To mix or not to mix : evidences for the unexpected high productivity of new agrivoltaic and agroforestry systems Christian Dupraz Grégoire Talbot Hélène Marrou Jacques Wery Sébastien Roux Fabien liagre Yoann Férard* Antoine Nogier* INRA, UMR System, Montpellier, France * SunR Company, Paris, France
Solar energy is plentiful and renewable3.5*1018MJ.year-1 = 8 000 times the current needs of humansPhotovoltaism is so far the best conversion processCapture efficiency : 15% versus less than 3% for biomassUse chain efficiency : 150 times higher for photovoltaism + electricengine compared to biomass + combustion engineBut would compete for crop land... 4
Crops and trees... : agroforestryCrops and photovoltaic panels... : agrivoltaism Heterogeneous and stratified systems Facilitation wanted Competition unavoidable
– Common observed values for LERs : – 0.9 to 1.1 for mixtures of annual species – 1.0 to 1.2 for legumes/non legumes species – What about mixtures of trees and crops, solar panels and crops? 0.9 < LER < 1.2 LER > 1 ?
AgroforestryMeasured LER 1.36 Poplars-winter cereals 12 year rotation
Walnut trees – winter cereals 40 Year rotation 1.3 to 1.6LERsPredictedby models
MeasuredLER foragrivoltaicsystemswithvariousdensitiesof PVpanels 1.3 to 1.7 Dupraz et al, 2011, Renewable Energy 36: 2725-2732
Surprising high yields of plants in the shade of solar panels or trees. High spatial variability of both light availability and plant yield (strip pattern of the shadeunder PVPs) which must be taken into account to assess the economic profitability of such systemsAV even more efficient than AF systems for land productivity
LER interpretationPractical : highWith a 1.4 LER, a 100 ha farm produces as much crop and tree products (or electricty) as a 140 ha farm where crops and trees (or solar panels) are separated in spaceTheoretical : poorLERs are « black box » indicators : they give no clues for the understanding of processes
Looking for explanations...Looking for facilitation...Light? Water? Nitrogen? First order interactions?When ?
Full Density of Half Density of panels panels 45 % RR 70% RR% annual relative radiation (RR) at ground level in an agrivoltaic system (43° Latitude North) 24
In agrivoltaic and agroforestry systems,competition for light is compensated... by what ??
LER interpretation• LERs can be simulated with process-based models• The simulations allow to decompose the LER in various interacting effects• This is a way to hierarchy factors of success in agroforestry and agrivoltaism Talbot, 2011, Ph. D. thesis (Submitted to Env. Model. Soft.)
LER interpretation for a walnut-wheat AF system
Decomposing the LER• LER= Rytree + Rycrop• RY = π[relative indexes]• Exemple of decomposition :• RY = Relative density x Size Memory x Light competition x Water competition x Harvest Index
Decomposing the Tree RY Tree Interpretation CommentsRY 0.52 RY > RD Tree dominant Depends on thinningRelative density RD 0.36 regime Major (feed forwardSize memory 1.50 +++ effect) Significant : less shadeLight competition 1.10 + from tree to tree Not significant : AF andWater competition 1.03 0 FC trees experience same WS Forest trees invest moreHarvest index 0.85 -- in the trunk Remember : value > 1 = [AF tree > FC tree]
Decomposing the Crop RY Crop Interpretation CommentsRY 0.76 RY < RD Crop dominatedRelative density RD 0.93 Postive impact on leafSize memory 0.99 0 area Significant : shade majorLight competition 0.74 --- effectWater competition 1.10 + Shaded crops protectedHarvest index (1) -- Not calculated so far Remember : value > 1 = [AF crop > Monocrop]
Hierarchy of components of a final LER 1,6 1,4 Tree Crop 1,2Multiplicative Component 1,0 0,8 0,6 0,4 0,2 0,0 Predicted = Relative X Size X Light X Water X Harvest RY density memory competition competition index
If the AF model is right… Advantages in AF Drawbacks in AF NeutralTrees Larger canopies Few trees Water stress Less light competition Invest C in rootsWinter Reduced water stress Less light HarvestCrops index Longer life of leaves Reduced cropped Reduced N stress area Reduced heat stress
If the AV model is right… Advantages in AV Drawbacks in AV NeutralSolar Improved ventilation Cost of supporting Lightpanels structure conversionSummer Reduced heat stress Reduced lightCrops Longer life of leaves Reduced cropped Reduced N stress area Reduced water stress
Integrated over 40 yearsBut what dynamics during the 40 years?
Absolute crop yields Relative crop yields Pure cropCrop yield stabilisation in temperate agroforestry systems
A synthesis of probable LERs of temperate AFsAssociation Plant Cycles Soil Root segre- Water Probable depth gation body in LER summerWalnut-cereal on Almost Deep High Yes 1.5plains with water complementartable ySorbus-wheat Lagged Medium Intermediate No 1.4Prunus-Medicago Close Deep Intermediate No 1.3Prunus-sunflower Synchronous Shallow Poor No 1.2Populus-maize Synchronous Poor No Yes 1.1Evergreen tree Superposed Any Variable No 1.0and summer crop Never demonstrated so far <1.0
Conclusions• LERs of AFs and AVs may be extremely high (part of a second green revolution?)• Process-based models can help in interpreting productivity results of AF and AV systems at various time steps.• Winter crops better in AF, summer crops better in AV (latitude-specific)
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