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Article

The Influence of Electrostatic Spraying with Waist-Shaped Charging Devices on the Distribution of Long-Range Air-Assisted Spray in Greenhouses

by
Jinlong Lin
1,2,
Jinping Cai
1,2,
Jingyi Ouyang
2,
Liping Xiao
1,2,* and
Baijing Qiu
3,4
1
Key Laboratory of Modern Agricultural Equipment of Jiangxi, Jiangxi Agricultural University, Nanchang 330045, China
2
School of Engineering, Jiangxi Agricultural University, Nanchang 330045, China
3
Key Laboratory of Plant Protection Engineering, Ministry of Agriculture and Rural Affairs, Jiangsu University, Zhenjiang 212013, China
4
Key Laboratory of Modern Agricultural Equipment and Technology, Ministry of Education, Jiangsu University, Zhenjiang 212013, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(10), 2278; https://doi.org/10.3390/agronomy14102278
Submission received: 30 August 2024 / Revised: 24 September 2024 / Accepted: 30 September 2024 / Published: 3 October 2024
(This article belongs to the Special Issue Unmanned Farms in Smart Agriculture)
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Abstract

:
Electrostatic spraying is considered an effective means to improve the efficacy of pesticide application and reduce pesticide consumption. However, the effectiveness of electrostatic spraying needs further validation in greenhouse environments, especially in long-range air-assisted spraying scenarios. A waist-shaped charging device has been improved to obtain a maximum charge-to-mass ratio of 4.4 mC/kg at an applied voltage of 6 kV in a laboratory setting, representing an increase of approximately 84.9% compared to a commercial circular charging electrode with a fan-shaped nozzle. A comparative air-assisted spray test between electrostatic deactivation (EDAS) and electrostatic activation (EAAS) was conducted on greenhouse tomato crops using a single hanging track autonomous sprayer equipped with a pair of waist-shaped charging devices. The results showed that EAAS yielded an overall average coverage of 28.4%, representing a significant 10.9% improvement over the 25.6% coverage achieved with EDAS. The overall coefficient of variation (CV) for EDAS and EAAS was 62.0% and 48.0%, respectively. Within these, the CV for the average coverage of the sample set reflecting axial distribution uniformity was 33.4% and 31.4%, respectively. Conversely, the CV for the average coverage of the sample group reflecting radial distribution uniformity was 33.7% and 17.9%, respectively. The results indicate that the waist-shaped charging device possesses remarkable charging capabilities, presenting favorable application prospects for long-range air-assisted spraying in greenhouses. The electrostatic application has a positive effect on enhancing the average coverage and improving the overall distribution uniformity. Notably, it significantly improves the radial distribution uniformity of the air-assisted spray at long range, albeit with limited improvement in the axial distribution uniformity.

