Science & Technology in Action

6th Edition

Crops for Energy

Teagasc Higher Education Authority

Agricultural Science
This lesson explores the potential of crops as sources of sustainable fuels. It analyses the energy efficiency of plants and outlines the methods used to recover plant energy, including combustion, gasification, fermentation and digestion. Additional costs such as planting, fertilisation and harvesting are also considered.
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Lesson excerpt

Over recent years there has been growing public awareness of the need to rationalise our use of fossil fuels and where possible to replace them with sustainable alternatives. A variety of crops can be used directly or indirectly as sources of fuel.

The Basic Principle
Plants need light; this is their ultimate source of energy. The living cells of a plant require a steady supply of energy but light is not available all the time and some cells never receive light (e.g. in the roots). Besides, very few of the thousands of biochemical reactions in plant cells can be 'powered' directly by light.

In the leaves of plants a process calledphotosynthesis takes place; in effect light energy is used to combine carbon dioxide and water to form glucose. Much of the glucose is distributed to other parts of the plant to power the complex life processes. What is not required immediately may be stored as starch. As plants grow new 'building materials' are required to form more roots, stems, leaves and fruit/seeds. These are composed of carbohydrates (such as cellulose), proteins and lipids (fats or oils). Over 90% of the dry mass of a plant is derived from the carbon dioxide it takes from the air.

If plant material is broken down (e.g., by digestion or combustion) the chemical energy 'stored' in the cellulose, oils etc. is released and may be used for heating or electricity generation.

True or False?

  1. Plants produce proteins by photosynthesis. false
  2. Global energy needs can be met by biofuels false
  3. Ethanol can be produced by fermentation of carbohydrates. true
  4. Ethanol production by cellulosic fermentation is still under development. true
  5. All the light energy entering a leaf is captured in carbohydrates such as glucose. false
  6. The photosynthetic efficiency of most plants is about 10%. false
  7. Chloroplasts absorb all the light energy that shines on a leaf. false
  8. About half of the glucose produced during photosynthesis is used to power other parts of the photosynthesis cycle. false
  9. Plants absorb carbon dioxide during photosynthesis. true
  10. The Sun is the ultimate source of energy for the Earth. true