Production Yield

In a very simplified form, the formation of ozone in the DBD discharge in the air can be represented as a two-step process. In the first step, the free charges generated in the discharge collide with oxygen molecules, which results in their dissociation into their constituent oxygen atoms (e + O₂ → e + O + O). The next step is a three-body reaction of one of these oxygen atoms with the oxygen molecules and another air molecule (usually oxygen or nitrogen) which leads to the formation of the ozone molecule (O + O₂ + M → O₃ + M). For a more detailed description of the plasma chemistry that leads to the formation of ozone and other oxygen and nitrogen species, see [11,15,62,63]. Figure 6 shows the ozone production yield as a function of the applied voltage for all three generator types. One has to keep in mind that the results presented in this paper are for a fixed electrode length (75 mm); however, as was shown by Hong et al. [40], the ozone production yield is a linear function of the electrode length. As expected, the ozone yield increases with increasing applied voltage in all studied cases. This is understandable since an increase in the applied voltage increases the number and energy of free charges, which promotes oxygen dissociation and the following three-body collision process. For a given value of applied voltage, the production yield from the generator with a smooth-edge and serrated electrode is similar, reaching maximally 46 mg/h and 44 mg/h, respectively, for 21.6 kV and is significantly lower for the one with the thin-wire electrode reaching maximally 36.3 mg/h. It can be assumed that the reduction of the ozone yield for the thin-wire electrode generator is due to discharge inhibition, which was experimentally confirmed in this research and is discussed in Section 3.1, Section 3.2 and Section 3.3.

Using the results of the active power measurements discussed in Section 3.2, in Figure 7, the ozone production yield is shown as the function of discharge active power. As can be seen, the data points of all three series, corresponding to the three types of generators, line up along a single, straight line, and were fitted with a linear function (marked with a dashed line in the graph). This suggests that although the ozone production yield varies with the electrode geometry and applied voltage, the ozone production efficiency can be expressed as a function of a single dependent parameter—the active discharge power. From the slope of the approximation function in Figure 7, we calculated the ozone production efficiency of the generators to be 9.66 g/kWh. This value is about 2 times smaller than the 21.2 g/kWh obtained by Hong et al. [40] from their SDBD actuator with a serrated electrode and 3 mm electrode gap, which is to our knowledge the smallest value obtained for an SDBD reactor. In comparison, Abdelaziz et al. [64] developed an SDBD plasma reactor for ozone generation, achieving a much higher energy efficiency of 52 g/kWh. Using an SDBD in a multi-electrode configuration, Xie et al. [20] obtained an efficiency of 131 g/kWh by optimizing the airflow in the reaction chamber. The relatively low ozone production efficiency of the generators in our studies is an advantage for our purposes, as it potentially allows it to be used in the vicinity of people.

3.5. Ozone Concentration Indoors

Finally, we studied the temporal variation of the ozone concentration in the room (with a volume of 60 m³) in which the SDBD generator was working. Since, as was shown in the previous subsection, the ozone production for all three generator types can be characterized by one parameter—ozone production efficiency, which we tested on only one generator—with a smooth-edge electrode for various active powers (Figure 8), and the conclusions can be extended to the other generators. In Figure 8, the SDBD was turned on at time t = 0 and turned off after 2.5 h of operation. We registered the increase in the ozone concentration during the operation of the generator, and next its decrease after turning it off, as the ozone decomposed until its concentration in the room dropped to 0 ppm.

After the generator is switched on, the concentration of ozone in the room increases until an equilibrium state is reached in which the amount of ozone generated in the SDBD discharge is balanced by the amount of ozone being decomposed, mainly through recombination in collisions with air molecules and unimolecular decay [65,66,67]. When the SDBD is turned off, the ozone concentration starts to decrease, since the decomposition processes become dominant. In the studied case, the equilibrium state was reached about 1.5 h after the generator was turned on. In this timeframe, the ozone concentration can be expressed as a function of time using Equation (1):

$$C\left(t\right)=C_e\left(1-e^{-\frac{t}{\tau }}\right)$$

(1)

where C_e—ozone concentration is in the equilibrium state, τ—ozone production growth factor [40]. Similarly, the ozone concentration over time, after the generator was turned off, can be expressed using Equation (2):

$$C\left(t\right)=C_{e^{\prime }}{e}^{-\frac{t-t_{off}}{{\tau }^{\prime }}}$$

(2)

where C_e′—initial ozone concentration, τ′—ozone decomposition growth factor, t_off—time at which the SDBD was turned off (in the studied case t_off = 2.5 h) [40,68]. The approximation function is shown with the solid lines in Figure 8, and as can be seen, the data points are reasonably well approximated with the functions described by Equations (1) and (2). The approximation parameters and the ozone half-life time calculated for the decomposition phase (T_1/2) can be found in

Table 1. Parameters for the approximation functions (1) and (2).

