Biohybrid Wind Energy Generators Based on Living Plants
235
take advantage of specific material/tissue properties and functionalities of living plants
with applications ranging from energy harvesting to sensing [1–8].
Several ways enable producing electricity with living plants such as plant microbial
fuel cells [9–15] and glucose biofuel cells [16, 17]. In the latter examples, the conversion
from organic matter into electricity is done by an artificial fuel cell that uses organic
molecules provided by the plant. However, we recently reported, that plants can directly
convert mechanical energy such as from wind into electricity [18, 19]. Plant leaves are
covered with a dielectric material, a thin polymer layer named cuticle [20]. This surface
accumulates charges upon contact with another material through contact electrification
and materials like PTFE and silicone rubber were shown to lead to significant charging
of the cuticle [18, 21]. The charges created on the cuticle are electrostatically induced
into the ion-conductive cellular tissue leading to a current that can be harvested by an
electrode at the plant tissue. In this manner, living plants constitute a triboelectric generator (TENG) with a comparable power output to similarly operated artificial TENGs
as tested in our previous study [18]. TENGs are not only considered a possible energy
source for the internet of things and sensor networks but are also being discussed for
application in larger scale energy harvesting farms with reported power densities of up to
several hundred watts per square meter of active surface area [22–24]. Using living plants
as opposed to generators based on only artificial materials has several added benefits.
Most materials and the structure are provided by the plant, moreover plant’s dynamic
self-repair and CO 2 compensation through ongoing growth would be extremely difficult
to realize in artificial generators. This offers a great opportunity to convert plants into
power sources. Yet, it still needs to be understood how to best construct and optimize
plant biohybrid systems in order to exploit environmental mechanical energy such as
wind.
Here, we report on essential components and behaviours of the plant-biohybrid
energy harvesting prototypes by investigating which electrode types at the plant tissue
are most efficient in terms of energy harvesting and the mechanism of charge transport
in the plant. In addition, we analyse how wind-induced voltages depend on mechanical
oscillations and contact of the plant leaves with the artificial components at different
wind speeds. Thus, we show how plant movements induced by wind influence charge
generation. Moreover, we demonstrate that plant biohybrid generators are capable of
directly powering several LEDs and a digital thermometer from wind energy.
2 Materials and Methods
2.1 Plant Species
Ficus microcarpa was purchased at a local plant nursery. Rhododendron yakushimanum
was provided by the Botanic Garden, University of Freiburg, Germany. Nerium oleander
was picked outdoor in the region of Tuscany, Italy.
2.2 Assembly of Plant-Biohybrid Energy Harvesters
To manufacture the ‘artificial leaves’ to be attached to the plants, transparent indium tin
oxide (ITO)-coated PET films (thickness 200 µm, nominal sheet resistance 350–500
235
take advantage of specific material/tissue properties and functionalities of living plants
with applications ranging from energy harvesting to sensing [1–8].
Several ways enable producing electricity with living plants such as plant microbial
fuel cells [9–15] and glucose biofuel cells [16, 17]. In the latter examples, the conversion
from organic matter into electricity is done by an artificial fuel cell that uses organic
molecules provided by the plant. However, we recently reported, that plants can directly
convert mechanical energy such as from wind into electricity [18, 19]. Plant leaves are
covered with a dielectric material, a thin polymer layer named cuticle [20]. This surface
accumulates charges upon contact with another material through contact electrification
and materials like PTFE and silicone rubber were shown to lead to significant charging
of the cuticle [18, 21]. The charges created on the cuticle are electrostatically induced
into the ion-conductive cellular tissue leading to a current that can be harvested by an
electrode at the plant tissue. In this manner, living plants constitute a triboelectric generator (TENG) with a comparable power output to similarly operated artificial TENGs
as tested in our previous study [18]. TENGs are not only considered a possible energy
source for the internet of things and sensor networks but are also being discussed for
application in larger scale energy harvesting farms with reported power densities of up to
several hundred watts per square meter of active surface area [22–24]. Using living plants
as opposed to generators based on only artificial materials has several added benefits.
Most materials and the structure are provided by the plant, moreover plant’s dynamic
self-repair and CO 2 compensation through ongoing growth would be extremely difficult
to realize in artificial generators. This offers a great opportunity to convert plants into
power sources. Yet, it still needs to be understood how to best construct and optimize
plant biohybrid systems in order to exploit environmental mechanical energy such as
wind.
Here, we report on essential components and behaviours of the plant-biohybrid
energy harvesting prototypes by investigating which electrode types at the plant tissue
are most efficient in terms of energy harvesting and the mechanism of charge transport
in the plant. In addition, we analyse how wind-induced voltages depend on mechanical
oscillations and contact of the plant leaves with the artificial components at different
wind speeds. Thus, we show how plant movements induced by wind influence charge
generation. Moreover, we demonstrate that plant biohybrid generators are capable of
directly powering several LEDs and a digital thermometer from wind energy.
2 Materials and Methods
2.1 Plant Species
Ficus microcarpa was purchased at a local plant nursery. Rhododendron yakushimanum
was provided by the Botanic Garden, University of Freiburg, Germany. Nerium oleander
was picked outdoor in the region of Tuscany, Italy.
2.2 Assembly of Plant-Biohybrid Energy Harvesters
To manufacture the ‘artificial leaves’ to be attached to the plants, transparent indium tin
oxide (ITO)-coated PET films (thickness 200 µm, nominal sheet resistance 350–500