The Effects of a Fishery Complementary Photovoltaic Power Plant on the Near-Surface Meteorology and Water Quality of Coastal Aquaculture Ponds

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The Effects of a Fishery Complementary Photovoltaic Power Plant on the Near-Surface Meteorology and Water Quality of Coastal Aquaculture Ponds


4.1. Effects of PV on Temperature

The average SAT in the PV area is 0.6 °C higher compared to the control area of the case study, while the average water temperature is 1.5 °C lower compared to the control area. The results in our case study are consistent with those of Li et al. [19]. They monitored air temperature and water temperature simultaneously and showed that the average air temperature at the PV site is 0.16 °C higher compared to that outside the PV site, and the water temperature is almost lower compared to that outside the PV site. What is the underlying cause for this phenomenon?
The coverage of PV power decreases the shortwave radiation within the PV area, which is conducive to a decrease in water and air temperature [20]. On the other hand, it should be noted that the PV panels emit long-wave radiation, thereby contributing to the heating of the surrounding atmosphere [19,21]. The average net shortwave radiation in the PV area (26 W/m2) decreased by 96% compared with that in the control area (717 W/m2) of the case study, while the net longwave radiation increased from −102 W/m2 in the control area to 44 W/m2. Because the reduction in shortwave radiation is much greater than the increase in longwave radiation, the Q in the PV area drops by 91%. There is a significant positive correlation between water temperature and Q (p ≤ 0.01) (Figure 6). In this regard, the decrease in water temperature in the PV area may be mainly caused by the reduction in shortwave radiation, which is consistent with the results of the reservoir PV simulation experiment by Ji et al. [22]. In addition, the increase in SAT mainly comes from the heating of PV panels.
However, the results by Yang et al. are not exactly consistent with our study. Yang et al. demonstrated that both air and water temperatures under PV panels are always higher than those in open water under the heating effect of PV panels with a peak temperature of 50 °C [21]. This phenomenon may be caused by different types of PV installations. The PV installation is fixed on a pile foundation of our case study, while it is floating in the their study [21]. The main difference between the two installations is the distance from the PV panels to the surface, which is connected to the wind speed below the PV panel. In our case study, the distances from the PV panel to the surface are 3.02 m, 2.35 m, and 2.50 m at S1, S2, and S3 corresponding to measured wind speeds of 4.0 m/s, 3.3 m/s, and 2.4 m/s, respectively. However, the distance from the floating PV installation to the surface is very small (0.15 m [21]), corresponding to a low wind speed (approximately 0 m/s) below it [22]. Evaporation is an important method of heat flux transfer at the air–water interface, and the change in wind speed affects the water temperature by influencing evaporation [21]. The water temperature probably increases when the drop in wind speed is much greater than the drop in solar radiation [23]. From the perspective of energy analysis, PV changes the energy balance of the water surface. In the study of Yang et al., the shortwave radiation energy in open water is balanced by sensible heat flux, latent heat flux, and longwave radiation. Since the wind speed under PV is 0 m/s and longwave radiation changes from negative to positive, longwave radiation is balanced by sensible heat flux [21]. Although the wind speed in this study is reduced by PV, the decrease does not exceed that of solar radiation, and the sensible heat flux and longwave radiation in the PV area are balanced by the latent heat flux.
In recent years, global climate change has had an impact on aquaculture, posing a threat to the security of fishery resources. For instance, the rising water temperature caused by global warming may exacerbate the incidence of red tides and severely affect fish production [24]. Therefore, it is necessary to take measures to mitigate the effects of climate change. In our research, we found that PV significantly reduces water temperature. Thus, adopting fishery complementary PV can alleviate water warming, which might be beneficial for aquaculture.

4.2. Effects of PV on Water Quality

The DO and salinity in the PV area are higher compared to the control area of the case study, while the pH, Chl-α, and some nutrients in the PV area are lower. In addition, the concentrations of nitrate and ammonium in the PV area are not significantly different from those in the control area.

