Biohybrid Wind Energy Generators Based on Living Plants
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3 Results and Discussion
3.1 Concept and Components of the Plant Biohybrid Wind Energy Harvesters
Figure 1 shows the concept and components of the plant biohybrid wind energy harvester.
The plants are equipped with the “artificial leaves” which consist of a triple layer of a
200 µm PET base sheet, coated with a ~50 nm ITO electrode, coated with a 500 µm
silicone rubber layer (see illustration in Fig. 1, right). These materials were selected based
on our previous studies in which we compared possible dielectric/electrode combinations
in more detail [18, 19]. In addition, a Velcro®-based system is used to fix the artificial
leaf to the petiole of the plant leaf so that the silicone rubber surface is oriented towards
the upper (adaxial) leaf surface. The photograph in Fig. 1 shows a typical assembly. In
addition, a cable connects the ITO electrode of the artificial leaf to the harvesting circuit.
The second main part of the generator is the plant itself taking advantage of wind-induced
leaf motions. When the plant and the artificial leaves move in the wind, the surfaces of
both structures come into transient contact leading to the contact electrification of the leaf
and the silicone rubber. Thereby opposite charges are created on both surfaces. When
the wind-induced leaf oscillations then force the two surfaces to separate, these charges
do not longer compensate each other and hence are electrostatically induced into the
ion-conductive plant tissue. The lower right illustration in Fig. 1 depicts the expected
charge distribution as well as a simple circuit used to measure the generated signals. The
plant-converted electricity can be harvested by an electrode in contact with the plant
tissue. In similar manner, the charges generated on the artificial leaf are induced into the
ITO electrode. The biohybrid device forms a triboelectric generator that uses the plant
structure to transduce mechanical into electrical energy and is driven by wind-induced
leaf motions.
3.2 Energy Harvesting Electrode at the Plant Tissue
A crucial component of the device is the electrode that connects the plant tissue to the
energy harvesting and signal acquisition circuit, respectively. Figure 2 shows voltage
and current measurements using four different electrode types placed in or on the stem
of a F. microcarpa at a distance of 120 mm from the leaf that is converting mechanical
excitation into electrical signals. The species was chosen as it was small enough to be
analyzed in our Faraday cage and stimulated in a controlled manner by our test apparatus
for controlling stimulus and environmental noise as good as possible. Electrodes used
(Fig. 2a) were (1) a gold coated pin electrode (0.5 mm diameter 10 mm length) penetrating the stem’s inner tissue through the bark; (2) a thin-temporary tattoo electrode
based on conductive polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
PEDOT:PSS as reported in Ref. [25] that was conformally transferred onto the bark
surface; (3) an AgCl coated Ag wire that contacts with a KCl gel solution on the bark
prepared as reported in Ref. [26]; and (4) a copper film (1.4 mm) attached onto the bark.
The measurements were performed while the F. microcarpa leaf was exposed to a contact
and release excitation at a constant force of 0.5 N and applied by a 25 mm 2 squared piece
of the artificial leaf using an actuator operating at a frequency of 5 Hz. This generated
electrical signals of constant magnitude in all tests and allowed to compare the role of
237
3 Results and Discussion
3.1 Concept and Components of the Plant Biohybrid Wind Energy Harvesters
Figure 1 shows the concept and components of the plant biohybrid wind energy harvester.
The plants are equipped with the “artificial leaves” which consist of a triple layer of a
200 µm PET base sheet, coated with a ~50 nm ITO electrode, coated with a 500 µm
silicone rubber layer (see illustration in Fig. 1, right). These materials were selected based
on our previous studies in which we compared possible dielectric/electrode combinations
in more detail [18, 19]. In addition, a Velcro®-based system is used to fix the artificial
leaf to the petiole of the plant leaf so that the silicone rubber surface is oriented towards
the upper (adaxial) leaf surface. The photograph in Fig. 1 shows a typical assembly. In
addition, a cable connects the ITO electrode of the artificial leaf to the harvesting circuit.
The second main part of the generator is the plant itself taking advantage of wind-induced
leaf motions. When the plant and the artificial leaves move in the wind, the surfaces of
both structures come into transient contact leading to the contact electrification of the leaf
and the silicone rubber. Thereby opposite charges are created on both surfaces. When
the wind-induced leaf oscillations then force the two surfaces to separate, these charges
do not longer compensate each other and hence are electrostatically induced into the
ion-conductive plant tissue. The lower right illustration in Fig. 1 depicts the expected
charge distribution as well as a simple circuit used to measure the generated signals. The
plant-converted electricity can be harvested by an electrode in contact with the plant
tissue. In similar manner, the charges generated on the artificial leaf are induced into the
ITO electrode. The biohybrid device forms a triboelectric generator that uses the plant
structure to transduce mechanical into electrical energy and is driven by wind-induced
leaf motions.
3.2 Energy Harvesting Electrode at the Plant Tissue
A crucial component of the device is the electrode that connects the plant tissue to the
energy harvesting and signal acquisition circuit, respectively. Figure 2 shows voltage
and current measurements using four different electrode types placed in or on the stem
of a F. microcarpa at a distance of 120 mm from the leaf that is converting mechanical
excitation into electrical signals. The species was chosen as it was small enough to be
analyzed in our Faraday cage and stimulated in a controlled manner by our test apparatus
for controlling stimulus and environmental noise as good as possible. Electrodes used
(Fig. 2a) were (1) a gold coated pin electrode (0.5 mm diameter 10 mm length) penetrating the stem’s inner tissue through the bark; (2) a thin-temporary tattoo electrode
based on conductive polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
PEDOT:PSS as reported in Ref. [25] that was conformally transferred onto the bark
surface; (3) an AgCl coated Ag wire that contacts with a KCl gel solution on the bark
prepared as reported in Ref. [26]; and (4) a copper film (1.4 mm) attached onto the bark.
The measurements were performed while the F. microcarpa leaf was exposed to a contact
and release excitation at a constant force of 0.5 N and applied by a 25 mm 2 squared piece
of the artificial leaf using an actuator operating at a frequency of 5 Hz. This generated
electrical signals of constant magnitude in all tests and allowed to compare the role of