Active Power [W] (Applied Voltage [kV])	During SDBD Operation (Equation (1))			After SDBD Is Turned Off (Equation (2))
	Ce (ppm)	τ (h)	χ2 *	Ce′ [ppm]	τ′ (h)	χ2 *	T1/2 (min)
0.94 (12.8)	0.059	0.572	1.97 × 10−5	0.063	0.237	0.89 × 10−5	10.4
1.35 (14.4)	0.081	0.452	0.38 × 10−5	0.076	0.325	0.54 × 10−5	11
1.81 (16)	0.111	0.571	1.57 × 10−5	0.104	0.304	0.72 × 10−5	11.4
2.51 (17.6)	0.153	0.386	1.16 × 10−5	0.149	0.374	0.7 × 10−5	14.1
3.51 (19.2)	0.215	0.439	2.71 × 10−5	0.222	0.357	1.55 × 10−5	14.8
4.52 (20.8)	0.289	0.437	2.9 × 10−5	0.288	0.393	0.58 × 10−5	15.8

* Reduced χ².

.

The average ozone production and decomposition growth factors are τ = 0.476 ± 0.08 h and τ′ = 0.332 ± 0.07 h, respectively, and the ozone half-life time varies from 10.4 min to 15.8 min. A similar ozone half-life time of 16 min was recently reported by Davoli et al. [69], which studied the ozone decomposition generated by a household dryer in a closed room. Baba et al. [70] reported ozone half-life time ranging from approximately 30 min to 60 min, and Weschler [71] from 6 min to 40 min, depending on the indoor environment. Using the approximation parameters (Table 1), the maximal ozone concentration in the room in the equilibrium state (C_e) can be expressed as a function of the active power (Figure 9). As expected, the maximal ozone concentration in the room is the linear function of the active discharge power, which can be extended for all three types of generators.

In closing, this study can be used as a guide in selecting the maximum power of the SDBD discharge that will allow safe work in an enclosed space (taking into account its volume). Various standards define the safe concentration of ozone differently. Usually, this value ranges from 0.05 ppm to 0.1 ppm [72]. It can be seen from Figure 9 that in order not to exceed the upper limit of the safe ozone concentration in the room, the SDBD generator must operate with an active power not exceeding a value of approximately 1.7 W. Although, as was shown above, the ozone production yield depends mainly on the active power of the SDBD discharge, the correct choice of the electrode geometry can help to reduce the ozone concentration in a specific application. For example, if the SDBD generator is to act as an EHD flow actuator, the optimal choice is to use the serrated electrode, which generates much higher EHD flows than the smooth-edge and thin-wire electrodes for the same active power [40,73]. On the other hand, if the SDBD generator is to be used as an ionizer, the use of a thin-wire electrode should be considered, since its construction inhibits the formation of the plasma streamers and allows higher values of applied voltage.

4. Conclusions

In this paper, we studied the SDBD discharge generated from smooth-edge, serrated and thin-wire high-voltage electrodes. Electrical measurements revealed that the discharge currents and the active power dissipated in the discharge were similar for the smooth-edge and serrated electrodes, and significantly lower for the thin-wire electrode, due to the absence of irregularities on the wire surface that could promote streamer formation. This was then confirmed using the time-resolved plasma imaging technique. We found that the ozone production yield increased with the applied voltage for all three generator types, but the production yield of the generator with the thin-wire electrode was significantly lower than that of the other two. We linked this difference to the previously observed suppression of the discharge plasma observed for this generator. Despite the differences in discharge, the ozone production yield was a linear function of the discharge active power, and from this, we calculated the efficiency of the ozone production to be 9.66 g/kWh for all three generators. Finally, we measured the temporal variation of the ozone concentration in an enclosed room (volume of 60 m³) in which the SDBD generator was working. We found that the maximum ozone concentration in the room increases linearly with the active discharge power. Our studies confirmed the SDBD discharge can be successfully used in a closed room when working with reduced dissipated power, without exceeding the safe level of ozone concentration. For the studied generators working in the 60 m³ room, the limit power was approximately 1.7 W, at which the ozone concentration in the room was below 0.1 ppm.