Abstract
As one of the main projects of facility agriculture promotion, the PV (photovoltaic) greenhouse has the problems of PV power generation competing for light with crop production, strong indoor chiaroscuro, and uneven light distribution. The internal light uniformity is tested by a zigzag greenhouse model to compare the light transmission effects of different light-transmitting materials applied to PV greenhouses. Altogether, 20 line/inch 3 mm and 30 line/inch 3 mm, 40 line/inch 2 mm, 25 line/inch 4 mm grating plates and 2 mm and 3 mm thick ordinary glass were used as light-transmitting components, and the light intensity and light uniformity in the greenhouse were the measurement indicators. The results show that the use of grating plates as covering material can improve the light intensity at the intersection of light and dark, but the overall light transmittance is not as good as glass because it is plastic, which ages easily with low light transmittance. It can also improve the use of land under the shade of PV modules to provide a better growth environment for crops. The test results show that using grating plates can maximize the light intensity of the greenhouse and solve the problem of uneven distribution of light inside the greenhouse caused by obstruction of PV equipment and greenhouse framework. In sunny weather, the light intensity in three rows of the measurement points at the north side in the greenhouse is greater than 20,000 Lx, and the light environment in other areas is between 5000 Lx and 20,000 Lx, which is suitable for planting shade-loving crops.
1. Introduction
At present, the energy structure at home and abroad is mainly based on fossil fuels. However, fossil fuels are non-renewable energy. With the increasing demand for energy and the continuous exploitation and consumption of fossil energy, it is urgent to develop renewable energies, such as wind power, hydropower, and solar energy. Among them, solar energy is the most popular energy, and PV power generation is one of the main forms of solar energy utilization. PV power generation includes rooftop PV, water PV, and traditional PV ground power stations. A PV greenhouse is an application form of PV power generation, which is mainly used in agricultural production, especially in areas with limited land resources or a lacking power supply. A PV greenhouse converts solar energy into electricity, which can be directly supplied to lighting, heating, ventilation, and other equipment and automation systems in the greenhouse and can also be connected to power grids [1]. PV greenhouse technology is closely related to sustainable development. The use of clean energy can reduce the dependence on traditional power grids, use less fossil energy, improve the utilization rate of solar energy, and contribute to environmental protection and sustainable economic development. In addition, when used properly, a PV greenhouse can also provide a good growing environment, such as suitable temperature, light, and ventilation conditions, to promote the growth of crops and increase yields. However, the shading of PV modules will inevitably reduce the light intensity in the greenhouse, change the light distribution characteristics in the greenhouse, and affect the growth of plants. At present, the scattering film is mainly used as the light-transmitting component, which can expand the illumination area and improve the light distribution characteristics in the greenhouse, but the effect is not satisfactory. At present, the main covering material for solar greenhouses is plastic film, and the most commonly used greenhouse film can be divided into three types based on resin raw materials: PVC (polyvinyl chloride) film, PE (polyethylene) film, and EVA (ethylene vinyl acetate) film. The performance of these three types of greenhouse films is different. PVC greenhouse films have the best insulation effect and are easy to stick and repair but are prone to pollution and have a rapid decrease in transmittance; PE greenhouse film has good transparency and is easy to clean dust and dirt, but its insulation performance is poor. The insulation and light transmittance of EVA greenhouse film are between PE and PVC greenhouse films [2]. There are still some challenges in PV greenhouse technology, such as the cost, efficiency, and reliability of PV panels, as well as technical issues in terms of light uniformity, ventilation, and temperature control inside the greenhouse, and solving these challenges requires continuous technological innovations and improvements.
