vol 8, issue 11

Electric crop fields

11 November 2013
By Sian Harris
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A graphic showing how electricity could be generated from plants

Tiny electrochemical reactions at plant roots could pave the way for cheap, sustainable electricity generation

Tiny electrochemical reactions at plant roots could pave the way for cheap, sustainable electricity generation if the plans of Dutch researchers are realised.

The completion of the years of research leading to a PhD often results in an evening of celebrating, but what is probably less common is for a PhD celebration to culminate in a product launch. However, this is exactly what happened at Marjolein Helder's party after she defended her PhD in November last year.

Helder is a researcher as part of the Environmental Technology Group at Wageningen University in the Netherlands, which for many years has been investigating the potential for electricity generation using the electrochemical reactions that occur at the roots of living plants. The group, which has received funding of '4m in recent years from an EU Seventh Framework grant, patented what it calls a Plant-Microbial Fuel Cell (Plant-MFC) in 2007.

The concept is straightforward: plants discharge waste organic matter into the soil via their roots in a process called rhizodeposition. This organic matter is broken down by micro-organisms in the soil to release electrons and protons and electricity is generated when these are recombined with oxygen from the air. The processes involved are continuous, which means that electricity could be generated during any season, throughout the day or the night.

Helder's PhD focused not only on the technology, but also on its potential for practical commercial applications. She says: "We've finished five years of research. I think we've unravelled quite a lot of what's going on and gained quite a lot of knowledge."

This preliminary research is crucial because in 2009 she and fellow researcher David Strik - one of the inventors of and early researchers in the Plant-MFC concept - founded a company to develop the technology. Plant-e currently operates from the university and Strik and Helder divide their time between Plant-e and the research centre. Despite being in its fledgling first three years of operation, the company already has ambitious plans to take the concept from lab to market.

The first step came in November with the first product announcement, the Plant-e Planet. This is a consumer gadget intended mainly to introduce the technology to consumers, familiarise the company with the market and perhaps most importantly to begin to generate revenue that can be invested in developing bigger-scale electricity generation.

The gadget, initially on sale only in the Netherlands thanks to challenges of transporting plants across borders, uses the electricity generated with one pair of electrodes from the roots of a plant, such as a bromeliad, banana plant or similar. The strength of this current of electricity is of that which will enable a globe to spin.

The company is currently taking orders for the Plant-e Planet and will begin production once it receives 325 orders. The company believes that this gadget is the first step towards visible green, clean and sustainable electricity for everyone. Their aim is that a decorative pot plant will provide enough electricity that can be used to put a globe - in their vision our planet - in motion. The Plant-e Planet is the first product in the world that runs entirely on electricity generated by living plants.

Improving performance

However, the electricity output of the gadget is currently relatively small. According to Helder, one pair of electrodes generates about 1V. To be useful beyond simple gadgets such as the spinning globe, 5V or 12V is required. Improving the output of the device is one of the product improvements that both Planet-e and the Environmental Technology Group are working on, and electrode design and positioning plays an important role in this. The Plant-MFC uses inert carbon electrodes of either graphite or activated carbon; these bring significant advantages over metal electrodes because, over time, metals are broken down and poison the plants.

"If electrodes corrode, that would cause problems for plants and pollute the soil. This would happen if we used metal," says Helder. "Carbon is inert so it doesn't corrode. It's a natural material and very cheap and at this point it's sufficient because we don't produce a lot of electricity."

Studies in the group have shown no adverse effects of the technology on plant growth. Strik explains, "Plant-e technology is different. We make use of inert carbon electrodes, self-repairing plants and bacteria to make sustainable and clean electricity."

However, further research is needed to improve the efficiency of these electrodes. "We are looking at new materials mainly for the cathode. There is still a lot of research to do on that side."

Another area for development is optimising how the electrodes are positioned around the plant as this can cause electricity production to rise and material use to drop by two-thirds. Helder points out that electrodes could also be used in series or they could be combined with power converters. "We still need to figure this out," she says.

Food and power

The potential choice of plants is wide according to Helder, but there is one common requirement: they need to be tolerant to their roots being waterlogged. This is important because water is generated in the process. The group is particularly interested in the potential of large areas of grassland, marshes and fields of rice and say that Plant-MFCs will work even in brackish or polluted water areas.

Eventually they envisage that some large areas could be used to generate electricity alongside crop production. This, according to the researchers, has the advantage over biofuels of farmers not being required to choose between growing plants for fuel or for food. Plant-MFCs would not require plants to be harvested or burned and the technology could also be integrated discreetly into the landscape.

In urban areas the team envisages applications too, with green roofs helping meet the electricity needs of their buildings, with green roofs bringing additional advantages such as insulation and water storage.

"In urban areas the roofs of houses and offices are extremely suitable," says Strik, "Equipped with this technology these roofs always generate electricity, day or night, summer or winter."

The group has already set up one such green roof as a pilot on a 15m2 area on the roof of the Netherlands Institute of Ecology. It has been set up as a modular system with just one pair of electrodes per square metre which currently gives an output of about 0.4W of direct-current electricity per square metre of plant growth, which means that this pilot roof generates about enough electricity to charge a mobile phone.

"When we scale up to MW size we would need a few kilometres of plants," Helders says. "That's the scale we want to work towards, scaling up is one of our big challenges."

The team believes that in the future, bio-electricity from plants could produce as much as 3.2W per square metre of plant growth. They anticipate that the technology has the potential to be more efficient than many other forms of electricity generation because of the simplicity of the process and the low molecular weight of the molecules produced. In particular, electricity is generated directly rather than going through an intermediate heat step. Research has been published showing high Coulombic efficiencies of up to 90'per cent from this type of process. In contrast, current biomass systems require harvesting, transportation and an intermediate heat stage.

The group also sees Plant-MFCs as potential economic rivals for solar panels. "At the moment it's quite expensive because it's quite labour-intensive," Helder says. "Nothing is pre-fabricated. It's going to take time to automate but when we can get the labour costs down it will probably be cheaper than solar panels."

The company predict that Plant-MFC technology can cover at least 20 per cent of Europe's primary energy need in a real clean and sustainable way.

Early days

Currently, the project is still mainly in a research phase. The researchers plan a second pilot green roof for use in 2013 and a first viable product in 2014.

The company is likely to develop early products themselves - perhaps the first 1,000 modules - and will then use a licensing model'to sell them to a larger company that would be able to scale up production, while Plant-e moves on to prototyping further products. "It's really a platform technology that enables us to develop many useful products. There's enough room for a lot of companies to work on this technology. We need to get familiar with each different product but will work with other companies to develop them," Helder adds.

In the meantime, Plant-e, like many other businesses, has had to keep an eye'on'its'sales figures in the consumer space. "Right now we are still running on grants and subsidies and we have subsidies'to'keep'developing'until'2014," said Helder. "Hopefully the gadget we're launching will generate cash flow." All of which means that the company's future hopes could depend on the sales of a spinning globe.

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