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Exact estimations for the construction and operation of biogas plants serve the following purposes:
- to compare the costs of alternative models (optimal project selection)
- for the information of the users as far as future financial burdens are concerned
- the calculation of financing needs including public subsidies (budget planning)
As far as costs are concerned there are three major categories:
The production costs include all expenses and lost income which are necessary for the construction of the plant e.g.: the land, excavation-work, construction of the digester and gas-holder, the piping system, the gas utilization system, the dung storage system and other buildings. The construction costs comprise wages and material.
The production costs of biogas plants are determined by the following factors
- purchasing costs or opportunity costs for land which is needed for the biogas plant and slurry storage;
- model of the biogas plant;
- size and dimensioning of the biogas unit
- amount and prices of material
- labor input and wages
- the degree of participation of the future biogas user and his opportunity costs for labor.
Calculating total production cost:
To gain a rough idea of the typical costs of a simple, unheated biogas plant, the following figures can be used: total cost for a biogas plant, including all essential installations but not including land, is between 50-75 US Dollar per m3 capacity. 35 - 40% of the total costs are for the digester.
The specific cost of gas production in community plants or large plants is generally lower compared with small family plants. The cost for the gas distribution (mainly piping) usually increases with the size of the plant. For communal plants with several end-users of biogas, the piping costs are high and compensate the degression by 'economics of size' partly or wholly. In regions where plant heating is necessary, large-scale plants would be more economical .
To keep the construction costs low, labor provided by the future biogas users is desirable. Often, the whole excavation work is done without hired labor. On the whole, a reduction of up to 15% of the wages can be effected by user-labor. If periods of low farm activities are chosen for the construction of the biogas plant, opportunity costs for labor can be kept low.
The operation and maintenance costs consist of wage and material cost for
- acquisition (purchase, collection and transportation) of the substrate;
- water supply for cleaning the stable and mixing the substrate;
- feeding and operating of the plant;
- supervision, maintenance and repair of the plant;
- storage and disposal of the slurry;
- gas distribution and utilization;
The running costs of a biogas plant with a professional management are just as important as the construction costs, for example for operation, maintenance, expenses for painting, service and repair.
Large-scale biogas plants have a high water consumption. Investigations are necessary, if the water quantity required causes additional costs in the long run. These could be construction costs for water piping or fees for public water supply. The question of water rights has to be clarified. Steps to be taken to cover the demand for water during dry periods require thorough planning.
Capital costs consist of redemption and interest for the capital taken up to finance the construction costs. For dynamic cost comparison the capital fixed in the plant is converted into equal annual amounts (see dynamic annuity calculation of costs).
The capital cost, apart from the depreciation rates or length of amortisation, is dependent on the interest rate at which the capital is provided. In each case current interest rates are to be laid down for the cost calculation, which reflect the opportunity costs of the invested capital. To avoid distortions of the financing costs the comparisons should always be calculated with the same interest rate.
Lifetime of Plants
In calculating the depreciation, the economic life-span of plants can be taken as 15 years, provided maintenance and repair are carried out regularly. Certain parts of the plant have to be replaced after 8 - l0 years, e.g. a steel gas holder. The steel parts need to be repainted every year or every second year. As a rule, real prices and interest rates should be used in the calculations. For cost calculation inflation rates are irrelevant as long as construction costs refer to one point of time. However, in calculating the cash reserves put aside for servicing and repair the inflation rate must be considered.
Average Cost of a Biogas Plant
The cost per cubic meter of digester volume decreases as volume rises. Therefore, the appropriate size of the biogas plant should be estimated.
For simple, unheated plants in tropical countries, the digester size is roughly:
- 12 to 20-fold the quantity of substrate put in daily at average expected digester temperatures over 25°C and
- 18 to 25-fold the quantity of daily feeding for temperature between 20 and 25°C.
Since the final method of construction is only determined during the first years of a biogas project, it is impossible to exactly calculate the building costs ahead of the actual implementation. The GTZ computer program called "BioCalc" (produced by BioSystem), can only provide an idea as it is based on only one type of plant.
Consequently, the following system is sufficient for a rough calculation:
- the cost of 6.5 sacks of cement x m3 digester volume plus
- the cost of 5 days work for a mason x m3 digester volume plus
- the costs of 100 m gas pipes (1/2"), plus
- the costs of two ball valves (1/2"), plus
- the cost of gas appliances which are feasible for this size.
The individual prices are to be determined for the project location. The sum then includes material and wages. The distance from the biogas plant to the point of gas consumption was assumed as being 25 m (the 100 m used in the calculation include costs for connectors and wages). Where greater distances are involved, the cost for gas pipes will have to be increased in proportion.
Cost and Benefit Relation of Biogas Plants
► Cost and Benefit Relation of Biogas Plants
As soon as the cost and benefit components of a biogas plant in planning can be quantified, and as soon as other important parameters (time horizon, interest rate, annual allowances, exchange rates, inflation rates) are determined, the economic viability of a biogas plant can be calculated.
Typically, the financial analysis of projects points out the financial viability of investment alternatives.
Three types of questions need to be answered:
- Which project is the least expensive among an array of options that produce the same output (least cost analysis)?
- Which project shows the highest net benefit (benefit minus cost) among an array of options (cost benefit analysis)?
- Is a project a financially viable solution to the problem on hand? (absolute viability, i.e. the question is dealt with whether the project's revenues are sufficiently high to meet capital cost and operating cost), and:
- Is a specific project more economical than others? (relative viability).
Procedure of Dynamic Approach
Due to the fact that the same amount of a credit or debit can have a very different value depending on when the transaction takes place, dynamic analysis differ from the static methods.
The need for a dynamic approach results from the fact that, as the costs and benefits of each option arise in different years, it is necessary to make them comparable.
The value which says how much a future or past payment is worth at the present time is described as its present value (PV).
Given an investment of a biogas plant of 2000 US$ in two years (discounting), having paid three years ago 120 US$ for the necessary landed property (compounding), with a given interest rate of 8%, the PV is as follows:
PV = [2000/(1,08)2 + 120*(1,08)3]
It is calculated from its past amount by compounding or from the future amount by discounting with the aid of a factor which depends on the interest rate adopted and the length of time between the payment and the present period.
The dynamic approach deals with a consideration of benefits and costs over several years and therefore shall be pointed out more detailed:
Investment criteria are, as follows:
Net Present Value (NPV)
The most common investment criteria is the NPV and is defined as follows:
NPV - Net Present Value
Ct - Costs in year t
Bt - Benefits in year t
k - discount rate
t - number of years from the present
n - total number of the years of the analysis period
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 AT Information - Biogas Digesters: http://www.fastonline.org/CD3WD_40/BIOGSHTM/EN/COSTBEN/COSTS.HTML