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Can renewable energy sources power the world?
Can renewable energy sources power the world?

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1 Introducing bioenergy

Bioenergy is the general term for energy derived from materials which are, or were recently, living matter, such as:

  • wood
  • straw
  • oilseeds
  • animal wastes.

They are collectively referred to as biomass.

All the Earth’s living matter, its total biomass, exists in the thin surface layer called the biosphere, which forms only a tiny fraction of the total mass of the Earth but represents an enormous store of chemical energy.

Although the majority of this is unavailable for human use, it is a store that is continually replenished by the flow of energy from the Sun, through the process of photosynthesis. As you can see from Figure 4.1, in this process carbon dioxide is taken from the air and combined with water from the surrounding environment to make living organic material, releasing oxygen in the process. In addition, a small fraction of the carbon from decomposing animal and plant residues (including roots) is transformed into soil organic matter, a significant carbon store.

The bioenergy cycle on a local scale
Figure 1 The bioenergy cycle on a local scale

If we intervene and ‘capture’ some of the biomass at the stage where it is acting as a store of chemical energy, we can use it as a fuel, that is, a material that can release useful energy through a change in its chemical composition, usually through burning. This generates heat that can be used directly or to raise steam for electricity generation.

Biomass can also be converted into intermediate biofuels such as:

  • charcoal
  • biogas
  • bioethanol
  • biodiesel.

Provided our consumption does not exceed the natural level of production, the burning of biomass should generate no more heat and create no more carbon dioxide than would have been formed in any case by natural processes.

Recent estimates of the annual contribution to world primary energy from traditional biomass fall in the range 40–60 EJ.

(Bauen et al., 2009)

The estimate given in Week 1 is based on a contribution of 30 EJ, with industrially processed biomass adding a further 9 EJ or so (BP, 2010).

Traditional biomass energy accounts for about a third of total primary energy consumption in the developing countries. In the industrialised world, the energy contribution from biomass is significant and rising, particularly in countries with large forestry industries such as Sweden and Finland.

Next you’ll look at how biomass works as a solar energy store.