In order to solve the problem of uneven light distribution in PV greenhouses caused by the shading of PV modules from the perspective of light-transmitting modules on the greenhouse roof, the grating plates with high scattering and high transmittance used as light-transmitting modules to improve the light distribution in zigzag PV greenhouses can effectively improve the light intensity in the greenhouse [3,4]. Liu Chengyu et al. pointed out that a large number of experimental studies were carried out on PV greenhouses. The experimental results of these studies summarized the problems and opinions regarding the research and development of PV greenhouses, as well as the relatively low utilization rate of light energy and the high maintenance price of PV equipment. It is particularly important to ensure that the light uniformity in the PV greenhouse is suitable for plant growth and ensures the thermal insulation function of the greenhouse while not affecting the operation of PV power generation in the greenhouse [5,6].
In this study, grating panels with different thicknesses and numbers of lines were used as PV greenhouse light-transmitting modules, and ordinary glass was used as the control to test the light intensity and uniformity in the greenhouse. The use of grating panels as greenhouse roof covering material was evaluated.
As shown in Figure 1, the grating plate is a plastic material with one side extruded into a cylindrical line and one side as a complete plane, and the cylindrical line spacing is equal to that called “grating”. The light transmittance of the grating plate used in this experiment is 93.77%, and the spectroscopic light waves of the grating will be diffracted at each slit, and the light waves diffracted through all slits will interfere to form interference fringes and be localized to infinity. When sunlight is incident on the grating plate, the area that can be irradiated by the light passing through the grating plate is greater than the area of the grating plate itself [7,8]. As a polymer light scattering material, the grating plate converts the parallel direct light into an isotropic surface light source and expands the illumination area, thus solving the problem of illumination uniformity to a certain extent.
Figure 1.
Grating plate.
2. Materials
2.1. Zigzag PV Greenhouse
The test greenhouse is a zigzag PV greenhouse [WS-GFJ-X.X(HD)] developed from the patented shed greenhouse of Professor Liu Jian of Hainan University: A Combined PV Greenhouse Roof Structure (ZL201621352420.7). The span of the PV greenhouse is 5.5–7.5 m, the bay is 4 m, the shoulder height is 2.2–2.5 m, and the top height is 3.4–3.7 m, as shown in Figure 2 and Figure 3.
Figure 2.
Schematic diagram of zigzag PV greenhouse structure (mm).
Figure 3.
A real picture of the zigzag PV greenhouse.
Combined with the climatic characteristics of the Hainan hot area and the needs of the solar and thermal environment of vegetable planting, the standard sawtooth PV vegetable greenhouse reasonably optimizes the layout of the roof structure and uses PV modules as roof covering materials to replace some traditional transparent covering materials such as films. Because of its reasonable structure, the ground planting utilization rate becomes higher, and the light and heat environment are more suitable, which can achieve the goal of not reducing the output of vegetable production compared with open field planting and can ensure the uninterrupted production of PV vegetable greenhouses [9].
The research on indoor lighting problems of zigzag PV greenhouses mainly focuses on the location of the greenhouse (latitude and longitude, altitude, etc.), orientation, structure, light characteristics of covering materials and enclosure materials (light transmittance, reflectivity, etc.), surrounding features, weather and other factors that have a great impact on the lighting in the greenhouse. The problem of shading and lighting in greenhouses also needs to solve the problem of reasonable distribution of sunlight in crops and PV power generation and maximize the benefits of agriculture and PV industry.
2.2. Zigzag PV Greenhouse Model
The model greenhouse is constructed according to the scale of 1:11 with a span of 500 mm, a column height of 160 mm, and a bay of 600 mm. Moreover, 20 mm × 20 mm aluminum profiles were used as the model greenhouse skeleton material. The azimuth of the greenhouse is set to due south, and the roof of the south slope is facing the sun [10]. Figure 4, Figure 5 and Figure 6 show the structural diagram and the physical drawing of the greenhouse model; the column and the roof beam of the model are connected by a rotating corner chain. The column and the beam part are connected by a vertical corner piece. The roof beam and the roof beam are connected by a vertical turning angle piece and are covered with 40 mesh insect nets around it. There are some slight differences between the model in the experiment and the actual PV greenhouse, but this experiment is to explore the selection of materials with different specifications of grating plates for the uniformity of internal lighting in the serrated PV greenhouse. (Simulate the sawtooth-shaped PV greenhouse skeleton model as European standard industrial aluminum profiles because this experiment used a sawtooth-shaped PV greenhouse simulated by Professor Liu’s patent, and the PV panels used were nontransparent).
