The world’s population expected to grow by 1.2 billion people within 15 years (1), coupled with an increasing demand for meat, eggs, and dairy products, that consume over 70 % of fresh water for cultivation (2), and electricity demand that is growing even faster than population growth (3). Moreover, the rise in the earth’s temperature is also an increasing problem, leading to climate change.
How could technological advancement in PV fix such issues?
Combining the two important needs of humans i.e., energy and food could be a vital solution to all problems. The concept of producing food and electricity together from the same area is known as Agri-photovoltaic (4). Agri-photovoltaic is a method in which electricity is generated from a photovoltaic system while food is grown on the same piece of land. The concept of Agri-Photovoltaic, also termed as APV, was developed by the German physicist Adolf Goetzberger in 1981 (5). Agri-Photovoltaic is also called as agrovoltaics.
The figure below represents differences in land use efficiency of a potato farm with and without integration of a PV power plant on the same land.
Figure 1: Through the combine use of land, the land use efficiency with APV on the test site in Heggelbach is up to 186 % (9).
Growth of the crops depend on many factors, such as the amount of sunlight, soil quality, amount of water etc. However, the amount of sunlight needed for plant growth is the most important factor. Most studies have shown that reduction in solar irradiance is a main factor for decrease in yield. Shade tolerant crops are able to reach their light saturation point at a lower total solar irradiance, such as crops even having a better yield performance under shade (6).
Light saturation Point
Plants need light for photosynthesis, and the ability to use incident light varies from plant to plant. Depending on the plant species, the rate of photosynthesis stagnates at a certain light intensity. The light saturation point is an important criterion for determining the suitability of plants for APV, as from this point on, the plants are no longer able to convert additional light into photosynthesis output and may even be damaged. The lower the light saturation point of a plant, the more suitable it is for cultivation under an APV system (7).
Figure 2: Schematic: Photosynthesis rate depending on the light intensity for sun and shade plants (9).
Types of APV Systems
There three types of APV, which are presented below.
1. High Elevated APV
One of the options to integrate PV systems into agriculture on the same piece of land are high elevated APVs. PV panels are installed at a high altitude, where rows of stable (steel) supports are placed at a freely adjustable distance. The distance between the rows is chosen in such a way that the plant fits into the surrounding agricultural practice and has a minimum impact. The system is erected using a wide variety of foundations, such as screw or pile-driven foundations being anchored so deeply into the ground to give the overall structure sufficient stability even under varying wind loads (8).
Figure 3: High Elevated APV System (own translation of (8)).
The height of the structure and the row spacing depend on chosen crop and then accordingly machinery is employed on the field afterwards. In this type of installation, a large row spacing, and a high elevation is chosen to ensure unrestricted access for large agricultural machinery for harvesting. Theoretically, the clearance height can be freely selected, but it usually corresponds to at least four meters. Similarly, the row spacing can also be freely chosen and depends on how wide the machines are for field cultivation and harvesting. The PV-plants are usually oriented the , south or southwest. The choice of crops can be relatively free in this type of application, since shading of the modules on the crops is excluded due to the high elevation (8). The main disadvantage of this type of APV is that the cost of building the mounting structure is relatively high compared to conventional PV system.
Figure 4: High Elevated APV System (16).
2. Vertical APV
Vertical APV is a type in which solar modules are installed in a vertical orientation and bifacial solar modules are mounted above and next to each other, with the lower edge of the bottom module row twenty to eighty centimetres above the ground. The rows can be up to three meters high. An even higher mounting offers too large attack surface for crosswinds and is not realised for reasons of statics, since the foundations would have to be disproportionately strong. Consequently, economical operation would not be rational due to the rising installation costs. Therefore, two modules are usually installed one above the other as with three or more modules, the wind load increases significantly. In addition, the row spacing would have to be very large to prevent self-shading of the modules. This would lead to a reduced area performance. In this type of system, bifacial modules are used to generate maximum electricity from both side of the panels. These modules usually have a bifaciality factor of 80 – 85 %. That means the back side of these modules can generate 80 – 85 % of power of the power of the front side (8).
The main advantage of vertical APV system is the peak energy production occurs in the morning and evening when the electricity prices are higher. Compared to conventional PV systems, which generates their maximum during midday, when the sun is at zenith.
Figure 5: Vertical APV System (own translation of (8)).
Figure 6: Vertical APV System (13).
