Technology and operationAnalysis
Operating costs (OPEX) of an agricultural biogas plant: structure and how to calculate them
What the cost of running a biogas plant consists of: substrates, CHP servicing, own energy, labour, insurance, digestate. How to calculate the cost per kWh and per m³ CH₄ and where to look for savings.
Other language versions: polski.
The operating costs (OPEX) of an agricultural biogas plant consist of several recurring groups: substrates, servicing and overhauls (mainly the CHP), labour, own energy and utilities, digestate management, insurance, taxes and fees, and analyses and monitoring. In many maize silage plants the feedstock costs the most; in waste plants its cost is often low or negative (a gate fee), but the expenditure on treatment, hygienisation and maintenance rises. Specific prices change from year to year, which is why below we show the structure and the method, not amounts.
Main cost groups
| Group | What it includes | What it depends on |
|---|---|---|
| Substrates | purchase, cultivation, harvest, ensiling, transport, losses | mix, distances, agricultural prices |
| Servicing and overhauls | oil, filters, spark plugs, major CHP overhauls, pumps, agitators, parts | operating hours, gas quality, service contract |
| Labour | crew, training, on-call duty | scale of the installation and automation |
| Own energy | mixing, pumps, blowers, heating (if not from the CHP) | technology, dry matter, weather |
| Digestate | storage, transport, application, separation | distances, area, legal requirements |
| Insurance and fees | policies, local taxes, environmental fees | location, value of the site |
| Analyses, monitoring, administration | laboratory, certification, accounting | legal and contractual requirements |
| Financing | interest, instalments | financing structure (usually outside OPEX) |
How to calculate the unit cost
For clarity, convert the costs in three ways:
- PLN/year — the sum of the items;
- PLN/kWh (or PLN/MWh) — OPEX divided by net electricity or by total energy (electrical and thermal), stating clearly what is in the denominator;
- PLN/m³ CH₄ — makes it possible to compare different technologies for using the gas, including upgrading.
Example (fictitious assumptions, for practice only)
An installation with an electrical capacity of 0.5 MW operates 8,000 h/year, producing 4,000 MWh of gross electricity. Own consumption: 8%, i.e. 3,680 MWh net. Let us assume the following OPEX breakdown: substrates 50%, servicing and overhauls 15%, labour 8%, own energy and utilities 5%, digestate 10%, insurance and fees 5%, analyses and administration 4%, other 3%. If total OPEX were 100 units, then 50 would go to substrates, 15 to servicing and so on; the cost per net MWh = 100 / 3,680 units per MWh. The figures do not correspond to real amounts, they only illustrate that a 10% change in the substrate price changes total OPEX by about 5%, and a 10% change in the cost of servicing — by 1.5%. That is why optimisation starts with the largest items.
Where to look for savings
- Substrates: better silage quality, lower storage losses, optimisation of the mix (e.g. a larger share of slurry and by-products), long-term contracts.
- Servicing: clean gas (desulphurisation), regular maintenance, a contract matched to operating hours, a parts store.
- Labour: automation, remote monitoring, training that shortens failure time.
- Own energy: intermittent mixing, frequency inverters, pump selection, tank insulation.
- Digestate: reducing transport, the application layout, cooperation with farmers nearby.
- Insurance: demonstrating inspections and safety systems.
Calculation pitfalls
- omitting the cost of one's own farm labour in growing substrates,
- calculating without major overhauls (spread their cost over the years),
- no reserve for unplanned repairs,
- mixing investment and operating costs,
- comparing installations without standardising the denominator.
Practical rule: monthly budget control
Keep a table of actual monthly costs divided into groups and compare it with the plan and with the production volume. A sudden deviation (e.g. a rise in servicing of a dozen or so per cent) is often the first sign of a technical problem. Once a year carry out a review: which items are growing faster than production and why. Such discipline makes it possible to maintain profitability even with variable energy and substrate prices.
Variable and fixed costs
It is worth dividing OPEX into variable costs (dependent on the amount of substrate processed and energy produced: substrates, digestate transport, the part of servicing calculated from operating hours) and fixed costs (labour, insurance, fees, analyses). This helps to understand how the unit cost changes when production falls: fixed costs are spread over fewer kWh. An engine shutdown lasting several weeks raises the unit cost not only through the loss of revenue, but also by spreading the fixed costs over lower production.
Major overhauls as a cost spread over time
A major overhaul of a gas engine takes place after several tens of thousands of operating hours (the scope depends on the manufacturer and the gas quality). Instead of booking it as a one-off expense, spread its cost over operating hours and plan a reserve in the budget. Proceed in the same way with replacing the roof membrane, agitators and pumps: their service life is known from experience, and setting aside a reserve protects financial liquidity.
Benchmarking
Compare your own indicators (cost per kWh, per tonne of substrate, service hours per year) with data from previous years and, if possible, with anonymised data from other installations. Publications of institutes (KTBL, FNR, DBFZ) give methodologies and reference data for German conditions, which after conversion can serve as a point of reference, but do not replace your own calculations.
Sources
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