Figure 4.
Schematic diagram of greenhouse model structure (mm).
Figure 5.
North end of the field experiment model (insect net can be opened).
Figure 6.
Side view of the field experiment model.
In order to simulate the problem of uneven distribution of illumination in the greenhouse caused by PV panel shading, we used black waterproof adhesive to simulate PV panels. Because the main focus of this study is on the impact of grating panels on the uniformity of illumination in the greenhouse, we mainly considered whether the shading is complete when simulating PV panels. Therefore, we chose to cover the lightweight wooden board with black tape for simulation.
3. Experimental Design and Field Management
3.1. Selection of Test Measurement Sites
As shown in Figure 7, six lines were set in the east–west direction of the greenhouse, which were, respectively, recorded as N1 to N6 at distances of 5.5 cm, 14.50 cm, 23.50 cm, 32.50 cm, 41.50 cm, and 50.50 cm of the gable on the east side. There were nine points on each line, which were evenly distributed in the east–west direction and north–south direction from the inside of the greenhouse. The distances were 3.0 cm, 8.0 cm, 13.0 cm, 18.0 cm, 23.0 cm, 28.0 cm, 33.0 cm, 38.0 cm, and 43.0 cm, which were recorded as point A to point I.
Figure 7.
Schematic diagram of the front and back sections of the light intensity measurement point (mm).
3.2. Experimental Time
The outdoor experiment was conducted at the Agricultural Science Base of Danzhou Campus of Hainan University (19°11′ N, 108°56′ E) from 10:00 to 16:00 on 1 June 2023 (sunny) and from 10:00 to 16:00 on 2 June (sunny).
Due to the fact that these two days are theoretically the period when Hainan Island is at its maximum solar altitude angle, the optimal experimental time range for selection is from June to July. Due to the fact that this experiment mainly discusses the influence of grating plates on the uniformity of indoor lighting in serrated PV cells, there is not much significance in selecting the appropriate time. The time required for readers to reproduce this experiment was also short, reflecting the repeatability of the experimental results.
U0 = Emin/Eav
3.3. Experimental Process
First of all, according to the formula mentioned in the Environmental Engineering of Facility Agriculture edited by Zou Zhirong and Shao Xiaohou [11], the number of inclination angles of the south roof β > 90° − 40° − α = 50° − α (α is the solar altitude angle at a certain time), the latitude of Danzhou at this time is 19°, and the solar regression movement from June 1 to 2 is moving from south to north, the days are getting longer, and the sunshine time in the northern hemisphere is all lengthened, we assume that the solar altitude angle is 38°, and 12° is selected as the inclination angle of the south roof; secondly, according to the test schedule in Table 1, the relationship between the light-transmitting components of different specifications and the light transmission of the point zone (the polymer of the grating plate is plastic) was tested one by one. Finally, Origin64 software was used for data analysis and comparison [12].
Table 1.
Experimental arrangement.
4. Results
4.1. Light Distribution Characteristics of the Corresponding Area of the Light-Transmitting Material in the Greenhouse
4.1.1. Characteristics of Light Distribution in the North–South Direction of the Greenhouse
As shown in Figure 8, the largest theoretical solar altitude angle is at 90 degrees when not considering the difference in the Tropic of Capricorn, and Figure 8 shows the north–south light distribution characteristics of the corresponding area of each material during this time, which is at 12:00. Combined with the analysis method of Li H et al. [13], the relevant lighting characteristics were analyzed by the one-day variation law and the significance Duncan analysis was performed on this basis.
