Energy crop

Energy crops are low-cost and low-maintenance crops grown solely for renewable bioenergy production (not for food). The crops are processed into solid, liquid or gaseous fuels, such as pellets, bioethanol or biogas. The fuels are burned to generate electrical power or heat.

The plants are generally categorized as woody or herbaceous. Woody plants include willow and poplar, herbaceous plants include Miscanthus x giganteus and Pennisetum purpureum (both known as elephant grass). Herbaceous crops, while physically smaller than trees, store roughly twice the amount of CO₂ (in the form of carbon) below ground compared to woody crops.

Through biotechnological procedures such as genetic modification, plants can be manipulated to create higher yields. Relatively high yields can also be realized with existing cultivars. However, some additional advantages such as reduced associated costs (i.e. costs during the manufacturing process) and less water use can only be accomplished by using genetically modified crops.

Solid biomass, often pelletized, is used for combustion in thermal power stations, either alone or co-fired with other fuels. Alternatively it may be used for heat or combined heat and power (CHP) production.

In short rotation coppice (SRC) agriculture, fast growing tree species like willow and poplar are grown and harvested in short cycles of three to five years. These trees grow best in wet soil conditions. An influence on local water conditions can not be excluded. Establishment close to vulnerable wetland should be avoided.

Whole crops such as maize, Sudan grass, millet, white sweet clover, and many others can be made into silage and then converted into biogas. Anaerobic digesters or biogas plants can be directly supplemented with energy crops once they have been ensiled into silage. The fastest-growing sector of German biofarming has been in the area of "Renewable Energy Crops" on nearly 500,000 ha (1,200,000 acres) of land (2006). Energy crops can also be grown to boost gas yields where feedstocks have a low energy content, such as manures and spoiled grain. It is estimated that the energy yield presently of bioenergy crops converted via silage to methane is about 2 GWh/km² (1.8×10¹⁰ BTU/sq mi) annually. Small mixed cropping enterprises with animals can use a portion of their acreage to grow and convert energy crops and sustain the entire farm's energy requirements with about one-fifth of the acreage. In Europe and especially Germany, however, this rapid growth has occurred only with substantial government support, as in the German bonus system for renewable energy. Similar developments of integrating crop farming and bioenergy production via silage-methane have been almost entirely overlooked in North America, where political and structural issues and a huge continued push to centralize energy production has overshadowed positive developments.^{[citation needed]}

European production of biodiesel from energy crops has grown steadily in the last decade, principally focused on rapeseed used for oil and energy. Production of oil/biodiesel from rape covers more than 12,000 km² in Germany alone, and has doubled in the past 15 years. Typical yield of oil as pure biodiesel is 100,000 L/km² (68,000 US gal/sq mi; 57,000 imp gal/sq mi) or higher, making biodiesel crops economically attractive, provided sustainable crop rotations are used that are nutrient-balanced and prevent the spread of disease such as clubroot. Biodiesel yield of soybeans is significantly lower than that of rape.

Two leading non-food crops for the production of cellulosic bioethanol are switchgrass and giant miscanthus. There has been a preoccupation with cellulosic bioethanol in America as the agricultural structure supporting biomethane is absent in many regions, with no credits or bonus system in place.^{[citation needed]} Consequently, a lot of private money and investor hopes are being pinned on marketable and patentable innovations in enzyme hydrolysis and similar processes. Grasses are also energy crops for biobutanol.