Environmental Engineering Reference
In-Depth Information
.
maximum energy demand (e.g. simultaneously running electrical
equipment; starting load of pumps/stirrers/CHP; maximum heat
demand in winter)
.
time dependency of energy demand (e.g. changes dependent on season,
over the course of a week and over the course of a day)
.
heat level(s) required
.
definition of single energy consumers with behaviour as described
above.
The provision of energy supply can then be planned on this basis. The first
issue to be determined is how much of the energy demand is to be met by
external sources (e.g. grid, generator, heating unit) and how much by
internal resources (e.g. biogas burner, excess heat from single aggregates
(compressors or CHP unit)). It is generally possible to run a biogas plant
isolated from energy grids, but running the plant within an electricity grid is
more convenient. The final solution for energy supply is determined not only
on the basis of technical and economic optimisation, but increasingly also
on ecological optimisation: today, the aim of reducing the greenhouse gas
emissions of a plant is an important aspect in the project planning process.
A biogas plant that is able to meet its own energy demand with biogas or
alternative renewable energy has a significantly better greenhouse gas
balance than a biogas plant supplied with fossil energy (e.g. from the
electricity or heat grid). An example is shown in Fig. 9.2.
9.3.1 Electricity supply
For most equipment used in industrial-scale biogas plants, a reliable
electricity supply is essential for correct operation. Electricity supply has to
be continuous and at a very uniform voltage, due to the fact that most
plants are controlled by a computer-based central control unit. If a reliable
electricity supply cannot be guaranteed, the electrical equipment selected
must be able to operate with the available supply; alternatively, additional
technical measures must be undertaken, such as the use of batteries or the
implementation of an uninterruptable power supply.
The main parameter for the electricity supply is the maximum
instantaneous load, which is in most cases connected to the starting current
of the motors (for example in pumps and mixers it can be as much as double
the power demand for each single piece of equipment). The simultaneous
starting and running of multiple units of equipment must also be taken into
account.
The simplest method for providing electricity supply is a connection to an
existing reliable electricity grid with sufficient capacity for the maximum
load of the biogas plant. The electricity for most biogas plants in Europe is
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