Environmental Engineering Reference
In-Depth Information
balance can therefore be carried out when the energy-producing and energy-
consuming devices have been established. On the basis of a detailed energy
balance, the different technological options can be evaluated and compared.
This section explains the basics and framework of energy balancing, along
with a specific example.
9.4.1 Basics and definitions
Independent evaluation of the energy flows in a biogas plant requires a clear
methodology and definitions. The crucial methodological
issues are as
follows.
.
For an exact calculation of an energy balance for a single biogas plant,
the correct boundaries have to be defined and correctly taken into
account. All flows of energy and material into, out of and within the
biogas plant must be described and all the boundaries correctly defined,
including the point of substrate import into the biogas plant, the point
of liquid/solid product output (e.g. including or excluding logistics), the
point of energy input (e.g. electricity before or after a transformer) and
the points of energy output. These should not be defined solely on the
basis of a flow diagram; instead, definitions should be established at the
plant itself, for example at a pipe connector. This is of particular
importance when comparing different biogas plants. It has to be clearly
stated which individual processes belong to the biogas plant and which
processes do not. The question of what equipment is necessary for
biogas production and what equipment is necessary for processes
external to the biogas production can help to identify suitable
boundaries.
.
The calculation method for analysing the energy content of solid,
gaseous and liquid material streams has to be established, particularly
with respect to chemically bound energy. One promising option is the
measurement or calculation of the heating value of the material flows.
.
The definition of references for the purpose of benchmarking and
evaluation in comparison with alternative technologies is extremely
important and helps to define suitable boundaries. Starting with
available data relating to the alternative technologies can be helpful
for methodological clarification.
.
The general rules for process modelling must be taken into account. This
implies the use of sound and consistent variables and units that are
always in correlation with the same reference. A reference can be a time
unit (e.g. day, month, year) or a mass unit (e.g. 1 t of substrate input).
Moreover, the main rules for energy balancing are the fundamental
theorems of energy. At the very least, the first theorem - in a closed
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