Page 53 - Energize March 2022

TECHNICAL

Figure 7: Semi-transparent panels (Groenleven, Brite Solar)
Design considerations 3
Design will depend on balance between the requirement of
the crops for photosynthetically active radiation (PAR) and
the requirements of the PV system for electricity generation.
The balance between local environmental conditions, farming
systems and socio-economic contexts will all influence the
design of an AGV system. Physically, the layout of the PV panels
will need to be optimised to allow sufficient sunlight to reach
the underlying crops. This would depend on both the amount
of solar radiation at the location and the shade tolerance of the
target crops.
An AGV system in an area receiving ample PAR (such as
East Africa) and growing shade-tolerant crops would be able
Figure 8: Lens based CPV system (Insolight)
to comprise a denser PV panel layout than an AGV system at
A second design uses lenses to focus concentrated direct higher latitudes and with less shade-tolerant crops underneath.
sunlight on small cells (CPV), but allows diffuse sunlight to The elevation of the panels to facilitate farming underneath
pass through. These cells are attached to a mobile frame would also depend on the planned activities; taller crops or
which moves a few mm during the day, adjusting to the sun’s mechanised farming would need a taller panel mounting
position, in order to maximise performance. The sun’s rays are structure than shorter, hand-picked crops. Tool use must also
either focused on the photovoltaic cells to ensure maximum be considered, with panels high enough to avoid damage. Taller
electricity production, or the structure moves slightly to allow systems would be more expensive to construct and laborious to
direct sunlight to be directed towards the crops. This makes it maintain and may be unnecessary if the farming system solely
possible to adjust the function of the system from electricity comprises short, hand-picked crops.
generation to crop growth. The orientation of AGV systems also need to consider
diurnal solar radiation patterns. Many conventional ground-
Transparent panels mounted solar parks comprise rows of PV panels on an east-
These are thin film PV panels similar to those used in BIPV west axis, with the panels either statically tilted at an optimal
installations (see Figure 9), which are used mainly for green angle for maximum solar radiation, or mechanically tracking the
house applications. The panels can also be tinted to provide a sun to further improve energy conversion.
filtering effect, allowing only those wavelengths beneficial to However, this orientation would mean some crops
plant growth to pass through. receive mainly direct PAR during the day, while those in the
shade receive mainly diffused PAR. AGV systems designed
with north-south oriented arrays (with panels still optimally
tilted), especially near the equator, would ensure a more
even distribution of sunlight throughout the day for the
underlying crops.

Pilot sites in East Africa 4
Solar radiation is abundant across East Africa with 4-7 kWh/
m /day. The potential for photovoltaic electricity generation is
2
therefore significant. Previously, solar panels were prohibitively
Figure 9: Tinted solar panels (Paolo Bombelli/Univ of Cambridge) expensive, but the rapidly decreasing costs have resulted in

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