Introduction and Structure - Renewable Energy: Technology, Economics and Environment - page 2

Against this background, the dimensions of global energy system are illustrated
and discussed as follows. However, first some energy terms are defined.
1.1.1 Energy terms
According to Max Planck, energy is defined as the ability of a system to cause ex-
ternal action. In this respect the following forms of energy are distinguished: me-
chanical energy (i.e. potential or kinetic energy), thermal, electric and chemical
energy, nuclear energy and solar energy. In practical energy appliances, the ability
to perform work becomes visible by force, heat and light. The ability to perform
work from chemical energy, as well as nuclear and solar energy is only given if
these forms of energy are transformed into mechanical and/or thermal energy.
The term energy carrier - thus a carrier of the above defined energy - is a sub-
stance that could be used to produce useful energy, either directly or by one or
several conversion processes. According to the degree of conversion, energy car-
riers are classified as primary or secondary energy carriers and as final energy car-
riers. The respective energy content of these energy carriers consists of primary
energy, secondary energy and final energy. Definitions of the individual terms are
as follows (Fig. 1.1) /1-1/, /1-2/.
− Primary energy carriers are substances which have not yet undergone any tech-
nical conversion, whereby the term primary energy refers to the energy content
of the primary energy carriers and the "primary" energy flows. From primary
energy (e.g. wind power, solar insulation) or primary energy carriers (e.g. hard
coal, lignite, crude oil, and biomass) secondary energy or secondary energy
carrier can either be produced directly or by one or several conversion steps.
− Secondary energy carriers are energy carriers that are produced from primary
or other secondary energy carriers, either directly or by one or several technical
conversion processes (e.g. gasoline, heating oil, rape oil, electrical energy),
whereby the term secondary energy refers to the energy content of the secon-
dary energy carrier and the corresponding energy flow. This processing of pri-
mary energy is subject to conversion and distribution losses. Secondary energy
carriers and secondary energies are available to be converted into other secon-
dary or final energy carriers or energies by the consumers.
− Final energy carrier and final energy respectively are energy streams directly
consumed by the final user (e.g. light fuel oil inside the oil tank of the house
owner, wood chips in front of the combustion oven, district heating at the
building substation). They result from secondary and possibly from primary
energy carriers, or energies, minus conversion and distribution losses, self-
consumption of the conversion system and non-energetic consumption. They
are available for the conversion into useful energy.
− Useful energy refers to the energy available to the consumer after the last con-
version step to satisfy the respective requirements or energy demands (e.g.