Agrivoltaic

Agrivoltaics (agrophotovoltaics, agrisolar, or dual-use solar) is the dual use of land for solar energy and agriculture. The technique was first conceived by Adolf Goetzberger and Armin Zastrow in 1981.

Many agricultural activities can be combined with solar, including plant crops, livestock, greenhouses, and wild plants to provide pollinator support. Agrivoltaic systems can include solar panels between crops, elevated above crops, or on greenhouses.

Solar panels help plants to retain moisture and lower temperatures, and can provide shelter for livestock animals. The dual use of land can also provide a diversified income stream for farmers.

Solar panels block light, which means that the design of dual use systems can require trade-offs between optimizing crop yield, crop quality, and energy production. Some crops and livestock benefit from the increased shade, lessening or even eliminating the trade-off.

Definition

Agrivoltaic practices vary from one country to another. In Europe and Asia, where the concept was first pioneered, the term ''agrivoltaics'' is applied to dedicated dual-use technology, generally a system of mounts or cables to raise the solar array some five metres above the ground in order to allow the land to be accessed by farm machinery, or a system where solar paneling is installed on the roofs of greenhouses.

By 2019, some authors had begun using the term ''agrivoltaics'' more broadly, so as to include any agricultural activity among solar arrays, including conventional solar arrays not originally intended for dual use. As an example, sheep can be grazed among conventional solar panels without any modification. Likewise, some conceive agrivoltaics so broadly as to include the mere installation of solar panels on the roofs of barns or livestock sheds.

System designs

The three basic types are:

* Interleaved arrays and crops
* Arrays elevated above crops/livestock
* Arrays on greenhouses

All three systems have several variables used to maximize solar energy absorbed in both the panels and the crops. The main variable taken into account for agrivoltaic systems is the tilt angle of the solar panels. Other variables taken into account for choosing the location of the agrivoltaic system are the crops chosen, panel heights, solar irradiance and climate of the area.

In their initial 1982 paper, Goetzberger and Zastrow published a number of ideas on how to optimize agrivoltaic installations.

* orientation of solar panels in the south for fixed or east–west panels for panels rotating on an axis,
* spacing between solar panels for sufficient light transmission to ground crops,
* elevation of the supporting structure of the solar panels to homogenize the amounts of radiation on the ground.

Experimental facilities often have a control agricultural area. The control zone is exploited under the same conditions as the agrivoltaic device in order to study the effects of the device on the development of crops.

Fixed solar panels over crops

The most conventional systems install fixed solar panels on agricultural greenhouses, above open fields crops or between open fields crops. It is possible to optimize the installation by modifying the density of solar panels or the inclination of the panels.

Vertical systems

Vertically mounted agrivoltaic systems with bifacial photovoltaic modules systems have been developed. Most agricultural fences can be used for vertical agrivoltaics. Overall, at least one PV module between posts is acceptable for most fences for $0.035/kWh for racking on existing fencing in the U.S.; although the yield for a vertical PV is only 76% facing south, the racking cost savings enable fence-retrofit agrivoltaics to often produce lower levelized cost electricity. For fence PV, microinverters had better performance when the cross-over fence length was under 30 m or when the system was small, whereas string inverters were a better selection for longer fences. Simulation results show that the row distance between bifacial photovoltaic module structures significantly affects the photosynthetically active radiation distribution. Next2Sun has commercialized vertical agrivoltaic systems in Europe. Open-source vertical wood-based PV racking has been designed for farms that is (i) constructed from locally accessible (domestic) renewable and sustainable materials, (ii) able to be made with hand tools by the average farmer on site, (iii) possesses a 25-year lifetime to match PV warranties, and (iv) is structurally sound, following Canadian building codes to weather high wind speeds and heavy snow loads. The results showed that the capital cost of the racking system is less expensive than the commercial equivalent and all of the previous wood-based rack designs, at a single unit retail cost of CAD 0.21.

Integrated systems

A standalone solar panel integrated system using a hydrogel can work as an atmospheric water generator, pulling in water vapor (usually at night) to produce fresh water to irrigate crops which can be enclosed beneath the panel (alternatively it can cool the panel).

Dynamic agrivoltaic

The simplest and earliest system was built in Japan using a rather flimsy set of panels mounted on thin pipes on stands without concrete footings. This system is dismountable and lightweight, and the panels can be moved around or adjusted manually during the seasons as the farmer cultivates the land. The spacing between the solar panels is wide in order to reduce wind resistance.

Some newer agrivoltaic system designs use a tracking system to automatically optimize the position of the panels to improve agricultural production or electricity production.

