Surrey of the monetary costs and benefits of a biogas plant
Figure 8.2 shows a breakdown of the basic investment-cost factors for a - presumedly -
standardized fixed-dome plant. The cost of material for building the digester, gasholder and
displacement pit (cement, bricks, blocks) can, as usual, be expected to constitute the biggest cost
item. At the same time, the breakdown shows that the cost of building the plant alone, i.e. without
including the peripherals (animal housing, gas appliances, piping) does not give a clear picture.
For a family-size plant, the user can expect to pay between 80 and 400 DM per m³ digester volume
(cf. table 8.2). This table shows the total-cost shares of various plant components for different types
of plant. While the average plant has a service life of 10-15 years, other costs may arise on a
recurrent basis, e.g. painting the drum of a floating-drum plant and replacing it after 4 - 5 years.
Otherwise, the operating costs consist mainly of maintenance and repair work needed for the gas
piping and gas appliances. At least 3% of the initial investment costs should be assumed for
maintenance and repair.
The main benefits of a biogas plant are:
- savings attributable to less (or no) consumption of conventional energy sources for cooking,
lighting or cooling
- the excess energy potential, which could be commercially exploited
- substitution of digested slurry in place of chemical fertilizers and/or financially noticeable
increases in crop yields
- savings on time that can be used for wage work, for example.
Usually, a biogas plant will only be profitable in terms of money if it yields considerable savings on
conventional sources of energy like firewood, kerosene or bottled gas (further assuming that they
are not subsidized).
Financially effective crop-yield increases thanks to fertilizing with digested slurry are hard to
quantify, i.e. their accurate registration requires intensive observation of the plant's operating
parameters.
Such limitations make it clear that many biogas plants are hardly profitable in monetary terms,
because the relatively high cost of investment is not offset by adequate financial returns.
Nonetheless, if the user considers all of the other (non-monetary) benefits, too, he may well find that
operating a biogas plant can be worth his while. The financial evaluation (micro-economic analysis),
the essential elements of which are discussed in the following chapter, therefore counts only as one
of several decision-making instruments to be presented to the potential user.
The main advisory objective is to assess the user's risk by calculating the payback period ("How
long will it take him to get back the money he invested?") and comparing it with the technical
service life of the plant. Also, the user must be given some idea of how much interest his capital
investment will carry (profitability calculation).
The micro-economic analytical methods described in the following subsections require the highest
achievable accuracy with regard to the identification of costs and benefits for the biogas plant under
consideration. Chapter 10.4 in the appendix includes an appropriate formsheet for data collection.
With a view to better illustrating the described analytical methods, the formsheet (table 10.10)
includes fictive, though quite realistic, data concerning a familysize biogas plant. Those data are
consistently referred to and included in the mathematical models for each of the various sample
analyses.
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