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

dupraz@supagro.inra.fr