Scope of these notes
The objective of the lecture notes is to give an introduction to the current issues of
energy engineering with a focus on the role of energy supply for development of the
human societies. Technical issues are treated only to the extent that this is necessary
for understanding of the limitations and potential of the current technologies.
These notes discuss the potential and technologies of renewable energy sources for
generation of energy carriers in industrial and developing countries. The approach is
source-oriented rather than user-oriented, but this does not mean that it is implied that
a switch from a “conventional” energy source to a renewable energy source requires
that the same energy carrier or the same amount of energy shall serve the needs of the
user after the switch. Since use of renewable energy is often more expensive than use
of the conventional energy sources, introduction of energy conservation measures is
often economically justified when the switch to renewable energy is made.
Definition of renewable energy
In the strict thermodynamic sense energy cannot be destroyed or produced, only
converted from one form to another. These conversions are associated with an
increase of the total entropy, which means a loss in the “quality” of the energy i.e. a
loss of exergy. Apparently, renewable energy is a contradiction from a strict
thermodynamic point of view.
The term is nevertheless used frequently, not only by politicians and laymen, but also
by those who understand their thermodynamics. Renewable energy is then understood
as energy that is supplied directly or indirectly from the Sun or the Moon and thermal
energy stored or generated below the crust of our planet, the Earth. In other words,
renewable energy represents energy that can be expected to be available for as long as
the Earth is habitable for the human race. Certainly these energy flows will deteriorate
at some stage, but in the human perspective they may be considered as perpetual. This
is in contrast with the large but much more limited deposits of fossil fuels, coal,
petroleum and natural gas and the likewise limited deposits of fissile uranium.
Even though renewable energy is based on energy sources that are expected to be
available very far into the future, there is no guarantee that an energy system is
sustainable just because it relies on renewable energy sources. A discussion of
different requirements for sustainability is the subject of a forthcoming lecture. It may
be sufficient at this stage to mention two requirements for sustainability that must be
fulfilled in addition to a sustainable energy source. These are economic sustainability
and environmental sustainability.
Economic sustainability requires that the resources generated by utilisation of the
energy source exceed those needed for the utilisation. Environmental sustainability
requires that first of all the resource base is not deteriorated by excessive utilisation.
This can easily be the case when biomass energy is over-utilised. Environmental
sustainability also requires that the impacts and risks caused by the utilisation are