The modern world claims for new sources of clean, renewable energy. The available bioenergy sources are not as green as they claimed to be. The current use of biomass usually produces some waste and competes with food crops for space. Scientists created a non-destructive technology that uses living plants, including food crops, for generating electricity from plants.
Plant use in energy production over time
Plants have the capacity to transform solar energy into chemical energy and store part of this energy in the form of sugars (carbohydrates). With the discovery of fire, the humankind found out how to use the energy stored in plants. Gathered around bonfires, our ancestors sought protection against wild animals, prepared their first cooked meals, and survived winters.
Many centuries later, wood regained its importance as an energy source throughout the second Industrial Revolution, feeding steam engines in steam-powered factories and steam-transport alongside with coal. It is worth mentioning that coal, so widely used for the production of electricity or heat, was once a plant too. Coal is nothing more than dead plant matter transformed by biological and geological processes over an incredibly long period.
In the present, humans still use burned biomass to produce energy, but the processes became more efficient than in the past. While during second Industrial Revolution steam-engines released toxic smoke into the air, modern power plants capture carbon dioxide emissions during electricity production from kernel burn.
In transport area, burned plants can produce electricity to power battery electric vehicles. It is also possible to convert plants into fuel (ethanol) through a fermentation process, and power internal combustion vehicles. The most used plants for ethanol production are corn and sugarcane.
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Plant potential to produce energy can be demonstrated with a simple school experiment: the potato battery. The energy stored in a single potato is enough to light an LED light. By using a copper wire, a copper coin as the cathode, and a galvanized nail as an anode, it is possible to produce electricity, as the chemical reaction within the potato creates an electrical current.
So far, I did not introduce you to anything new, right? Very well, the potato is a tuber. Tubers are modified plant structures used for the storage of nutrients. The potato contains high amounts of starch, which is a type of carbohydrate. So how can the plant produce those stored nutrients? In the photosynthesis, plants use sunlight to turn carbon dioxide and water into glucose (also a carbohydrate) and release oxygen into the atmosphere. This reaction occurs on the leaves, more precisely in organelles called chloroplasts. Inside chloroplasts, there are tiny membrane-bound compartments called thylakoids, with approximately 3µm ± 0.5 in length. Thylakoids are responsible for the capture and storage of solar energy during photosynthesis.
Artificial photosynthetic system
Scientists from the University of Georgia, USA, found a way to interrupt the photosynthesis, more specifically the pathway along with electrons flow, and redirect those electrons before they are used to produce glucose. They removed thylakoids from cells of spinach leaves and created an artificial photosynthetic system. The system comprises photosynthetic fuel cells composed of multi-walled nanotubes (MWNT) to which the scientists added biological counterparts. To produce the anode they immobilized thylakoids on the MWNT backing, forming a thylakoid-MWNT anode. Similarly, scientists used Laccase, a copper-containing enzyme, naturally found in several plants to produce a Laccase-MWNT cathode. The system outcome is a photo-induced water-splitting reaction, which releases electrons. The anode functions as a biocatalyst that oxidizes and splits water, whereas, the cathode operates as an enzymatic bioelectro-catalyst that reduces oxygen and regenerates water. The photoelectrochemical activity in the system produces high-energy photocurrents.
This promising new technology demonstrates power generation enough to charge remote sensors or portable electronic equipment. However, protein damage in thylakoids over time can decrease the efficient of the system. In the wild, plants can repair their photo-damaged proteins, whereas, in an artificial photosynthetic system, this drawback still requires fixing.
Green power plants
An alternative technology generating electricity from plants. This approach requires no plant combustion, produces no greenhouse emission, is a renewable energy source, and on the top of that, do not harm plants. The principle lies on the use of a by-product of the photosynthesis that can be harnessed for electricity. Plants use approximately half of the glucose produced in photosynthesis and excrete the excess through their roots in the form of rhizodeposits.
In the soil, there are beneficial bacteria that protect plants against pathogens and sometimes form a biofilm around roots. These bacteria are also able to metabolize rhizodeposits, and break down those sugars, releasing protons and electrons into the soil. Scientists used that knowledge to produce a system that captures those electrons, transfers them to a power harvester, and produces in situ energy: the microbial fuel cell (MFC).