Li et al. conducted a PV simulation experiment in a pond with a sunshade net and found that when there is PV coverage, DO increases, which is consistent with our results [25]. It is worth noting that when PV coverage is in the range of 0~50%, the DO increases with increasing PV coverage; when PV coverage is greater than 50%, the DO decreases with increasing PV coverage [25]. It is well known that DO is mainly related to the photosynthesis of phytoplankton, the respiration of organisms, and the temperature of the water. In our study, the respiration of organisms is weak at low temperatures (13.3~15.1 °C). In addition, the intensity of phytoplankton photosynthesis is weak due to the low E value below PV. Therefore, it is likely that the high DO concentration in the PV area of the case study is mainly caused by low water temperatures.
For shallow aquatic ecosystems, nutrient and light competition between phytoplankton and benthic algae can affect turbidity. Mei et al. found that elevated temperature promotes the growth of phytoplankton but inhibits that of benthic algae, resulting in an increase in the number of total suspended solids and a significant decrease in the light intensity of the sediment surface [26]. PV coverage is conducive to the increase in subsurface E of the case study, but PV coverage reduces the light incident on the water body; thus, the final E of the water body depends on the relative effect of the above two factors. The turbidity of the PV area is 70% lower compared to the control area of the case study. The E of the above surface and surface in the PV area is lower compared to the control area of the case study by 51,050 and 3745 lux, respectively, while the E of the subsurface in the PV area is 140 lux higher compared to the control area. This finding shows that close to the bottom, the influence of turbidity on E is relatively great. Therefore, the low turbidity in the PV area is conducive to the acquisition and utilization of light by organisms in the water body and, to a certain extent, the adverse impact of reduced E on primary production [27], further reducing the impact of phytoplankton photosynthesis on DO.
The concentration of Chl-α in the PV area is significantly lower compared to the control area of the case study, which is consistent with the conclusions of existing studies. For example, in the modelling study of Yang et al., under 30% PV coverage, the concentration of Chl-α decreased by 30% [28]. Hass et al. conducted a detailed modelling experiment on Chl-α with 0–100% PV coverage (increasing by 10%) and found that the concentration of Chl-α decreases with increasing coverage. When the coverage reaches 20~30%, the concentration of Chl-α decreases significantly, and when the coverage exceeds 70%, the concentration of Chl-α decreases significantly. The concentration of Chl-α is below the threshold of 0.4 μg/L in oligotrophic lakes [29]. Although PV coverage reaches 80%, the lowest Chl-α concentration is still greater than 1 μg/L of the case study. This phenomenon may occur because our study area is a closed water body with low flow rates. However, turbines are deployed in the research area of Hass et al., which can maintain a relatively high flow rate and strong mixing of water bodies [29]. Exley et al. conducted PV simulation experiments under three flow rate scenarios at high, middle, and low levels and found that the concentration of Chl-α decreases exponentially with increasing PV coverage at high and middle flow rates, and the concentration of Chl-α with only 60% coverage drops below 1 μg/L at high flow rates. However, reducing the low flow rate to the same Chl-α concentration requires at least 90% coverage [30]. This finding further indicates that a high flow rate can enhance the effect of PV on Chl-α concentration reduction.
In the study of Li et al., pH decreases with increasing PV coverage, but the decrease is very small. Compared with the control area, the pH decreases by only 0.2 units under 100% coverage [25]. The average pH of the PV area is reduced by only 0.28 units compared to the control area. The pH of the water body is affected by the difference in photosynthesis and respiration intensity. Both photosynthesis and respiration in the PV area are weakened due to the decrease in E and water temperature, and the decrease in pH indicates that the effects of PV on respiration may be higher than those on photosynthesis. Experiments on the effects of evaporation are popular topics in recent water-based PV research. PV reduces solar radiation and wind speed, reducing evaporation in water bodies to varying degrees [31,32,33]. From the perspective of evaporation, the salinity of the PV area should be lower compared to the control area, but the salinity of the PV area of the case study is significantly higher compared to the control area, and the specific reasons remain to be studied.
In the modelling analysis of the impact of the floating PV installation on water quality, compared with the control area, nitrate in the PV area decreases and ammonium increases, but the magnitude is small (0.16% and 0.4% in summer; 0.05% and 0.24% in winter) [34]. In the PV simulation experiment of the sunshade grid, the concentrations of ammonium and active phosphorus in water with a 75% coverage rate decrease by 43.1% and 24.9%, respectively, and the concentration of nitrite increases by 45.5% [25]. This finding shows that inorganic nutrients vary greatly in different types of PV power or under different scenarios. According to the results of the t-test, there are no significant differences between the concentrations of nitrate and ammonium in the PV area and the control area in our study. The nitrite concentration is extremely low in the PV area, and the highest value in the control area is only 0.0025 mg/L, which may be influenced by the high DO concentration in the water body in the study area; a high DO concentration can promote the production of nitrate. The concentrations of labile phosphate, active silicate, TN, and TP in the PV area are lower compared to the control area of the case study. This finding is consistent with the research results of Li et al. [25]. Labile phosphate is more easily adsorbed under aerobic conditions [35]. The DO in the PV area is significantly higher compared to the control area of the case study, which may be one of the reasons for the concentration of labile phosphate in the PV area being lower compared to the control area. Temperature affects the labile phosphate concentration. The rising water temperature accelerates the degradation of sediment humus by microorganisms, and organic matter is released into the water layer in the form of phosphate [36]. The temperature of the PV area is reduced, and the labile phosphate concentration is reduced. In addition, rainwater erosion enhances phosphorus release from sediments [37]. PV power may reduce the concentration of labile phosphate in the PV area by attenuating the scouring effect. Some studies have shown that the species composition of phytoplankton in the water in PV areas changes. For example, Exley et al. found through modelling that under the scenario of a low flow rate and high coverage rate (90%), diatoms occupy a dominant position in the phytoplankton community most of the time, while under the scenario of a low coverage rate (30%), green algae are dominant [30]. This study area exhibits a low flow rate and high PV coverage. The decline in active silicate concentration within the PV area can potentially be attributed to the rise in diatoms and other groups. Despite a significant decrease in Chl-α concentration within the PV area, the increased proportion of diatoms may still be the primary factor contributing to the decline in active silicate concentration. Furthermore, the Pearson correlation analysis revealed a noteworthy positive correlation between water temperature and active silicate. The potential influence of water temperature on active silicate through its impact on diatoms necessitates further investigation to elucidate the specific underlying mechanism.
The overall concentrations of TN, TP, and TOC area are high in the case study, especially TOC. This finding indicates that the inorganic process of the water body is relatively slow, which is consistent with the characteristics of slow biological activity in winter. In addition, the TN, TP, and TOC in the PV area are significantly lower compared to the control area. In the modelling study of Yang et al., when the PV coverage rate is 30%, the TOC concentration of the reservoir decreases by 15% compared with the control area [28]. The relatively low TN, TP, and TOC values in the PV area may be related to the relatively low primary production and slow accumulation of organic matter. According to Pearson correlation analysis, there is a significant positive correlation between water temperature and TP, indicating that a decrease in water temperature can lead to a decrease in TP.
Previous studies have demonstrated that the coverage of PV panels can influence the production of fish and crabs. The installation of PV panels may have a negative impact on milkfish (Chanos chanos) production and a positive impact on Chinese Mitten Crab (Eriocheir sinensis) production [13,38]. Further investigations will be focused on the yield and size of mud crab in the study area.

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