In 2004 Günter Czaloun proposed a photovoltaic tracking system with a rope rack system. Panels can be oriented to improve power generation or shade crops as needed. The first prototype was built in 2007 in Austria. The company REM TEC deployed several plants equipped with dual-axis tracking systems in Italy and China. They have also developed an equivalent system used for agricultural greenhouses.

In France, Sun'R and Agrivolta companies are developing single-axis tracking systems. According to them, their systems can be adapted to the plant needs. The Sun'R system is east–west axis tracking system. According to the company, complex plant growth models, weather forecasts, calculation and optimization software are used. The device from Agrivolta is equipped with south-facing solar panels that can be removed by a sliding system. A Japanese company has also developed a tracking system to follow the sun.

In Switzerland, the company Insolight is developing translucent solar modules with an integrated tracking system that allows the modules to remain static. The module uses lenses to concentrate light onto solar cells and a dynamic light transmission system to adjust the amount of transmitted light and adapt to agricultural needs.

The Artigianfer company developed a photovoltaic greenhouse whose solar panels are installed on movable shutters. The panels can follow the course of the sun along an east–west axis.

In 2015 Wen Liu from the University of Science and Technology in Hefei, China, proposed a new agrivoltaic concept: curved glass panels covered with a dichroitic polymer film that selectively transmits blue and red wavelengths which are necessary for photosynthesis. All other wavelengths are reflected and concentrated on solar cells for power generation using a dual tracking system. Shadow effects arising from regular solar panels above the crop field are eliminated since the crops continue to receive the blue and red wavelength necessary for photosynthesis. Several awards have been granted for this new type of agrivoltaic, among others the R&D100 prize in 2017.

The difficulty of such systems is to find the mode of operation to maintain the good balance between the two types of production according to the goals of the system. Fine control of the panels to adapt shading to the need of plants requires advanced agronomic skills to understand the development of plants. Experimental devices are usually developed in collaboration with research centers.

Greenhouses with spectrally selective solar modules

Potential new photovoltaic technologies which let through the colors of light needed by the interior plants, but use the other wavelengths to generate electricity, might one day have some future use in greenhouses. Selecting the optimum color tint and transparency for crop yield and electricity generation requires experimentation. There are prototypes of such greenhouses. "Semi-transparent" PV panels used in agrivoltaics increase the spacing between solar cells and use clear backsheets enhancing food production below. In this option, the fixed PV panels enable the east–west movement of the sun to "spray sunlight" over the plants below, thereby reducing "over-exposure" due to the day-long sun as in transparent greenhouses, as they generate electricity above.

Solar grazing

Perhaps the easiest use of agriculture and PV is allowing sheep or cows to graze under solar panels. The sheep control vegetation, which would otherwise shade the PV. Sheep even do a more thorough job than lawnmowers as they can reach around the legs of the structures. In return, sheep or goats receive forage and a shady place to rest. Sheep may be cheaper than mowing. In general PV system operators pay shepherds to transport sheep. Some experimental sheep agrivoltaics found higher herbage mass available in solar pastures, and while others had lower herbage, this was offset by higher forage quality, resulting in similar spring lamb production to open pastures. Agrivoltaics also can be used to shade cows. Solar grazing is popular in the U.S. and an organization has formed to support it.

Effects

The solar panels of agrivoltaics remove light and space from the crops, but they also affect crops and land they cover in other ways. Two possible effects are water and heat.

In northern latitude climates, agrivoltaics are expected to change the microclimate for crops in both positive and negative manners with no net benefit, reducing quality by increasing humidity and disease, and requiring a higher expenditure on pesticides, but mitigating temperature fluctuations and thus increasing yields. In countries with low or unsteady precipitation, high temperature fluctuation and fewer opportunities for artificial irrigation, such systems are expected to beneficially affect the quality of the microclimate.

Water

In experiments testing evaporation levels under solar panels for shade resistant crops cucumber watered by irrigation in a California desert, a 14–29% savings in evaporation was found, and similar research in the Arizona desert demonstrated water savings of 50% for certain crops. Australian trials found that solar panels can keep grass watered through condensation below the panels.

Heat

A study was done on the heat of the land, air and crops under solar panels for a growing season. It was found that while the air beneath the panels stayed consistent, the land and plants had lower temperatures recorded.

Advantages

Dual use in land for agriculture and energy production could alleviate competition for land resources and allow for less pressure to develop farmland or natural areas into solar farms, or to convert natural areas into more farmland. Initial simulations performed by Dupraz et al. in 2011, where the word 'agrivoltaics' was first coined, calculated that the land use efficiency may increase by 60–70% (mostly in terms of usage of solar irradiance). The central socio-political opportunities of agrivoltaics include income diversification for farmers, enhanced community relations and acceptance for PV developers, and energy demand and emissions reduction for the global population.

