CHP Efficiency for Biogas
Combined heat and power plant efficiency is a function of the conversion efficiency of the energy in the fuel gas to useful energy in the form of electricity and heat.
A combined heat and power (CHP) plant is typically a reciprocating gas engine that uses the energy in the gas to drive a crank shaft. The crank shaft turns an alternator to produce electricity. Heat is released during the gas combustion process. This heat can be recovered during cogeneration in order to maximise the heating value of the system.
Energy conversion and biogas CHP plant efficiency
The conversion efficiency of a combined heat and power plant can be defined as
Electrical output from an alternator + utilised thermal output from gas engine
Divided by
Energy input to gas engine based upon LHV of gas
X 100
Alternatively it is defined as the electrical efficiency of the CHP plus, the thermal efficiency of the CHP.
Before conversion into electrical efficiency the energy in the biogas must first be converted into mechanical electricity. There is a small amount of loss during this process.
Gross electrical efficiency is straight forward to calculate. For a high efficiency biogas engine this can reach 43% depending upon the model and electrical output.