1. Introduction

Electrostatic spraying technology significantly improves atomization quality by charging the spray liquid or droplets through a specialized charging device, effectively modulating their surface tension. Concurrently, it leverages electrostatic field forces to optimize the spatial distribution of droplets, improving the uniformity of droplet deposition, and under the attraction of the electrostatic field, it enhances the adhesion of droplets to both the frontal and dorsal surfaces of crop leaves [1].
In recent years, numerous studies have shown that electrostatic spraying effectively increases the deposition of droplets on targets in various scenarios. X. He et al. assessed the application of combining electrostatic spraying with infrared target detection technology in orchards [2]. The results showed that electrostatic spraying significantly increased the deposition of droplets and greatly improved the efficiency of pesticide utilization. Gitirana Neto et al. conducted an evaluation of electrostatic spraying for coffee crops, revealing that it substantially enhanced droplet deposition in the lower canopy of the crops while reducing deposition on the soil to some extent [3]. An evaluation of the application effect of electrostatic spray on greenhouse pepper crops has revealed that, in comparison to traditional handheld sprayers, the electrostatic handheld sprayer enhances the canopy deposition by 1.48 times and the deposition under the leaf by 1.78 times [4]. In applications on guava, electrostatic spraying achieved double the deposition compared to traditional spraying and significantly suppressed ground deposition [5]. Similar improvements were also confirmed in evaluations of electrostatic spraying for cotton crops [6]. In the evaluation of the “tendone” vineyard, it was found that electrostatic spraying significantly increased the deposition of the lower leaf area, but had no noticeable effect on the upper leaf area [7]. This may be related to the structure of grape crops and the operation method of the sprayer. However, another trial on multi-row vineyards indicated that, in addition to the deposition of droplets, activating electrostatic spraying also effectively increased droplet coverage and achieved substantial improvements in coverage uniformity [8]. The effectiveness of electrostatic spraying in improving application results was confirmed through the evaluation of insects killed in cotton crops [9]. The same potential for improved pest control efficacy was also demonstrated in strawberry fields [10]. Furthermore, Wei et al. discovered the potential of electrostatic spray to suppress droplet drift during wind tunnel experiments with conical electrostatic nozzles [11]. However, the effectiveness of electrostatic spraying in applications does not always seem to be so significant. Hoffmann et al. did not find significant effects of electrostatic spraying on the penetration and deposition levels in vegetation [12]. Similarly, Salcedo conducted experiments on spraying applications in orchards [13], which also indicated that there was no significant effect of electrostatic activation on deposition coverage rates across different configurations of multi-nozzle sprayers, although similar technology yielded positive effects in applications for multi-row vineyards [8].
The charging capability of electrostatic spray nozzles is a significant factor influencing their application performance. Inductive charging, known for its high cost-effectiveness and excellent safety [14], has been thoroughly investigated by researchers in terms of its charging performance in electrostatic nozzle [15,16]. A multitude of studies have consistently demonstrated that, within a defined range, an increase in voltage under either an annular or conical inductive electrode configuration can significantly enhance the charge-to-mass ratio (CMR) of the droplet group [17,18,19,20,21]. Interestingly, upon reaching a certain voltage threshold, the CMR of the droplet group exhibits a trend in initial stabilization followed by a gradual decline. Notably, the varying voltage thresholds identified in these studies to achieve peak CMRs, ranging from 2.5 kV to 12 kV, can be attributed to the disparities in the materials utilized, structural dimensions, and nozzle flow rates of the inductive electrodes employed in the respective experiments.
The atomization process of droplets inherently exhibits variability stemming from the diverse nozzle structures employed, and inductive charging is notably susceptible to these fluctuations in the atomization dynamics. In response, a multitude of researchers have designed and developed a range of inductive nozzle charging devices that are specifically tailored to meet the distinct requirements of various spraying scenarios [6,17,18,22]. These innovative devices have consistently demonstrated comparable or even enhanced deposition performance in comparison to conventional nozzles, showcasing their adaptability and effectiveness.
Electrode materials and structures are recognized as pivotal factors influencing charging capacity [23]. Patel et al. have argued that copper, nickel, and nickel-plated copper exhibit superior inductive effects compared to other comparative materials. Additionally, nickel, carbon, and other materials have been employed as electrostatic induction electrode materials [24,25]. Circular electrode represents a common electrode structure, widely adopted in mature commercial products [17,24,25,26]. However, research focusing on the development of electrostatic induction nozzles for fan-shaped sprays is relatively scarce. Jia et al. developed a parallel clamping electrostatic induction device tailored for the ST110-01 fan-shaped nozzle (Lechler, Metzingen, Germany) with a spray angle of 110° and evaluated its electrostatic induction performance within an air curtain spray boom setup [27]. While the parallel clamping induction electrode boasts good processability, the reduction in the induction electrode area somewhat compromises charging efficiency.
The gap between the liquid flow or film and the electrode in electrostatic charging systems has also garnered attention. Wang et al. [20] have verified that the CMR of droplet swarms increases as the diameter of the circular induction electrode decreases. Nevertheless, excessively narrow spacing can lead to fine droplets adhering to the electrode, causing electrode discharge and, in severe cases, electrostatic breakdown, significantly impairing or even eliminating electrostatic induction capabilities. Furthermore, the charging capability is closely related to parameters such as flow rate and electrical conductivity of the liquid. Numerous studies have confirmed that the CMR of the droplet group initially increases and then decreases or stabilizes as the liquid flow rate increases [20,28]. However, there are also research results indicating that the CMR first decreases rapidly and then stabilizes with the increase in liquid flow rate [29].
Although the application effect of handheld electrostatic spraying in greenhouses has been verified, the manual application of pesticides in a high-temperature and high-humidity greenhouse environment poses potential health risks [30,31], which is effectively overcome by the unmanned self-propelled sprayer [32]. Restricted by the high-density planting habits and complex ancillary facilities in greenhouses, the autonomous sprayer on the ground lacks predictable application prospects in China. Lin et al. reported an autonomous air-assisted sprayer based on a single hanging track, which adopts three-dimensional spraying motion to overcome the constraints of narrow-row spacing and ancillary facilities in greenhouses [33]. This sprayer demonstrated acceptable spraying quality in greenhouse cucumber crops, providing a promising solution for pesticide application in greenhouse vegetables in China.
Due to the scarcity of research on the application of electrostatic spraying in autonomous greenhouse sprayers, particularly the unclear effectiveness in long-range air-assisted spraying scenarios, long-range air-assisted sprayers, the development of suitable electrostatic charging devices, and the investigation of the effects of electrostatic spraying applied to long-range air-assisted sprayers are expected to further enhance the quality of spraying [2].The primary objectives of this study endeavor are (1) to evaluate the charging capability of a simplified waist-shape charging device designed for the FFS80-01 nozzle (Lechler, Metzingen, Germany) and study the effect of applied voltage on CMR; and (2) to evaluate the application performance of a waist-shaped charging device in a greenhouse autonomous air-assisted sprayer and study the impact of electrostatic activation on coverage and deposition distribution under long-range air-assisted spraying conditions.

