Science - September 15, 2011

A planter as socket

Is it really possible - plants that produce electricity? Plants that recharge your mobile phone, light up streets and keep your fridge running? Wageningen pioneers are well on the way to making this pipe dream come true.

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Getting electricity from plants seems like science fiction, but David Strik, Environmental Technology researcher and part owner of startup Plant-e, takes a different view. ‘Twenty percent of the people on this earth have no access to electricity. A lot of them are living in wetlands. Those are people we could supply with electricity using Plant Power. Then they will be able to use a few square metres to light LED lamps, charge their mobile phone or run a super-efficient laptop. Plant Power is not a fantasy, we've shown that.'
It is already possible to charge a mobile phone using plants, so the principle does work. That principle (see info graphics) was discovered by Bert Hamelers, the head of the Environmental Technology group, who works on new technologies for generating clean energy. Plant power involves plants and bacteria working together to generate electricity. The plant provides the fuel and bacteria convert that into electricity. It's called a plant-microbial fuel cell. The ‘fuel' is the organic compounds discharged by the plant's roots into the soil. Sugars and organic acids as well as hydrocarbon polymers, enzymes and dead cell material make excellent fuel, for example. Electrochemical bacteria in the soil then break down this material (through oxidation) into CO2, H+ ions and electrons. You harvest those electrons and hey presto, you've got electricity.
Black box
So the whole process takes place in the soil. The plant-microbial fuel cell is essentially a planter with a few simple technical fittings to collect the electricity. The details of the process are still not fully understood. Plant power is pioneering work. Marjolein Helder, a PhD student and partner in Plant-e, is quite open about the fact that the system is partly a black box. ‘We have some insight into which bacteria are doing the work but that still needs to be published so I can't say anything yet. It is known that bacteria of the Geobacter genus are able to make electricity and we have seen them in our system, but we don't know how much fuel (exudates) a plant produces. The exudates are difficult to measure as they are broken down in the soil straight away. But the amount of fuel is obviously a deciding factor in the system. One of our challenges is to increase that concentration, for example by feeding the plant on certain nutrients.'
Of course, the plant itself is also important as it produces the fuel through photosynthesis. All it needs for that is CO2, water and sunlight. The plants being used are reed meadow grass and cord grass. Helder: ‘Those are our model plants. Reed meadow grass is a freshwater plant that can be found all over campus in the ditches. Cord grass likes salty conditions and is found in coastal areas. Any plant will do in principle as long as it grows in waterlogged, anaerobic soil. Oxygen is disastrous as it attracts the electrons being released. Boggy areas, wetlands, deltas and paddy fields could well be highly suitable for this technology.'
According to Helder, plant power is the greenest energy imaginable. ‘You could generate electricity from plants eleven months a year in our climate. Day and night, because it works at night as well. The process only stops when there's frost on the ground and the system is frozen.'
‘I didn't really want to do a PhD', reveals Helder. ‘But I was tempted into it by this project. You rarely get the opportunity to work on something so special. A project with so much potential that could also get results so quickly.' And there is also Plant-e (pronounced "plenty"), the spin-off Strik and Helder set up exactly two years ago to commercialize the new technique. At the time, Helder promised to develop the technology so that within a year they would be able to charge a mobile phone. Now, two years later, they have managed that.
Electrical roof
The setup that makes that possible has been running on the roof of the new NIOO building in Mansholtlaan since August. NIOO has been flaunting the fact that it has the first green electricity roof in the Netherlands. At present it has a total of sixteen square metres for its 'green battery' consisting of cord grass and reed meadow grass. It is Plant-e's first large-scale experiment and the first time 'useful electricity' is being produced, ‘with a voltage and electrical current we can do something with. For example, we are going to use it to charge mobile phones.' Helder praises NIOO for its pioneer's role. ‘We would like to do a feasibility study for Wageningen UR's Orion but that's still at the discussion stage. NIOO did provide us with the space, the opportunity and some of the money to do this.'
The NIOO trial should open the way to developing a real product. It is not yet clear what form that will take. Helder: ‘Should we start with a gadget for a window box? Should it be a build-it-yourself pack or should we market ready-made planters? It all depends on the scale. This should become clearer over the coming year.' She sees this as ultimately the biggest challenge. ‘Of course there are countless technical difficulties but these are known and we have thought of solutions for them. But we haven't really started on a large-scale application.'


Roof electricity

How much electricity does a plant generate? The Environmental Technology number crunchers say an average flat roof of fifty square metres could in theory generate 150 Watts continuously, about a third of what a household consumes. Assuming 25 cents per kWh, this would save the average household 330 euros a year.

Half a million?
Plant-e could well be half a million euros richer by the time you read this, as the winner of the 2011 Green Challenge is due to be announced on Thursday 15 September. Plant-e is one of the six candidates that made it to the final round of this sustainability competition, organized by the Postcode Lottery. ‘I was staggered when we heard that', says Marjolein Helder. ‘After all, it is an acknowledgement of the value of our work. People sometimes sneer about what we do here but now we are one of only two Dutch finalists.' Plant-e and the other five finalists were chosen from 717 submissions from around the world.

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