The MFC is also a bio-electrochemical system composed of anode and cathode. In the anode compartment, scientists inoculated bacteria as biocatalysts and introduced plant roots. The electrons released from the bacteria action flow from the anode to the cathode due to a potential difference. As the system requires bacteria to work, the plant used in it should be able to grow under anaerobic conditions.
Scientists from the Institute of Ecology of Wageningen University, Netherlands, tested the potential for real world application of the Plant-MFCs. They used reed mannagrass (Glyceria maxima), a native perennial grass that grows in wet areas. They obtained a maximum energy production of 67 megawatts per square meter.
The spin-off of the studies by the Dutch scientists was the creation of a company, which currently holds the patent on this technology. In Hembrug, Netherlands, this company made a demo of this green technology in 2014, in which green electricity harvested from plants charged 300 LED lights. The company released to the market wi-fi hotspots, mobile charges, rooftop electricity modules, and science kits, all running on green power.
Biochemists from Cambridge University, England, showed another practical application of the concept. They constructed a green bus shelter for the Science Festival in March 2015 using a hybrid energy source: solar panels combined with plants on alternate walls of the bus shelter. The installation, located in the University’s Botanical Gardens had plants growing in pockets attached to walls, containing 1l of soil. The back of the pockets was made of carbon fiber to catch the electric current.
So what happens now?
The next step is large scale testing. Scientists from the University of Pharmacy and Life Sciences, in Tokyo, Japan tested the plant-MFC system in rice paddy fields. The energy production generated in their experiment, 80 megawatts per square meter, surpassed that of small scale production. In addition to the large potential of rice paddy fields, mangroves, river deltas, and all sorts of wetlands, even polluted areas, are suitable energy sources.
At the same time, scientist from another research group tested MFC with different plant species and obtained encouraging results. The common cordgrass (Spartina anglica), a herbaceous perennial plant native to England, achieved 222 megawatts per square meter.
Although the energy as green electricity generated cannot meet world demand yet, its use as a complementary energy source is promising. Due to its simple process, the potential for application for generating electricity from plants is huge. It can grant energy to poor people that live in remote rural areas and currently have no energy at all. Moreover, as it is a new technology, the process will certainly be improved in the next few years and efficiency in green electricity production can be even higher.
Suggestions for further reading:
Calkins, J. O., Umasankar, Y., O’Neill, H., and Ramasamy, R. P, 2013. High photo-electrochemical activity of thylakoidcarbon nanotube composites for photosynthetic energy conversion. Energy &Environmental Sciences 6, 1891-1900.
Campbell, J.E., Lobell, D.B., and Field, C.B., 2009. Greater transportation energy and GHG offsets from bioelectricity than ethanol. Science, 324(5930), pp.1055-1057.
Kouzuma, A., Kaku, N. and Watanabe, K., 2014. Microbial electricity generation in rice paddy fields: recent advances and perspectives in rhizosphere microbial fuel cells. Applied microbiology and biotechnology, 98(23), pp.9521-9526.
Helder, M., Strik, D.P.B.T.B., Hamelers, H.V.M., Kuhn, A.J., Blok, C. and Buisman, C.J.N., 2010. Concurrent bio-electricity and biomass production in three Plant-Microbial Fuel Cells using Spartina anglica, Arundinella anomala and Arundo donax. Bioresource technology, 101(10), pp.3541-3547.
Helder, M., Chen, W.S., Harst, E.J., Strik, D.P., Hamelers, H.B.V., Buisman, C.J. and Potting, J., 2013. Electricity production with living plants on a green roof: environmental performance of the plant‐microbial fuel cell. Biofuels, Bioproducts and Biorefining, 7(1), pp.52-64.
Strik, D.P.B.T.B., Hamelers, H.V.M., Snel, J.F.H., Buisman, C.J.N., 2008. Green electricity production with living plants and bacteria in a fuel cell. International Journal of Energy Research, 32(9), pp.870-876.