Introduction
The increased demand for renewable energy as the global population continues to increase over time and as society works towards fighting climate change has sparked creative ways to meet demands while reducing fossil fuel emissions. Agrivoltaics, the practice of using land to simultaneously house solar photovoltaic (PV) panels and grow crops or have livestock grazing, has been on the rise since it was established in 1981 (David, 2021). In certain countries like Japan, agrivoltaic farms have evolved and grown within the past few decades. In 2021, there were 1,992 agrivoltaic farms covering 560 hectares in Japan, providing 500,000 to 600,000 megawatt hours of power for the country (Tajima and Iida, 2021). By combining agriculture and solar energy, these demands can be met while reducing fossil fuel emissions by providing clean, renewable energy. In this review, I argue that agrivoltaics is an effective strategy for combating climate change by maximizing land use and reducing greenhouse gas emissions without adding stress to water resources or food production.
Combatting Climate Change with Agrivoltaics
There are multiple benefits to employing agrivoltaics as a climate change mitigation strategy including the maximization of land use, reducing greenhouse gas emissions, and not adding stress to water resources or food production. These benefits are further explored and discussed below.
Maximization of Land Use
Maximizing land use to the highest potential is becoming increasingly important as the global population continues to grow since access to land is heavily relied on for housing, agriculture, and renewable energy technologies. Approximately 12% of land globally is used by the agricultural sector to grow crops (Mamun et al., 2022). Additionally, solar PV development is facing challenges such as land conflict and social resistance as solar PV farms continue to grow in size and require large plots of land (Pascaris et al., 2021). As the global population continues to increase by approximately 1.15% each year, competition for land across multiple industries will be exacerbated (Mamun et al., 2022). In countries that are densely populated, there may be challenges associated with choosing land for certain resources over others. The large area of land required for both the cultivation of crops and the installation of solar PV farms suggest that the incorporation of these two industries on the same plot of land would be helpful for maximizing land use in a time when populations are rising and demands for resources are higher than ever. A study conducted by Dupraz et al. (2011) showed that land productivity in certain settings has increased by 60 to 70% in the presence of agrivoltaic systems, as opposed to when the two systems operate independent of each other.
There are several ways in which solar energy can be produced in conjunction with the use of agricultural land through agrivoltaics. These options include intensive horticulture, which uses temperature-controlled conditions to grow crops that would benefit from the presence of solar PV panels to increase productivity and reduce potential damage caused by weather; broad acre, which consists of farms that grow grain, oilseeds, and other large-scale production crops; perennial pastures, which grow crops that are used for livestock feed, including grasses and legumes; and the grazing of livestock, which allows for animals to graze on land in the presence of solar PV panels (Mamun et al., 2022). Integrating various agricultural practices with the production of solar energy provides benefits that would not occur if these systems were to operate independently.
Maximizing land use with agrivoltaics is beneficial for plant productivity, reducing the need for vegetation removal and maintenance, and providing much-needed relief for livestock in extreme heat conditions. Several plants, including bottle gourd, cucumber, grape, lettuce, and tomato can adapt to the presence of solar PV panels by maintaining productivity levels under increased shade (Mamun et al., 2022). Additionally, the yield of crops can be higher in drought conditions when conserving water is imperative, as shown in a study conducted by Graham et al. (2021) where several crops had a higher crop yield in the agrivoltaic system compared to the unshaded reference area during an unusually hot summer. The presence of livestock can also be advantageous since the grazing animals eliminate the need for vegetation removal and maintenance (Mamun et al., 2022). While the livestock help maintain vegetation growth, the presence of solar PV panels benefits the wellbeing of livestock by providing additional shade. When agrivoltaic practices were incorporated into a pasture, the solar PV panels reduced respiration rates and body temperatures for the dairy cows, showing the additional shade reduced the effects of heat stress for these grazing animals (Sharpe et al., 2021). Because climate change is causing global surface temperatures to rise, additional shade will help the livelihood of livestock that may be impacted by extreme heat conditions (Heins et al., 2022). Both crops and livestock can benefit from the integration of solar PV panels, making this a feasible and desirable solution for maximizing land use.
