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Floating Photovoltaics (FPVs): Impacts on Algal Growth in Reservoir Systems

/, Technology, Undergraduate Research/Floating Photovoltaics (FPVs): Impacts on Algal Growth in Reservoir Systems

Floating Photovoltaics (FPVs): Impacts on Algal Growth in Reservoir Systems

2023-05-22T16:54:15-07:00 May 19th, 2023|Biology, Technology, Undergraduate Research|

By Benjamin Narwold, Environmental Science and Management major ’23

Author’s Note: I wrote this review paper to learn more about the environmental impacts of floating photovoltaics (FPVs) because this topic directly applies to my work as an undergraduate researcher position with the Global Ecology and Sustainability Lab at UC Davis. I wanted to focus specifically on the impacts of FPV on algae because of the biological implications of disturbing ecologically important photosynthesizers in reservoirs. I want readers to develop an understanding of FPVs as a climate change mitigation solution, how these systems may disturb algae, and the uncertainties in whether expected and observed changes in algae growth are beneficial or detrimental to the aquatic environment.

ABSTRACT

Floating photovoltaics (FPVs) are typical photovoltaics mounted on plastic pontoon floats and deployed on man-made water bodies. If FPVs are developed to cover 27% of the surface area of US reservoirs, they would provide 10% of the electricity in the US. Freshwater reservoirs are host to vulnerable ecosystems; therefore, understanding the water quality impacts of FPVs is necessary for sustainable development. This review aimed to fingerprint the impacts of FPVs on reservoir aquatic ecology in terms of algal growth and identify the uncertainties in FPV-induced algae reduction to present our current understanding of the environmental impacts of reservoir-based FPVs. The UC Davis Library database was searched for papers from peer-reviewed journals published from 2018 to 2022 that covered “floating photovoltaics”, “algae reduction”, and “environmental impacts”. A consistent result across studies was that FPVs reduce algal growth by reducing the sunlight entering the host waterbody, and this can disrupt phytoplankton dynamics and have cascading effects on the broader ecosystem. Modeling and experimental approaches found that 40% coverage of the reservoir by FPVs is optimal for energy production while maintaining the necessary algae levels to support the local ecosystem. The lack of research on the ideal percent coverage of FPVs to reduce algal growth but not disrupt ecosystem dynamics emphasizes the need for future research that addresses FPV disturbance of local microclimates, algae response to reduced sunlight, and the corresponding cascading impacts on other organisms dependent on the products of algal photosynthesis.

Keywords: floating photovoltaics, algae reduction, environmental impacts, water and ecology management, energy and water nexus

Caption: Floating photovoltaic (FPV) system in Altamonte Springs, Florida. One of four sites monitored by the Global Ecology and Sustainability Lab for water quality impacts of FPV.

INTRODUCTION

Climate change is a global problem of increasing intensity and poses challenges to food, water, and energy security. Global climate models predict a 2-4°C increase in global temperatures from now until 2100, which will degrade human health and threaten ecosystems [1]. Renewable energy is a critical component of reducing anthropogenic greenhouse gas emissions, and the widespread transition away from fossil fuels is becoming increasingly feasible with new technologies. One of these new renewable energy systems is floating photovoltaics (FPVs), standard photovoltaic (solar panel) modules mounted to a polyethylene pontoon float system, positioned off the water’s surface, and anchored to the bottom or shore of the host waterbody [2]. FPVs represent an intriguing and novel renewable energy solution because they can be deployed on human-constructed water bodies and improve land-use efficiency. Ground-mounted solar projects compete for land against agricultural and urbanization interests, whereas many artificial and semi-natural water bodies, such as wastewater discharge pools, have no conflicting human interests [3]. FPV development thus presents an opportunity to sustainably increase solar energy production without interfering with agricultural and urban development, which will continue to expand as world populations increase. In addition to optimizing land use, FPVs can produce up to 22% more power than conventional solar due to evaporative cooling [4]. The solar panels are located just above the water’s surface, so the local water evaporation contributes to a reduction in solar panel temperature, thus increasing efficiency. Generating electricity using FPVs is intended to augment solar power generation capacity and supply more renewable energy to the grid for households and industry.

Among the most abundantly available space to develop this pivotal land-use optimization and climate change mitigation solution are reservoirs, lakes formed from damming a river for water storage and hydropower production. A GIS analysis found that covering 27% of the surface area of reservoirs in the United States with FPVs would generate enough electricity to meet 9.6% (2116 Gigawatts) of the country’s 2016 energy demands [4]. But reservoirs and similar bodies of water nevertheless represent vulnerable freshwater ecosystems, so developing an understanding of the water quality and species impacts of FPVs represents the primary hurdle to informing sustainable development of these systems.

