Brittany Kelp Biogas Cooperative Profit Modeler

Calculator explanation (model, formulas, and assumptions)

What this model is for

This calculator is a quick project-finance model for a kelp-to-biogas cooperative. It converts harvested wet kelp into dry matter, estimates biogas and methane production, converts methane to energy, and then estimates annual cash flow from three revenue channels (electricity, heat, digestate) minus operating and maintenance costs. It then projects those annual benefits over your chosen horizon to compute cumulative cash flow, NPV, and a simple payback year.

Units and inputs (what each field means)

• Kelp harvested per week (tonnes wet): average weekly wet mass delivered to the digester. The model multiplies by 52 to estimate annual wet tonnage.
• Average dry matter content (%): fraction of wet mass that is dry matter (e.g., 18% means 0.18). Annual dry tonnes = annual wet tonnes × dry matter fraction.
• Biogas yield (m³ per tonne dry): biogas volume per tonne of dry matter.
• Methane share of biogas (%): methane fraction of biogas volume.
• CHP electrical efficiency (%): fraction of methane energy converted to electricity. The remainder is treated as recoverable heat.
• Feed-in tariff (EUR per kWh): electricity price per kWh exported.
• Heat sales price (EUR per MWh): heat price per MWh (note the unit is MWh, not kWh).
• Digestate value (EUR per tonne wet): digestate revenue per tonne of wet kelp processed (a simplified proxy for digestate output and pricing).
• Operating cost (EUR per tonne wet): variable cost per tonne wet (collection, transport, handling, consumables).
• Annual maintenance and staffing (EUR): fixed annual cost for maintenance contracts, staffing, monitoring, and compliance.
• Capital expenditure (EUR) and grant support (EUR): net upfront investment = capex − grant.
• Cooperative members sharing profits: used only to compute an indicative payout per member (annual benefit ÷ members).
• Analysis horizon (years) and discount rate (%): used for NPV and discounted cash flow.

Core formulas used

The calculator uses the following steps (annualized):

1. Annual wet tonnes = harvest per week × 52
2. Annual dry tonnes = annual wet tonnes × (dry matter % ÷ 100)
3. Biogas volume (m³) = annual dry tonnes × biogas yield
4. Methane volume (m³) = biogas volume × (methane share % ÷ 100)
5. Energy (kWh) = methane volume × 10 Assumption: 1 m³ CH₄ ≈ 10 kWh (lower heating value approximation).
6. Electricity output (kWh) = energy × (CHP electrical efficiency % ÷ 100)
7. Heat output (MWh) = (energy × (1 − electrical efficiency)) ÷ 1000
8. Revenues:
   • Electricity revenue = electricity output × feed-in tariff
   • Heat revenue = heat output (MWh) × heat price
   • Digestate revenue = annual wet tonnes × digestate value
9. Costs:
   • Operating cost = annual wet tonnes × operating cost per tonne
   • Maintenance = annual maintenance and staffing
10. Annual net benefit = total revenues − operating cost − maintenance
11. NPV = −(capex − grant) + Σ(annual net benefit ÷ (1 + r)^year)

Worked example (using the default inputs)

If you keep the default values (40 t/week wet, 18% dry matter, 320 m³/t dry biogas yield, 60% methane, 35% electrical efficiency, €0.185/kWh tariff, €32/MWh heat, €8/t digestate, €45/t operating cost, €110k maintenance, €2.4M capex, €0.4M grant, 24 members, 12 years, 5.5% discount), the model will:

• Process about 2,080 t/year wet kelp and 374.4 t/year dry matter.
• Produce about 119,808 m³/year biogas and 71,885 m³/year methane.
• Estimate electricity and heat revenues based on the CHP split, then subtract variable and fixed costs.
• Compute an indicative per-member payout by dividing annual net benefit by 24 (this is not a governance rule—just a simple split for planning discussions).

Use the results panel to sanity-check scale: doubling weekly harvest should roughly double wet tonnes, dry tonnes, biogas, methane, and the variable parts of revenue and operating cost.

Important limitations (what this model does not include)

• No debt service: the model does not include loan schedules, interest, DSCR, or taxes.
• Constant annual performance: it assumes the same annual net benefit each year (no degradation, downtime, inflation, or tariff step-downs).
• Simplified digestate economics: digestate revenue is modeled as a simple €/t wet coefficient; real projects depend on nutrient analysis, dewatering, storage, and hauling distance.
• Heat sales depend on offtake: heat revenue assumes you can sell the modeled heat at the stated price; in practice, heat offtake can be seasonal and constrained by network capacity.