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One of the consequences of the 2nd law is that it is impossible for a power plant to achieve 100% efficiency. In fact the maximum efficiency is limited by the temperature of the boiler and temperature of the condenser for power plants powered by heat (like coal, gas fired, and nuclear).

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12y ago
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13y ago

The 2nd law of thermodynamics basically states that there is always increasing entropy in the universe. Although the total quantity of energy is conserved, there is always a breakdown of useful energy into less useful forms. Usable energy, when processed, turns into a less efficient form. Entropy is the measure of unusable energy in a closed system, so therefore, entropy is increasing (2nd law).

As for the efficiency of power plants, that depends on the type. Coal-burning power plants have between 32 % to 42 % efficiency, natural gas fired ones have 32 % to 38 % efficiency, and nuclear power plants only have 0.27 % efficiency.

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14y ago

The efficiency of a power plant is usually measured as a ration of its electrical output to to amount of heat used, expressed as a percentage. So if we have a plant that has operates at 2000 mWt (thermal megawatts) to produce 600 mWe (electrical megawatts), its efficiency is 30%. Typical commercial plants range from about 30% to 65% efficiency. The more efficient plants cost more to build.

Efficiency depends more on how the energy is used rather than how it is produced because ratings are based on conversion of heat to electrical power, rather than the energy in the fuel to electricity or cost of fuel to electricity.

Thermal engines typically use less that 50% of the heat they generate. In a combined cycle plant, some part of the remainder of the heat is captured. In some plants, there are three ways for the heat to be captured. Combined cycle gas turbines can be 65% efficient, with the gas combustion driving a turbine, the exhaust heat driving a secondary electrical generator, and the residual heat used to heat buildings. This is called cogeneration - the production of two types of power from a single source.

Nevertheless, the method of production does had an effect on the efficiency of a combined cycle system. For example, natural gas can be used very effectively for cogeneration because the levels of pollution and danger of accident from natural gas are relatively low. Coal can be used for cogeneration, but not as efficiently, because the exhaust must be treated, which requires energy. Nuclear power is problematical for cogeneration because the plants cannot be sited in dense population centers due to needs to account for evacuation in case of accident.

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11y ago

Because Energy flow is from a high energy state to a low energy state the direction of flow of heat is from a high temperature to a low temperature. Because the second law states that no system can perfectly maintain it's total energy, the efficiency of a heat engine must necessarily be reduced by the entropy of the energy transfer.

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Q: Why does the 2nd law of thermodynamics relate to the direction of heat flow and efficiency of heat engines?
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