Thermal Devices

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The first law of thermodynamics showed that we could convert energy from one form to another. When its brakes are applied, a car will slow down because of friction. As a result, both the tires and the road become a bit warmer. In this example, work has been turned to thermal energy or heat; therefore, work and heat are quantitatively the same. Experience tells us the reverse, cooling the tires and road to move the car backward, is not possible. This shows that thermal energy (heat) and mechanical energy (work) are qualitatively different - work can be turned entirely to heat, while the reverse is not true. This simple observation is a direct consequence of the second law of thermodynamics, which states that processes can occur naturally in one direction and not the other, although energy expenditure is exactly the same in both cases.

One of the applications of thermodynamics is in designing devices that transform one form of energy to a more useful form. Of course we wish to design these devices using the least amount of energy and with the highest efficiencies possible. The first law of thermodynamics requires a minimum amount of energy to achieve a task. The second law of thermodynamics puts a limit on how efficient a device can be. From a theoretical standpoint, a device is most efficient (ideal) when it operates with no frictional losses; in reality, most systems have much lower efficiencies.

Some examples of thermal devices we use or are impacted by in everyday life are engines, power plants, refrigerators, heat pumps, and air conditioners. In all these devices, some form of energy (fuel) is consumed. Whether used in automobiles or jet aircrafts, heat engines convert part of this energy to shaft work that eventually runs the vehicle (Figure 1a). Power plants work in a similar fashion, but their work output is mainly in the form of electricity. Refrigerators, air conditioners, and heat pumps work in essentially the reverse direction; they use fuel energy to “pump” heat away from the space we want to cool or “pump” heat into the space that we want to heat (Figure 1b). No matter what the application, part of the energy is always discarded as waste energy into the surrounding atmosphere. In other words, it is impossible to build devices that convert 100% of the input energy into useful forms. Furthermore, because there are always some frictional losses, actual efficiency is always less than the maximum theoretical efficiency dictated by the laws of thermodynamics. Table 1 shows the typical efficiencies of several machines.

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Figure 1: Heat Engines and Refrigerators

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Table 1: Typical Efficiencies of Some Complex Machines

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References

Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

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