Reducing Greenhouse Gas Emissions
Reducing greenhouse gas emissions is imperative for mitigating climate change. The Representative Concentration Pathway (RCP) 8.5 represents a “business as usual” emissions scenario and under these circumstances, cumulative carbon dioxide emissions are expected to reach 7,300 gigatons of carbon dioxide over the course of the century (Riahi et al., 2011), which is equivalent to greenhouse gas emissions from approximately 1.6 trillion gasoline-powered passenger vehicles driven for one year (US EPA). Implementing agrivoltaics can be an effective way in reducing the likelihood of reaching emissions levels predicted by the RCP 8.5 scenario. Agrivoltaics can reduce greenhouse gas emissions in various ways, including the maximization of land use by housing solar PV panels in close proximity to agricultural land. In a case study conducted with a PV greenhouse, carbon dioxide emissions were reduced by up to 12% when agrivoltaics were incorporated compared to traditional growing practices (Leon and Ishihara, 2018). The reduction in carbon dioxide emissions could have been attributed to the on-site PV-generated electricity powering irrigation procedures (Mamun et al., 2022). Additionally, in a life cycle assessment conducted on an agrivoltaic farm that incorporated a rabbit farm and solar PV panels, the agrivoltaic system produced 69.3% less emissions and demanded 82.9% less fossil fuel energy than when the rabbit farm and solar PV panels operated independent of each other (Pascaris et al., 2021). The reduction in greenhouse gas emissions and fossil fuel energy demand was attributed to the synergistic relationship between the rabbits, the land, and the solar PV system. The rabbits were able to feed and graze off the grass present in the pasture, therefore eliminating the need for production and transportation of rabbit feed. This also eliminated the need for PV maintenance such as mowing and herbicide application. Because these processes were eliminated and there was less transportation required and resources used, greenhouse gas emissions were significantly reduced.
Agrivoltaic systems also prevent further greenhouse gas emissions because the act of sharing land between these two industries could prevent further deforestation (Agostini et al., 2021). Forests and grasslands are experiencing deforestation as the competition for viable land for agriculture and solar PV farms continues. In the tropics, approximately 90 to 99% of deforested land that was cleared between 2011 and 2015 was cleared for agricultural purposes (Pendrill et al., 2022). Forested lands and grasslands act as carbon sinks and emit carbon when they are cleared due to the lack of space (Gatti et al., 2021). If agriculture and solar PV farms can coexist on the same land, less carbon will be emitted by deforestation and the existing forests can continue to act as carbon sinks.
The process of replacing fossil fuel-produced energy with solar PV panels will also significantly reduce greenhouse gas emissions. According to the National Renewable Energy Laboratory, by 2050, approximately 80% of the electricity demand within the United States has the potential to be met with renewable energy technologies, with solar PV being able to accommodate 12.7% of the total demand (Proctor et al., 2021). With renewable energy technologies on the rise, it is important to consider the emission reductions that would accompany these changes. By replacing traditional fossil fuel-produced energy for electricity with energy provided by solar PV panels, carbon dioxide emissions could be reduced by up to 330,470 metric tons each year, which is equivalent to removing more than 71,000 cars from the road each year (Proctor et al., 2021).
Water Conservation
As global surface temperatures continue to increase with climate change, access to water will become more challenging. An estimated average global temperature increase of 1.4 to 5.8° Celsius will lead to a significant reduction in the availability of fresh water resources, which will have a detrimental effect on agricultural yield (Misra, 2014). Because of this, conserving water in an agricultural setting will be important to keep the agricultural industry going during drought conditions. Crops receive more shade when solar PV panels are present than they would in typical agricultural settings. The shade provided by elevated solar PV panels in agrivoltaic farms can increase daily water use efficiency by preventing evaporation from soil and transpiration from crops (Barron-Gafford et. al., 2019). In a study conducted on a solar PV farm with cultivated lettuces, a 20% reduction in plant water demands was observed, along with a 5- to 7-day delay in plant maturity (Elamri et al., 2018). The water use reduction was attributed to additional shade provided by the solar PV panels and the study indicated that the crops were not exposed to water stresses during the cropping cycles. The delay in plant maturity was not noted as a negative effect. The observed delay in plant maturity could be considered a strategic element for commercial reasons and it allows for the agrivoltaic installations to reduce irrigation amounts while reducing the effects of heat waves or drought periods on the development and health