Quality and Second-Law Efficiency

It may be seen from the above that when transform­ing thermal energy into other forms of energy, the amount available is not the total energy available, but is smaller. However, if work is done by an agent, all the work is available; if electrical energy is available, all of it may be transformed directly into other, usable, forms of energy. Thermal energy is said to be low-quality energy because of its limitations in the conversion process, while work and electrical energy are high-quality energy.

Thermal energy may be used directly in space heating and in cooking, but if we want to run a motor with it, not all the thermal energy would be available to us. It would make sense to use forms of energy (if available) that could deliver all their energy to the motor. Conversely, if we want to increase the temperature of a room, it is silly to use high-quality energy to do it.

While the equation for efficiency is found using the first law of thermodynamics, the American Physical Society study ‘‘Efficient Uses of Energy’’ points out that the second law of thermodynamics, which shows that the Carnot engine operating between two temperatures is the most efficient, allows us to compare the actual efficiency to the theoretical maximum efficiency for producing a given amount of work; this is called the second-law efficiency for the system. For real thermal systems, the second-law efficiency measures how much room for improvement remains.

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