Thermal Energy Summary

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Understanding laws of thermodynamics is fundamental to understanding the workings of the universe. In this chapter we covered these laws and their relevance in designing useful thermal devices. In describing the working principle of such devices (a thermal power plant, a heat engine, a refrigerator, or any other thermodynamic system for that matter) we concluded that:

1) At least two reservoirs, one at a high temperature and the other at a low temperature, are needed. This is one of the important consequences of the second law of thermodynamics, which states that we cannot convert heat to work with a single heat source. In the case of a steam power plant, the steam generator and condenser are the two reservoirs. In internal combustion engines, the two heat reservoirs are hot combustion gases and cold exhaust gases.

2) Some form of energy is always needed to provide heat to the high-temperature heat reservoir. For power plants, energy needed to heat water comes from an external source which could be fossil, nuclear, or less commonly solar or geothermal.

3) To increase efficiency, we must either increase the temperature of the source or lower the temperature of the sink. We are limited to atmospheric temperatures for the sink, but we can increase the source temperature. In power plants, we can raise steam temperature by increasing the boiler pressure (water boils at a higher temperature if the pressure is greater). Raising the compression ratio, minimizing heat losses, and using better fuels can similarly increase internal combustion efficiency.

4) Higher efficiencies not only reduce fuel consumption, but also protect our environment by producing fewer pollutants. In addition, higher efficiency means a smaller fraction of energy must be disposed of as waste heat, commonly termed “thermal pollution,” which is a major source of global warming.


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

Further Reading

El-Sayed, Y., The Thermodynamics of Energy Conversions, Elsevier Direct Science, 2003.

Cengel, Y. A., Heat Transfer: A Practical Approach, McGraw-Hill, Inc., 1998.

Rifkin, J., Entropy, The Viking Press, 1980.

El-Wakil, M/ M., Power Plant Technology, McGraw-Hill, Inc., 1984.

Energy and Buildings, Science Direct Elsevier Publishing Company. An international journal publishing articles about energy use in buildings and indoor environment quality.

Energy Conversion and Management, Science Direct Elsevier Publishing Company. This journal focuses on energy efficiency and management; heat pipes; space and terrestrial power systems; hydrogen production and storage; renewable energy; nuclear power; fuel cells and advanced batteries.

Energy and Buildings, Science Direct Elsevier Publishing Company, An international journal dedicated to investigations of energy use and efficiency in buildings.

External Links

How Things Work (

How Stuff Works (

California Energy Commission Consumer Energy Center (