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Engr.Azher
What is difference between thermal efficieny,overall efficiency and relative efficieny ? View All


13 years ago - 2 weeks left to answer. - 1 response - Report Abuse
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Niel
Azher:

The three types of efficiency you mention are used in different applications.

Relative Efficiency is not generally an engineering term. It is a statistical term [1]. It is used as a method to evaluate the efficiency of two different procedures.

Thermal efficiency and overall efficiency are engineering terms.

Thermal efficiency relates to processes where the input is thermal energy as in a fired power plant, a internal combustion engine or solar energy systems. It is usually based on the Carnot Cycle for the system in question [2]. It is the ratio of the mechanical output to the thermal input.

Overall efficiency looks at entire systems from the initial input to the final output. The input does not necessarily have to be thermal in nature. Examples of systems where the input is not thermal include hydro-power systems, electrical motors, and gear trains. Again it is the ratio of energy output to energy input.

Yes, you can measure relative efficiencies of two different devices, but most of the time engineers do don't look at this. One example would be to look at two engines.

If one has a thermal efficiency of 33% and the other has an efficiency of 30% the relative efficiency of the second engine only 90.9% (30/33) that of the first engine. Alternately the first engine is 10% (33/30) more efficient than the second engine.

Niel


13 years ago

Source: [1] http://en.wikipedia.org/wiki/Efficiency_(statistics)

[2] http://en.wikipedia.org/wiki/Carnot_cycle


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