FPVs reduce the amount of sunlight reaching the surface of their host waterbody, which reduces the amount of evaporative water loss and results in significant changes to algae growth [5]. Several studies have found that FPVs alter phytoplankton dynamics and can have cascading effects on the other organisms in the ecosystem [6–8]. A key agent of uncertainty surrounding reservoir FPVs is determining the equilibrium range of algal growth needed to support reservoir food webs. In some reservoir systems, we see strong summertime algal blooms. An algal bloom is a rapid increase in or overaccumulation of an algal population that can result in oxygen-depleted waterbodies called “dead zones,” where the algae eventually die and decompose [9]. FPV-induced shading can counter harmful algal blooms, providing environmental benefits to augment renewable energy generation. Alternatively, in reservoirs that do not have problematic algal blooms, adding an FPV system may reduce healthy algal populations and cause adverse rippling effects to other species in the ecosystem. Developing an understanding of what percent of the total water surface area of the reservoir covered by FPVs is enough to reduce algal growth and bloom potential but not too large to disrupt ecosystem dynamics will require further research. Specifically, assessing the disturbance of local microclimates caused by FPVs, algae response to reduced sunlight conditions, and the impact on other aquatic species dependent on the ecosystem functioning provided by algae. Due to climate change, we predict an increase in temperature and shifting precipitation patterns; therefore, it is important to contextualize the water quality impacts of FPV and its influence on algae, given this variability.

Figure 1. Impact of FPVs on algal in reservoir ecosystems. FPV-induced shading can provide additional environmental benefits in reservoirs with algal blooms and may cause adverse effects in healthy reservoirs.

Methods

This review surveys what we know regarding the impacts of FPVs on algal growth in reservoir systems. The UC Davis Library database was searched for papers from peer-reviewed journals using the following keywords: “floating photovoltaics,” “algae reduction,” and “environmental impacts.” I looked at experiments on reservoir-based FPVs from 2018-2022 to analyze plot scale impacts on algal growth, quantified with chlorophyll-a monitoring data, and assessed global-scale changes in algal growth from a climate change perspective, with consideration of FPV materials and design. Although a study on crystalline solar cells incorporated in this review is from 2016 and falls outside the 5-year range of focus, it represents a necessary juxtaposition to the semitransparent polymer cell technology. Overall, I analyzed the methods and results of site-specific, laboratory, and global-scale studies to fingerprint the current state of knowledge on the impacts of FPVs on algae and algal blooms to inform reservoir management.

Algal Growth and FPV Coverage Scenarios

Algae are responsible for producing oxygen in the waterbody, and the impact of FPVs on algae growth depends on the percentage of the waterbody covered by the FPV and is measured by looking at chlorophyll-a (ch-a) differences. Ch-a, a pigment present in all photosynthetically active algae, is often used as a proxy measurement to assess algal growth dynamics within a waterbody [10]. Ch-a is measured using optical sensors and wavelengths of light, so it is an indirect measurement of algal concentration. FPVs reduce the amount of sunlight reaching the surface of their host waterbody and disrupt phytoplankton dynamics. Hass et al. (2020) and Wang et al. (2022) investigated different FPV coverage scenarios and used ch-a as a proxy for algal growth. Hass et al. used the ELCOM-CAED model to evaluate ten different FPV coverage scenarios, and Wang et al. simulated 40% coverage relative to 0% coverage control ponds using black polyethylene weaving nets as a proxy for an FPV array. Both the model output and experiment-based approach settled on 40% FPV coverage as an equilibrium development target [7, 11]. The results of these studies show continuity; however, Hass et al. did not consider the difference in absorption wavelength range for different microalgal taxa, and Wang et al. did not use actual solar panels in their experimental design. Additionally, Andini et al. (2022) investigated the difference in algae between 0% and 100% coverage at Mahoni Lake in Indonesia by experimenting with mesocosms, isolated systems that mimic real-world environmental conditions while allowing control for biological composition by taking samples at the same water depths. These researchers found that 100% FPV coverage reduced ch-a between 0 and 1.25 mg/L, average temperature between 0 and 2.5℃, dissolved oxygen between 0 and 1.5 mg/L, and electrical conductivity categorically in the waterbody. However, the researchers only considered directly measured water quality variables and did not assess the long-term trophic consequences of 100% FPV coverage [6]. Clearly, the study was designed to show the polarity between 0% and 100% coverage in terms of several water quality parameters; however, realistic intermediate FPV coverages incorporated into both Hass et al. and Wang et al. were absent from this study. Given these compiled results, future research can continue to work toward the broader question of determining what percent FPV coverage can be applied to a reservoir to maximize energy production and minimize environmental disturbance.