of crops. Another study focusing on peppers and tomatoes in an agrivoltaic system showed that there was an increase in water use efficiency of 157% and 65% for jalapeño peppers and cherry tomatoes, respectively (Barron-Gafford et al., 2019). These results suggest that certain crops are able to adapt to or prefer the additional shade provided by solar PV panels and prioritizing these crops in agrivoltaic settings will help reduce water consumption. Additionally, in several irrigation settings tested in this study, soil moisture content remained higher in agrivoltaic systems as compared to the traditional (control) growing areas. When the soil was irrigated every two days, soil moisture in the agrivoltaic system was 15% higher than in the control setting. When irrigation was conducted every day, soil moisture in the agrivoltaic system was 5% higher than in the control setting. Soil that was irrigated every two days under an agrivoltaic setting had higher moisture levels than the soil irrigated daily under the control setting, showing that less frequent irrigation under an agrivoltaic setting is helpful for water conservation. A study conducted in Germany’s largest agrivoltaic research facility aimed to understand the productivity of potato, celeriac, clover grass, and winter wheat in an agrivoltaic system under drought conditions (Graham et al., 2021). Winter wheat, potato, and celeriac had a higher crop yield of 3%, 11%, and 12%, respectively, in the agrivoltaic system compared to the unshaded reference area during an exceptionally hot summer in 2018. The study found that agrivoltaics produces a higher crop yield under dry and warm climates by preventing evaporation and transpiration, which decreases water use and irrigation. Drought conditions, including aridity, frequency, and severity, are expected to change as the climate warms. As drought conditions worsen due to climate change, utilizing agrivoltaic farms with shade-tolerant crops will help society adapt to changing conditions (Trommsdorff et al., 2021). The presence of the solar PV panels preventing further evaporation from the soil and transpiration from crops is an integral component for conserving water in agrivoltaic settings, and this will be important moving forward as precipitation amounts and access to fresh water are uncertain with a changing climate.
Solar PV panels also help reduce water consumption by intentionally distributing water to maximize water use (Dinesh and Pearce, 2016). This can be done by orienting the solar PV panels in a way that would drain rainwater directly onto the crops. Additionally, if an irrigation system with sprinklers is present, the water distributed by sprinklers can clean the solar PV panels and then drain onto the crops. Since dust is spread during farming activities, solar PV panels must be kept clean to maximize the energy that they can provide. By orienting the solar PV panels to drain directly on top of crops, water can be used for both irrigation and cleaning purposes, which would reduce the amount of water required to keep the agrivoltaic system going and improve effective water usage.
The spacing and orientation of solar PV panels will need to be modified to allow for the highest crop yield and retention of soil moisture. There are three zones associated with ground-mounted solar PV panels: zone 1, directly under the panel which has a low irradiance and high humidity level; zone 2, directly behind the panel that has regular light exposure and sufficient soil moisture; and zone 3, the area directly between panels that has the highest irradiation and lowest humidity (Toledo and Scognamiglio, 2021). The varying irradiance and humidity at each zone and the type of crop need to be considered when developing agrivoltaic farms since certain crops respond differently to varying levels of sunlight and moisture. Understanding and utilizing these zones to maximize crop health and growth will be an important component of agrivoltaic systems.
Food Production
A major concern that comes with agrivoltaics is not knowing how food production will be impacted in an agrivoltaic system. By 2050, food demands are expected to increase by 25 to 56% as compared to 2010, meaning a reduction in food production would be detrimental for society (Chae et al., 2022). Several studies have shown that multiple crops are shade-tolerant and can adapt to receiving less sunlight during the growing process. The radiation interception efficiency (RIE) impacts the shade-tolerance of a crop (Dinesh and Pearce, 2016). Lettuce, for example, is able to adapt to the constant RIE by increasing its leaf area to maximize the amount of incoming solar radiation it receives. A study conducted by Dinesh and Pearce (2016) compared half density and full density agrivoltaic configurations, where half density had two solar PV arrays spaced 6.4 meters apart and full density had four solar PV arrays spaced 3.2 meters apart. The lettuce yield in the spring in the half density configuration was not significantly impacted as compared to the control. This indicates that the time of year, configuration, and density of solar PV panels have an impact on crop yield, and these factors should be considered when determining which crops to use in an agrivoltaic setting.
