What are energy harvests?

Rapeseed sprig, which is used to make biodiesel.

Energy crops are plants grown and harvested with the specific intention of using them as fuel. These plants are typically divided into two categories: herbaceous, such as grasses, and woody, such as trees and shrubs. The energy stored in the plants can be accessed by direct combustion, gasification and conversion into liquid fuels. Energy crops are generally chosen for their energy content, the ease with which they can be grown and harvested, as well as the associated costs of processing the final product. Inedible parts of food plants used for the same purpose are considered agricultural by-products, not energy crops.

A large rapeseed field.

Several types of grass have potential as an energy source, with switchgrass generating more interest; yields are typically higher and production costs lower than for other herbaceous plants. Switchgrass can reach full production capacity in three years and grows from a permanent root system, requiring no replanting for up to 15 years. Only a quarter of the amount of water and fertilizer needed for most food crops is needed and the grass is remarkably resistant to pests and droughts. Standard farm equipment can be used to harvest switchgrass, which is baled like hay.

Some crops are used to make biodiesel fuel.

Trees and shrubs used for energy crops usually do not grow to maturity as lumber is not the final product. Short rotation hardwood crops (SRWC) are grown with an expected harvest within four to ten years, while fast-growing hardwoods such as poplar and willow can grow up to ten feet a year under ideal conditions. Cultivated tree groups also produce up to ten times more wood per acre than natural forests.

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The two main uses of energy crops are the generation of electricity and as a feedstock for the production of biofuels. A power plant can use crops in a steam boiler, burning them directly or along with coal, a practice called cofiring. Through the gasification process, biomass can be processed into syngas, a mixture of carbon monoxide and hydrogen or methane. Both can be used to power steam turbines or as a power source for manufacturing.

Butanol, a long-chain hydrocarbon similar to gasoline, can be produced from herbaceous plants such as switchgrass, miscanthus and elephant grass. Ethanol, an alcohol fuel, is made from wheat, corn, sugar cane, or any plant that can be used to make an alcoholic beverage, while biodiesel can be made from vegetable oils produced from energy crops like soy, rapeseed and hemp. The lipids, or fats, in the oil are reacted with alcohol to produce biodiesel. While some biofuels can power vehicles directly, most are used in blends with traditional fuels.

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