Agrovoltaics: A synergistic relationship between agriculture and electricity production, but is it true?

The development of photovoltaic energy generators requires the use of large areas of land. Agrovoltaic systems, if designed to optimize land use, harmonize agricultural and electricity production on the same unit of land.

The development of renewable energy aims to meet global energy demand by replacing fossil fuels. Nonetheless, it requires large areas, often agricultural.

At the same time, food security is threatened by the impact of climate change and the growth of the world population. This is creating growing competition for limited land resources. In this context, the combination of agriculture and photovoltaic generators - often referred to as agro-photovoltaic (APV) or agrivoltaic systems - is seen as an opportunity for the synergistic combination of renewable energy and food production.

Although this technology has already been applied in various commercial projects, its practicability and impact on agricultural production today is the subject of technological and design studies and efforts. In this article, we want to briefly represent the current state of the art and the potential opportunities for the application of APV systems.

Our experience leads us to say that cultivation under APV, if well designed, can lead to an insignificant drop in yields even when solar radiation is reduced by as much as a third under photovoltaic panels.

While, through the combined production of energy and crops, APV can increase land productivity up to 70%. Given the impacts of climate change and conditions in arid climates, potential benefits for agricultural production are likely through additional shading and improved water management. In addition, APV increases the value of agriculture and can contribute to the decentralized and off-grid electrification of some rural and developing areas, thereby further improving agricultural productivity. As such, APV can be a valuable technical approach to more sustainable agriculture, helping to meet current and future energy and food production needs while at the same time saving land resources. 

The concept of APV

The concept of agro-photovoltaics (APV) had already been introduced in 1982 by Goetzberger and Zastrow as a means of modifying solar power plants and at the same time allowing further agricultural production on the same area. The idea was to raise the solar collectors 2m above the ground and increase the distance between them to avoid excessive shading of the crops. They speculated that these systems would only capture a third of the incoming radiation and that further technical improvements could increase their suitability for application in agricultural production.

It took about three decades before this concept, referred to as agrofotovoltaics, agroPV, agrivoltaics or solar sharing, was implemented in various projects and pilot plants around the world.

Calculations have shown that application with this technical approach can increase farm incomes by more than 30%, when yield losses due to shading effects are minimized by the selection of suitable crops. Later, other researchers worked on the Land Equivalent Ratio (LER), a method for evaluating the productivity of a companion system compared to a monoculture system, to determine the advantages of a dual-use APV system over a single crop and production. photovoltaic. Their simulations demonstrated that overall land productivity can be increased up to 70% in APV systems.

In a recent modeling study concerning the production of corn for biogas, Amaducci, Colauzzi and Yin, three researchers (the first two of the Catholic University of the Sacred Heart of Piacenza and the third of the Department of Plant Sciences, Wageningen University & Research - Netherlands (in 2018) demonstrated that renewable energy soil productivity can even be doubled by APV compared to separate maize and energy production with ground-mounted photovoltaic modules. In 2010, a research group led by INRA's Dupraz, UMR System France, built a test facility for the APV to test their hypothesis. To find a well-balanced combination of food and energy production, they tested two different densities of photovoltaic modules. From this test they found that the yield photovoltaic increases with the density of the panels, while the optimal conditions for the simultaneous production of crops were found with the solution at low density of photovoltaic modules. In this test the solar panels were raised to a clear height of 4m to allow the passage of common agricultural machinery.

The technical characteristics of the APV systems are constantly improved and vary according to the regions and companies. Some APV projects already use mobile photovoltaic modules that enable solar tracking. These maximize the photovoltaic yield and at the same time improve the availability of light allowing sufficient crop growth. This approach has recently been studied with 1-axis orientable photovoltaic systems and different tracking settings. The study showed that both energy and agricultural production performance can actually be further increased by applying dynamic photovoltaic modules. In normal solar tracking mode, the modules automatically adapted to the solar altitude, optimizing electricity production and also increasing solar radiation at the plant level compared to fixed photovoltaic modules. To increase the radiation to the crop and thus further improve its productivity, they also tested a controlled tracking mode that incorporates diurnal changes in solar radiation. In the morning and late afternoon, the position of the photovoltaic panels was changed to reduce the shading of the crops, while at solar noon the shading was increased to reduce evapotranspiration and the negative effects of high temperature and excessive radiation on plant growth. As a result, crop biomass increased with controlled monitoring, but obviously electricity production decreased compared to normal solar tracking mode. Solar tracking technology has already been implemented in various commercial APV plants and has recently also been investigated in photovoltaic greenhouses. However, the amount of radiation available below the APV array is affected more by the density of the panel than by the mobility of the panel.

In addition to improving the efficiency of light use for both photovoltaics and agricultural production, mobile photovoltaic panels can also be used to improve the distribution of rainfall under APV systems. The use of APV systems has recently also been considered in cultivation systems such as viticulture and in intensive fruit production, where the use of support structures is already common practice and synergistic effects may exist. A study, by US researchers from Michigan Technological University's Department of Electrical & Computer Engineering, models the APV potential of wineries in India and found that the annual income of these farms could be multiplied compared to conventional farms without APV, although keeping the grape yields unaltered. Now, extrapolating on a national scale (i.e. taking into account the entire vineyard area in India which is about 34,000 hectares), they have calculated an APV production of 16,000 GWh, enough to meet the energy demand of over 15 million people. .

Thus, the most promising potential of APV systems can be easily predicted in arid regions where various synergistic effects can occur. Agricultural production can benefit from greater water savings thanks to the reduction of evapotranspiration and the negative effects of excessive radiation, while economic profitability increases and rural electrification is made possible, moreover. the reduced evaporation of the soil in the presence of APV can also decrease the yield losses in dry years and improve the stability of the crop yield.

