{"id":16999,"date":"2018-03-09T13:44:38","date_gmt":"2018-03-09T13:44:38","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=16999"},"modified":"2022-11-10T09:05:29","modified_gmt":"2022-11-10T09:05:29","slug":"heat-engines","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/thermodynamics\/laws-of-thermodynamics\/heat-engines\/","title":{"rendered":"Heat Engines"},"content":{"rendered":"
In general, a heat engine<\/strong> is a device that converts chemical energy to heat or thermal energy and then to mechanical energy or electrical energy. For example, automotive engines are heat engines that can consume primary energy sources.<\/p>\n

All conventional thermal power plants are heat engines<\/strong> subject to the efficiency limitations imposed by the second law of thermodynamics<\/strong>.<\/p>\n

A typical gasoline automotive engine<\/strong> operates at around 25% to 30%<\/strong> of thermal efficiency. About 70-75% is rejected as waste heat without being converted into useful work, i.e., work delivered to wheels.<\/p>\n<\/div><\/div>\n

Energy sources<\/strong><\/a> have always played a very important role in the development of human society. Energy<\/a><\/strong> is generally defined as the potential to do work<\/a><\/strong> or produce heat<\/a><\/strong>. Sometimes it is like the \u201ccurrency\u201d for performing work. One of the most wonderful properties of the universe is that energy can be transformed from one type to another<\/strong> and transferred from one object to another<\/strong>.<\/p>\n

In general, it is easy to produce thermal energy<\/strong><\/a> by doing work<\/strong>, for example, by any frictional process. But to get work from thermal energy<\/strong> is more difficult<\/strong>. It is closely associated with the concept of entropy<\/a>. For example, electricity is particularly useful since it has very<\/strong> low entropy<\/strong> (is highly ordered) and can be converted into other forms of energy very efficiently<\/strong>.<\/p>\n

Sometimes, mechanical energy<\/a> is directly available, for example, wind power and hydropower. But most of our energy comes from burning fossil fuels<\/strong> (coal, oil, and gas) and nuclear reactions<\/strong><\/a>. At present, fossil fuel is still the world\u2019s predominant energy source. But the burning of fossil fuels generates only thermal energy<\/strong>. Therefore these energy sources are so-called \u201cprimary energy sources<\/strong><\/a>\u201d that must be converted<\/strong> to the secondary energy source<\/a><\/strong>, so-called energy carriers <\/strong>(electrical energy<\/a>, etc.). A heat engine<\/strong> must be used to convert thermal energy into another form of energy.<\/p>\n

In general, a heat engine<\/strong> is a device that converts chemical energy to heat or thermal energy and then to mechanical energy or electrical energy.<\/p>\n

\"Example<\/a>
The Rankine cycle closely describes the processes in steam-operated heat engines commonly found in most thermal power plants.<\/figcaption><\/figure>\n

Many heat engines<\/strong> operate cyclically, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle.
\nFor example, as is typical in all conventional thermal power plants,<\/strong>\u00a0the heat is used to generate steam<\/a> which drives a steam turbine<\/strong> connected to a generator that produces electricity. Steam generators<\/a>, steam turbines, condensers, and feedwater pumps constitute a heat engine<\/strong> subject to the efficiency limitations imposed by the second law of thermodynamics<\/strong>. In modern nuclear power plants<\/a>, the overall thermodynamic efficiency is about one-third <\/strong>(33%), so 3000 MWth<\/strong> of thermal power from the fission reaction is needed to generate 1000 MWe<\/strong> of electrical power.<\/p>\n

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Heat Engines and the Second Law<\/h2>\n

The second law of thermodynamics<\/strong><\/a> may be expressed in many specific ways. Each statement expresses the same law. Listed below are three that are often encountered.<\/p>\n