A presentation from the WCCA 2011 event held in Brisbane, Australia.

1. 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
* Sun'R Company, Paris, France

2. To mix or not to mix?

3. To mix or not to mix?

4. 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

5. Crops and trees... : agroforestry
Crops and photovoltaic panels... :
agrivoltaism
Heterogeneous and stratified systems
Facilitation wanted
Competition unavoidable

6. Agroforestry system (AF)

7. Agrivoltaic system (AV) ?)
Virtual images, Sun’R corporation
7

13. 13

14. 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

15. Maize and beans Cassava and peanuts

16. – 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 ?

17. Agroforestry
Measured LER
1.36
Poplars-winter cereals
12 year rotation

18. Walnut trees – winter cereals
40 Year rotation
1.3 to 1.6
LERs
Predicted
by models

19. 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

20. 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

21. 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

22. Looking for explanations...
Looking for facilitation...
Light? Water? Nitrogen? First order interactions?
When ?

23. 2003
2009

24. 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

25. In agrivoltaic and agroforestry systems,
competition for light is compensated... by
what ??

27. 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.)

28. LER interpretation for a walnut-wheat AF system

29. 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

30. Decomposing the Tree RY
Tree Interpretation Comments
RY 0.52 RY > RD Tree dominant
Depends on thinning
Relative density RD 0.36 regime
Major (feed forward
Size memory 1.50 +++ effect)
Significant : less shade
Light competition 1.10 + from tree to tree
Not significant : AF and
Water competition 1.03 0 FC trees experience
same WS
Forest trees invest more
Harvest index 0.85 -- in the trunk
Remember : value > 1 = [AF tree > FC tree]

31. Decomposing the Crop RY
Crop Interpretation Comments
RY 0.76 RY < RD Crop dominated
Relative density RD 0.93
Postive impact on leaf
Size memory 0.99 0 area
Significant : shade major
Light competition 0.74 --- effect
Water competition 1.10 + Shaded crops protected
Harvest index (1) -- Not calculated so far
Remember : value > 1 = [AF crop > Monocrop]

32. Hierarchy of components of a final LER
1,6
1,4 Tree
Crop
1,2
Multiplicative 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

33. If the AF model is right…
Advantages in AF Drawbacks in AF Neutral
Trees Larger canopies Few trees Water stress
Less light competition Invest C in roots
Winter Reduced water stress Less light Harvest
Crops index
Longer life of leaves Reduced cropped
Reduced N stress area
Reduced heat stress

34. If the AV model is right…
Advantages in AV Drawbacks in AV Neutral
Solar Improved ventilation Cost of supporting Light
panels structure conversion
Summer Reduced heat stress Reduced light
Crops
Longer life of leaves Reduced cropped
Reduced N stress area
Reduced water stress

35. Integrated over 40 years
But what dynamics during the 40 years?

36. 2,0
1,8
Land Equivalent Ratio
1,5
1,3
1,0
0,8
0,5 Annual
Integrated
0,3
0,0
0 5 10 15 20 25 30 35 40
Years

37. Absolute crop yields Relative crop yields
Pure crop
Crop yield stabilisation in temperate agroforestry systems

38. A synthesis of probable LERs of temperate AFs
Association Plant Cycles Soil Root segre- Water Probable
depth gation body in LER
summer
Walnut-cereal on Almost Deep High Yes 1.5
plains with water complementar
table y
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 Superposed Any Variable No 1.0
and summer crop
Never demonstrated so far <1.0

39. 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)

40. 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
dupraz@supagro.inra.fr