Figure 8.
At 12:00 a.m. The light distribution of each point in the north–south direction of the corresponding area in each plate greenhouse. (Where there is a single marked letter, the difference is not significant, and where there are different marked letters, the difference is significant).
On the whole, the light intensity from point A to point E is significantly lower than that from point F to point I and the highest light intensity is at point G or H. In the southern area, the scattered light entering from the south increases the illumination intensity of point A, which is obviously higher than that of points B and C. In the middle area, the growth trend from point D to point E is lower than that from point E to point F, and the illuminance of the glass at point F is greater than that of the grating plate. The light intensity from point G to point H in the back-end area is generally greater than that of the grating plate, while point I is the opposite [14].
4.1.2. North–South Light Distribution in the Greenhouse
The distribution of light intensity in the north–south direction in different time periods in the greenhouse is shown in Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13, and in general, the influence of cloud cover is excluded Choab N et al. [15] Bulik, T, Piacentini et al. [16] The distribution trend of light intensity in the north–south direction of each light-transmitting material in each time period is as follows: the light intensity from point B to point G or H gradually increases and reaches the peak value. According to the analysis methods of Igoe, D, Turner et al. [17], and Ayet A et al. [18], it is inferred that point A is affected by the scattered light from the ground to the south, and the light intensity is greater than that of point B. The light intensity of glass as a light-transmitting component of a greenhouse is generally greater than that of grating panels [19].
Figure 9.
At 10:30 a.m. Average light intensity at each point. (Where there is a single marked letter, the difference is not significant, and where there are different marked letters, the difference is significant).
Figure 10.
At 11:30 a.m. Average light intensity at each point. (Where there is a single marked letter, the difference is not significant, and where there are different marked letters, the difference is significant).
Figure 11.
At 12:30 p.m. Average light intensity at each point. (Where there is a single marked letter, the difference is not significant, and where there are different marked letters, the difference is significant).
Figure 12.
At 13:30. Average light intensity at each point. (Where there is a single marked letter, the difference is not significant, and where there are different marked letters, the difference is significant).
Figure 13.
At 14:30. Average light intensity at each point. (Where there is a single marked letter, the difference is not significant, and where there are different marked letters, the difference is significant).
4.2. Data Processing and Analysis
As shown in Figure 14, Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19, the variation of the illuminance of each material over time is distributed over time and appears in a “W” shape, assuming that cloud interference is excluded [19,20,21,22]. The illumination intensity of each dot zone of each material will reach a small peak at 12:30 because the solar altitude angle is the highest and the light intensity is strongest at noon, and the illumination of each dot zone at 11:30 and 13:30 is lower than the value of 12:30. The illumination of each point at 10:30 and 14:30 is greater than that at 11:30 and 13:30 because the light measured at the N1 or N6 lines at the measurement points in the greenhouse is directly through the insect net and is not refracted through the light transmitting element and obscured by the greenhouse skeleton [23]. In fact, the light intensity of the outdoors changes at any time [24], the cloud layer is always moving, and the measurement process takes time, which makes the measurement results not exactly the same as the conjecture. Meanwhile, considering the evaporation of water vapor is the highest when the solar altitude angle is at noon, the blocking of water vapor will also affect the measurement results [25,26].
Figure 14.
The average illumination intensity of each point of the 20 line/inch 3 mm thick grating plate at different times (illuminance at different time points from point A to point I).
Figure 15.
The average illumination intensity of each point of the 25 line/inch 4 mm thick grating plate at different times (illuminance at different time points from point A to point I).
Figure 16.
The average illumination intensity of each point of the 30 line/inch 3 mm thick grating plate at different times (illuminance at different time points from point A to point I).
Figure 17.
The average illumination intensity of each point of the 40 line/inch 2 mm thick grating plate at different times (illuminance at different time points from point A to point I).
Figure 18.
The average light intensity of each dot band of 2 mm thick glass at different times (illuminance at different time points from point A to point I).