3. High row-spaced APV
The third option of the APV system is a system in which the panels are installed in such a way the row spacing is kept maximum for agricultural use. This type of design was developed in Althegnenberg in North Bavaria and has only been installed there so far. In this project, PV panels were installed on a single-axis tracker mounting structure and the row spacing of 14 m was kept significantly larger than conventional fixed tilt PV systems. This is also helpful for mechanised cultivation of the intervening agricultural land. In addition, the elevation is higher than conventional tracked PV systems to minimise shading of the modules by the crops (8).
Figure 7: High row-spaced APV System (own translation of (8)).
In normal operation, the tracking function is used to optimise the electricity yield while influencing the growth of the intervening plants as little as possible. For agricultural cultivation, the modules can be rotated by more than 60 degrees. In this almost vertical orientation, the farmer has more space to sow, cultivate and harvest the intermediate areas without interference. This system design is therefore suitable for use in many traditional cash crops. However, even with this type of system, the choice of suitable crops to be grown between the rows of modules is limited. Higher elevation to PV modules is possible, but only up to a certain limit. Such an elevation significantly increases the cost of mounting system and consequently worsens the overall economic efficiency of the plant. The main advantage of this type of APV is the cost of PV system is almost equal to the cost of conventional PV systems (8).
Figure 8: High row-spaced APV System (14).
Potential of APV in Germany
Figure 9: Land allocation in Germany (9)
APV has the most potential of all the integrated PV systems. On balance, just about 4 % of arable land is needed to cover the current total electricity demand (final energy) in Germany (about 500 GWp installed capacity). According to an initial evaluation by Fraunhofer ISE, APV has a technical potential of around 1700 GWp in Germany alone, based primarily on shade-tolerant crops. If only ten percent of these 1700 GWp were used, the current PV capacity in Germany would more than triple. From an energy perspective, dual use of arable land for cultivation and energy production is much more efficient than growing plants for energy alone, such as rapeseed for biodiesel, which presently amounts to 14 % of the agricultural land in Germany (9).
Figure 10: Worldwide installed capacity of APV systems (15).
As the potato is considered as a shade-tolerant crop, an experiment (without a PV system) was conducted by the University of Hohenheim to evaluate the impact of four different levels of shading (0 %, 12 %, 26 %, 50 %) on potato growth, tuber yield and quality parameters under the given total irradiance of southwestern Germany. The field experiments were carried out over three years from 2015 to 2017. The results show that the yield can be achieved with reduced solar radiation of up to 26% (6) and thus give an extremely good prognosis for potato cultivation in combination with a PV system.
1. High Elevated APV
In the research project APV-RESOLA conducted by Fraunhofer ISE, a sequence of several crops consisting of a grass/clover mixture, winter wheat, potatoes, and celery were grown under biodynamic principles at the pilot plant in Heggelbach. The suitability ability of growing plants under APV was successfully demonstrated. Potato yields were also found to be highly dependent on weather fluctuations, with crop yield under the APV system varying from -20 % in 2017 to +11 % in the hot, dry year of 2018, compared to the reference system (without solar panels). The initial results for yields on the test plots in 2017 were promising: For the grass-clover mixture, crop yield only fell slightly by 5.3 % compared to the reference plot. For potatoes, wheat, and celery, on the other hand, the yield reduction due to shading was somewhat more pronounced at 18 – 19 % percent. In the very dry year of 2018, higher yields were obtained for winter wheat, potatoes, and celery than to the reference plots without PV modules. Celery benefited the most, with a yield increase of twelve percent. The yields of potatoes and winter wheat increased by eleven and three percent, respectively. For the grass-clover mixture, the yield dropped by 8 % compared to the reference plot (9).
2. High Row spaced APV
Another theoretical study has recently been conducted by SRH Berlin University of Applied Science on the 15 ha of potato farm at Künzelsau, Germany. The focus was on the performance analysis of single-axis trackers and fixed inclined PV systems. The row spacing between the fixed tilt PV systems and single axis tracking system was set at 13 m and 15 m, respectively, to avoid interrow shading and smooth movement of agricultural equipment. The result showed that potato yield under a fixed APV system is 106,14 tons (compared to 165 tons yield in open field), while the APV system provides 9,403 MWh of electricity per year, implicating that the utilisation of land is more than 100 %. The increase in land use efficiency is measured by the land equivalent ratio (LER). This concept is adopted from agroforestry to calculate the land use efficiency by APV System (see Fig. 1). The LER of the fixed tilt APV system is 1.26, meaning that a 10 ha APV system would produce as much potato and electricity as 12.6 ha of mono productions. Similarly, the single axis tracker APV system provides potato crop yield of 113 tons and 8400 MWh of electricity per year. The LER is 1.31 (10), implying that a 10 ha APV system would produce as much potato and electricity as 13.1 ha of mono productions. This study concludes that the single axis tracker APV system is better compared to fixed tilt APV system.