Fraunhofer Institute for Solar Energy Systems ISE - Agrophotovoltaic pilot system in Heggelbach

Existing projects and technologies

In recent years, several commercial and research APV structures have been built.

Since 2004, numerous small-scale APV plants have been built in Japan. These systems, called 'solar sharing', consist of photovoltaic panels mounted on poles with a height of 3 m from the ground. They combine solar energy production with the cultivation of various local food crops such as peanuts, sweet potatoes, eggplant, cucumbers, tomatoes, taros and cabbage.

Some APV projects have also been implemented in Europe. In addition to various research facilities in France and Germany, three commercial APV projects, patented as 'Agrovoltaic', have been implemented in northern Italy.

Installed systems have capacities up to 1500 kWp using mounted solar modules (4–5 m high) with solar tracking technology. Another APV field in Abruzzo uses 67 stand-alone solar trackers with various underlying crops such as tomatoes, watermelons and wheat and generates a total power of 800 kWp.

Fraunhofer Institute for Solar Energy Systems ISE - Agrophotovoltaic pilot system in Heggelbach

In Germany, the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) is at the forefront of APV research. In 2016, the Fraunhofer ISE built an APV research facility in southern Germany. It has a total size of 0.3 ha and a capacity of 194 kWp. The solar panels are mounted on stilts with a vertical distance of 5m. The structure has a number of specific features to allow uniform light distribution for simultaneous optimization of photovoltaic and photosynthetic yield. The fixed photovoltaic panels are oriented in the south-west direction with an inclination angle of 20 ° and a distance between the rows of 6.3 m. The photosynthetically active radiation (PAR) available to the underlying plants is expected to reach values of approximately 60% of the total PAR above the array (with variations between winter and summer). Double-sided photovoltaic modules are used to further improve the photovoltaic energy yield. These, in fact, are able to use the light from both sides and therefore also intercept the reflected radiation. The system was built on a arable field of a farm managed according to the principles of biodynamics in order to study its practical suitability for agricultural machinery and the impact on crop rotation.

What was noted:

  • Application of photovoltaic panels can lead to increased water runoff, causing unbalanced water distribution with distinct wetlands below the bottom edge of the panel and sheltered areas directly below the panel. During heavy rains, heavy runoff from photovoltaic modules can lead to soil erosion and the formation of badlands. However, the problem only occurred in the early stages of wheat, potato and turnip development, i.e. when the soil was not covered or was barely covered by crops.
  • The density of photovoltaic modules must be lower than that of conventional ground-mounted photovoltaic systems to maintain acceptable agricultural yields. A row spacing of approximately 3m is assumed to be adequate to allow sufficient light to reach the crop canopy while achieving satisfactory energy yields.
  • Orienting the photovoltaic modules to the south-west or south-east was more suitable for achieving uniform light conditions under the panels. This also resulted in a reduction in the electrical output of the 5% compared to the densities of conventional south-facing ground systems. 
  • It should be noted that a small angle of inclination can lead to more dust deposits as these are not easily washed off by rain. The same goes for the snowpack. Indeed it is appropriate modify the inclination of the panel during certain periods of the year that correspond to stages of development of crops sensitive to light. The mobile photovoltaic modules allow you to automatically control the solar tracking to meet both the specific needs of the crops and the diurnal and seasonal variations in terms of light intensity.
  • A particular concern is the decline in electrical performance through the deposition of dust on the surface of the panel as a result of agricultural management, such as tillage and harvesting.

 

Microclimatic alterations and their impact on crops

In addition to the field management aspects mentioned above, one of the most important problems for agricultural practice under an APV array is the alteration of the microclimate conditions and the resulting consequences for the cultivation of crops.

While the reduction in solar radiation under the APV cover should be the most noticeable change, many other microclimatic factors could also be altered. 

On some days with low wind speeds or strong solar radiation, temperatures under the panel tend to be higher. However, other studies found that the soil temperature and the maximum air temperature decreased in shade compared to full sun conditions. This inconsistency may be due to the direct effects of solar panels on air temperature observed in studies with ground-mounted solar farms and the heterogeneous shading conditions below the APV systems.

Excessive heat can have negative effects on crops, as has been shown for example for potatoes, where marketable tuber yields have decreased. Ltemperature can affect the nutritional quality, for example the fatty acid composition of rapeseed and the starch content of potatoes. While air temperatures tended to be higher, soil temperatures fell below the APV, while the temperatures of durum wheat, lettuce and cucumber crops grown under the APV decreased during the day, increased during the night.

In addition to the potential problems related to water distribution, the water balance in general can change with an APV system. LEvapotranspiration is reduced under photovoltaic arrays due to both the decrease in evaporation and transpiration as a consequence of the reduction in light. However, they found that the effect depended on the cultivated species, as evaporation is favored by the crop coverage rate. Under the APV, the crop coverage rate increased for lettuce, for example, but decreased for cucumber. So APV systems can improve water use efficiency (WUE) and help prevent water loss in dry climates if suitable crop species are chosen. This is in agreement with the results for citrus grown under shade nets, where the WUE increased with less solar irradiation.. LGrowing maize with APV in non-irrigated conditions reduces soil evaporation and also increases average yield. The highest yield variation was obtained in full sun conditions. Therefore, APV can lead to yield stabilization, mitigating yield losses in drought years

Source  

Agrivoltaic systems to optimize land use for electric energy production
Authors: Stefano Amaducci, Xinyou Yinb, Michele Colauzzi.

How does a shelter of solar panels influence water flows in a soil – crop system?
Authors H. Marrou L. Dufour J.Wery

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