2. Materials and Methods

2.1. CMR Test

2.1.1. Structure of Waist-Shape Charging Device

Circular and conical electrodes represent the most prevalent forms of electrostatic induction electrodes, primarily compatible with hollow or solid conical nozzles. They have evolved into mature commercial products and have been applied in numerous studies (Figure 1a) [16,21,26,34]. Prior research has indicated that the misaligned and stacked arrangement of dual flat-fan nozzles (Hypro 80-01, Agratech, Rossendale, UK) on an autonomous air-assisted sprayer based a single hanging track in greenhouses had achieved acceptable axial distribution uniformity within an effective range of 7 m. Circular induction electrodes, suitable for conical nozzles, effectively form capacitors with conical liquid films. However, for flat-fan nozzles, the arched space between the curved plate and the flat-fan liquid film hinders the enhancement of capacitance capacity.
In this study, building upon the foundation of circular induction electrodes, and drawing inspiration from the design concept of parallel clamp-type induction electrodes, aims to reduce the distance between the electrode and the liquid film by adopting a waist-shaped electrode design. Additionally, by retaining the connecting portion between the plates, the equivalent liquid film area is increased, ultimately resulting in a higher induced charge. The kidney-shaped induction electrode utilized in this study is crafted from a 1.5 mm thick copper strip [25], featuring an inner diameter D of 38 mm and a height B of 12 mm, consistent with circular induction electrodes (Figure 2).
Research indicates that the electric field between the liquid film and electrode is unstable [35], with a breakdown field strength of 10 kV/cm [36]. In this study, the inner width K of the induction electrode was determined to be 18 mm to ensure that the capacitor does not undergo breakdown at an applied voltage of 9 kV. The distance X from the bottom edge of the electrode to the nozzle was set at 5 mm to maintain a uniform and unbroken state of the liquid film within the waist-shaped electrode under 0.3 MPa [37]. The nozzle base was 3D-printed with resin material and installed on the spray head through a snap-fit connection. Both the circular and waist-shaped charging devices are equipped with flat-fan nozzles with a spray angle of 80°.

2.1.2. CMR Measurement

The CMR measurement experiments for both the circular and waist-shaped charging devices were conducted in the Key Laboratory of Plant Protection, Ministry of Agriculture and Rural Affairs, Jiangsu University, under controlled conditions of 10 ± 2 °C room temperature and 60 ± 3% relative humidity. The CMR measurement system consisted of a picoammeter, a Faraday cylinder, a personal computer, digital electronic scales, etc. (Figure 3), and the performance parameters are listed in Table 1. The spray system utilized the HYPRO 80-01 nozzle, which was equipped with a circular-shaped induction electrode and a waist-shaped induction electrode. The spray liquid used in the test was clean tap water with a conductivity of 0.202 mS/cm, a density of 0.998 g/mL, and a surface tension of 0.074 N/m. The spray pressure is 0.3 MPa, and the applied voltages are 4, 5, 6, 7, and 8 kV. A timer-controlled circuit was employed to regulate each spray session to last for 60 s. During this period, the charged current of the droplet cloud was measured, and the mass of the sprayed liquid was weighed. Each voltage condition was repeated three times.

