What Is the Electrical Efficiency of a Biogas CHP Engine?

A biogas CHP engine typically converts 35–45% of the gas energy into electricity, with the recovered heat lifting total CHP efficiency to 80–90%. The exact electrical figure tracks engine size: small units below 100 kWe land at 30–38%, mid-size 100–500 kWe at 38–42%, and large units above 1 MWe at 42–45%. Efficiency also depends on biogas methane content, because a lean 50% CH₄ gas simply carries less usable energy than a 60% one.

Why Electrical Efficiency Rises With Size

Larger reciprocating gas engines spread their fixed friction, auxiliary, and cooling losses over many more kilowatts, and they run at higher compression ratios and better specific output. A 50 kWe unit may waste a larger share of its gas just keeping its own systems alive, while a 1 MW engine approaches the thermodynamic ceiling of the Otto or lean-burn cycle. Heat recovery bridges the gap: even a modestly efficient engine delivers most of its lost energy back as 80–95 °C water or 500–550 °C exhaust.

Comparative Data Table: Electrical Efficiency by Engine Size

What Pushes Real Efficiency Below the Nameplate

Three factors erode the brochure number. Biogas quality is first: H₂S and siloxanes foul surfaces and force derating, and low methane content cuts the fuel heating value. Load factor is second—an engine throttled to 60% of rated output runs well below its peak efficiency, so a stable, year-round gas supply matters more than peak rating. Third, exhaust and jacket heat that has no user simply goes unrecovered, so a CHP plant without a heat customer is effectively a low-efficiency power-only unit.

Frequently Asked Questions (FAQ)

Q1: What is the electrical efficiency of a biogas CHP engine?
A: Typically 35–45%, rising from about 30–38% on units under 100 kWe to 42–45% above 1 MWe. Total combined heat-and-power efficiency reaches 80–90% with heat recovery.

Q2: How efficient is a small biogas genset?
A: Around 30–38% electrical for sub-100 kWe units, because fixed losses weigh more heavily on a smaller machine. That is why small plants should secure a heat user to recover the large thermal fraction.

Q3: What is total CHP efficiency?
A: Usually 80–90%, combining the electrical output with recovered heat from jacket water and exhaust. The heat share is often the larger part of the usable energy.

Q4: Does engine size change efficiency?
A: Yes, materially. Larger engines approach the thermodynamic ceiling of the cycle and spread losses over more output, so a 1 MW engine is typically 5–10 points more efficient electrically than a 50 kW unit.

Project Case Reference

Malaysia Biogas Project  —  Malaysia · 2026

5 GFS Tanks   ·   27,000 m³ Total Volume   ·   22,000 m³ Biogas/Day

A large-scale biogas project in Malaysia featuring 5 Glass-Fused-to-Steel (GFS) tanks. The project achieves approximately 80% digestibility, with each single tank producing about 4,400 m³ of biogas daily, totaling 22,000 m³ per day across all 5 tanks.

Technical Specifications

Single Tank Volume: 5,400 m³ (Ø24.46 × 12 m)

Total Effective Volume: 27,000 m³ (5 tanks)

Daily Biogas Production: 22,000 m³ total

Digestibility: ≈ 80%

Gas Production Rate: 0.45 m³ / kg COD removed

Tank Type: Glass-Fused-to-Steel (GFS)

Water Quality Data