Pennisetum purpureum, also known as elephant grass (Fig. 4.14), is a species of grass native to the tropical grasslands of Africa. It is a tall (2-4.5 m) perennial plant with a very high productivity, both as a forage grass for livestock and as a biofuel crop. It is usually harvested before winter, so it can be burnt in power plants (Langeland et al., 2008).

Helianthus tuberosus

Helianthus tuberosus, also called Jerusalem artichoke, sunroot, sunchoke, earth apple and topinambur, is a species of sunflower native to the eastern United States that can grow up to 3 m high (Fig. 4.15). Due to its high alcohol yield (5000-6000 l/ha in Spain), Jerusalem artichoke also has an unused great potential as a producer of ethanol fuel from stems. Tubers are an important source of fructose for industry (Huxley et al., 1992; Davidson et al, 2006).