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2.3 Efficiency & Cost

Electrical energy results from converting one form of energy, generally mechanical energy, into another form, electricity. Not all of the original energy input into an energy converter changes to electricity. This leads to the idea of efficiency, or a rating of how well a converter changes energy from one form to another form. Efficiency has different meanings in different types of physical systems. In mechanical systems, use is made of thermal efficiency and mechanical efficiency, which describe the efficiency of the two corresponding phases of a given process. In the electrical systems that will be discussed, efficiency is defined as

This can also be expressed in terms of mechanical and electrical losses in either energy or power terms as

The energy use or efficiency of an electric machine is becoming increasingly significant. It is one of the more important design criteria today. Approximate maximum efficiencies of various types of energy converters are shown in Table 2.2.

Table 2.2 Approximate maximum efficiencies of energy converters.

Energy Converter Efficency, percent
Synchronous electric generator greater than 95
Large electric motor 90
Small electric motor 65
Fuel cell 60
Steam turbine 45
Steam power plant 40
Diesel engine 38
Automobile engine 30
Solar cell 10

The efficiency of energy converters has a direct effect on the financial cost to produce electricity. More efficient converters use less fuel or less space to produce the same amount of electricity. This also decreases the associated financial costs of the system. Current standards necessiate maximizing efficiency and minimizing the financial costs for electric systems.

In the next section, Electromechanical Energy Conversion, general conversion processes will be discussed. This section is coupled with a section on Direct Energy Conversion, where energy conversion occurs without using a mechnical turbine to electricity. Following these sections, conversion types are discussed more specifically. These sections lead us into more of the physical and engineering concepts behind electricity.



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