Figure 19.
The average light intensity of each dot band of 3 mm thick glass at different times (illuminance at different time points from point A to point I).
4.3. Light Uniformity in the Greenhouse
Light Uniformity and Variation Coefficient
Table 2, Table 3, Table 4, Table 5, Table 6 and Table 7 show the light uniformity and variation coefficient in the model greenhouse at different time periods when the grating plate and glass are used as light-transmitting components. The data varied a lot due to the different measurement times of each point and the rapid change of the atmospheric cloud layer. However, it is not difficult to see that the coefficient of variation of the illumination uniformity of the grating plate is relatively small, indicating that the illumination uniformity of the grating plate is superior to that of glass [27,28,29,30]. The results of data processing are as follows:
Table 2.
Light uniformity and variation coefficient of 20 line/inch 3 mm grating transmittance component.
Table 3.
Light uniformity and variation coefficient of 25 line/inch 4 mm grating transmittance component.
Table 4.
Light uniformity and variation coefficient of 30 line/inch 3 mm grating transmittance component.
Table 5.
Light uniformity and variation coefficient of 40 line/inch 2 mm grating transmittance component.
Table 6.
Light uniformity and variation coefficient of 2 mm thick glass transmittance component.
Table 7.
Light uniformity and variation coefficient of 3 mm thick glass transmittance component.
In Table 2, the illumination uniformity of the 20-line 3 mm thick grating plate as the light transmitting module from point A to point I and from 10:30 to 14:30 increased with time, the variation coefficient increased with time, and the variance decreased with the increase in time. The coefficient of variation of each point fluctuates little, and its own value is also very small, indicating a relatively stable change.
In Table 3, the illumination uniformity of the 25-line 4 mm thick grating plate as the light transmitting module from point A to point I and from 10:30 to 14:30 decreased with the increase in time, the variation coefficient increased with the increase in time, the variance increased with the increase in time, the average point G, point H and point I decreased with the increase in time, and the other points increased with the increase in time. The coefficient of variation of each point fluctuates little, and its own value is also very small, indicating a relatively stable change.
In Table 4, the illumination uniformity of the 30-line 3 mm thick grating plate as the light transmitting module from point A to point I and from 10:30 to 14:30 decreased with the increase in time, the variation coefficient increased with time, the variance increased with the increase in time, and the variance increased with the increase in time. The coefficient of variation of each point fluctuates little, and its own value is also very small, indicating a relatively stable change.
In Table 5, the illumination uniformity of the 40-line 2 mm thick grating plate as the light transmitting module from point A to point I and from 10:30 to 14:30 increased with time, the variation coefficient decreased with the increase in time, and the variance decreased with the increase in time. The coefficient of variation of each point fluctuates little, and its own value is also very small, indicating a relatively stable change.
In Table 6, the illumination uniformity of 2 mm thick glass as the light transmitting module from point A to point I and from 10:30 to 14:30 increased with the increase in time; the variation coefficient decreased slightly with the increase in time; the rest of the points increased with time; the variance except for point A increased slightly with time; the variance decreased slightly with the increase in time; the variance except for point A increased slightly with time and decreased with the increase in time; the average point A, point F, point G, point H increased with the increase in time; and point B, point C, point D, point E, and point I decreased with the increase in time.
In Table 7, the illumination uniformity of the 3 mm thick glass translucent module from point A to point I and from 10:30 to 14:30 decreased with the increase in time; the variation coefficient increased with the increase in time; the variance increased with the increase in time; the average points A, B, C, D, E, and F increased with the increase in time; and the average points G, H, and I decreased with the increase in time.