2.2. Field Test

2.2.1. Autonomous Air-Assisted Sprayer

Concurrently conducted with the spray angle correction field test [38], the electrostatic spraying field test utilized an upgraded automatic air-assisted sprayer based on a single hanging track. This sprayer has been significantly improved from previous studies [33], featuring the substitution of lead-acid batteries with an AC 220V power supply and the incorporation of an additional pair of downward-facing nozzles to address the deposition blind area beneath the sprayer. The spray boom of this sprayer is designed with a stacked configuration, each side equipped with a pair of electrostatic charging devices, operating at an electrostatic applied voltage of 6 kV. And the electrostatic charge device can be conveniently set to either electrostatic deactivated or activated modes. The main operating parameters of the sprayer are shown in Table 2.

2.2.2. Test Greenhouse and Crop

The field test was conducted at the Breeding Experimental Base of the Chinese Academy of Agricultural Sciences in Shouguang City, Shandong Province. The greenhouse measures 120 m in length, and 14 m in width, with a maximum height of 5.8 m and an effective planting width of 13.2 m. Along the length direction of the greenhouse, the main steel wire with a height of 2.3 m and an interval of 2.5 m is erected through pillars. The auxiliary steel wire is supported by the main steel wire and erected above each row of crops. Fine ropes are tied to the auxiliary steel wire for the crops to climb and grow. The track was installed at a height of 3.2 m, and its projection on the ground was 7 m away from the northern boundary of the planting area. The greenhouse crop was tomato, and no defoliation operation was performed 50–54 days after transplantation. The technical parameters of tomato planting are shown in Table 3.

2.2.3. Test Zone and Sample Layout

The northern area adjacent to the track was designated as the test zone, with the 9th to 10th and 16th to 17th rows of crops selected for data collection. Within this zone, using identical operational parameters, electrostatic deactivated air-assisted spraying (EDAS) and electrostatic activated air-assisted spraying (EAAS) were performed separately, with each type of spraying repeated three times.
Water-sensitive paper (WSP) was employed to evaluate spray deposition coverage. The sampling locations were strategically positioned at the top, middle, and bottom of the crop canopy on both sides of the wide rows, as well as in the middle of the narrow-row canopy on the left. These locations were labeled as LT (Left Top), LM (Left Middle), LB (Left Bottom), RT (Right Top), RM (Right Middle), RB (Right Bottom), and IM (Inner Middle), totaling 7 sample groups. The heights of the locations (HT, HM, and HB) were set at 1.9 m, 1.1 m, and 0.3 m, respectively (Figure 4c).
The sample set was defined by its equal distance from the track’s projection on the ground. The first set was positioned 0.7 m from the track’s projection, and subsequent sets were arranged every 2 crop plants (0.9 m) northward, resulting in a total of 8 sets (Figure 4d). Near the designated points, WSP was securely attached to the leaves using paperclips and placed close to the petioles. This setup yielded a total of 56 samples per repetition. Plastic sheets were laid beneath the WSP to prevent contamination from leaf transpiration and water vapor.

2.2.4. Data Processing

The WSP samples were processed in accordance with the treatment method outlined in a previous study [33]. The deposition performance was evaluated based on the average coverage rate and coefficient of variation (CV) of coverage. The average coverage rate of the overall sample (CaO) and its coefficient of variation (CVO), the coefficient of variation of average coverage rate for sample sets (CVaS), reflecting axial distribution uniformity, and the coefficient of variation of average coverage rate for sample groups (CVaG), reflecting radial distribution uniformity, were calculated. Furthermore, the differences between electrostatic deactivation and activation conditions were compared.

