Environmental Engineering Reference
In-Depth Information
cent per year over the past decade, and has an annual turnover of around
A$15 billion per year.
Biomass energy
Biomass refers to combustible or fermentable biomaterials created via photo-
synthesis. It can be derived from waste materials such as sugar cane bagasse,
garbage, sawdust and sewerage or from energy crops such as trees or canola.
Firewood is another common form of biomass energy. When biomass produc-
tion for energy is combined with other useful purposes the economic viability
of biomass energy can be substantially improved. For example, the growth of
Eucalyptus Mallee trees in the corridors between fields allows the harvesting of
wood by coppicing. The Mallee also serves other purposes including as a
windbreak, shade and an ecological corridor. In a country such as Australia,
where the population density is low, multiple-purpose energy crops may be
viable. In developing countries, where energy use per capita is low, biomass
can be a substantial fraction of total energy use.
Unfortunately, the conversion of solar energy into chemical biomass
energy has a very low overall efficiency. The conversion of solar energy to
chemical energy has an efficiency of less than 0.5 per cent, and conversion to
electricity has efficiency below 0.2 per cent (which is two orders of magni-
tude smaller than from photovoltaics, solar heat or wind energy). When
used on a very large scale, biomass competes with food and timber produc-
tion or with ecosystem preservation for the supply of arable land, water, pes-
ticides and fertiliser. The notion that biomass can be grown at low cost on
waste land with small environmental impact is incorrect. The use of low
quality agricultural land results in low yields and high costs, whether the
crop is for food, timber or for energy. Suggestions that genetic engineering
can be used to greatly increase the energy conversion efficiency of biomass
are far from current reality.
Other forms of solar energy
Indirect solar energy sources such as waves, ocean thermal gradients, ocean
currents and hydro sources are geographically limited. Wave energy technol-
ogy is still in the development phase, but could be important in the future in
regions with large areas of shallow sea and frequent storms. Temperature
differences between the deep and surface layers of the oceans can be used to
create electrical energy, although there are severe technical problems. Hydro
