Plant Generated electricity

Reporter: Sebastian Ordoñez

 

UN-Bogotá Team

 

International Physicists' Tournament-Colombia

Online version

2021

statement

Some internet post suggest that it is possible to generate electricity from photosynthesis by charging a battery connected to a metallic network that winds through the roots of a plant. In response, a skeptic may claim that the setup is just a voltaic pile, and therefore, photosynthesis is irrelevant.  

statement

Some internet post suggest that it is possible to generate electricity from photosynthesis by charging a battery connected to a metallic network that winds through the roots of a plant. In response, a skeptic may claim that the setup is just a voltaic pile, and therefore, photosynthesis is irrelevant.  

  •  Explain the phenomenon .
  •  Determine whether photosynthesis is indeed involved.
  •  Determine the main factors affecting the phenomenon

PhotoSynthesis

The oxigenic photosynthesis in plants is described  by the overall equation

\boxed{6\text{H}_{2}\text{O}+6\text{CO}_{2}+\gamma\Longrightarrow \text{C}_{6}\text{H}_{12}\text{O}_{6}+6\text{O}_{2}}
  • This process takes place in organelles called choloroplasts.

G.S Singhal. et al. Concepts in photobiology. Photosyntesis and Photomorphogenesis. Springer, 1999.

Solar energy is used to fix carbon dioxide in the form of carbohydrates.

  • Where is the chemical energy going?
  • How is related photosynthesis with the surrounding soil environment?

Rhizodepositon

Rhizodeposition  drives the interactions between plant, soil, and microbial populations.

  • "Depending on plant species, age, and environmental conditions up to 60% of the net fixed carbon can be transferred from its leaves to the roots" (Strik, 2008)
  • Rhizodeposits contain carbon that can be employed by microorganisms.

K. Hassan et. al.  The interactions of Rhizodeposits with Plant Growth-Promoting Rhizobacteria in the Rhizosphere,  Int. J. Energy Res. 2019.

Release of carbon compounds from plant roots into the surrounding soil environment.

Soil & microorganisms

Mixture of organic matter, minerals, liquids and organisms that together support life.

  • Organic Matter and living micoorganisms
  • pH, moisture, temperature

"It has been estimated that bacterial populations in the rhizosphere are 10-100 times higher than in bulk soil" (Strik, 2018)

In  bacteria  we have, as result of respiration,  the temporary release of electrons which are handy for generating electricity.

\boxed{\text{C}_{6}\text{H}_{12}\text{O}_{6}+6\text{O}_{2} \Longrightarrow 6\text{CO}_{2} + 24\text{H}^{+}+24\text{e}^{-}}

There is a symbiosis between plants and bacteria in the rhizospehere.

Voltaic Pile

In order to have a simple voltaic pile we need two metals with different electronegativity, and a medium to increase the electrolyte conductivity.

The strength of the pile is expressed in terms of its electromotive force given in volts.

Mechanism

B. Strik. Green electricity production with living plants and bacteria in a fuel cell,  Int. J. Energy Res. 2008.

  1. The plant produces organic matter from sunlight and CO2 via photosynthesis.
  2. Up to 60% of this organic mather ends up in soil as rhizodeposits.
  3. This organic matter is oxidized by bacteria, releasing CO2, protons and electrons.
  4. These electrons flow due to a potential difference, from the anode through an electrical circuit.

In summary we have:

Photosynthesis is indirectly involved in  the generation of electricity on the roots

Experimental setup: Prototype

  • Cupper and zinc electrodes connected in serie in each row. 
  • Cell tray with the metallic network.  On the right, flower pot version.
  • Final prototype version including plant

Experimental setup: Prototype

  • Cell trays with soil added. 

Measuring instruments

Soil tester

  • Moisture
  • Temperature
  • pH 

Multimeter

  • Voltage: uncertainty \(\pm 0.01  V\)

The plant

  • "Perennial plants, however, can transfer 70-80% of the net fixed carbon to the roots, of which 8-65% is released as rhizodeposition" .(Helder, 2012)

M. Helder. Design criteria for the Plant-Microbial Fuel Cell. Electricity generation with living plants – from lab to application. PhD thesis. 2012

THE PLANT

A special slide for the plant and its properties....

Groups under consideration

  • Plants light: prototypes with plants and exposure to light throughout the experiment.
  • Plants no light: prototypes with plants and no exposure to light throughout the experiment.

We consider four different gruops and for each of them we use five replicas

  •  Dead soil: high temperature sanitization process, negligible content of microorganisms.
  • Living soil: coming directly from the healthy rizosphere of plants, high content of bacteria.

DARk ROOM 

Temperature inside is the same than for other groups

 DEAD Soil

We are able to see a first clear effect of humidity on voltage.

From the simplest group to the most complex

Living soil and DEAD Soil

Now, we are able to see the effect of having microorganisms.

Living soil and plants No light

Living soil and plants light

 Plants with and without light 

 Plants with and without light 

Multiple comparison test

By using a two-way ANOVA we are able to make a multiple comparison test between our gruops in different time points, with a 95% confidence interval.

MD: mean difference

CI: confidence interval of difference

BT: below treshold

Sum: summary

P value: adjusted p-value

 Null hypothesis significance testing

A very small p-value means that  an extreme observed outcome would be very unlikely under the null hypothesis

Multiple comparison test

Multiple comparison test

Multiple comparison test

Multiple comparison test

By using a two-way ANOVA we are able to make a multiple comparison test between our gruops in different time points, with a 95% confidence interval.

 Compared groups  MD             95 % CI   BT Sum P value
Living Soil vs. Plants Ligth 0,4915 -0.2826 to 1.266 No ns 0,3108
Living Soil vs. Plants No Light 0,4789 -0.3028 to 1.261 No ns 0,3431
Plants Ligth vs. Plants No Light -0,0126 -0.1556 to 0.1304 No ns 0,995

Multiple comparison test

By using a two-way ANOVA we are able to make a multiple comparison test between our gruops in different time points, with a 95% confidence interval.

 Compared groups  MD             95 % CI   BT Sum P value
Dead Soil vs. Living Soil -1.012 -1.804 to -0.2203 Yes ** 0.0091
Dead Soil vs. Plants Light -0.5206 -0.7285 to -0.312 Yes **** <0.0001
Dead Soil vs. Plants No Light -0.5332 -0.7732 to -0.293 Yes **** <0.0001

Conclusions

  • We conclude that  photosynthesis in those setups plays the role of keeping the plant and the environment healthy, but is not a determinant factor beyond that for electricity .

 

  • We find that the main parameter affecting electricity generation is the amount of electrolytes, in this case humidity coming from watering

 

  • We observe that the presence of microorganisms in the environment is an important factor in electricity production.

 

  • We obtain electricity without the presence of plants, then, in principle  effects of photosinthesys is secondary.