3. Results and Discussion

3.1. Effect of Voltage on Charging Capacity

The results of the CMR measurements for the waist-shaped and circular charging devices under various voltages are illustrated in Figure 5. Both inductive charging devices exhibit a similar trend in their CMRs, which begin to increase approximately linearly as the applied voltage rises from 4 kV. When the applied voltage reaches 6 kV, both charging devices achieve their peak CMRs. Specifically, the waist-shaped charging device reaches a maximum CMR of 4.4 mC/kg at 6 kV, marking an 84.9% increase compared to the 2.4 mC/kg of the circular one. Subsequently, as the applied voltage continues to increase, the CMR of the waist-shaped charging device experiences a slight decline, while the decline in the circular charging devices is more gradual. The variation trend in the CMR with respect to the applied voltage aligns consistently with previous research findings. Notably, the observed peak CMR value occurs at an applied voltage of 6 kV, which falls well within the range reported in previous studies. It is noteworthy that under other applied voltages, the CMR of the former is also 73.8–94.7% higher than that of the latter, indicating that the simple waist-shaped charging device possesses excellent charging capabilities for sprayed liquids.
Additionally, the electrostatic field effect enlarges the liquid film spray angle of the planar fan-shaped nozzle to a certain extent. After relatively prolonged spraying, some fine droplets are adsorbed onto the inductive electrode under the electrostatic force, and the resulting humid environment can diminish the electrode’s charging capacity [39]. In extreme cases, the accumulation of excessive droplets may lead to electrical breakdown between the electrode and the nozzle liquid film plate, ultimately resulting in the loss of charging capability. Without the optimization to prevent the wetting of fine droplets, the waist-shaped charging device can only maintain stable operation for a limited period of 10 min. Therefore, to ensure the normal operation of the inductive electrode, it is imperative to adopt appropriate measures to prevent droplet adsorption onto the electrode.

3.2. Differences in the Overall Performance

The test results reveal that, in comparison to electrostatic deactivation, electrostatic activation has effectively increased the overall average coverage. Table 4 presents data indicating that the CaO and CVO for EDAS are 27.7% and 62.6%, respectively, while those for EAAS are 28.1% and 48.3%, respectively. Since the nozzles beneath the sprayer, covering sample set 1, were not equipped with a charging system, the data from sample set 1 were removed for correction. After data correction, the CaO for EAAS and EDAS were recorded as 28.4% and 25.6% respectively, marking a 10.9% increase following electrostatic activation. This signifies that electrostatic application also plays a significant role in enhancing deposition coverage within the context of long-range air-assisted spraying in greenhouses. This finding echoes the similar conclusion drawn by Sánchez-Hermosilla in their study on handheld electrostatic spraying of peppers in greenhouses [4]. However, a slight variation exists in that the latter reported a maximum increase of 1.47 times in deposition, whereas, in this study, the electrostatic activation in long-range air-assisted spraying yielded a more modest 10.9% improvement in coverage rate. Despite this, for greenhouse vegetable farming that necessitates frequent pesticide applications, it not only translates to savings of over 10% in pesticide usage but also signifies more efficient pest control and reduced soil pollution.
Electrostatic spraying has also demonstrated a positive effect in improving the uniformity of overall coverage. Although the effectiveness of electrostatic spraying in improving the overall distribution uniformity has been verified in multi-row vineyards [8], the application effect of long-range air-assisted spraying in greenhouses has not been conclusively proven. Table 5 shows that after electrostatic activation, the CVO of EAAS decreased significantly from 62.0% in EDAS to 48.0%. This result provides strong evidence that electrostatics improve distribution uniformity in long-range air-assisted spraying. Furthermore, this finding allows for a detailed analysis of the specific mechanisms by which electrostatic activation impacts distribution uniformity.