Algal Blooms and Mitigation Potential

Algal blooms are a product of high productivity conditions that favor rapid algae growth, and the shading provided by FPV systems could mitigate the intensity and negative impacts of summertime algal blooms. High productivity conditions include high water temperature, intense sunlight, and abundant nutrients such as nitrogen and phosphorus. The first two variables can be controlled by FPV coverage. In a study of the global change in phytoplankton blooms since the 1980s, Ho et al. (2019) found that most of the 71 large lakes sampled saw an increase in peak summertime bloom intensity over the past three decades, and the lakes that showed improvement in bloom conditions experienced little to no warming. Temperature, precipitation, and fertilizer inputs were the considered variables, and this study could not find significant correspondence of blooms to any of these variables exclusively [12]. This insignificant result suggests a diversity of causal agents on a per-lake basis. Thus, conducting site-specific studies and monitoring these water quality variables will help establish algal bloom causation and the relative intensity of the confounding variables and, therefore, whether FPV coverage would be an effective mitigation agent. If the algae in a reservoir are linked to less-controllable variables like carbon dioxide concentration in the water or nutrient loading from agricultural runoff, FPV-shading will have a negligible effect on algae [6, 7]. Such considerations are critical to informing the potential environmental co-benefits of an FPV installation.

FPV Solar Cell Design

The properties of solar cells within the photovoltaic panels themselves are instrumental in determining what wavelengths of light interact with the surface of the host waterbody under the panels. Crystalline silicon solar cells absorb radiation wavelengths from 300-1300 nm and have a thick active layer of about 300 µm, responsible for high photon absorption [13]. These properties result in opaque solar panels that do not allow photons to travel through the panel and interact with the waterbody. Conversely, semitransparent polymer solar cells (ST-PSCs) represent an alternative material and technological approach, and algae growth can be regulated by engineering the panels to provide specific transmission windows and light intensities. Zhang et al., 2020 found that the growth rate for the algal genus Chlorella was minimized under the opaque treatment; however, the changes in photosynthetic efficiencies did not significantly affect the growth rate of Chlorella during the 24-hour experimentation window. While the researchers were able to show the variability in the number of photons penetrating the panels from 300-1000 nm across three treatments of different layering of material within the ST-PSCs, they were unable to yield a significant result in their study [5]. These results have limited scope because this study was conducted in a lab and did not assess real-sized PV panels in the field; however, it highlights how algae species may prefer different light wavelengths for photosynthesis that may be discontinuous with the wavelengths an FPV system best controls. Therefore, it is vital to coordinate solar panel material design in order to reflect and absorb the primary wavelengths that support algal photosynthesis. The viability of prioritizing this component of FPV is uncertain; however, new materials and technologies are being developed and utilized, and this relationship must be considered as we work to maximize FPV coverage in reservoir systems with minimal ecological complications (Figure 2).

Figure 2. Relationship between FPV transparency, light profiles entering the waterbody and interacting with algae, and FPV coverage optimization. Solar cell design influences light transmission, and photosynthetic rates in algae vary with light wavelength and intensity, providing site-specific design opportunities.

CONCLUSION

FPVs are relatively untapped climate change mitigation solutions and can potentially reduce algae, benefitting water quality in freshwater ecosystems and reservoirs that suffer from strong summertime algal blooms. Algae are critical primary producers in reservoir ecosystems; therefore, areas for future research include microalgae response to the reduced sunlight conditions created by FPVs and the ecological role of algal taxa within the reservoir ecosystem. Further laboratory studies of solar panel designs in this context are needed. Future research on FPVs and water quality must also account for climate change, shifting baselines, and environmental variables. From a reservoir management viewpoint, this includes studying whether reservoirs have lower nutrient loading and whether the algae can be managed with FPV arrays, fingerprinting the inter-reservoir variability to determine where we should spatially place FPV arrays and localize impacts, and further modeling the relationship between warming and algal blooms to understand the long-term effectiveness of FPV-based algae management. Climate change will continue to operate in the background, and energy security issues will intensify. Our understanding of the environmental impacts of FPVs is currently limited to the point where we cannot safely approve and construct these systems on most reservoirs; therefore, future studies are needed to incorporate this modern technology into the global renewable energy portfolio.

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