| Plant -MFC Concept |
Plant-MFC TechnologyThe novel technology we study in this project is the Plant Microbial Fuel cell. The concept involves living plants that harvest solar energy with their leaves to produce carbohydrates from carbon dioxide.
These carbohydrates are transported from the production site (leaves) to the underground roots. A part (up to 40% or more was reported (Bais et al., 2006) of these carbohydrates and other molecules can be excreted into the surrounding soil. These so-called exudates or rhizodeposits, mainly low-molecular weight organic acids and carbohydrates, are in nature used by micro-organisms in the complex soil ecosystem (rhizosphere). Our approach is to efficiently convert in-situ the chemical energy of exudates into electrical energy using a emerging technology, the microbial fuel cell (MFC). Principally, this can be accomplished by placing the plant with its roots in the bioanode, containing the recently discovered electrochemically active micro-organisms, of the microbial fuel cell. Three members of the PlantPower team were recently & independently able to deliver the first 2 ‘proofs of principle’ (Strik et al. 2008; De Schamphelaire et al., 2008) by showing that electricity was generated with living plants and microbes in a fuel cell. These were the first demonstrations of the direct use of rhizodeposist as bioenergy source. The Plant-MFC is expected to also produce hydrogen, a much desired biofuel. This is possible as the electrons produced have sufficient energy for biocatalysed hydrogen production, (Rozendal, 2007).
Proof-of-principle
Societal impact: clean, renewable, sustainable, efficient, in-situ & social production
Societal impactPlantPower project deliverables have potentially a high impact to solve societal problems caused by current energy systems. The current pollution, high energy prices, increasing food prices & climate change have all a direct negative impact on the quality of life, health, safety and the environment. The Plant-MFC concept does not have these negative impacts and can improve the quality of life. The Plant-MFC may mature to a very competitive technology towards other bioenergy systems as it can deliver net 5 times higher energy than other systems. It is in potential a clean, renewable, sustainable, efficient, in-situ & social electricity and fuel production system as explained next CleanFirst of all, the Plant-MFC concept produces bioenergy in clean way. No combustion or extra greenhouse gas emissions (toxic fine particles and environment damaging SOx and NOx) will be produced during production. The Plant-MFC has a short carbon cycle; the plant takes up CO2 which is, after current generation, by the bacteria in the fuel cell returned towards the atmosphere. The Plant-MFC has even the potential to reduce methane emission when implemented into rice fields. Rice plants excrete many organics which are now transformed into methane and contribute to an estimated 20 percent of all methane emission world wide (Crutzen, 1991). When these energy carriers are oxidized by electrochemical active bacteria, no methane will be produced, but CO2, which is the original gas taken up by the plant. However, we are aware of the possibility that by future Plant-MFC implementations the total methane emission may increase. Therefore, this project includes RTD on solutions regarding the competition and methane production in rhizosphere processes. RenewableThe Plant-MFC concept captures solar energy and transforms it into useful bioenergy. This source is considered renewable since it will be available for an estimated 5.5 billion years. The Plant-MFC concept produces bioenergy in a sustainable manner as no pollutants are produced and nutrients will be preserved in the technology. E.g. when perennial plants are used in (sub) tropical regions, plants can produce exudates for decades. So, in this technology plants are not harvested and burned which results in often non reusable ashes. Efficient & in-situThe Plant-MFC is also efficient. The main reason is that the plants in the Plant-MFC produce low molecular weight molecules which are efficient transformable into useful energy carriers. This is much more energy efficient then producing the complex molecules forming the plant biomass. The micro-organisms in the Plant-MFC are especially efficient in transforming low molecular compounds like exudates. Transformation has been reported with high Coulombic efficiencies up to 90% (Logan et al., 2006). Total MFC energy recoveries while treating artificial wastewater of up to 60% were achieved in the laboratories of WU. (not published) This high efficiency is possible due too the fact that electricity production is possible without the intermediate producti on of heat. This way the process is not limited by Carnot’s theorem. How productive the Plant-MFC will become is prospected in the next paragraph. For now we want to emphasize on the in principle efficient energy transformations present in the Plant-MFC. This in sharp contrast to current bio-energy systems, where pools of complex molecules need to be processed at lower efficiencies (UNDP, 2001; EPSO, 2007). Furthermore, in a Plant-MFC, no harvesting and transport of biomass is needed. As no biomass is harvested also no nutrients will be taken away from the Plant-MFC. A fully matured Plant-MFC system could thus in principle produce in-situ bioenergy with a low almost negligible fossil energy input, as no harvesting, transport and fertilization are needed. SocialFrom a social point of view the plant-MFC has several advantages. E.g., Plant-MFCs can be implemented in (semi-)natural environments such as recreation areas, rice-fields, saline soils, energy islands, selected wetlands, biorefineries, with most likely a minimal disturbance of the scenery & primary functions and so without being competitive with agricultural lands that are needed for food and without damaging the environment. Actually the work of Schamplelaire et al. (2008) with rice plants did not show significant difference of rice production with or without MFC integration. So that’s the first proof that food and bioenergy can be combined without competing with each other.
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