3.3. Effect of Electrostatic Spray on Axial Coverage and Distribution Uniformity

Electrostatic spraying enhances the average coverage of sample sets along the axial direction. Figure 6a shows the average coverage rate of different sample sets (CaS) under both EDAS and EAAS modes, after removing data from set 1. Whether it is EDAS or EAAS, the average coverage rate of the sample set along the airflow axis generally shows a gradual downward trend. In the EDAS, the average coverage of each sample set demonstrated a gradually decreasing trend from set 2, initially at a slower pace and then more rapidly, until reaching 12.5% for set 8. Upon electrostatic activation, it is evident that the average coverage of set 2 significantly decreased from 37.5% to 26.2%, while downstream sample sets, including set 3, set 4, set 6, and set 7, experienced notable increases in average coverage ranging from 18.4% to 29.0%. Additionally, set 5 and the farthest set 8 showed increments of 6.0% and 10.8%, respectively. This phenomenon can be attributed to the electrostatic field’s refinement of droplet sizes, enabling more fine droplets to be transported and deposited onto the downstream crop canopies under the influence of airflow. Consequently, this process reduces upstream coverage while enhancing downstream coverage.
Regarding axial distribution uniformity, the CV for the average coverage rates across various sample sets (CVaS) under EDAS and EAAS conditions were 33.4% and 31.4%, respectively, indicating that although electrostatic assistance increased the downstream average coverage rate to some extent, its impact on the overall axial distribution was not significant. This observation can be further corroborated from another perspective. Figure 6b illustrates the distribution of average coverage rates among different sample groups, with error bars indicating the intra-group CV of coverage rates (CViG), which describes the uniformity of the sample group at the same position within the crop canopy. Analysis of the differences in CViG between the two spray modes revealed a p-value of 0.351, suggesting that in the context of long-range air-assisted spraying in greenhouses, while electrostatic assistance enhanced the axial average coverage rate to a certain degree, it did not show a significant positive effect on the uniformity of the axial distribution.

3.4. Effect of Electrostatic Spray on Radial Coverage and Distribution Uniformity

Electrostatic activation equalizes the average coverage of radial sample groups. Figure 6b shows the average coverage rates of various sample groups under both EDAS and EAAS conditions, illustrating the radial distribution pattern of the pneumatic long-range air-assisted sprayer. Before electrostatic activation, the average coverage rates of the three sample groups within the right-side canopy exhibit a pronounced step-like variation, with a sharp decline from 32.7% in the RT group to 19.9% in the RB group. In contrast, the LM and LB groups exhibit an average coverage of approximately 24.0%, while the upper sample LT group registers the highest average coverage of 43.7%. Concurrently, the IM group displays the lowest average coverage rate of 13.2%, indicating that the penetration ability of the long-range air-assisted sprayer using two-dimensional contour-following spraying motion still needs improvement.
For the sample groups on the right side of the canopy, after electrostatic activation, the average coverage rates of the sample groups RM and RB significantly increased, essentially aligning with those of RT. For the sample groups on the left side of the canopy, while electrostatic activation slightly restrained the coverage of sample groups LM and LB, it significantly reduced the coverage of sample group LT from 41.8% to 31.5%, thereby achieving a more balanced deposition performance. Simultaneously, the average coverage rate of the sample group IM reached 25.5%, marking a 92.8% improvement compared to EDAS. While the influence of electrostatic activation on the intra-group coverage rate of all samples does not exhibit a statistically significant impact, for the long-range air-assisted spraying, electrostatic spraying has contributed to enhancing the CaO to a certain extent and has also improved the deposition coverage rate of specific sample groups, such as the lower and inner parts of the canopy.
Electrostatic activation significantly improves the uniformity of distribution among radial sample groups. By analyzing the coefficients of variation for the average coverage rates across various sample groups (CVaG) under EDAS and EAAS conditions, we found them to be 33.7% and 17.9%, respectively. This indicates that electrostatic activation has a significant effect in solving the radial coverage imbalance under non-electrostatic conditions.
Furthermore, the numbers on the error bars in Figure 6a represent the CV of intra-set coverage rates for the corresponding sample sets (CViS), reflecting the radial distribution within each set. With the exception of set 2, which exhibits inferior uniformity, the CViS for EAAS is consistently lower than that of non-electrostatic spraying across all other sample sets. Through an analysis of the differences in CViS between EDAS and EAAS conditions, it has been further confirmed that electrostatic activation exerts a significant influence on the radial distribution uniformity of spray applications, with a statistical significance level of p = 1.2%. This unequivocally underscores the profound influence of electrostatic activation in substantially enhancing the uniformity of radial deposition distribution in long-range air-assisted spray applications. This is mainly because the atomization process, with the intervention of static electricity, reduces the droplet size to a certain extent [17,19,40]. Meanwhile, droplets carrying the same charge are more evenly distributed in space under the repulsive force of the electric field, thereby significantly improving the radial deposition distribution of the sprayer.

