Introduction and Structure - Renewable Energy: Technology, Economics and Environment - page 10

different utilisation methods in a flexible manner. Therefore the basic procedure
of addressing the various options is outlined in the following. Furthermore, the
main terms on which the respective explanations are based are defined.
1.3.1 Principles
The possibilities and boundaries to convert renewable energies into end or useful
energy largely depend on the respective physical and technical conditions. They
will thus be explained and discussed in detail for each utilisation method. When-
ever possible, among others, also the theoretically or technically maximum
achievable efficiencies and technical availabilities will be indicated. These techni-
cal key figures are defined as follows.
− Efficiency. The efficiency is defined as the ratio of useful power output (e.g.
electricity, heat) to the power input (e.g. solar radiation, geothermal energy). It
depends on the respective operating conditions of the conversion plant, as well
as a series of other factors, which vary over time (e.g. in case of a heating
boiler the efficiency varies with ambient temperature, among other factors).
− Utilisation ratio. The utilisation ratio is defined as the ratio of the total output
of useful energy to the total energy input within a certain period of time (e.g.
one year). The observed time periods may include part load periods and breaks
as well as start-up and shutdown times. Utilisation ratios are thus usually
smaller compared to the efficiency of conversion plants indicated for the design
point at full load.
− Technical availability. The technical availability describes that portion of the
time period under observation, within which a plant has actually been available
for its intended purpose and thus considers time periods during which the plant
has been unavailable due to malfunctions.
1.3.2 Technical description
The renewable energy supply, whose physical principles and supply characteris-
tics are described first (Chapter 2), can be transformed by appropriate conversion
plants into secondary or end energy carriers, or directly into useful energy. The
methods and procedures appropriate under the given conditions are described
within the following chapters; explanations are based on state-of-the-art technol-
ogy and current conditions.
First, the system components referring to the respective utilisation method are
discussed and subsequently explained within the context of the overall energy
conversion system. This includes the characteristic curve, the energy flow as well
as the respective losses given within the entire provision or conversion chain. In