Environmental Engineering Reference
In-Depth Information
Table 2.3
Energy ratio of several crops
Crop
Ratio
Miscanthus
32.5
Willow
30.0
Hemp (straw)
8.5
Wheat (grain)
8.8
Oilseed rape
3.8
number of crops based on DTI report URN 01/797, [16]. Miscanthus and willow are the
preferred crops in European and other similar latitudes. Currently, the main use of miscanthus
in the UK is in co-fi ring with coal in existing power stations.
Large scale biofuel production is not only energy intensive but it could have an adverse
environmental and social impact. Such development requires substantial water resources with
the result that water tables in areas of intense cultivation have been lowered to unacceptable
levels. Expansion of biofuel crops could also speed up tropical deforestation with the associ-
ated lowering of CO
2
absorption and threat to extinction of thousands of species of animals
and plants. If such crops are encouraged through subsidies, food shortages may occur if land
previously used to produce food is lost. It could be concluded that the biomass path, unless
used wisely, may cause serious environmental impacts.
2.9 Summary of Power Generation Characteristics
This chapter has reviewed the main forms of electricity generation. For the renewable energy
forms that are the main topic of this topic, the resource characteristics have been presented
and discussed in some detail since they impact signifi cantly on the manner in which these
sources can be integrated into the power system. This chapter concludes by summarizing the
key characteristics of the different forms of generation in Table 2.4.
In this table typical unit sizes for the different forms of electricity generation are referred
to. These unit sizes are in all cases the nameplate rating of the prime mover/generator
package. Power stations may contain one or several such units. The table also describes the
nature of the energy resource in terms of availability. Traditional generators supplied from
coal, oil, gas or nuclear energy score highly here as the primary energy resource is continu-
ously available. However, their capability to be dispatched, which requires that their output
can be changed automatically or at will by control engineers to follow the demand variations,
depends crucially on their thermal/mechanical nature and differs from plant to plant as
described in Chapter 3.
One characteristic shared by all the renewable sources, excluding biomass and tidal, is
their variability and relative unpredictability. This presents a challenge in integrating such
sources in electrical power networks that have been designed to operate with traditional
generators whose availability appears certain. In reality, however, no plant can be completely
available; there is always some probability of breakdown, the need for maintenance, etc. This
topic is complex and often misunderstood. It is dealt with in some detail in Chapter 3.