3.5. Effect of Electrostatic Spray on Penetration

Electrostatic spraying is generally recognized as having good penetration in short-distance applications [41], yet its effectiveness in long-range applications, particularly within greenhouse environments, lacks sufficient evidence. Figure 6b demonstrates that the sample group IM achieves comparable uniformity under both experimental conditions, but the average coverage rate under the EAAS condition is 25.5%, almost double that of the EDAS condition, which stands at 13.2%. Positioned at the midpoint of the interior canopy of narrow-row crops, the sample group IM offers a vantage point to observe the penetration capability of spray applications to a certain extent. Through long-range air-assisted spraying experiments, this study detected a substantial increase in the average coverage rate of IM after electrostatic activation, signifying that electrostatic activation also enhances the penetration capability of droplets in long-range air-assisted spraying. This enhancement stems from the electrostatic effects during atomization and transportation, which result in smaller droplet sizes that are less prone to collision and aggregation. These smaller droplets exhibit better flow following the airstream, enabling them to diffuse deeper into the interior and bottom of the canopy, leading to more effective deposition [42]. Consequently, this improves droplet penetration and enhances deposition coverage within the inner canopy layers.

4. Conclusions

This study introduces the design of a simplified waist-shaped charging device specifically tailored for the autonomous long-range air-assisted sprayer based on a single hanging track in greenhouses. Furthermore, the impact of applied voltage on its charging capacity has been studied. Using this spray nozzle, spray application tests were conducted on tomato crops in greenhouses, focusing on the effects of electrostatic deactivation and electrostatic activation on coverage and deposition distribution. The primary conclusions are summarized as follows:
At an applied voltage of 6 kV, the simplified waist-shaped electrostatic induction device achieved the highest CMR, surpassing commercial circular electrostatic induction charging devices by 84.9%, demonstrating its exceptional charging capability. This underscores the feasibility of designing induction charging devices with a waist-shaped induction electrode. However, further enhancements in insulation capacity are necessary, along with measures to ensure the induction electrode remains dry during prolonged operation. This is crucial to prevent the adsorption of fine droplets onto the electrode, which could lead to electrode wetting, subsequent breakdown, and ultimately, the loss of charging capability.
Electrostatic activation has improved the average coverage rate of the overall samples to a certain extent and significantly improved the overall distribution uniformity. Specifically, electrostatic activation has suppressed the average coverage rate of sample set 2, which is positioned closest to the nozzle, while simultaneously enhancing the average coverage rate of downstream sample sets. This observation underscores the benefit of employing electrostatic spraying in enhancing deposition performance downstream in the context of long-range air-assisted spraying. Notably, the impact of electrostatic activation on the consistency or uniformity of average coverage rates across all sample sets, particularly in terms of axial distribution, is not significant.
In long-range air-assisted spraying, electrostatic activation significantly inhibits the abnormally high coverage of radial sample groups and enhances the abnormally low coverage, indicating that electrostatic spraying primarily improves the overall distribution performance by significantly enhancing the uniformity of radial deposition distribution. Furthermore, the application of electrostatic spraying in long-range air-assisted spraying has a positive impact on enhancing penetration.
There are certain limitations in assessing application performance solely through water-sensitive paper. It is essential to evaluate the quality of electrostatic-activated long-range air-assisted spraying in greenhouse scenarios by assessing the deposition amount. Additionally, investigating the deposition distribution performance under different CMRs in long-range air-assisted spraying scenarios will further reveal the mechanisms behind how electrostatic spraying influences deposition quality.

Author Contributions

Conceptualization, J.L. and J.C.; methodology, J.L. and L.X.; software, J.O.; validation, L.X. and J.C.; formal analysis, L.X. and B.Q.; investigation, J.L. and J.O.; resources, L.X. and B.Q.; data curation, J.L. and J.C.; writing—original draft preparation, J.L.; writing—review and editing, J.L., J.C. and J.O.; visualization, J.L. and L.X.; supervision, L.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC Grant No. 31971790) and the Project of Post Expert in the Industrial Technology System of Agricultural Machinery and Equipment Application in Jiangxi Province.

