The ‘water footprint’ of bioenergy, i.e. the amount of water required to cultivate crops for biomass, is much greater than for other forms of energy. The generation of bioelectricity is significantly more water-efficient in the end, however – by a factor of two – than the production of biofuel. By establishing the water footprint for thirteen crops, researchers at the University of Twente were able to make an informed choice of a specific crop and production region. They published their results in the Proceedings of the National Academy of Sciences (PNAS) of 2 June.
In their article, the researchers show the water footprint of thirteen crops: the volume of water – rainwater and irrigation – required per gigajoule of energy production. In respect of various applications of biomass, the researchers present the impact that cultivation of the crops has on water consumption. By linking the water consumption to the location and climate data, it is possible to select the optimum production region for each crop. This makes it easier to prevent biomass cultivation from jeopardizing food production in regions where water is already in short supply, according to the researchers.
This lends an extra dimension to the bioenergy debate: until now, the discussion has mainly focused on the question of whether it should be allowed to use food crops for fuel. But underlying this is the question of how we should deploy our limited supplies of fresh water. Water that is used for bioenergy – whether it be for a food crop such as maize or a non-food crop such as jatropha – cannot be used for food production, for drinking water or for maintaining natural eco-systems. The water footprint, developed by Prof. Arjen Hoekstra, one of the authors of the PNAS article, is a powerful tool for surveying this.
1 litre of diesel, 14,000 litres of water
An example is biodiesel, which is made from rapeseed, soya or jatropha. On average, it takes 14,000 litres of water to produce one litre of biodiesel from rapeseed or soya. However, the water footprint for rapeseed in Western Europe is significantly smaller than in Asia. For soya, India has a large water footprint, while the figures for countries such as Italy and Paraguay are more favourable. Jatropha, which is increasingly used for biomass production, has an even less favourable water footprint of 20,000 litres of water on average for one litre of biodiesel.
The research shows that generation of bioelectricity has a smaller water footprint than the production of biofuels. A significant cause is that in the case of the former, the whole plant is used and in the case of the latter, only the sugar, starch or oil from the seeds is used. As regards the new generation of biofuels, ethanol can also be made from the stalk and leaves; this will have a favourable effect on the water consumption.
Sugar beet a favourite
In the generation of bioelectricity, too, there are big differences between the crops: sugar beet has by far the smallest water footprint – jatropha is 10 times less water-efficient. For the production of bioethanol, sugar beet is again by far the favourite: one litre of bioethanol made from sugar beet takes 1,400 litres of water, as against 2,500 litres for sugarcane, which is widely cultivated in Brazil.
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