Recently, new trails have been created for special crops such as raspberry cultivation in the Netherlands and grapes in France by the German company BayWa r.e. and its subsidiary GroenLeven. In this trail’s, the height structure is integrated into the existing support. The modules were built like a roof over the plants and protect them from heavy rainfall and excessive solar radiation. The row spacing is decided on the basis on sufficient solar irradiance for the crop. The type of elevation creates a favourable microclimate that cools off during the day through a chimney effect and can maintain higher temperatures than the surrounding area at night. The results show that this type of PV installation not only protects the crops from weather events such as heat or hail, but also creates an environment that stimulates growth and yields. However, the cost of these structures is very high and therefore not suitable for particularly large agricultural areas (8).
Figure 11: Raspberry cultivation under PV in France (15).
A preliminary study by Fraunhofer ISE at a site in the Indian federal state of Maharashtra indicates that the effects of shading and reduced evaporation from APV systems can boost tomato and cotton yields by up to 40 % (11). Researchers expect the land use efficiency for this region to nearly double (9).
International Development in APV
China has the highest share of APV worldwide. With a total of around 2.8 GWp of APV installations built, APV in China accounts for roughly 1.9 GWp (as of 2020). China is home to the largest APV system in the world, with photovoltaic modules with a capacity of 700 MWp towering over berry crops on the edges of the Gobi Desert, contributing to the fight against desertification (9).
Japan and South Korea are also taking advantage of the opportunities that photovoltaics have on agriculture. However, both countries are currently focusing on smaller systems. In Japan, more than 1800 systems have currently been installed. In South Korea, where migration to cities is rampant, the government is planning to build 100.000 APV systems on farms as a retirement provision for farmers (monthly income of around 1.000 US-Dollars from electricity sales), in order to slow down the extinction of farm communities (9).
France has also been promoting APV with its own tenders since 2017; an installation capacity of 15 MW per year is planned. The focus of the award of the contracts is mainly on the offer price and the innovative nature of the projects. The maximum project size is 3 MWp. In the first public tenders that have already taken place, only contracts for greenhouses were awarded, in the second and third round, however, 140 MWp each is to be tendered for APV systems with an output between 100 kWp and 3 MWp. Granted projects can obtain feed-in tariff for 20 years. 40 MWp were awarded for APV projects in March 2020 (9).
APV systems are also being installed in the USA. For example, a research plant in Massachusetts was able to demonstrate the dual use of crop production and power generation. The state provided funding for dual use starting in 2018 (9). A study conducted by Michigan Technological University shows an increase in PV capacity between 40 and 70 GW in the case that only lettuce cultivation in the USA is converted to APV systems (4).
Challenges: Barriers to Implementation
While the technical and economic feasibility of APV has been demonstrated in many countries, the current regulatory framework is probably the greatest hurdle to realising its potential. In Germany, for example, the dual use of land for photovoltaics and agriculture is currently not defined in law and the Renewable Energy Sources Act (EEG) does not provide for adequate compensation. Social acceptance is certainly another challenge for the use of APV systems in some regions. The early involvement of stakeholders and the citizens of municipalities on whose territory the planned APV system is to be built is therefore one of the important fields of action. In order to be able to make more reliable statements about the different approaches, possible synergy effects and questions of acceptance, it is necessary to install the first larger pilot plants and to carry out further research projects. In this way, not only the ecological and economic opportunities and risks but also the non-technical, social success factors can be examined in greater detail. At the same time, approaches can be developed to promote the willingness to invest, and stakeholders, citizens and economic enterprises can be encouraged to develop creative solutions.
The application of APV systems offers a range of opportunities that vary according to regional and climatic conditions. The real added value of the APV technology is that it enables the simultaneous production of food and energy, which offers undeniable economic benefits for farmers, with additional potential synergistic effects.
APV can be an important component of future agricultural systems and address some of the most important current and prospective societal and environmental challenges, such as climate change, global energy demand, food security and land use (12).
By: Omkar R. Paygude
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