Data Availability Statement

Dataset available on request from the authors. The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Figure 1. Electrostatic charging device (a); commercial circular electrode (b); and waist-shaped electrode.
Figure 1. Electrostatic charging device (a); commercial circular electrode (b); and waist-shaped electrode.
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Figure 2. Schematic diagram of waist-shape charging device.
Figure 2. Schematic diagram of waist-shape charging device.
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Figure 3. CMR measurement setup.
Figure 3. CMR measurement setup.
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Figure 4. Sprayer, crop structure, spray path, and sample arrangement scheme. Crop structure and spray patterns: (a) rear view of sprayer; (b) spraying in crop rows; (c) spray path and arrangement of sample groups; and (d) arrangement of sample sets.
Figure 4. Sprayer, crop structure, spray path, and sample arrangement scheme. Crop structure and spray patterns: (a) rear view of sprayer; (b) spraying in crop rows; (c) spray path and arrangement of sample groups; and (d) arrangement of sample sets.
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Figure 5. The CMR of waist-shaped and circular induction electrodes under different voltages.
Figure 5. The CMR of waist-shaped and circular induction electrodes under different voltages.
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Figure 6. Average coverage and distribution within the sample group and set between EDAS and EAAS: (a) average coverage and distribution of sample sets; (b) average coverage and distribution of sample groups (number on error bar indicates CV of coverage rate within sample set or group).
Figure 6. Average coverage and distribution within the sample group and set between EDAS and EAAS: (a) average coverage and distribution of sample sets; (b) average coverage and distribution of sample groups (number on error bar indicates CV of coverage rate within sample set or group).
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Table 1. The main components of the CMR measurement system.
Table 1. The main components of the CMR measurement system.
ComponentsTypeParameters
Digital high-voltage electrostatic generatorTCM6000iN (Teslaman Ltd., Dalian, China)0–30 kV
PicoammeterKeithley 6485 (Tektronix China Ltd., Shanghai, China)10 fA–20 mA
Digital electronic scalesLQ-A10001 (Licheng China Ltd., Shanghai, China)0.1–1000 g
Table 2. The main operating parameters of the sprayer.
Table 2. The main operating parameters of the sprayer.
ParameterValueParameterValue
Horizontal traveling stroke SH (m) 1.4Average traveling velocity v T (m/s)0.30
Vertical lifting stroke SV (m) 1.5Average lift velocity v L (m/s)0.35
Pressure P (Mpa)0.3Flow rate (L/min)1.56
Table 3. Main parameters and experimental environment of tomato crops.
Table 3. Main parameters and experimental environment of tomato crops.
ItemsValueItemsValue
Wide line spacing (m)0.85Average number of leaves (Ps)187.4
Narrow line spacing (m)0.55Average leaf area (cm2)112.3
Plant spacing (m)0.45Leaf area index LAI7.47
Plants per row Np (Ps)30Relative humidity (%)66–75
Average plant height (m)2.16Operating temperature (°C)26–31.2
Table 4. The performance of EDAS and EAAS in overall coverage rate and uniformity *.
Table 4. The performance of EDAS and EAAS in overall coverage rate and uniformity *.
Spray PatternCaO (%)CVO (%)
Before correctionEDAS27.762.6
EAAS28.148.3
After correctionEDAS25.662.0
EAAS28.448.0
* No electrostatic spraying was performed in the set 1 area of the sample, and the set 1 data were removed for correction.
Table 5. CV of average coverage rate between sample set and sample group of EDAS and EAAS *.
Table 5. CV of average coverage rate between sample set and sample group of EDAS and EAAS *.
Spray PatternCVaS (%)CVaG (%)
EDAS33.433.7
EAAS31.417.9
* Electrostatic spraying was not implemented in the region of the sample set 1 and the set 1 data were deleted.
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Lin, J.; Cai, J.; Ouyang, J.; Xiao, L.; Qiu, B. The Influence of Electrostatic Spraying with Waist-Shaped Charging Devices on the Distribution of Long-Range Air-Assisted Spray in Greenhouses. Agronomy 2024, 14, 2278. https://doi.org/10.3390/agronomy14102278

AMA Style

Lin J, Cai J, Ouyang J, Xiao L, Qiu B. The Influence of Electrostatic Spraying with Waist-Shaped Charging Devices on the Distribution of Long-Range Air-Assisted Spray in Greenhouses. Agronomy. 2024; 14(10):2278. https://doi.org/10.3390/agronomy14102278

Chicago/Turabian Style

Lin, Jinlong, Jinping Cai, Jingyi Ouyang, Liping Xiao, and Baijing Qiu. 2024. "The Influence of Electrostatic Spraying with Waist-Shaped Charging Devices on the Distribution of Long-Range Air-Assisted Spray in Greenhouses" Agronomy 14, no. 10: 2278. https://doi.org/10.3390/agronomy14102278

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