Can triboelectricity enhance plant growth?

1. Synopsis

Growth enhancement in plants is crucial to increase the food production in crops. This growth enhancement has traditionally been achieved through the use of fertilisers and control over the environmental conditions using systems such as hydroponics. However, these methods are energy-intensive, or require the use of non-sustainable compounds.

An emerging field in the improvement of plant growth is the use of electroculture. This method has been reported to increase the production by 20 to 30%, according to a recent article in the World Economic Forum. The technology involves the use of a voltage that can kill pathogens, and increase the bioavailability of certain nutrients. In addition, electroculture has already been proven to improve the growth rates of several plants when used at a certain voltage and within specific time intervals.

Despite the potential advantages of combining traditional methods for growth enhancement with electroculture, the high energy required can limit their use and increase the production costs. In this project, we explore this technology, and combine it with a simple and low-cost triboelectric generator, to enhance the growth of aquatic plants. Triboelectric generators can produce an output power by collecting mechanical energy from the environment. This technology was employed to harvest the energy from the friction between two working peristaltic pumps, allowing a recycling of the energy from the pumps, without the need for an additional power source.

2. Triboelectric generator

Since 2012, the field of triboelectric generators has experienced an exponential growth. To date, multiple low-cost setups have been developed, with a relatively high power. For the purpose of this project, we followed the video tutorial developed by EPFL, which can be found in this link. Briefly, two paper sheets were covered in graphite from a pencil on one side, and one of the electrodes was covered in kapton tape and a teflon sheet on the electrode side. The final devices are shown below:

In this case, we employed a conductive copper sticky tape instead of a wire, and the devices were glued onto a rigid methacrylate sheet that was fixed onto the peristaltic pumps. As a consequence, the friction between the two electrodes generated a voltage and a current. After powering up the peristaltic pumps with 12 V, both the Open Circuit Voltage and Short Circuit current were measured using arduino-based devices (PH-4502C, and ACS712 Current Sensor respectively). The results are shown below:

As observed, an alternating voltage in the range of ~1 V was generated using this method. The changes in the voltage were a consequence of the moving pumps. However, the current could not be accurately measured directly due to its low magnitude.

3. Final setup design

For the final testing of the devices, the triboelectric generators were assembled onto the peristaltic pumps using the methacrylate sheets as the support. The system is shown below:

The electrical output from the triboelectric generators was connected to the plants through the conductive wires. In this case, we chose Lemna Minor plants as the culture tested plants due to their fast growth and To study the effects of applying this voltage to the plants, we run two plant cultures simultaneously. One of the plant cultures was subjected to the voltage, whilst the other one was not modified. The pristine version of the plants was used a s a control sample, that would allow a comparison of the effects of applying a voltage to the plants. For the testing, we used a setup as shown below, where both plant cultures were kept in a water:

In both cases, peristaltic pumps were used to move the water from the culture zone up to the reservoir area. In the case of the electrocultured plants, the voltage was only used for 2 hours each day, and the number of plants was quantified within 7 days from the start of the experiment.

4. System testing

To study possible effects of the application of a voltage to the aquatic plants, three different parameters were assessed; water acidity (pH), water conductivity, and number of plants. The obtained results from the testing are shown below:

As observed, in both cases, the number of plants greatly increased after a few days, given the fast growth of the Lemna Minor species. However, in the case of the electroculture, the number of plants after a week of testing was 11, while the culture tested under normal conditions has 6 plants. Although further tests should be conducted to prove the improvements in the plant growth due to electroculture, this preliminary result shows promise in the development of low cost systems for enhanced plant growth. To determine possible changes in the growth medium of the plants, we also analysed different water parameters. In the case of conductivity, although in the case of the electroculture we observed a slight increase initially, which can be indicative of a higher presence of ions in water, the observed change was relatively small, in the range of μS. In the case of the pH, within the normal culture, with no applied voltage, the pH remained approximately constant during the testing time. On the contrary, it decreased in the case of the electroculture, up to a value of 4.5.

5. Conclusions

An increase in the growth of the tested plants subjected to a small voltage for 7 days was observed. Thus, although in a first instance, the application of a small voltage obtained through a simple triboelectric generator seems to stimulate the growth of certain aquatic plants, further experiments are required to confirm this hypothesis. More importantly, this project shows an alternative for the harvesting of energy from peristaltic pumps, that could improve the growth of plants without the need for additional systems, or expensive fertilisers.