Chiltepin peppers, jalapeño peppers, and cherry tomatoes are viable candidates for agrivoltaic farms as they thrive in dryland environments (Barron-Gafford et al., 2019). In addition to the increased water use efficiency, cumulative carbon dioxide uptake and fruit production were increased under the solar PV panels for several of these varieties in the study. The cumulative carbon dioxide uptake was increased by 33% and fruit production was increased by 300% under the solar PV panels for chiltepin peppers. While the cumulative carbon dioxide uptake for jalapeño peppers was decreased by 11% in an agrivoltaic settings, the total fruit production was not significantly impacted. For cherry tomatoes, the cumulative carbon dioxide uptake was increased by 65% and the total fruit production was increased by 200% in an agrivoltaic setting as compared to a traditional growing environment. The increased fruit production for chiltepin peppers and cherry tomatoes, and the lack of change within fruit production for jalapeño peppers, indicate food production for certain crops will be unaffected or even improved by the presence of solar PV panels.
Another factor influencing crop production is the height at which the solar PV panels are mounted. In a study conducted on an agrivoltaic farm, a 100 square meter size plot of land with no solar PV modules, low-density modules, and high-density modules was investigated to understand the yield of corn (Sekiyama and Nagashima, 2019). In the low-density configuration, the biomass of corn stover (the leaves and stalks) was 4.9% larger and the corn yield per square meter was 5.6% larger than the control (no solar PV module) configuration. While corn is typically a shade-intolerant crop, the study focused on incorporated stilted solar PV panels instead of installing panels closer to the ground. The stilt-mounted solar PV panels allowed additional sunlight to reach the crops as the sun moves from east to west across the sky. Raising the solar PV panels helped maintain food production in this setting while also producing clean, renewable energy.
While certain crops and plants cannot adapt to and/or do not thrive under the additional shaded conditions provided by PV panels, agrivoltaic farms should focus on growing plants that have shown to be unimpaired by the presence of solar PV panels. Utilizing shade-tolerant crops for agrivoltaic farms will allow crops that need full sunlight to have priority over land that will be used solely for agricultural purposes.
Implementation
In order to successfully implement agrivoltaic systems around the world, several factors must be considered, including density, height, and orientation of solar PV panels, and the type of crop that will be grown and/or the kind of animal that will be grazing. The original conception of agrivoltaics included elevated solar PV panels approximately 2 meters off the ground and an increased distance between rows to allow for maximum solar radiation to reach the ground (Toledo and Scognamiglio, 2021). While mounting solar PV panels higher off the ground allows more sunlight to reach the ground, there are public concerns with reduced visibility by the obstruction of the panels that will potentially impact recreation and tourism. Additionally, there is an increase in cost associated with higher panels. Finding a balance between allowing plants to receive as much sunlight as they need to successfully grow, keeping costs to a minimum, and not creating additional problems with the public will allow agrivoltaic systems to be a successful alternative to independently operated agricultural farms and solar PV farms.
An implementation practice that will help agrivoltaics succeed is utilizing semi-transparent PV (STPV) panels as opposed to the traditional opaque PV panels, which can cause the microclimate under PV panels to change (Gorjian et al., 2022). In a study conducted by Uchanski et al. (2023), plants under STPV panels received more intense light without increasing the soil or air temperatures as compared to opaque panels, which can benefit plants. STPV panels, specifically ones that utilize crystalline silicon technologies, are the most beneficial in terms of cost, stability, and efficiency because they are widely available and can absorb light from a wide spectrum (Gorjian et al., 2022). Accounting for the nuances associated with agrivoltaic systems will be tedious, but necessary for allowing the production of clean energy without adding stress to water resources or food production.
Conclusion
Implementing agrivoltaics is an effective way to work towards mitigating climate change by maximizing land use and reducing greenhouse gas emissions without putting additional stress on water resources or food production. A synergistic relationship occurs when solar PV panels and livestock or crops exist on the same land through landuse maximization. Additional shade from solar PV panels provides relief for livestock in extreme heat conditions and can result in a higher yield for certain crops. Greenhouse gas emissions are reduced through the production of renewable energy, the reduction of emissions from unneeded transportation and production of goods, and the prevention of deforestation. Water is conserved as the additional shade from solar PV panels prevents evaporation from the soil and transpiration from the crops, while the orientation of the panels can be configured to distribute rainwater to drain directly onto the crops. Food production for certain crops can increase under solar PV panels, especially under drought conditions. As the global population continues to increase and competition for land intensifies, utilizing land to its highest potential will ensure that enough resources are produced to sustain the population. Multiple factors need to be considered when installing an agrivoltaic system, such as the configuration of solar PV panels and the type of agriculture that will be incorporated into the system. When these factors are properly implemented, agrivoltaics has the potential to significantly change the agricultural and solar PV industries for the better.