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| Sat, 21st Sep 2019 17:44:00 |
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Agrivoltaics : a win-win system to combine food and energy production? |
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Agrivoltaics are innovative production systems combining energy production and food production. Below an elevated solar array, crops can be grown in the partial and fluctuating shade. This audacious idea first emerged in 1982, and was published by a duo of researchers in physical sciences (Goetzber and Zaztrow, 1982). Unfortunately, the topic did not retain much interest at that time and the actual possibility of growing crops under solar panels was not explored further before another couple of decades.
It’s only in the early 2010 that new global and local challenges brought agrivoltaics back to the scientific stage. With the XXIst century, global awareness about climate change arose and progressively became a crucial challenge together with food security. To bend down carbon emission, first generation biofuel brought quickly an easy alternative to fossil fuels. But deeper assessment soon revealed the carbon cost of biofuel production and the growing competition between biomass crop and food crop. Less than decade later, solar energy was estimated to be “cleaner” but soon triggered a new crisis. In different countries, especially in Southern Europe, farmers started protesting against land recuperation by solar investors while solar plants flourished in the landscape. In late 2009, a partial ban on solar arrays construction on agricultural land was enforced in France.
The tense and competing context called for a new paradigm, to overcome land sharing and splitting between different productions. The idea of sharing landwas proposed by a private French company, Sun’R SAS www.sunr.fr). The National Institute of Agronomic Research in France (INRA) accepted to explore these systems, considering agrivoltaism as a mixed system similar to agroforestry. INRA and Sun’R built jointly a 860 m² and 4 m high prototype of agrivoltaic system, in Southern France. The idea was to produce the first references about growing crops under the fluctuating shade of solar panels, so we choose a range of crop production, including cereals (wheat and barley) and vegetables (lettuce, common bean, and cucumber).
This first three-year study provided detailed analysis of microclimate under the solar panels, a major result that was consolidated by further studies from other groups around the world. Although air temperature measured at 2m was homogenous under the agrivoltaic set up and similar to that in the full sun, crop temperature showed a different pattern during day-time (24h). This finding resulted from complex modifications of the energy balance under the solar panels, including fluxes of latent energy.
Although we suspected these changes in microclimate did affect stomata aperture and photosynthesis, we were not able to evidence changes in radiation use efficiency of the crops. Yet, yields were only slightly reduced, or even maintained despite reduced incident light. In the case of lettuce, our results showed that yield was maintained through an improved Radiation Interception Efficiency in the shade. Lettuce plants achieved this adaptation through (i) an increase in the total leaf area per plant, (ii) a different distribution of leaf area among the pool of leaves. Regarding the role of agrivoltaics in water saving, our finding were less encouraging: although climatic demand for water was lower in the partial shade, water use efficiency was not clearly increased.
After this first “brush cutting” seminal study, different groups in France and in the USA went on, deciphering the soil-plant atmosphere interactions in agrivoltaics. Barron-Gafford and co-authors in their recent publication provided more insight on the effect of fluctuating shade and reduced air vapor pressure deficit under solar panel on stomatal gaz exchanges. They also demonstrated that elevating solar panels and growing crops underneath could also improve energy production compared to traditional photovoltaic systems. Solar panels in their agrivoltaic setting were on average 8.9+0.2 °C cooler in the daylight hours than solar panels in a classical ground mounted solar plant in Arizona which could lead to 1% increase in power generation annually. Therefore, agrivoltaics start now to stand now as win-win systems in which both crops and solar panels have beneficial effects on each other.
Agrivoltaic studies all converge towards a very encouraging diagnosis of the capacity of agricoltaic systems to achieve satisfactory crop production and electricity production on the same land. Recent advances in photovoltaic technologies, including solar trackers and translucid photovoltaic cells may even contribute to improve the total efficiency of such systems. However, initial hopes, i.e. that photovoltaic shelters could substantially improve crop production underneath have been found to happen only in very high radiation and temperature environments. In temperate and northern countries, agrivoltaics systems implementation should still be strictly ruled and controlled to avoid agricultural land hijack. It should be ensured that the geometry of agrivoltaic device allow sufficient light for crop and good cropping conditions are guaranteed. It’s in sub-Saharan Africa, and in tropical islands that agrivoltaic would make most sense: they would provide a shelter and improved microclimatic conditions for the crops and field laborers and provide a readily usable and locally produced electricity source to communities that are often off the grid. Technical solutions for maintenance or elevated solar panels and resistance to climatic event should be a priority asset for the development of agrivoltaics in tropical developing countries. Finally, although photovoltaic panels price has dramatically decreased over the past decade, it is still illusory for most farmers to own, build, and maintain their own agrivoltaic system. Serious economic studies are needed to design ethical contracts between farmholders and photovoltaic investors.
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