A large advantage of agrivoltaics is that it can overcome NIMBYism for PV systems, which has been becoming an issue. A U.S. survey study assessed if public support for solar development increases when energy and agricultural production are combined in an agrivoltaic system and found 81.8% of respondents would be more likely to support solar development in their community if it integrated agricultural production. Dinesh et al.'s model claims that the value of solar generated electricity coupled to shade-tolerant crop production created an over 30% increase in economic value from farms deploying agrivoltaic systems instead of conventional agriculture.
Agrivoltaics may be beneficial for summer crops due to the microclimate they create and the side effect of heat and water flow control. Agrivoltaics is environmentally superior to conventional agriculture or PV systems; a life cycle analysis study found the pasture-based agrivoltaic system features a dual synergy that consequently produces 69.3% less greenhouse gas emissions and demands 82.9% less fossil energy compared to non-integrated production.

Increased crop yield has been shown for a number of crops:

* Basil
* Broccoli
* Celery
* Chiltepin peppers
* Corn/maize
* Lettuce
* Pasture grass
* Potatoes
* Spinach
* Strawberries
* Tomatoes
* Wheat
Sheep grazing around solar panels in Australia produce a higher volume of wool, at better quality.

Disadvantages

A disadvantage often cited as an important factor in photovoltaics in general is the substitution of food-producing farmland with solar panels. Cropland is the same type of land on which solar panels are the most efficient. Despite allowing for some agriculture to occur on the solar power plant, agrivoltaics may be accompanied by a drop in production. Although some crops in some situations, such as lettuce in California, do not appear to be affected by shading in terms of yield, some land will be sacrificed for mounting structures and systems equipment.

Agrivoltaics will only work well for plants that require shade and where sunlight is not a limiting factor. Shade crops represent only a tiny percentage of agricultural productivity. For instance, wheat crops have been shown to produce lower yield in a low-light environment.

Agrivoltaic greenhouses are inefficient; in one study, greenhouses with half of the roof covered in panels were simulated, and the resulting crop output reduced by 64% and panel productivity reduced by 84%.

A study identified barriers to adoption of agrivoltaics among farmers that include (i) desired certainty of long-term land productivity, (ii) market potential, (iii) just compensation and (iv) a need for predesigned system flexibility to accommodate different scales, types of operations, and changing farming practices.

Agrivoltaics require a large investment, not only in the solar arrays, but in different farming machinery and electrical infrastructure. The potential for farm machinery to damage the infrastructure can also drive up insurance premiums as opposed to conventional solar arrays. In Germany, the high mounting costs could make such systems difficult to finance for farmers based on convention farming loans, but it is possible that in the future governmental regulations, market changes and subsidies may create a new market for investors in such schemes, potentially giving future farmers completely different financing opportunities.

Photovoltaic systems are technologically complex, meaning farmers will be unable to fix some things that may break down or be damaged, and requiring a sufficient pool of professionals. In the case of Germany the average increase in labour costs due to agrivoltaic systems are expected to be around 3%. Allowing sheep to graze among the solar panels may be an attractive option to extract extra agriculture usage from conventional solar arrays, but there may not be enough shepherds available.

Economics

The shade produced by systems located on top of crops can reduce production of some crops, but such losses may be offset by the energy produced. Many experimental plots have been installed by various organisations around the world, but no such systems are known to be commercially viable outside China and Japan. Agrivoltaics is more advantageous in arid regions.

The most important factor in the economic viability of agrivoltaics is the cost of installing the photovoltaic panels.

History

Adolf Goetzberger, founder of the Fraunhofer Institute for Solar Energy Systems in 1981, together with Armin Zastrow, theorised about dual usage of arable land for solar energy production and plant cultivation in 1982, which would address the problem of competition for the use of arable land between solar energy production and crops. The light saturation point is the maximum amount of photons absorbable by a plant species: more photons will not increase the rate of photosynthesis (see also photorespiration). Recognising this, Akira Nagashima also suggested combining photovoltaic (PV) systems and farming to use the excess light, and developed the first prototypes in Japan in 2004.

The term "agrivoltaic" may have been used for the first time in a 2011 publication. The concept has been called "agrophotovoltaics" in a German report, and a term translating as "solar sharing" has been used in Japanese. Facilities such as photovoltaic greenhouses can be considered agrivoltaic systems.

In Europe in the early 2000s, experimental photovoltaic greenhouses have been built, with part of the greenhouse roof replaced by solar panels. In Austria, a small experimental open field agrivoltaic system was built in 2007, followed by two experiments in Italy. Experiments in France and Germany then followed.

See also

* Floating solar
* Growth of photovoltaics
* Solar canal

References

External links

American Solar Grazing Association

Conference

{{Solar energy

Agriculture
Solar energy