To mix or not to mix : evidences for the unexpected high productivity of new agrivoltaic and agroforestry systems
Christian DuprazGrégoire TalbotHélène MarrouJacques Wery
Sébastien RouxFabien liagre
Yoann Férard*Antoine Nogier*
INRA, UMR System, Montpellier, France
* Sun'R Company, Paris, France
To mix or not to mix?
To mix or not to mix?
Solar energy is plentiful and renewable
3.5*1018MJ.year-1 = 8 000 times the current needs of humans
Photovoltaism is so far the best conversion process
Capture efficiency : 15% versus less than 3% for biomass
Use chain efficiency : 150 times higher for photovoltaism + electric engine compared to biomass + combustion engine
But would compete for crop land...
4
To mix or not to mix?
Crops and trees... : agroforestry
Crops and photovoltaic panels... : agrivoltaism
Heterogeneous and stratified systemsFacilitation wanted
Competition unavoidable
Agroforestry system (AF)
Agrivoltaic system (AV) ?)
7Virtual images, Sun’R corporation
13
How to measure the productivity of mixtures
• Land Equivalent Ratio LER (Mead and Willey, 1980)
• LER = Sum (Relative Yields) = Rytree or solar panels + RYcrop
LER = 1
Maize and beans Cassava and peanuts
– 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?
LER > 1 ?0.9 < LER < 1.2
1.36Agroforestry Measured LER
Poplars-winter cereals
12 year rotation
1.3 to 1.6 LERs Predicted by models
Walnut trees – winter cereals
40 Year rotation
Measured LER for agrivoltaic systems with various densities of PV panels
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 shade under PVPs) which must be taken into account to assess the economic
profitability of such systems
AV even more efficient than AF systems for land productivity
LER interpretation
Practical : high
With 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 space
Theoretical : poor
LERs 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 ?
2003
2009
Full Density of panels
Half Density of panels
% annual relative radiation (RR) at ground level in an agrivoltaic system (43° Latitude North)
70% RR45 % RR
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 Comments
RY 0.52 RY > RD Tree dominant
Relative density RD 0.36 Depends on thinning regime
Size memory 1.50 +++ Major (feed forward effect)
Light competition 1.10 + Significant : less shade from tree to tree
Water competition 1.03 0Not significant : AF and
FC trees experience same WS
Harvest index 0.85 -- Forest trees invest more in the trunk
Remember : value > 1 = [AF tree > FC tree]
Decomposing the Crop RY
Crop Interpretation Comments
RY 0.76 RY < RD Crop dominated
Relative density RD 0.93
Size memory 0.99 0 Postive impact on leaf area
Light competition 0.74 --- Significant : shade major effect
Water competition 1.10 + Shaded crops protected
Harvest index (1) -- Not calculated so far
Remember : value > 1 = [AF crop > Monocrop]
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
PredictedRY
Relativedensity
Sizememory
Lightcompetition
Watercompetition
Harvestindex
Mul
tiplic
ativ
e C
ompo
nent
TreeCrop
X XXX=
Hierarchy of components of a final LER
If the AF model is right…
Advantages in AF Drawbacks in AF Neutral
Trees Larger canopies
Less light competition
Few trees
Invest C in roots
Water stress
WinterCrops
Reduced water stress
Longer life of leaves
Reduced N stress
Reduced heat stress
Less light
Reduced cropped
area
Harvest index
If the AV model is right…
Advantages in AV Drawbacks in AV Neutral
Solar panels
Improved ventilation Cost of supporting structure
Light conversion
Summer Crops
Reduced heat stress
Longer life of leaves
Reduced N stress
Reduced water stress
Reduced light
Reduced cropped
area
Integrated over 40 years
But what dynamics during the 40 years?
0,0
0,3
0,5
0,8
1,0
1,3
1,5
1,8
2,0
0 5 10 15 20 25 30 35 40Years
Land
Equ
ival
ent R
atio
AnnualIntegrated
Crop yield stabilisation in temperate agroforestry systems
Absolute crop yields Relative crop yields
Pure crop
Association Plant Cycles Soil depth
Root segre-gation
Water body in summer
Probable LER
Walnut-cereal on plains with water table
Almost complementar
y
Deep High Yes 1.5
Sorbus-wheat Lagged Medium Intermediate No 1.4
Prunus-Medicago Close Deep Intermediate No 1.3
Prunus-sunflower Synchronous Shallow Poor No 1.2
Populus-maize Synchronous Poor No Yes 1.1
Evergreen tree and summer crop
Superposed Any Variable No 1.0
Never demonstrated so far <1.0
A synthesis of probable LERs of temperate AFs
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)
Now available…
A Textbook + 1 hour DVD (with English, Spanish and Dutch sub-titles)
Available at a special discount price today
Book + DVD : 50 €
Contact :
Christian Dupraz
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