5. Discussion
- The use of a grating plate as the light transmitting module can improve the light uniformity of the light and dark zone junction area in the low light area caused by the shading of PV modules in the zigzag PV greenhouse. However, the light transmittance of the grating plate is lower than that of the translucent glass, and the light entering through the translucent roof will be reduced, causing a low utilization rate of sunlight and lower light intensity under the grating plate in the greenhouse compared with ordinary translucent glass. From the perspective of light distribution characteristics, the grating plate has a high scattering feature to refract the light to an area larger than its own size, which improves the light intensity of some dark band areas, and the increased intensity would decrease with distance. Therefore, grating plates are a good way to deal with the need to block a part of the light and increase the light intensity near the band area. At the same time, it provides ideas for improving the light environment in the greenhouse by using the optical path of light transmitting materials to light.
- In addition to this experiment, the greenhouse model that can change the inclination angle of the roof can also be used to determine the lighting environment in the greenhouse under different roof coverage rates. Since the model greenhouse is based on the size of the actual greenhouse and is scaled down, the light intensity in the room is affected by the skeleton. At the same time, due to the volume of the measuring instrument, the existing model fails to measure the light intensity in the greenhouse from different heights. The model of the measuring instrument illuminometer is TES-1330A.
6. Conclusions
This study is based on the construction of a zigzag PV greenhouse model, using grating panels and ordinary glass as light-transmitting modules. On this basis, the illumination intensity of the model greenhouse was measured when the grating plate and ordinary glass were used as the light-transmitting components in different time periods of the day, and the illuminance of each measurement point was obtained and compared. The following main conclusions were obtained:
- (1)
- In the outdoor experiment, the roof inclination angle of the model greenhouse is 12 degrees, and the roof coverage rate is 41.92%. From the experimental results, it is not difficult to see that the light in the greenhouse with the light transmitting component is ordinary glass, the dark band is concentrated in the front end (A, B, C points) area, the bright band is concentrated in the rear end (G, H, I points) area, and the middle (D, E, F points) area belongs to the light and dark junction zone. At noon, when the grating plate is used as the light-transmitting component, the uniformity of the points (A to I) in the north–south direction is the same as in the east–west direction. The light intensity in the front area of the greenhouse is the same as that of the grating group and the glass group. The grating plate can be used as the light transmitting module to reduce the light intensity of the bright belt, improve the light intensity at the intersection of light and dark, and expand the planting area in the PV greenhouse.
- (2)
- When the grating plate is used as the light transmitting module, the light intensity from point G to point I in the greenhouse is greater than 20,000 Lx, and the light environment in other areas is less than 20,000 lx and greater than 5000 Lx, which is suitable for planting shade-loving crops, and the light intensity of the 40-line specification with a thickness of 2 mm can be maximized to improve the light intensity of the greenhouse. At present, the cost of grating plates on the market is about 90–120 CNY per square meter, which is roughly the same as the cost of glass, and there is the possibility of actual production. Why is this specification of grating most suitable for improving indoor lighting? It may be because the diffraction effect brought on by the grating itself leads to an increase in indoor lighting, which is very interesting.
In summary, grating panels can be used as a greenhouse covering materials to improve the light intensity at the intersection of light and dark bands in PV greenhouses.
7. Patents
One patent has been applied in China in this manuscript (Patent No. CN201320738494.4)
Author Contributions
Conceptualization, J.L.; methodology, B.W.; software, Y.S.; validation, Y.S.; formal analysis, B.W.; investigation, Y.S.; resources, B.W.; data curation, B.W.; writing—original draft, Y.S.; writing—review and editing, B.W.; visualization, Y.S.; project administration, J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.
Funding
This research was supported by China Huaneng Group Co., Ltd. Headquarters Technology Project, grant number HNKJ22-HF77; Hainan Provincial Natural Science Foundation of China, grant number 322RC583.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
Acknowledgments
We would like to thank the Innovation and Utilization team of tropical melon crop genetic germplasm, Hainan University.
Conflicts of Interest
The authors declare no conflicts of interest.
Nomenclature
| U0 | Illumination uniformity |
| Emin | Minimum illumination value |
| Eav | Average illuminance value |
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