I think that engine efficiency comes from the thermodynamics, as the ratio of work done to the heat absorbed: e=Q1-Q2/Q1, see https://en.wikipedia.org/wiki/Engine_efficiency For all internal combustion engines there exists a cycle in p-V coordinates; see https://en.wikipedia.org/wiki/Internal_combustion_engine Even for ideal Carnot cycle efficiency is e=1-TC/TH, where TC, TH are temperatures of cooler and heater; see https://en.wikipedia.org/wiki/Carnot_cycle . Real engines have typically lower efficiency, in the range 30-50%.

Electric engine efficiency is defined as the ratio of  produced mechanical work to incoming electric energy. It can be very close to 100%; see https://en.wikipedia.org/wiki/Electric_motor

Actually, electrical efficiency refers to the electrically how useful the system is. In other words, how much electrical losses are there in the systems. It relates electrical input to electrical output from the system under consideration.

If a system is 95% electrically efficient, the there is only 5% electrical loss and thus a 100 unit of input electrical power will results into 95 units of electrical power.

Regards,

Mahamad Nabab Alam

Thank you for your replies.

For example, initially I have 15 MWh of energy as biogas. If I were to use it in a gas engine-generator set with electrical efficiency rating of 45%, does it mean that the electrical energy output is 6.75 MWh?

Or do I need to further multiple by the generator efficiency (assume 90%) giving an electrical energy output of 6.075 MWh?

Low efficiency is not due to electrical generator which works at about 95%, heat losses in combustion of gas/oil and flow of heat with exhaust gases from the engine brings down the net efficiency to 45%.

Electric power plant efficiency η is defined as the ratio between the useful electricity output from the generating unit, in a specific time, and the energy value of the energy source supplied to the unit in the same time period.

Efficiency Comparisons

The table below shows the theoretical efficiency of converting various energy sources by a variety of methods into useful electrical energy.

please refer: http://www.mpoweruk.com/energy_efficiency.htm

I guess that 45% efficiency is not electrical (usually not less than 95%) but overall genset efficiency. It means the ratio of the obtained electric energy to energy from the combusted fuel

You are right,  Jerzy Kowalski, 45% is total/overall efficiency of the genset with electrical generator efficiency at 95%.