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Euphorbia tirucalli and prickly pear cactus growing at a test site in Kenya © Tessie George of LiveEnergies GmbH
Euphorbia tirucalli and prickly pear cactus growing at a test site in Kenya

Researchers from Plant Sciences and Engineering Science are collaborating on ways to release the huge energy potential of succulent plants that can survive in hostile environments.

Climate change demands that we move towards more sustainable forms of energy generation, and plants with high biomass certainly have the ability to make major contributions to the world’s need for sustainable sources of energy. But for bioenergy crops to make a major impact on climate change they will need extensive areas of land to grow. This has led to the charge that bioenergy crops are not really sustainable at all: traditional biofuels are made from food crops like maize or sugarcane, and are often grown on good-quality agricultural land, competing with food production. 40% of the USA’s maize crop, for example, is currently used to make bioethanol. Some bioenergy crops also involve the destruction of pristine natural habitats for their cultivation. Worldwide, meanwhile, there are hundreds of millions of hectares of degraded land (such as abandoned agricultural land or already deforested areas) which remain completely unused.

Researchers from the University of Oxford, Tropical Power, Imperial College London and the Royal Botanic Gardens at Kew are addressing this problem by turning their attention to a group of plants which have been very little studied in an agricultural context, and which humanity has barely begun to exploit. In a recent paper published in the journal Energy and Environmental Science the authors argue that succulent plants could be used to generate globally significant quantities of renewable electricity without displacing food crops, and indeed might even increase food supply.

This is because plants such as cacti, pineapples, agaves and species of Euphorbia (the spurge family) have evolved a remarkable solution to the problem of survival in hostile environments. They use a form of photosynthesis called crassulacean acid metabolism (CAM), which allows plants to shut their stomata during the day to minimise water loss. As this also prevents the plants taking in CO2 for photosynthesis, they capture CO2 at night instead by synthesising malic acid, which is then broken down the following day to release CO2 so the plant can photosynthesise. This adaptation allows CAM plants to make very efficient use of scarce water supplies. Because most CAM plants originate from tropical regions, they can also tolerate intense light and high temperatures.

The enormous advantage of CAM plants as bioenergy crops is that they can be grown on marginal or degraded land with levels of rainfall that are too low or where rainfall is too unreliable to support conventional agriculture, thereby avoiding the conflict between ‘food or fuel’. They are cheap to grow, and because of their natural tolerance of harsh conditions they will also be resilient to the hotter and drier conditions predicted for many parts of the world under future climate-change scenarios.

Succulent plants are often rich in simple sugars, which help protect the plant in extreme conditions and assist with water retention, and these sugars would provide an easy route for conversion to bioethanol by fermentation. However, the researchers are exploring another avenue that may be more energy-efficient: using the biomass for anaerobic digestion to produce biogas and thence electricity. Anaerobic digestion simulates a process that occurs in nature – for example, in a cow’s stomach, where microorganisms digest and decompose organic matter by feeding on it in the absence of oxygen. In the process, they produce biogas, a mixture of 60 per cent methane, 40 per cent carbon dioxide and traces of other gases. The presence of methane means biogas can be combusted in a combined heat and power gas engine to produce electricity, heat, or both.

Two plants in particular are being investigated: Euphorbia tirucalli, an African species of spurge, and the prickly pear cactus. The paper’s lead author, engineer Mike Mason, estimates that by growing these plants on anything from 100 – 380 million hectares of semi-arid land – between four per cent and 15 per cent of the total available – it would be possible to produce enough biogas to generate five petawatt hours (five trillion kilowatt hours) of electricity per year, equivalent to that generated from natural gas. An added benefit is that CAM plants are so good at storing water that large quantities of nutrient-rich waste water would be created as a by-product. This could be used in irrigation, as fertiliser, or for highly productive aquaculture for growing algae or other protein-rich food sources.

Compared to ethanol which needs large-scale, high-technology processing plants, biogas can be produced with relatively simple technology at modest scale – and this puts it within reach of some of the world’s poorest communities who are most in need of cheap sources of energy.